Turbulent flame propagation in partially premixed flames
NASA Technical Reports Server (NTRS)
Poinsot, T.; Veynante, D.; Trouve, A.; Ruetsch, G.
1996-01-01
Turbulent premixed flame propagation is essential in many practical devices. In the past, fundamental and modeling studies of propagating flames have generally focused on turbulent flame propagation in mixtures of homogeneous composition, i.e. a mixture where the fuel-oxidizer mass ratio, or equivalence ratio, is uniform. This situation corresponds to the ideal case of perfect premixing between fuel and oxidizer. In practical situations, however, deviations from this ideal case occur frequently. In stratified reciprocating engines, fuel injection and large-scale flow motions are fine-tuned to create a mean gradient of equivalence ratio in the combustion chamber which provides additional control on combustion performance. In aircraft engines, combustion occurs with fuel and secondary air injected at various locations resulting in a nonuniform equivalence ratio. In both examples, mean values of the equivalence ratio can exhibit strong spatial and temporal variations. These variations in mixture composition are particularly significant in engines that use direct fuel injection into the combustion chamber. In this case, the liquid fuel does not always completely vaporize and mix before combustion occurs, resulting in persistent rich and lean pockets into which the turbulent flame propagates. From a practical point of view, there are several basic and important issues regarding partially premixed combustion that need to be resolved. Two such issues are how reactant composition inhomogeneities affect the laminar and turbulent flame speeds, and how the burnt gas temperature varies as a function of these inhomogeneities. Knowledge of the flame speed is critical in optimizing combustion performance, and the minimization of pollutant emissions relies heavily on the temperature in the burnt gases. Another application of partially premixed combustion is found in the field of active control of turbulent combustion. One possible technique of active control consists of pulsating
NASA Astrophysics Data System (ADS)
Duchamp de Lageneste, Laurent; Pitsch, Heinz
2001-11-01
Level-set methods (G-equation) have been recently used in the context of RANS to model turbulent premixed (Hermann 2000) or partially premixed (Chen 1999) combustion. By directly taking into account unsteady effects, LES can be expected to improve predictions over RANS. Since the reaction zone thickness of premixed flames in technical devices is usually much smaller than the LES grid spacing, chemical reactions completely occur on the sub-grid scales and hence have to be modeled entirely. In the level-set methodology, the flame front is represented by an arbitrary iso-surface G0 of a scalar field G whose evolution is described by the so-called G-equation. This equation is only valid at G=G_0, and hence decoupled from other G levels. Heat release is then modeled using a flamelet approach in which temperature is determined as a function of G and the mixture-fraction Z. In the present study, the proposed approach has been formulated for LES and validated using data from a turbulent Bunsen burner experiment (Chen, Peters 1996). Simulation of an experimental Lean Premixed Prevapourised (LPP) dump combustor (Besson, Bruel 1999, 2000) under different premixed or partially premixed conditions will also be presented.
LES-Modeling of a Partially Premixed Flame using a Deconvolution Turbulence Closure
NASA Astrophysics Data System (ADS)
Wang, Qing; Wu, Hao; Ihme, Matthias
2015-11-01
The modeling of the turbulence/chemistry interaction in partially premixed and multi-stream combustion remains an outstanding issue. By extending a recently developed constrained minimum mean-square error deconvolution (CMMSED) method, to objective of this work is to develop a source-term closure for turbulent multi-stream combustion. In this method, the chemical source term is obtained from a three-stream flamelet model, and CMMSED is used as closure model, thereby eliminating the need for presumed PDF-modeling. The model is applied to LES of a piloted turbulent jet flame with inhomogeneous inlets, and simulation results are compared with experiments. Comparisons with presumed PDF-methods are performed, and issues regarding resolution and conservation of the CMMSED method are examined. The author would like to acknowledge the support of funding from Stanford Graduate Fellowship.
NASA Astrophysics Data System (ADS)
Juddoo, Mrinal; Masri, Assaad R.; Pope, Stephen B.
2011-12-01
This paper reports measured stability limits and PDF calculations of piloted, turbulent flames of compressed natural gas (CNG) partially-premixed with either pure oxygen, or with varying levels of O2/N2. Stability limits are presented for flames of CNG fuel premixed with up to 20% oxygen as well as CNG-O2-N2 fuel where the O2 content is varied from 8 to 22% by volume. Calculations are presented for (i) Sydney flame B [Masri et al. 1988] which uses pure CNG as well as flames B15 to B25 where the CNG is partially-premixed with 15-25% oxygen by volume, respectively and (ii) Sandia methane-air (1:3 by volume) flame E [Barlow et al. 2005] as well as new flames E15 and E25 that are partially-premixed with 'reconstituted air' where the O2 content in nitrogen is 15 and 25% by volume, respectively. The calculations solve a transported PDF of composition using a particle-based Monte Carlo method and employ the EMST mixing model as well as detailed chemical kinetics. The addition of oxygen to the fuel increases stability, shortens the flames, broadens the reaction zone, and shifts the stoichiometric mixture fraction towards the inner side of the jet. It is found that for pure CNG flames where the reaction zone is narrow (∼0.1 in mixture fraction space), the PDF calculations fail to reproduce the correct level of local extinction on approach to blow-off. A broadening in the reaction zone up to about 0.25 in mixture fraction space is needed for the PDF/EMST approach to be able to capture these finite-rate chemistry effects. It is also found that for the same level of partial premixing, increasing the O2/N2 ratio increases the maximum levels of CO and NO but shifts the peak to richer mixture fractions. Over the range of oxygenation investigated here, stability limits have shown to improve almost linearly with increasing oxygen levels in the fuel and with increasing the contribution of release rate from the pilot.
Coriton, Bruno; Frank, Jonathan H.
2016-02-16
In turbulent flows, the interaction between vorticity, ω, and strain rate, s, is considered a primary mechanism for the transfer of energy from large to small scales through vortex stretching. The ω-s coupling in turbulent jet flames is investigated using tomographic particle image velocimetry (TPIV). TPIV provides a direct measurement of the three-dimensional velocity field from which ω and s are determined. The effects of combustion and mean shear on the ω-s interaction are investigated in turbulent partially premixed methane/air jet flames with high and low probabilities of localized extinction as well as in a non-reacting isothermal air jet withmore » Reynolds number of approximately 13,000. Results show that combustion causes structures of high vorticity and strain rate to agglomerate in highly correlated, elongated layers that span the height of the probe volume. In the non-reacting jet, these structures have a more varied morphology, greater fragmentation, and are not as well correlated. The enhanced spatiotemporal correlation of vorticity and strain rate in the stable flame results in stronger ω-s interaction characterized by increased enstrophy and strain-rate production rates via vortex stretching and straining, respectively. The probability of preferential local alignment between ω and the eigenvector of the intermediate principal strain rate, s2, which is intrinsic to the ω-s coupling in turbulent flows, is larger in the flames and increases with the flame stability. The larger mean shear in the flame imposes a preferential orientation of ω and s2 tangential to the shear layer. The extensive and compressive principal strain rates, s1 and s3, respectively, are preferentially oriented at approximately 45° with respect to the jet axis. As a result, the production rates of strain and vorticity tend to be dominated by instances in which ω is parallel to the s1¯-s2¯ plane and orthogonal to s3¯.« less
Dhamale, N.; Parthasarathy, R. N.; Gollahalli, S. R.
2011-01-01
Canola methyl ester (CME) is a biofuel that is a renewable alternative energy resource and is produced by the transesterification of canola oil. The objective of this study was to document the effects of turbulence on the combustion characteristics of blends of CME and No 2 diesel fuel in a partially-premixed flame environment. The experiments were conducted with mixtures of pre-vaporized fuel and air at an initial equivalence ratio of 7 and three burner exit Reynolds numbers, 2700, 3600, and 4500. Three blends with 25, 50, and 75% volume concentration of CME were studied. The soot volume fraction was highestmore » for the pure diesel flames and did not change significantly with Reynolds number due to the mutually compensating effects of increased carbon input rate and increased air entrainment as the Reynolds number was increased. The global NOx emission index was highest and the CO emission index was the lowest for the pure CME flame, and varied non-monotonically with biofuel content in the blend The mean temperature and the NOx concentration at three-quarter flame height were generally correlated, indicating that the thermal mechanism of NOx formation was dominant in the turbulent biofuel flames also.« less
Turbulent transport in premixed flames
NASA Technical Reports Server (NTRS)
Rutland, C. J.; Cant, R. S.
1994-01-01
Simulations of planar, premixed turbulent flames with heat release were used to study turbulent transport. Reynolds stress and Reynolds flux budgets were obtained and used to guide the investigation of important physical effects. Essentially all pressure terms in the transport equations were found to be significant. In the Reynolds flux equations, these terms are the major source of counter-gradient transport. Viscous and molecular terms were also found to be significant, with both dilatational and solenoidal terms contributing to the Reynolds stress dissipation. The BML theory of premixed turbulent combustion was critically examined in detail. The BML bimodal pdf was found to agree well with the DNS data. All BML decompositions, through the third moments, show very good agreement with the DNS results. Several BML models for conditional terms were checked using the DNS data and were found to require more extensive development.
Coriton, Bruno; Frank, Jonathan H.
2016-02-16
In turbulent flows, the interaction between vorticity, ω, and strain rate, s, is considered a primary mechanism for the transfer of energy from large to small scales through vortex stretching. The ω-s coupling in turbulent jet flames is investigated using tomographic particle image velocimetry (TPIV). TPIV provides a direct measurement of the three-dimensional velocity field from which ω and s are determined. The effects of combustion and mean shear on the ω-s interaction are investigated in turbulent partially premixed methane/air jet flames with high and low probabilities of localized extinction as well as in a non-reacting isothermal air jet with Reynolds number of approximately 13,000. Results show that combustion causes structures of high vorticity and strain rate to agglomerate in highly correlated, elongated layers that span the height of the probe volume. In the non-reacting jet, these structures have a more varied morphology, greater fragmentation, and are not as well correlated. The enhanced spatiotemporal correlation of vorticity and strain rate in the stable flame results in stronger ω-s interaction characterized by increased enstrophy and strain-rate production rates via vortex stretching and straining, respectively. The probability of preferential local alignment between ω and the eigenvector of the intermediate principal strain rate, s_{2}, which is intrinsic to the ω-s coupling in turbulent flows, is larger in the flames and increases with the flame stability. The larger mean shear in the flame imposes a preferential orientation of ω and s_{2} tangential to the shear layer. The extensive and compressive principal strain rates, s_{1} and s_{3}, respectively, are preferentially oriented at approximately 45° with respect to the jet axis. As a result, the production rates of strain and vorticity tend to be dominated by instances in which ω is parallel to the s_{1}¯-s_{2}¯ plane and orthogonal
Premixed autoignition in compressible turbulence
NASA Astrophysics Data System (ADS)
Konduri, Aditya; Kolla, Hemanth; Krisman, Alexander; Chen, Jacqueline
2016-11-01
Prediction of chemical ignition delay in an autoignition process is critical in combustion systems like compression ignition engines and gas turbines. Often, ignition delay times measured in simple homogeneous experiments or homogeneous calculations are not representative of actual autoignition processes in complex turbulent flows. This is due the presence of turbulent mixing which results in fluctuations in thermodynamic properties as well as chemical composition. In the present study the effect of fluctuations of thermodynamic variables on the ignition delay is quantified with direct numerical simulations of compressible isotropic turbulence. A premixed syngas-air mixture is used to remove the effects of inhomogeneity in the chemical composition. Preliminary results show a significant spatial variation in the ignition delay time. We analyze the topology of autoignition kernels and identify the influence of extreme events resulting from compressibility and intermittency. The dependence of ignition delay time on Reynolds and turbulent Mach numbers is also quantified. Supported by Basic Energy Sciences, Dept of Energy, United States.
Turbulent Premixed Flames in Microgravity
NASA Technical Reports Server (NTRS)
Menon, Suresh
1996-01-01
The experimental cold-flow facility is now full operational and is currently being used to obtain baseline turbulence data in a Couette flow. The baseline turbulence data is necessary to confirm the capability of the chosen device to generate and maintain the required turbulence intensity. Subsequent reacting flow studies will assume that a similar turbulent flow field exists ahead of the premixed flame. Some modifications and refinements had to be made to enable accurate measurements. It consists of two rollers, one (driven by a motor) which drives a continuous belt and four smaller rollers used to set the belt spacing and tension to minimize belt flutter. The entire assemble is enclosed in a structure that has the dimensions to enable future drop tower experiments of the hot facility. All critical dimensions are the same as the original plans except for the pulley ratio which has been changed to enable a wider operating regime in terms of the Reynolds number. With the current setup, Reynolds numbers as low as 100 and as high as 14,000 can be achieved. This is because the in-between belt spacing can be varied from 1 cm to 7.6 cm, and the belt speed can be accurately varied from .15 m/sec to 3.1 m/sec.
Gravity Effects Observed In Partially Premixed Flames
NASA Technical Reports Server (NTRS)
Puri, Ishwar K.; Aggarwal, Suresh K.; Lock, Andrew J.; Gauguly, Ranjan; Hegde, Uday
2003-01-01
Partially premixed flames (PPFs) contain a rich premixed fuel air mixture in a pocket or stream, and, for complete combustion to occur, they require the transport of oxidizer from an appropriately oxidizer-rich (or fuel-lean) mixture that is present in another pocket or stream. Partial oxidation reactions occur in fuel-rich portions of the mixture and any remaining unburned fuel and/or intermediate species are consumed in the oxidizer-rich portions. Partial premixing, therefore, represents that condition when the equivalence ratio (phi) in one portion of the flowfield is greater than unity, and in another section its value is less than unity. In general, for combustion to occur efficiently, the global equivalence ratio is in the range fuel-lean to stoichiometric. These flames can be established by design by placing a fuel-rich mixture in contact with a fuel-lean mixture, but they also occur otherwise in many practical systems, which include nonpremixed lifted flames, turbulent nonpremixed combustion, spray flames, and unwanted fires. Other practical applications of PPFs are reported elsewhere. Although extensive experimental studies have been conducted on premixed and nonpremixed flames under microgravity, there is a absence of previous experimental work on burner stabilized PPFs in this regard. Previous numerical studies by our group employing a detailed numerical model showed gravity effects to be significant on the PPF structure. We report on the results of microgravity experiments conducted on two-dimensional (established on a Wolfhard-Parker slot burner) and axisymmetric flames (on a coannular burner) that were investigated in a self-contained multipurpose rig. Thermocouple and radiometer data were also used to characterize the thermal transport in the flame.
Premixed turbulent flame propagation in microgravity
NASA Technical Reports Server (NTRS)
Menon, S.; Jagoda, J.; Sujith, R.
1995-01-01
To reduce pollutant formation there is, at present, an increased interest in employing premixed fuel/air mixture in combustion devices. It is well known that greater control over local temperature can be achieved with premixed flames and with lean premixed mixtures, significant reduction of pollutants such as NO(x) can be achieved. However, an issue that is still unresolved is the predictability of the flame propagation speed in turbulent premixed mixtures, especially in lean mixtures. Although substantial progress has been made in recent years, there is still no direct verification that flame speeds in turbulent premixed flows are highly predictable in complex flow fields found in realistic combustors. One of the problems associated with experimental verification is the difficulty in obtaining access to all scales of motion in typical high Reynolds number flows, since, such flows contain scales of motion that range from the size of the device to the smallest Kolmogorov scale. The overall objective of this study is to characterize the behavior of turbulent premixed flames at reasonable high Reynolds number, Re(sub L). Of particular interest here is the thin flame limit where the laminar flame thickness is much smaller than the Kolmogorov scale. Thin flames occur in many practical combustion devices and will be numerically studied using a recently developed new formulation that is briefly described.
Dynamics and structure of turbulent premixed flames
NASA Technical Reports Server (NTRS)
Bilger, R. W.; Swaminathan, N.; Ruetsch, G. R.; Smith, N. S. A.
1995-01-01
In earlier work (Mantel & Bilger, 1994) the structure of the turbulent premixed flame was investigated using statistics based on conditional averaging with the reaction progress variable as the conditioning variable. The DNS data base of Trouve and Poinsot (1994) was used in this investigation. Attention was focused on the conditional dissipation and conditional axial velocity in the flame with a view to modeling these quantities for use in the conditional moment closure (CMC) approach to analysis of kinetics in premixed flames (Bilger, 1993). Two remarkable findings were made: there was almost no acceleration of the axial velocity in the flame front itself; and the conditional scalar dissipation remained as high, or higher, than that found in laminar premixed flames. The first finding was surprising since in laminar flames all the fluid acceleration occurs through the flame front, and this could be expected also for turbulent premixed flames at the flamelet limit. The finding gave hope of inventing a new approach to the dynamics of turbulent premixed flames through use of rapid distortion theory or an unsteady Bernoulli equation. This could lead to a new second order closure for turbulent premixed flames. The second finding was contrary to our measurements with laser diagnostics in lean hydrocarbon flames where it is found that conditional scalar dissipation drops dramatically below that for laminar flamelets when the turbulence intensity becomes high. Such behavior was not explainable with a one-step kinetic model, even at non-unity Lewis number. It could be due to depletion of H2 from the reaction zone by preferential diffusion. The capacity of the flame to generate radicals is critically dependent on the levels of H2 present (Bilger, et al., 1991). It seemed that a DNS computation with a multistep reduced mechanism would be worthwhile if a way could be found to make this feasible. Truly innovative approaches to complex problems often come only when there is the
Internal structure of a premixed turbulent flame
Rajan, S.; Smith, J.R.; Rambach, G.D.
1982-10-01
A pulsed laser and a multielement detector have been used to make instantaneous Rayleigh profiles along a line through a turbulent flame front thus eliminating the effects of flame front motion. The flame front in a premixed turbulent flame moves randomly about a mean position, giving rise to the visually observed flame brush or time-averaged flame thickness which is larger than the instantaneous thickness of the reaction zone. The physical characteristics and statistical properties of such turbulent flames reported previously were deduced from the time histories of Rayleigh scattered laser light at one or two points within the reaction zone. The study was conducted on a premixed propane-air flame stabilized on a rod at the exit plane of a square burner. Turbulence-producing screens below the burner exit controlled turbulent length scales while intensity was controlled with inlet mixture velocity. Turbulence properties of the cold reactants were determined by hot-wire anemometry. Mean and fluctuating velocity in the unburnt and burnt gases were measured using laser Doppler velocimetry. At the low level of turbulence studied, the instantaneous flame front thickness was found to be only slightly greater than the laminar flame thickness, and the magnitude of the density fluctuations only slightly greater than the cold flow turbulence intensity. Mean and rms values of density and velocity; density and velocity probability density functions; spatial density correlations; and comparison of data with the Bray-Moss-Libby model are presented.
Numerical simulation of premixed turbulent methane combustion
Day, Marc S.; Bell, John B.; Almgren, Ann S.; Beckner, Vincent E.; Lijewski, Michael J.; Cheng, Robert; Shepherd, Ian; Johnson, Matthew
2003-06-14
With adaptive-grid computational methodologies and judicious use of compressible and low Mach number combustion models, we are carrying out three-dimensional, time-dependent direct numerical simulations of a laboratory-scale turbulent premixed methane burner. In the laboratory experiment, turbulence is generated by a grid located in the throat of a 50mm diameter circular nozzle; swirl is be introduced by four tangential air jets spaced uniformly around the circumference of the nozzle just above the turbulence grid. A premixed methane flame is stabilized above the nozzle in the central core region where a velocity deficit is induced7the swirling flow. The time-dependent flow field inside the nozzle, from the turbulence grid and the high-speed jets, to the nozzle exit plane is simulated using an adaptive-grid embedded-boundary compressible Navier-Stokes solver. The compressible calculation then provides time-dependent boundary conditions for an adaptive low Mach number model of the swirl-stabilized premixed flame. The low Mach model incorporates detailed chemical kinetics and species transport using 20 species and 84 reactions. Laboratory diagnostics available for comparisons include characterizations of the flow field just down stream of the nozzle exit plane, and flame surface statistics, such as mean location, wrinkling and crossing frequencies.
Premixed Turbulent Flame Propagation in Microgravity
NASA Technical Reports Server (NTRS)
Menon, Suresh
1999-01-01
A combined numerical-experimental study has been carried out to investigate the structure and propagation characteristics of turbulent premixed flames with and without the influence of buoyancy. Experimentally, the premixed flame characteristics are studied in the wrinkled regime using a Couette flow facility and an isotropic flow facility in order to resolve the scale of flame wrinkling. Both facilities were chosen for their ability to achieve sustained turbulence at low Reynolds number. This implies that conventional diagnostics can be employed to resolve the smallest scales of wrinkling. The Couette facility was also built keeping in mind the constraints imposed by the drop tower requirements. Results showed that the flow in this Couette flow facility achieves full-developed turbulence at low Re and all turbulence statistics are in good agreement with past measurements on large-scale facilities. Premixed flame propagation studies were then carried out both using the isotropic box and the Couette facility. Flame imaging showed that fine scales of wrinkling occurs during flame propagation. Both cases in Ig showed significant buoyancy effect. To demonstrate that micro-g can remove this buoyancy effect, a small drop tower was built and drop experiments were conducted using the isotropic box. Results using the Couette facility confirmed the ability to carry out these unique reacting flow experiments at least in 1g. Drop experiments at NASA GRC were planned but were not completed due to termination of this project.
Displacement speeds in turbulent premixed flame simulations
Day, Marcus S.; Shepherd, Ian G.; Bell, J.; Grcar, Joseph F.; Lijewski, Michael J.
2007-07-01
The theory of turbulent premixed flames is based on acharacterization of the flame as a discontinuous surface propagatingthrough the fluid. The displacement speed, defined as the local speed ofthe flame front normal to itself, relative to the unburned fluid,provides one characterization of the burning velocity. In this paper, weintroduce a geometric approach to computing displacement speed anddiscuss the efficacy of the displacement speed for characterizing aturbulent flame.
Ignition transition in turbulent premixed combustion
Shy, S.S.; Liu, C.C.; Shih, W.T.
2010-02-15
Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion in a centrally-ignited, fan-stirred cruciform burner capable of generating intense isotropic turbulence. Using the same methodology, this paper presents new complete MIE data sets for stoichiometric and rich cases at three different equivalence ratios {phi} = 1.0, 1.2 and 1.3, each covering a wide range of a turbulent Karlovitz number (Ka) indicating a time ratio between chemical reaction and turbulence. Thus, ignition transition in premixed turbulent combustion depending on both Ka and {phi} can be identified for the first time. It is found that there are two distinct modes on ignition in randomly stirred methane-air mixtures (ignition transition) separated by a critical Ka where values of Ka{sub c} {approx} 8-26 depending on {phi} with the minimum Ka{sub c} occurring near {phi} = 1. For Ka < Ka{sub c}, MIE increases gradually with Ka, flame kernel formation is similar to laminar ignition remaining a torus, and 2D laser tomography images of subsequent outwardly-propagating turbulent flames show sharp fronts. For Ka > Ka{sub c}, MIE increases abruptly with Ka, flame kernel is disrupted, and subsequent randomly-propagating turbulent flames reveal distributed-like fronts. Moreover, we introduce a reaction zone Peclet number (P{sub RZ}) indicating the diffusivity ratio between turbulence and chemical reaction, such that the aforementioned very scattering MIE data depending on Ka and {phi} can be collapsed into a single curve having two drastically different increasing slopes with P{sub RZ} which are separated by a critical P{sub RZ} {approx} 4.5 showing ignition transition. Finally, a physical model is proposed to explain these results. (author)
Simulation of lean premixed turbulent combustion
NASA Astrophysics Data System (ADS)
Bell, J.; Day, M.; Almgren, A.; Lijewski, M.; Rendleman, C.; Cheng, R.; Shepherd, I.
2006-09-01
There is considerable technological interest in developing new fuel-flexible combustion systems that can burn fuels such as hydrogen or syngas. Lean premixed systems have the potential to burn these types of fuels with high efficiency and low NOx emissions due to reduced burnt gas temperatures. Although traditional Scientific approaches based on theory and laboratory experiment have played essential roles in developing our current understanding of premixed combustion, they are unable to meet the challenges of designing fuel-flexible lean premixed combustion devices. Computation, with its ability to deal with complexity and its unlimited access to data, has the potential for addressing these challenges. Realizing this potential requires the ability to perform high fidelity simulations of turbulent lean premixed flames under realistic conditions. In this paper, we examine the specialized mathematical structure of these combustion problems and discuss simulation approaches that exploit this structure. Using these ideas we can dramatically reduce computational cost, making it possible to perform high-fidelity simulations of realistic flames. We illustrate this methodology by considering ultra-lean hydrogen flames and discuss how this type of simulation is changing the way researchers study combustion.
Nongradient diffusion in premixed turbulent flames
NASA Technical Reports Server (NTRS)
Libby, Paul A.
1988-01-01
Recent theoretical and experimental results demonstrating the interaction between force fields and density inhomogeneities as they arise in premixed turbulent flames are discussed. In such flames, the density fluctuates between two levels, the high density in reactants rho sub r and the low density in products rho sub p, with the ratio rho sub r/rho sub p on the order of five to ten in flows of applied interest. The force fields in such flames arise from the mean pressure drop across the flame or from the Reynolds shear stresses in tangential flames with constrained streamlines. The consequence of the interaction is nongradient turbulent transport, countergradient in the direction normal to the flame and nongradient in the tangential direction. The theoretical basis for these results, the presently available experimental support therefore and the implications for other variable density turbulent flows are discussed.
Numerical simulation of premixed turbulent methane combustion
Bell, John B.; Day, Marcus S.; Grcar, Joseph F.
2001-12-14
In this paper we study the behavior of a premixed turbulent methane flame in three dimensions using numerical simulation. The simulations are performed using an adaptive time-dependent low Mach number combustion algorithm based on a second-order projection formulation that conserves both species mass and total enthalpy. The species and enthalpy equations are treated using an operator-split approach that incorporates stiff integration techniques for modeling detailed chemical kinetics. The methodology also incorporates a mixture model for differential diffusion. For the simulations presented here, methane chemistry and transport are modeled using the DRM-19 (19-species, 84-reaction) mechanism derived from the GRIMech-1.2 mechanism along with its associated thermodynamics and transport databases. We consider a lean flame with equivalence ratio 0.8 for two different levels of turbulent intensity. For each case we examine the basic structure of the flame including turbulent flame speed and flame surface area. The results indicate that flame wrinkling is the dominant factor leading to the increased turbulent flame speed. Joint probability distributions are computed to establish a correlation between heat release and curvature. We also investigate the effect of turbulent flame interaction on the flame chemistry. We identify specific flame intermediates that are sensitive to turbulence and explore various correlations between these species and local flame curvature. We identify different mechanisms by which turbulence modulates the chemistry of the flame.
Simulation of lean premixed turbulent combustion
Bell, John B.; Day, Marcus S.; Almgren, Ann S.; Lijewski, MichaelJ.; Rendleman, Charles A.; Cheng, Robert K.; Shepherd, Ian G.
2006-06-25
There is considerable technological interest in developingnew fuel-flexible combustion systems that can burn fuels such ashydrogenor syngas. Lean premixed systems have the potential to burn thesetypes of fuels with high efficiency and low NOx emissions due to reducedburnt gas temperatures. Although traditional scientific approaches basedon theory and laboratory experiment have played essential roles indeveloping our current understanding of premixed combustion, they areunable to meet the challenges of designing fuel-flexible lean premixedcombustion devices. Computation, with itsability to deal with complexityand its unlimited access to data, hasthe potential for addressing thesechallenges. Realizing this potential requires the ability to perform highfidelity simulations of turbulent lean premixed flames under realisticconditions. In this paper, we examine the specialized mathematicalstructure of these combustion problems and discuss simulation approachesthat exploit this structure. Using these ideas we can dramatically reducecomputational cost, making it possible to perform high-fidelitysimulations of realistic flames. We illustrate this methodology byconsidering ultra-lean hydrogen flames and discuss how this type ofsimulation is changing the way researchers study combustion.
Flame propagation under partially-premixed conditions
NASA Technical Reports Server (NTRS)
Ruetsch, Gregory R.
1994-01-01
This study concentrates on developing a better understanding of triple flames. We relax the assumption of zero heat release, address the issue of stabilization, and, in order to investigate the role that heat release plays in flame propagation in partially premixed combustion, we return to a simple flow field and investigate the behavior of flames in a laminar environment. We solve the compressible Navier-Stokes equations in a two-dimensional domain. At the boundaries, we use an inflow boundary condition on the left and nearly-perfect reflective boundary conditions, required to avoid pressure drift, at the outflow and sides. After the flow and flame are initialized, the mixture fraction is varied at the inlet from its uniform stoichiometric value to a tanh profile varying from zero to one. As the mixture fraction gradient reaches the flame surface only the centerline is exposed to the stoichionetric mixture fraction and locally maintains the planar flame speed and reaction rate. Above this point the mixture is fuel rich, and below fuel lean. As a result, these regions of non-unity equivalence ratio burn less, the reaction rate drops, and the local flame speed is reduced. The excess fuel and oxidizer then combine behind the premixed flame along the stoichiometric surface and burn in a trailing diffusion flame. Thus the 'triple' flame refers to the fuel-rich premixed flame, the fuel-lean premixed flame, and the trailing diffusion flame. Due to heat release, the normal velocity across the flame is increased, whereas the tangential component remains unchanged. Far-field flame speed, local flame speed, and their differences are shown as a function of the local mixing thickness. It was also determined that the lateral position of the flame affects stabilization, and the distribution of the reaction rate along the premixed wings of triple flames affects flame propagation.
The behavior of partially premixed flames
NASA Astrophysics Data System (ADS)
Choi, Chun Wai
In this investigation, we have characterized the structure of two-dimensional partially-premixed slot burner flames through the measurement of the heat release topography, and the temperature and velocity distribution. The measurements were used to infer the flame stretch and the response of the local propagation speed of the inner rich premixed reaction zone in these flames to stretch rate variations due to hydrodynamic and curvature effects. The inner premixed reaction zone of the PPFs exhibits a highly curved portion near its tip and planar topography along its lower portion. An "effective flame speed" was characterized for two flames beyond the rich flammability limit that can only burn in a partially-premixed mode due to the synergy between the inner premixed and outer nonpremixed reaction zones. The reaction zone speed in the curved region increases significantly during the transition from a planar to curved topology due to curvature effects. The Markstein relation must be suitably modified to account for the curvature of the reaction zones for flame with negative curvature. Negative curvature increases the local value of the flame speed above the unstretched flame speed Su o while positive curvature decreases it below that value. Although curvature effects are included in the definition of stretch, they are not fully accounted for by the Su(kappa) Markstein linear relation. This implies that the curvature has an influence on Su through kappa and another more explicit effect. The propagation of triple flames in premixed and nonpremixed jet modes was investigated. The response of flame speed at the triple point to stretch has a turning behavior due to the variation of the radius of curvature while the flame is propagating downward. In normal gravity, the buoyant gases accelerate the flow in a direction opposite to the gravity vector, causing air entrainment, which enhances the mixing of the reactants with ambient laboratory air and consequently, influences the
Mixing Model Performance in Non-Premixed Turbulent Combustion
NASA Astrophysics Data System (ADS)
Pope, Stephen B.; Ren, Zhuyin
2002-11-01
In order to shed light on their qualitative and quantitative performance, three different turbulent mixing models are studied in application to non-premixed turbulent combustion. In previous works, PDF model calculations with detailed kinetics have been shown to agree well with experimental data for non-premixed piloted jet flames. The calculations from two different groups using different descriptions of the chemistry and turbulent mixing are capable of producing the correct levels of local extinction and reignition. The success of these calculations raises several questions, since it is not clear that the mixing models used contain an adequate description of the processes involved. To address these questions, three mixing models (IEM, modified Curl and EMST) are applied to a partially-stirred reactor burning hydrogen in air. The parameters varied are the residence time and the mixing time scale. For small relative values of the mixing time scale (approaching the perfectly-stirred limit) the models yield the same extinction behavior. But for larger values, the behavior is distictly different, with EMST being must resistant to extinction.
Studies of premixed laminar and turbulent flames at microgravity
NASA Technical Reports Server (NTRS)
Ronney, Paul D.
1993-01-01
A two and one-half year experimental and theoretical research program on the properties of laminar and turbulent premixed gas flames at microgravity was conducted. Progress during this program is identified and avenues for future studies are discussed.
Turbulent premixed flames on fractal-grid-generated turbulence
NASA Astrophysics Data System (ADS)
Soulopoulos, N.; Kerl, J.; Sponfeldner, T.; Beyrau, F.; Hardalupas, Y.; Taylor, A. M. K. P.; Vassilicos, J. C.
2013-12-01
A space-filling, low blockage fractal grid is used as a novel turbulence generator in a premixed turbulent flame stabilized by a rod. The study compares the flame behaviour with a fractal grid to the behaviour when a standard square mesh grid with the same effective mesh size and solidity as the fractal grid is used. The isothermal gas flow turbulence characteristics, including mean flow velocity and rms of velocity fluctuations and Taylor length, were evaluated from hot-wire measurements. The behaviour of the flames was assessed with direct chemiluminescence emission from the flame and high-speed OH-laser-induced fluorescence. The characteristics of the two flames are considered in terms of turbulent flame thickness, local flame curvature and turbulent flame speed. It is found that, for the same flow rate and stoichiometry and at the same distance downstream of the location of the grid, fractal-grid-generated turbulence leads to a more turbulent flame with enhanced burning rate and increased flame surface area.
Flame front geometry in premixed turbulent flames
Shepherd, I.G.; Ashurst, W.T.
1991-12-01
Experimental and numerical determinations of flame front curvature and orientation in premixed turbulent flames are presented. The experimental data is obtained from planar, cross sectional images of stagnation point flames at high Damkoehler number. A direct numerical simulation of a constant energy flow is combined with a zero-thickness, constant density flame model to provide the numerical results. The computational domain is a 32{sup 3} cube with periodic boundary conditions. The two-dimensional curvature distributions of the experiments and numerical simulations compare well at similar q{prime}/S{sub L} values with means close to zero and marked negative skewness. At higher turbulence levels the simulations show that the distributions become symmetric about zero. These features are also found in the three dimensional distributions of curvature. The simulations support assumptions which make it possible to determine the mean direction cosines from the experimental data. This leads to a reduction of 12% in the estimated flame surface area density in the middle of the flame brush. 18 refs.
Large eddy simulation of bluff body stabilized premixed and partially premixed combustion
NASA Astrophysics Data System (ADS)
Porumbel, Ionut
Large Eddy Simulation (LES) of bluff body stabilized premixed and partially premixed combustion close to the flammability limit is carried out in this thesis. The main goal of the thesis is the study of the equivalence ratio effect on flame stability and dynamics in premixed and partially premixed flames. An LES numerical algorithm able to handle the entire range of combustion regimes and equivalence ratios is developed for this purpose. The algorithm has no ad-hoc adjustable model parameters and is able to respond automatically to variations in the inflow conditions, without user intervention. Algorithm validation is achieved by conducting LES of reactive and non-reactive flow. Comparison with experimental data shows good agreement for both mean and unsteady flow properties. In the reactive flow, two scalar closure models, Eddy Break-Up (EBULES) and Linear Eddy Mixing (LEMLES), are used and compared. Over important regions, the flame lies in the Broken Reaction Zone regime. Here, the EBU model assumptions fail. In LEMLES, the reaction-diffusion equation is not filtered, but resolved on a linear domain and the model maintains validity. The flame thickness predicted by LEMLES is smaller and the flame is faster to respond to turbulent fluctuations, resulting in a more significant wrinkling of the flame surface when compared to EBULES. As a result, LEMLES captures better the subtle effects of the flame-turbulence interaction, the flame structure shows higher complexity, and the far field spreading of the wake is closer to the experimental observations. Three premixed (φ = 0.6, 0.65, and 0.75) cases are simulated. As expected, for the leaner case (φ = 0.6) the flame temperature is lower, the heat release is reduced and vorticity is stronger. As a result, the flame in this case is found to be unstable. In the rich case (φ = 0.75), the flame temperature is higher, and the spreading rate of the wake is increased due to the higher amount of heat release. The ignition
Kim, Jong-Ryul; Choi, Gyung-Min; Kim, Duck-Jool
2011-01-15
The influence of varying combustor pressure on flame oscillation and emission characteristics in the partially premixed turbulent flame were investigated. In order to investigate combustion characteristics in the partially premixed turbulent flame, the combustor pressure was controlled in the range of -30 to 30 kPa for each equivalence ratio ({phi} = 0.8-1.2). The r.m.s. of the pressure fluctuations increased with decreasing combustor pressure for the lean condition. The combustor pressure had a sizeable influence on combustion oscillation, whose dominant frequency varied with the combustor pressure. Combustion instabilities could be controlled by increasing the turbulent intensity of the unburned mixture under the lean condition. An unstable flame was caused by incomplete combustion; hence, EICO greatly increased. Furthermore, EINO{sub x} simply reduced with decreasing combustor pressure at a rate of 0.035 g/10 kPa. The possibility of combustion control on the combusting mode and exhaust gas emission was demonstrated. (author)
Experimental study of premixed flames in intense isotropic turbulence
Bedat, B.; Cheng, R.K.
1994-04-01
A methodology for investigating premixed turbulent flames propagating in intense isotropic turbulence has been developed. The burner uses a turbulence generator developed by Videto and Santavicca and the flame is stabilized by weak-swirl generated by air injectors. This set-up produces stable premixed turbulent flames under a wide range of mixture conditions and turbulence intensities. The experiments are designed to investigate systematically the changes in flame structures for conditions which can be classified as wrinkled laminar flames, corrugated flames and flames with distributed reaction zones. Laser Doppler anemometry and Rayleigh scattering techniques are used to determine the turbulence and scalar statistics. In the intense turbulence, the flames are found to produce very little changes in the mean and rams velocities. Their flame speed increase linearly with turbulence intensity as for wrinkled laminar flames. The Rayleigh scattering pdfs for flames within the distributed reaction zone regime are distinctly bimodal. The probabilities of the reacting states (i.e. contributions from within the reaction zone) is not higher than those of wrinkled laminar flame. These results show that there is no drastic changes in flame structures at Karlovitz number close to unity. This suggest that the Klimov-Williams criterion under-predicts the resilience of wrinkled flamelets to intense turbulence.
Active Control for Statistically Stationary Turbulent PremixedFlame Simulations
Bell, J.B.; Day, M.S.; Grcar, J.F.; Lijewski, M.J.
2005-08-30
The speed of propagation of a premixed turbulent flame correlates with the intensity of the turbulence encountered by the flame. One consequence of this property is that premixed flames in both laboratory experiments and practical combustors require some type of stabilization mechanism to prevent blow-off and flashback. The stabilization devices often introduce a level of geometric complexity that is prohibitive for detailed computational studies of turbulent flame dynamics. Furthermore, the stabilization introduces additional fluid mechanical complexity into the overall combustion process that can complicate the analysis of fundamental flame properties. To circumvent these difficulties we introduce a feedback control algorithm that allows us to computationally stabilize a turbulent premixed flame in a simple geometric configuration. For the simulations, we specify turbulent inflow conditions and dynamically adjust the integrated fueling rate to control the mean location of the flame in the domain. We outline the numerical procedure, and illustrate the behavior of the control algorithm on methane flames at various equivalence ratios in two dimensions. The simulation data are used to study the local variation in the speed of propagation due to flame surface curvature.
DNS of turbulent premixed slot flames with mixture inhomogeneity: a study of NOx formation
NASA Astrophysics Data System (ADS)
Luca, Stefano; Attili, Antonio; Bisetti, Fabrizio
2016-11-01
A set of Direct Numerical Simulations of three-dimensional methane/air lean flames in a spatially developing turbulent slot burner are performed. The flames are in the thin-reaction zone regimes and the jet Reynolds number is 5600. This configuration is of interest since it displays turbulent production by mean shear as in real devices. The gas phase hydrodynamics are modeled with the reactive, unsteady Navier-Stokes equations in the low Mach number limit. Combustion is treated with finite-rate chemistry. The jet is characterized by a non-uniform equivalence ratio at the inlet and varying levels of incomplete premixing for the methane/air mixture are considered. The global equivalence ratio is 0.7 and temperature is 800 K. All simulations are performed at 4 atm. The instantaneous profiles of the mass fractions of methane and air at the inlet are sampled from a set of turbulent channel simulations that provide realistic, fully turbulent fields. The data are analyzed to study the influence of partial premixing on the flame structure. Particular focus is devoted to the assessment of heat release rate fluctuations and NOx formation. In particular, the effects of partial premixing on the production rates for the various pathways to NOx formation are investigated.
Spectral kinetic energy transfer in turbulent premixed reacting flows.
Towery, C A Z; Poludnenko, A Y; Urzay, J; O'Brien, J; Ihme, M; Hamlington, P E
2016-05-01
Spectral kinetic energy transfer by advective processes in turbulent premixed reacting flows is examined using data from a direct numerical simulation of a statistically planar turbulent premixed flame. Two-dimensional turbulence kinetic-energy spectra conditioned on the planar-averaged reactant mass fraction are computed through the flame brush and variations in the spectra are connected to terms in the spectral kinetic energy transport equation. Conditional kinetic energy spectra show that turbulent small-scale motions are suppressed in the burnt combustion products, while the energy content of the mean flow increases. An analysis of spectral kinetic energy transfer further indicates that, contrary to the net down-scale transfer of energy found in the unburnt reactants, advective processes transfer energy from small to large scales in the flame brush close to the products. Triadic interactions calculated through the flame brush show that this net up-scale transfer of energy occurs primarily at spatial scales near the laminar flame thermal width. The present results thus indicate that advective processes in premixed reacting flows contribute to energy backscatter near the scale of the flame.
PDF Modeling of Turbulent Lean Premixed Combustion
Yilmaz, S.L.; •Givi, P.; Strakey, P.A.
2007-10-01
The joint velocity-scalar-frequency probability density function (PDF) methodology is employed for prediction of a bluff-body stabilized lean premixed methane-air flame. A reduced mechanism with CO and NO chemistry is used to describe fuel oxidation. The predicted mean and rms values of the velocity, temperature and concentrations of major and minor species are compared with laboratory measurements. This technical effort was performed in support of the National Energy Technology Laboratory’s on-going research in “Assessment of Turbo-Chemistry Models for Gas Turbine Combustion Emissions” under the RDS contract DE-AC26-04NT41817.
Computations of turbulent lean premixed combustion using conditional moment closure
NASA Astrophysics Data System (ADS)
Amzin, Shokri; Swaminathan, Nedunchezhian
2013-12-01
Conditional Moment Closure (CMC) is a suitable method for predicting scalars such as carbon monoxide with slow chemical time scales in turbulent combustion. Although this method has been successfully applied to non-premixed combustion, its application to lean premixed combustion is rare. In this study the CMC method is used to compute piloted lean premixed combustion in a distributed combustion regime. The conditional scalar dissipation rate of the conditioning scalar, the progress variable, is closed using an algebraic model and turbulence is modelled using the standard k-ɛ model. The conditional mean reaction rate is closed using a first order CMC closure with the GRI-3.0 chemical mechanism to represent the chemical kinetics of methane oxidation. The PDF of the progress variable is obtained using a presumed shape with the Beta function. The computed results are compared with the experimental measurements and earlier computations using the transported PDF approach. The results show reasonable agreement with the experimental measurements and are consistent with the transported PDF computations. When the compounded effects of shear-turbulence and flame are strong, second order closures may be required for the CMC.
Field Effects of Buoyancy on Lean Premixed Turbulent Flames
NASA Technical Reports Server (NTRS)
Cheng, R. K.; Johnson, M. R.; Greenberg, P. S.; Wernet, M. P.
2003-01-01
The study of field effects of buoyancy on premixed turbulent flames is directed towards the advancement of turbulent combustion theory and the development of cleaner combustion technologies. Turbulent combustion is considered the most important unsolved problem in combustion science and laboratory studies of turbulence flame processes are vital to theoretical development. Although buoyancy is dominant in laboratory flames, most combustion models are not yet capable to consider buoyancy effects. This inconsistency has impeded the validation of theories and numerical simulations with experiments. Conversely, the understanding of buoyancy effects is far too limited to help develop buoyant flame models. Our research is also relevant to combustion technology because lean premixed combustion is a proven method to reduce the formation of oxides of nitrogen (NOx). In industrial lean premixed combustion systems, their operating conditions make them susceptible to buoyancy thus affecting heat distribution, emissions, stability, flashback and blowoff. But little knowledge is available to guide combustion engineers as to how to avoid or overcome these problems. Our hypothesis is that through its influence on the mean pressure field, buoyancy has direct and indirect effects on local flame/turbulence interactions. Although buoyancy acts on the hot products in the farfield the effect is also felt in the nearfield region upstream of the flame. These changes also influence the generation and dissipation of turbulent kinetic energy inside the flame brush and throughout the flowfield. Moreover, the plume of an open flame is unstable and the periodic fluctuations make additional contributions to flame front dynamics in the farfield. Therefore, processes such as flame wrinkling, flow acceleration due to heat release and flame- generated vorticity are all affected. Other global flame properties (e.g. flame stabilization limits and flame speed) may all be coupled to buoyancy. This
Analysis of the flamelet concept in the numerical simulation of laminar partially premixed flames
Consul, R.; Oliva, A.; Perez-Segarra, C.D.; Carbonell, D.; de Goey, L.P.H.
2008-04-15
The aim of this work is to analyze the application of flamelet models based on the mixture fraction variable and its dissipation rate to the numerical simulation of partially premixed flames. Although the main application of these models is the computation of turbulent flames, this work focuses on the performance of flamelet concept in laminar flame simulations removing, in this way, turbulence closure interactions. A well-known coflow methane/air laminar flame is selected. Five levels of premixing are taken into account from an equivalence ratio {phi}={infinity} (nonpremixed) to {phi}=2.464. Results obtained using the flamelet approaches are compared to data obtained from the detailed solution of the complete transport equations using primitive variables. Numerical simulations of a counterflow flame are also presented to support the discussion of the results. Special emphasis is given to the analysis of the scalar dissipation rate modeling. (author)
Accelerative propagation and explosion triggering by expanding turbulent premixed flames.
Akkerman, V'yacheslav; Chaudhuri, Swetaprovo; Law, Chung K
2013-02-01
The dynamics and morphology of outwardly propagating, accelerating turbulent premixed flames and the effect of flame acceleration on explosion triggering are analyzed. Guided by recent theoretical results and substantiated by experiments, we find that an expanding flame front in an externally forced, near-isotropic turbulent environment exhibits accelerative propagation given by a well-defined power law based on the average global flame radius. In this context the limits of the power-law exponent and the effective turbulence intensity experienced by the flame are derived. The power-law exponent is found to be substantially larger than that for the hydrodynamically unstable cellular laminar flames, hence facilitating the possibility of detonation triggering in turbulent environments. For large length scales, hydrodynamic instability is expected to provide additional acceleration, thus further favoring the attainment of detonation triggering.
NASA Astrophysics Data System (ADS)
Lewandowski, M. T.; Pozorski, J.
2016-10-01
The present work reports on the assessment of different turbulence-chemistry interaction closures for the modelling of turbulent non-premixed combustion. Two-dimensional axisymmetric simulations have been carried out based on three different laboratory flames. The methane fueled, piloted jet flame Sandia D, the simple jet syngas flame and the so-called Delft Jet-in-Hot Coflow flame are studied. All the flames can be characterised as non-premixed but differ by some features which are taken into account through appropriate modelling approach.
Direct numerical simulation of turbulent non-premixed methane-air flames
Chen, J.H.; Card, J.M.; Day, M.; Mahalingam, S.
1995-07-01
Turbulent non-premixed stoichiometric methane-air flames have been studied using the direct numerical simulation approach. A global one- step mechanism is used to describe the chemical kinetics, and molecular transport is modeled with constant Lewis numbers for individual species. The effect of turbulence on the internal flame structure and extinction characteristics of methane-air flames is evaluated. The flame is wrinkled and in some regions extinguished by the turbulence, while the turbulence is weakened in the vicinity of the flame due to a combination of dilatation and a 25:1 increase in kinematic viscosity across the flame. Reignition followed by partially-premixed burning is observed in the present results. Local curvature effects are found to be important in determining the local stoichiometry of the flame, and hence, the location of the peak reaction rate relative to the stoichiometric surface. The results presented in this study demonstrate the feasibility of incorporating global-step kinetics for the oxidation of methane into direct numerical simulations of homogeneous turbulence to study the flame structure.
Partially Premixed Flame (PPF) Research for Fire Safety
NASA Technical Reports Server (NTRS)
Puri, Ishwar K.; Aggarwal, Suresh K.; Lock, Andrew J.; Hegde, Uday
2004-01-01
Incipient fires typically occur after the partial premixing of fuel and oxidizer. The mixing of product species into the fuel/oxidizer mixture influences flame stabilization and fire spread. Therefore, it is important to characterize the impact of different levels of fuel/oxidizer/product mixing on flame stabilization, liftoff and extinguishment under different gravity conditions. With regard to fire protection, the agent concentration required to achieve flame suppression is an important consideration. The initial stage of an unwanted fire in a microgravity environment will depend on the level of partial premixing and the local conditions such as air currents generated by the fire itself and any forced ventilation (that influence agent and product mixing into the fire). The motivation of our investigation is to characterize these impacts in a systematic and fundamental manner.
Temperature response of turbulent premixed flames to inlet velocity oscillations
NASA Astrophysics Data System (ADS)
Ayoola, B.; Hartung, G.; Armitage, C. A.; Hult, J.; Cant, R. S.; Kaminski, C. F.
2009-01-01
Flame-turbulence interactions are at the heart of modern combustion research as they have a major influence on efficiency, stability of operation and pollutant emissions. The problem remains a formidable challenge, and predictive modelling and the implementation of active control measures both rely on further fundamental measurements. Model burners with simple geometry offer an opportunity for the isolation and detailed study of phenomena that take place in real-world combustors, in an environment conducive to the application of advanced laser diagnostic tools. Lean premixed combustion conditions are currently of greatest interest since these are able to provide low NO x and improved increased fuel economy, which in turn leads to lower CO2 emissions. This paper presents an experimental investigation of the response of a bluff-body-stabilised flame to periodic inlet fluctuations under lean premixed turbulent conditions. Inlet velocity fluctuations were imposed acoustically using loudspeakers. Spatially resolved heat release rate imaging measurements, using simultaneous planar laser-induced fluorescence (PLIF) of OH and CH2O, have been performed to explore the periodic heat release rate response to various acoustic forcing amplitudes and frequencies. For the first time we use this method to evaluate flame transfer functions and we compare these results with chemiluminescence measurements. Qualitative thermometry based on two-line OH PLIF was also used to compare the periodic temperature distribution around the flame with the periodic fluctuation of local heat release rate during acoustic forcing cycles.
RANS/PDF and LES/FDF for prediction of turbulent premixed flames
NASA Astrophysics Data System (ADS)
Yilmaz, Server Levent
Probability density function (PDF) and filtered density function (FDF) methodologies are developed and implemented, respectively, for Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) of turbulent premixed flames. RANS predictions are made of a lean premixed bluff-body flame via the joint velocity-scalar-frequency PDF model. LES of a premixed Bunsen-burner flame is conducted via the scalar FDF methodology. Both simulations employ finite rate kinetics via a reduced methane chemistry mechanism to account for combustion. Prediction results are compared with experimental data, and are shown to capture some of the intricate physics of turbulent premixed combustion. Keywords. large eddy simulation, filtered density function, Reynolds-averaged Navier-Stokes, probability density function, turbulent reacting flows, lean premixed combustion.
Characteristics of Non-Premixed Turbulent Flames in Microgravity
NASA Technical Reports Server (NTRS)
Hegde, U.; Yuan, Z. G.; Stocker, D. P.; Bahadori, M. Y.
2001-01-01
This project is concerned with the characteristics of turbulent hydrocarbon (primarily propane) gas-jet diffusion flames in microgravity. A microgravity environment provides the opportunity to study the structure of turbulent diffusion flames under momentum-dominated conditions (large Froude number) at moderate Reynolds number which is a combination not achievable in normal gravity. This paper summarizes progress made since the last workshop. Primarily, the features of flame radiation from microgravity turbulent jet diffusion flames in a reduced gravity environment are described. Tests were conducted for non-premixed, nitrogen diluted propane flames burning in quiescent air in the NASA Glenn 5.18 Second Zero Gravity Facility. Measured flame radiation from wedge-shaped, axial slices of the flame are compared for microgravity and normal gravity flames. Results from numerical computations of the flame using a k-e model for the turbulence are also presented to show the effects of flame radiation on the thermal field. Flame radiation is an important quantity that is impacted by buoyancy as has been shown in previous studies by the authors and also by Urban et al. It was found that jet diffusion flames burning under microgravity conditions have significantly higher radiative loss (about five to seven times higher) compared to their normal gravity counterparts because of larger flame size in microgravity and larger convective heat loss fraction from the flame in normal gravity. These studies, however, were confined to laminar flames. For the case of turbulent flames, the flame radiation is a function of time and both the time-averaged and time-dependent components are of interest. In this paper, attention is focused primarily on the time-averaged level of the radiation but the turbulent structure of the flame is also assessed from considerations of the radiation power spectra.
Turbulent premixed combustion; Further discussions on the scales of fluctuations
Borghi, R. )
1990-06-01
The prediction of turbulent combustion is classically performed by numerical integration of modeled equations. For each existing approach, a crucial quantity is a time scale for the destruction of temperature (or concentration) fluctuations. Usually, this time scale is simply taken as proportional to the corresponding time scale for velocity fluctuations, but the question of the influence of the reaction on this scale arises, and has already been discussed. The authors discuss this point in the particular case of wrinkled premixed flames. Algebraic closure formulas, as well as closures based on the so-called {epsilon}{sub y} equation, or on the equation for the flame surface by unit of volume of Marble and Broadwell, are proposed and discussed. It is found, in particular, that in the case of wrinkled flames with infinite Damkohler and Reynolds numbers the reaction does not play any direct role on the time scale.
Flame surface density and burning rate in premixed turbulent flames
Shepherd, I.G.
1995-10-01
The flame surface density has been measured in hydrocarbon/air stagnation point and v-shaped premixed turbulent flames. A method is proposed to determine the flame surface density from the data obtained by laser sheet tomography. The average flame length and flame zone area as a function of the progress variable are calculated from a map of progress variable and a set of flame edges obtained from the tomographs. From these results a surface density estimate in two dimensions is determined. By this technique it is possible to avoid the difficulties which arise when using an algebraic model based on the measurement of the flame front geometry and a scalar length scale. From these results the burning rate can be obtained which compares well with estimates calculated using the fractal technique. The present method, however, is not constrained by a minimum window size as is the case for the fractal determinations.
Direct simulations of premixed turbulent flames with nonunity Lewis numbers
NASA Technical Reports Server (NTRS)
Rutland, C. J.; Trouve, A.
1993-01-01
A principal effect of turbulence on premixed flames in the flamelet regime is to wrinkle the flame fronts. For nonunity Lewis numbers, Le is not equal to 1, the local flame structure is altered in curved regions. This effect is examined using direct numerical simulations of 3D isotropic turbulence with constant density, single-step Arrhenius kinetics chemistry. Simulations of Lewis numbers 0.8, 1.0, and 1.2 are compared. At the local level, curvature effects dominated changes to the flame structure while strain effects were insignificant. A strong Lewis-number-dependent correlation was found between surface curvature and the local flame speed. The correlation was positive for Le less than 1 and negative for Le greater than 1. At the global level, strain-related effects were more significant than curvature effects. The turbulent flame speed changed significantly with Lewis number, increasing as Le decreased. This was found to be due to strain effects that have a nonzero mean over the flame surface, rather than to curvature effects that have a nearly zero mean. The mean product temperature was also found to vary with Lewis number, being higher for Le greater than 1 and lower for Le less than 1.
An Investigation of a Hybrid Mixing Model for PDF Simulations of Turbulent Premixed Flames
NASA Astrophysics Data System (ADS)
Zhou, Hua; Li, Shan; Wang, Hu; Ren, Zhuyin
2015-11-01
Predictive simulations of turbulent premixed flames over a wide range of Damköhler numbers in the framework of Probability Density Function (PDF) method still remain challenging due to the deficiency in current micro-mixing models. In this work, a hybrid micro-mixing model, valid in both the flamelet regime and broken reaction zone regime, is proposed. A priori testing of this model is first performed by examining the conditional scalar dissipation rate and conditional scalar diffusion in a 3-D direct numerical simulation dataset of a temporally evolving turbulent slot jet flame of lean premixed H2-air in the thin reaction zone regime. Then, this new model is applied to PDF simulations of the Piloted Premixed Jet Burner (PPJB) flames, which are a set of highly shear turbulent premixed flames and feature strong turbulence-chemistry interaction at high Reynolds and Karlovitz numbers. Supported by NSFC 51476087 and NSFC 91441202.
Measurements of turbulent premixed flame dynamics using cinema stereoscopic PIV
NASA Astrophysics Data System (ADS)
Steinberg, Adam M.; Driscoll, James F.; Ceccio, Steven L.
2008-06-01
A new experimental method is described that provides high-speed movies of turbulent premixed flame wrinkling dynamics and the associated vorticity fields. This method employs cinema stereoscopic particle image velocimetry and has been applied to a turbulent slot Bunsen flame. Three-component velocity fields were measured with high temporal and spatial resolutions of 0.9 ms and 140 μm, respectively. The flame-front location was determined using a new multi-step method based on particle image gradients, which is described. Comparisons are made between flame fronts found with this method and simultaneous CH-PLIF images. These show that the flame contour determined corresponds well to the true location of maximum gas density gradient. Time histories of typical eddy-flame interactions are reported and several important phenomena identified. Outwardly rotating eddy pairs wrinkle the flame and are attenuated at they pass through the flamelet. Significant flame-generated vorticity is produced downstream of the wrinkled tip. Similar wrinkles are caused by larger groups of outwardly rotating eddies. Inwardly rotating pairs cause significant convex wrinkles that grow as the flame propagates. These wrinkles encounter other eddies that alter their behavior. The effects of the hydrodynamic and diffusive instabilities are observed and found to be significant contributors to the formation and propagation of wrinkles.
Blowoff dynamics of bluff body stabilized turbulent premixed flames
Chaudhuri, Swetaprovo; Kostka, Stanislav; Renfro, Michael W.; Cetegen, Baki M.
2010-04-15
This article concerns the flame dynamics of a bluff body stabilized turbulent premixed flame as it approaches lean blowoff. Time resolved chemiluminescence imaging along with simultaneous particle image velocimetry and OH planar laser-induced fluorescence were utilized in an axisymmetric bluff body stabilized, propane-air flame to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental results. It was found that as lean blowoff is approached by reduction of equivalence ratio, flame speed decreases and the flame shape progressively changes from a conical to a columnar shape. For a stably burning conical flame away from blowoff, the flame front envelopes the shear layer vortices. Near blowoff, the columnar flame front and shear layer vortices overlap to induce high local stretch rates that exceed the extinction stretch rates instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following shear layer extinction, fresh reactants can pass through the shear layers to react within the recirculation zone with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for a few milliseconds and can reignite the shear layers such that the entire flame is reestablished for a short period. This extinction and reignition event can happen several times before final blowoff which occurs when the flame kernel fails to reignite the shear layers and ultimately leads to total flame extinguishment. (author)
Spatial resolution effects of CARS in turbulent premixed combustion thermometry
Shepherd, I.G. . Applied Science Div.); Porter, F.M.; Greenhalgh, D.A. )
1990-10-01
Coherent Anti-Stokes Raman Scattering CARS nitrogen thermometry has proved to be a successful technique to obtain point temperature measurements with high temporal resolution in hot reactive gases. It has significant advantages over other probe methods, such as thermocouples, where access, physical and/or chemical perturbation and a lack of sufficient temperature range may create problems. The longitudinal or axial resolution of a CARS measurement, in the preferred BOXCARS geometry, is determined by the zone of perfect overlap of the intersecting laser beams (typically 1-5 mm). The length squared dependence of the CARS signal, however, can limit the spatial resolution if usable signal-to-noise rations are to be obtained. This can be a particular problem when probing systems where steep temperature gradients exist. In this numerical study of a premixed turbulent flame zone the effect of spatial resolution on CARS temperature probability density functions (pdfs) and mean temperature profile measurements is assessed, and possible means to overcome this problem are presented.
DNS of premixed turbulent V-flame: coupling spectral and finite difference methods
NASA Astrophysics Data System (ADS)
Hauguel, Raphael; Vervisch, Luc; Domingo, Pascale
2005-01-01
To allow for a reliable examination of the interaction between velocity fluctuations, acoustics and combustion, a novel numerical procedure is discussed in which a spectral solution of the Navier-Stokes equations is directly associated to a high-order finite difference fully compressible DNS solver (sixth order PADE). Using this combination of high-order solvers with accurate boundary conditions, simulations have been performed where a turbulent premixed V-shape flame develops in grid turbulence. In the light of the DNS results, a sub-model for premixed turbulent combustion is analyzed. To cite this article: R. Hauguel et al., C. R. Mecanique 333 (2005).
An Investigation of a Hybrid Mixing Timescale Model for PDF Simulations of Turbulent Premixed Flames
NASA Astrophysics Data System (ADS)
Zhou, Hua; Kuron, Mike; Ren, Zhuyin; Lu, Tianfeng; Chen, Jacqueline H.
2016-11-01
Transported probability density function (TPDF) method features the generality for all combustion regimes, which is attractive for turbulent combustion simulations. However, the modeling of micromixing due to molecular diffusion is still considered to be a primary challenge for TPDF method, especially in turbulent premixed flames. Recently, a hybrid mixing rate model for TPDF simulations of turbulent premixed flames has been proposed, which recovers the correct mixing rates in the limits of flamelet regime and broken reaction zone regime while at the same time aims to properly account for the transition in between. In this work, this model is employed in TPDF simulations of turbulent premixed methane-air slot burner flames. The model performance is assessed by comparing the results from both direct numerical simulation (DNS) and conventional constant mechanical-to-scalar mixing rate model. This work is Granted by NSFC 51476087 and 91441202.
The dynamics of turbulent premixed flames: Mechanisms and models for turbulence-flame interaction
NASA Astrophysics Data System (ADS)
Steinberg, Adam M.
The use of turbulent premixed combustion in engines has been garnering renewed interest due to its potential to reduce NOx emissions. However there are many aspects of turbulence-flame interaction that must be better understood before such flames can be accurately modeled. The focus of this dissertation is to develop an improved understanding for the manner in which turbulence interacts with a premixed flame in the 'thin flamelet regime'. To do so, two new diagnostics were developed and employed in a turbulent slot Bunsen flame. These diagnostics, Cinema-Stereoscopic Particle Image Velocimetry and Orthogonal-Plane Cinema-Stereoscopic Particle Image Velocimetry, provided temporally resolved velocity and flame surface measurements in two- and three-dimensions with rates of up to 3 kHz and spatial resolutions as low as 280 mum. Using these measurements, the mechanisms with which turbulence generates flame surface area were studied. It was found that the previous concept that flame stretch is characterized by counter-rotating vortex pairs does not accurately describe real turbulence-flame interactions. Analysis of the experimental data showed that the straining of the flame surface is determined by coherent structures of fluid dynamic strain rate, while the wrinkling is caused by vortical structures. Furthermore, it was shown that the canonical vortex pair configuration is not an accurate reflection of the real interaction geometry. Hence, models developed based on this geometry are unlikely to be accurate. Previous models for the strain rate, curvature stretch rate, and turbulent burning velocity were evaluated. It was found that the previous models did not accurately predict the measured data for a variety of reasons: the assumed interaction geometries did not encompass enough possibilities to describe the possible effects of real turbulence, the turbulence was not properly characterized, and the transport of flame surface area was not always considered. New models
Recent Advances in Understanding of Thermal Expansion Effects in Premixed Turbulent Flames
NASA Astrophysics Data System (ADS)
Sabelnikov, Vladimir A.; Lipatnikov, Andrei N.
2017-01-01
When a premixed flame propagates in a turbulent flow, not only does turbulence affect the burning rate (e.g., by wrinkling the flame and increasing its surface area), but also the heat release in the flame perturbs the pressure field, and these pressure perturbations affect the turbulent flow and scalar transport. For instance, the latter effects manifest themselves in the so-called countergradient turbulent scalar flux, which has been documented in various flames and has challenged the combustion community for approximately 35 years. Over the past decade, substantial progress has been made in investigating (a) the influence of thermal expansion in a premixed flame on the turbulent flow and turbulent scalar transport within the flame brush, as well as (b) the feedback influence of countergradient scalar transport on the turbulent burning rate. The present article reviews recent developments in this field and outlines issues to be solved in future research.
Combustion dynamics linked to flame behaviour in a partially premixed swirled industrial burner
Biagioli, Fernando; Guethe, Felix; Schuermans, Bruno
2008-07-15
Previous work [Biagioli, F., Stabilization mechanism of turbulent premixed flames in strongly swirled flows, Combustion, Theory and Modelling 10 (3) (2006) 389-412; Guethe, F., Lachner, R., Schuermans, B., Biagioli, F., Geng, W., Inauen, A., Schenker, S., Bombach, R., Hubschmid, W., Flame imaging on the ALSTOM EV-burner: thermo acoustic pulsations and CFD-validation, in: AIAA Paper 2006-437 presented at the 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, January 9-12, 2006] has shown that turbulent dry low NO{sub x} (partially premixed) flames in high swirl conical burners may be subject to a large change of their anchoring location at the symmetry axis when a critical value of the bulk equivalence ratio is reached, i.e. they are bi-stable. This flame behavior is linked here to combustion pressure dynamics measured in an atmospheric test rig for a prototype version of the Alstom EnVironmental (EV) conical burner. The link is made via the solution of the problem of the 'travelling flameholder', which shows that the unsteady displacement of the flame anchoring location implies an unsteady variation of the flame surface area and therefore unsteady heat release. The relevance of this source of unsteady heat release - which is different from more usual ones due to variations in turbulent burning rate and in the sensible enthalpy jump across the flame - to the generation of combustion dynamics in strongly swirled flows is confirmed here by the strong positive correlation between the tendency of the flame to be displaced and the measured amplitude of pressure pulsations. (author)
Analysis of Turbulent Scales of Motion in Premixed Flames Using Structure Functions
NASA Astrophysics Data System (ADS)
Hamlington, Peter; Whitman, Samuel; Towery, Colin; Poludnenko, Alexei
2016-11-01
Recently, multiscale turbulence-flame interactions in premixed reacting flows have been examined using both physical space and spectral approaches. However, there remains relatively little understanding of how turbulent scales of motion vary through the internal structure of the flame itself (i.e., through premixed flamelets). Such an analysis is made difficult by the inhomogeneity, small scale, and spatial locality of many premixed flames, particularly at high Damköhler and low Karlovitz numbers. Conditional structure functions provide a possible solution to this analysis challenge, and in this talk we present results from the calculation of structure functions using data from highly-resolved direct numerical simulations (DNS) of turbulent premixed flames. The high resolution of the DNS allows structure functions to be calculated normally and tangentially to the local flame surface, revealing the specific effects of the flame on turbulent scales of motion near the scale of the local flame width. Moreover, the conditional nature of the analysis allows the effects of different flame regions (e.g., the preheat and reaction zones) on turbulence to be isolated. The implications of these results for the theory and modeling of turbulent flame physics are outlined.
High frequency acoustic wave scattering from turbulent premixed flames
NASA Astrophysics Data System (ADS)
Narra, Venkateswarlu
This thesis describes an experimental investigation of high frequency acoustic wave scattering from turbulent premixed flames. The objective of this work was to characterize the scattered incoherent acoustic field and determine its parametric dependence on frequency, flame brush thickness, incident and measurement angles, mean velocity and flame speed. The experimental facility consists of a slot burner with a flat flame sheet that is approximately 15 cm wide and 12 cm tall. The baseline cold flow characteristics and flame sheet statistics were extensively characterized. Studies were performed over a wide range of frequencies (1-24 kHz) in order to characterize the role of the incident acoustic wave length. The spectrum of the scattered acoustic field showed distinct incoherent spectral sidebands on either side of the driving frequency. The scattered incoherent field was characterized in terms of the incoherent field strength and spectral bandwidth and related to the theoretical predictions. The role of the flame front wrinkling scale, i.e., flame brush thickness, was also studied. Flame brush thickness was varied independent of the mean velocity and flame speed by using a variable turbulence generator. Results are reported for five flame brush thickness cases, ranging from 1.2 mm to 5.2 mm. Some dependence of scattered field characteristics on flame brush thickness was observed, but the magnitude of the effect was much smaller than expected from theoretical considerations. The spatial dependence of the scattered field was investigated by measuring the scattered field at four measurement angles and exciting the flame at four incident angles. Theory predicts that these variations influence the spatial scale of the acoustic wave normal to the flame, a result confirmed by the measurements. Measurements were performed for multiple combinations of mean velocities and flame speeds. The scattered field was observed to depend strongly on the flame speed. Further analysis
Studies of Premixed Laminar and Turbulent Flames at Microgravity
NASA Technical Reports Server (NTRS)
Ronney, Paul D.
1993-01-01
The work of the Principal Investigator (PI) has encompassed four topics related to the experimental and theoretical study of combustion limits in premixed flames at microgravity, as discussed in the following sections. These topics include: (1) radiation effects on premixed gas flames; (2) flame structure and stability at low Lewis number; (3) flame propagation and extinction is cylindrical tubes; and (4) experimental simulation of combustion processes using autocatalytic chemical reactions.
Pdf modeling for premixed turbulent combustion based on the properties of iso-concentration surfaces
NASA Technical Reports Server (NTRS)
Vervisch, L.; Kollmann, W.; Bray, K. N. C.; Mantel, T.
1994-01-01
In premixed turbulent flames the presence of intense mixing zones located in front of and behind the flame surface leads to a requirement to study the behavior of iso-concentration surfaces defined for all values of the progress variable (equal to unity in burnt gases and to zero in fresh mixtures). To support this study, some theoretical and mathematical tools devoted to level surfaces are first developed. Then a database of direct numerical simulations of turbulent premixed flames is generated and used to investigate the internal structure of the flame brush, and a new pdf model based on the properties of iso-surfaces is proposed.
Flamelet Model Application for Non-Premixed Turbulent Combustion
NASA Technical Reports Server (NTRS)
Secundov, A.; Bezgin, L.; Buriko, Yu.; Guskov, O.; Kopchenov, V.; Laskin, I.; Lomkov, K.; Tshepin, S.; Volkov, D.; Zaitsev, S.
1996-01-01
The current Final Report contains results of the study which was performed in Scientific Research Center 'ECOLEN' (Moscow, Russia). The study concerns the development and verification of non-expensive approach for modeling of supersonic turbulent diffusion flames based on flamelet consideration of the chemistry/turbulence interaction (FL approach). Research work included: development of the approach and CFD tests of the flamelet model for supersonic jet flames; development of the simplified procedure for solution of the flamelet equations based on partial equilibrium chemistry assumption; study of the flame ignition/extinction predictions provided by flamelet model. The performed investigation demonstrated that FL approach allowed to describe satisfactory main features of supersonic H 2/air jet flames. Model demonstrated also high capabilities for reduction of the computational expenses in CFD modeling of the supersonic flames taking into account detailed oxidation chemistry. However, some disadvantages and restrictions of the existing version of approach were found in this study. They were: (1) inaccuracy in predictions of the passive scalar statistics by our turbulence model for one of the considered test cases; and (2) applicability of the available version of the flamelet model to flames without large ignition delay distance only. Based on the results of the performed investigation, we formulated and submitted to the National Aeronautics and Space Administration our Project Proposal for the next step research directed toward further improvement of the FL approach.
Conditional budgets of second-order statistics in nonpremixed and premixed turbulent combustion
NASA Astrophysics Data System (ADS)
Macart, Jonathan F.; Grenga, Temistocle; Mueller, Michael E.
2016-11-01
Combustion heat release modifies or introduces a number of new terms to the balance equations for second-order turbulence statistics (turbulent kinetic energy, scalar variance, etc.) compared to incompressible flow. A major modification is a significant increase in viscosity and dissipation in the high-temperature combustion products, but new terms also appear due to density variation and gas expansion (dilatation). Previous scaling analyses have hypothesized that dilatation effects are important in turbulent premixed combustion but are unimportant in turbulent nonpremixed combustion. To explore this hypothesis, a series of DNS calculations have been performed in the low Mach number limit for spatially evolving turbulent planar jet flames of hydrogen and air in both premixed and nonpremixed configurations. Unlike other studies exploring the effects of heat release on turbulence, the turbulence is not forced, and detailed chemical kinetics are used to describe hydrogen-air combustion. Budgets for second-order statistics are computed conditioned on progress variable in the premixed flame and on mixture fraction in the nonpremixed flame in order to locate regions with respect to the flame structure where dilatation effects are strongest.
Study of Turbulent Premixed Flame Propagation using a Laminar Flamelet Model
NASA Technical Reports Server (NTRS)
Im, H. G.
1995-01-01
The laminar flamelet concept in turbulent reacting flows is considered applicable to many practical combustion systems (Linan & Williams 1993). For turbulent premixed combustion, the laminar flamelet regime is valid when turbulent Karlovitz number is less than unity, which is equivalent to stating that the characteristic thickness of the flame is less than that of a Kolmogorov eddy; this is known as the Klimov-Williams criterion (Williams 1985). In such a case, the flame maintains its laminar structure, and the effect of turbulent flow is merely to wrinkle and strain the flame front. The propagating wrinkled premixed flame can then be described as an infinitesimally thin surface dividing the unburnt fresh mixture and the burnt product.
Subfilter Scale Combustion Modelling for Large Eddy Simulation of Turbulent Premixed Flames
NASA Astrophysics Data System (ADS)
Shahbazian, Nasim
Large eddy simulation (LES) is a powerful computational tool for modelling turbulent combustion processes. However, for reactive flows, LES is still under significant development. In particular, for turbulent premixed flames, a considerable complication of LES is that the flame thickness is generally much smaller than the LES filter width such that the flame front and chemical reactions cannot be resolved on the grid. Accurate and robust subfilter-scale (SFS) models of the unresolved turbulence-chemistry interactions are therefore required and studies are needed to evaluate and improve them. In this thesis, a detailed comparison and evaluation of five different SFS models for turbulence- chemistry interactions in LES of premixed flames is presented. These approaches include both flamelet- and non-flamelet-based models, coupled with simple or tabulated chemistry. The mod- elling approaches considered herein are: algebraic- and transport-equation variants of the flame surface density (FSD) model, the presumed conditional moment (PCM) with flame prolongation of intrinsic low-dimensional manifold (FPI) tabulated chemistry, or PCM-FPI approach, evaluated with two different presumed probability density function (PDF) models; and conditional source-term estimation (CSE) approach. The predicted LES solutions are compared to the existing laboratory-scale experimental observation of Bunsen-type turbulent premixed methane-air flames, corresponding to lean and stoichiometric conditions lying from the upper limit of the flamelet regime to well within the thin reaction zones regime of the standard regimes diagram. Direct comparison of different SFS approaches allows investigation of stability and performance of the models, while the weaknesses and strengths of each approach are identified. Evaluation of algebraic and transported FSD models highlights the importance of non-equilibrium transport in turbulent premixed flames. The effect of the PDF type for the reaction progress
Subfilter scale combustion modelling for large eddy simulation of turbulent premixed flames
NASA Astrophysics Data System (ADS)
Shahbazian, Nasim
Large eddy simulation (LES) is a powerful computational tool for modelling turbulent combustion processes. However, for reactive flows, LES is still under significant development. In particular, for turbulent premixed flames, a considerable complication of LES is that the flame thickness is generally much smaller than the LES filter width such that the flame front and chemical reactions cannot be resolved on the grid. Accurate and robust subfilter-scale (SFS) models of the unresolved turbulence-chemistry interactions are therefore required and studies are needed to evaluate and improve them. In this thesis, a detailed comparison and evaluation of five different SFS models for turbulence-chemistry interactions in LES of premixed flames is presented. These approaches include both flamelet- and non-flamelet-based models, coupled with simple or tabulated chemistry. The modelling approaches considered herein are: algebraic- and transport-equation variants of the flame surface density (FSD) model, the presumed conditional moment (PCM) with flame prolongation of intrinsic low-dimensional manifold (FPI) tabulated chemistry, or PCM-FPI approach, evaluated with two different presumed probability density function (PDF) models; and conditional source-term estimation (CSE) approach. The predicted LES solutions are compared to the existing laboratory-scale experimental observation of Bunsen-type turbulent premixed methane-air flames, corresponding to lean and stoichiometric conditions lying from the upper limit of the flamelet regime to well within the thin reaction zones regime of the standard regimes diagram. Direct comparison of different SFS approaches allows investigation of stability and performance of the models, while the weaknesses and strengths of each approach are identified. Evaluation of algebraic and transported FSD models highlights the importance of non-equilibrium transport in turbulent premixed flames. The effect of the PDF type for the reaction progress
The evolution equation for the flame surface density in turbulent premixed combustion
NASA Technical Reports Server (NTRS)
Trouve, Arnaud
1993-01-01
The mean reaction rate in flamelet models for turbulent premixed combustion depends on two basic quantities: a mean chemical rate, called the flamelet speed, and the flame surface density. Our previous work had been primarily focused on the problem of the structure and topology of turbulent premixed flames, and it was then determined that the flamelet speed, when space-averaged, is only weakly sensitive to the turbulent flow field. Consequently, the flame surface density is the key quantity that conveys most of the effects of the turbulence on the rate of energy release. In flamelet models, this quantity is obtained via a modeled transport equation called the Sigma-equation. Past theoretical work has produced a rigorous approach that leads to an exact but unclosed formulation for the turbulent Sigma-equation. In the exact Sigma-equation, it appears that the dynamical properties of the flame surface density are determined by a single parameter, namely the turbulent flame stretch. Unfortunately, the turbulent flame stretch as well as the flame surface density is not available from experiments, and, in the absence of experimental data, little is known on the validity of the closure assumptions used in current flamelet models. Direct Numerical Simulation (DNS) is the alternative approach to get basic information on these fundamental quantities. In the present work, three-dimensional DNS of premixed flames in isotropic turbulent flow is used to estimate the different terms appearing in the Sigma-equation. A new methodology is proposed to provide the source and sink terms for the flame surface density, resolved both temporally and spatially throughout the turbulent flame brush. Using this methodology, our objective is to extract the turbulent flame stretch from the DNS data base and then perform extensive comparisons with flamelet models. Thanks to the detailed information produced by the DNS-based analysis, it is expected that this type of comparison will not only
Consistent flamelet modeling of differential molecular diffusion for turbulent non-premixed flames
NASA Astrophysics Data System (ADS)
Wang, Haifeng
2016-03-01
Treating differential molecular diffusion correctly and accurately remains as a great challenge to the modeling of turbulent non-premixed combustion. The aim of this paper is to develop consistent modeling strategies for differential molecular diffusion in flamelet models. Two types of differential molecular diffusion models are introduced, linear differential diffusion models and nonlinear differential diffusion models. A multi-component turbulent mixing layer problem is analyzed in detail to gain insights into differential molecular diffusion and its characteristics, particularly the dependence of differential molecular diffusion on the Reynolds number and the Lewis number. These characteristics are then used to validate the differential molecular diffusion models. Finally, the new models are applied to the modeling of a series of laboratory-scale turbulent non-premixed jet flames with different Reynolds number (Sandia Flames B, C, and D) to further assess the models' performance.
The Behavior of Methane-Air Partially Premixed Flames Under Normal- and Zero-G Conditions
NASA Technical Reports Server (NTRS)
Puri, Ishwar K.; Aggarwal, Suresh K.; Choi, Chun Wai; Hegde, Uday
2001-01-01
Partially premixed flames (PPFs) represent a class of hybrid flames containing multiple reaction zones. These flames are established when less than stoichiometric quantity of oxidizer is molecularly mixed with the fuel stream before entering the reaction zone where additional oxidizer is available for complete combustion. This mode of combustion can be used to exploit the advantages of both nonpremixed and premixed flames regarding operational safety, lower pollutant emissions and flame stabilization. A double flame containing a fuel-rich premixed reaction zone, which is anchored by a nonpremixed reaction zone, is one example of a partially premixed flame. A triple flame is also a PPF that contains three reaction zones, namely, a fuel-rich premixed zone, a fuel-lean premixed zone, and a nonpremixed reaction zone. Herein we focus on two aspects of our investigation, one involving the development of optical diagnostics that can be used on a microgravity rig, which has been recently fabricated, and the other on the numerically predicted differences between normal- and zero-gravity PPFs. Both the measurements and simulations examine the detailed structure of methane-air PPFs stabilized on a Wolfhard-Parker slot burner.
NASA Astrophysics Data System (ADS)
Chen, Zhi; Ruan, Shaohong; Swaminathan, Nedunchezhian
2016-07-01
Three-dimensional (3D) unsteady Reynolds-averaged Navier-Stokes simulations of a spark-ignited turbulent methane/air jet flame evolving from ignition to stabilisation are conducted for different jet velocities. A partially premixed combustion model is used involving a correlated joint probability density function and both premixed and non-premixed combustion mode contributions. The 3D simulation results for the temporal evolution of the flame's leading edge are compared with previous two-dimensional (2D) results and experimental data. The comparison shows that the final stabilised flame lift-off height is well predicted by both 2D and 3D computations. However, the transient evolution of the flame's leading edge computed from 3D simulation agrees reasonably well with experiment, whereas evident discrepancies were found in the previous 2D study. This difference suggests that the third physical dimension plays an important role during the flame transient evolution process. The flame brush's leading edge displacement speed resulting from reaction, normal and tangential diffusion processes are studied at different typical stages after ignition in order to understand the effect of the third physical dimension further. Substantial differences are found for the reaction and normal diffusion components between 2D and 3D simulations especially in the initial propagation stage. The evolution of reaction progress variable scalar gradients and its interaction with the flow and mixing field in the 3D physical space have an important effect on the flame's leading edge propagation.
Turbulence-flame interactions in DNS of a laboratory high Karlovitz premixed turbulent jet flame
NASA Astrophysics Data System (ADS)
Wang, Haiou; Hawkes, Evatt R.; Chen, Jacqueline H.
2016-09-01
In the present work, direct numerical simulation (DNS) of a laboratory premixed turbulent jet flame was performed to study turbulence-flame interactions. The turbulent flame features moderate Reynolds number and high Karlovitz number (Ka). The orientations of the flame normal vector n, the vorticity vector ω and the principal strain rate eigenvectors ei are examined. The in-plane and out-of-plane angles are introduced to quantify the vector orientations, which also measure the flame geometry and the vortical structures. A general observation is that the distributions of these angles are more isotropic downstream as the flame and the flow become more developed. The out-of-plane angle of the flame normal vector, β, is a key parameter in developing the correction of 2D measurements to estimate the corresponding 3D quantities. The DNS results show that the correction factor is unity at the inlet and approaches its theoretical value of an isotropic distribution downstream. The alignment characteristics of n, ω and ei, which reflect the interactions of turbulence and flame, are also studied. Similar to a passive scalar gradient in non-reacting flows, the flame normal has a tendency to align with the most compressive strain rate, e3, in the flame, indicating that turbulence contributes to the production of scalar gradient. The vorticity dynamics are examined via the vortex stretching term, which was found to be the predominant source of vorticity generation balanced by dissipation, in the enstrophy transport equation. It is found that although the vorticity preferentially aligns with the intermediate strain rate, e2, the contribution of the most extensive strain rate, e1, to vortex stretching is comparable with that of the intermediate strain rate, e2. This is because the eigenvalue of the most extensive strain rate, λ1, is always large and positive. It is confirmed that the vorticity vector is preferentially positioned along the flame tangential plane, contributing
A simulation of a bluff-body stabilized turbulent premixed flame using LES-PDF
NASA Astrophysics Data System (ADS)
Kim, Jeonglae; Pope, Stephen
2013-11-01
A turbulent premixed flame stabilized by a triangular cylinder as a flame-holder is simulated. The computational condition matches the Volvo experiments (Sjunnesson et al. 1992). Propane is premixed at a fuel lean condition of ϕ = 0 . 65 . For this reactive simulation, LES-PDF formulation is used, similar to Yang et al. (2012). The evolution of Lagrangian particles is simulated by solving stochastic differential equations modeling transport of the composition PDF. Mixing is modeled by the modified IEM model (Viswanathan et al. 2011). Chemical reactions are calculated by ISAT and for the good load balancing, PURAN distribution of ISAT tables is applied (Hiremath et al. 2012). To calculate resolved density, the two-way coupling (Popov & Pope 2013) is applied, solving a transport equation of resolved specific volume to reduce statistical noise. A baseline calculation shows a good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. Chemical reaction does not significantly contribute to the overall computational cost, in contrast to non-premixed flame simulations (Hiremath et al. 2013), presumably due to the restricted manifold of the purely premixed flame in the composition space.
Local curvature measurements of a lean, partially premixed swirl-stabilised flame
NASA Astrophysics Data System (ADS)
Bayley, Alan E.; Hardalupas, Yannis; Taylor, Alex M. K. P.
2012-04-01
A swirl-stabilised, lean, partially premixed combustor operating at atmospheric conditions has been used to investigate the local curvature distributions in lifted, stable and thermoacoustically oscillating CH4-air partially premixed flames for bulk cold-flow Reynolds numbers of 15,000 and 23,000. Single-shot OH planar laser-induced fluorescence has been used to capture instantaneous images of these three different flame types. Use of binary thresholding to identify the reactant and product regions in the OH planar laser-induced fluorescence images, in order to extract accurate flame-front locations, is shown to be unsatisfactory for the examined flames. The Canny-Deriche edge detection filter has also been examined and is seen to still leave an unacceptable quantity of artificial flame-fronts. A novel approach has been developed for image analysis where a combination of a non-linear diffusion filter, Sobel gradient and threshold-based curve elimination routines have been used to extract traces of the flame-front to obtain local curvature distributions. A visual comparison of the effectiveness of flame-front identification is made between the novel approach, the threshold binarisation filter and the Canny-Deriche filter. The novel approach appears to most accurately identify the flame-fronts. Example histograms of the curvature for six flame conditions and of the total image area are presented and are found to have a broader range of local flame curvatures for increasing bulk Reynolds numbers. Significantly positive values of mean curvature and marginally positive values of skewness of the histogram have been measured for one lifted flame case, but this is generally accounted for by the effect of flame brush curvature. The mean local flame-front curvature reduces with increasing axial distance from the burner exit plane for all flame types. These changes are more pronounced in the lifted flames but are marginal for the thermoacoustically oscillating flames. It is
The ``turbulent flame speed'' of wrinkled premixed flames
NASA Astrophysics Data System (ADS)
Matalon, Moshe; Creta, Francesco
2012-11-01
The determination of the turbulent flame speed is a central problem in combustion theory. Early studies by Damköhler and Shelkin resorted to geometrical and scaling arguments to deduce expressions for the turbulent flame speed and its dependence on turbulence intensity. A more rigorous approach was undertaken by Clavin and Williams who, based on a multi-scale asymptotic approach valid for weakly wrinkled flames, derived an expression that apart from a numerical factor recaptures the early result by Damköhler and Shelkin. The common denominator of the phenomenological and the more rigorous propositions is an increase in turbulent flame speed due solely to an increase in flame surface area. Various suggestions based on physical and/or experimental arguments have been also proposed, incorporating other functional parameters into the flame speed relation. The objective of this work is to extend the asymptotic results to a fully nonlinear regime that permits to systematically extract scaling laws for the turbulent flame speed that depend on turbulence intensity and scale, mixture composition and thermal expansion, flow conditions including effects of curvature and strain, and flame instabilities. To this end, we use a hybrid Navier-Stokes/front-capturing methodology, which consistently with the asymptotic model, treats the flame as a surface of density discontinuity separating burned and unburned gases. The present results are limited to positive Markstein length, corresponding to lean hydrocarbon-air or rich hydrogen-air mixtures, and to wrinkled flames of vanishingly small thickness, smaller that the smallest fluid scales. For simplicity we have considered here two-dimensional turbulence, which although lacks some features of real three-dimensional turbulence, is not detrimental when using the hydrodynamic model under consideration, because the turbulent flame retains its laminar structure and its interaction with turbulence is primarily advective/kinematic in
NASA Astrophysics Data System (ADS)
Lee, Chin Yik; Li, Larry Kin Bong; Juniper, Matthew P.; Cant, Robert Stewart
2016-01-01
Turbulent premixed flames often experience thermoacoustic instabilities when the combustion heat release rate is in phase with acoustic pressure fluctuations. Linear methods often assume a priori that oscillations are periodic and occur at a dominant frequency with a fixed amplitude. Such assumptions are not made when using nonlinear analysis. When an oscillation is fully saturated, nonlinear analysis can serve as a useful avenue to reveal flame behaviour far more elaborate than period-one limit cycles, including quasi-periodicity and chaos in hydrodynamically or thermoacoustically self-excited system. In this paper, the behaviour of a bluff-body stabilised turbulent premixed propane/air flame in a model jet-engine afterburner configuration is investigated using computational fluid dynamics. For the frequencies of interest in this investigation, an unsteady Reynolds-averaged Navier-Stokes approach is found to be appropriate. Combustion is represented using a modified laminar flamelet approach with an algebraic closure for the flame surface density. The results are validated by comparison with existing experimental data and with large eddy simulation, and the observed self-excited oscillations in pressure and heat release are studied using methods derived from dynamical systems theory. A systematic analysis is carried out by increasing the equivalence ratio of the reactant stream supplied to the premixed flame. A strong variation in the global flame structure is observed. The flame exhibits a self-excited hydrodynamic oscillation at low equivalence ratios, becomes steady as the equivalence ratio is increased to intermediate values, and again exhibits a self-excited thermoacoustic oscillation at higher equivalence ratios. Rich nonlinear behaviour is observed and the investigation demonstrates that turbulent premixed flames can exhibit complex dynamical behaviour including quasiperiodicity, limit cycles and period-two limit cycles due to the interactions of various
Turbulent combustion of premixed flames in closed vessels
Checkel, M.D. . Mechanical Engineering); Thomas, A. . Mechanical Engineering)
1994-03-01
An extensive series of experiments on spark-ignited explosion of propane-air mixtures in both turbulent and quiescent conditions in a cubic closed vessel is described. Turbulence was produced by a moving grid, and the development with time of pressure and of flame area (from light emission) recorded. The effects of grid-hole diameter, grid velocity, spark timing after passage of the grid, equivalence ratio, and initial pressure were investigated. Estimates of the rate of strain in the unburnt gases were derived from hot-wire anemometry. Results indicated that rate of strain was a major factor governing the rate of combustion. Theoretical simulations of explosions with a simple model were made, in which turbulence was characterized solely by the rate of strain, and in which the decay of turbulence during explosions and the effect of changes in pressure on both burning velocity and flow field were taken into account. The simulations were compared with experimental results, and reinforced the idea that turbulence has the dual effect of causing wrinkling of flames and, especially for weaker mixtures, reducing the burning velocity. An empirical relationship was found in which the logarithmic rate of wrinkling was proportional to the square root of the rate of strain. Some simple conclusions are drawn regarding practical application of the results.
Conditional statistics in a turbulent premixed flame derived from direct numerical simulation
NASA Technical Reports Server (NTRS)
Mantel, Thierry; Bilger, Robert W.
1994-01-01
The objective of this paper is to briefly introduce conditional moment closure (CMC) methods for premixed systems and to derive the transport equation for the conditional species mass fraction conditioned on the progress variable based on the enthalpy. Our statistical analysis will be based on the 3-D DNS database of Trouve and Poinsot available at the Center for Turbulence Research. The initial conditions and characteristics (turbulence, thermo-diffusive properties) as well as the numerical method utilized in the DNS of Trouve and Poinsot are presented, and some details concerning our statistical analysis are also given. From the analysis of DNS results, the effects of the position in the flame brush, of the Damkoehler and Lewis numbers on the conditional mean scalar dissipation, and conditional mean velocity are presented and discussed. Information concerning unconditional turbulent fluxes are also presented. The anomaly found in previous studies of counter-gradient diffusion for the turbulent flux of the progress variable is investigated.
Interaction of turbulent premixed flames with combustion products: Role of stoichiometry
Coriton, Bruno Rene Leon; Frank, Jonathan H.; Gomez, Alessandro
2016-05-30
Stabilization methods of turbulent flames often involve mixing of reactants with hot products of combustion. The stabilizing effect of combustion product enthalpy has been long recognized, but the role played by the chemical composition of the product gases is typically overlooked. We employ a counterflow system to pinpoint the effects of the combustion product stoichiometry on the structure of turbulent premixed flames under conditions of both stable burning and local extinction. To that end, a turbulent jet of lean-to-rich, CH4/O2/N2-premixed reactants at a turbulent Reynolds number of 1050 was opposed to a stream of hot products of combustion that weremore » generated in a preburner. While the combustion product stream temperature was kept constant, its stoichiometry was varied independently from that of the reactant stream, leading to reactant-to-product stratification of relevance to practical combustion systems. The detailed structure of the turbulent flame front was analyzed in two series of experiments using laser-induced fluorescence (LIF): joint CH2O LIF and OH LIF measurements and joint CO LIF and OH LIF measurements. Results revealed that a decrease in local CH2O+OH and CO+OH reaction rates coincide with the depletion of OH radicals in the vicinity of the combustion product stream. These critical combustion reaction rates were more readily quenched in the presence of products of combustion from a stoichiometric flame, whereas they were favored by lean combustion products. As a result, stoichiometric combustion products contributed to a greater occurrence of local extinction. Furthermore, they limited the capacity of premixed reactants to ignite and of the turbulent premixed flames to stabilize. In contrast, lean and rich combustion products facilitated flame ignition and stability and reduced the rate of local extinction. The influence of the combustion product stream on the turbulent flame front was limited to a zone of approximately two millimeters
Interaction of turbulent premixed flames with combustion products: Role of stoichiometry
Coriton, Bruno Rene Leon; Frank, Jonathan H.; Gomez, Alessandro
2016-05-30
Stabilization methods of turbulent flames often involve mixing of reactants with hot products of combustion. The stabilizing effect of combustion product enthalpy has been long recognized, but the role played by the chemical composition of the product gases is typically overlooked. We employ a counterflow system to pinpoint the effects of the combustion product stoichiometry on the structure of turbulent premixed flames under conditions of both stable burning and local extinction. To that end, a turbulent jet of lean-to-rich, CH_{4}/O_{2}/N_{2}-premixed reactants at a turbulent Reynolds number of 1050 was opposed to a stream of hot products of combustion that were generated in a preburner. While the combustion product stream temperature was kept constant, its stoichiometry was varied independently from that of the reactant stream, leading to reactant-to-product stratification of relevance to practical combustion systems. The detailed structure of the turbulent flame front was analyzed in two series of experiments using laser-induced fluorescence (LIF): joint CH_{2}O LIF and OH LIF measurements and joint CO LIF and OH LIF measurements. Results revealed that a decrease in local CH_{2}O+OH and CO+OH reaction rates coincide with the depletion of OH radicals in the vicinity of the combustion product stream. These critical combustion reaction rates were more readily quenched in the presence of products of combustion from a stoichiometric flame, whereas they were favored by lean combustion products. As a result, stoichiometric combustion products contributed to a greater occurrence of local extinction. Furthermore, they limited the capacity of premixed reactants to ignite and of the turbulent premixed flames to stabilize. In contrast, lean and rich combustion products facilitated flame ignition and stability and reduced the rate of local extinction. The influence of the combustion product stream on the turbulent flame front was
Study and modeling of finite rate chemistry effects in turbulent non-premixed flames
NASA Technical Reports Server (NTRS)
Vervisch, Luc
1993-01-01
The development of numerical models that reflect some of the most important features of turbulent reacting flows requires information about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between turbulent and chemical processes is so strong that it is extremely difficult to isolate the role played by one individual physical phenomenon. Direct numerical simulation (hereafter DNS) allows us to study in detail the turbulence-chemistry interaction in some restricted but completely defined situations. Globally, non-premixed flames are controlled by two limiting regimes: the fast chemistry case, where the turbulent flame can be pictured as a random distribution of local chemical equilibrium problems; and the slow chemistry case, where the chemistry integrates in time the turbulent fluctuations. The Damkoehler number, ratio of a mechanical time scale to chemical time scale, is used to distinguish between these regimes. Today most of the industrial computer codes are able to perform predictions in the hypothesis of local equilibrium chemistry using a presumed shape for the probability density function (pdt) of the conserved scalar. However, the finite rate chemistry situation is of great interest because industrial burners usually generate regimes in which, at some points, the flame is undergoing local extinction or at least non-equilibrium situations. Moreover, this variety of situations strongly influences the production of pollutants. To quantify finite rate chemistry effect, the interaction between a non-premixed flame and a free decaying turbulence is studied using DNS. The attention is focused on the dynamic of extinction, and an attempt is made to quantify the effect of the reaction on the small scale mixing process. The unequal diffusivity effect is also addressed. Finally, a simple turbulent combustion model based on the DNS observations and tractable in real flow configurations is proposed.
Flow field studies of a new series of turbulent premixed stratified flames
Seffrin, F.; Fuest, F.; Dreizler, A.; Geyer, D.
2010-02-15
This paper presents a new burner design for lean premixed stratified combustion for experiments to validate models for numerical simulations. The burner demonstrates combustion phenomena relevant to technological applications, where flames are often turbulent, lean premixed, and stratified. The generic burner was designed for high Reynolds number flows and can stabilize a variety of different lean premixed flames. The burner's design and its versatile operational conditions are introduced. Shear, stratification, and fuel type are parametrically varied to provide a sound database of related flow configurations. Reacting and corresponding non-reacting configurations are examined. Experimental setups and the results of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) are presented and discussed. LDV measurements provide radial profiles of mean axial velocity, mean radial velocity, and turbulent kinetic energy as well as integral time scales. High-speed PIV is introduced as a novel technique to determine integral time and length scales and provide 2D 2-component velocity fields and related quantities, such as vorticity. (author)
Support for Advanced Imaging of Premixed Turbulent Combustion Processes
2007-11-02
quantifies mean flame properties (e.g., mean progress variable fields ), chemical reaction sheet wrinkling by turbulence (e. g., reaction sheet...orientation statistics, curvatures and surface density data) and important velocities (e. g., velocity field of reactants, reaction sheet displacement speeds...with stereo particle image velocimetry (SPIV) to measure flamelet wrinkling, flamelet speeds and the reactant velocity field in order to relate
Surface properties of turbulent premixed propane/air flames at various Lewis numbers
Lee, T.W.; North, G.L.; Santavicca, D.A. )
1993-06-01
Surface properties of turbulent premixed flames including the wrinkled flame perimeter, fraction of the flame pocket perimeter, flame curvature, and orientation distributions have been measured for propane-air flames at Lewis numbers ranging from 0.98 to 1.86 and u[prime]/S[sub L] = 1.42-5.71. The wrinkled flame perimeter is found to be greater for the thermodiffusively unstable Lewis number (Le < 1) by up to 30% in comparison to the most stable condition (Le = 1.86) tested, while the fraction of the flame pocket perimeter shows a similar tendency to be greater for Le < 1. The flame curvature probability density functions are nearly symmetric with respect to the zero mean at all Lewis numbers throughout the range of u[prime]/S[sub L] tested, and show a much stronger dependence on the turbulence condition than on the Lewis number. Similarly, the flame orientation distributions show a trend from anisotropy toward a more uniform distribution with increasing u[prime]/S[sub L] at a similar rate for all Lewis numbers. Thus, for turbulent premixed propane/air flames for a practical range of Lewis number from 0.98 to 1.86, the effect of Lewis number is primarily to affect the flame structures and thereby flame surface areas and flame pocket areas, while the flame curvature and orientation statistics are essentially determined by the turbulence properties.
The evolution equation for the flame surface density in turbulent premixed combustion
NASA Technical Reports Server (NTRS)
Trouve, A.; Poinsot, T.
1992-01-01
One central ingredient in flamelet models for turbulent premixed combustion is the flame surface density. This quantity conveys most of the effects of the turbulence on the rate of energy release and is obtained via a modeled transport equation, called the Sigma-equation. Past theoretical work has produced a rigorous approach that leads to an exact, but unclosed, formulation for the turbulent Sigma-equation. In this exact Sigma-equation, it appears that the dynamical properties of the flame surface density are determined by a single parameter, namely the turbulent flame stretch. Unfortunately, the flame surface density and the turbulent flame stretch are not available from experiments and, in the absence of experimental data, little is known on the validity of the closure assumptions used in current flamelet models. Direct Numerical Simulation (DNS) is the obvious, complementary approach to get basic information on these fundamental quantities. Three-dimensional DNS of premixed flames in isotropic turbulent flow is used to estimate the different terms appearing in the Sigma-equation. A new methodology is proposed to provide the source and sink terms for the flame surface density, resolved both temporally and spatially throughout the turbulent flame brush. Using this methodology, the effects of the Lewis number on the rate of production of flame surface area are described in great detail and meaningful comparisons with flamelet models can be performed. The analysis reveals in particular the tendency of the models to overpredict flame surface dissipation as well as their inability to reproduce variations due to thermo-diffusive phenomena. Thanks to the detailed information produced by a DNS-based analysis, this type of comparison not only underscores the shortcomings of current models but also suggests ways to improve them.
Geometrical properties of turbulent premixed flames and other corrugated interfaces.
Thiesset, F; Maurice, G; Halter, F; Mazellier, N; Chauveau, C; Gökalp, I
2016-01-01
This study focuses on the geometrical properties of turbulent flame fronts and other interfaces. Toward that end, we use an original tool based on proper orthogonal decomposition (POD), which is applied to the interface spatial coordinates. The focus is mainly on the degree of roughness of the flame front, which is quantified through the scale dependence of its coverage arclength. POD is first validated by comparing with the caliper technique. Fractal characteristics are extracted in an unambiguous fashion using a parametric expression which appears to be impressively well suited for representing Richardson plots. Then it is shown that, for the range of Reynolds numbers investigated here, the scale-by-scale contribution to the arclength does not comply with scale similarity, irrespectively of the type of similarity which is invoked. The finite ratios between large and small scales, referred to as finite Reynolds number effects, are likely to explain this observation. In this context, the Reynolds number that ought to be achieved for a proper inertial range to be discernible, and for scale similarity to be likely to apply, is calculated. Fractal characteristics of flame folding are compared to available predictions. It is confirmed that the inner cutoff satisfactorily correlates with the Kolmogorov scale while the outer cutoff appears to be proportional to the integral length scale. However, the scaling for the fractal dimension is much less obvious. It is argued that much higher Reynolds numbers have to be reached for drawing firm statements about the evolution (or constancy) of the fractal dimension with respect to flame and flow parameters. Finally, a heuristic phenomenology of corrugated interfaces is highlighted. The degree of generality of the latter phenomenology is confirmed by comparing the folding of different interfaces including a turbulent-nonturbulent interface, a liquid jet destabilized by a surrounding air jet, a cavitating flow, and an isoscalar
Field Effects of Buoyancy on Lean Premixed Turbulent Flames
NASA Technical Reports Server (NTRS)
Cheng, R. K.; Dimalanta, R.; Wernet, M. P.; Greenberg, P. S.
2001-01-01
Buoyancy affects the entire flowfield of steady turbulent flames and this aspect of flame buoyancy coupling is largely unexplored by experiments or by theory. Open flames and flames within large confinements are free to expand and interact with the surrounding environment. In addition to fluid and combustion conditions, their aerodynamic flowfields are determined by the flame brush orientation and geometry, wake of the stabilizer, enclosure size, and of course, the gravitational field. Because the flowfield consists mainly of cold reactants (mostly in the nearfield) and hot products (mostly in the farfield), buoyancy effects are manifested in the farfield region. In upward pointing flames, an obvious effect is a favorable axial pressure gradient that accelerates the products thereby increasing the axial aerodynamic stretch rate. Intrinsic to turbulent flows, changes in mean aerodynamic stretch also couple to the fluctuating pressure field. Consequently, buoyancy can influence the turbulence intensities upstream and downstream of the flame. Flame wrinkling process, and heat release rate are also directly affected. This backward coupling mechanism is the so-called elliptic problem. To resolve the field effects of buoyancy would require the solution of three-dimensional non-linear Navier Stokes equations with full specification of the upstream, wall and downstream boundary conditions.
Wavelet multi-resolution analysis of energy transfer in turbulent premixed flames
NASA Astrophysics Data System (ADS)
Kim, Jeonglae; Bassenne, Maxime; Towery, Colin; Poludnenko, Alexei; Hamlington, Peter; Ihme, Matthias; Urzay, Javier
2016-11-01
Direct numerical simulations of turbulent premixed flames are examined using wavelet multi-resolution analyses (WMRA) as a diagnostics tool to evaluate the spatially localized inter-scale energy transfer in reacting flows. In non-reacting homogeneous-isotropic turbulence, the net energy transfer occurs from large to small scales on average, thus following the classical Kolmogorov energy cascade. However, in turbulent flames, our prior work suggests that thermal expansion leads to a small-scale pressure-work contribution that transfers energy in an inverse cascade on average, which has important consequences for LES modeling of reacting flows. The current study employs WMRA to investigate, simultaneously in physical and spectral spaces, the characteristics of this combustion-induced backscatter effect. The WMRA diagnostics provide spatial statistics of the spectra, scale-conditioned intermittency of velocity and vorticity, along with energy-transfer fluxes conditioned on the local progress variable.
Effects of Buoyancy on the Flowfields of Lean Premixed Turbulent V-Flames
NASA Technical Reports Server (NTRS)
Cheng, R. K.; Bedat, B.; Yegian, D. T.; Greenberg, P.
1999-01-01
Open laboratory turbulent flames used for investigating fundamental flame turbulence interactions are greatly affected by buoyancy. Though much of our current knowledge is based on observations made in open flames, buoyancy effects are usually not considered in data interpretation, numerical analysis or theories. This inconsistency remains an obstacle to merging experimental observations and theoretical predictions. To better understanding the effects of buoyancy, our research focuses on steady lean premixed flames propagating in fully developed turbulence. We hypothesize that the most significant role of buoyancy forces on these flames is to influence their flowfields through a coupling with the mean and the fluctuating pressure fields. This coupling relates to the elliptical problem that emphasizes the importance of the upstream, wall and downstream boundary conditions in determining all aspects of flame propagation. Therefore, buoyancy has the same significance as other parameters such as flow configuration, and flame geometry.
NASA Astrophysics Data System (ADS)
Wacks, Daniel H.; Chakraborty, Nilanjan; Klein, Markus; Arias, Paul G.; Im, Hong G.
2016-12-01
The distributions of flow topologies within the flames representing the corrugated flamelets, thin reaction zones, and broken reaction zone regimes of premixed turbulent combustion are investigated using direct numerical simulation data of statistically planar turbulent H2-air flames with an equivalence ratio ϕ =0.7 . It was found that the diminishing influence of dilatation rate with increasing Karlovitz number has significant influences on the statistical behaviors of the first, second, and third invariants (i.e., P ,Q , and R ) of the velocity gradient tensor. These differences are reflected in the distributions of the flow topologies within the flames considered in this analysis. This has important consequences for those topologies that make dominant contributions to the scalar-turbulence interaction and vortex-stretching terms in the scalar dissipation rate and enstrophy transport equations, respectively. Detailed physical explanations are provided for the observed regime dependences of the flow topologies and their implications on the scalar dissipation rate and enstrophy transport.
Interplay of Darrieus-Landau instability and weak turbulence in premixed flame propagation
NASA Astrophysics Data System (ADS)
Creta, Francesco; Lamioni, Rachele; Lapenna, Pasquale Eduardo; Troiani, Guido
2016-11-01
In this study we investigate, both numerically and experimentally, the interplay between the intrinsic Darrieus-Landau (DL) or hydrodynamic instability of a premixed flame and the moderately turbulent flow field in which the flame propagates. The objective is threefold: to establish, unambiguously, through a suitably defined marker, the presence or absence of DL-induced effects on the turbulent flame, to quantify the DL effects on the flame propagation and morphology and, finally, to asses whether such effects are mitigated or suppressed as the turbulence intensity is increased. The numerical simulations are based on a deficient reactant model which lends itself to a wealth of results from asymptotic theory, such as the determination of stability limits. The skewness of the flame curvature probability density function is identified as an unambiguous morphological marker for the presence or absence of DL effects in a turbulent environment. In addition, the turbulent propagation speed is shown to exhibit a distinct dual behavior whereby it is noticeably enhanced in the presence of DL instability while it is unchanged otherwise. Furthermore, increasing the turbulence intensity is found to be mitigating with respect to DL-induced effects such as the mentioned dual behavior which disappears at higher intensities. Experimental propane and/or air Bunsen flames are also investigated, utilizing two distinct diameters, respectively, above and below the estimated DL cutoff wavelength. Curvature skewness is still clearly observed to act as a marker for DL instability while the turbulent propagation speed is concurrently enhanced in the presence of the instability.
Interplay of Darrieus-Landau instability and weak turbulence in premixed flame propagation.
Creta, Francesco; Lamioni, Rachele; Lapenna, Pasquale Eduardo; Troiani, Guido
2016-11-01
In this study we investigate, both numerically and experimentally, the interplay between the intrinsic Darrieus-Landau (DL) or hydrodynamic instability of a premixed flame and the moderately turbulent flow field in which the flame propagates. The objective is threefold: to establish, unambiguously, through a suitably defined marker, the presence or absence of DL-induced effects on the turbulent flame, to quantify the DL effects on the flame propagation and morphology and, finally, to asses whether such effects are mitigated or suppressed as the turbulence intensity is increased. The numerical simulations are based on a deficient reactant model which lends itself to a wealth of results from asymptotic theory, such as the determination of stability limits. The skewness of the flame curvature probability density function is identified as an unambiguous morphological marker for the presence or absence of DL effects in a turbulent environment. In addition, the turbulent propagation speed is shown to exhibit a distinct dual behavior whereby it is noticeably enhanced in the presence of DL instability while it is unchanged otherwise. Furthermore, increasing the turbulence intensity is found to be mitigating with respect to DL-induced effects such as the mentioned dual behavior which disappears at higher intensities. Experimental propane and/or air Bunsen flames are also investigated, utilizing two distinct diameters, respectively, above and below the estimated DL cutoff wavelength. Curvature skewness is still clearly observed to act as a marker for DL instability while the turbulent propagation speed is concurrently enhanced in the presence of the instability.
Local Extinction Mechanisms in Non-Premixed Turbulent Combustion
1991-08-31
Navier-Stokes-for-mean-flow/pdf- transport method , as outlined above. I I I I I I I I 64... 3 Section 4 REFERENCES 1. Amano, R.S. and Kodali , V.S., (1984...obtained by the moment-equation/assumed- shape method . 5. Calculations compare favorably with the Raman data. 6. Work on pdf transport/Monte-Carlo... methods in recirculation-stabilized flames has begun. 7. The range of turbulence-chemistry interactions in combustion has been quantified, in an attempt to
Steinberg, Adam M.; Driscoll, James F.
2010-07-15
Temporally resolved measurements of turbulence-flame interaction were used to experimentally determine relationships for the strain-rate and curvature stretch-rate exerted on a premixed flame surface. These relationships include a series of transfer functions that are analogous to, but not equal to, stretch-efficiency functions. The measurements were obtained by applying high-repetition-rate particle image velocimetry in a turbulent slot Bunsen flame and were able to resolve the range of turbulent scales that cause flame surface straining and wrinkling. Fluid control masses were tracked in a Lagrangian manner as they interacted with the flame surface. From each interaction, the spatially and temporally filtered subgrid strain-rate and curvature stretch-rate were measured. By analyzing the statistics of thousands of turbulence-flame interactions, relationships for the strain-rate and curvature stretch-rate were determined that are appropriate for Large Eddy Simulation. It was found that the strain-rate exerted on the flame during these interactions was better correlated with the strength of the subgrid fluid-dynamic strain-rate field than with previously used characteristic strain-rates. Furthermore, stretch-efficiency functions developed from simplified vortex-flame interactions significantly over-predict the measurements. Hence, the proposed relationship relates the strain-rate on the flame to the filtered subgrid fluid-dynamic strain-rate field during real turbulence-flame interactions using an empirically determined Strain-Rate Transfer function. It was found that the curvature stretch-rate did not locally balance the strain-rate as has been proposed in previous models. A geometric relationship was found to exist between the subgrid flame surface wrinkling factor and subgrid curvature stretch-rate, which could be expressed using an empirically determined wrinkling factor transfer function. Curve fits to the measured relationships are provided that could be
DNS and modeling of the interaction between turbulent premixed flames and walls
NASA Technical Reports Server (NTRS)
Poinsot, T. J.; Haworth, D. C.
1992-01-01
The interaction between turbulent premixed flames and walls is studied using a two-dimensional full Navier-Stokes solver with simple chemistry. The effects of wall distance on the local and global flame structure are investigated. Quenching distances and maximum wall heat fluxes during quenching are computed in laminar cases and are found to be comparable to experimental and analytical results. For turbulent cases, it is shown that quenching distances and maximum heat fluxes remain of the same order as for laminar flames. Based on simulation results, a 'law-of-the-wall' model is derived to describe the interaction between a turbulent premixed flame and a wall. This model is constructed to provide reasonable behavior of flame surface density near a wall under the assumption that flame-wall interaction takes place at scales smaller than the computational mesh. It can be implemented in conjunction with any of several recent flamelet models based on a modeled surface density equation, with no additional constraints on mesh size or time step.
Stochastic modeling of unsteady extinction in turbulent non-premixed combustion
Lackmann, T.; Hewson, J. C.; Knaus, R. C.; Kerstein, A. R.; Oevermann, M.
2016-07-19
Turbulent fluctuations of the scalar dissipation rate have a major impact on extinction in non-premixed combustion. Recently, an unsteady extinction criterion has been developed (Hewson, 2013) that predicts extinction dependent on the duration and the magnitude of dissipation rate fluctuations exceeding a critical quenching value; this quantity is referred to as the dissipation impulse. Furthermore, the magnitude of the dissipation impulse corresponding to unsteady extinction is related to the difficulty with which a flamelet is exintguished, based on the steady-state S-curve.
Understanding and predicting soot generation in turbulent non-premixed jet flames.
Wang, Hai; Kook, Sanghoon; Doom, Jeffrey; Oefelein, Joseph Charles; Zhang, Jiayao; Shaddix, Christopher R.; Schefer, Robert W.; Pickett, Lyle M.
2010-10-01
This report documents the results of a project funded by DoD's Strategic Environmental Research and Development Program (SERDP) on the science behind development of predictive models for soot emission from gas turbine engines. Measurements of soot formation were performed in laminar flat premixed flames and turbulent non-premixed jet flames at 1 atm pressure and in turbulent liquid spray flames under representative conditions for takeoff in a gas turbine engine. The laminar flames and open jet flames used both ethylene and a prevaporized JP-8 surrogate fuel composed of n-dodecane and m-xylene. The pressurized turbulent jet flame measurements used the JP-8 surrogate fuel and compared its combustion and sooting characteristics to a world-average JP-8 fuel sample. The pressurized jet flame measurements demonstrated that the surrogate was representative of JP-8, with a somewhat higher tendency to soot formation. The premixed flame measurements revealed that flame temperature has a strong impact on the rate of soot nucleation and particle coagulation, but little sensitivity in the overall trends was found with different fuels. An extensive array of non-intrusive optical and laser-based measurements was performed in turbulent non-premixed jet flames established on specially designed piloted burners. Soot concentration data was collected throughout the flames, together with instantaneous images showing the relationship between soot and the OH radical and soot and PAH. A detailed chemical kinetic mechanism for ethylene combustion, including fuel-rich chemistry and benzene formation steps, was compiled, validated, and reduced. The reduced ethylene mechanism was incorporated into a high-fidelity LES code, together with a moment-based soot model and models for thermal radiation, to evaluate the ability of the chemistry and soot models to predict soot formation in the jet diffusion flame. The LES results highlight the importance of including an optically-thick radiation model
Kinetic Effects of Non-Equilibrium Plasma on Partially Premixed Flame Extinction
2011-01-01
dissociative attachment processes. The Boltzmann equation calculates the rate coefficients of the electron impact elementary reactions by averaging the...ion-ion neutralization processes, ion-molecule reactions, and electron attachment and detachment processes. Note that the present model does not solve...partially premixed methane flames was studied at 60 Torr by blending 2% CH4 into the oxidizer stream. The non-equilibrium discharge accelerated
Chen, Yung-Cheng; Kim, Munki; Han, Jeongjae; Yun, Sangwook; Yoon, Youngbin
2008-08-15
A new definition of turbulent consumption speed is proposed in this work that is based on the heat release rate integral, rather than the mass burning rate integral. Its detailed derivation and the assumptions involved are discussed in a general context that applies to all properly defined reaction progress variables. The major advantage of the proposed definition is that it does not require the thin-flame assumption, in contrast to previous definitions. Experimental determination of the local turbulent displacement speed, S{sub D}, and the local turbulent consumption speed, S{sub C}, is also demonstrated with the particle image velocimetry technique in three turbulent premixed stagnation flames. The turbulence intensity of these flames is of the same order of the laminar burning velocity. Based on the current data, a model equation for the local mean heat release rate is proposed. The relationship between S{sub D} and S{sub C} is discussed along with a possible modeling approach for the turbulent displacement speed. (author)
Lagrangian analysis of premixed turbulent combustion in hydrogen-air flames
NASA Astrophysics Data System (ADS)
Darragh, Ryan; Poludnenko, Alexei; Hamlington, Peter
2016-11-01
Lagrangian analysis has long been a tool used to analyze non-reacting turbulent flows, and has recently gained attention in the reacting flow and combustion communities. The approach itself allows one to separate local molecular effects, such as those due to reactions or diffusion, from turbulent advective effects along fluid pathlines, or trajectories. Accurate calculation of these trajectories can, however, be rather difficult due to the chaotic nature of turbulent flows and the added complexity of reactions. In order to determine resolution requirements and verify the numerical algorithm, extensive tests are described in this talk for prescribed steady, unsteady, and chaotic flows, as well as for direct numerical simulations (DNS) of non-reacting homogeneous isotropic turbulence. The Lagrangian analysis is then applied to DNS of premixed hydrogen-air flames at two different turbulence intensities for both single- and multi-step chemical mechanisms. Non-monotonic temperature and fuel-mass fraction evolutions are found to exist along trajectories passing through the flame brush. Such non-monotonicity is shown to be due to molecular diffusion resulting from large spatial gradients created by turbulent advection. This work was supported by the Air Force Office of Scientific Research (AFOSR) under Award No. FA9550-14-1-0273, and the Department of Defense (DoD) High Performance Computing Modernization Program (HPCMP) under a Frontier project award.
Simulations and experiments on the ignition probability in turbulent premixed bluff-body flames
NASA Astrophysics Data System (ADS)
Sitte, Michael Philip; Bach, Ellen; Kariuki, James; Bauer, Hans-Jörg; Mastorakos, Epaminondas
2016-05-01
The ignition characteristics of a premixed bluff-body burner under lean conditions were investigated experimentally and numerically with a physical model focusing on ignition probability. Visualisation of the flame with a 5 kHz OH* chemiluminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work on non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability Pign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. Flows corresponding to flames close to the blow-off limit could not be ignited, although such flames were stable if reached from a richer already ignited condition. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high Pign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. A modelling effort that takes convection and localised flame quenching into account by tracking stochastic virtual flame particles, previously validated for non-premixed and spray ignition, was used to estimate the ignition probability. The applicability of this approach to premixed flows was first evaluated by investigating the model's flame propagation mechanism in a uniform turbulence field, which showed that the model reproduces the bending behaviour of the ST-versus-u‧ curve. Then ignition simulations of the bluff-body burner were carried out. The ignition probability map was computed and it was found that the model reproduces all main trends found in the experimental study.
Characterization of flame front surfaces in turbulent premixed methane/air combustion
Smallwood, G.J.; Guelder, Oe.L.; Snelling, D.R.; Deschamps, B.M.; Goekalp, I.
1995-06-01
A detailed experimental investigation of the application of fractal geometry concepts in determining the turbulent burning velocity in the wrinkled flame regime of turbulent premixed combustion was conducted. The fractal dimension and cutoff scales were determined for six different turbulent flames in the wrinkled flame regime, where the turbulence intensity, turbulent length scale, and equivalence ratio were varied. Unlike previous reports, it has proved possible to obtain the fractal dimension and inner and outer cutoffs from individual flame images. From this individual data, the pdf distributions of all three fractal parameters, along with the distribution of the predicted increase in surface area, may be determined. The analysis of over 300 flame images for each flame condition provided a sufficient sample size to accurately define the pdf distributions and their means. However, the predicted S{sub T}/S{sub L}, calculated using fractal parameters, was significantly below the measured values. For conical flames, a geometrical modification factor was employed to predict S{sub T}/S{sub L}, however, this did little to improve the predictions. There appeared to be no dependence of the predicted S{sub T}/S{sub L} on the approach flow turbulence. The cutoffs did not seem to vary significantly with any of the length scales in the approach flow turbulence, although the fractal dimension did appear to have a weak dependence on u{prime}/S{sub L} and Re{sub {lambda}}. The probable reasons that fractal geometry does not correctly predict S{sub T}/S{sub L} are that S{sub T}/S{sub L} = A{sub w}/A{sub 0} does not hold in wrinkled turbulent premixed flames, that the flame front surface cannot be described by a single scaling exponent, or that these are not wrinkled flames. S{sub T} = turbulent burning velocity, S{sub L} = laminar burning velocity, A{sub w} = wrinkled flame surface area, and A{sub 0} = flow cross section area.
Experimental study of the flowfield of a V-shaped premixed turbulent flame
Cheng, R.K.; Ng, T.T.
1981-01-01
The flowfield of a V-shaped, premixed ethylene/air flame in grid induced turbulence has been studied using Laser Doubler Velocimetry. The experimental conditions covered free-stream velocities of 5 and 7 m/s and equivalence ratios ranging from 0.6 to 0.78. The two-dimensional velocity vectors obtained indicate that flow deflection in the free stream was significant and seemed to correlate with the flame angle. The influence of the flame holder wake on the flame was demonstrated. In the presence of the flame, an increase in the turbulence level in the free stream was found and was attributed to fluctuations in flow deflection induced by the fluctuating flame.
NASA Astrophysics Data System (ADS)
Thiesset, Fabien; Maurice, Guillaume; Halter, Fabien; Mazellier, Nicolas; Chauveau, Christian; Gökalp, Iskender
2016-05-01
We propose a model for assessing the unresolved wrinkling factor in the large eddy simulation of turbulent premixed combustion. It relies essentially on a power-law dependence of the wrinkling factor on the filter size and an original expression for the 'active' corrugating strain rate. The latter is written as the turbulent strain multiplied by an efficiency function that accounts for viscous effects and the kinematic constraint of Peters. This yields functional expressions for the fractal dimension and the inner cut-off length scale, the latter being (i) filter-size independent and (ii) consistent with the Damköhler asymptotic behaviours at both large and small Karlovitz numbers. A new expression for the wrinkling factor that incorporates finite Reynolds number effects is further proposed. Finally, the model is successfully assessed on an experimental filtered database.
Turbulent non-premixed combustion driven by the Richtmyer-Meshkov instability
NASA Astrophysics Data System (ADS)
Varshochi, Hilda; Ramaprabhu, Praveen; Attal, Nitesh
2016-11-01
We report on 3D high resolution numerical simulations of a non-premixed, reacting Richmyer-Meshkov (RM) instability performed using the FLASH code. In the simulations, a Mach 1.6 shock traverses a diffuse, corrugated material interface separating Hydrogen at 1000 K and Oxygen at 300 K, so that local misalignments between pressure and density gradients induce baroclinic vorticity at the contact line. The vorticity deposition drives the RM instability, which in turn results in combustion and flame formation. We study the evolution of the interface and the flame as the resulting RM instability grows through linear, nonlinear and turbulent stages. We develop a detailed understanding of the effects of heat release and combustion on the underlying flow properties by comparing our results with a baseline non-reacting RM flow. We document the properties of the instability (growth rates, pdfs, spectra) and the flame (scalar dissipation rate, flame surface area, heat release rate) as well as the nature of the coupling between the two. Our findings are relevant to supernovae detonation, knocking in IC engines and scramjet performance, while the underlying flow problem defined here represents a novel canonical framework to understand the broader class of non-premixed turbulent flames.
Effects of Buoyancy on the Flowfields of Lean Premixed Turbulent V-Flames
NASA Technical Reports Server (NTRS)
Cheng, R. K.; Greenberg, P.; Bedat, B.; Yegian, D. T.
1999-01-01
Open laboratory turbulent flames used for investigating fundament flame turbulence interactions are greatly affected by buoyancy. Though much of our current knowledge is based on observations made in these open flames, the effects of buoyancy are usually not included in data interpretation, numerical analysis or theories. This inconsistency remains an obstacle to merging experimental observations and theoretical predictions. To better understanding the effects of buoyancy, our research focuses on steady lean premixed flames propagating in fully developed turbulence. We hypothesize that the most significant role of buoyancy forces on these flames is to influence their flowfields through a coupling with mean and fluctuating pressure fields. Changes in flow pattern alter the mean aerodynamic stretch and in turn affect turbulence fluctuation intensities both upstream and downstream of the flame zone. Consequently, flame stabilization, reaction rates, and turbulent flame processes are all affected. This coupling relates to the elliptical problem that emphasizes the importance of the upstream, wall and downstream boundary conditions in determining all aspects of flame propagation. Therefore, buoyancy has the same significance as other parameters such as flow configuration, flame geometry, means of flame stabilization, flame shape, enclosure size, mixture conditions, and flow conditions.
NASA Technical Reports Server (NTRS)
Card, J. M.; Chen, J. H.; Day, M.; Mahalingam, S.
1994-01-01
Turbulent non-premixed stoichiometric methane-air flames modeled with reduced kinetics have been studied using the direct numerical simulation approach. The simulations include realistic chemical kinetics, and the molecular transport is modeled with constant Lewis numbers for individual species. The effect of turbulence on the internal flame structure and extinction characteristics of methane-air flames is evaluated. Consistent with earlier DNS with simple one-step chemistry, the flame is wrinkled and in some regions extinguished by the turbulence, while the turbulence is weakened in the vicinity of the flame due to a combination of dilatation and an increase in kinematic viscosity. Unlike previous results, reignition is observed in the present simulations. Lewis number effects are important in determining the local stoichiometry of the flame. The results presented in this work are preliminary but demonstrate the feasibility of incorporating reduced kinetics for the oxidation of methane with direct numerical simulations of homogeneous turbulence to evaluate the limitations of various levels of reduction in the kinetics and to address the formation of thermal and prompt NO(x).
Field Effects of Buoyancy on a Premixed Turbulent Flame Studied by Particle Image Velocimetry
NASA Technical Reports Server (NTRS)
Cheng, Robert K.
2003-01-01
Typical laboratory flames for the scientific investigation of flame/turbulence interactions are prone to buoyancy effects. Buoyancy acts on these open flame systems and provides upstream feedbacks that control the global flame properties as well as local turbulence/flame interactions. Consequently the flame structures, stabilization limits, and turbulent reaction rates are directly or indirectly coupled with buoyancy. The objective of this study is to characterize the differences between premixed turbulent flames pointing upwards (1g), pointing downwards (-1g), and in microgravity (mg). The configuration is an inverted conical flame stabilized by a small cone-shaped bluff body that we call CLEAN Flames (Cone-Stabilized Lean Flames). We use two laser diagnostics to capture the velocity and scalar fields. Particle image velocimetry (PIV) measures the mean and root mean square velocities and planar imaging by the flame fronts method outlines the flame wrinkle topology. The results were obtained under typical conditions of small domestic heating systems such as water heaters, ovens, and furnaces. Significant differences between the 1g and -1g flames point to the need for including buoyancy contributions in theoretical and numerical calculations. In Earth gravity, there is a complex coupling of buoyancy with the turbulent flow and heat release in the flame. An investigation of buoyancy-free flames in microgravity will provide the key to discern gravity contributions. Data obtained in microgravity flames will provide the benchmark for interpreting and analyzing 1g and -1g flame results.
Steinberg, Adam M.; Driscoll, James F.
2009-12-15
The dynamical processes of flame surface straining and wrinkling that occur as turbulence interacts with a premixed flame were measured using cinema-stereoscopic PIV (CS-PIV) and orthogonal-plane cinema-stereoscopic PIV (OPCS-PIV). These diagnostics provided temporally resolved measurements of turbulence-flame interaction at frame rates of up to 3 kHz and spatial resolutions as small as 280{mu} m. Previous descriptions of flame straining and wrinkling have typically been derived based on a canonical interaction between a pair of counter-rotating vortices and a planar flame surface. However, it was found that this configuration did not properly represent real turbulence-flame interaction. Interactions resembling the canonical configuration were observed in less than 10% of the recorded frames. Instead, straining and wrinkling were generally caused more geometrically complex turbulence, consisting of large groups of structures that could be multiply curved and intertwined. The effect of the interaction was highly dependent on the interaction geometry. Furthermore, even when the turbulence did exist in the canonical geometry, the straining and wrinkling of the flame surface were not well characterized by the vortical structures. A new mechanistic description of the turbulence-flame interaction was therefore identified and confirmed by the measurements. In this description, flame surface straining is caused by coherent structures of fluid-dynamic strain-rate (strain-rate structures). The role of vortical structures is to curve existing flame surface, creating wrinkles. By simultaneously considering both forms of turbulent structure, turbulence-flame interactions in both the canonical configuration and more complex geometries could be understood. (author)
Studies in premixed combustion
Sivashinsky, G.I.
1992-01-01
This report discusses the following topics on premixed combustion: theory of turbulent flame propagation; pattern formation in premixed flames and related problems; and pattern formation in extended systems. (LSP)
Krishnan, S. R.; inivasan, K. K.
2010-09-14
Detailed results from a multi-zone phenomenological simulation of partially premixed advanced-injection low-pilot-ignited natural-gas low-temperature combustion are presented with a focus on early injection timings (the beginning of (pilot) injection (BOI)) and very small diesel quantities (2-3 per cent of total fuel energy). Combining several aspects of diesel and spark ignition engine combustion models, the closed-cycle simulation accounted for diesel autoignition, diesel spray combustion, and natural-gas combustion by premixed turbulent flame propagation. The cylinder contents were divided into an unburned zone, several pilot fuel zones (or 'packets') that modelled diesel evaporation and ignition, a flame zone for natural-gas combustion, and a burned zone. The simulation predicted the onset of ignition, cylinder pressures, and heat release rate profiles satisfactorily over a wide range of BOIs (20-60° before top dead centre (before TDC)) but especially well at early BOIs. Strong coupling was observed between pilot spray combustion in the packets and premixed turbulent combustion in the flame zone and, therefore, the number of ignition centres (packets) profoundly affected flame combustion. The highest local peak temperatures (greater than 2000 K) were observed in the packets, while the flame zone was much cooler (about 1650 K), indicating that pilot diesel spray combustion is probably the dominant source of engine-out emissions of nitrogen oxide (NO_{x}). Further, the 60° before TDC BOI yielded the lowest average peak packet temperatures (about 1720 K) compared with the 20° before TDC BOI (about 2480 K) and 40° before TDC BOI (about 2700 K). These trends support experimental NO_{x} trends, which showed the lowest NO_{x} emissions for the 60°, 20°, and 40° before TDC BOIs in that order. Parametric studies showed that increasing the intake charge temperature, pilot quantity, and natural-gas equivalence ratio all led to higher peak
NASA Astrophysics Data System (ADS)
Manickam, B.; Franke, J.; Muppala, S. P. R.; Dinkelacker, F.
In this LES study, an algebraic flame surface wrinkling model based on the progress variable gradient approach is validated for lean premixed turbulent propane/air flames measured on VOLVO test rig. These combustion results are analyzed for uncertainty in the solution using two quality assessment techniques.
Rayleigh/Raman/LIF measurements in a turbulent lean premixed combustor
Nandula, S.P.; Pitz, R.W.; Barlow, R.S.; Fiechtner, G.J.
1995-12-31
Much of the industrial electrical generation capability being added worldwide is gas-turbine engine based and is fueled by natural gas. These gas-turbine engines use lean premixed (LP) combustion to meet the strict NO{sub x} emission standards, while maintaining acceptable levels of CO. In conventional, diffusion flame gas turbine combustors, large amount of NO{sub x} forms in the hot stoichiometric zones via the Zeldovich (thermal) mechanism. Hence, lean premixed combustors are rapidly becoming the norm, since they are specifically designed to avoid these hot stoichiometric zones and the associated thermal NO{sub x}. However, considerable research and development are still required to reduce the NO{sub x} levels (25-40 ppmvd adjusted to 15% O{sub 2} with the current technology), to the projected goal of under 10 ppmvd by the turn of the century. Achieving this objective would require extensive experiments in LP natural gas (or CH{sub 4}) flames for understanding the combustion phenomena underlying the formation of the exhaust pollutants. Although LP combustion is an effective way to control NO{sub x}, the downside is that it increases the CO emissions. The formation and destruction of the pollutants (NO{sub x} and CO) are strongly affected by the fluid mechanics, the finite-rate chemistry, and their (turbulence-chemistry) interactions. Hence, a thorough understanding of these interactions is vital for controlling and reducing the pollutant emissions. The present research is contributing to this goal by providing a detailed nonintrusive laser based data set with good spatial and temporal resolutions of the pollutants (NO and CO) along with the major species, temperature, and OH. The measurements reported in this work, along with the existing velocity data on a turbulent LP combustor burning CH{sub 4}, would provide insight into the turbulence-chemistry interactions and their effect on pollutant formation.
Rayleigh/Raman/LIF measurements in a turbulent lean premixed combustor
Nandula, S.P.; Pitz, R.W.; Barlow, R.S.
1995-10-01
Much of the industrial electrical generation capability being added worldwide is gas-turbine engine based and is fueled by natural gas. These gas-turbine engines use lean premixed (LP) combustion to meet the strict NO{sub x} emission standards, while maintaining acceptable levels of CO. In conventional, diffusion flame gas turbine combustors, large amount of NO{sub x} forms in the hot stoichiometric zones via the Zeldovich (thermal) mechanism. Hence, lean premixed combustors are rapidly becoming the norm, since they are specifically designed to avoid these hot stoichiometric zones and the associated thermal NO, However, considerable research and development are still required to reduce the NO{sub x} levels (25-40 ppmvd adjusted to 15% O{sub 2} with the current technology), to the projected goal of under 10 ppmvd by the turn of the century. Achieving this objective would require extensive experiments in LP natural gas (or CH{sub 4}) flames for understanding the combustion phenomena underlying the formation of the exhaust pollutants. Although LP combustion is an effective way to control NO{sub x}, the downside is that it increases the CO emissions. The formation and destruction of the pollutants (NO{sub x} and CO) are strongly affected by the fluid mechanics, the finite-rate chemistry, and their (turbulence-chemistry) interactions. Hence, a thorough understanding of these interactions is vital for controlling and reducing the pollutant emissions. The present research is contributing to this goal by providing a detailed nonintrusive laser based data set with good spatial and temporal resolutions of the pollutants (NO and CO) along with the major species, temperature, and OH. The measurements reported in this work, along with the existing velocity data on a turbulent LP combustor burning CH{sub 4}, would provide insight into the turbulence-chemistry interactions and their effect on pollutant formation.
Armitage, C.A.; Mastorakos, E.; Cant, R.S.; Balachandran, R.
2006-08-15
Acoustically forced lean premixed turbulent bluff-body stabilized flames are investigated using turbulent combustion CFD. The calculations simulate aspects of the experimental investigation by Balachandran et al. [R. Balachandran, B. Ayoola, C. Kaminski, A. Dowling, E. Mastorakos, Combust. Flame 143 (2005) 37-55] and focus on the amplitude dependence of the flame response. For the frequencies of interest in this investigation an unsteady Reynolds-averaged Navier-Stokes (URANS) approach is appropriate. The combustion is represented using a modified laminar flamelet approach with an algebraic representation of the flame surface density. The predictions are compared with flame surface density (FSD) and OH* chemiluminescence measurements. In the experiments the response of the flame has been quantified by means of a number of single-frequency, amplitude-dependent transfer functions. The predicted flame shape and position are in good agreement with the experiment. The dynamic response of the flame to inlet velocity forcing is also well captured by the calculations. At moderate frequencies nonlinear behavior of the transfer functions is observed as the forcing amplitude is increased. In the experiments this nonlinearity was attributed in part to the rollup of the reacting shear layer into vortices and in part to the collision of the inner and outer flame sheets. This transition to nonlinearity is also observed in the transfer functions obtained from the predictions. Furthermore, the vortex shedding and flame-sheet collapse may be seen in snapshots of the predicted flow field taken throughout the forcing cycle. The URANS methodology successfully predicts the behavior of the forced premixed turbulent flames and captures the effects of saturation in the transfer function of the response of the heat release to velocity fluctuations. (author)
Numerical simulations of turbulent premixed H2/O2/N2 flames with complex chemistry
NASA Technical Reports Server (NTRS)
Baum, M.; Poinsot, T. J.; Haworth, D. C.
1992-01-01
Premixed stoichiometric H2/O2/N2 flames propagating in two-dimensional turbulence were studied using direct numerical simulation (simulations in which all fluid and thermochemical scales are fully resolved) including realistic chemical kinetics and molecular transport. Results are compared with earlier zero-chemistry (flame sheet) and one-step chemistry simulations. Consistent with the simpler models, the turbulent flame with realistic chemistry aligns preferentially with extensive strain rates in the tangent plane and flame curvature probability density functions are close to symmetric with near-zero means. By contrast to simple-chemistry results with non-unity Lewis numbers (ratio of thermal to species diffusivity), local flame structure does not correlate with curvature but rather with tangential strain rate. Turbulent straining results in substantial thinning of the flame relative to the steady unstrained laminar case. Heat release and H2O2 contours remain thin and connected ('flamelet-like') while species including H-atom and OH are more diffuse. Peak OH concentration occurs well behind the peak heat-release zone. The feasibility of incorporating realistic chemistry into full turbulence simulations to address issues such as pollutant formation in hydrocarbon-air flames is suggested.
Impact of chemical kinetic model reduction on premixed turbulent flame characteristics
NASA Astrophysics Data System (ADS)
Fillo, Aaron; Niemeyer, Kyle
2016-11-01
The use of detailed chemical kinetic models for direct numerical simulations (DNS) is prohibitively expensive. Current best practice for the development of reduced models is to match laminar burning parameters such as flame speed, thickness, and ignition delay time to predictions of the detailed chemical kinetic models. Prior studies using reduced models implicitly assumed that matching the homogeneous and laminar properties of the detailed model will result in similar behavior in a turbulent environment. However, this assumption has not been tested. Fillo et al. recently demonstrated experimentally that real jet fuels with similar chemistry and laminar burning parameters exhibit different turbulent flame speeds under the same flow conditions. This result raises questions about the validity of current best practices for the development of reduced chemical kinetic models for turbulent DNS. This study will investigate the validity of current best practices. Turbulent burning parameters, including flame speed, thickness, and stretch rate, will be compared for three skeletal mechanisms of the Princeton POSF 4658 mechanism, reduced using current best practice methods. DNS calculations of premixed, high-Karlovitz flames will be compared to determine if these methods are valid. This material is based upon work supported by the National Science Foundation under Grant No. 1314109-DGE.
Large scale Direct Numerical Simulation of premixed turbulent jet flames at high Reynolds number
NASA Astrophysics Data System (ADS)
Attili, Antonio; Luca, Stefano; Lo Schiavo, Ermanno; Bisetti, Fabrizio; Creta, Francesco
2016-11-01
A set of direct numerical simulations of turbulent premixed jet flames at different Reynolds and Karlovitz numbers is presented. The simulations feature finite rate chemistry with 16 species and 73 reactions and up to 22 Billion grid points. The jet consists of a methane/air mixture with equivalence ratio ϕ = 0 . 7 and temperature varying between 500 and 800 K. The temperature and species concentrations in the coflow correspond to the equilibrium state of the burnt mixture. All the simulations are performed at 4 atm. The flame length, normalized by the jet width, decreases significantly as the Reynolds number increases. This is consistent with an increase of the turbulent flame speed due to the increased integral scale of turbulence. This behavior is typical of flames in the thin-reaction zone regime, which are affected by turbulent transport in the preheat layer. Fractal dimension and topology of the flame surface, statistics of temperature gradients, and flame structure are investigated and the dependence of these quantities on the Reynolds number is assessed.
Flame front surface characteristics in turbulent premixed propane/air combustion
Guelder, O.L.; Smallwood, G.J.; Wong, R.; Snelling, D.R.; Smith, R.; Deschamps, B.M.; Sautet, J.C.
2000-03-01
The characteristics of the flame front surfaces in turbulent premixed propane/air flames were investigated. Flame front images were obtained using laser-induced fluorescence (LIF) of OH and Mie scattering on two Bunsen-type burners of 11.2-mm and 22.4-mm diameters. Nondimensional turbulence intensity, u{prime}/S{sub L}, was varied from 0.9 to 15, and the Reynolds number, based on the integral length scale, varied from 40 to 467. Approximately 100 images were recorded for each experimental condition. Fractal parameters (fractal dimension, inner and outer cutoffs) and corresponding standard deviations were determined by analysis of the flame front images using the caliper technique. The fractal dimensions derived from OH and Mie scattering images are almost identical. However, inner and outer cutoffs from OH images are consistently higher than those obtained from Mie scattering. The self-similar region of the flame front wrinkling is about a decade for all flames studied. In the nondimensional turbulence intensity range from 1 to 15, it was found that the mean fractal dimension is about 2.2 and it does not show any dependence on turbulence intensity. This contradicts the findings of the previous studies that showed that the fractal dimension asymptotically reaches to 2.35--2.37 when the nondimensional turbulence intensity u{prime}/S{sub L} exceeds 3. It is shown that the reason for this discrepancy is the image analysis method used in the previous studies. Examples are given to show the inadequacy of the circle method used in previous studies for extraction of fractal parameters from flame front images. The fractal parameters obtained so far, in this and previous studies, are not capable of correctly predicting the turbulent burning velocity using the available fractal area closure model.
Characterization and modeling of premixed turbulent n-heptane ames in the thin reaction zone regime
NASA Astrophysics Data System (ADS)
Savard, Bruno
n-heptane/air premixed turbulent flames in the high-Karlovitz portion of the thin reaction zone regime are characterized and modeled in this thesis using Direct Numerical Simulations (DNS) with detailed chemistry. In order to perform these simulations, a time-integration scheme that can efficiently handle the stiffness of the equations solved is developed first. A first simulation with unity Lewis number is considered in order to assess the effect of turbulence on the flame in the absence of differential diffusion. A second simulation with non-unity Lewis numbers is considered to study how turbulence affects differential diffusion. In the absence of differential diffusion, minimal departure from the 1D unstretched flame structure (species vs. temperature profiles) is observed. In the non-unity Lewis number case, the flame structure lies between that of 1D unstretched flames with "laminar'' non-unity Lewis numbers and unity Lewis number. This is attributed to effective Lewis numbers resulting from intense turbulent mixing and a first model is proposed. The reaction zone is shown to be thin for both flames, yet large chemical source term fluctuations are observed. The fuel consumption rate is found to be only weakly correlated with stretch, although local extinctions in the non-unity Lewis number case are well correlated with high curvature. These results explain the apparent turbulent flame speeds. Other variables that better correlate with this fuel burning rate are identified through a coordinate transformation. It is shown that the unity Lewis number turbulent flames can be accurately described by a set of 1D (in progress variable space) flamelet equations parameterized by the dissipation rate of the progress variable. In the non-unity Lewis number flames, the flamelet equations suggest a dependence on a second parameter, the diffusion of the progress variable. A new tabulation approach is proposed for the simulation of such flames with these dimensionally
Scaling and efficiency of PRISM in adaptive simulations of turbulent premixed flames
Tonse, Shaheen R.; Bell, J.B.; Brown, N.J.; Day, M.S.; Frenklach, M.; Grcar, J.F.; Propp, R.M.
1999-12-01
The dominant computational cost in modeling turbulent combustion phenomena numerically with high fidelity chemical mechanisms is the time required to solve the ordinary differential equations associated with chemical kinetics. One approach to reducing that computational cost is to develop an inexpensive surrogate model that accurately represents evolution of chemical kinetics. One such approach, PRISM, develops a polynomial representation of the chemistry evolution in a local region of chemical composition space. This representation is then stored for later use. As the computation proceeds, the chemistry evolution for other points within the same region are computed by evaluating these polynomials instead of calling an ordinary differential equation solver. If initial data for advancing the chemistry is encountered that is not in any region for which a polynomial is defined, the methodology dynamically samples that region and constructs a new representation for that region. The utility of this approach is determined by the size of the regions over which the representation provides a good approximation to the kinetics and the number of these regions that are necessary to model the subset of composition space that is active during a simulation. In this paper, we assess the PRISM methodology in the context of a turbulent premixed flame in two dimensions. We consider a range of turbulent intensities ranging from weak turbulence that has little effect on the flame to strong turbulence that tears pockets of burning fluid from the main flame. For each case, we explore a range of sizes for the local regions and determine the scaling behavior as a function of region size and turbulent intensity.
Effects of pressure fluctuations on the combustion process in turbulent premixed flames
NASA Astrophysics Data System (ADS)
Beardsell, Guillaume; Lapointe, Simon; Blanquart, Guillaume
2016-11-01
The need for a thorough understanding of turbulence-combustion interactions in compressible flows is driven by recent technological developments in propulsion as well as renewed interest in the development of next generation supersonic and hypersonic vehicles. In such flows, pressure fluctuations displaying a wide range of length and timescales are present. These fluctuations are expected to impact the combustion process to varying degrees, depending amongst other things on the amplitude of the pressure variations and the timescales of the chemical reactions taking place in the flame. In this context, numerical simulations of these flows can provide insight into the impact of pressure fluctuations on the combustion process. In the present work, we analyze data from simulations of statistically-flat premixed n-heptane/air flames at high Karlovitz numbers. The compressible Navier-Stokes equations are solved exactly (DNS) and results obtained with both detailed kinetic modeling and one-step chemistry are considered. The effects of pressure fluctuations on the fuel burning rate are investigated. The findings are compared with results obtained from simulations of one-dimensional premixed flames subjected to various pressure waves.
Large eddy simulation of soot formation in a turbulent non-premixed jet flame
El-Asrag, Hossam; Menon, Suresh
2009-02-15
A recently developed subgrid model for soot dynamics [H. El-Asrag, T. Lu, C.K. Law, S. Menon, Combust. Flame 150 (2007) 108-126] is used to study the soot formation in a non-premixed turbulent flame. The model allows coupling between reaction, diffusion and soot (including soot diffusion and thermophoretic forces) processes in the subgrid domain without requiring ad hoc filtering or model parameter adjustments. The combined model includes the entire process, from the initial phase, when the soot nucleus diameter is much smaller than the mean free path, to the final phase, after coagulation and aggregation, where it can be considered in the continuum regime. A relatively detailed but reduced kinetics for ethylene-air is used to simulate an experimentally studied non-premixed ethylene/air jet diffusion flame. Acetylene is used as a soot precursor species. The soot volume fraction order of magnitude, the location of its maxima, and the soot particle size distribution are all captured reasonably. Along the centerline, an initial region dominated by nucleation and surface growth is established followed by an oxidation region. The diffusion effect is found to be most important in the nucleation regime, while the thermophoretic forces become more influential downstream of the potential core in the oxidation zone. The particle size distribution shows a log-normal distribution in the nucleation region, and a more Gaussian like distribution further downstream. Limitations of the current approach and possible solution strategies are also discussed. (author)
Experimental Study of the Flowfield of a Two-Dimensional Premixed Turbulent Flame
Ganji, A. R.; Sawyer, R. F.
1980-07-01
A turbulent reacting shear layer in a premixed propane/air flow has been studied in a two dimensional combustor, with the flame stabilized behind a rearward facing streamlined step. Spark shadowgraphs show that in the range of velocities (7.5 to 22.5 m/sec corresponding to Reynolds numbers of .5 x 10{sup 4} cm{sup -1} to 1. 5 x 10{sup 4} cm{sup -1} ) and equivalence ratios (0.4 to 0.7) studied, the mixing layer is dominated by Brown~ Roshko type large coherent structures in both reacting and nonreacting flows. High speed schlieren movies show that these eddies are convected downstream and increase their size and spacing by combustion and coalescence with neighboring eddies. Tracing individual eddies shows, in the reacting shear layer, that, on the average, eddies accelerate as they move downstream with the highest acceleration close to the origin of the shear layer. Combustion is confined to these large structures which develop as a result of vortical action of the shear flow. On the average, the reacting eddies have a lower growth rate than nonreacting eddies. A turbulent boundary layer created by means of a tripping wire upstream of the edge of the step virtually eliminates the large coherent structures in the shear layer, while for the case in which the wire could not trigger the transition to turbulence, the large coherent structures dominated the reacting and nonreacting flows.
Conditional Reynolds stress in a strongly heated turbulent boundary layer with premixed combustion
Cheng, R.K.; Ng, T.T.
1985-02-01
A two-color laser Doppler anemometry technique has been used to measure velocity statistics and Reynolds stress in a turbulent boundary layer over a strongly heated wall (T/sub w/ = 1100 K) with premixed ethylene--air combustion. Measurements in the isothermal and nonreacting heated wall turbulent boundary layers were also made for comparison. Conditional velocity covariance -approx.(uv) are deduced using the quadrant and ''hole'' analysis methods. The conditional data obtained in the isothermal layer are in exellent agreement with those deduced by others based on hot-wire data. The results obtained in the heated and reacting layers indicate that the decrease in mean Reynolds stress can be attributed to a reduction in the -approx.(uv) contributions associated with a burst of low momentum fluid form the wall. The reduction is much more significant in the reacting layer than in the heated layer. Comparison with previous density shows that the changes in conditional -approx.(uv) contributions occur mostly within the constant density sublayer of combustion products adjacent to the wall. In the region where combustion reaction takes place, no significant change in the turbulence intensities or Reynolds stress is found.
NASA Technical Reports Server (NTRS)
Trouve, A.; Veynante, D.; Bray, K. N. C.; Mantel, T.
1994-01-01
Current flamelot models based on a description of the flame surface dynamics require the closure of two inter-related equations: a transport equation for the mean reaction progress variable, (tilde)c, and a transport equation for the flame surface density, Sigma. The coupling between these two equations is investigated using direct numerical simulations (DNS) with emphasis on the correlation between the turbulent fluxes of (tilde)c, bar(pu''c''), and Sigma, (u'')(sub S)Sigma. Two different DNS databases are used in the present work: a database developed at CTR by A. Trouve and a database developed by C. J. Rutland using a different code. Both databases correspond to statistically one-dimensional premixed flames in isotropic turbulent flow. The run parameters, however, are significantly different, and the two databases correspond to different combustion regimes. It is found that in all simulated flames, the correlation between bar(pu''c'') and (u'')(sub S)Sigma is always strong. The sign, however, of the turbulent flux of (tilde)c or Sigma with respect to the mean gradients, delta(tilde)c/delta(x) or delta(Sigma)/delta(x), is case-dependent. The CTR database is found to exhibit gradient turbulent transport of (tilde)c and Sigma, whereas the Rutland DNS features counter-gradient diffusion. The two databases are analyzed and compared using various tools (a local analysis of the flow field near the flame, a classical analysis of the conservation equation for (tilde)(u''c''), and a thin flame theoretical analysis). A mechanism is then proposed to explain the discrepancies between the two databases and a preliminary simple criterion is derived to predict the occurrence of gradient/counter-gradient turbulent diffusion.
Monte-Carlo computation of turbulent premixed methane/air ignition
NASA Astrophysics Data System (ADS)
Carmen, Christina Lieselotte
The present work describes the results obtained by a time dependent numerical technique that simulates the early flame development of a spark-ignited premixed, lean, gaseous methane/air mixture with the unsteady spherical flame propagating in homogeneous and isotropic turbulence. The algorithm described is based upon a sub-model developed by an international automobile research and manufacturing corporation in order to analyze turbulence conditions within internal combustion engines. Several developments and modifications to the original algorithm have been implemented including a revised chemical reaction scheme and the evaluation and calculation of various turbulent flame properties. Solution of the complete set of Navier-Stokes governing equations for a turbulent reactive flow is avoided by reducing the equations to a single transport equation. The transport equation is derived from the Navier-Stokes equations for a joint probability density function, thus requiring no closure assumptions for the Reynolds stresses. A Monte-Carlo method is also utilized to simulate phenomena represented by the probability density function transport equation by use of the method of fractional steps. Gaussian distributions of fluctuating velocity and fuel concentration are prescribed. Attention is focused on the evaluation of the three primary parameters that influence the initial flame kernel growth-the ignition system characteristics, the mixture composition, and the nature of the flow field. Efforts are concentrated on the effects of moderate to intense turbulence on flames within the distributed reaction zone. Results are presented for lean conditions with the fuel equivalence ratio varying from 0.6 to 0.9. The present computational results, including flame regime analysis and the calculation of various flame speeds, provide excellent agreement with results obtained by other experimental and numerical researchers.
A mixing timescale model for TPDF simulations of turbulent premixed flames
Kuron, Michael; Ren, Zhuyin; Hawkes, Evatt R.; ...
2017-02-06
Transported probability density function (TPDF) methods are an attractive modeling approach for turbulent flames as chemical reactions appear in closed form. However, molecular micro-mixing needs to be modeled and this modeling is considered a primary challenge for TPDF methods. In the present study, a new algebraic mixing rate model for TPDF simulations of turbulent premixed flames is proposed, which is a key ingredient in commonly used molecular mixing models. The new model aims to properly account for the transition in reactive scalar mixing rate behavior from the limit of turbulence-dominated mixing to molecular mixing behavior in flamelets. An a priorimore » assessment of the new model is performed using direct numerical simulation (DNS) data of a lean premixed hydrogen–air jet flame. The new model accurately captures the mixing timescale behavior in the DNS and is found to be a significant improvement over the commonly used constant mechanical-to-scalar mixing timescale ratio model. An a posteriori TPDF study is then performed using the same DNS data as a numerical test bed. The DNS provides the initial conditions and time-varying input quantities, including the mean velocity, turbulent diffusion coefficient, and modeled scalar mixing rate for the TPDF simulations, thus allowing an exclusive focus on the mixing model. Here, the new mixing timescale model is compared with the constant mechanical-to-scalar mixing timescale ratio coupled with the Euclidean Minimum Spanning Tree (EMST) mixing model, as well as a laminar flamelet closure. It is found that the laminar flamelet closure is unable to properly capture the mixing behavior in the thin reaction zones regime while the constant mechanical-to-scalar mixing timescale model under-predicts the flame speed. Furthermore, the EMST model coupled with the new mixing timescale model provides the best prediction of the flame structure and flame propagation among the models tested, as the dynamics of reactive
Turbulence, combustion, pollutant, and stability characterization of a premixed, step combustor
NASA Technical Reports Server (NTRS)
Ganji, A. T.; Sawyer, R. F.
1980-01-01
A two dimensional combustion tunnel was constructed to study a lean premixed turbulent propane/air flame stablized behind a rearward facing step. Studied were: (1) the existence and importance of large coherent structures in turbulent reacting and nonreacting free shear layers behind the steps; (2) the effect of inlet temperature and reference velocity on combustion efficiency; (3) CO, NO2 and NO sub x production in the flame; and (4) the blowout and upstream propagation of the flame. In the ranges studied, the large coherent structures dominated both the reacting and the nonreacting free shear layers behind the step. The growth of the vortices and the propagation of the flamer were intimately linked. Vortex pairing was observed to be one of the mechanisms for introduction of fresh reactants into the shear layer and growth of the shear layer. Probe composition measurements of the flame showed that, in the recirculation zone, the reaction was above 99 percent complete, CO and unburnt hydrocarbons were above the equilibrium level NO sub x concentration was far below the equilibrium level and NO2 comprised a negligible fraction of NO sub x.
Three-dimensional DNS of turbulent premixed flames in a constant volume vessel
NASA Astrophysics Data System (ADS)
Fukushima, Naoya; Tsunemi, Akihiko; Shimura, Masayasu; Shim, Youngsam; Tanahashi, Mamoru; Miyauchi, Toshio
2010-11-01
Clarification of flame behaviors in a vessel is of great importance for high efficiency of combustors, especially in SI engines. Direct numerical simulation of turbulent hydrogen-air premixed flames in a constant volume rectangular vessel at relatively high Reynolds number has been conducted by considering detailed kinetic mechanism. At first, flame ignites and propagates from the ignition kernel. When the flame approaches a wall, the flame displacement speed normal to the wall decreases gradually. After the flame impingement on the wall, the flame propagates along the wall and the flame displacement speed parallel to the wall becomes higher than that of freely propagating flames. The flame is also strongly affected by internal pressure rise in the vessel. Since the pressure increase makes flame thickness thin, heat release rate of each flame element is augmented. The local pressure rise due to dilatation also enhances turbulence and finer scale vortices appear, which makes flame surface more complicated and results in increase of the flame surface area.
Numerical prediction of turbulent flame stability in premixed/prevaporized (HSCT) combustors
NASA Technical Reports Server (NTRS)
Winowich, Nicholas S.
1990-01-01
A numerical analysis of combustion instabilities that induce flashback in a lean, premixed, prevaporized dump combustor is performed. KIVA-II, a finite volume CFD code for the modeling of transient, multidimensional, chemically reactive flows, serves as the principal analytical tool. The experiment of Proctor and T'ien is used as a reference for developing the computational model. An experimentally derived combustion instability mechanism is presented on the basis of the observations of Proctor and T'ien and other investigators of instabilities in low speed (M less than 0.1) dump combustors. The analysis comprises two independent procedures that begin from a calculated stable flame: The first is a linear increase of the equivalence ratio and the second is the linear decrease of the inflow velocity. The objective is to observe changes in the aerothermochemical features of the flow field prior to flashback. It was found that only the linear increase of the equivalence ratio elicits a calculated flashback result. Though this result did not exhibit large scale coherent vortices in the turbulent shear layer coincident with a flame flickering mode as was observed experimentally, there were interesting acoustic effects which were resolved quite well in the calculation. A discussion of the k-e turbulence model used by KIVA-II is prompted by the absence of combustion instabilities in the model as the inflow velocity is linearly decreased. Finally, recommendations are made for further numerical analysis that may improve correlation with experimentally observed combustion instabilities.
Effects of nucleating species on soot formation in turbulent non-premixed sooting jet flames
NASA Astrophysics Data System (ADS)
Jain, Abhishek; Xuan, Yuan
2016-11-01
Soot nucleation is one of the most unknown processes in the soot life cycle, and it is believed to occur from Polycyclic Aromatic Hydrocarbons (PAH) generated from the combustion of various fuel sources under locally fuel-rich conditions. Current soot nucleation models may include as few as one (typically naphthalene) or as many as a dozen of nucleating species. In this study, the effects of PAH inclusion in the soot nucleation model on soot yield and distribution are studied by means of Large-Eddy Simulations (LES) of two piloted turbulent non-premixed sooting jet flames, using ethylene and a jet fuel surrogate, respectively. Two sets of simulations are performed for each flame, one considering only a single nucleating PAH (naphthalene) and the other one considering a range of nucleating PAH from naphthalene to cyclopenta[cd]pyrene. Flamelet-based chemistry tabulation is used for the major thermochemical quantities, and a recently developed relaxation model is used for PAH species to account for the interactions between turbulence and their chemistry. The effects of nucleating PAH species on soot are highlighted by comparing the mean soot volume fraction distributions and statistical characteristics of soot obtained from both sets of simulations against experimental measurements. Graduate Student, MNE.
Scalar mixing in LES/PDF of a high-Ka premixed turbulent jet flame
NASA Astrophysics Data System (ADS)
You, Jiaping; Yang, Yue
2016-11-01
We report a large-eddy simulation (LES)/probability density function (PDF) study of a high-Ka premixed turbulent flame in the Lund University Piloted Jet (LUPJ) flame series, which has been investigated using direct numerical simulation (DNS) and experiments. The target flame, featuring broadened preheat and reaction zones, is categorized into the broken reaction zone regime. In the present study, three widely used mixing modes, namely the Interaction by Exchange with the Mean (IEM), Modified Curl (MC), and Euclidean Minimum Spanning Tree (EMST) models are applied to assess their performance through detailed a posteriori comparisons with DNS. A dynamic model for the time scale of scalar mixing is formulated to describe the turbulent mixing of scalars at small scales. Better quantitative agreement for the mean temperature and mean mass fractions of major and minor species are obtained with the MC and EMST models than with the IEM model. The multi-scalar mixing in composition space with the three models are analyzed to assess the modeling of the conditional molecular diffusion term. In addition, we demonstrate that the product of OH and CH2O concentrations can be a good surrogate of the local heat release rate in this flame. This work is supported by the National Natural Science Foundation of China (Grant Nos. 11521091 and 91541204).
Computational study of lean premixed turbulent flames using RANSPDF and LESPDF methods
NASA Astrophysics Data System (ADS)
Rowinski, David H.; Pope, Stephen B.
2013-08-01
A computational study is performed on a series of four piloted, lean, premixed turbulent jet flames. These flames use the Sydney Piloted Premixed Jet Burner (PPJB), and with jet velocities of 50, 100, 150 and 200 m/s are denoted PM150, PM1100, PM1150 and PM1200, respectively. Calculations are performed using the RANSPDF and LESPDF methodologies, with different treatments of molecular diffusion, with detailed chemistry and flamelet-based chemistry modelling, and using different imposed boundary conditions. The sensitivities of the calculations to these different aspects of the modelling are compared and discussed. Comparisons are made to experimental data and to previously-performed calculations. It is found that, given suitable boundary conditions and treatment of molecular diffusion, excellent agreement between the calculations and experimental measurements of the mean and variance fields can be achieved for PM150 and PM1100. The application of a recently developed implementation of molecular diffusion results in a large improvement in the computed variance fields in the LESPDF calculations. The inclusion of differential diffusion in the LESPDF calculations provides insight on the behaviour in the near-field region of the jet, but its effects are found to be confined to this region and to the species CO, OH and H2. A major discrepancy observed in many previous calculations of these flames is an overprediction of reaction progress in PM1150 and PM1200, and this discrepancy is also observed in the LESPDF calculations; however, a parametric study of the LESPDF mixing model reveals that, with a sufficiently large mixing frequency, calculations of these two flames are capable of yielding improved reaction progress in good qualitative agreement with the mean and RMS scalar measurements up to an x/D of 30. Lastly, the merits of each computational methodology are discussed in light of their computational costs.
Kemenov, Konstantin A.; Calhoon, William H.
2015-03-24
Large-scale strain rate field, a resolved quantity which is easily computable in large-eddy simulations (LES), could have profound effects on the premixed flame properties by altering the turbulent flame speed and inducing local extinction. The role of the resolved strain rate has been investigated in a posterior LES study of GE lean premixed dry low NOx emissions LM6000 gas turbine combustor model. A novel approach which is based on the coupling of the lineareddy model with a one-dimensional counter-flow solver has been applied to obtain the parameterizations of the resolved premixed flame properties in terms of the reactive progress variable, the local strain rate measure, and local Reynolds and Karlovitz numbers. The strain rate effects have been analyzed by comparing LES statistics for several models of the turbulent flame speed, i.e, with and without accounting for the local strain rate effects, with available experimental data. The sensitivity of the simulation results to the inflow velocity conditions as well as the grid resolution have been also studied. Overall, the results suggest the necessity to represent the strain rate effects accurately in order to improve LES modeling of the turbulent flame speed.
Kemenov, Konstantin A.; Calhoon, William H.
2015-03-24
Large-scale strain rate field, a resolved quantity which is easily computable in large-eddy simulations (LES), could have profound effects on the premixed flame properties by altering the turbulent flame speed and inducing local extinction. The role of the resolved strain rate has been investigated in a posterior LES study of GE lean premixed dry low NOx emissions LM6000 gas turbine combustor model. A novel approach which is based on the coupling of the lineareddy model with a one-dimensional counter-flow solver has been applied to obtain the parameterizations of the resolved premixed flame properties in terms of the reactive progress variable,more » the local strain rate measure, and local Reynolds and Karlovitz numbers. The strain rate effects have been analyzed by comparing LES statistics for several models of the turbulent flame speed, i.e, with and without accounting for the local strain rate effects, with available experimental data. The sensitivity of the simulation results to the inflow velocity conditions as well as the grid resolution have been also studied. Overall, the results suggest the necessity to represent the strain rate effects accurately in order to improve LES modeling of the turbulent flame speed.« less
Multi-kHz temperature imaging in turbulent non-premixed flames using planar Rayleigh scattering
NASA Astrophysics Data System (ADS)
Patton, R. A.; Gabet, K. N.; Jiang, N.; Lempert, W. R.; Sutton, J. A.
2012-08-01
In this manuscript, we describe the development of two-dimensional, high-repetition-rate (10-kHz) Rayleigh scattering imaging as applied to turbulent combustion environments. In particular, we report what we believe to be the first sets of high-speed planar Rayleigh scattering images in turbulent non-premixed flames, yielding temporally correlated image sequences of the instantaneous temperature field. Sample results are presented for the well-characterized DLR flames A and B (CH4/H2/N2) at Reynolds numbers of 15,200 and 22,800 at various axial positions downstream of the jet exit. The measurements are facilitated by the use of a user-calibrated, intensified, high-resolution CMOS camera in conjunction with a unique high-energy, high-repetition-rate pulse-burst laser system (PBLS) at Ohio State University, which yields output energies up to 200 mJ/pulse at 532 nm with 100-μs laser pulse spacing. The spatial and temporal resolution of the imaging system and acquired images are compared to the finest spatial and temporal scales expected within the turbulent flames. One of the most important features of the PBLS is the ability to readily change the pulse-to-pulse spacing as the required temporal resolution necessitates it. The quality and accuracy of the high-speed temperature imaging results are assessed by comparing derived statistics (mean and standard deviation) to that of previously reported point-based reference data acquired at Sandia National Laboratories and available within the TNF workshop. Good agreement between the two data sets is obtained providing an initial indication of quantitative nature of the planar, kHz-rate temperature imaging results.
Impact of heat release on strain rate field in turbulent premixed Bunsen flames
Coriton, Bruno Rene Leon; Frank, Jonathan H.
2016-08-10
The effects of combustion on the strain rate field are investigated in turbulent premixed CH_{4}/air Bunsen flames using simultaneous tomographic PIV and OH LIF measurements. Tomographic PIV provides three-dimensional velocity measurements, from which the complete strain rate tensor is determined. The OH LIF measurements are used to determine the position of the flame surface and the flame-normal orientation within the imaging plane. This combination of diagnostic techniques enables quantification of divergence as well as flame-normal and tangential strain rates, which are otherwise biased using only planar measurements. Measurements are compared in three lean-to-stoichiometric flames that have different amounts of heat release and Damköhler numbers greater than unity. The effects of heat release on the principal strain rates and their alignment relative to the local flame normal are analyzed. The extensive strain rate preferentially aligns with the flame normal in the reaction zone, which has been indicated by previous studies. The strength of this alignment increases with increasing heat release and, as a result, the flame-normal strain rate becomes highly extensive. These effects are associated with the gas expansion normal to the flame surface, which is largest for the stoichiometric flame. In the preheat zone, the compressive strain rate has a tendency to align with the flame normal. Away from the flame front, the flame – strain rate alignment is arbitrary in both the reactants and products. The flame-tangential strain rate is on average positive across the flame front, and therefore the turbulent strain rate field contributes to the enhancement of scalar gradients as in passive scalar turbulence. As a result, increases in heat release result in larger positive values of the divergence as well as flame-normal and tangential strain rates, the tangential strain rate has a weaker dependence on heat release than the flame-normal strain rate and the
Impact of heat release on strain rate field in turbulent premixed Bunsen flames
Coriton, Bruno Rene Leon; Frank, Jonathan H.
2016-08-10
The effects of combustion on the strain rate field are investigated in turbulent premixed CH4/air Bunsen flames using simultaneous tomographic PIV and OH LIF measurements. Tomographic PIV provides three-dimensional velocity measurements, from which the complete strain rate tensor is determined. The OH LIF measurements are used to determine the position of the flame surface and the flame-normal orientation within the imaging plane. This combination of diagnostic techniques enables quantification of divergence as well as flame-normal and tangential strain rates, which are otherwise biased using only planar measurements. Measurements are compared in three lean-to-stoichiometric flames that have different amounts of heatmore » release and Damköhler numbers greater than unity. The effects of heat release on the principal strain rates and their alignment relative to the local flame normal are analyzed. The extensive strain rate preferentially aligns with the flame normal in the reaction zone, which has been indicated by previous studies. The strength of this alignment increases with increasing heat release and, as a result, the flame-normal strain rate becomes highly extensive. These effects are associated with the gas expansion normal to the flame surface, which is largest for the stoichiometric flame. In the preheat zone, the compressive strain rate has a tendency to align with the flame normal. Away from the flame front, the flame – strain rate alignment is arbitrary in both the reactants and products. The flame-tangential strain rate is on average positive across the flame front, and therefore the turbulent strain rate field contributes to the enhancement of scalar gradients as in passive scalar turbulence. As a result, increases in heat release result in larger positive values of the divergence as well as flame-normal and tangential strain rates, the tangential strain rate has a weaker dependence on heat release than the flame-normal strain rate and the
Sankaran, Ramanan; Hawkes, Evatt R.; Yoo, Chun Sang; ...
2015-06-22
Direct numerical simulations of three-dimensional spatially-developing turbulent Bunsen flames were performed at three different turbulence intensities. We performed these simulations using a reduced methane–air chemical mechanism which was specifically tailored for the lean premixed conditions simulated here. A planar-jet turbulent Bunsen flame configuration was used in which turbulent preheated methane–air mixture at 0.7 equivalence ratio issued through a central jet and was surrounded by a hot laminar coflow of burned products. The turbulence characteristics at the jet inflow were selected such that combustion occured in the thin reaction zones (TRZ) regime. At the lowest turbulence intensity, the conditions fall onmore » the boundary between the TRZ regime and the corrugated flamelet regime, and progressively moved further into the TRZ regime by increasing the turbulent intensity. The data from the three simulations was analyzed to understand the effect of turbulent stirring on the flame structure and thickness. Furthermore, statistical analysis of the data showed that the thermal preheat layer of the flame was thickened due to the action of turbulence, but the reaction zone was not significantly affected. A global and local analysis of the burning velocity of the flame was performed to compare the different flames. Detailed statistical averages of the flame speed were also obtained to study the spatial dependence of displacement speed and its correlation to strain rate and curvature.« less
Sankaran, Ramanan; Hawkes, Evatt R.; Yoo, Chun Sang; Chen, Jacqueline H.
2015-06-22
Direct numerical simulations of three-dimensional spatially-developing turbulent Bunsen flames were performed at three different turbulence intensities. We performed these simulations using a reduced methane–air chemical mechanism which was specifically tailored for the lean premixed conditions simulated here. A planar-jet turbulent Bunsen flame configuration was used in which turbulent preheated methane–air mixture at 0.7 equivalence ratio issued through a central jet and was surrounded by a hot laminar coflow of burned products. The turbulence characteristics at the jet inflow were selected such that combustion occured in the thin reaction zones (TRZ) regime. At the lowest turbulence intensity, the conditions fall on the boundary between the TRZ regime and the corrugated flamelet regime, and progressively moved further into the TRZ regime by increasing the turbulent intensity. The data from the three simulations was analyzed to understand the effect of turbulent stirring on the flame structure and thickness. Furthermore, statistical analysis of the data showed that the thermal preheat layer of the flame was thickened due to the action of turbulence, but the reaction zone was not significantly affected. A global and local analysis of the burning velocity of the flame was performed to compare the different flames. Detailed statistical averages of the flame speed were also obtained to study the spatial dependence of displacement speed and its correlation to strain rate and curvature.
Correlation of flame speed with stretch in turbulent premixed methane/air flames
Chen, J.H.; Im, H.G.
1998-03-01
Direct numerical simulations of two-dimensional unsteady premixed methane/air flames are performed to determine the correlation of flame speed with stretch over a wide range of curvatures and strain rates generated by intense two-dimensional turbulence. Lean and stoichiometric premixtures are considered with a detailed C{sub 1}-mechanism for methane oxidation. The computed correlation shows the existence of two distinct stable branches. It further shows that exceedingly large negative values of stretch can be obtained solely through curvature effects which give rise to an overall nonlinear correlation of the flame speed with stretch. Over a narrower stretch range, {minus}1 {le} Ka {le} 1, which includes 90% of the sample, the correlation is approximately linear, and hence, the asymptotic theory for stretch is practically applicable. Overall, one-third of the sample has negative stretch. In this linear range, the Markstein number associated with the positive branch is determined and is consistent with values obtained from comparable steady counterflow computations. In addition to this conventional positive branch, a negative branch is identified. This negative branch occurs when a flame cusp, with a center of curvature in the burnt gases, is subjected to intense compressive strain, resulting in a negative displacement speed. Negative flame speeds are also encountered for extensive tangential strain rates exceeding a Karlovitz number of unity, a value consistent with steady counterflow computations.
Han, Chao; Lignell, David O.; Hawkes, Evatt R.; ...
2017-02-09
Here, the effect of differential molecular diffusion (DMD) in turbulent non-premixed flames is studied by examining two previously reported DNS of temporally evolving planar jet flames, one with CO/H2 as the fuel and the other with C2H4 as the fuel. The effect of DMD in the CO/H2 DNS flames in which H2 is part of fuel is found to behave similar to laminar flamelet, while in the C2H4 DNS flames in which H2 is not present in the fuel it is similar to laminar flamelet in early stages but becomes different from laminar flamelet later. The scaling of the effectmore » of DMD with respect to the Reynolds number Re is investigated in the CO/H2 DNS flames, and an evident power law scaling (~Re–a with a a positive constant) is observed. The scaling of the effect of DMD with respect to the Damkohler number Da is explored in both laminar counter-flow jet C2H4 diffusion flames and the C2H4 DNS flames. A power law scaling (~Daa with a a positive constant) is clearly demonstrated for C2H4 nonpremixed flames.« less
An Experimental Study of the Structure of Turbulent Non-Premixed Jet Flames in Microgravity
NASA Astrophysics Data System (ADS)
Boxx, Isaac; Idicheria, Cherian; Clemens, Noel
2000-11-01
The aim of this work is to investigate the structure of transitional and turbulent non-premixed jet flames under microgravity conditions. The microgravity experiments are being conducted using a newly developed drop rig and the University of Texas 1.5 second drop tower. The rig itself measures 16”x33”x38” and contains a co-flowing round jet flame facility, flow control system, CCD camera, and data/image acquisition computer. These experiments are the first phase of a larger study being conducted at the NASA Glenn Research Center 2.2 second drop tower facility. The flames being studied include methane and propane round jet flames at jet exit Reynolds numbers as high as 10,000. The primary diagnostic technique employed is emission imaging of flame luminosity using a relatively high-speed (350 fps) CCD camera. The high-speed images are used to study flame height, flame tip dynamics and burnout characteristics. Results are compared to normal gravity experimental results obtained in the same apparatus.
Influence of Aerodynamic Strain Rate on Local Extinction in Turbulent Non-premixed Jet Flames
NASA Astrophysics Data System (ADS)
Ramachandran, Aravind; Narayanaswamy, Venkateswaran; Lyons, Kevin
2016-11-01
2-D velocity field measurements obtained from Particle Image Velocimetry (PIV) are used to obtain aerodynamic strain rate information in regions of local extinction in lifted turbulent non-premixed methane jet flames in coflow. Diluting the coflow to reduce the oxygen molefraction results in increased occurrences of local extinction. Statistical analysis is performed to correlate regions of high local strain rate with local extinctions in both air coflow and diluted coflow cases to study the influence of strain rate against vortical structures in extinguishing the flame front. A comparison is also made with heated and vitiated coflow cases, where autoignition is a flame stabilization mechanism and influenced by local strain rate. At high jet exit velocities (Ux > > Ur), the out-of-plane strain rate component can be neglected but the convection of extinguished pockets into the measurement plane needs to be resolved by stereoscopic (3-D) measurements which will be done in a future work. This work has been supported by the U.S. Army Research Office (Contracts W911NF1210140 and W911NF1610087) Dr. Ralph Anthenien, Technical Monitor, ARO.
Chaudhuri, Swetaprovo; Cetegen, Baki M.
2009-03-15
Response of bluff-body stabilized conical turbulent premixed flames was experimentally studied for a range of excitation frequencies (10-400 Hz), mean flow velocities (5, 10 and 15 m/s) and three different spatial mixture distributions (uniform, inner and outer enrichment). Upstream excitation was provided by a loudspeaker producing velocity oscillation amplitudes of about 8% of the mean flow velocity. Flame response was detected by a photomultiplier observing the CH{sup *} emission from the flame. The studied turbulent flames exhibited transfer function characteristics of a low-pass filter with a cutoff Strouhal number between 0.08 and 0.12. The amplification factors at low frequencies ranged from 2 to 20 and generally increased for mean flow velocities from 5 to 15 m/s. The highest levels of amplification were found for the outer mixture enrichment followed in decreasing order by uniform and inner mixture gradient cases. The high levels of flame response for the outer enrichment case were attributed to the enhanced flame-vortex interaction in outer jet shear layer. At high excitation levels (u{sup '}/U{sub m}{approx}0.3) for U{sub m}=5 m/ s where non-linear flame response is expected, the flame exhibited a reduced amplitude response in the frequency range between 40 and 100 Hz for the uniform and outer equivalence ratio gradient cases and no discernible effect for the inner equivalence ratio gradient. In all cases, transfer function phase was found to vary linearly with excitation frequency. Finally, a relationship between the amplitude characteristics of the bluff-body wake transfer function and flame blowoff equivalence ratio was presented. (author)
NASA Technical Reports Server (NTRS)
Cooper, L. P.
1981-01-01
An analysis was conducted of the effect of flameholding devices on the precombustion fuel-air characteristics and on oxides of nitrogen (NOx) emissions for combustion of premixed partially vaporized mixtures. The analysis includes the interrelationships of flameholder droplet collection efficiency, reatomization efficiency and blockage, and the initial droplet size distribution and accounts for the contribution of droplet combustion in partially vaporized mixtures to NOx emissions. Application of the analytical procedures is illustrated and parametric predictions of NOx emissions are presented.
Cyclic Combustion Variations in Dual Fuel Partially Premixed Pilot-Ignited Natural Gas Engines
Srinivasan, K. K.; Krishnan, S. R.; Qi, Y.
2012-05-09
Dual fuel pilot ignited natural gas engines are identified as an efficient and viable alternative to conventional diesel engines. This paper examines cyclic combustion fluctuations in conventional dual fuel and in dual fuel partially premixed low temperature combustion (LTC). Conventional dual fueling with 95% (energy basis) natural gas (NG) substitution reduces NOx emissions by almost 90%t relative to straight diesel operation; however, this is accompanied by 98% increase in HC emissions, 10 percentage points reduction in fuel conversion efficiency (FCE) and 12 percentage points increase in COVimep. Dual fuel LTC is achieved by injection of a small amount of diesel fuel (2-3 percent on an energy basis) to ignite a premixed natural gas₋air mixture to attain very low NOx emissions (less than 0.2 g/kWh). Cyclic variations in both combustion modes were analyzed by observing the cyclic fluctuations in start of combustion (SOC), peak cylinder pressures (Pmax), combustion phasing (Ca50), and the separation between the diesel injection event and Ca50 (termed "relative combustion phasing" ). For conventional dual fueling, as % NG increases, Pmax decreases, SOC and Ca50 are delayed, and cyclic variations increase. For dual fuel LTC, as diesel injection timing is advanced from 20° to 60° BTDC, the relative combustion phasing is identified as an important combustion parameter along with SoC, Pmax, and CaPmax. For both combustion modes, cyclic variations were characterized by alternating slow and fast burn cycles, especially at high %NG and advanced injection timings. Finally, heat release return maps were analyzed to demonstrate thermal management strategies as an effective tool to mitigate cyclic combustion variations, especially in dual fuel LTC.
NASA Astrophysics Data System (ADS)
Nikolaevich Lipatnikov, Andrei; Nishiki, Shinnosuke; Hasegawa, Tatsuya
2015-05-01
The linear relation between the mean rate of product creation and the mean scalar dissipation rate, derived in the seminal paper by K.N.C. Bray ['The interaction between turbulence and combustion', Proceedings of the Combustion Institute, Vol. 17 (1979), pp. 223-233], is the cornerstone for models of premixed turbulent combustion that deal with the dissipation rate in order to close the reaction rate. In the present work, this linear relation is straightforwardly validated by analysing data computed earlier in the 3D Direct Numerical Simulation (DNS) of three statistically stationary, 1D, planar turbulent flames associated with the flamelet regime of premixed combustion. Although the linear relation does not hold at the leading and trailing edges of the mean flame brush, such a result is expected within the framework of Bray's theory. However, the present DNS yields substantially larger (smaller) values of an input parameter cm (or K2 = 1/(2cm - 1)), involved by the studied linear relation, when compared to the commonly used value of cm = 0.7 (or K2 = 2.5). To gain further insight into the issue and into the eventual dependence of cm on mixture composition, the DNS data are combined with the results of numerical simulations of stationary, 1D, planar laminar methane-air flames with complex chemistry, with the results being reported in terms of differently defined combustion progress variables c, i.e. the normalised temperature, density, or mole fraction of CH4, O2, CO2 or H2O. Such a study indicates the dependence of cm both on the definition of c and on the equivalence ratio. Nevertheless, K2 and cm can be estimated by processing the results of simulations of counterpart laminar premixed flames. Similar conclusions were also drawn by skipping the DNS data, but invoking a presumed beta probability density function in order to evaluate cm for the differently defined c's and various equivalence ratios.
Control of the low-load region in partially premixed combustion
NASA Astrophysics Data System (ADS)
Ingesson, Gabriel; Yin, Lianhao; Johansson, Rolf; Tunestal, Per
2016-09-01
Partially premixed combustion (PPC) is a low temperature, direct-injection combustion concept that has shown to give promising emission levels and efficiencies over a wide operating range. In this concept, high EGR ratios, high octane-number fuels and early injection timings are used to slow down the auto-ignition reactions and to enhance the fuel and are mixing before the start of combustion. A drawback with this concept is the combustion stability in the low-load region where a high octane-number fuel might cause misfire and low combustion efficiency. This paper investigates the problem of low-load PPC controller design for increased engine efficiency. First, low-load PPC data, obtained from a multi-cylinder heavy- duty engine is presented. The data shows that combustion efficiency could be increased by using a pilot injection and that there is a non-linearity in the relation between injection and combustion timing. Furthermore, intake conditions should be set in order to avoid operating points with unfavourable global equivalence ratio and in-cylinder temperature combinations. Model predictive control simulations were used together with a calibrated engine model to find a gas-system controller that fulfilled this task. The findings are then summarized in a suggested engine controller design. Finally, an experimental performance evaluation of the suggested controller is presented.
Blowoff behavior of bluff body stabilized flames in vitiated and partially premixed flows
NASA Astrophysics Data System (ADS)
Tuttle, Steven G.
Turbulent flame holding and blowoff characteristics of bluff body stabilized flames were measured in an enclosed rectangular duct with a triangular flame holder in vitiated, premixed and unvitiated, asymmetrically stratified flows. Blowoff stability margins were characterized, with chemiluminescence measurements performed by high-speed imaging to capture flame dynamics during blow off. As the equivalence ratio was decreased, local extinction along the shear layer flames occurred with greater frequency and proximity to the wake stagnation zone. Decreased equivalence ratio resulted in extinction events at the trailing edge of the stagnation zone, where reactants were convected into the recirculation zone and burned. Eventually, increasing reactant dilution of the recirculation zone either increased the ignition time scale or the lowered the strain tolerance of the propagating flames in the flame anchoring region, resulting in lift-off or extinction, and the near field shear layer flames convected to the wake stagnation zone, where they continued to propagate. From there, the flames were convected upstream into the recirculation zone, where they were eventually quenched. Simultaneous PIV and OH PLIF measurements captured the flame edge location and aerodynamic behavior as blowoff was approached. Two-dimensional hydrodynamic stretch alone the flame front and flow field vorticity maps were extracted from the combined PIV/OH PLIF data. The distribution of flame stretch shifted to greater values as the equivalence ratio decreased. Asymmetric fuel distributions, measured with acetone LW, were found to increase the equivalence ratio at blow off from that found with uniformly-fueled flows. This was attributed to the greater wake instability and extinction of the lean-side flames. The asymmetrically fueled flames were more susceptible to thermoacoustic instabilities when the shedding frequency was near an acoustic eigenmode of the exhaust duct, due to the decreased
Dempsey, Adam B.; Curran, Scott; Wagner, Robert M.; Cannella, William C.
2015-05-12
Gasoline compression ignition concepts with the majority of the fuel being introduced early in the cycle are known as partially premixed combustion (PPC). Previous research on single- and multi-cylinder engines has shown that PPC has the potential for high thermal efficiency with low NOx and soot emissions. A variety of fuel injection strategies has been proposed in the literature. These injection strategies aim to create a partially stratified charge to simultaneously reduce NOx and soot emissions while maintaining some level of control over the combustion process through the fuel delivery system. The impact of the direct injection strategy to create a premixed charge of fuel and air has not previously been explored, and its impact on engine efficiency and emissions is not well understood. This paper explores the effect of sweeping the direct injected pilot timing from -91° to -324° ATDC, which is just after the exhaust valve closes for the engine used in this study. During the sweep, the pilot injection consistently contained 65% of the total fuel (based on command duration ratio), and the main injection timing was adjusted slightly to maintain combustion phasing near top dead center. A modern four cylinder, 1.9 L diesel engine with a variable geometry turbocharger, high pressure common rail injection system, wide included angle injectors, and variable swirl actuation was used in this study. The pistons were modified to an open bowl configuration suitable for highly premixed combustion modes. The stock diesel injection system was unmodified, and the gasoline fuel was doped with a lubricity additive to protect the high pressure fuel pump and the injectors. The study was conducted at a fixed speed/load condition of 2000 rpm and 4.0 bar brake mean effective pressure (BMEP). The pilot injection timing sweep was conducted at different intake manifold pressures, swirl levels, and fuel injection GTP-15-1067, Dempsey 2 pressures. The gasoline used in this study has
Dempsey, Adam B.; Curran, Scott; Wagner, Robert M.; ...
2015-05-12
Gasoline compression ignition concepts with the majority of the fuel being introduced early in the cycle are known as partially premixed combustion (PPC). Previous research on single- and multi-cylinder engines has shown that PPC has the potential for high thermal efficiency with low NOx and soot emissions. A variety of fuel injection strategies has been proposed in the literature. These injection strategies aim to create a partially stratified charge to simultaneously reduce NOx and soot emissions while maintaining some level of control over the combustion process through the fuel delivery system. The impact of the direct injection strategy to createmore » a premixed charge of fuel and air has not previously been explored, and its impact on engine efficiency and emissions is not well understood. This paper explores the effect of sweeping the direct injected pilot timing from -91° to -324° ATDC, which is just after the exhaust valve closes for the engine used in this study. During the sweep, the pilot injection consistently contained 65% of the total fuel (based on command duration ratio), and the main injection timing was adjusted slightly to maintain combustion phasing near top dead center. A modern four cylinder, 1.9 L diesel engine with a variable geometry turbocharger, high pressure common rail injection system, wide included angle injectors, and variable swirl actuation was used in this study. The pistons were modified to an open bowl configuration suitable for highly premixed combustion modes. The stock diesel injection system was unmodified, and the gasoline fuel was doped with a lubricity additive to protect the high pressure fuel pump and the injectors. The study was conducted at a fixed speed/load condition of 2000 rpm and 4.0 bar brake mean effective pressure (BMEP). The pilot injection timing sweep was conducted at different intake manifold pressures, swirl levels, and fuel injection GTP-15-1067, Dempsey 2 pressures. The gasoline used in this study
NASA Astrophysics Data System (ADS)
Kwon, Jaesung; Huh, Kang Y.
2014-09-01
DNS is performed to analyse the effects of Lewis number (Le), density ratio and gravity in stagnating turbulent premixed flames. The results show good agreement with those of Lee and Huh (Combustion and Flame, Vol. 159, 2012, pp. 1576-1591) with respect to the turbulent burning velocity, ST, in terms of turbulent diffusivity, flamelet thickness, mean curvature and displacement speed at the leading edge. In all four stagnating flames studied, a mean tangential strain rate resulting in a mean flamelet thickness smaller than the unstretched laminar flame thickness leads to an increase in ST. A flame cusp of positive curvature involves a superadiabatic burned gas temperature due to diffusive-thermal instability for an Le less than unity. Wrinkling tends to be suppressed at a larger density ratio, not enhanced by hydrodynamic instability, in the stagnating flow configuration. Turbulence is produced, resulting in highly anisotropic turbulence with heavier unburned gas accelerating through a flame brush by Rayleigh-Taylor instability. Results are also provided on brush thickness, flame surface density and conditional velocities in burned and unburned gas and on flame surfaces to represent the internal brush structures for all four test flames.
NASA Technical Reports Server (NTRS)
Cooper, L. P.
1980-01-01
An experimental and analytical study of the combustion of partially vaporized fuel-air mixtures was performed to assess the impact of the degree of fuel vaporization upon emissions for a premixing-prevaporizing flametube combustor. Data collected in this study showed near linear increases in nitric oxide emissions with decreasing vaporization at equivalence ratios of 0.6. For equivalence ratios of 0.72, the degree of vaporization had very little impact on nitric oxide emissions. A simple mechanism which accounts for the combustion of liquid droplets in partially vaporized mixtures was found to agree with the measured results with fair accuracy with respect to both trends and magnitudes.
Computational analysis of some physical issues in nonpremixed and premixed turbulent flames
NASA Astrophysics Data System (ADS)
Steinberger, Craig J.
1997-07-01
Results are presented of direct numerical simulations (DNS) of a randomly perturbed spatially developing planar jet under the influence of a finite rate chemical reaction of the type F+O/to Product with initially nonpremixed reactants. The objectives of the simulations are to assess the compositional structure of the flame and to determine the influence of reaction exothermicity. It is shown that as the intensity of mixing is increased and the effect of finite rate chemistry is more pronounced, the magnitudes of the ensemble mean and root mean square of the product mass fraction decrease and those of the reactants mass fraction increase. At higher mixing rates the joint probability density functions of the reactants' mass fractions shift towards higher values within the composition domain indicating a lower reactedness. These trends are consistent with those observed experimentally and are useful in portraying the statistical structure of non-equilibrium diffusion flames. The DNS generated data are also utilized to examine the applicability of the 'laminar diffusion flamelet model' in predicting the rate of the reactant conversion with finite rate chemistry. This examination indicates that the performance of the model is improved as the value of the local Damkohler number is increased. In the setting of a 'turbulent' flame, the effect of the heat liberated by the chemical reaction is shown to increase the rate of reactant conversion. This finding is different from those of earlier DNS results and laboratory investigations. Lagrangian simulations are conducted of unsteady premixed flames in a spatially developing planar mixing layer. The flames are simulated via the FLAIR (Flux Line-Segment Model for Advection and Interface Reconstruction) algorithms combined with a vortex method flow solver. The objective of the simulations is to capture the structure of the flame front, ascertain the influences of exothermicity and baroclinic torque on the flame, and to assess the
2016-06-23
iodine and the simple structure of the I atom , the signal is very sensitive to the laser wavelength. Thus the measurement is sensitive to the stability of...new method based on photodissociation of iodine containing species, two-photon laser-induced fluorescence (LIF) of atomic iodine, and Rayleigh...therefore I element only exists as atomic I in the flow field. Because the PD is complete, the mass fraction of these foreign I atoms represents that of
NASA Astrophysics Data System (ADS)
Askari, Omid
This dissertation investigates the combustion and injection fundamental characteristics of different alternative fuels both experimentally and theoretically. The subjects such as lean partially premixed combustion of methane/hydrogen/air/diluent, methane high pressure direct-injection, thermal plasma formation, thermodynamic properties of hydrocarbon/air mixtures at high temperatures, laminar flames and flame morphology of synthetic gas (syngas) and Gas-to-Liquid (GTL) fuels were extensively studied in this work. These subjects will be summarized in three following paragraphs. The fundamentals of spray and partially premixed combustion characteristics of directly injected methane in a constant volume combustion chamber have been experimentally studied. The injected fuel jet generates turbulence in the vessel and forms a turbulent heterogeneous fuel-air mixture in the vessel, similar to that in a Compressed Natural Gas (CNG) Direct-Injection (DI) engines. The effect of different characteristics parameters such as spark delay time, stratification ratio, turbulence intensity, fuel injection pressure, chamber pressure, chamber temperature, Exhaust Gas recirculation (EGR) addition, hydrogen addition and equivalence ratio on flame propagation and emission concentrations were analyzed. As a part of this work and for the purpose of control and calibration of high pressure injector, spray development and characteristics including spray tip penetration, spray cone angle and overall equivalence ratio were evaluated under a wide range of fuel injection pressures of 30 to 90 atm and different chamber pressures of 1 to 5 atm. Thermodynamic properties of hydrocarbon/air plasma mixtures at ultra-high temperatures must be precisely calculated due to important influence on the flame kernel formation and propagation in combusting flows and spark discharge applications. A new algorithm based on the statistical thermodynamics was developed to calculate the ultra-high temperature plasma
Dec, John E.; Yang, Yi; Ji, Chunsheng; Dernotte, Jeremie
2015-04-14
Low-temperature gasoline combustion (LTGC), based on the compression ignition of a premixed or partially premixed dilute charge, can provide thermal efficiencies (TE) and maximum loads comparable to those of turbo-charged diesel engines, and ultra-low NOx and particulate emissions. Intake boosting is key to achieving high loads with dilute combustion, and it also enhances the fuel's autoignition reactivity, reducing the required intake heating or hot residuals. These effects have the advantages of increasing TE and charge density, allowing greater timing retard with good stability, and making the fuel Φ- sensitive so that partial fuel stratification (PFS) can be applied for higher loads and further TE improvements. However, at high boost the autoignition reactivity enhancement can become excessive, and substantial amounts of EGR are required to prevent overly advanced combustion. Accordingly, an experimental investigation has been conducted to determine how the tradeoff between the effects of intake boost varies with fuel-type and its impact on load range and TE. Five fuels are investigated: a conventional AKI=87 petroleum-based gasoline (E0), and blends of 10 and 20% ethanol with this gasoline to reduce its reactivity enhancement with boost (E10 and E20). Furthermore, a second zero-ethanol gasoline with AKI=93 (matching that of E20) was also investigated (CF-E0), and some neat ethanol data are also reported.
Dec, John E.; Yang, Yi; Ji, Chunsheng; ...
2015-04-14
Low-temperature gasoline combustion (LTGC), based on the compression ignition of a premixed or partially premixed dilute charge, can provide thermal efficiencies (TE) and maximum loads comparable to those of turbo-charged diesel engines, and ultra-low NOx and particulate emissions. Intake boosting is key to achieving high loads with dilute combustion, and it also enhances the fuel's autoignition reactivity, reducing the required intake heating or hot residuals. These effects have the advantages of increasing TE and charge density, allowing greater timing retard with good stability, and making the fuel Φ- sensitive so that partial fuel stratification (PFS) can be applied for highermore » loads and further TE improvements. However, at high boost the autoignition reactivity enhancement can become excessive, and substantial amounts of EGR are required to prevent overly advanced combustion. Accordingly, an experimental investigation has been conducted to determine how the tradeoff between the effects of intake boost varies with fuel-type and its impact on load range and TE. Five fuels are investigated: a conventional AKI=87 petroleum-based gasoline (E0), and blends of 10 and 20% ethanol with this gasoline to reduce its reactivity enhancement with boost (E10 and E20). Furthermore, a second zero-ethanol gasoline with AKI=93 (matching that of E20) was also investigated (CF-E0), and some neat ethanol data are also reported.« less
NASA Astrophysics Data System (ADS)
Hartung, G.; Hult, J.; Balachandran, R.; Mackley, M. R.; Kaminski, C. F.
2009-09-01
This paper describes the simultaneous application of time-sequenced laser-induced fluorescence imaging of OH radicals and stereoscopic particle image velocimetry for measurements of the flame front dynamics in lean and premixed LP turbulent flames. The studied flames could be acoustically driven, to simulate phenomena important in LP combustion technologies. In combination with novel image post processing techniques we show how the data obtained can be used to track the flame front contour in a plane defined by the illuminating laser sheets. We consider effects of chemistry and convective fluid motion on the dynamics of the observed displacements and analyse the influence of turbulence and acoustic forcing on the observed contour velocity, a quantity we term as s {/d 2D}. We show that this quantity is a valuable and sensitive indicator of flame turbulence interactions, as (a) it is measurable with existing experimental methodologies, and (b) because computational data, e.g. from large eddy simulations, can be post processed in an identical fashion. s {/d 2D} is related (to a two-dimensional projection) of the three-dimensional flame displacement speed s d , but artifacts due to out of plane convective motion of the flame surface and the uncertainty in the angle of the flame surface normal have to be carefully considered. Monte Carlo simulations were performed to estimate such effects for several distributions of flame front angle distributions, and it is shown conclusively that s {/d 2D} is a sensitive indicator of a quantity related to s d in the flames we study. s {/d 2D} was shown to increase linearly both with turbulent intensity and with the amplitude of acousting forcing for the range of conditions studied.
NASA Technical Reports Server (NTRS)
Nguyen, H. Lee; Wey, Ming-Jyh
1990-01-01
Two-dimensional calculations were made of spark ignited premixed-charge combustion and direct injection stratified-charge combustion in gasoline fueled piston engines. Results are obtained using kinetic-controlled combustion submodel governed by a four-step global chemical reaction or a hybrid laminar kinetics/mixing-controlled combustion submodel that accounts for laminar kinetics and turbulent mixing effects. The numerical solutions are obtained by using KIVA-2 computer code which uses a kinetic-controlled combustion submodel governed by a four-step global chemical reaction (i.e., it assumes that the mixing time is smaller than the chemistry). A hybrid laminar/mixing-controlled combustion submodel was implemented into KIVA-2. In this model, chemical species approach their thermodynamics equilibrium with a rate that is a combination of the turbulent-mixing time and the chemical-kinetics time. The combination is formed in such a way that the longer of the two times has more influence on the conversion rate and the energy release. An additional element of the model is that the laminar-flame kinetics strongly influence the early flame development following ignition.
NASA Astrophysics Data System (ADS)
Liu, Yu
2015-09-01
The spectral characteristics of combustion noise are dictated by the temporal correlation of the overall change of heat release rate fluctuations which has not received sufficient attention in prior studies. In this work, the two-time correlation of the volumetric heat release rate fluctuations within the flame brush and its role in modeling combustion noise spectrum are investigated by analyzing direct numerical simulation (DNS) data of turbulent premixed V-flames. This two-time correlation can be well represented by Gaussian-type functions and it captures the slow global variation of the fluctuating heat release rate and hence the low-frequency noise sources of unsteady combustion. The resulting correlation model is applied to predict the far-field noise spectrum from test open flames, and different reference time scales are used to scale this correlation from the DNS data to the test flames. The comparison between predictions and measurements indicates that the correlation models of all reference time scales are capable of reproducing the essential spectral shape including the low- and high-frequency dependencies. Reasonable agreement in the peak frequency, peak sound pressure level, and the Strouhal number scaling of peak frequency is also achieved for two turbulent time scales. A promising convective time scale shows great potential for characterizing the spectral features, yet its predictive capabilities are to be further verified through a longer DNS signal of a bounded flame configuration.
NASA Technical Reports Server (NTRS)
Nguyen, H. Lee; Wey, Ming-Jyh
1990-01-01
Two dimensional calculations were made of spark ignited premixed-charge combustion and direct injection stratified-charge combustion in gasoline fueled piston engines. Results are obtained using kinetic-controlled combustion submodel governed by a four-step global chemical reaction or a hybrid laminar kinetics/mixing-controlled combustion submodel that accounts for laminar kinetics and turbulent mixing effects. The numerical solutions are obtained by using KIVA-2 computer code which uses a kinetic-controlled combustion submodel governed by a four-step global chemical reaction (i.e., it assumes that the mixing time is smaller than the chemistry). A hybrid laminar/mixing-controlled combustion submodel was implemented into KIVA-2. In this model, chemical species approach their thermodynamics equilibrium with a rate that is a combination of the turbulent-mixing time and the chemical-kinetics time. The combination is formed in such a way that the longer of the two times has more influence on the conversion rate and the energy release. An additional element of the model is that the laminar-flame kinetics strongly influence the early flame development following ignition.
Premixed conical flame stabilization
NASA Astrophysics Data System (ADS)
Krikunova, A. I.; Son, E. E.; Saveliev, A. S.
2016-11-01
In the current work, stabilization of premixed laminar and lean turbulent flames for wide range of flow rates and equivalence ratios was performed. Methane-air mixture was ignited after passing through premixed chamber with beads and grids, and conical nozzle (Bunsen-type burner). On the edge of the nozzle a stabilized body-ring was mounted. Ring geometry was varied to get the widest stable flame parameters. This work was performed as part of the project on experimental investigation of premixed flames under microgravity conditions.
NASA Astrophysics Data System (ADS)
Keppeler, Roman; Pfitzner, Michael
2015-01-01
An algebraic model is derived that accounts for the effects of non-resolved Landau-Darrieus and thermo-diffusive instabilities on the propagation speed of fully premixed laminar and turbulent flame fronts in the Large Eddy Simulation (LES) context provided that the laminar flame speed appears as a model parameter in the LES combustion model. The model is derived assuming fractal characteristics of flames which exhibit cellular structures due to instabilities. The smallest and largest unstable wavelengths are computed employing a dispersion relation for nominally planar flames. Values for the fractal dimension characterising the flame structures are taken from the literature. A phenomenological model accounts for the stabilising effect of strain. Based on experimental data, a correlation for a critical strain rate, which indicates the onset of instabilities, is formulated. To validate the new model which accounts for instabilities on the effective speed of laminar flame propagation, laminar expanding spherical methane-air flames at p = 5 bar and p = 10 bar are simulated in the LES context. Values for the fractal dimension, as proposed in the literature, are varied. The predicted flame propagation speed is in very good agreement with experimental data when applying a fractal dimension of about D = 2.06. The critical strain turns out to be a suitable parameter to indicate the onset of instabilities and to quantify the influence of instabilities. Simulations applying a second model proposed by Bradley and valid for spherically expanding flames show similar results. LES of turbulent Bunsen flames at 1, 5 and 10 bar, which are characterised by u‧/s0L < 1, are performed to evaluate the derived instability model for turbulent flames. The simulated flames (from the Kobayashi database) have already been experimentally investigated in the context of Landau-Darrieus and thermo-diffusive instabilities. In agreement with conclusions from these investigations, for the
NASA Astrophysics Data System (ADS)
Arun, C. R.; Raghavan, Vasudevan
2012-09-01
Interactions of fuel-rich and fuel-lean mixtures and formation of interlinked multiple flame zones are observed in gas turbines and industrial furnaces. For fundamentally understanding such flames, numerical investigation of heat and mass transport, and chemical reaction processes, in laminar, counter flowing partially premixed rich and lean streams of methane and air mixtures, is presented. An axisymmetric numerical reactive flow model, with C2 detailed mechanism for describing methane oxidation in air and an optically thin radiation sub-model, is used in simulations. The numerical results are validated against the experimental results from literature. The equivalence ratios of counter flowing rich and lean reactant streams and the resulting strain rates have been varied. The effect of these parameters on the flame structure is presented. For a given rich and lean side equivalence ratios, by varying the strain rates, triple, double and single flame zones are obtained.
Assessing Model Assumptions for Turbulent Premixed Combustion at High Karlovitz Number
2015-09-03
1309. [26] P. Carroll , G. Blanquart, A proposed modification to Lundgren’s physical space velocity forcing method for isotropic turbulence, Phys...Combust. Flame 159 (2012) 317–335. [30] P. Carroll , G. Blanquart, The effect of velocity field forcing techniques on the Karman-Howarth equation, J...331–368. [43] S. K. Lele, Compressibility effects on turbulence, Ann. Rev. Fluid Mech. 26 (1994) 211 – 254. [44] P. L. Carroll , G. Blanquart, A
NASA Astrophysics Data System (ADS)
Kim, Jeonglae; Pope, Stephen B.
2014-05-01
A turbulent lean-premixed propane-air flame stabilised by a triangular cylinder as a flame-holder is simulated to assess the accuracy and computational efficiency of combined dimension reduction and tabulation of chemistry. The computational condition matches the Volvo rig experiments. For the reactive simulation, the Lagrangian Large-Eddy Simulation/Probability Density Function (LES/PDF) formulation is used. A novel two-way coupling approach between LES and PDF is applied to obtain resolved density to reduce its statistical fluctuations. Composition mixing is evaluated by the modified Interaction-by-Exchange with the Mean (IEM) model. A baseline case uses In Situ Adaptive Tabulation (ISAT) to calculate chemical reactions efficiently. Its results demonstrate good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. For dimension reduction, 11 and 16 represented species are chosen and a variant of Rate Controlled Constrained Equilibrium (RCCE) is applied in conjunction with ISAT to each case. All the quantities in the comparison are indistinguishable from the baseline results using ISAT only. The combined use of RCCE/ISAT reduces the computational time for chemical reaction by more than 50%. However, for the current turbulent premixed flame, chemical reaction takes only a minor portion of the overall computational cost, in contrast to non-premixed flame simulations using LES/PDF, presumably due to the restricted manifold of purely premixed flame in the composition space. Instead, composition mixing is the major contributor to cost reduction since the mean-drift term, which is computationally expensive, is computed for the reduced representation. Overall, a reduction of more than 15% in the computational cost is obtained.
Dan, Youquan; Zeng, Shuguang; Hao, Bangyuan; Zhang, Bin
2010-03-01
Two characteristic distances for partially coherent beams propagating in atmospheric turbulence have been proposed. The turbulent Rayleigh range is used for characterizing the range over which the beams propagate in turbulence without spreading appreciably; i.e., the concept of the well-known Rayleigh range in free space is extended to the case of turbulence. In this paper the range of turbulence-independent propagation of the beams, in contrast to similar characteristic distances in previous published works, is based on the formula of the beam propagation factor (M(2) factor) and is used for describing the range over which the spatial and angular spreading and the M(2) factor increase due to turbulence are sufficiently small and negligible. Several simple formulas used for calculating the approximate values of these distances are given, and the formulas are applied to Gaussian Schell-model (GSM) beams and illustrated by examples. Furthermore, as a typical example, the effect of the angular spread of GSM beams in turbulence on a thin-lens optical system is also discussed. We show that the turbulent Rayleigh range depends on the Rayleigh range in free space, the waist width, and the spatial power spectrum of the refractive-index fluctuations of the turbulent atmosphere, and that the range of turbulence-independent propagation depends on the waist width, the initial angular spread in the waist plane, and the spatial power spectrum.
Structure of turbulent non-premixed flames modeled with two-step chemistry
NASA Technical Reports Server (NTRS)
Chen, J. H.; Mahalingam, S.; Puri, I. K.; Vervisch, L.
1992-01-01
Direct numerical simulations of turbulent diffusion flames modeled with finite-rate, two-step chemistry, A + B yields I, A + I yields P, were carried out. A detailed analysis of the turbulent flame structure reveals the complex nature of the penetration of various reactive species across two reaction zones in mixture fraction space. Due to this two zone structure, these flames were found to be robust, resisting extinction over the parameter ranges investigated. As in single-step computations, mixture fraction dissipation rate and the mixture fraction were found to be statistically correlated. Simulations involving unequal molecular diffusivities suggest that the small scale mixing process and, hence, the turbulent flame structure is sensitive to the Schmidt number.
Kim, Munki; Choi, Youngil; Oh, Jeongseog; Yoon, Youngbin
2009-12-15
This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen non-premixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NO{sub x} emissions. Acoustic excitation causes the flame length to decrease by 15% and consequently, a 25% reduction in EINO{sub x} is achieved, compared to coaxial air flames without acoustic excitation at the same coaxial air to fuel velocity ratio. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NO{sub x} emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface. (author)
Chaparro, Andres; Landry, Eric; Cetegen, Baki M.
2006-04-15
The response of bluff-body stabilized conical V-shaped premixed flames to periodic upstream velocity oscillations was characterized as a function of oscillation frequency, mean flow velocity, and equivalence ratio. The flame heat release response to the imposed velocity oscillations was determined from the CH* chemiluminescence captured by two photomultiplier (PMT) detectors at a wavelength of 430 nm. One of the PMTs viewed flame radiation in a 10-mm horizontal slice, 50 mm above the bluff-body. The second PMT observed the overall flame radiation. The flame transfer function characteristics were determined from the spectral analysis of the velocity and PMT signals. It was found that the flame heat release amplitude response is confined to low-frequency excitation below a Strouhal number of 4. The phase relationship of the transfer function for these turbulent flames was evaluated using the signal from the spatially masked PMT. The transfer function estimate based on these data exhibits second-order characteristics with a phase lag between the velocity and heat release signals. The localized heat-release response contains frequencies that are multiples of the excitation frequency, suggesting splitting and tilting of flame structures as well as some nonlinear effects. Increase of flame equivalence ratio from lean toward stoichiometric resulted in slight amplification of the high-frequency response. (author)
Measurements of fuel mixture fraction oscillations of a turbulent jet non-premixed flame
Kanga, D.M.; Fernandez, V.; Culick, F.E.C.; Ratner, A.
2009-01-15
This work describes new type of combustion instability for which the 3-way coupling between mixing, flame heat release, and acoustics is modified by local buoyancy effects. Measurements of fuel mixture fraction are made for a non-premixed jet flame in a combustion chamber to assess the dynamics of mixing under imposed acoustic oscillations (22-55 Hz). Infrared laser absorption and phase resolved acetone-planar laser induced fluorescence are used to measure the fuel mixture fraction and then the degree of fuel/air mixing is calculated by determining the unmixedness. Results show acoustic excitation causes oscillations in the degree of fuel/air mixing at the driving frequency, which results in oscillatory flame behavior. This oscillatory flame behavior couples to the buoyancy and this in turn affects the mixing. Results also show that the mixing becomes less effective when the excitation frequency is increased or when the flame is present, compared to the non-reacting case. This work describes a key coupling mechanism that occurs when buoyancy is a significant factor in the flow field. (author)
NASA Astrophysics Data System (ADS)
Fries, Dan; Ochs, Bradley; Ranjan, Devesh; Menon, Suresh
2016-11-01
A new facility has been developed at the Georgia Institute of Technology to study sub- and supersonic combustion, which is based on classical flame bomb studies but incorporates a mean flow, allowing for a wider variety of turbulent conditions and the inclusion of effects like compressibility, while supporting shear-free spherical flames. Homogeneous, isotropic turbulence is generated via an active vane grid. Methane-air flame kernels advecting with the mean flow are generated using Laser Induced Breakdown ignition. The facility is accessing the thin reaction zone regime with uRMS' /SL0 = 6 . 9 - 22 , L11 /δF = 44 - 68 and Reλ = 190 - 550 . The flame kernels are probed with OH-Planar Laser Induced Fluorescence (PLIF). To validate the facility, results at Ū = 30 m/s are compared to existing data using a scaling derived from a spectral closure of the G-equation. This indicates the reacting flow remains Galilean invariant under the given conditions. The differences between global and local turbulent consumption speeds derived from OH-PLIF results are discussed with a focus on modeling efforts. The curvature of flame wrinkles is evaluated to examine the impact of different turbulent scales on flame development. This work was supported by the Air Force Office of Scientific Research under basic research Grant FA9550-15-1-0512 (Project monitor: Dr. Chiping Li).
Joint PDF Modelling of Local Extinction and Pollutant Formation in Non-premixed Turbulent Flames
NASA Astrophysics Data System (ADS)
Tang, Qing; Xu, Jun; Pope, Stephen B.
2000-11-01
A velocity-composition-turbulence frequency joint PDF approach is applied to model piloted methane/air turbulent diffusion flames investigated experimentally by Barlow and Frank. These flames exhibit an increasing amount of local extinction with increasing jet velocity, and are good cases to test the capabilities of turbulence-chemistry and combustion-chemistry models to account for local extinction and pollutant formation. In this study, the chemistry is an augmented reduced mechanism (19 species and 15 reaction steps) derived from the GRI2.11 detailed mechanism for methane oxidation by Sung and co-workers. The mechanism takes account of C2 chemistry, and the formation of oxides of nitrogen is treated by the inclusion of NO, NH3 and HCN. The turbulence models include the simplified Langevin model (SLM) for velocity, a stochastic model of Jayesh and Pope for turbulence frequency, the EMST model of Subramaniam and Pope for molecular mixing. The computational method for the solution of the modeled joint PDF equation features moving particles in a Lagrangian framework. The reaction calculations are performed via the in situ adaptive tabulation (ISAT) algorithm of Pope. The calculation results show good agreement with the experimental data, including the minor species NO and CO. The increase of local extinction (quantitatively characterized by a single variable - burning index) with increasing jet velocity is also accurately predicted by the calculations. It is founded that a small change of the inlet pilot temperature has a significant influence on the calculations and a systematic study has been made to investigate this sensitivity. For the flame with lowest velocity, the large influence is mainly observed close to the nozzle, while for the flame close to extinction, the calculated behavior is exquisitely sensitive to the pilot temperature, i.e., a 10K lower pilot temperature may cause global extinction.
Design of "model-friendly" turbulent non-premixed jet burners for C2+ hydrocarbon fuels.
Zhang, Jiayao; Shaddix, Christopher R; Schefer, Robert W
2011-07-01
Experimental measurements in laboratory-scale turbulent burners with well-controlled boundary and flow configurations can provide valuable data for validating models of turbulence-chemistry interactions applicable to the design and analysis of practical combustors. This paper reports on the design of two canonical nonpremixed turbulent jet burners for use with undiluted gaseous and liquid hydrocarbon fuels, respectively. Previous burners of this type have only been developed for fuels composed of H(2), CO, and/or methane, often with substantial dilution. While both new burners are composed of concentric tubes with annular pilot flames, the liquid-fuel burner has an additional fuel vaporization step and an electrically heated fuel vapor delivery system. The performance of these burners is demonstrated by interrogating four ethylene flames and one flame fueled by a simple JP-8 surrogate. Through visual observation, it is found that the visible flame lengths show good agreement with standard empirical correlations. Rayleigh line imaging demonstrates that the pilot flame provides a spatially homogeneous flow of hot products along the edge of the fuel jet. Planar imaging of OH laser-induced fluorescence reveals a lack of local flame extinction in the high-strain near-burner region for fuel jet Reynolds numbers (Re) less than 20,000, and increasingly common extinction events for higher jet velocities. Planar imaging of soot laser-induced incandescence shows that the soot layers in these flames are relatively thin and are entrained into vortical flow structures in fuel-rich regions inside of the flame sheet.
NASA Astrophysics Data System (ADS)
Lee, Jaeseo; Lee, Gwang G.; Huh, Kang Y.
2014-12-01
This paper presents validation of new analytical expressions for the turbulent burning velocity, ST, based on asymptotic behavior at the leading edge (LE) in turbulent premixed combustion. Reaction and density variation are assumed to be negligible at the LE to avoid the cold boundary difficulty in the statistically steady state. Good agreement is shown for the slopes, dST/du', with respect to Lc/δf at low turbulence, with both normalized by those of the reference cases. δf is the inverse of the maximum gradient of reaction progress variable through an unstretched laminar flame, and Lc is the characteristic length scale given as burner diameter or measured integral length scale. Comparison is made for thirty-five datasets involving different fuels, equivalence ratios, H2 fractions in fuel, pressures, and integral length scales from eight references [R. C. Aldredge et al., "Premixed-flame propagation in turbulent Taylor-Couette flow," Combust. Flame 115, 395 (1998); M. Lawes et al., "The turbulent burning velocity of iso-octane/air mixtures," Combust. Flame 159, 1949 (2012); H. Kido et al., "Influence of local flame displacement velocity on turbulent burning velocity," Proc. Combust. Inst. 29, 1855 (2002); J. Wang et al., "Correlation of turbulent burning velocity for syngas/air mixtures at high pressure up to 1.0 MPa," Exp. Therm. Fluid Sci. 50, 90 (2013); H. Kobayashi et al., "Experimental study on general correlation of turbulent burning velocity at high pressure," Proc. Combust. Inst. 27, 941 (1998); C. W. Chiu et al., "High-pressure hydrogen/carbon monoxide syngas turbulent burning velocities measured at constant turbulent Reynolds numbers," Int. J. Hydrogen Energy 37, 10935 (2012); P. Venkateswaran et al., "Pressure and fuel effects on turbulent consumption speeds of H2/CO blends," Proc. Combust. Inst. 34, 1527 (2013); M. Fairweather et al., "Turbulent burning rates of methane and methane-hydrogen mixtures," Combust. Flame 156, 780 (2009)]. The turbulent
Li, Zheming; Rosell, Joakim; Aldén, Marcus; Richter, Mattias
2016-11-18
Spatially and temporally resolved measurements are of great importance in turbulent premixed flame studies, especially when investigating rapid processes such as when flame local extinction, re-ignition, or flashback occur in a reacting flow. Here, an experimental approach for simultaneously probing two different species at high frame rates (50 kHz) is presented by employing a multi-YAG laser system. The laser radiation at 355 nm generated by a multi-YAG laser system was split into two beam paths: one beam for exciting formaldehyde and the other for pumping an optical parametric oscillator (OPO). To be able to capture the resulting fluorescence at such a high frame rate without significant loss in spatial resolution, two framing cameras, containing a total of 16 intensified charge-coupled devices (CCDs), were employed. In principle, the proposed setup provides the possibility of probing formaldehyde and simultaneously accessing the distribution of one other relevant species at this high frame rate. In this demonstration, the laser wavelength was tuned to 283 nm and, in conjunction with the 355 nm beam path, simultaneously high speed two-dimensional (2D) visualization of OH-radicals and formaldehyde was achieved. A modified flat flame, McKenna-type burner was used to provide a turbulent premixed jet-flame supported by a surrounding pilot flame. Local flame extinction and re-ignition processes were recorded for fuel/air jet speeds of 120 m/s.
NASA Astrophysics Data System (ADS)
El-Asrag, Hossam A.; Braun, Markus; Masri, Assaad R.
2016-07-01
The paper presents Large Eddy Simulations (LESs) for the Sydney ethanol piloted turbulent dilute spray flames ETF2, ETF6, and ETF7. The Flamelet Generated Manifold (FGM) approach is employed to predict mixing and burning of the evaporating fuel droplets. A methodology to match the experimental inflow spray profiles is presented. The spray statistical time-averaged results show reasonable agreement with mean and RMS data. The Particle Size Distribution (PSD) shows a good match downstream of the nozzle exit and up to x/D = 10. At x/D = 20 and 30 the PSD is under-predicted for droplets with mean diameter D10 > 20μm and over-predicted for the smaller size droplets. The simulations reasonably predict the reported mean flame structure and length. The effect of increasing the carrier velocity (ETF2-ETF7) or decreasing the liquid fuel injection mass flow rate (ETF2-ETF6) is found to result in a leaner, shorter flame and stronger spray-flow interactions. Higher tendency to local extinction is observed for ETF7 which is closer to blow-off compared to ETF2 and has higher scalar dissipation rates, higher range of Stokes number, and faster droplet response. The possible sources of LES-FGM deviations from the measurements are discussed and highlighted. In particular, the spray time-averaged statistical error contribution is quantified and the impact of the inflow uncertainty is studied. Sensitivity analysis to the pre-vaporized nozzle fuel mass fraction show that such small inflow perturbations (by ± 2% for the ETF2 flame) have a strong impact on the flame structure, and the droplets' dynamics. Conditional scatter plots show that the flame exhibits wide range of mixing conditions and bimodal mixing lines particularly at upstream locations (x/D < 20), where the injected droplets are still penetrating the centerline. This is relaxed further downstream as droplets gradually evaporate and burn in a diffusion like mode.
Two-Point Scalar Time-Series Measurements in Turbulent Partially Premixed Flames
2009-02-01
AUTHOR(S) Galen B. King , Normand M. Laurendeau, and Michael W. Renfro 5d. PROJECT NUMBER 2308 5e. TASK NUMBER BX 5f. WORK UNIT NUMBER 7. PERFORMING...Report Prepared by Galen B. King and Normand M. Laurendeau School of Mechanical Engineering Purdue University West Lafayette, IN 47907-1288...Professors Galen B. King , Normand M. Laurendeau, and Michael W. Renfro are co- principal investigators for this research program. Mr. Andrew Noble and
Partially turbulated trailing edge cooling passages for gas turbine nozzles
Thatcher, Jonathan Carl; Burdgick, Steven Sebastian
2001-01-01
A plurality of passages are spaced one from the other along the length of a trailing edge of a nozzle vane in a gas turbine. The passages lie in communication with a cavity in the vane for flowing cooling air from the cavity through the passages through the tip of the trailing edge into the hot gas path. Each passage is partially turbulated and includes ribs in an aft portion thereof to provide enhanced cooling effects adjacent the tip of the trailing edge. The major portions of the passages are smooth bore. By this arrangement, reduced temperature gradients across the trailing edge metal are provided. Additionally, the inlets to each of the passages have a restriction whereby a reduced magnitude of compressor bleed discharge air is utilized for trailing edge cooling purposes.
Turbulence-induced degradation properties of partially coherent flat-topped beams
NASA Astrophysics Data System (ADS)
Alavinejad, M.; Ghafary, B.
2008-05-01
Propagation of partially coherent flat-topped (PCFT) beams in a turbulent atmosphere is investigated and analytical formula for the average intensity is derived. Detailed analysis of PCFT beams through atmospheric turbulence with various correlation length indicated that PCFT beams with smaller correlation length are less affected by atmospheric turbulence. Analytical formula for beam width and power in bucket (PIB) of PCFT propagated through turbulence media are derived. The investigation showed that the beam width and PIB of PCFT with higher correlation length are more affected by turbulence.
Observations of turbulence in a partially stratified estuary
Stagey, M.T.; Monismith, Stephen G.; Burau, J.R.
1999-01-01
The authors present a field study of estuarine turbulence in which profiles of Reynolds stresses were directly measured using an ADCP throughout a 25-h tidal day. The dataset that is discussed quantifies turbulent mixing for a water column in northern San Francisco Bay that experiences a sequence of states that includes a weak ebb and flood that are stratified, followed by a strong, and eventually unstratified, ebb and flood. These measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification. Examination of individual Reynolds stress profiles along with profiles of Richardson number and turbulent Froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects. Using the measured turbulence production rate P, the dissipation rate e. is estimated. The observed turbulence had values of e/vN2 > 20 all of the time and e/vN2 > 200 most of the time, suggesting that the observed motions were buoyancy affected turbulence rather than internal waves. However, at times, turbulent Froude numbers in much of the upper-water column were less than one, indicating important stratification effects. Taken as a whole, the data show that stratification affects the turbulent velocity variance q2 most severely; that is, observed reductions in u'w' are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes. Finally, the dataset is compared to predictions made using the popular Mellor-Yamada level 2.5 closure. These comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions. The length scale does not appear to compensate for these errors, and, as a result, similar errors are seen in the eddy viscosity
Studies in premixed combustion. Progress report, November 1, 1990--October 31, 1992
Sivashinsky, G.I.
1992-08-01
This report discusses the following topics on premixed combustion: theory of turbulent flame propagation; pattern formation in premixed flames and related problems; and pattern formation in extended systems. (LSP)
Twist phase-induced reduction in scintillation of a partially coherent beam in turbulent atmosphere.
Wang, Fei; Cai, Yangjian; Eyyuboğlu, Halil T; Baykal, Yahya
2012-01-15
The scintillation index of a Gaussian Schell-model beam with twist phase (i.e., twisted GSM beam) in weak turbulent atmosphere is formulated with the help of a tensor method. Variations of the scintillation index of a twisted GSM beam on propagation in turbulent atmosphere are studied in detail. It is interesting to find that the scintillation index of a twisted GSM beam can be smaller than that without twist phase in weak turbulent atmosphere. Thus, modulation of the twist phase of a partially coherent beam provides a new way to reduce turbulence-induced scintillation.
Yang, Ailin; Zhang, Entao; Ji, Xiaoling; Lü, Baida
2008-06-09
The propagation of partially coherent Hermite-cosh-Gaussian (H-ChG)beams through atmospheric turbulence is studied in detail. The analytical expression for the angular spread of partially coherent H-ChG beams in turbulence is derived. It is shown that the angular spread of partially coherent H-ChG beams with smaller spatial correlation length sigma0, smaller waist width w0, smaller beam parameter Omega0, and larger beam orders m, n is less affected by turbulence than that of partially coherent H-ChG beams with larger sigma0, w0, Omega0, and smaller m, n. Under a certain condition partially coherent H-ChG beams may generate the same angular spread as a fully coherent Gaussian beam in free space and also in atmospheric turbulence. The angular spread of partially coherent Hermite-Gaussian (H-G), cosh-Gaussian (ChG), Gaussian Schell-model (GSM) beams, and fully coherent H-ChG, H-G, ChG, Gaussian beams is studied and treated as special cases of partially coherent H-ChG beams. The results are interpreted physically.
Propagation properties of partially coherent four-petal Gaussian vortex beams in oceanic turbulence
NASA Astrophysics Data System (ADS)
Liu, Dajun; Wang, Yaochuan; Wang, Guiqiu; Luo, Xixian; Yin, Hongming
2017-01-01
Based on the extended Huygens-Fresnel principle, the analytical expressions for partially coherent four-petal Gaussian vortex beams propagating in oceanic turbulence are obtained, and the influence of the coherence length, beam order N, topological charge M and oceanic turbulence parameters on the evolution properties of beams are discussed in detail using numerical examples. The results show that the beam will evolve into a Gauss-like beam rapidly with decreasing coherence length σ and oceanic parameters \\varsigma and \\varepsilon , or increasing oceanic parameter {χ\\text{T}} in the far field due to the influence of the coherence length and oceanic turbulence.
Mode analysis of spreading of partially coherent beams propagating through atmospheric turbulence.
Shirai, Tomohiro; Dogariu, Aristide; Wolf, Emil
2003-06-01
The spreading of partially coherent beams propagating through atmospheric turbulence is studied by use of the coherent-mode representation of the beams. Specifically, we consider partially coherent Gaussian Schell-model beams entering the atmosphere, and we examine the spreading of each coherent mode, represented by a Hermite-Gaussian function, on propagation. We find that in atmospheric turbulence the relative spreading of higher-order modes is smaller than that of lower-order modes, whereas the relative spreading of all order modes is the same as in free space. This modal behavior successfully explains why under certain circumstances partially coherent beams are less affected by atmospheric turbulence than are fully spatially coherent laser beams.
NASA Technical Reports Server (NTRS)
1996-01-01
Topics considered include: New approach to turbulence modeling; Second moment closure analysis of the backstep flow database; Prediction of the backflow and recovery regions in the backward facing step at various Reynolds numbers; Turbulent flame propagation in partially premixed flames; Ensemble averaged dynamic modeling. Also included a study of the turbulence structures of wall-bounded shear flows; Simulation and modeling of the elliptic streamline flow.
NASA Astrophysics Data System (ADS)
Liu, Dajun; Wang, Yaochuan; Yin, Hongming
2016-04-01
The partially coherent four-petal Gaussian vortex beam is introduced and described by analytical expressions. The analytical propagation equation for partially coherent four-petal Gaussian vortex beam in turbulent atmosphere is derived by using the extended Huygens-Fresnel diffraction integral formula. The influences of refraction index structure, beam order n, topological charge M and the coherence length on the average intensity distributions of beam are investigated by numerical examples.
Casanova, S.; Schlickeiser, R.
2012-02-01
Recently, a new transport theory of cosmic rays in magnetized space plasmas extending the quasilinear approximation to the particle orbit has been developed for the case of an axisymmetric incompressible magnetic turbulence. Here, we generalize the approach to the important physical case of a compressible plasma. As previously obtained in the case of an incompressible plasma, we allow arbitrary gyrophase deviations from the unperturbed spiral orbits in the uniform magnetic field. For the case of quasi-stationary and spatially homogeneous magnetic turbulence we derive, in the small Larmor radius approximation, gyrophase-averaged cosmic-ray Fokker-Planck coefficients. Upper limits for the perpendicular and pitch-angle Fokker-Planck coefficients and for the perpendicular and parallel spatial diffusion coefficients are presented.
NASA Astrophysics Data System (ADS)
Yan, Xiang; Zhang, Peng-Fei; Zhang, Jing-Hui; Qiao, Chun-Hong; Fan, Cheng-Yu
2016-08-01
Non-classical polarization properties of dark hollow beams propagating through non-Kolmogorov turbulence are studied. The analytic equation for the polarization degree of the quantization partially coherent dark hollow beams is obtained. It is found that the polarization fluctuations of the quantization partially coherent dark hollow beams are dependent on the turbulence factors and beam parameters with the detection photon numbers. Furthermore, an investigation of the changes in the on-axis propagation point and off-axis propagation point shows that the polarization degree of the quantization partially coherent dark hollow beams presents oscillation for a short propagation distance and gradually returns to zero for a sufficiently long distance. Project supported by the Major Research Plan of the National Natural Science Foundation of China (Grant No. 61405205).
Multiple tube premixing device
Uhm, Jong Ho; Varatharajan, Balachandar; Ziminsky, Willy Steve; Kraemer, Gilbert Otto; Yilmaz, Ertan; Lacy, Benjamin; Stevenson, Christian; Felling, David
2012-12-11
The present application provides a premixer for a combustor. The premixer may include a fuel plenum with a number of fuel tubes and a burner tube with a number of air tubes. The fuel tubes extend about the air tubes.
Multiple tube premixing device
Uhm, Jong Ho; Naidu, Balachandar; Ziminksy, Willy Steve; Kraemer, Gilbert Otto; Yilmaz, Ertan; Lacy, Benjamin; Stevenson, Christian; Felling, David
2013-08-13
The present application provides a premixer for a combustor. The premixer may include a fuel plenum with a number of fuel tubes and a burner tube with a number of air tubes. The fuel tubes extend about the air tubes.
Zhou, Guoquan; Cai, Yangjian; Chu, Xiuxiang
2012-04-23
The propagation of a partially coherent hollow vortex Gaussian beam through a paraxial ABCD optical system in turbulent atmosphere has been investigated. The analytical expressions for the average intensity and the degree of the polarization of a partially coherent hollow vortex Gaussian beam through a paraxial ABCD optical system are derived in turbulent atmosphere, respectively. The average intensity distribution and the degree of the polarization of a partially coherent hollow vortex Gaussian beam in turbulent atmosphere are numerically demonstrated. The influences of the beam parameters, the topological charge, the transverse coherent lengths, and the structure constant of the atmospheric turbulence on the propagation of a partially coherent hollow vortex Gaussian beam in turbulent atmosphere are also examined in detail. This research is beneficial to the practical applications in free-space optical communications and the remote sensing of the dark hollow beams.
Dou, Lingyu; Ji, Xiaoling; Li, Peiyun
2012-04-09
The model of partially coherent annular beams with linear non-uniformity field profile in the x direction is set up. The analytic expressions for the average intensity and the centre of gravity of partially coherent annular beams with decentered field propagating through atmospheric turbulence along a slant path are derived. The propagation equation governing the position of the intensity maximum is also given. It is found that the beam non-uniformity is amended gradually as the propagation distance and the strength of turbulence increase. The centre of beam gravity is independent of both the propagation distance and the turbulence. However, the position of the intensity maximum changes versus the propagation distance and the turbulence, and is farthest away from the propagation z-axis at a certain propagation distance. When the propagation distance is large enough, the position of the intensity maximum reaches an asymptotic value which increases with decreasing the zenith angle and is largest for the free space case. When the propagation distance is large enough, the position of the intensity maximum is not on the propagation z-axis, and is nearer to the propagation z-axis than the centre of beam gravity. On the other hand, changes in the intensity maximum in the far field are also examined in this paper.
Beam propagation factor of partially coherent flat-topped beams in a turbulent atmosphere.
Dan, Youquan; Zhang, Bin
2008-09-29
The Wigner distribution function (WDF) has been used to study the beam propagation factor (M(2)-factor) for partially coherent flat-topped (PCFT) beams with circular symmetry in a turbulent atmosphere. Based on the extended Huygens-Fresnel principle and the definition of the WDF, an expression for the WDF of PCFT beams in turbulence has been given. By use of the second-order moments of the WDF, the analytical formulas for the root-mean-square (rms) spatial width, the rms angular width, and the M(2)-factor of PCFT beams in turbulence have been derived, which can be applied to cases of different spatial power spectra of the refractive index fluctuations. The rms angular width and the M(2)-factor of PCFT beams in turbulence have been discussed with numerical examples. It can be shown that the M(2)-factor of PCFT beams in turbulence depends on the beam order, degree of global coherence of the source, waist width, wavelength, spatial power spectrum of the refractive index fluctuations, and propagation distance.
Chaos in an imperfectly premixed model combustor
Kabiraj, Lipika Saurabh, Aditya; Paschereit, Christian O.; Karimi, Nader; Sailor, Anna; Mastorakos, Epaminondas; Dowling, Ann P.
2015-02-15
This article reports nonlinear bifurcations observed in a laboratory scale, turbulent combustor operating under imperfectly premixed mode with global equivalence ratio as the control parameter. The results indicate that the dynamics of thermoacoustic instability correspond to quasi-periodic bifurcation to low-dimensional, deterministic chaos, a route that is common to a variety of dissipative nonlinear systems. The results support the recent identification of bifurcation scenarios in a laminar premixed flame combustor (Kabiraj et al., Chaos: Interdiscip. J. Nonlinear Sci. 22, 023129 (2012)) and extend the observation to a practically relevant combustor configuration.
Mao, Haidan; Zhao, Daomu
2010-01-18
Based on the intensity moments and Wigner distribution function, the second-order moments for broadband partially coherent flat-topped (BPCFT) beams in atmospheric turbulence are studied. The beam width of BPCFT beams in atmospheric turbulence is larger than that in free space. The beam width of BPCFT beams in atmospheric turbulence is larger than that of broadband fully coherent flat-topped (BFCFT) beams in atmospheric turbulence. The broader the bandwidth is, the larger the beam width of BPCFT beams becomes. Similar conclusion can be obtained by analyzing the divergence angle and beam propagation factor of BPCFT beams. The beam width of BPCFT beams in atmospheric turbulence is less affected by the broad spectral bandwidth than that in free space. The beam width of BFCFT beams in atmospheric turbulence is less affected by the broad spectral bandwidth than that of BPCFT beams in atmospheric turbulence.
Cheng, Mingjian; Guo, Lixin; Li, Jiangting; Huang, Qingqing; Cheng, Qi; Zhang, Dan
2016-06-10
The analytical formulas for the orbital angular momentum (OAM) mode probability density, signal OAM mode detection probability, and spiral spectrum of partially coherent Laguerre-Gaussian (LG) beams with optical vortices propagation in weak horizontal oceanic turbulent channels were developed, based on the Rytov approximation theory. The effect of oceanic turbulence and beam source parameters on the propagation behavior of the optical vortices carried by partially coherent LG beams was investigated in detail. Our results indicated that optical turbulence in an ocean environment produced a much stronger effect on the optical vortex than that in an atmosphere environment; the effective range of the signal OAM mode of LG beams with a smaller ratio of the mode crosstalk was limited to only several tens of meters in turbulent ocean. The existence of oceanic turbulence evidently induced OAM mode crosstalk and spiral spectrum spread. The effects of oceanic turbulence on the OAM mode detection probability increased with the increase of radial and azimuthal mode orders, oceanic turbulent equivalent temperature structure parameter, and temperature-salinity balance parameter. The spatial partial coherence of the beam source would enhance the effect of turbulent aberrations on the signal OAM mode detection probability, and fully coherent vortex beams provided better performance than partially coherent ones. Increasing wavelength of the vortex beams would help improve the performance of this quantum optical communication system. These results might be of interest for the potential application of optical vortices in practical underwater quantum optical communication among divers, submarines, and sensors in the ocean environment.
NASA Astrophysics Data System (ADS)
Ruan, Shaohong; Swaminathan, Nedunchezhian; Darbyshire, Oliver
2014-03-01
This study focuses on the modelling of turbulent lifted jet flames using flamelets and a presumed Probability Density Function (PDF) approach with interest in both flame lift-off height and flame brush structure. First, flamelet models used to capture contributions from premixed and non-premixed modes of the partially premixed combustion in the lifted jet flame are assessed using a Direct Numerical Simulation (DNS) data for a turbulent lifted hydrogen jet flame. The joint PDFs of mixture fraction Z and progress variable c, including their statistical correlation, are obtained using a copula method, which is also validated using the DNS data. The statistically independent PDFs are found to be generally inadequate to represent the joint PDFs from the DNS data. The effects of Z-c correlation and the contribution from the non-premixed combustion mode on the flame lift-off height are studied systematically by including one effect at a time in the simulations used for a posteriori validation. A simple model including the effects of chemical kinetics and scalar dissipation rate is suggested and used for non-premixed combustion contributions. The results clearly show that both Z-c correlation and non-premixed combustion effects are required in the premixed flamelets approach to get good agreement with the measured flame lift-off heights as a function of jet velocity. The flame brush structure reported in earlier experimental studies is also captured reasonably well for various axial positions. It seems that flame stabilisation is influenced by both premixed and non-premixed combustion modes, and their mutual influences.
NASA Astrophysics Data System (ADS)
Wang, Haiyan; Li, Xiangyin
2010-11-01
With the help of the tensor method, the analytical expression for the cross-spectral density of the radial partially coherent flat-topped array (RPCFTA) beams propagating in a turbulent atmosphere is derived, where the correlated superposition and uncorrelated superposition are considered. The average intensity, the spatial coherence properties and power in bucket (PIB) of these kinds of beams are investigated in detail. It is shown by numerical results and analysis that the average intensity and the spatial coherence of the correlated or uncorrelated RPCFTA beams will change on propagation and this change is dependent upon the correlation of the source's beamlets and atmospheric turbulence. In addition, the comparisons of the average intensity and the spatial coherence between the correlated and the uncorrelated RPCFTA beams propagating both in turbulent atmosphere and in free space are also given, and some interesting results are obtained. The laser power of focus ability of the single PCFT beam is worse than that of the correlated RPCFTA beam and but better than that of the uncorrelated RPCFTA beam when propagation distance in turbulent atmosphere is far-field plane.
Self-consistent Simulations of Plasma-Neutral in a Partially Ionized Astrophysical Turbulent Plasma
NASA Astrophysics Data System (ADS)
Shaikh, Dastgeer; Zank, G. P.
2010-03-01
A local turbulence model is developed to study energy cascades in the heliosheath and outer heliosphere (OH) based on self-consistent two-dimensional fluid simulations. The model describes a partially ionized magnetofluid OH that couples a neutral hydrogen fluid with a plasma primarily through charge-exchange interactions. Charge-exchange interactions are ubiquitous in warm heliospheric plasma, and the strength of the interaction depends largely on the relative speed between the plasma and the neutral fluid. Unlike small-length scale linear collisional dissipation in a single fluid, charge-exchange processes introduce channels that can be effective on a variety of length scales that depend on the neutral and plasma densities, temperature, relative velocities, charge-exchange cross section, and the characteristic length scales. We find, from scaling arguments and nonlinear coupled fluid simulations, that charge-exchange interactions modify spectral transfer associated with large-scale energy-containing eddies. Consequently, the turbulent cascade rate prolongs spectral transfer among inertial range turbulent modes. Turbulent spectra associated with the neutral and plasma fluids are therefore steeper than those predicted by Kolmogorov's phenomenology. Our work is important in the context of the global heliospheric interaction, the energization and transport of cosmic rays, gamma-ray bursts, interstellar density spectra, etc. Furthermore, the plasma-neutral coupling is crucial in understanding the energy dissipation mechanism in molecular clouds and star formation processes.
NASA Astrophysics Data System (ADS)
Wu, Yuqian; Zhang, Yixin; Wang, Qiu; Hu, Zhengda
2016-11-01
For Gaussian beams with three different partially coherent models, including Gaussian-Schell model (GSM), Laguerre-Gaussian Schell-model (LGSM) and Bessel-Gaussian Schell-model (BGSM) beams propagating through a biological turbulent tissue, the expression of the spatial coherence radius of a spherical wave propagating in a turbulent biological tissue, and the average intensity and beam spreading for GSM, LGSM and BGSM beams are derived based on the fractal model of power spectrum of refractive-index variations in biological tissue. Effects of partially coherent model and parameters of biological turbulence on such beams are studied in numerical simulations. Our results reveal that the spreading of GSM beams is smaller than LGSM and BGSM beams on the same conditions, and the beam with larger source coherence width has smaller beam spreading than that with smaller coherence width. The results are useful for any applications involved light beam propagation through tissues, especially the cases where the average intensity and spreading properties of the light should be taken into account to evaluate the system performance and investigations in the structures of biological tissue.
NASA Astrophysics Data System (ADS)
Zhang, Yongtao; Zhao, Zhiguo; Ding, Chaoliang; Pan, Liuzhan
2017-02-01
We have investigated the correlation singularities of a partially coherent radially polarized beam propagating through non-Kolmogorov turbulence. An analytical expression for the radius of a ring dislocation is derived. It is shown that the dependence of the radius of a ring dislocation on spatial coherence width in non-Kolmogorov turbulence is quite different from that in free space. The relation between spatial coherence width and beam width affects the change trends of the radius of a ring dislocation versus spatial coherence width. For different value ranges of the power law, the change of the radius of a ring dislocation with power law has an opposite trend. It is also found that the propagation distance plays an important role in determining the change of the radius of a ring dislocation. Our results will be useful in measuring the statistical properties of a random medium or a random field.
Turbulent Nonpremixed Flames (TNF): Experimental Data Archives and Computational Submodels
In the 1990s an international collaboration formed around a series of workshops that became known collectively as the International Workshop on Measurement and Computation of Turbulent Non-Premixed Flames (TNF). An online library, hosted by Sandia National Laboratory (California) was established that provides data sets and submodels or "mechanisms" for the study of turbulence-chemistry interactions in turbulent nonpremixed and partially premixed combustion. Data are organized by flame types: simple jet flames, piloted jet flames, bluff body flames, and swirl flames. These data sets provide a means for collaborative comparisons of both measured and simulated/modeled research results and also assist scientists in determining priorities for further research. More than 20 data sets or databases are available from this website, along with various downloadable files of chemical mechanisms. The website also provides an extensive bibliography and the proceedings of the workshops themselves from 1996 through 2012. Information continues to be added to this collection.
Premixed flame stabilization on a bluff body
Hertzberg, J.R.; Talbot, L.
1986-01-01
This paper studies the effects of fluid mechanics on combustion, the density and velocity fields of a turbulent premixed flame stabilized on a bluff-body flameholder observed by using Rayleigh scattering for single point measurements of density and laser Doppler velocimetry for velocity data. The stabilization region near the flameholder is the focus of this work. There are several motivations for a study of this nature. First, this configuration, in which a premixed flame is stabilized in the free shear layer of a separated wake behind a bluff body has implications for both mixing layer and basic flame anchoring questions, making this a fundamental problem. Second, since most premixed flames require some form of stabilization for laboratory study, understanding the interaction of the stabilization region and the propagating premixed flame is essential for the interpretation of any resultant data. Third, flame stabilization is of ongoing concern for ramjets, turbojet afterburners and other practical combustion systems. Finally, global models of flame stabilization are based on assumptions, such as the presence of stable recirculating vortices and high turbulence in the recirculation zone, which require verification.
NASA Astrophysics Data System (ADS)
Luo, Dahai; Yan, Chao; Wang, Xiaoyong
2015-02-01
Separation commonly exists in the flows around flight vehicles and also in the internal combustor flows. Simulation of high-speed turbulent-separated flows using a reliable computational design tool is crucial for the development of supersonic and hypersonic vehicles. In this paper, we present the computational results of supersonic base and ramped-cavity flows at high Reynolds numbers using the partially averaged Navier-Stokes (PANS) method. The current PANS models are based on the Menter SST turbulence model and also the Wilcox k-ω model. Results from PANS simulations are compared in detail with the available experimental data. The effect of the resolution control parameter fk (the ratio of unresolved-to-total kinetic energy) relevant to the PANS method is investigated. More turbulent flow structures are resolved as expected with decreasing fk, but it does not mean better results can be obtained. Spatially varying and dynamically updated fk in PANS simulations has been performed. Results from variable fk PANS simulations show good agreement with the experiment and great improvement when compared to Reynolds averaged Navier-Stokes (RANS) computation and constant fk PANS simulations.
Modeling of turbulent chemical reaction
NASA Technical Reports Server (NTRS)
Chen, J.-Y.
1995-01-01
Viewgraphs are presented on modeling turbulent reacting flows, regimes of turbulent combustion, regimes of premixed and regimes of non-premixed turbulent combustion, chemical closure models, flamelet model, conditional moment closure (CMC), NO(x) emissions from turbulent H2 jet flames, probability density function (PDF), departures from chemical equilibrium, mixing models for PDF methods, comparison of predicted and measured H2O mass fractions in turbulent nonpremixed jet flames, experimental evidence of preferential diffusion in turbulent jet flames, and computation of turbulent reacting flows.
Gershgorin, B.; Majda, A.J.
2011-02-20
A statistically exactly solvable model for passive tracers is introduced as a test model for the authors' Nonlinear Extended Kalman Filter (NEKF) as well as other filtering algorithms. The model involves a Gaussian velocity field and a passive tracer governed by the advection-diffusion equation with an imposed mean gradient. The model has direct relevance to engineering problems such as the spread of pollutants in the air or contaminants in the water as well as climate change problems concerning the transport of greenhouse gases such as carbon dioxide with strongly intermittent probability distributions consistent with the actual observations of the atmosphere. One of the attractive properties of the model is the existence of the exact statistical solution. In particular, this unique feature of the model provides an opportunity to design and test fast and efficient algorithms for real-time data assimilation based on rigorous mathematical theory for a turbulence model problem with many active spatiotemporal scales. Here, we extensively study the performance of the NEKF which uses the exact first and second order nonlinear statistics without any approximations due to linearization. The role of partial and sparse observations, the frequency of observations and the observation noise strength in recovering the true signal, its spectrum, and fat tail probability distribution are the central issues discussed here. The results of our study provide useful guidelines for filtering realistic turbulent systems with passive tracers through partial observations.
Huang, Yongping; Wang, Fanhou; Gao, Zenghui; Zhang, Bin
2015-01-26
Propagation properties of partially coherent electromagnetic hyperbolic-sine-Gaussian (PCESHG) vortex beams through non-Kolmogorov atmospheric turbulence, including the spectral degree of polarization and evolution behavior of coherent vortices and average intensity are investigated in detail by using the extended Huygens-Fresnel principle and the spatial power spectrum of the refractive index of non-Kolmogorov turbulence. It is shown that the motion, creation and annihilation of the coherent vortices of PCESHG vortex beams in non-Kolmogorov turbulence may appear with the increasing propagation distance, and the distance for the conservation of the topological charge depends on the turbulence parameters and beam parameters. In additions, the evolution behavior of coherent vortices, average intensity and spectral degree of polarization vary significantly for different values of the generalized exponent parameter and the generalized refractive-index structure parameter of non-Kolmogorov turbulence, and the beam parameters as well as the propagation distance.
NASA Technical Reports Server (NTRS)
Chen, J. H.; Mahalingam, S.; Puri, I. K.; Vervisch, L.
1992-01-01
The interaction between a quasi-laminar flame and a turbulent flowfield is investigated through direct numerical simulations (DNS) of reacting flow in two- and three-dimensional domains. Effects due to finite-rate chemistry are studied using a single step global reaction A (fuel) + B (oxidizer) yields P (product), and by varying a global Damkoehler number, as a result of which the turbulence-chemistry interaction in the flame is found to generate a wide variety of conditions, ranging from near-equilibrium to near-extinction. Differential diffusion effects are studied by changing the Schmidt number of one reactive species to one-half. It is observed that laminar flamelet response is followed within the turbulent flowfield, except in regions where transient effects seem to dominate.
Turbulent Dynamo in a Conducting Fluid and a Partially Ionized Gas
NASA Astrophysics Data System (ADS)
Xu, Siyao; Lazarian, A.
2016-12-01
By following the Kazantsev theory and taking into account both microscopic and turbulent diffusion of magnetic fields, we develop a unified treatment of the kinematic and nonlinear stages of a turbulent dynamo process, and we study the dynamo process for a full range of magnetic Prandtl number P m and ionization fractions. We find a striking similarity between the dependence of dynamo behavior on P m in a conducting fluid and { R } (a function of ionization fraction) in a partially ionized gas. In a weakly ionized medium, the kinematic stage is largely extended, including not only exponential growth but a new regime of dynamo characterized by a linear-in-time growth of magnetic field strength, and the resulting magnetic energy is much higher than the kinetic energy carried by viscous-scale eddies. Unlike the kinematic stage, the subsequent nonlinear stage is unaffected by microscopic diffusion processes and has a universal linear-in-time growth of magnetic energy with the growth rate as a constant fraction 3/38 of the turbulent energy transfer rate, showing good agreement with earlier numerical results. Applying the analysis to the first stars and galaxies, we find that the kinematic stage is able to generate a field strength only an order of magnitude smaller than the final saturation value. But the generation of large-scale magnetic fields can only be accounted for by the relatively inefficient nonlinear stage and requires longer time than the free-fall time. It suggests that magnetic fields may not have played a dynamically important role during the formation of the first stars.
Numerical investigation of turbulent bubbly wakes created by the ventilated partial cavity
NASA Astrophysics Data System (ADS)
Xiang, Min; Zhang, WeiHua; Cheung, S. C. P.; Tu, JiYuan
2012-02-01
This paper presents a numerical study on the turbulent bubbly wakes created by the ventilated partial cavity. A semi-empirical approach is introduced to model the discrete interface of the ventilated cavity and its complex gas leakage rate induced by the local turbulent shear stress. Based on the Eulerian-Eulerian two-fluid modeling framework, a population balance approach based on MUltiple-SIze-Group (MUSIG) model is incorporated to simulate the size evolution of the sheared off microbubbles and its complex interactions with the two-phase flow structure in the wake region. Numerical predictions at various axial locations downstream of the test body were in satisfactory agreement with the experimental measurements. The captured bubbly wake structure illustrates that the bubbles may disperse as a twin-vortex tube driven by gravity effect. The predicted Sauter mean bubble diameter has confirmed the dominance of the coleascense process in the axial direction. As the bubbles develop downstream, the coleascense and breakup rate gradually reach balance, resulting in the stable bubble diameter. A close examination of the flow structures, gas void fraction distributions and the bubble size evolution provides valuable insights into the complex physical phenomenon induced by ventilated cavity.
Partially-Averaged Navier Stokes Model for Turbulence: Implementation and Validation
NASA Technical Reports Server (NTRS)
Girimaji, Sharath S.; Abdol-Hamid, Khaled S.
2005-01-01
Partially-averaged Navier Stokes (PANS) is a suite of turbulence closure models of various modeled-to-resolved scale ratios ranging from Reynolds-averaged Navier Stokes (RANS) to Navier-Stokes (direct numerical simulations). The objective of PANS, like hybrid models, is to resolve large scale structures at reasonable computational expense. The modeled-to-resolved scale ratio or the level of physical resolution in PANS is quantified by two parameters: the unresolved-to-total ratios of kinetic energy (f(sub k)) and dissipation (f(sub epsilon)). The unresolved-scale stress is modeled with the Boussinesq approximation and modeled transport equations are solved for the unresolved kinetic energy and dissipation. In this paper, we first present a brief discussion of the PANS philosophy followed by a description of the implementation procedure and finally perform preliminary evaluation in benchmark problems.
Wu, Yuqian; Zhang, Yixin; Zhu, Yun
2016-08-01
We studied Gaussian beams with three different partially coherent models, including the Gaussian-Schell model (GSM), Laguerre-Gaussian Schell model (LGSM), and Bessel-Gaussian Schell model (BGSM), propagating through oceanic turbulence. The expressions of average intensity, beam spreading, and beam wander for GSM, LGSM, and BGSM beams in the paraxial channel are derived. We make a contrast for the three models in numerical simulations and find that the GSM beam has smaller spreading than the others, and the LGSM beam needs longer propagation distance to transform into a well-like profile of average intensity than the BGSM beam in the same conditions. The salinity fluctuation has a greater contribution to the wander of LGSM and BGSM beams than that of the temperature fluctuation. Our results can be helpful in the design of an optical wireless communication link operating in oceanic environment.
Kraemer, Gilbert Otto; Varatharajan, Balachandar; Evulet, Andrei Tristan; Yilmaz, Ertan; Lacy, Benjamin Paul
2013-12-31
Methods and systems are provided for premixing combustion fuel and air within gas turbines. In one embodiment, a combustor includes an upstream mixing panel configured to direct compressed air and combustion fuel through premixing zone to form a fuel-air mixture. The combustor includes a downstream mixing panel configured to mix additional combustion fuel with the fule-air mixture to form a combustion mixture.
NASA Astrophysics Data System (ADS)
Kuttieri, R. A.; Sinha, M.
2012-07-01
An approach based on neural partial differentiation is suggested for aircraft parameter estimation using the flight data gathered under turbulent atmospheric conditions. The classical methods such as output error and equation error methods suffer from severe convergence issues; resulting in biased, inaccurate, and inconsistent estimates. Though filter error method yields better estimates while dealing with the flight data having process noise, it has few demerits like computational overheads and it allows estimation of a single set of process noise distribution matrix. The proposed neural method does not face any such problem of the classical methods. Moreover, the neural method does not require parameter initialization and a priori knowledge of the model structure. The neural network maps the aircraft state and control variables into the output variables corresponding to aerodynamic forces and moments. The parameter estimation, pertaining to lateral-directional motion, of the research aircraft de Havilland DHC-2 with simulated process noise, is presented. The results obtained using the neural partial differentiation are compared with the nominal values given in literature and with the classical methods. The neural method yields the aerodynamic derivatives very close to the nominal values and having quite low standard deviation. The neural methodology is also validated by comparing actual output variables with the neural predicted and neural reconstructed variables.
Multiscale Interactions and Backscatter in Premixed Combustion
NASA Astrophysics Data System (ADS)
Hamlington, Peter; Towery, Colin; O'Brien, Jeffrey; Poludnenko, Alexei; Urzay, Javier; Ihme, Matthias
2015-11-01
Multiscale interactions and energy transfer between turbulence and flames are fundamental to understanding and modeling premixed turbulent reacting flows. To investigate such flows, direct numerical simulations of statistically planar turbulent premixed flames have been performed, and the dynamics of kinetic energy transfer are examined in both spectral and physical spaces. In the spectral analysis, two-dimensional kinetic energy spectra and triadic interactions are computed through the flame brush. It is found that there is suppression of turbulent small-scale motions in the combustion products, along with backscatter of energy for a range of scales near the thermal laminar flame width. In the physical-space analysis, a differential filter is applied to examine the transfer of kinetic energy between subgrid and resolved scales in the context of large eddy simulations. Subgrid-scale backscatter of kinetic energy driven by combustion is found to prevail over forward scatter throughout the flame brush. The spectral- and physical-space analyses thus both suggest an enhancement of reverse-cascade phenomena in the flame brush, which is possibly driven by accumulation of kinetic energy in the scales where combustion-induced heat release is preferentially deployed.
Qian, Xianmei; Zhu, Wenyue; Rao, Ruizhong
2009-03-02
The propagation effects of spatially pseudo-partially coherent Gaussian Schell-model beams in atmosphere are investigated numerically. The characteristics of beam spreading, beam wandering and intensity scintillation are analyzed respectively. It is found that the degradation of degree of source coherence may cause reductions of relative beam spreading and scintillation index, which indicates that partially coherent beams are more resistant to atmospheric turbulence than fully coherent beams. And beam wandering is not much sensitive to the change of source coherence. However, a partially coherent beam have a larger spreading than the fully coherent beam both in free space and in atmospheric turbulence. The influences of changing frequency of random phase screen which models the source coherence on the final intensity pattern are also discussed.
NASA Astrophysics Data System (ADS)
Zhang, En-Tao; Ji, Xiao-Ling; Lü, Bai-Da
2009-02-01
The propagation properties of the off-axis superposition of partially coherent beams through atmospheric turbulence and their beam quality in terms of the mean-squared beam width w(z) and the power in the bucket (PIB) are studied in detail, where the effects of partial coherence, off-axis beam superposition and atmospheric turbulence are considered. The analytical expressions for the intensity, the beam width and the PIB are derived, and illustrative examples are given numerically. It is shown that the maximum intensity Imax and the PIB decrease and w(z) increases as the refraction index structure constant Cn2 increases. Therefore, the turbulence results in a degradation of the beam quality. However, the resulting partially coherent beam with a smaller value of spatial correlation parameter γ and larger values of separate distance xd and beam number M is less affected by the turbulence than that with a larger value of γ and smaller values of xd and M. The main results obtained in this paper are explained physically.
Impact of multi-component diffusion in turbulent combustion using direct numerical simulations
Bruno, Claudio; Sankaran, Vaidyanathan; Kolla, Hemanth; ...
2015-08-28
This study presents the results of DNS of a partially premixed turbulent syngas/air flame at atmospheric pressure. The objective was to assess the importance and possible effects of molecular transport on flame behavior and structure. To this purpose DNS were performed at with two proprietary DNS codes and with three different molecular diffusion transport models: fully multi-component, mixture averaged, and imposing the Lewis number of all species to be unity.
Wu, Zhen-Sen; Li, Ya-Qing
2011-07-01
On the basis of the extended Huygens-Fresnel principle, the scattering of partially coherent Gaussian-Schell-model (GSM) beams from a diffuse target in slant double-passage atmospheric turbulence is studied and compared with that of fully coherent Gaussian beams. Using the cross-spectral density function of the GSM beams, we derive the expressions of the mutual coherence function, angle-of-arrival fluctuation, and covariance and variance of the intensity of the scattered field, taking into account the fluctuations of both the log-amplitude and phase. The numerical results are presented, and the influences of the wavelength, propagation distance, and waist radius on scattering properties are discussed. The perturbation region of the normalized intensity variance of the partially coherent GSM beam is smaller than that of the fully coherent Gaussian beam at the middle turbulence level. The normalized intensity variance of long-distance beam propagation is smaller than that of beam propagation along a short distance.
Premixing quality and flame stability: A theoretical and experimental study
NASA Technical Reports Server (NTRS)
Radhakrishnan, K.; Heywood, J. B.; Tabaczynski, R. J.
1979-01-01
Models for predicting flame ignition and blowout in a combustor primary zone are presented. A correlation for the blowoff velocity of premixed turbulent flames is developed using the basic quantities of turbulent flow, and the laminar flame speed. A statistical model employing a Monte Carlo calculation procedure is developed to account for nonuniformities in a combustor primary zone. An overall kinetic rate equation is used to describe the fuel oxidation process. The model is used to predict the lean ignition and blow out limits of premixed turbulent flames; the effects of mixture nonuniformity on the lean ignition limit are explored using an assumed distribution of fuel-air ratios. Data on the effects of variations in inlet temperature, reference velocity and mixture uniformity on the lean ignition and blowout limits of gaseous propane-air flames are presented.
Spectral Kinetic Energy Transfer Through a Premixed Flame Brush
NASA Astrophysics Data System (ADS)
Towery, Colin A. Z.; Poludnenko, Alexei Y.; Hamlington, Peter E.
2014-11-01
Turbulence-flame interactions are of fundamental importance for understanding and modeling premixed turbulent reacting flows. These interactions can result in nonlinear feedback leading to large changes in both the turbulence and flame. Recent computational studies have indicated, however, that not all scales of turbulent motion are affected equally. Small-scale motions appear to be suppressed while larger-scale motions are unaffected or even enhanced. In order to determine the scale-dependence of turbulence-flame interactions, direct numerical simulations of statistically planar, premixed flames have been performed and analyzed. Two-dimensional kinetic energy spectra, conditioned on the planar-averaged fuel mass-fraction, are measured through the flame brush and compared to both compressible and incompressible non-reacting flow spectra. Changes in the spectra with respect to fuel mass-fraction are then connected to the dynamics of the kinetic energy spectrum transport equation. Particular focus is placed on understanding triadic velocity, pressure, and dilatation interactions, including the characterization of backscatter due to heat release and compressibility. Finally, the implications of these results for modeling practical premixed combustion problems are outlined.
Lean premixed flames for low NO{sub x} combustors
Sojka, P.; Tseng, L.; Bryjak, J.
1995-10-01
Gas turbines are being used throughout the world to generate electricity. Due to increasing fuel costs and environmental concerns, gas turbines must meet stringent performance requirements, demonstrating high thermal efficiencies and low pollutant emissions. In order for U.S. manufactured gas turbines to stay competitive, their NO{sub x} levels must be below 10 ppm and their thermal efficiencies should approach 60%. Current technology is being stretched to achieve these goals. The twin goals of high efficiency and low NO{sub x} emissions require extending the operating range of current gas turbines. Higher efficiency requires operation at higher pressures and temperatures. Lower NO{sub x} emissions requires lower flame temperatures. Lower flame temperatures can be achieved through partially to fully pre-mixed combustion. However, increased performance and lower emissions result in a set of competing goals. In order to achieve a successful compromise between high efficiency and low NO{sub x} emissions, advanced design tools must be developed. One key design tool is a computationally efficient, high pressure, turbulent flow, combustion model capable of predicting pollutant formation in an actual gas turbine. Its development is the goal of this program. Achieving this goal requires completion of three tasks. The first task is to develop a reduced chemical kinetics model describing N{sub O}x formation in natural gas-air systems. The second task is to develop a computationally efficient model that describes turbulence-chemistry interactions. The third task is to incorporate the reduced chemical kinetics and turbulence-chemistry interaction models into a commercially available flow solver and compare its predictions with experimental data obtained under carefully controlled conditions so that the accuracy of model predictions can be evaluated.
NASA Technical Reports Server (NTRS)
Sohrab, Siavash H.
1999-01-01
Counterflow premixed flames play a significant role in the modeling of laminar flames. This is in part motivated by the fact that stretched premixed flames simulate local flamelet dynamics within turbulent premixed flames. In the present study, the modified form of the Navier-Stokes equation for reactive fields introduced earlier is employed to investigate the hydrodynamics of spherical flows embedded within counterflows. The geometry of premixed flames near the stagnation point is also determined. The predictions are in favorable agreement with the experimental observations and prior numerical studies.
NASA Technical Reports Server (NTRS)
Cho, John Y. N.; Swartz, Wesley E.; Kelley, Michael C.; Miller, Clark A.
1993-01-01
During the first rocket sequence (called Salvo B) of the NLC-91 campaign, the Cornell University Portable Radar Interferometer (CUPRI) observed two simultaneously occurring layers of Polar Mesophere Summer Echoes (PMSE). during the time of the Turbo B flight, the high time-resolution CUPRI Doppler spectra exhibited sawtooth-like discontinuities in the lower layer which we interpret to be a distorted partial reflection layer which was advected across the radar beam. The upper layer, on the other hand, appeared to be caused by turbulent scatter and we estimate the turbulence energy dissipation rate in the upper layer at the time of the Turbo B flight to have been approximately 0.04 W/kg. Futhermore, a shift in the antenna beam direction from vertical to 8 deg off zenith revealed an aspect sensitivity of approximately 5 dB in the lower layer but none in the upper layer. We conclude that, at this particular time, turbulent scatter was responsible for the upper layer while some form of partial reflection was dominant in the lower layer.
NASA Astrophysics Data System (ADS)
Cho, John Y. N.; Swartz, Wesley E.; Kelley, Michael C.; Miller, Clark A.
1993-10-01
During the first rocket sequence (called Salvo B) of the NLC-91 campaign, the Cornell University Portable Radar Interferometer (CUPRI) observed two simultaneously occurring layers of Polar Mesophere Summer Echoes (PMSE). during the time of the Turbo B flight, the high time-resolution CUPRI Doppler spectra exhibited sawtooth-like discontinuities in the lower layer which we interpret to be a distorted partial reflection layer which was advected across the radar beam. The upper layer, on the other hand, appeared to be caused by turbulent scatter and we estimate the turbulence energy dissipation rate in the upper layer at the time of the Turbo B flight to have been approximately 0.04 W/kg. Futhermore, a shift in the antenna beam direction from vertical to 8 deg off zenith revealed an aspect sensitivity of approximately 5 dB in the lower layer but none in the upper layer. We conclude that, at this particular time, turbulent scatter was responsible for the upper layer while some form of partial reflection was dominant in the lower layer.
NASA Astrophysics Data System (ADS)
Frisch, Uriel
1996-01-01
Written five centuries after the first studies of Leonardo da Vinci and half a century after A.N. Kolmogorov's first attempt to predict the properties of flow, this textbook presents a modern account of turbulence, one of the greatest challenges in physics. "Fully developed turbulence" is ubiquitous in both cosmic and natural environments, in engineering applications and in everyday life. Elementary presentations of dynamical systems ideas, probabilistic methods (including the theory of large deviations) and fractal geometry make this a self-contained textbook. This is the first book on turbulence to use modern ideas from chaos and symmetry breaking. The book will appeal to first-year graduate students in mathematics, physics, astrophysics, geosciences and engineering, as well as professional scientists and engineers.
Vorticity isotropy in high Karlovitz number premixed flames
NASA Astrophysics Data System (ADS)
Bobbitt, Brock; Blanquart, Guillaume
2016-10-01
The isotropy of the smallest turbulent scales is investigated in premixed turbulent combustion by analyzing the vorticity vector in a series of high Karlovitz number premixed flame direct numerical simulations. It is found that increasing the Karlovitz number and the ratio of the integral length scale to the flame thickness both reduce the level of anisotropy. By analyzing the vorticity transport equation, it is determined that the vortex stretching term is primarily responsible for the development of any anisotropy. The local dynamics of the vortex stretching term and vorticity resemble that of homogeneous isotropic turbulence to a greater extent at higher Karlovitz numbers. This results in small scale isotropy at sufficiently high Karlovitz numbers and supports a fundamental similarity of the behavior of the smallest turbulent scales throughout the flame and in homogeneous isotropic turbulence. At lower Karlovitz numbers, the vortex stretching term and the vorticity alignment in the strain-rate tensor eigenframe are altered by the flame. The integral length scale has minimal impact on these local dynamics but promotes the effects of the flame to be equal in all directions. The resulting isotropy in vorticity does not reflect a fundamental similarity between the smallest turbulent scales in the flame and in homogeneous isotropic turbulence.
Beam wander of partially coherent array beams through non-Kolmogorov turbulence.
Huang, Yongping; Zeng, Anping; Gao, Zenghui; Zhang, Bin
2015-04-15
Based on the theory of second moments and non-Kolmogorov spectrum, the beam wander theory is extend to non-Kolmogorov turbulence, the general analytical expression of beam wander in non-Kolmogorov turbulence is derived. Beam wander depends on the non-Kolmogorov turbulence parameters and the initial second moments of the laser beam at the input plane. Taking the Gaussian Schell model array beams as an example, the effects of the generalized exponent parameter, inner scale, and outer scale of non-Kolmogorov turbulence and the beam separation distance, beam number, and coherence degree on the beam wander are studied in detail. It has been shown that the beam wander varies non-monotonically with increasing generalized exponent parameter of the turbulence. Furthermore, it increases as the inner scale decreases or outer scale increases, and decreases as the beam separation distance and beam number increase and the coherence of the beam becomes weaker. Our results also indicate that the beam wander could be reduced by adjusting the beam parameters appropriately.
Premixed direct injection disk
York, William David; Ziminsky, Willy Steve; Johnson, Thomas Edward; Lacy, Benjamin; Zuo, Baifang; Uhm, Jong Ho
2013-04-23
A fuel/air mixing disk for use in a fuel/air mixing combustor assembly is provided. The disk includes a first face, a second face, and at least one fuel plenum disposed therebetween. A plurality of fuel/air mixing tubes extend through the pre-mixing disk, each mixing tube including an outer tube wall extending axially along a tube axis and in fluid communication with the at least one fuel plenum. At least a portion of the plurality of fuel/air mixing tubes further includes at least one fuel injection hole have a fuel injection hole diameter extending through said outer tube wall, the fuel injection hole having an injection angle relative to the tube axis. The invention provides good fuel air mixing with low combustion generated NOx and low flow pressure loss translating to a high gas turbine efficiency, that is durable, and resistant to flame holding and flash back.
Lean premixed/prevaporized combustion
NASA Technical Reports Server (NTRS)
Lefebvre, A. H. (Editor)
1977-01-01
Recommendations were formulated on the status and application of lean premixed/prevaporized combustion to the aircraft gas turbine for the reduction of pollutant emissions. The approach taken by the NASA Stratospheric Cruise Emission Reduction Program (SCERP) in pursuing the lean premixed/prevaporized combustion technique was also discussed. The proceedings contains an overview of the SCERP program, the discussions and recommendations of the participants, and an overall summary.
NASA Astrophysics Data System (ADS)
Li, Ya-Qing; Wu, Zhen-Sen; Zhang, Yuan-Yuan; Wang, Ming-Jun
2014-07-01
Based on the modified Rytov theory and the international telecommunication union-radio (ITU-R) slant atmospheric structure constant model, the uniform scintillation index of partially coherent Gaussian—Schell model (GSM) beam propagation in the slant path is derived from weak- to strong-turbulence regions considering inner- and outer-scale effects. The effects of wavelength of beams and inner- and outer-scale of turbulence on scintillation are analyzed numerically. Comparison between the scintillation of GSM beams under the von Karman spectrum and that of beams under the modified Hill spectrum is made. The results obtained show that the scintillation index obtained under the von Karman spectrum is smaller than that under the modified Hill spectrum. This study can find theory bases for the experiments of the partially coherent GSM beam propagation through atmospheric turbulence.
Direct simulation of turbulent combustion
NASA Technical Reports Server (NTRS)
Poinsot, T. J.
1990-01-01
Understanding and modeling of turbulent combustion are key-problems in the computation of numerous practical systems. Because of the lack of analytical theories in this field and of the difficulty of performing precise experiments, direct simulation appears to be one of the most attractive tools to use in addressing this problem. The present work can be split into two parts: (1) Development and validation of a direct simulation method for turbulent combustion; (2) Applications of the method to premixed turbulent combustion problems. The goal of part 1 is to define and to test a numerical method for direct simulation of reacting flows. A high level of confidence should be attached to direct simulation results, and this can only be achieved through extensive validation tests. In part 2, direct simulation is used to address some of the many critical problems related to turbulent combustion. At the present time, I have limited this work to premixed combustion and considered only four basic issues: (1) The effect of pressure waves on flame propagation; (2) The interaction between flame fronts and vortices; (3) The influence of curvature on premixed flame fronts; and (4) The validation of flamelet models for premixed turbulent combustion.
NASA Astrophysics Data System (ADS)
Wu, Hao; Ihme, Matthias
2016-11-01
High-fidelity turbulent reactive flow simulations are typically associated with small time step sizes (h <=10-8 sec) due to the CFL condition imposed by the fine gird. Although the step size is not sufficiently small to allow fully explicit time integration in the presence of stiff chemistry, it makes the use of classical implicit multi-step ODE solvers (e.g. VODE) an inefficient approach in combustion simulations due to the reduced number of internal iterations and excessive implicitness. In this study, an improved 4th-order Rosenbrock-Krylov (ROK4L) scheme is developed for the system of chemical reactions. This class of schemes replaces the Jacobian matrix by its low-rank Krylov approximation, thus introducing partial implicitness. The scheme is improved in both accuracy and efficiency by fulfilling additional order conditions and reducing the number of function evaluations. The ROK4L scheme is demonstrated to possess superior efficiency in comparison to CVODE due to the minimal degree of implicitness for small time-step sizes and the avoidance of other overhead associated with the start-up process of multi-step methods. Financial support from NASA Transformational Tools and Technologies Project with Award No. NNX15AV04A.
Near field flow structure of isothermal swirling flows and reacting non-premixed swirling flames
Olivani, Andrea; Solero, Giulio; Cozzi, Fabio; Coghe, Aldo
2007-04-15
Two confined lean non-premixed swirl-stabilized flame typologies were investigated in order to achieve detailed information on the thermal and aerodynamic field in the close vicinity of the burner throat and provide correlation with the exhaust emissions. Previous finding indicated the generation of a partially premixed flame with radial fuel injection and a purely diffusive flame with co-axial injection in a swirling co-flow. In the present work, the experimental study is reported which has been conducted on a straight exit laboratory burner with no quarl cone, fuelled by natural gas and air, and fired vertically upwards with the flame stabilized at the end of two concentric pipes with the annulus supplying swirled air and the central pipe delivering the fuel. Two fuel injection typologies, co-axial and radial (i.e., transverse), leading to different mixing mechanisms, have been characterized through different techniques: particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) for a comprehensive analysis of the velocity field, still photography for the detection of flame front and main visible features, and thermocouples for the temperature distribution. Isothermal flow conditions have been included in the experimental investigation to provide a basic picture of the flow field and to comprehend the modifications induced by the combustion process. The results indicated that, although the global mixing process and the main flame structure are governed by the swirl motion imparted to the air stream, the two different fuel injection methodologies play an important role on mixture formation and flame stabilization in the primary mixing zone. Particularly, it has been found that, in case of axial injection, the turbulent interaction between the central fuel jet and the backflow generated by the swirl can induce an intermittent fuel penetration in the recirculated hot products and the formation of a central sooting luminous plume, a phenomenon totally
Premixed Prevaporized Combustor Technology Forum
NASA Technical Reports Server (NTRS)
1979-01-01
The Forum was held to present the results of recent and current work intended to provide basic information required for demonstration of lean, premixed prevaporized combustors for aircraft gas turbine engine application. Papers are presented which deal with the following major topics: (1) engine interfaces; (2) fuel-air preparation; (3) autoignition; (4) lean combustion; and (5) concept design studies.
Statistics of premixed flame cells
NASA Technical Reports Server (NTRS)
Noever, David A.
1991-01-01
The statistics of random cellular patterns in premixed flames are analyzed. Agreement is found with a variety of topological relations previously found for other networks, namely, Lewis's law and Aboav's law. Despite the diverse underlying physics, flame cells are shown to share a broad class of geometric properties with other random networks-metal grains, soap foams, bioconvection, and Langmuir monolayers.
Statistics of premixed flame cells
Noever, D.A. )
1991-07-15
The statistics of random cellular patterns in premixed flames are analyzed. Agreement is found with a variety of topological relations previously found for other networks, namely, Lewis's law and Aboav's law. Despite the diverse underlying physics, flame cells are shown to share a broad class of geometric properties with other random networks---metal grains, soap foams, bioconvection, and Langmuir monolayers.
Chen, Chunyi; Yang, Huamin; Zhou, Zhou; Zhang, Weizhi; Kavehrad, Mohsen; Tong, Shoufeng; Wang, Tianshu
2013-12-02
The temporal covariance function of irradiance-flux fluctua-tions for Gaussian Schell-model (GSM) beams propagating in atmospheric turbulence is theoretically formulated by making use of the method of effective beam parameters. Based on this formulation, new expressions for the root-mean-square (RMS) bandwidth of the irradiance-flux temporal spectrum due to GSM beams passing through atmospheric turbulence are derived. With the help of these expressions, the temporal fade statistics of the irradiance flux in free-space optical (FSO) communication systems, using spatially partially coherent sources, impaired by atmospheric turbulence are further calculated. Results show that with a given receiver aperture size, the use of a spatially partially coherent source can reduce both the fractional fade time and average fade duration of the received light signal; however, when atmospheric turbulence grows strong, the reduction in the fractional fade time becomes insignificant for both large and small receiver apertures and in the average fade duration turns inconsiderable for small receiver apertures. It is also illustrated that if the receiver aperture size is fixed, changing the transverse correlation length of the source from a larger value to a smaller one can reduce the average fade frequency of the received light signal only when a threshold parameter in decibels greater than the critical threshold level is specified.
NASA Astrophysics Data System (ADS)
Yousefi, M.; Kashani, F. D.; Mashal, A.
2017-02-01
In this research, an analytical expression for cross-spectral density matrix elements (and consequently, average intensity) of partially coherent flat-topped (PCFT) radial array laser beams in weak oceanic turbulence are derived based on the extended Huygens-Fresnel principle and the previously developed knowledge of the propagation of a partially coherent beam in atmosphere. Mean-squared beam width is calculated analytically using average intensity formula. The simulation is done by considering the effects of source parameters (such as the radius of the array setup’s circle and effective width of spectral degree of coherence) and turbulent ocean factors (such as the rate of dissipation of the turbulent kinetic energy per unit mass of fluid and relative strength of temperature-salinity fluctuations, Kolmogorov micro-scale, and the rate of dissipation of the mean squared temperature) in detail. It is found that when salinity fluctuations in the ocean dominate temperature fluctuations, the average intensity of the PCFT array beam becomes more broad and the array beam profile conversion process to a single wider Gaussian beam profile will occur at a faster rate. For the same turbulent conditions and the same initial beam width, the divergence of a flat-topped array beam is less than the Gaussian array beam. The simulation and calculation results are shown by graphs.
Kashani, Fatemeh Dabbagh; Yousefi, Masoud
2016-08-10
In this research, based on an analytical expression for cross-spectral density (CSD) matrix elements, coherence and polarization properties of phase-locked partially coherent flat-topped (PCFT) radial array laser beams propagating through weak oceanic turbulence are analyzed. Spectral degrees of coherence and polarization are analytically calculated using CSD matrix elements. Also, the effective width of spatial degree of coherence (EWSDC) is calculated numerically. The simulation is done by considering the effects of source parameters (such as radius of the array setup's circle, effective width of the spectral degree of coherence, and wavelength) and turbulent ocean factors (such as the rate of dissipation of the turbulent kinetic energy per unit mass of fluid and relative strength of temperature and salinity fluctuations, Kolmogorov micro-scale, and rate of dissipation of the mean squared temperature) in detail. Results indicate that any change in the amount of turbulence factors that increase the turbulence power reduces the EWSDC significantly and causes the reduction in the degree of polarization, and occurs at shorter propagation distances but with smaller magnitudes. In addition, being valid for all conditions, the degradation rate of the EWSDC of Gaussian array beams are more in comparison with the PCFT ones. The simulation and calculation results are shown by graphs.
Premixed burner experiments: Geometry, mixing, and flame structure issues
Gupta, A.K.; Lewis, M.J.; Gupta, M.
1995-10-01
This research program is exploring techniques for improved fuel-air mixing, with the aim of achieving combustor operations up to stoichiometric conditions with minimal NO x and maximum efficiency. The experimental studies involve the use of a double-concentric natural gas burner that is operable in either premixed or non-premixed modes, and the system allows systematic variation of equivalence ratio, swirl strength shear length region and flow momentum in each annulus. Flame structures formed with various combinations of swirl strengths, flow throughput and equivalence ratios in premixed mode show the significant impact of swirl flow distribution on flame structure emanating from the mixedness. This impact on flame structure is expected to have a pronounced effect on the heat release rate and the emission of NO{sub x}. Thus, swirler design and configuration remains a key factor in the quest for completely optimized combustion. Parallel numerical studies of the flow and combustion phenomena were carried out, using the RSM and thek-{epsilon} turbulence models. These results have not only indicated the strengths and limitations of CFD in performance and pollutants emission predictions, but have provided guidelines on the size and strength of the recirculation produced and the spatio-temporal structure of the combustion flowfield. The first stage of parametric studies on geometry and operational parameters at Morgan State University have culminated in the completion of a one-dimensional flow code that is integrated with a solid, virtual model of the existing premixed burner. This coupling will provide the unique opportunity to study the impact of geometry on the flowfield and vice-versa, with particular emphasis on concurrent design optimization.
NASA Astrophysics Data System (ADS)
Chaouat, Bruno; Schiestel, Roland
2012-08-01
The basis of the partially integrated transport modeling (PITM) method was introduced by Schiestel and Dejoan ["Towards a new partially integrated transport model for coarse grid and unsteady turbulent flow simulations," Theor. Comput. Fluid Dyn. 18, 443 (2005)], 10.1007/s00162-004-0155-z and Chaouat and Schiestel ["A new partially integrated transport model for subgrid-scale stresses and dissipation rate for turbulent developing flows," Phys. Fluids 17, 065106 (2005)], 10.1063/1.1928607. This method provides a continuous approach for hybrid RANS-LES (Reynolds averaged Navier-Stokes equations-large eddy simulations) simulations with seamless coupling between RANS and LES regions. The main ingredient of the method is the new dissipation-rate equation that can be applied as a subfilter scale turbulence model. Then, it becomes easy to convert almost any usual RANS transport model into a subfilter scale model. In particular, the method can be applied to two equation models and to stress transport models as well. In the derivation of the method, the partial integration technique allows to keep a link between the spectral space and the physical space of the resulting model. The physical turbulent processes involving the production, dissipation, and flux transfer of the turbulent energy are introduced in the equations. The present work, after recalling the main building steps of the PITM method, brings further insight into the physical interpretation of the method, its underlying hypotheses and its internal acting mechanisms. In particular, the finiteness of the coefficients used in the dissipation-rate equation is discussed in detail from a theoretical point of view. Then, we consider the analytical example of self-similar turbulent flow for analyzing the dissipation-rate equation. From an analytical solution obtained by Taylor series expansions taking into account the Kovasznay hypothesis for evaluating the transfer term, we compute the functional coefficients c
Direct Numerical Simulation of a Cavity-Stabilized Ethylene/Air Premixed Flame
NASA Astrophysics Data System (ADS)
Chen, Jacqueline; Konduri, Aditya; Kolla, Hemanth; Rauch, Andreas; Chelliah, Harsha
2016-11-01
Cavity flame holders have been shown to be important for flame stabilization in scramjet combustors. In the present study the stabilization of a lean premixed ethylene/air flame in a rectangular cavity at thermo-chemical conditions relevant to scramjet combustors is simulated using a compressible reacting multi-block direct numerical simulation solver, S3D, incorporating a 22 species ethylene-air reduced chemical model. The fuel is premixed with air to an equivalence ratio of 0.4 and enters the computational domain at Mach numbers between 0.3 and 0.6. An auxiliary inert channel flow simulation is used to provide the turbulent velocity profile at the inlet for the reacting flow simulation. The detailed interaction between intense turbulence, nonequilibrium concentrations of radical species formed in the cavity and mixing with the premixed main stream under density variations due to heat release rate and compressibility effects is quantified. The mechanism for flame stabilization is quantified in terms of relevant non-dimensional parameters, and detailed analysis of the flame and turbulence structure will be presented. We acknowledge the sponsorship of the AFOSR-NSF Joint Effort on Turbulent Combustion Model Assumptions and the DOE Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences.
Premixed Parenteral Nutrition Solution Use in Children
Crill, Catherine M.
2015-01-01
OBJECTIVES: In response to national drug shortages, our institution established criteria for the use of commercial premixed parenteral nutrition (PN) solutions in select pediatric patients. Although these solutions have been marketed for use in children, there are no data in this patient population. The objective of this study was to review our use of commercial premixed PN solutions in children. METHODS: This was a retrospective review of patients ≤18 years of age who received a premixed PN solution from October 2010 to April 2012. All premixed PN courses were assessed for incidence of premixed PN discontinuation due to laboratory abnormalities. Estimated goal and actual protein and total caloric intake were evaluated for premixed PN courses that were continued for >48 hours. RESULTS: Sixty-nine patients received 74 courses of premixed PN solutions for a mean duration of 5.6 ± 6.2 (range, 1–31) days. Fifteen courses (20%) required discontinuation of premixed PN as a result of mild laboratory abnormalities. No changes in clinical status were observed in patients and all abnormalities were corrected after switching to individualized PN. In patients receiving PN for >48 hours, premixed PN solutions provided goal protein in 48/49 (98%) courses and goal calories in 33/49 (67%) courses. CONCLUSIONS: Premixed PN solutions were used in a wide range of pediatric patients and provide a potential option for PN support in pediatric patients when drug shortages limit PN product supply. Close monitoring for electrolyte abnormalities and protein and caloric intake is recommended when using premixed PN solutions in children. PMID:26472952
NASA Astrophysics Data System (ADS)
Smolyakov, A. I.; Chapurin, O.; Frias, W.; Koshkarov, O.; Romadanov, I.; Tang, T.; Umansky, M.; Raitses, Y.; Kaganovich, I. D.; Lakhin, V. P.
2017-01-01
Partially-magnetized plasmas with magnetized electrons and non-magnetized ions are common in Hall thrusters for electric propulsion and magnetron material processing devices. These plasmas are usually in strongly non-equilibrium state due to presence of crossed electric and magnetic fields, inhomogeneities of plasma density, temperature, magnetic field and beams of accelerated ions. Free energy from these sources make such plasmas prone to various instabilities resulting in turbulence, anomalous transport, and appearance of coherent structures as found in experiments. This paper provides an overview of instabilities that exist in such plasmas. A nonlinear fluid model has been developed for description of the Simon-Hoh, lower-hybrid and ion-sound instabilities. The model also incorporates electron gyroviscosity describing the effects of finite electron temperature. The nonlinear fluid model has been implemented in the BOUT++ framework. The results of nonlinear simulations are presented demonstrating turbulence, anomalous current and tendency toward the formation of coherent structures.
Premixed flame propagation in vertical tubes
NASA Astrophysics Data System (ADS)
Kazakov, Kirill A.
2016-04-01
Analytical treatment of the premixed flame propagation in vertical tubes with smooth walls is given. Using the on-shell flame description, equations for a quasi-steady flame with a small but finite front thickness are obtained and solved numerically. It is found that near the limits of inflammability, solutions describing upward flame propagation come in pairs having close propagation speeds and that the effect of gravity is to reverse the burnt gas velocity profile generated by the flame. On the basis of these results, a theory of partial flame propagation driven by a strong gravitational field is developed. A complete explanation is given of the intricate observed behavior of limit flames, including dependence of the inflammability range on the size of the combustion domain, the large distances of partial flame propagation, and the progression of flame extinction. The role of the finite front-thickness effects is discussed in detail. Also, various mechanisms governing flame acceleration in smooth tubes are identified. Acceleration of methane-air flames in open tubes is shown to be a combined effect of the hydrostatic pressure difference produced by the ambient cold air and the difference of dynamic gas pressure at the tube ends. On the other hand, a strong spontaneous acceleration of the fast methane-oxygen flames at the initial stage of their evolution in open-closed tubes is conditioned by metastability of the quasi-steady propagation regimes. An extensive comparison of the obtained results with the experimental data is made.
Lean Premixed Combustion Stabilized by Low Swirl a Promising Concept for Practical Applications
NASA Technical Reports Server (NTRS)
Cheng, R. K.
1999-01-01
Since its inception, the low-swirl burner (LSB) has shown to be a useful laboratory apparatus for fundamental studies of premixed turbulent flames. The LSB operates under wide ranges of equivalence ratios, flow rates, and turbulence intensities. Its flame is lifted and detached from the burner and allows easy access for laser diagnostics. The flame brush is axisymmetric and propagates normal to the incident reactants. Therefore, the LSB is well suited for investigating detailed flame structures and empirical coefficients such as flame speed, turbulence transport, and flame generated turbulence. Due to its capability to stabilize ultra-lean premixed turbulent flames (phi approx. = 0.55), the LSB has generated interest from the gas appliance industry for use as an economical low-NO(x) burner. Lean premixed combustion emits low levels of NO(x), due primarily to the low flame temperature. Therefore, it is a very effective NO(x) prevention method without involving selective catalytic reduction (SCR), fuel-air staging, or flue gas recirculation (FGR). En the gas turbine industry, substantial research efforts have already been undertaken and engines with lean premixed combustors are already in use. For commercial and residential applications, premixed pulsed combustors and premixed ceramic matrix burners are commercially available. These lean premixed combustion technologies, however, tend to be elaborate but have relatively limited operational flexibility, and higher capital, operating and maintenance costs. Consequently, these industries are continuing the development of lean premixed combustion technologies as well as exploring new concepts. This paper summarizes the research effects we have undertaken in the past few years to demonstrate the feasibility of applying the low-swirl flame stabilization method for a wide range of heating and power generation systems. The principle of flame stabilization by low-swirl is counter to the conventional high-swirl methods that
Gas turbine premixer with internal cooling
York, William David; Johnson, Thomas Edward; Lacy, Benjamin Paul; Stevenson, Christian Xavier
2012-12-18
A system that includes a turbine fuel nozzle comprising an air-fuel premixer. The air-fuel premixed includes a swirl vane configured to swirl fuel and air in a downstream direction, wherein the swirl vane comprises an internal coolant path from a downstream end portion in an upstream direction through a substantial length of the swirl vane.
Flashback resistant pre-mixer assembly
Laster, Walter R [Oviedo, FL; Gambacorta, Domenico [Oviedo, FL
2012-02-14
A pre-mixer assembly associated with a fuel supply system for mixing of air and fuel upstream from a main combustion zone in a gas turbine engine. The pre-mixer assembly includes a swirler assembly disposed about a fuel injector of the fuel supply system and a pre-mixer transition member. The swirler assembly includes a forward end defining an air inlet and an opposed aft end. The pre-mixer transition member has a forward end affixed to the aft end of the swirler assembly and an opposed aft end defining an outlet of the pre-mixer assembly. The aft end of the pre-mixer transition member is spaced from a base plate such that a gap is formed between the aft end of the pre-mixer transition member and the base plate for permitting a flow of purge air therethrough to increase a velocity of the air/fuel mixture exiting the pre-mixer assembly.
NASA Technical Reports Server (NTRS)
Shih, Tsan-Hsing; Liu, Nan-Suey
2008-01-01
This paper describes an approach which aims at bridging the gap between the traditional Reynolds-averaged Navier-Stokes (RANS) approach and the traditional large eddy simulation (LES) approach. It has the characteristics of the very large eddy simulation (VLES) and we call this approach the partially-resolved numerical simulation (PRNS). Systematic simulations using the National Combustion Code (NCC) have been carried out for fully developed turbulent pipe flows at different Reynolds numbers to evaluate the PRNS approach. Also presented are the sample results of two demonstration cases: nonreacting flow in a single injector flame tube and reacting flow in a Lean Direct Injection (LDI) hydrogen combustor.
Fuel premixing module for gas turbine engine combustor
NASA Technical Reports Server (NTRS)
Chin, Jushan (Inventor); Rizk, Nader K. (Inventor); Razdan, Mohan K. (Inventor); Marshall, Andre W. (Inventor)
2005-01-01
A fuel-air premixing module is designed to reduce emissions from a gas turbine engine. In one form, the premixing module includes a central pilot premixer module with a main premixer module positioned thereround. Each of the portions of the fuel-air premixing module include an axial inflow swirler with a plurality of fixed swirler vanes. Fuel is injected into the main premixer module between the swirler vanes of the axial inflow swirler and at an acute angle relative to the centerline of the premixing module.
Strakey, Peter A.; Eggenspieler, Gilles
2010-04-07
The development of a dynamic thickened flame (TF) turbulence-chemistry interaction model is presented based on a novel approach to determine the subfilter flame wrinkling efficiency. The burner is based on an enclosed scaled-down version of the low swirl injector developed at Lawrence Berkeley National Laboratory (LBNL). A perfectly premixed lean methane-air flame was studied, as well as the cold-flow characteristics of the combustor.
Yousefi, Masoud; Golmohammady, Shole; Mashal, Ahmad; Kashani, Fatemeh Dabbagh
2015-11-01
In this paper, on the basis of the extended Huygens-Fresnel principle, a semianalytical expression for describing on-axis scintillation index of a partially coherent flat-topped (PCFT) laser beam of weak to moderate oceanic turbulence is derived; consequently, by using the log-normal intensity probability density function, the bit error rate (BER) is evaluated. The effects of source factors (such as wavelength, order of flatness, and beam width) and turbulent ocean parameters (such as Kolmogorov microscale, relative strengths of temperature and salinity fluctuations, rate of dissipation of the mean squared temperature, and rate of dissipation of the turbulent kinetic energy per unit mass of fluid) on propagation behavior of scintillation index, and, hence, on BER, are studied in detail. Results indicate that, in comparison with a Gaussian beam, a PCFT laser beam with a higher order of flatness is found to have lower scintillations. In addition, the scintillation index and BER are most affected when salinity fluctuations in the ocean dominate temperature fluctuations.
Turbulent Flame Processes Via Diffusion Flame-Vortex Ring Interactions
NASA Technical Reports Server (NTRS)
Dahm, Werner J. A.; Chen, Shin-Juh; Silver, Joel A.; Piltch, Nancy D.; VanderWal, Randall L.
2001-01-01
Flame-vortex interactions are canonical configurations that can be used to study the underlying processes occurring in turbulent reacting flows. This configuration contains many of the fundamental aspects of the coupling between fluid dynamics and combustion that could be investigated with more controllable conditions than are possible under direct investigations of turbulent flames. Diffusion flame-vortex ring interaction contains many of the fundamental elements of flow, transport, combustion, and soot processes found in turbulent diffusion flames. Some of these elements include concentrated vorticity, entrainment and mixing, strain and nonequilibrium phenomena, diffusion and differential diffusion, partial premixing and diluent effects, soot formation and oxidation, and heat release effects. Such simplified flowfield allows the complex processes to be examined more closely and yet preserving the physical processes present in turbulent reacting flows. Furthermore, experimental results from the study of flame-vortex interactions are useful for the validation of numerical simulations and more importantly to deepen our understanding of the fundamental processes present in reacting flows. Experimental and numerical results obtained under microgravity conditions of the diffusion flame-vortex ring interaction are summarized in this paper. Results are obtained using techniques that include Flame Luminosity Imaging (FLI), Laser Soot-Mie Scattering (LSMS), Computational Fluid Dynamics and Combustion (CFDC), and Diode Laser Spectroscopy/Iterative Temperature with Assumed Chemistry (DLS/ITAC).
Lewis, Elliot Sullivan-; McDonell, Vincent G.
2014-12-01
Due to increasingly stringent air quality requirements stationary power gas turbines have moved to lean-premixed operation, which reduces pollutant emissions but can result in flashback. Flashback can cause serious damage to the premixer hardware. Curtailing flashback can be difficult with hydrocarbon fuels and becomes even more challenging when hydrogen is used as the fuel. The two main approaches for coping with flashback are either to design a combustor that is resistant to flashback, or to design a premixer that will not anchor a flame if flashback occurs. Even with a well-designed combustor flashback can occur under certain circumstances, thus it is necessary to determine how to avoid flameholding within the premixer passageways of a gas turbine. To this end, an experiment was designed that would determine the flameholding propensities at elevated pressures and temperatures of three different classes of geometric features commonly found in gas turbine premixers, with both natural gas and hydrogen fuel. Experiments to find the equivalence ratio at blow off were conducted within an optically accessible test apparatus with four flameholders: 0.25 and 0.50 inch diameter cylinders, a reverse facing step with a height of 0.25 inches, and a symmetric airfoil with a thickness of 0.25 inches and a chord length of one inch. Tests were carried out at temperatures between 300 K and 750 K, at pressures up to 9 atmospheres. Typical bulk velocities were between 40 and 100 m/s. The effect of airfoil’s angle of rotation was also investigated. Blow off for hydrogen flames was found to occur at much lower adiabatic flame temperatures than natural gas flames. Additionally it was observed that at high pressures and high turbulence intensities, reactant velocity does not have a noticeable effect on the point of blow off due in large part to corresponding increases in turbulent flame speed. Finally a semi empirical correlation was developed that predicts flame extinction for both
A premixed hydrogen/oxygen catalytic igniter
NASA Technical Reports Server (NTRS)
Green, James M.
1989-01-01
The catalytic ignition of hydrogen and oxygen propellants was studied using a premixing hydrogen/oxygen injector. The premixed injector was designed to eliminate problems associated with catalytic ignition caused by poor propellant mixing in the catalyst bed. Mixture ratio, mass flow rate, and propellant inlet temperature were varied parametrically in testing, and a pulse mode life test of the igniter was conducted. The results of the tests showed that the premixed injector eliminated flame flashback in the reactor and increased the life of the igniter significantly. The results of the experimental program and a comparison with data collected in a previous program are given.
2015-01-19
The purpose of LEM-CF Premixed Tool Kit is to process premixed flame simulation data from the LEM-CF solver (https://fileshare.craft-tech.com/clusters/view/lem-cf) into a large-eddy simulation (LES) subgrid model database. These databases may be used with a user-defined-function (UDF) that is included in the Tool Kit. The subgrid model UDF may be used with the ANSYS FLUENT flow solver or other commercial flow solvers.
A numerical investigation of premixed combustion in wave rotors
NASA Technical Reports Server (NTRS)
Nalim, M. Razi; Paxson, Daniel E.
1996-01-01
Wave rotor cycles which utilize premixed combustion processes within the passages are examined numerically using a one-dimensional CFD-based simulation. Internal-combustion wave rotors are envisioned for use as pressure-gain combustors in gas turbine engines. The simulation methodology is described, including a presentation of the assumed governing equations for the flow and reaction in the channels, the numerical integration method used, and the modeling of external components such as recirculation ducts. A number of cycle simulations are then presented which illustrate both turbulent-deflagration and detonation modes of combustion. Estimates of performance and rotor wall temperatures for the various cycles are made, and the advantages and disadvantages of each are discussed.
Improved Modeling of Finite-Rate Turbulent Combustion Processes in Research Combustors
NASA Technical Reports Server (NTRS)
VanOverbeke, Thomas J.
1998-01-01
The objective of this thesis is to further develop and test a stochastic model of turbulent combustion in recirculating flows. There is a requirement to increase the accuracy of multi-dimensional combustion predictions. As turbulence affects reaction rates, this interaction must be more accurately evaluated. In this work a more physically correct way of handling the interaction of turbulence on combustion is further developed and tested. As turbulence involves randomness, stochastic modeling is used. Averaged values such as temperature and species concentration are found by integrating the probability density function (pdf) over the range of the scalar. The model in this work does not assume the pdf type, but solves for the evolution of the pdf using the Monte Carlo solution technique. The model is further developed by including a more robust reaction solver, by using accurate thermodynamics and by more accurate transport elements. The stochastic method is used with Semi-Implicit Method for Pressure-Linked Equations. The SIMPLE method is used to solve for velocity, pressure, turbulent kinetic energy and dissipation. The pdf solver solves for temperature and species concentration. Thus, the method is partially familiar to combustor engineers. The method is compared to benchmark experimental data and baseline calculations. The baseline method was tested on isothermal flows, evaporating sprays and combusting sprays. Pdf and baseline predictions were performed for three diffusion flames and one premixed flame. The pdf method predicted lower combustion rates than the baseline method in agreement with the data, except for the premixed flame. The baseline and stochastic predictions bounded the experimental data for the premixed flame. The use of a continuous mixing model or relax to mean mixing model had little effect on the prediction of average temperature. Two grids were used in a hydrogen diffusion flame simulation. Grid density did not effect the predictions except
Coaxial fuel and air premixer for a gas turbine combustor
York, William D; Ziminsky, Willy S; Lacy, Benjamin P
2013-05-21
An air/fuel premixer comprising a peripheral wall defining a mixing chamber, a nozzle disposed at least partially within the peripheral wall comprising an outer annular wall spaced from the peripheral wall so as to define an outer air passage between the peripheral wall and the outer annular wall, an inner annular wall disposed at least partially within and spaced from the outer annular wall, so as to define an inner air passage, and at least one fuel gas annulus between the outer annular wall and the inner annular wall, the at least one fuel gas annulus defining at least one fuel gas passage, at least one air inlet for introducing air through the inner air passage and the outer air passage to the mixing chamber, and at least one fuel inlet for injecting fuel through the fuel gas passage to the mixing chamber to form an air/fuel mixture.
The role of reactant unmixedness, strain rate, and length scale on premixed combustor performance
Samuelsen, S.; LaRue, J.; Vilayanur, S.; Guillaume, D.
1995-12-31
Lean premixed combustion provides a means to reduce pollutant formation and increase combustion efficiency. However, fuel-air mixing is rarely uniform in space and time. This nonuniformity in concentration will lead to relative increases in pollutant formation and decreases in combustion efficiency. The nonuniformity of the concentration at the exit of the premixer has been defined by Lyons (1981) as the ``unmixedness.`` Although turbulence properties such as length scales and strain rate are known to effect unmixedness, the exact relationship is unknown. Evaluating this relationship and the effect of unmixedness in premixed combustion on pollutant formation and combustion efficiency are an important part of the overall goal of US Department of Energy`s Advanced Turbine System (ATS) program and are among the goals of the program described herein. The information obtained from ATS is intended to help to develop and commercialize gas turbines. The contributions to the program which the University of California (Irvine) Combustion Lab (UCICL) will provide are: (1) establish the relationship of inlet unmixedness, length scales, and mean strain rate to performance, (2) determine the optimal levels of inlet unmixedness, length scales, and mean strain rates to maximize combustor performance, and (3) identify efficient premixing methods for achieving the necessary inlet conditions. The program during this reporting period is focused on developing a means to measure and qualify different degrees of temporal and spatial unmixedness. Laser diagnostic methods for planer unmixedness measurements are being developed and preliminary results are presented herein. These results will be used to (1), aid in the design of experimental premixers, and (2), determine the unmixedness which will be correlated with the emissions of the combustor. This measure of unmixedness coupled with length scale, strain rate and intensity information is required to attain the UCI goals.
Design factors for stable lean premix combustion
Richards, G.A.; Yip, M.J.; Gemmen, R.S.
1995-10-01
The Advanced Turbine Systems (ATS) program includes the development of low-emission combustors. Low emissions have already been achieved by premixing fuel and air to avoid the hot gas pockets produced by nozzles without premixing. While the advantages of premixed combustion have been widely recognized, turbine developers using premixed nozzles have experienced repeated problems with combustion oscillations. Left uncontrolled, these oscillations can lead to pressure fluctuations capable of damaging engine hardware. Elimination of such oscillations is often difficult and time consuming - particularly when oscillations are discovered in the last stages of engine development. To address this issue, METC is studying oscillating combustion from lean premixing fuel nozzles. These tests are providing generic information on the mechanisms that contribute to oscillating behavior in gas turbines. METC is also investigating the use of so-called {open_quotes}active{close_quotes} control of combustion oscillations. This technique periodically injects fuel pulses into the combustor to disrupt the oscillating behavior. Recent results on active combustion control are presented in Gemmen et al. (1995) and Richards et al. (1995). This paper describes the status of METC efforts to avoid oscillations through simple design changes.
Soot Formation in Laminar Premixed Flames
NASA Technical Reports Server (NTRS)
Xu, F.; Krishnan, S. S.; Faeth, G. M.
1999-01-01
Soot processes within hydrocarbon-fueled flames affect emissions of pollutant soot, thermal loads on combustors, hazards of unwanted fires and capabilities for computational combustion. In view of these observations, the present study is considering processes of soot formation in both burner-stabilized and freely-propagating laminar premixed flames. These flames are being studied in order to simplify the interpretation of measurements and to enhance computational tractability compared to the diffusion flame environments of greatest interest for soot processes. In addition, earlier studies of soot formation in laminar premixed flames used approximations of soot optical and structure properties that have not been effective during recent evaluations, as well as questionable estimates of flow residence times). The objective of present work was to exploit methods of avoiding these difficulties developed for laminar diffusion flames to study soot growth in laminar premixed flames. The following description of these studies is brief.
Velocity and turbulence measurements in combustion systems
NASA Astrophysics Data System (ADS)
Goldstein, R. J.; Lau, K. Y.; Leung, C. C.
1983-06-01
A laser-Doppler velocimeter is used in the measurement of high-temperature gas flows. A two-stage fluidization particle generator provides magnesium oxide particles to serve as optical scattering centers. The one-dimensional dual-beam system is frequency shifted to permit measurements of velocities up to 300 meters per second and turbulence intensities greater than 100 percent. Exiting flows from can-type gas turbine combustors and burners with pre-mixed oxy-acetylene flames are described in terms of the velocity, turbulence intensity, and temperature profiles. The results indicate the influence of the combustion process on turbulence.
Representation of the Essential Flame-Turbulence Dynamics using Specific Flame-Vortex Interactions
NASA Astrophysics Data System (ADS)
Paes, Paulo L. K.; Brasseur, James; Xuan, Yuan
2016-11-01
Many engineering applications involve turbulent reacting flows, where nonlinear, multi-scale turbulence-combustion couplings are important. Directly resolving the complex fluid dynamics involved in these applications is associated with prohibitive computational costs, which makes it necessary to employ turbulent closure models and turbulent combustion models to account for the effects of unresolved scales on resolved scales. Most of these existent closure models rely on some assumptions about the turbulence dynamics and the scale separation between turbulence and the different combustion processes. A better understanding of the turbulence-combustion interactions is required for the development of more accurate, physics-based sub-grid-scale models for turbulent reacting flows. Instead of developing an extreme-resolution, high Reynolds number turbulent flame simulation that is limited to a localized part of the regime diagram, in this work, we propose to develop a series of numerical experiments of simplified interactions between a laminar premixed flame and specified vortex distributions of varying strengths and scales to capture the essential flame-turbulence dynamics over distinct premixed turbulent combustion regimes. The response of the laminar flame to different vortex time and length scales is investigated and the physical relevance of each dataset to practical turbulent premixed flames is discussed.
LES of a bluff-body stabilized premixed flame using discontinuous Galerkin scheme
NASA Astrophysics Data System (ADS)
Lv, Yu; Ihme, Matthias; Stanford University Team
2016-11-01
This talk focuses on the development of a high-order discontinuous Galerkin (DG) method for application to chemically reacting flows. To enable these simulations, several algorithmic aspects are addressed, including the time-integration of multi-step chemical reactions, the incorporation of detailed thermo-viscous transport properties, and the stabilization of high-order solution representation. This DG solver is applied in implicit LES of a turbulent bluff-body stabilized propane/air premixed flame. The simulation results for cold-flow and reacting conditions are reported and compared to experimental data.
The role of reactant unmixedness, strain rate, and length scale on premixed combustor performance
Samuelsen, S.; LaRue, J.; Vilayanur, S.
1995-10-01
Lean premixed combustion provides a means to reduce pollutant formation and increase combustion efficiency. However, fuel-air mixing is rarely uniform in space and time. This nonuniformity in concentration will lead to relative increases in pollutant formation and decreases in combustion efficiency. The nonuniformity of the concentration at the exit of the premixer has been defined by Lyons (1981) as the {open_quotes}unmixedness.{close_quotes} Although turbulence properties such as length scales and strain rate are known to effect unmixedness, the exact relationship is unknown. Evaluating this relationship and the effect of unmixedness in premixed combustion on pollutant formation and combustion efficiency are an important part of the overall goal of US Department of Energy`s Advanced Turbine Systems (ATS) program and are among the goals of the program described herein. The information obtained from ATS is intended to help to develop and commercialize gas turbines which have (1) a wide range of operation/stability, (2) a minimal amount of pollutant formation, and (3) high combustion efficiency. Specifically, with regard to pollutants, the goals are to reduce the NO{sub x} emissions by at least 10%, obtain less than 20 PPM of both CO and UHC, and increase the combustion efficiency by 5%.
Local Limit Phenomena, Flow Compression, and Fuel Cracking Effects in High-Speed Turbulent Flames
2015-06-01
previously generated at Sandia are employed for the analysis: a non-premixed temporal jet flame of ethylene-air diluted with N2 (DNS by D.O. Lignell...flame temperature). Both the DNS datasets were generated from Sandia’s multi-million-CPU-hour supercomputing and are high fidelity data sources for...computational diagnostic benchmarking and turbulent combustion model creation and validation. The CEMA result for the non-premixed temporal jet flame
Effect of degree of fuel vaporization upon emissions for a premixed prevaporized combustion system
NASA Technical Reports Server (NTRS)
Cooper, L. P.
1979-01-01
An experimental and analytical study of the combustion of partially vaporized fuel/air mixtures was performed to assess the impact of the degree of fuel vaporization upon emissions for a premixing-prevaporizing flametube combustor. Data collected showed near-linear increases in NOx emmissions with decreasing vaporization at equivalence ratios of 0.6. For equivalence ratios of 0.72, the degree of vaporization had very little impact on NOx emissions. A simple mechanism which accounts for the combustion of liquid droplets in partially vaporized mixtures was found to agree with the measured results with fair accuracy with respect to both trends and magnitudes.
Premixed-Gas Flame Propagation in Hele-Shaw Cells
NASA Technical Reports Server (NTRS)
Sharif, J.; Abid, M.; Ronney, P. D.
1999-01-01
It is well known that buoyancy and thermal expansion affect the propagation ra and shapes of premixed gas flames. The understanding of such effects is complicated by the large density ratio between the reactants and products, which induces a baroclinic production of vorticity due to misalignment of density and pressure gradients at the front, which in turn leads to a complicated multi-dimensional flame/flow interaction. The Hele-Shaw cell, i.e., the region between closely-spaced flat parallel plates, is probably the simplest system in which multi-dimensional convection is presents consequently, the behavior of fluids in this system has been studied extensively (Homsy, 1987). Probably the most important characteristic of Hele-Shaw flows is that when the Reynolds number based on gap width is sufficiently small, the Navier-Stokes equations averaged over the gap reduce to a linear relation, namely a Laplace equation for pressure (Darcy's law). In this work, flame propagation in Hele-Shaw cells is studied to obtain a better understanding of buoyancy and thermal expansion effects on premixed flames. This work is also relevant to the study of unburned hydrocarbon emissions produced by internal combustion engines since these emissions are largely a result of the partial burning or complete flame quenching in the narrow, annular gap called the "crevice volume" between the piston and cylinder walls (Heywood, 1988). A better understanding of how flames propagate in these volumes through experiments using Hele-Shaw cells could lead to identification of means to reduce these emissions.
High-speed tomographic PIV and OH PLIF measurements in turbulent reactive flows
NASA Astrophysics Data System (ADS)
Coriton, Bruno; Steinberg, Adam M.; Frank, Jonathan H.
2014-06-01
High-speed tomographic particle image velocimetry (TPIV) is demonstrated in turbulent reactive flows at acquisition rates ranging from 10 to 16 kHz. The 10-kHz TPIV measurements are combined with planar laser-induced fluorescence (PLIF) imaging of OH to mark the high-temperature reaction zone of the flame. Simultaneous TPIV/OH PLIF measurements are applied to the stabilization region of a weakly turbulent lifted dimethyl ether (DME)/air jet flame ( Re D = 7,600) and the mixing layer of a turbulent partially premixed DME/air jet flame ( Re D = 29,300). In the lifted jet flame, vortical structures exhibit time-dependent morphological changes and eventually dissipate as they approach the flame. In the near field of the turbulent jet flame, dynamics of localized extinction are captured as coherent structures with high compressive strain rates interact with the reaction zone and subsequently break apart. The principal axis of compressive strain has a strong preferential orientation at 45° with respect to the jet axis. The three-dimensional velocity field measurements are used to evaluate biases in two-dimensional (2D) measurements of compressive strain rates in a turbulent jet flame. The biases in the 2D measurements primarily stem from out-of-plane orientation of the principal axis of compressive strain. Comparisons with a constant density turbulent non-reactive jet ( Re D = 22,600) show that the jet flame has larger coherent structures that are confined near the reaction zone. Data from the non-reactive jet are also used to evaluate effects of noise, bias, and spatial averaging on measurements of the velocity and velocity gradients.
Filtered chemical source term modeling for LES of high Karlovitz number premixed flames
NASA Astrophysics Data System (ADS)
Lapointe, Simon; Blanquart, Guillaume
2015-11-01
Tabulated chemistry with the transport of a single progress variable is a popular technique for large eddy simulations of premixed turbulent flames. Since the reaction zone thickness is usually smaller than the LES grid size, modeling of the filtered progress variable reaction rate is required. Most models assume that the filtered progress variable reaction rate is a function of the filtered progress variable and its variance where the dependence can be obtained through the probability density function (PDF) of the progress variable. Among the most common approaches, the PDF can be presumed (usually as a β-PDF) or computed using spatially filtered one dimensional laminar flames (FLF). Models for the filtered source term are studied a priori using results from DNS of turbulent n-heptane/air premixed flames at varying Karlovitz numbers. Predictions from the optimal estimator and models based on laminar flames using a β-PDF or a FLF-PDF are compared to the exact filtered source term. For all filter widths and Karlovitz numbers, the optimal estimator yields small errors while β-PDF and FLF-PDF approaches present larger errors. Sources of differences are discussed.
NASA Astrophysics Data System (ADS)
Kwon, Oh Chae
Recent work shows that preferential-diffusion/stretch interactions of laminar premixed flames are sufficiently robust to affect the stability of practical strongly-turbulent flames. In addition, past measurements of laminar burning velocities should be re-assessed because there generally was no attempt to control flame stretch. Finally, the sensitivity of laminar premixed flames to stretch (represented by the Markstein number) should be studied to better understand and model the properties of laminar premixed flames. Motivated by these considerations, an experimental and computational study of preferential-diffusion/stretch interactions for laminar premixed flames, for both alkane/alcohol-fuel-vapor-fueled flames (as practical fuels) and hydrogen-fueled flames (considering diluent-variation effects) was carried out during the present investigation. Considering outwardly-propagating spherical laminar premixed flames, laminar burning velocities of fuel-vapor/oxygen/nitrogen flames and hydrogen/oxygen/diluent (nitrogen, argon or helium) flames were measured for various values of stretch, fuel-equivalence ratios (0.6--4.5) and pressures (0.3--3 atm). The measurements were reduced to find fundamental unstretched laminar burning velocities and Markstein numbers. The measurements were also used to evaluate corresponding numerical simulations of the experimentally-observed flames, based on contemporary detailed H2/O2 reaction mechanisms. Both measured and predicted ratios of unstretched to stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), yielding a constant Markstein number for a particular reactant mixture. The present flames were very sensitive to flame stretch (i.e., they had large Markstein numbers with significant ratios of unstretched to stretched laminar burning velocities) for levels of flame stretch well below quenching conditions. Increasing flame temperatures tended to reduce flame sensitivity to
Tabulated Combustion Model Development For Non-Premixed Flames
NASA Astrophysics Data System (ADS)
Kundu, Prithwish
Turbulent non-premixed flames play a very important role in the field of engineering ranging from power generation to propulsion. The coupling of fluid mechanics and complicated combustion chemistry of fuels pose a challenge for the numerical modeling of these type of problems. Combustion modeling in Computational Fluid Dynamics (CFD) is one of the most important tools used for predictive modeling of complex systems and to understand the basic fundamentals of combustion. Traditional combustion models solve a transport equation of each species with a source term. In order to resolve the complex chemistry accurately it is important to include a large number of species. However, the computational cost is generally proportional to the cube of number of species. The presence of a large number of species in a flame makes the use of CFD computationally expensive and beyond reach for some applications or inaccurate when solved with simplified chemistry. For highly turbulent flows, it also becomes important to incorporate the effects of turbulence chemistry interaction (TCI). The aim of this work is to develop high fidelity combustion models based on the flamelet concept and to significantly advance the existing capabilities. A thorough investigation of existing models (Finite-rate chemistry and Representative Interactive Flamelet (RIF)) and comparative study of combustion models was done initially on a constant volume combustion chamber with diesel fuel injection. The CFD modeling was validated with experimental results and was also successfully applied to a single cylinder diesel engine. The effect of number of flamelets on the RIF model and flamelet initialization strategies were studied. The RIF model with multiple flamelets is computationally expensive and a model was proposed on the frame work of RIF. The new model was based on tabulated chemistry and incorporated TCI effects. A multidimensional tabulated chemistry database generation code was developed based on the 1
Spectral formulation of turbulent flame speed with consideration of hydrodynamic instability.
Chaudhuri, Swetaprovo; Akkerman, V'yacheslav; Law, Chung K
2011-08-01
Effects of Darrieus-Landau (DL) instability on the structure and propagation of turbulent premixed flame fronts are considered. By first hypothesizing separation of time scales of instability and turbulence, we estimate whether the instability can develop in the presence of turbulence of given flow rms-velocity and integral length scale. As a result, we modify the standard turbulent premixed combustion regime diagram by introducing new boundaries, limiting the domain where the instability influences the global flame shape and speed. Based on this analysis, a "turbulence-induced DL cutoff" as a function of turbulence and instability parameters is introduced, which when combined with a turbulent flame speed without DL instability yields the turbulent flame speed accounting for the instability. The consumption turbulent flame speed for no DL instability is formulated from the spectral closure of the G equation, thus accounting for the scale-dependent "turbulent" nature of the problem. Finally, an analytical form of the turbulent flame speed is derived, which is found to agree well with the corresponding experimentally measured turbulent flame speed from literature over wide ranges of normalized turbulence intensities and length scales.
The effect of background turbulence on the propagation of large-scale flames
NASA Astrophysics Data System (ADS)
Matalon, Moshe
2008-12-01
This paper is based on an invited presentation at the Conference on Turbulent Mixing and Beyond held in the Abdus Salam International Center for Theoretical Physics, Trieste, Italy (August 2007). It consists of a summary of recent investigations aimed at understanding the nature and consequences of the Darrieus-Landau instability that is prominent in premixed combustion. It describes rigorous asymptotic methodologies used to simplify the propagation problem of multi-dimensional and time-dependent premixed flames in order to understand the nonlinear evolution of hydrodynamically unstable flames. In particular, it addresses the effect of background turbulent noise on the structure and propagation of large-scale flames.
Preparation of feed premix for veterinary purposes.
Franc, Aleš; Lehocký, Róbert; Muselík, Jan; Vetchý, David; Dobšíková, Radka; Modrá, Helena
2014-10-01
This experimental study describes the preparation of a veterinary medicated premix containing tetracycline hydrochloride for oral administration to aquatic animals. For the manufacture of the premix, commercially produced animal feed is used, which is intended for consumption in the form of pellets that were coated with a mixture of chlortetracycline hydrochloride and other excipients. Feed pellets were combined with a mixture of an active substance and excipients with a large specific surface (colloidal silica - Aerosil® 200) allowing an easy adhesion to the surface of the pellets, and a solid polymer with a low glass transition point (Eudragit® E) which ensures the formation of a hard coat. A mixture of these substances has been applied to the surface of the pellets either A) in the solid state simply by dry adhesion; B) by coating the pellets with the mixture and additional impregnation with ethanol; or C) the polymer was subsequently applied in solution. In the final stage, the pellets were heated in order to achieve the glass transition point of the polymer to create a solid and mechanically resistant coating. Coated pellets prepared by three methods described above are almost identical in their physical properties. With this technology it is possible to produce a feed mixture with a very low content of the active substance in situ without the need for a complex technological equipment.
A model for premixed combustion oscillations
Janus, M.C.; Richards, G.A.
1996-03-01
Combustion oscillations are receiving renewed research interest due to increasing application of lean premix (LPM) combustion to gas turbines. A simple, nonlinear model for premixed combustion is described; it was developed to explain experimental results and to provide guidance for developing active control schemes based on nonlinear concepts. The model can be used to quickly examine instability trends associated with changes in equivalence ratio, mass flow rate, geometry, ambient conditions, etc. The model represents the relevant processes occurring in a fuel nozzle and combustor analogous to current LPM turbine combustors. Conservation equations for the nozzle and combustor are developed from simple control volume analysis, providing ordinary differential equations that can be solved on a PC. Combustion is modeled as a stirred reactor, with bimolecular reaction between fuel and air. Although focus is on the model, it and experimental results are compared to understand effects of inlet air temperature and open loop control schemes. The model shows that both are related to changes in transport time.
NASA Astrophysics Data System (ADS)
Yu, R.; Lipatnikov, A. N.; Bai, X. S.
2014-08-01
In order to gain further insight into (i) the use of conditioned quantities for characterizing turbulence within a premixed flame brush and (ii) the influence of front propagation on turbulent scalar transport, a 3D Direct Numerical Simulation (DNS) study of an infinitely thin front that self-propagates in statistically stationary, homogeneous, isotropic, forced turbulence was performed by numerically integrating Navier-Stokes and level set equations. While this study was motivated by issues relevant to premixed combustion, the density was assumed to be constant in order (i) to avoid the influence of the front on the flow and, therefore, to know the true turbulence characteristics as reference quantities for assessment of conditioned moments and (ii) to separate the influence of front propagation on turbulent transport from the influence of pressure gradient induced by heat release. Numerical simulations were performed for two turbulence Reynolds numbers (50 and 100) and four ratios (1, 2, 5, and 10) of the rms turbulent velocity to the front speed. Obtained results show that, first, the mean front thickness is decreased when a ratio of the rms turbulent velocity to the front speed is decreased. Second, although the gradient diffusion closure yields the right direction of turbulent scalar flux obtained in the DNS, the diffusion coefficient Dt determined using the DNS data depends on the mean progress variable. Moreover, Dt is decreased when the front speed is increased, thus, indicating that the front propagation affects turbulent scalar transport even in a constant-density case. Third, conditioned moments of the velocity field differ from counterpart mean moments, thus, disputing the use of conditioned velocity moments for characterizing turbulence when modeling premixed turbulent combustion. Fourth, computed conditioned enstrophies are close to the mean enstrophy in all studied cases, thus, suggesting the use of conditioned enstrophy for characterizing turbulence
Direct numerical simulation of turbulent reacting flows
Chen, J.H.
1993-12-01
The development of turbulent combustion models that reflect some of the most important characteristics of turbulent reacting flows requires knowledge about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between the turbulence and the chemistry is so strong in certain regimes that is is very difficult to isolate the role played by one individual phenomenon. Direct numerical simulation (DNS) is an extremely useful tool to study in detail the turbulence-chemistry interactions in certain well defined regimes. Globally, non-premixed flames are controlled by two limiting cases: the fast chemistry limit, where the turbulent fluctuations. In between these two limits, finite-rate chemical effects are important and the turbulence interacts strongly with the chemical processes. This regime is important because industrial burners operate in regimes in which, locally the flame undergoes extinction, or is at least in some nonequilibrium condition. Furthermore, these nonequilibrium conditions strongly influence the production of pollutants. To quantify the finite-rate chemistry effect, direct numerical simulations are performed to study the interaction between an initially laminar non-premixed flame and a three-dimensional field of homogeneous isotropic decaying turbulence. Emphasis is placed on the dynamics of extinction and on transient effects on the fine scale mixing process. Differential molecular diffusion among species is also examined with this approach, both for nonreacting and reacting situations. To address the problem of large-scale mixing and to examine the effects of mean shear, efforts are underway to perform large eddy simulations of round three-dimensional jets.
Direct spectral/hp element simulation of piloted jet non-premixed flames
NASA Astrophysics Data System (ADS)
Nastase, Cristian R.
2004-11-01
The spectral/hp element method is used for direct numerical simulation (DNS) of piloted non premixed methane jet flames. This method combines the accuracy of spectral methods with versatility of finite element methods, and allows accurate simulations of complex flows on structured and unstructured grids. Here, the methodology is extended for simulation of multi-species, reactive flows using the discontinuous Galerkin formulation. Parallel computation is performed via MPI standards coupled with a domain decomposition methodology. The overall computational scheme allows for an efficient partitioning of the flow configuration. Tests performed with up to 64 processors show quasi-linear parallel performance and scalability. The flame configurations are similar to the piloted jet non-premixed flame considered at the Combustion Research Facility at the Sandia National Laboratories. For a momentum dominated flame, the simulated results portray many of the features observed experimentally. This pertains to both the spatial and the compositional structures of the flow. For a buoyancy controlled flame (at elevated gravity levels), the results indicate an increase in both the turbulence levels and flow acceleration. Departure from equilibrium, including localized extinction is observed on a significant portion of this flame.
Premixed flames in closed cylindrical tubes
NASA Astrophysics Data System (ADS)
Metzener, Philippe; Matalon, Moshe
2001-09-01
We consider the propagation of a premixed flame, as a two-dimensional sheet separating unburned gas from burned products, in a closed cylindrical tube. A nonlinear evolution equation, that describes the motion of the flame front as a function of its mean position, is derived. The equation contains a destabilizing term that results from the gas motion induced by thermal expansion and has a memory term associated with vorticity generation. Numerical solutions of this equation indicate that, when diffusion is stabilizing, the flame evolves into a non-planar form whose shape, and its associated symmetry properties, are determined by the Markstein parameter, and by the initial data. In particular, we observe the development of convex axisymmetric or non-axisymmetric flames, tulip flames and cellular flames.
The structure of particle cloud premixed flames
NASA Technical Reports Server (NTRS)
Seshadri, K.
1993-01-01
The aim of this study is to provide a numerical and asymptotic description of the structure of planar laminar flames, propagating in a medium containing a uniform cloud of fuel-particles premixed with air. Attention is restricted here to systems where the fuel-particles first vaporize to form a known gaseous fuel, which is then oxidized in the gas-phase. This program is supported for the period September 14, 1991 to September 13, 1992. Some results of the study is shown in Ref. 1. The work summarized in Ref. 1 was initiated prior to September 14, 1991 and was completed on February 1992. Research performed in addition to that described in Ref. 1 in collaboration with Professor A. Linan, is summarized here.
The premixed flame in uniform straining flow
NASA Technical Reports Server (NTRS)
Durbin, P. A.
1982-01-01
Characteristics of the premixed flame in uniform straining flow are investigated by the technique of activation-energy asymptotics. An inverse method is used, which avoids some of the restrictions of previous analyses. It is shown that this method recovers known results for adiabatic flames. New results for flames with heat loss are obtained, and it is shown that, in the presence of finite heat loss, straining can extinguish flames. A stability analysis shows that straining can suppress the cellular instability of flames with Lewis number less than unity. Strain can produce instability of flames with Lewis number greater than unity. A comparison shows quite good agreement between theoretical deductions and experimental observations of Ishizuka, Miyasaka & Law (1981).
Implementation of Premixed Equilibrium Chemistry Capability in OVERFLOW
NASA Technical Reports Server (NTRS)
Olsen, Mike E.; Liu, Yen; Vinokur, M.; Olsen, Tom
2004-01-01
An implementation of premixed equilibrium chemistry has been completed for the OVERFLOW code, a chimera capable, complex geometry flow code widely used to predict transonic flowfields. The implementation builds on the computational efficiency and geometric generality of the solver.
Implementation of Premixed Equilibrium Chemistry Capability in OVERFLOW
NASA Technical Reports Server (NTRS)
Olsen, M. E.; Liu, Y.; Vinokur, M.; Olsen, T.
2003-01-01
An implementation of premixed equilibrium chemistry has been completed for the OVERFLOW code, a chimera capable, complex geometry flow code widely used to predict transonic flowfields. The implementation builds on the computational efficiency and geometric generality of the solver.
Mechanisms of combustion limits in premixed gas flames at microgravity
NASA Technical Reports Server (NTRS)
Ronney, Paul D.
1991-01-01
A three-year experimental and theoretical research program on the mechanisms of combustion limits of premixed gasflames at microgravity was conducted. Progress during this program is identified and avenues for future studies are discussed.
Transient response of premixed methane flames
Vagelopoulos, Christina M.; Frank, Jonathan H.
2006-08-15
The response of premixed methane-air flames to transient strain and local variations in equivalence ratio is studied during isolated interactions between a line-vortex pair and a V-flame. The temporal evolution of OH and CH is measured with planar laser-induced fluorescence for N{sub 2}-diluted flames with equivalence ratios ranging from 0.8 to 1.2. One-dimensional laminar flame calculations are used to simulate the flame response to unsteady strain and variations in reactant composition. When the reactant composition of the vortex pair and the V-flame are identical, the measurements and predictions show that the peak mole fractions of OH and CH decay monotonically in lean, stoichiometric, and rich flames. We also investigate the effects of a vortex pair with a leaner composition than the V-flame. In a stoichiometric flame, the leaner vortex enhances the decay of both OH and CH. In a rich flame, we observe an abrupt increase in OH-LIF signal and a disappearance of CH-LIF signal that are consistent with a previous experimental investigation. Our results indicate that the previously observed OH burst and CH breakage were caused by a difference in the equivalence ratios of the vortex pair and the main reactant flow. A numerical study shows that N{sub 2} dilution enhances the response of premixed flames to unsteady strain and variations in stoichiometry. Reaction-path and sensitivity analyses indicate that the peak OH and CH mole fractions exhibit significant sensitivity to the main branching reaction, H+O{sub 2} {r_reversible}OH+O. The sensitivity of OH and CH to this and other reactions is enhanced by N{sub 2} dilution. As a result, N{sub 2}-diluted flames provide a good test case for studying the reliability of chemical kinetic and transport models. (author)
Flames in fractal grid generated turbulence
NASA Astrophysics Data System (ADS)
Goh, K. H. H.; Geipel, P.; Hampp, F.; Lindstedt, R. P.
2013-12-01
Twin premixed turbulent opposed jet flames were stabilized for lean mixtures of air with methane and propane in fractal grid generated turbulence. A density segregation method was applied alongside particle image velocimetry to obtain velocity and scalar statistics. It is shown that the current fractal grids increase the turbulence levels by around a factor of 2. Proper orthogonal decomposition (POD) was applied to show that the fractal grids produce slightly larger turbulent structures that decay at a slower rate as compared to conventional perforated plates. Conditional POD (CPOD) was also implemented using the density segregation technique and the results show that CPOD is essential to segregate the relative structures and turbulent kinetic energy distributions in each stream. The Kolmogorov length scales were also estimated providing values ∼0.1 and ∼0.5 mm in the reactants and products, respectively. Resolved profiles of flame surface density indicate that a thin flame assumption leading to bimodal statistics is not perfectly valid under the current conditions and it is expected that the data obtained will be of significant value to the development of computational methods that can provide information on the conditional structure of turbulence. It is concluded that the increase in the turbulent Reynolds number is without any negative impact on other parameters and that fractal grids provide a route towards removing the classical problem of a relatively low ratio of turbulent to bulk strain associated with the opposed jet configuration.
Lean Combustion Limits of a Confined Premixed-Prevaporized Propane Jet
NASA Technical Reports Server (NTRS)
Huck, K. L.; Marek, C. J.
1978-01-01
Lean blowout limits were reported for a premixed prevaporized propane jet issuing into a cylindrical combustor. A single hole in a flat plate was used as a flameholder. Flameholders with various hole diameters were used. Jet velocities were varied from 3 to 290 meters per second. The combustor cross sectional area was changed by using different quartz liners of 12.7 and 22.2 millimeters diameters. As a result the combustor Reynolds number varied from 1000 to 9000. Stability was achieved at laminar as well as turbulent conditions. Three zones of flame stability were observed. The blowout equivalence ratio varied with step size and the combustor and jet Reynolds numbers. The combustor inlet mixture temperature was 395 K, and the combustor pressure was 1 atmosphere.
Premixes production for synthesis of wear-resistant composite materials
NASA Astrophysics Data System (ADS)
Kontsevoi, Yu V.; Meilakh, A. G.; Shubin, A. B.; Pastukhov, E. A.; Dolmatov, A. V.; Sipatov, I. S.
2017-01-01
State of the art line of powder metallurgy is application of initial powders as micro-composites with additional components - premixes. Usage of premixes inhibits segregation of added components and implies the homogeneity of powder charge composition, and finally it has a significant impact on structure formation and properties of end products. The aim of the present work was to design the new production technology of premixes based on iron powder which is layer-by-layer plated by aluminium and copper. We propose to carry out production of Cu-Al-Fe premixes in two stages: cladding of iron powder by aluminium and coating of the obtained composite by copper. The self-developed technique of vibration treatment of iron and aluminium powder mixture was chosen for this purpose. The uniform in thickness and unbroken copper-plating of Fe-Al powders were carried out by chemical technique. Physico-chemical properties and production conditions of premixes-powders were studied, besides optimal parameters of production and further heat-treatment were selected. In the result of the present study the Fe-Al-Cu premixes with laminated structure comprising of iron core, Fe-Al and Cu-Al intermetallide shells were synthesised.
Fully Premixed Low Emission, High Pressure Multi-Fuel Burner
NASA Technical Reports Server (NTRS)
Nguyen, Quang-Viet (Inventor)
2012-01-01
A low-emissions high-pressure multi-fuel burner includes a fuel inlet, for receiving a fuel, an oxidizer inlet, for receiving an oxidizer gas, an injector plate, having a plurality of nozzles that are aligned with premix face of the injector plate, the plurality of nozzles in communication with the fuel and oxidizer inlets and each nozzle providing flow for one of the fuel and the oxidizer gas and an impingement-cooled face, parallel to the premix face of the injector plate and forming a micro-premix chamber between the impingement-cooled face and the in injector face. The fuel and the oxidizer gas are mixed in the micro-premix chamber through impingement-enhanced mixing of flows of the fuel and the oxidizer gas. The burner can be used for low-emissions fuel-lean fully-premixed, or fuel-rich fully-premixed hydrogen-air combustion, or for combustion with other gases such as methane or other hydrocarbons, or even liquid fuels.
Pre-mixed flame simulations for non-unity Lewis numbers
NASA Technical Reports Server (NTRS)
Rutland, C. J.; Trouve, A.
1990-01-01
A principal effect of turbulence on premixed flames in the flamelet region is to wrinkle the flame fronts. For non-unity Lewis numbers (Le), the local flame structure is altered in curved regions. This effect is examined using direct numerical simulations of the three dimensional, constant density, decaying isotropic turbulence with a single step, finite rate chemical reaction. Simulations of Lewis numbers 0.8, 1.0, and 1.2 are compared. The turbulent flame speed, S(sub T), increases as Le decreases. The correlation between S(sub T) and u prime found in previous Le = 1 simulations has a strong Lewis number dependency. The variance of the pdf of the flame curvature increases as Le decreases, indicating that the flames become more wrinkled. A strong correlation between local flame speed and curvature was found. For Le greater than 1, the flame speed increases in regions concave towards the products and decreases in convex regions. The opposite correlation was found for Le less than 1. The mean temperature of the products was also found to vary with Lewis number. For Le = 0.8, it is less than the adiabatic flame temperature and for Le = 1.2 it is greater.
NASA Technical Reports Server (NTRS)
Cooper, L. P.
1979-01-01
An experimental and analytical study of the combustion of partially vaporized fuelair mixtures was performed to assess the impact of the degree of fuel vaporization upon emissions for a premixing-prevaporizing flametube combustor. Data collected show near linear increases in nitrogen oxide emissions with decreasing vaporization at equivalence ratios of 0.6. For equivalence ratio of 0.72, the degree of vaporization had very little impact on nitrogen oxide emissions. A simple mechanism which accounts for the combustion of liquid droplets in partially vaporized mixtures was found to agree with the measured results with fair accuracy with respect to both trends and magnitudes.
Premixer Design for High Hydrogen Fuels
Benjamin P. Lacy; Keith R. McManus; Balachandar Varatharajan; Biswadip Shome
2005-12-16
This 21-month project translated DLN technology to the unique properties of high hydrogen content IGCC fuels, and yielded designs in preparation for a future testing and validation phase. Fundamental flame characterization, mixing, and flame property measurement experiments were conducted to tailor computational design tools and criteria to create a framework for predicting nozzle operability (e.g., flame stabilization, emissions, resistance to flashback/flame-holding and auto-ignition). This framework was then used to establish, rank, and evaluate potential solutions to the operability challenges of IGCC combustion. The leading contenders were studied and developed with the most promising concepts evaluated via computational fluid dynamics (CFD) modeling and using the design rules generated by the fundamental experiments, as well as using GE's combustion design tools and practices. Finally, the project scoped the necessary steps required to carry the design through mechanical and durability review, testing, and validation, towards full demonstration of this revolutionary technology. This project was carried out in three linked tasks with the following results. (1) Develop conceptual designs of premixer and down-select the promising options. This task defined the ''gap'' between existing design capabilities and the targeted range of IGCC fuel compositions and evaluated the current capability of DLN pre-mixer designs when operated at similar conditions. Two concepts (1) swirl based and (2) multiple point lean direct injection based premixers were selected via a QFD from 13 potential design concepts. (2) Carry out CFD on chosen options (1 or 2) to evaluate operability risks. This task developed the leading options down-selected in Task 1. Both a GE15 swozzle based premixer and a lean direct injection concept were examined by performing a detailed CFD study wherein the aerodynamics of the design, together with the chemical kinetics of the combustion process, were
Numerical simulation of turbulent combustion: Scientific challenges
NASA Astrophysics Data System (ADS)
Ren, ZhuYin; Lu, Zhen; Hou, LingYun; Lu, LiuYan
2014-08-01
Predictive simulation of engine combustion is key to understanding the underlying complicated physicochemical processes, improving engine performance, and reducing pollutant emissions. Critical issues as turbulence modeling, turbulence-chemistry interaction, and accommodation of detailed chemical kinetics in complex flows remain challenging and essential for high-fidelity combustion simulation. This paper reviews the current status of the state-of-the-art large eddy simulation (LES)/prob-ability density function (PDF)/detailed chemistry approach that can address the three challenging modelling issues. PDF as a subgrid model for LES is formulated and the hybrid mesh-particle method for LES/PDF simulations is described. Then the development need in micro-mixing models for the PDF simulations of turbulent premixed combustion is identified. Finally the different acceleration methods for detailed chemistry are reviewed and a combined strategy is proposed for further development.
Flame structure and chemiluminescence in premixed flames
NASA Astrophysics Data System (ADS)
Grana-Otero, Jose; Mahmoudi, Siamak
2016-11-01
The quantitative use of chemiluminescence requires the knowledge of the relationship between the concentration of excited species with flame properties such as the equivalency ratio, the burning rate or the heat release rate. With the aim of rigorously finding from first principles these relations we have analyzed, numerically and analytically, the distribution of the excited species OH* and CH* in steady hydrogen and methane planar premixed flames. Their mass fractions turn out to be extremely small; thus, a kinetic mechanism describing their dynamics in the flame can be obtained by simply adding the kinetic mechanism describing the excitation and de-excitation to the mechanism of the base flame. Due also to their small concentrations, the excited species are in steady state, facilitating a simple analytical description. The analyses show that OH*, both in hydrogen and methane flames, can be found broadly distributed downstream the preheat region, in a three-layer structure that is analytically described. The distribution of CH* is much simpler, being always in equilibrium with CH, whose concentration is in turn proportional to that of CH4. As a result, CH* is confined to the methane consumption layer in lean flames, but broadly distributed in rich flames.
The structure of particle cloud premixed flames
NASA Technical Reports Server (NTRS)
Seshadri, K.; Berlad, A. L.
1992-01-01
The structure of premixed flames propagating in combustible systems containing uniformly distributed volatile fuel particles in an oxidizing gas mixture is analyzed. This analysis is motivated by experiments conducted at NASA Lewis Research Center on the structure of flames propagating in combustible mixtures of lycopodium particles and air. Several interesting modes of flame propagation were observed in these experiments depending on the number density and the initial size of the fuel particle. The experimental results show that steady flame propagation occurs even if the initial equivalence ratio of the combustible mixture based on the gaseous fuel available in the particles, phi sub u, is substantially larger than unity. A model is developed to explain these experimental observations. In the model, it is presumed that the fuel particles vaporize first to yield a gaseous fuel of known chemical composition which then reacts with oxygen in a one-step overall process. The activation energy of the chemical reaction is presumed to be large. The activation energy characterizing the kinetics of vaporization is also presumed to be large. The equations governing the structure of the flame were integrated numerically. It is shown that the interplay of vaporization kinetics and oxidation process can result in steady flame propagation in combustible mixtures where the value of phi sub u is substantially larger than unity. This prediction is in agreement with experimental observations.
Aerodynamic properties of turbulent combustion fields
NASA Technical Reports Server (NTRS)
Hsiao, C. C.; Oppenheim, A. K.
1985-01-01
Flow fields involving turbulent flames in premixed gases under a variety of conditions are modeled by the use of a numerical technique based on the random vortex method to solve the Navier-Stokes equations and a flame propagation algorithm to trace the motion of the front and implement the Huygens principle, both due to Chorin. A successive over-relaxation hybrid method is applied to solve the Euler equation for flows in an arbitrarily shaped domain. The method of images, conformal transformation, and the integral-equation technique are also used to treat flows in special cases, according to their particular requirements. Salient features of turbulent flame propagation in premixed gases are interpreted by relating them to the aerodynamic properties of the flow field. Included among them is the well-known cellular structure of flames stabilized by bluff bodies, as well as the formation of the characteristic tulip shape of flames propagating in ducts. In its rudimentary form, the mechanism of propagation of a turbulent flame is shown to consist of: (1) rotary motion of eddies at the flame front, (2) self-advancement of the front at an appropriate normal burning speed, and (3) dynamic effects of expansion due to exothermicity of the combustion reaction. An idealized model is used to illustrate these fundamental mechanisms and to investigate basic aerodynamic features of flames in premixed gases. The case of a confined flame stabilized behind a rearward-facing step is given particular care and attention. Solutions are shown to be in satisfactory agreement with experimental results, especially with respect to global properties such as the average velocity profiles and reattachment length.
21 CFR 170.60 - Nitrites and/or nitrates in curing premixes.
Code of Federal Regulations, 2014 CFR
2014-04-01
... 21 Food and Drugs 3 2014-04-01 2014-04-01 false Nitrites and/or nitrates in curing premixes. 170... nitrates in curing premixes. (a) Nitrites and/or nitrates are food additives when combined in curing... nitrates, when packaged separately from flavoring and seasoning in curing premixes, may continue to be...
Annular fuel and air co-flow premixer
Stevenson, Christian Xavier; Melton, Patrick Benedict; York, William David
2013-10-15
Disclosed is a premixer for a combustor including an annular outer shell and an annular inner shell. The inner shell defines an inner flow channel inside of the inner shell and is located to define an outer flow channel between the outer shell and the inner shell. A fuel discharge annulus is located between the outer flow channel and the inner flow channel and is configured to inject a fuel flow into a mixing area in a direction substantially parallel to an outer airflow through the outer flow channel and an inner flow through the inner flow channel. Further disclosed are a combustor including a plurality of premixers and a method of premixing air and fuel in a combustor.
Flame Propagation in Low-Intensity Turbulence under Microgravity Conditions
NASA Technical Reports Server (NTRS)
Aldredge, R. C.
2001-01-01
The goal of the research is to understand the influences of the hydrodynamic instability on premixed-flame propagation. It is known that coupling between flame and flow-field dynamics in association with the hydrodynamic instability may lead to flame-generated turbulence, flame acceleration and enhancement of burning rates. As a result of such hydrodynamic coupling the transition from initially planar or wrinkled laminar flames to fast turbulent flames or detonations is possible, even when diffusive-thermal effects associated with non-unity reactant Lewis numbers are not destabilizing. It is important to identify methods of suppressing the hydrodynamic instability so as to insure fire safety, particularly in space.
NASA Technical Reports Server (NTRS)
Ibarreta, Alfonso F.; Driscoll, James F.; Feikema, Douglas A.; Salzman, Jack (Technical Monitor)
2001-01-01
The effect of flame stretch, composed of strain and curvature, plays a major role in the propagation of turbulent premixed flames. Although all forms of stretch (positive and negative) are present in turbulent conditions, little research has been focused on the stretch due to curvature. The present study quantifies the Markstein number (which characterizes the sensitivity of the flame propagation speed to the imposed stretch rate) for an inwardly-propagating flame (IPF). This flame is of interest because it is negatively stretched, and is subjected to curvature effects alone, without the competing effects of strain. In an extension of our previous work, microgravity experiments were run using a vortex-flame interaction to create a pocket of reactants surrounded by an IPF. Computations using the RUN-1DL code of Rogg were also performed in order to explain the measurements. It was found that the Markstein number of an inwardly-propagating flame, for both the microgravity experiment and the computations, is significantly larger than that of an outwardly-propagating flame. Further insight was gained by running the computations for the simplified (hypothetical) cases of one step chemistry, unity Lewis number, and negligible heat release. Results provide additional evidence that the Markstein numbers associated with strain and curvature have different values.
Analytical Modeling of Operating Characteristics of Premixing-prevaporizing Fuel-air Mixing Passages
NASA Technical Reports Server (NTRS)
Anderson, O. L.
1983-01-01
A model for predicting the distribution of liquid fuel droplets and fuel vapor in premixing-prevaporizing fuel-air mixing passages of the direct injection type is described. This model consists of three computer programs: a calculation of the two dimensional or axisymmetric air flow field neglecting the effects of fuel; a calculation of the three dimensional fuel droplet trajectories and evaporation rates in a known, moving air flow; and a calculation of fuel vapor diffusing into a moving three dimensional air flow with source terms dependent on the droplet evaporation rates. The air flow calculation can treat compressible swirling flows in arbitrary ducts with arbitrary distributions of temperature and velocity as initial conditions. The fuel droplets are treated as initial conditions. The fuel droplets are treated as individual particle classes each satisfying Newton's law, a heat transfer, and a mass transfer equation. The vapor diffusion calculation treats three dimensional, gas phase, turbulent diffusion processes with the turbulence level determined by the air flow calculations and the source terms determined by the droplet evaporation rates.
Vaporization of droplets in premixing chambers
NASA Technical Reports Server (NTRS)
Yule, A. J.; Chigier, N. A.
1980-01-01
Detailed measurements were made of the structures of turbulent fuel sprays vaporizing in heated airstreams. The measurements show the size dependent vaporization and dispersion of the droplets and the important influence of the large eddies in the turbulence. The measurements form a data base for the development of models of fuel spray vaporization. Two laser techniques were specially developed for the investigation. A laser tomography technique converts line-of-sight light scattering measurements into time averaged 'point' measurements of droplet size distribution and volume concentration. A laser anemometer particle sizing technique was further developed to permit accurate measurements of individual particle sizes and velocities, with backscatter collection of light. The experiments are combined with heat transfer models to analyze the performance of miniature thermocouples in liquid sprays.
NASA Technical Reports Server (NTRS)
Chandler, C. L.
1987-01-01
In order to forecast turbulence, one needs to have an understanding of the cause of turbulence. Therefore, an attempt is made to show the atmospheric structure that often results when aircraft encounter moderate or greater turbulence. The analysis is based on thousands of hours of observations of flights over the past 39 years of aviation meteorology.
Towards understanding turbulent scalar mixing
NASA Technical Reports Server (NTRS)
Girimaji, Sharath S.
1992-01-01
In an effort towards understanding turbulent scalar mixing, we study the effect of molecular mixing, first in isolation and then by accounting for the effects of the velocity field. The chief motivation for this approach stems from the strong resemblance of the scalar probability density function (PDF) obtained from the scalar field evolving from the heat conduction equation that arises in a turbulent velocity field. However, the evolution of the scalar dissipation is different for the two cases. We attempt to account for these differences, which are due to the velocity field, using a Lagrangian frame analysis. After establishing the usefulness of this approach, we use the heat-conduction simulations (HCS), in lieu of the more expensive direct numerical simulations (DNS), to study many of the less understood aspects of turbulent mixing. Comparison between the HCS data and available models are made whenever possible. It is established that the beta PDF characterizes the evolution of the scalar PDF during mixing from all types of non-premixed initial conditions.
NASA Astrophysics Data System (ADS)
Umeh, Chukwueloka O. U.
Increasingly stringent emissions regulations on fossil fuel-burning combustors in land-based and aviation gas turbine engines has led engine manufacturers to turn to lean burning premixed combustors. In modern lean premixed gas turbine combustors, flame stabilization is achieved by the use of a high level of inlet swirl and an expansion into a larger chamber. This predisposes the inlet vortex flow to transition to a vortex breakdown state, which is characterized by a stagnation and recirculation zone near the exit plane of the inlet region. A compact, turbulent and highly mixed flame results. Although this breakdown phenomenon helps reduce emissions drastically compared to rich-burn engines, it also contributes to the formation of thermo-acoustic instabilities (or combustion dynamics), flashback and lean blow out, which hinder performance and in extreme cases, causes very expensive damage to the combustion system. Theoretical, computational and experimental studies of vortex breakdown in a finite length, axisymmetric chamber with a swirling inlet flow demonstrate that critical conditions for the first appearance of breakdown in the combustion chamber as well as in the combustor's inlet region govern the flow's behavior. These critical conditions are satisfied for both the average and the instantaneous behavior of the flow for ambient temperature, preheated and lean premixed reacting flows. Good agreement is found between the theoretical and numerical simulations, as well as the experimental results. Results show that these critical conditions, as well as the appearance, location, size and stability of the breakdown are affected by inlet airflow rate, inlet air temperature, flame temperature (in reacting flow), inlet tube length, dump plane configuration and chamber air leakage. In the presence of combustion, experimental results show that the existence of a breakdown zone, its shape, location and stability influence flame stabilization, combustion dynamics
The multispecies modeling of the premixed, laminar steady-state ozone flame
NASA Technical Reports Server (NTRS)
Heimerl, J. M.; Coffee, T. P.
1980-01-01
Species dependent kinetic, transport and thermodynamic coefficients were employed in a one dimensional model of the premixed, laminar, steady state ozone flame. Convenient expressions for these coefficients are reported. They are based on independent measurements, no arbitrary parameters are used. The governing equations are solved using a relaxation technique and the partial differential equation package, PDECOL. Species and temperature profiles and the burning velocities are found over the range of initial ozone mole fraction of 0.25 to 1.00. The computed burning velocities are no more than 30% greater than the measurements of Streng and Grosses. Comparison with the computed results of Warnatz shows agreement within + or - 12%, even though quite different expressions for some of the kinetic coefficients were used. These differences are most obvious in the atomic oxygen and temperature profiles at an initial ozone mole fraction of unity.
Premixing and flash vaporization in a two-stage combustor
Sjoblom, B.G.A.
1982-01-01
A double recirculation zone two-stage combustor fitted with airblast atomizers has been investigated in a previous work. This paper describes further tests with premixing tubes in the secondary combustion zone. Flash vaporization was employed to ensure complete vaporization of the secondary fuel, which was heated to 600K by the combustor inlet air. 9 refs.
NO{sub x}-abatement potential of lean-premixed GT combustors
Sattelmayer, T.; Polifke, W.; Winkler, D.; Doebbeling, K.
1998-01-01
The influence of the structure of perfectly premixed flames on NO{sub x} formation is investigated theoretically. Since a network of reaction kinetics modules and model flames is used for this purpose, the results obtained are independent of specific burner geometries. Calculations are presented for a mixture temperature of 630 K, an adiabatic flame temperature of 1840 K, and 1 and 15 bars combustor pressure. In particular, the following effects are studied separately from each other: molecular diffusion of temperature and species, flame strain, local quench in highly strained flames and subsequent reignition, turbulent diffusion (no preferential diffusion), and small scale mixing (stirring) in the flame front. Either no relevant influence or an increase in NO{sub x} burners is to avoid excessive turbulent stirring in the flame front. Turbulent flames that exhibit locally and instantaneously near laminar structures (flamelets) appear to be optimal. Using the same methodology, the scope of the investigation is extended to lean-lean staging, since a higher NO{sub x}-abatement potential can be expected in principle. As long as the chemical reactions of the second stage take place in the boundary between the fresh mixture of the second stage and the combustion products from upstream, no advantage can be expected from lean-lean staging. Only if the preliminary burner exhibits much poorer mixing than the second stage can lean-lean staging be beneficial. In contrast, if full mixing between the two stages prior to afterburning can be achieved (lean-mix-lean technique), the combustor outlet temperature can in principle be increased somewhat without NO penalty.
Analysis of homogeneous turbulent reacting flows
NASA Technical Reports Server (NTRS)
Leonard, A. D.; Hill, J. C.; Mahalingam, S.; Ferziger, J. H.
1988-01-01
Full turbulence simulations at low Reynolds numbers were made for the single-step, irreversible, bimolecular reaction between non-premixed reactants in isochoric, decaying homogeneous turbulence. Various initial conditions for the scalar field were used in the simulations to control the initial scalar dissipation length scale, and simulations were also made for temperature-dependent reaction rates and for non-stoichiometric and unequal diffusivity conditions. Joint probability density functions (pdf's), conditional pdf's, and various statistical quantities appearing in the moment equations were computed. Preliminary analysis of the results indicates that compressive strain-rate correlates better than other dynamical quantities with local reaction rate, and the locations of peak reaction rates seem to be insensitive to the scalar field initial conditions.
Localized flame extinction and re-ignition in turbulent jet ignition assisted combustion
NASA Astrophysics Data System (ADS)
Validi, Abdoulahad; Schock, Harold; Jaberi, Farhad; Computational Fluid Dynamics Laboratory Team
2016-11-01
Direct numerical simulations (DNS) of turbulent jet ignition (TJI)-assisted combustion of ultra-lean fuel-air is performed in a three-dimensional planar jet configuration. TJI is a novel ignition enhancement method which facilitates the combustion of lean and ultra-lean mixtures by rapidly exposing them to high temperature combustion products. Fully compressible gas dynamics and species equations are solved with high order finite difference methods. The hydrogen-air reaction is simulated with a detailed chemical kinetics mechanism consisting of 9 species and 38 elementary reactions. The interesting phenomena involved in TJI combustion including localized premixed flame extinction/re-ignition and simultaneous premixed/non-premixed flames are investigated by using the flame heat release, temperature, species concentrations, and a newly defined TJI progress variable.
Di Sarli, Valeria; Di Benedetto, Almerinda; Russo, Gennaro
2010-08-15
In this work, an assessment of different sub-grid scale (sgs) combustion models proposed for large eddy simulation (LES) of steady turbulent premixed combustion (Colin et al., Phys. Fluids 12 (2000) 1843-1863; Flohr and Pitsch, Proc. CTR Summer Program, 2000, pp. 61-82; Kim and Menon, Combust. Sci. Technol. 160 (2000) 119-150; Charlette et al., Combust. Flame 131 (2002) 159-180; Pitsch and Duchamp de Lageneste, Proc. Combust. Inst. 29 (2002) 2001-2008) was performed to identify the model that best predicts unsteady flame propagation in gas explosions. Numerical results were compared to the experimental data by Patel et al. (Proc. Combust. Inst. 29 (2002) 1849-1854) for premixed deflagrating flame in a vented chamber in the presence of three sequential obstacles. It is found that all sgs combustion models are able to reproduce qualitatively the experiment in terms of step of flame acceleration and deceleration around each obstacle, and shape of the propagating flame. Without adjusting any constants and parameters, the sgs model by Charlette et al. also provides satisfactory quantitative predictions for flame speed and pressure peak. Conversely, the sgs combustion models other than Charlette et al. give correct predictions only after an ad hoc tuning of constants and parameters.
Turbulent transport measurements in a model of GT-combustor
NASA Astrophysics Data System (ADS)
Chikishev, L. M.; Gobyzov, O. A.; Sharaborin, D. K.; Lobasov, A. S.; Dulin, V. M.; Markovich, D. M.; Tsatiashvili, V. V.
2016-10-01
To reduce NOx formation modern industrial power gas-turbines utilizes lean premixed combustion of natural gas. The uniform distribution of local fuel/air ratio in the combustion chamber plays one of the key roles in the field of lean combustion to prevent thermo-acoustic pulsations. Present paper reports on simultaneous Particle Image Velocimetry and acetone Planar Laser Induced Fluorescence measurements in a cold model of GT-combustor to investigate mixing processes which are relevant to the organization of lean premixed combustion. Velocity and passive admixture pulsations correlations were measured to verify gradient closer model, which is often used in Reynolds-Averaged Navier-Stokes (RANS) simulation of turbulent mixing.
Experimental analysis of an oblique turbulent flame front propagating in a stratified flow
Galizzi, C.; Escudie, D.
2010-12-15
This paper details the experimental study of a turbulent V-shaped flame expanding in a nonhomogeneous premixed flow. Its aim is to characterize the effects of stratification on turbulent flame characteristics. The setup consists of a stationary V-shaped flame stabilized on a rod and expanding freely in a lean premixed methane-air flow. One of the two oblique fronts interacts with a stratified slice, which has an equivalence ratio close to one and a thickness greater than that of the flame front. Several techniques such as PIV and CH{sup *} chemiluminescence are used to investigate the instantaneous fields, while laser Doppler anemometry and thermocouples are combined with a concentration probe to provide information on the mean fields. First, in order to provide a reference, the homogeneous turbulent case is studied. Next, the stratified turbulent premixed flame is investigated. Results show significant modifications of the whole flame and of the velocity field upstream of the flame front. The analysis of the geometric properties of the stratified flame indicates an increase in flame brush thickness, closely related to the local equivalence ratio. (author)
Staged multi-tube premixing injector
Zuo, Baifang; Khan, Abdul Rafey; York, William David; Ziminsky, Willy Steve
2012-10-02
A fuel injection nozzle includes a body member having an upstream wall opposing a downstream wall, and an internal wall disposed between the upstream wall and the downstream wall, a first chamber partially defined by the an inner surface of the upstream wall and a surface of the internal wall, a second chamber partially defined by an inner surface of the downstream wall and a surface of the internal wall a first gas inlet communicative with the first chamber operative to emit a first gas into the first chamber, a second gas inlet communicative with the second chamber operative to emit a second gas into the second chamber, and a plurality of mixing tubes, each of the mixing tubes having a tube inner surface, a tube outer surface, a first inlet communicative with an aperture in the upstream wall operative to receive a third gas.
Gupta, Kamlesh G.; Echekki, Tarek
2011-02-15
The autoignition of hydrogen/carbon monoxide in a turbulent jet with preheated co-flow air is studied using the one-dimensional turbulence (ODT) model. The simulations are performed at atmospheric pressure based on varying the jet Reynolds number and the oxidizer preheat temperature for two compositions corresponding to varying the ratios of H{sub 2} and CO in the fuel stream. Moreover, simulations for homogeneous autoignition are implemented for similar mixture conditions for comparison with the turbulent jet results. The results identify the key effects of differential diffusion and turbulence on the onset and eventual progress of autoignition in the turbulent jets. The differential diffusion of hydrogen fuels results in a reduction of the ignition delay relative to similar conditions of homogeneous autoignition. Turbulence may play an important role in delaying ignition at high-turbulence conditions, a process countered by the differential diffusion of hydrogen relative to carbon monoxide; however, when ignition is established, turbulence enhances the overall rates of combustion of the non-premixed flame downstream of the ignition point. (author)
Commercial premixed parenteral nutrition: Is it right for your institution?
Miller, Sarah J
2009-01-01
Two-compartment premixed parenteral nutrition (PN) products are heavily promoted in the United States. These products may present safety advantages over PN solutions mixed by a local pharmacy, although clinical data to support this assertion are scarce. Multicompartment products can be labor-saving for pharmacy and therefore may be cost-effective for some institutions. Before adopting such products for use, an institution must determine that standardized PN solutions are acceptable for many or most of their patients compared with customized PN compounded specifically for individual patients. A larger selection of premixed products is available in Europe and some other parts of the world compared with the United States. Availability of a broader selection of products in the United States, including 3-compartment bags and a wider range of macronutrient concentrations and volumes, may make the use of such products more desirable in the future.
Tripathi, Markandey M.; Krishnan, Sundar R.; Srinivasan, Kalyan K.; Yueh, Fang-Yu; Singh, Jagdish P.
2011-09-07
Chemiluminescence emissions from OH*, CH*, C2, and CO2 formed within the reaction zone of premixed flames depend upon the fuel-air equivalence ratio in the burning mixture. In the present paper, a new partial least square regression (PLS-R) based multivariate sensing methodology is investigated and compared with an OH*/CH* intensity ratio-based calibration model for sensing equivalence ratio in atmospheric methane-air premixed flames. Five replications of spectral data at nine different equivalence ratios ranging from 0.73 to 1.48 were used in the calibration of both models. During model development, the PLS-R model was initially validated with the calibration data set using the leave-one-out cross validation technique. Since the PLS-R model used the entire raw spectral intensities, it did not need the nonlinear background subtraction of CO2 emission that is required for typical OH*/CH* intensity ratio calibrations. An unbiased spectral data set (not used in the PLS-R model development), for 28 different equivalence ratio conditions ranging from 0.71 to 1.67, was used to predict equivalence ratios using the PLS-R and the intensity ratio calibration models. It was found that the equivalence ratios predicted with the PLS-R based multivariate calibration model matched the experimentally measured equivalence ratios within 7%; whereas, the OH*/CH* intensity ratio calibration grossly underpredicted equivalence ratios in comparison to measured equivalence ratios, especially under rich conditions ( > 1.2). The practical implications of the chemiluminescence-based multivariate equivalence ratio sensing methodology are also discussed.
Lean, Premixed-Prevaporized (LPP) combustor conceptual design study
NASA Technical Reports Server (NTRS)
Dickman, R. A.; Dodds, W. J.; Ekstedt, E. E.
1979-01-01
Four combustion systems were designed and sized for the energy efficient engine. A fifth combustor was designed for the cycle and envelope of the twin-spool, high bypass ratio, high pressure ratio turbofan engine. Emission levels, combustion performance, life, and reliability assessments were made for these five combustion systems. Results of these design studies indicate that cruise NOx emission can be reduced by the use of lean, premixed-prevaporaized combustion and airflow modulation.
Pulsed jet combustion generator for non-premixed charge engines
Oppenheim, A. K.; Stewart, H. E.
1990-01-01
A device for introducing fuel into the head space of cylinder of non-premixed charge (diesel) engines is disclosed, which distributes fuel in atomized form in a plume, whose fluid dynamic properties are such that the compression heated air in the cylinder head space is entrained into the interior of the plume where it is mixed with and ignites the fuel in the plume interior, to thereby control combustion, particularly by use of a multiplicity of individually controllable devices per cylinder.
Premixed intravenous admixtures: a positive development for hospital pharmacy.
Lee, H E
1983-06-01
The development of premixed intravenous admixtures is reviewed in a historical context, and its effects on hospital pharmacy practice are discussed. As pharmaceutical manufacturers introduce more i.v. medications in ready-to-use containers, the same complaints that were voiced by pharmacists about unit dose packaging and ready-to-dispense tablets and capsules are being aired. But premixed i.v. admixtures are a logical extension of the basic unit dose principle of providing a readily identifiable and ready-to-administer dose. The time and cost savings these products offer are needed in hospital pharmacies. Some of the disadvantages of these products--including storage and freezer space and multiplicity of administration systems--are overcome by proper planning and education of personnel. If fewer personnel are now needed to prepare i.v. admixtures, then those personnel should be used to improve patient care in other ways. The use of premixed i.v. admixtures is a positive technological advance in drug packaging. Its advantages outweight its disadvantages, and it will soon be become the universally accepted form of i.v. drug packaging.
Deur, J.M.; Penko, P.F.; Cline, M.C.
1995-07-01
Requirements to reduce pollutant emissions from gas turbines used in aircraft propulsion and ground-based power generation have led to consideration of lean premixed/prevaporized (LPP) combustion concepts. This paper describes a series of the LPP combustor analyses performed with KIVA-II, a multi-dimensional CFD code for problems involving sprays, turbulence, and combustion. Modifications to KIVA-II`s boundary condition and chemistry treatments have been made to meet the needs of the present study. The study examines the relationships between fuel vaporization, fuel/air mixing, and combustion in a generic LPP combustor. Parameters considered include: mixer tube diameter, mixer tube length, mixer tube configuration (straight versus converging/diverging tubes), air inlet velocity, air inlet swirl angle, secondary air injection (dilution holes), fuel injection velocity, fuel injection angle, number of fuel injection ports, fuel spray cone angle, and fuel droplet size. Cases have been run with and without combustion to examine the variations in fuel/air mixing and potential for flashback due to the above parameters. The degree of fuel/air mixing is judged by comparing average, minimum, and maximum fuel/air ratios at the exit of the mixer tube, while flame stability is monitored by following the location of the flame front as the solution progresses from ignition to steady state.
NASA Technical Reports Server (NTRS)
Reisel, John R.; Laurendeau, Normand M.
1994-01-01
Laser-induced fluorescence (LIF) has been applied to the quantitative measurement of nitric oxide (NO) in premixed, laminar, high-pressure flames. Their chemistry was also studied using three current kinetics schemes to determine the predictive capabilities of each mechanism with respect to NO concentrations. The flames studied were low-temperature (1600 less than T less than 1850K) C2H6/O2/N2 and C2H6/O2/N2 flames, and high temperature (2100 less than T less than 2300K) C2H6/O2/N2 flames. Laser-saturated fluorescence (LSF) was initially used to measure the NO concentrations. However, while the excitation transition was well saturated at atmospheric pressure, the fluorescence behavior was basically linear with respect to laser power at pressures above 6 atm. Measurements and calculations demonstrated that the fluorescence quenching rate variation is negligible for LIF measurements of NO at a given pressure. Therefore, linear LIF was used to perform quantitative measurements of NO concentration in these high-pressure flames. The transportability of a calibration factor from one set of flame conditions to another also was investigated by considering changes in the absorption and quenching environment for different flame conditions. The feasibility of performing LIF measurements of (NO) in turbulent flames was studied; the single-shot detection limit was determined to be 2 ppm.
NASA Astrophysics Data System (ADS)
Tsubota, Makoto
2008-11-01
The present article reviews the recent developments in the physics of quantum turbulence. Quantum turbulence (QT) was discovered in superfluid 4He in the 1950s, and the research has tended toward a new direction since the mid 90s. The similarities and differences between quantum and classical turbulence have become an important area of research. QT is comprised of quantized vortices that are definite topological defects, being expected to yield a model of turbulence that is much simpler than the classical model. The general introduction of the issue and a brief review on classical turbulence are followed by a description of the dynamics of quantized vortices. Then, we discuss the energy spectrum of QT at very low temperatures. At low wavenumbers, the energy is transferred through the Richardson cascade of quantized vortices, and the spectrum obeys the Kolmogorov law, which is the most important statistical law in turbulence; this classical region shows the similarity to conventional turbulence. At higher wavenumbers, the energy is transferred by the Kelvin-wave cascade on each vortex. This quantum regime depends strongly on the nature of each quantized vortex. The possible dissipation mechanism is discussed. Finally, important new experimental studies, which include investigations into temperature-dependent transition to QT, dissipation at very low temperatures, QT created by vibrating structures, and visualization of QT, are reviewed. The present article concludes with a brief look at QT in atomic Bose-Einstein condensates.
NASA Astrophysics Data System (ADS)
Dai, Jian; Yu, NanJia; Cai, GuoBiao
2015-12-01
Single-element combustor experiments are conducted for three shear coaxial geometry configuration injectors by using gaseous oxygen and gaseous hydrogen (GO2/GH2) as propellants. During the combustion process, several spatially and timeresolved non-intrusive optical techniques, such as OH planar laser induced fluorescence (PLIF), high speed imaging, and infrared imaging, are simultaneously employed to observe the OH radical concentration distribution, flame fluctuations, and temperature fields. The results demonstrate that the turbulent flow phenomenon of non-premixed flame exhibits a remarkable periodicity, and the mixing ratio becomes a crucial factor to influence the combustion flame length. The high speed and infrared images have a consistent temperature field trend. As for the OH-PLIF images, an intuitionistic local flame structure is revealed by single-shot instantaneous images. Furthermore, the means and standard deviations of OH radical intensity are acquired to provide statistical information regarding the flame, which may be helpful for validation of numerical simulations in future. Parameters of structure configurations, such as impinging angle and oxygen post thickness, play an important role in the reaction zone distribution. Based on a successful flame contour extraction method assembled with non-linear anisotropic diffusive filtering and variational level-set, it is possible to implement a fractal analysis to describe the fractal characteristics of the non-premixed flame contour. As a result, the flame front cannot be regarded as a fractal object. However, this turbulent process presents a self-similarity characteristic.
The turbulent flame speed for low-to-moderate turbulence intensities
NASA Astrophysics Data System (ADS)
Matalon, Moshe; Fogla, Navin; Creta, Francesco
2014-11-01
Premixed flame propagation in two-dimensional turbulent flows is examined within the context of a hydrodynamic model. The flame is treated as a surface of density discontinuity and propagates against a turbulent flow of prescribed intensity and scale. A hybrid Navier-Stokes/interface capturing technique is used to describe the flow field throughout the entire domain and track the highly-fluctuating flame front which is allowed to attain folded conformations and form pockets of unburned gases that detach from the main flame surface and are rapidly consumed. A parametric study is conducted to examine the effects of the turbulence parameters: intensity and scale, and the combustion parameters: thermal expansion and mixture composition (or Markstein length). Markstein lengths are varied in order to span both, the Darrieus-Landau (DL) instability-free subcritical and the DL instability-affected supercritical regimes. Scaling laws for the turbulent flame speed, exhibiting explicit dependence on the system parameters, are proposed for low-to-moderate turbulence intensities.
Mixing and chemical reaction in sheared and nonsheared homogeneous turbulence
NASA Technical Reports Server (NTRS)
Leonard, Andy D.; Hill, James C.
1992-01-01
Direct numerical simulations were made to examine the local structure of the reaction zone for a moderately fast reaction between unmixed species in decaying, homogeneous turbulence and in a homogeneous turbulent shear flow. Pseudospectral techniques were used in domains of 64 exp 3 and higher wavenumbers. A finite-rate, single step reaction between non-premixed reactants was considered, and in one case temperature-dependent Arrhenius kinetics was assumed. Locally intense reaction rates that tend to persist throughout the simulations occur in locations where the reactant concentration gradients are large and are amplified by the local rate of strain. The reaction zones are more organized in the case of a uniform mean shear than in isotropic turbulence, and regions of intense reaction rate appear to be associated with vortex structures such as horseshoe vortices and fingers seen in mixing layers. Concentration gradients tend to align with the direction of the most compressive principal strain rate, more so in the isotropic case.
Spontaneous Transition of Turbulent Flames to Detonations in Unconfined Media
NASA Astrophysics Data System (ADS)
Poludnenko, Alexei Y.; Gardiner, Thomas A.; Oran, Elaine S.
2011-07-01
A deflagration-to-detonation transition (DDT) can occur in environments ranging from experimental and industrial systems to astrophysical thermonuclear (type Ia) supernovae explosions. Substantial progress has been made in explaining the nature of DDT in confined systems with walls, internal obstacles, or preexisting shocks. It remains unclear, however, whether DDT can occur in unconfined media. Here we use direct numerical simulations (DNS) to show that for high enough turbulent intensities unconfined, subsonic, premixed, turbulent flames are inherently unstable to DDT. The associated mechanism, based on the nonsteady evolution of flames faster than the Chapman-Jouguet deflagrations, is qualitatively different from the traditionally suggested spontaneous reaction-wave model. Critical turbulent flame speeds, predicted by this mechanism for the onset of DDT, are in agreement with DNS results.
Premixed calcium phosphate cements: Synthesis, physical properties, and cell cytotoxicity
Xu, Hockin H.K.; Carey, Lisa E.; Simon, Carl G.; Takagi, Shozo; Chow, Laurence C.
2009-01-01
Objectives Calcium phosphate cement (CPC) is a promising material for dental, periodontal, and craniofacial repairs. However, its use requires on-site powder–liquid mixing that increases the surgical placement time and raises concerns of insufficient and inhomogeneous mixing. The objective of this study was to determine a formulation of premixed CPC (PCPC) with rapid setting, high strength, and good in vitro cell viability. Methods PCPCs were formulated from CPC powder + non-aqueous liquid + gelling agent + hardening accelerator. Five PCPCs were thus developed: PCPC-Tartaric, PCPC-Malonic, PCPC-Citric, PCPC-Glycolic, and PCPC-Malic. Formulations and controls were compared for setting time, diametral tensile strength, and osteoblast cell compatibility. Results Setting time (mean ± S.D.; n = 4) for PCPC-Tartaric was 8.2 ± 0.8 min, significantly less than the 61.7 ± 1.5 min for the Premixed Control developed previously (p < 0.001). On 7th day immersion, the diametral tensile strength of PCPC-Tartaric reached 6.5 ± 0.8 MPa, higher than 4.5 ± 0.8 MPa of Premixed Control (p = 0.036). Osteoblast cells displayed a polygonal morphology and attached to the nano-hydroxyapatite crystals in the PCPCs. All cements had similar live cell density values (p = 0.126), indicating that the new PCPCs were as cell compatible as a non-premixed CPC control known to be biocompatible. Each of the new PCPCs had a cell viability that was not significantly different (p > 0.1) from that of the non-premixed CPC control. Significance PCPCs will eliminate the powder–liquid mixing during surgery and may also improve the cement performance. The new PCPCs supported cell attachment and yielded a high cell density and viability. Their mechanical strengths approached the reported strengths of sintered porous hydroxyapatite implants and cancellous bone. These nano-crystalline hydroxyapatite cements may be useful in dental, periodontal, and craniofacial repairs. PMID:16678895
Premixed Atmosphere and Convection Influences on Flame Inhibition and Combustion (Pacific)
NASA Technical Reports Server (NTRS)
Honda, Linton K.; Ronney, Paul D.
1997-01-01
Flame spread over flat solid fuel beds is a useful paradigm for studying the behavior of more complex two-phase nonpremixed flames. For practical applications, two of the most important elements of flame spreading are the effects of (1) the ambient atmosphere (e.g. pressure and composition) and (2) the flow environment on the spread rate and extinction conditions. Concerning (1), studies of flame spread in vitiated air and non-standard atmospheres such as those found in undersea vessels and spacecraft are particularly important for the assessment of fire hazards in these environments as well as determination of the effectiveness of fire suppressants. Concerning (2), the flow environment may vary widely even when no forced flow is present because of buoyancy effects. Consequently, the goal of this work is to employ microgravity (micro g) experiments to extend previous studies of the effects of ambient atmosphere and the flow environment on flame spread through the use of microgravity (micro g) experiments. Because of the considerable differences between upward (concurrent-flow) and downward (opposed-flow) flame spread at 1g (Williams, 1976, Fernandez-Pello, 1984), in this work both upward and downward 1g spread are tested. Two types of changes to the oxidizing atmosphere are considered in this work. One is the addition of sub-flammability-limit concentrations of a gaseous fuel ('partially premixed' atmospheres). This is of interest because in fires in enclosures, combustion may occur under poorly ventilated conditions, so that oxygen is partially depleted from the air and is replaced by combustible gases such as fuel vapors, H2 or CO. Subsequent fire spread over the solid fuel could occur under conditions of varying oxygen and gaseous fuel content. The potential significance of flame spread under vitiated or partially premixed conditions has been noted previously (Beyler, 1984). The second change is the diluent type, which affects the radiative properties of the
Modeling complex chemical effects in turbulent nonpremixed combustion
NASA Technical Reports Server (NTRS)
Smith, Nigel S. A.
1995-01-01
Virtually all of the energy derived from the consumption of combustibles occurs in systems which utilize turbulent fluid motion. Since combustion is largely related to the mixing of fluids and mixing processes are orders of magnitude more rapid when enhanced by turbulent motion, efficiency criteria dictate that chemically powered devices necessarily involve fluid turbulence. Where combustion occurs concurrently with mixing at an interface between two reactive fluid bodies, this mode of combustion is called nonpremixed combustion. This is distinct from premixed combustion where flame-fronts propagate into a homogeneous mixture of reactants. These two modes are limiting cases in the range of temporal lag between mixing of reactants and the onset of reaction. Nonpremixed combustion occurs where this lag tends to zero, while premixed combustion occurs where this lag tends to infinity. Many combustion processes are hybrids of these two extremes with finite non-zero lag times. Turbulent nonpremixed combustion is important from a practical standpoint because it occurs in gas fired boilers, furnaces, waste incinerators, diesel engines, gas turbine combustors, and afterburners etc. To a large extent, past development of these practical systems involved an empirical methodology. Presently, efficiency standards and emission regulations are being further tightened (Correa 1993), and empiricism has had to give way to more fundamental research in order to understand and effectively model practical combustion processes (Pope 1991). A key element in effective modeling of turbulent combustion is making use of a sufficiently detailed chemical kinetic mechanism. The prediction of pollutant emission such as oxides of nitrogen (NO(x)) and sulphur (SO(x)) unburned hydrocarbons, and particulates demands the use of detailed chemical mechanisms. It is essential that practical models for turbulent nonpremixed combustion are capable of handling large numbers of 'stiff' chemical species
ERIC Educational Resources Information Center
Hanratty, Thomas J.
1980-01-01
This paper gives an account of research on the structure of turbulence close to a solid boundary. Included is a method to study the flow close to the wall of a pipe without interferring with it. (Author/JN)
NASA Astrophysics Data System (ADS)
Nazarenko, Sergey
2015-07-01
Wave turbulence is the statistical mechanics of random waves with a broadband spectrum interacting via non-linearity. To understand its difference from non-random well-tuned coherent waves, one could compare the sound of thunder to a piece of classical music. Wave turbulence is surprisingly common and important in a great variety of physical settings, starting with the most familiar ocean waves to waves at quantum scales or to much longer waves in astrophysics. We will provide a basic overview of the wave turbulence ideas, approaches and main results emphasising the physics of the phenomena and using qualitative descriptions avoiding, whenever possible, involved mathematical derivations. In particular, dimensional analysis will be used for obtaining the key scaling solutions in wave turbulence - Kolmogorov-Zakharov (KZ) spectra.
Numerical simulation of a laboratory-scale turbulent V-flame
Bell, J.B.; Day, M.S.; Shepherd, I.G.; Johnson, M.; Cheng, R.K.; Grcar,J.F.; Beckner, V.E.; Lijewski, M.J.
2005-02-07
We present a three-dimensional, time-dependent simulation of a laboratory-scale rod-stabilized premixed turbulent V-flame. The simulations are performed using an adaptive time-dependent low Mach number model with detailed chemical kinetics and a mixture model for differential species diffusion. The algorithm is based on a second-order projection formulation and does not require an explicit subgrid model for turbulence or turbulence chemistry interaction. Adaptive mesh refinement is used to dynamically resolve the flame and turbulent structures. Here, we briefly discuss the numerical procedure and present detailed comparisons with experimental measurements showing that the computation is able to accurately capture the basic flame morphology and associated mean velocity field. Finally, we discuss key issues that arise in performing these types of simulations and the implications of these issues for using computation to form a bridge between turbulent flame experiments and basic combustion chemistry.
A priori analysis of a LES subfilter model for soot-turbulence-chemistry interactions
NASA Astrophysics Data System (ADS)
Lew, Jeffry K.; Mueller, Michael E.
2016-11-01
In a turbulent flame, soot interacts with turbulence and combustion chemistry at the smallest scales. An existing LES subfilter model proposes that soot-turbulence interactions are independent of chemistry due to the time scale separation between slow soot formation and rapid heat-releasing reactions. However, interactions between soot, turbulence, and chemistry occur even after the nucleation of soot from polycyclic aromatic hydrocarbon (PAH) dimers. In fact, the interplay of soot and gas-phase chemistry may be intensified during oxidation and surface growth. To capture these effects, a dependence on the local mixture fraction has been introduced into the subfilter model. This modified model is evaluated a priori using a direct numerical simulation (DNS) database of soot evolution in a turbulent non-premixed n-heptane/air jet flame.
Controllability of flow turbulence.
Guan, Shuguang; Wei, G W; Lai, C-H
2004-06-01
In this paper, we study the controllability of real-world flow turbulence governed by the two-dimensional Navier-Stokes equations, using strategies developed in chaos control. A case of control/synchronization of turbulent dynamics is observed when only one component of the velocity field vector is unidirectionally coupled to a target state, while the other component is uncoupled. Unlike previous results, it is shown that the dynamics of the whole velocity field cannot be completely controlled/synchronized to the target, even in the limit of long time and strong coupling strength. It is further revealed that the controlled component of the velocity field can be fully controlled/synchronized to the target, but the other component, which is not directly coupled to the target, can only be partially controlled/synchronized to the target. By extending an auxiliary method to distributed dynamic systems, the partial synchronization of two turbulent orbits in the present study can be categorized in the domain of generalized synchronization of spatiotemporal dynamics.
Modelling of Turbulent Scalar Fluxes in the Broken Reaction Zones Regime
NASA Astrophysics Data System (ADS)
Im, Hong G.; Chakraborty, Nilanjan; Klein, Markus; Kasten, Christian; Arias, Paul
2016-11-01
The LES filtered species transport equation in turbulent reacting flow simulations contains the unclosed turbulent scalar flux that needs to be modelled. It is well known that the statistical behavior of this term and its alignment characteristics with resolved scalar gradient depend on the relative importance of heat release and turbulent velocity fluctuations. Counter-gradient transport has been reported in several earlier studies where the flames under investigation were located either in the corrugated flamelets or thin reaction zones regime of premixed turbulent combustion. Therefore it is useful to understand the statistical behavior of turbulent scalar fluxes if the flame represents the broken reaction zones regime (BRZR). The present analysis aims to provide improved understanding on this subject through an a-priori analysis of a detailed chemistry database consisting of three freely-propagating statistically planar turbulent H2-air premixed flames representing three different regimes of combustion. Results indicate that heat release effects weaken with increasing Karlovitz number, but that counter-gradient transport can still occur for large LES filter size in the BRZR. Furthermore the behaviour of the flux and in particular its sign are different for reactant and product species. KAUST, EPSRC, KAUST Supercomputing Lab, N8, Archer.
Workshop on Engineering Turbulence Modeling
NASA Technical Reports Server (NTRS)
Povinelli, Louis A. (Editor); Liou, W. W. (Editor); Shabbir, A. (Editor); Shih, T.-H. (Editor)
1992-01-01
Discussed here is the future direction of various levels of engineering turbulence modeling related to computational fluid dynamics (CFD) computations for propulsion. For each level of computation, there are a few turbulence models which represent the state-of-the-art for that level. However, it is important to know their capabilities as well as their deficiencies in order to help engineers select and implement the appropriate models in their real world engineering calculations. This will also help turbulence modelers perceive the future directions for improving turbulence models. The focus is on one-point closure models (i.e., from algebraic models to higher order moment closure schemes and partial differential equation methods) which can be applied to CFD computations. However, other schemes helpful in developing one-point closure models, are also discussed.
Numerical methods for turbulent flow
NASA Technical Reports Server (NTRS)
Turner, James C., Jr.
1988-01-01
It has generally become accepted that the Navier-Strokes equations predict the dynamic behavior of turbulent as well as laminar flows of a fluid at a point in space away form a discontinuity such as a shock wave. Turbulence is also closely related to the phenomena of non-uniqueness of solutions of the Navier-Strokes equations. These second order, nonlinear partial differential equations can be solved analytically for only a few simple flows. Turbulent flow fields are much to complex to lend themselves to these few analytical methods. Numerical methods, therefore, offer the only possibility of achieving a solution of turbulent flow equations. In spite of recent advances in computer technology, the direct solution, by discrete methods, of the Navier-Strokes equations for turbulent flow fields is today, and in the foreseeable future, impossible. Thus the only economically feasible way to solve practical turbulent flow problems numerically is to use statistically averaged equations governing mean-flow quantities. The objective is to study some recent developments relating to the use of numerical methods to study turbulent flow.
Effects of pressure on syngas/air turbulent nonpremixed flames
NASA Astrophysics Data System (ADS)
Lee, Bok Jik; Im, Hong G.; Ciottoli, Pietro Paolo; Valorani, Mauro
2016-11-01
Large eddy simulations (LES) of turbulent non-premixed jet flames were conducted to investigate the effects of pressure on the syngas/air flame behavior. The software to solve the reactive Navier-Stokes equations was developed based on the OpenFOAM framework, using the YSLFM library for the flamelet-based chemical closure. The flamelet tabulation is obtained by means of an in-house code designed to solve unsteady flamelets of both ideal and real fluid mixtures. The validation of the numerical setup is attained by comparison of the numerical results with the Sandia/ETH-Zurich experimental database of the CO/H2/N2 non-premixed, unconfined, turbulent jet flame, referred to as "Flame A". Two additional simulations, at pressure conditions of 2 and 5 atm, are compared and analyzed to unravel computational and scientific challenges in characterizing turbulent flames at high pressures. A set of flamelet solutions, representative of the jet flames under review, are analyzed following a CSP approach. In particular, the Tangential Stretching Rate (TSR), representing the reciprocal of the most energetic time scale associated with the chemical source term, and its extension to reaction-diffusion systems (extended TSR), are adopted.
Dry low NOx combustion system with pre-mixed direct-injection secondary fuel nozzle
Zuo, Baifang; Johnson, Thomas; Ziminsky, Willy; Khan, Abdul
2013-12-17
A combustion system includes a first combustion chamber and a second combustion chamber. The second combustion chamber is positioned downstream of the first combustion chamber. The combustion system also includes a pre-mixed, direct-injection secondary fuel nozzle. The pre-mixed, direct-injection secondary fuel nozzle extends through the first combustion chamber into the second combustion chamber.
21 CFR 170.60 - Nitrites and/or nitrates in curing premixes.
Code of Federal Regulations, 2012 CFR
2012-04-01
... 21 Food and Drugs 3 2012-04-01 2012-04-01 false Nitrites and/or nitrates in curing premixes. 170... and Decisions § 170.60 Nitrites and/or nitrates in curing premixes. (a) Nitrites and/or nitrates are.... (b) Nitrites and/or nitrates, when packaged separately from flavoring and seasoning in...
21 CFR 170.60 - Nitrites and/or nitrates in curing premixes.
Code of Federal Regulations, 2011 CFR
2011-04-01
... 21 Food and Drugs 3 2011-04-01 2011-04-01 false Nitrites and/or nitrates in curing premixes. 170... and Decisions § 170.60 Nitrites and/or nitrates in curing premixes. (a) Nitrites and/or nitrates are.... (b) Nitrites and/or nitrates, when packaged separately from flavoring and seasoning in...
21 CFR 170.60 - Nitrites and/or nitrates in curing premixes.
Code of Federal Regulations, 2013 CFR
2013-04-01
... 21 Food and Drugs 3 2013-04-01 2013-04-01 false Nitrites and/or nitrates in curing premixes. 170... and Decisions § 170.60 Nitrites and/or nitrates in curing premixes. (a) Nitrites and/or nitrates are.... (b) Nitrites and/or nitrates, when packaged separately from flavoring and seasoning in...
21 CFR 170.60 - Nitrites and/or nitrates in curing premixes.
Code of Federal Regulations, 2010 CFR
2010-04-01
... 21 Food and Drugs 3 2010-04-01 2009-04-01 true Nitrites and/or nitrates in curing premixes. 170.60... Decisions § 170.60 Nitrites and/or nitrates in curing premixes. (a) Nitrites and/or nitrates are food.... (b) Nitrites and/or nitrates, when packaged separately from flavoring and seasoning in...
Combustion mechanism of ultralean rotating counterflow twin premixed flame
NASA Astrophysics Data System (ADS)
Uemichi, Akane; Nishioka, Makihito
2015-01-01
In our previous numerical studies [Nishioka Makihito, Zhenyu Shen, and Akane Uemichi. "Ultra-lean combustion through the backflow of burned gas in rotating counterflow twin premixed flames." Combustion and Flame 158.11 (2011): 2188-2198. Uemichi Akane, and Makihito Nishioka. "Numerical study on ultra-lean rotating counterflow twin premixed flame of hydrogen-air." Proceedings of the Combustion Institute 34.1 (2013): 1135-1142]. we found that methane- and hydrogen-air rotating counterflow twin flames (RCTF) can achieve ultralean combustion when backward flow of burned gas occurs due to the centrifugal force created by rotation. In this study, we investigated the mechanisms of ultralean combustion in these flames by the detailed numerical analyses of the convective and diffusive transport of the main species. We found that, under ultralean conditions, the diffusive transport of fuel exceeds its backward convective transport in the flame zone, which is located on the burned-gas side of the stagnation point. In contrast, the relative magnitudes of diffusive and convective transport for oxygen are reversed compared to those for the fuel. The resulting flows for fuel and oxygen lead to what we call a 'net flux imbalance'. This net flux imbalance increases the flame temperature and concentrations of active radicals. For hydrogen-air RCTF, a very large diffusivity of hydrogen enhances the net flux imbalance, significantly increasing the flame temperature. This behaviour is intrinsic to a very lean premixed flame in which the reaction zone is located in the backflow of its own burned gas.
Jet flow and premixed jet flame control by plasma swirler
NASA Astrophysics Data System (ADS)
Li, Gang; Jiang, Xi; Zhao, Yujun; Liu, Cunxi; Chen, Qi; Xu, Gang; Liu, Fuqiang
2017-04-01
A swirler based on dielectric barrier discharge plasma actuators is designed and its effectiveness in both jet flow and premixed jet flame control is demonstrated. In contrast to traditional spanwise-oriented actuators, plasma actuators are placed along the axial direction of the injector to induce a circumferential velocity to the main flow and create a swirl flow without any insertion or moving part. In the DBD plasma swirl injector, the discharge does not ignite the mixture nor does it induce flashback. Flame visualization is obtained by cameras while velocity profiles are obtained by Laser Doppler Anemometry measurements. The results obtained indicate the effectiveness of the new design.
Heat Transfer Effects on a Fully Premixed Methane Impinging Flame
2014-10-30
HEAT TRANSFER EFFECTS ON A FULLY PREMIXED METHANE IMPINGING FLAME D. Mira1, M. Zavala1, M. Avila1, H. Owen1, J.C. Cajas1, G. Houzeaux1 and M...to evaluate the numeri- cal algorithms and the effects of the thermal coupling with the flow dynamics is the case of a jet flame im- pinging on a...investigate the heat transfer effects and flow dynamics of an imping- ing flame with low nozzle-to-plate distance when the solid plate is considered non
Response mechanisms of attached premixed flames subjected to harmonic forcing
NASA Astrophysics Data System (ADS)
Shreekrishna
The persistent thrust for a cleaner, greener environment has prompted air pollution regulations to be enforced with increased stringency by environmental protection bodies all over the world. This has prompted gas turbine manufacturers to move from nonpremixed combustion to lean, premixed combustion. These lean premixed combustors operate quite fuel-lean compared to the stochiometric, in order to minimize CO and NOx productions, and are very susceptible to oscillations in any of the upstream flow variables. These oscillations cause the heat release rate of the flame to oscillate, which can engage one or more acoustic modes of the combustor or gas turbine components, and under certain conditions, lead to limit cycle oscillations. This phenomenon, called thermoacoustic instabilities, is characterized by very high pressure oscillations and increased heat fluxes at system walls, and can cause significant problems in the routine operability of these combustors, not to mention the occasional hardware damages that could occur, all of which cumulatively cost several millions of dollars. In a bid towards understanding this flow-flame interaction, this research works studies the heat release response of premixed flames to oscillations in reactant equivalence ratio, reactant velocity and pressure, under conditions where the flame preheat zone is convectively compact to these disturbances, using the G-equation. The heat release response is quantified by means of the flame transfer function and together with combustor acoustics, forms a critical component of the analytical models that can predict combustor dynamics. To this end, low excitation amplitude (linear) and high excitation amplitude (nonlinear) responses of the flame are studied in this work. The linear heat release response of lean, premixed flames are seen to be dominated by responses to velocity and equivalence ratio fluctuations at low frequencies, and to pressure fluctuations at high frequencies which are in the
Premixer assembly for mixing air and fuel for combustion
York, William David; Johnson, Thomas Edward; Keener, Christopher Paul
2016-12-13
A premixer assembly for mixing air and fuel for combustion includes a plurality of tubes disposed at a head end of a combustor assembly. Also included is a tube of the plurality of tubes, the tube including an inlet end and an outlet end. Further included is at least one non-circular portion of the tube extending along a length of the tube, the at least one non-circular portion having a non-circular cross-section, and the tube having a substantially constant cross-sectional area along its length
NASA Technical Reports Server (NTRS)
Tchen, C. M.
1986-01-01
Theoretical and numerical works in atmospheric turbulence have used the Navier-Stokes fluid equations exclusively for describing large-scale motions. Controversy over the existence of an average temperature gradient for the very large eddies in the atmosphere suggested that a new theoretical basis for describing large-scale turbulence was necessary. A new soliton formalism as a fluid analogue that generalizes the Schrodinger equation and the Zakharov equations has been developed. This formalism, processing all the nonlinearities including those from modulation provided by the density fluctuations and from convection due to the emission of finite sound waves by velocity fluctuations, treats large-scale turbulence as coalescing and colliding solitons. The new soliton system describes large-scale instabilities more explicitly than the Navier-Stokes system because it has a nonlinearity of the gradient type, while the Navier-Stokes has a nonlinearity of the non-gradient type. The forced Schrodinger equation for strong fluctuations describes the micro-hydrodynamical state of soliton turbulence and is valid for large-scale turbulence in fluids and plasmas where internal waves can interact with velocity fluctuations.
NASA Astrophysics Data System (ADS)
Cao, Hong-Jun; Zhang, Hui-Qiang; Lin, Wen-Yi
2012-05-01
Four kinds of presumed probability-density-function (PDF) models for non-premixed turbulent combustion are evaluated in flames with various stoichiometric mixture fractions by using large eddy simulation (LES). The LES code is validated by the experimental data of a classical turbulent jet flame (Sandia flame D). The mean and rms temperatures obtained by the presumed PDF models are compared with the LES results. The β-function model achieves a good prediction for different flames. The predicted rms temperature by using the double-δ function model is very small and unphysical in the vicinity of the maximum mean temperature. The clip-Gaussian model and the multi-δ function model make a worse prediction of the extremely fuel-rich or fuel-lean side due to the clip at the boundary of the mixture fraction space. The results also show that the overall prediction performance of presumed PDF models is better at mediate stoichiometric mixture fractions than that at very small or very large ones.
Scaling of turbulent flame speed for expanding flames with Markstein diffusion considerations
NASA Astrophysics Data System (ADS)
Chaudhuri, Swetaprovo; Wu, Fujia; Law, Chung K.
2013-09-01
In this paper we clarify the role of Markstein diffusivity, which is the product of the planar laminar flame speed and the Markstein length, on the turbulent flame speed and its scaling, based on experimental measurements on constant-pressure expanding turbulent flames. Turbulent flame propagation data are presented for premixed flames of mixtures of hydrogen, methane, ethylene, n-butane, and dimethyl ether with air, in near-isotropic turbulence in a dual-chamber, fan-stirred vessel. For each individual fuel-air mixture presented in this work and the recently published iso-octane data from Leeds, normalized turbulent flame speed data of individual fuel-air mixtures approximately follow a ReT,f0.5 scaling, for which the average radius is the length scale and thermal diffusivity is the transport property of the turbulence Reynolds number. At a given ReT,f, it is experimentally observed that the normalized turbulent flame speed decreases with increasing Markstein number, which could be explained by considering Markstein diffusivity as the leading dissipation mechanism for the large wave number flame surface fluctuations. Consequently, by replacing thermal diffusivity with the Markstein diffusivity in the turbulence Reynolds number definition above, it is found that normalized turbulent flame speeds could be scaled by ReT,M0.5 irrespective of the fuel, equivalence ratio, pressure, and turbulence intensity for positive Markstein number flames.
Scaling of turbulent flame speed for expanding flames with Markstein diffusion considerations.
Chaudhuri, Swetaprovo; Wu, Fujia; Law, Chung K
2013-09-01
In this paper we clarify the role of Markstein diffusivity, which is the product of the planar laminar flame speed and the Markstein length, on the turbulent flame speed and its scaling, based on experimental measurements on constant-pressure expanding turbulent flames. Turbulent flame propagation data are presented for premixed flames of mixtures of hydrogen, methane, ethylene, n-butane, and dimethyl ether with air, in near-isotropic turbulence in a dual-chamber, fan-stirred vessel. For each individual fuel-air mixture presented in this work and the recently published iso-octane data from Leeds, normalized turbulent flame speed data of individual fuel-air mixtures approximately follow a Re_{T,f}^{0.5} scaling, for which the average radius is the length scale and thermal diffusivity is the transport property of the turbulence Reynolds number. At a given Re_{T,f}^{}, it is experimentally observed that the normalized turbulent flame speed decreases with increasing Markstein number, which could be explained by considering Markstein diffusivity as the leading dissipation mechanism for the large wave number flame surface fluctuations. Consequently, by replacing thermal diffusivity with the Markstein diffusivity in the turbulence Reynolds number definition above, it is found that normalized turbulent flame speeds could be scaled by Re_{T,M}^{0.5} irrespective of the fuel, equivalence ratio, pressure, and turbulence intensity for positive Markstein number flames.
NASA Astrophysics Data System (ADS)
Skrbek, L.
2011-12-01
We review physical properties of quantum fluids He II and 3He-B, where quantum turbulence (QT) has been studied experimentally. Basic properties of QT in these working fluids are discussed within the phenomenological two-fluid model introduced by Landau. We consider counterflows in which the normal and superfluid components flow against each other, as well as co-flows in which the direction of the two fluids is the same. We pay special attention to the important case of zero temperature limit, where QT represents an interesting and probably the simplest prototype of three-dimensional turbulence in fluids. Experimental techniques to explore QT such as second sound attenuation, Andreev reflection, NMR, ion propagation are briefly introduced and results of various experiments on so-called Vinen QT and Kolmogorov QT both in He II and 3He are discussed, emphasizing similarities and differences between classical and quantum turbulence.
Turbulence in Compressible Flows
NASA Technical Reports Server (NTRS)
1997-01-01
Lecture notes for the AGARD Fluid Dynamics Panel (FDP) Special Course on 'Turbulence in Compressible Flows' have been assembled in this report. The following topics were covered: Compressible Turbulent Boundary Layers, Compressible Turbulent Free Shear Layers, Turbulent Combustion, DNS/LES and RANS Simulations of Compressible Turbulent Flows, and Case Studies of Applications of Turbulence Models in Aerospace.
Correlation of Flame Speed with Stretch in Turbulent Premixed Methane/Air Flames
NASA Astrophysics Data System (ADS)
Chen, Jacqueline H.; Im, Hong G.
1997-11-01
Flame speed correlation with stretch is obtained from direct numerical simulations of lean to stoichiometric methane/air flames over a broad range of Karlovitz numbers. The correlation is interpreted in terms of local tangential strain rate and curvature effects. DNS results show that there exist two distinct branches in the correlation curve depending on the sign of the displacement speed. For small Karlovitz numbers with positive displacement speed, the estimated Markstein length from the DNS results agrees well with that obtained from steady strained laminar flame calculations as well as with experimental studies. Larger values of Karlovitz numbers observed in the DNS results are found to be mainly due to the effect of strong curvatures; for those cases the correlation shows nonlinear behavior. The sensitivity of the correlation to the definition of the flame front and the statistical importance of particular branches in the correlation are also discussed.
2016-03-24
and heat transfer within engines are important to Air Force interests. A flamlelet combustion sub-model [1] may be most appropriate for the flamelet...large eddies are more efficient at extinguishing a flame than small eddies. The reasons are not clear, but it is likely that local extinction...show that heat release rate profile falls nearly on top of the Overlap profile. It also shows that formaldehyde exists throughout the preheat zone
Establishing bioequivalence of veterinary premixes (Type A medicated articles).
Hunter, R P; Lees, P; Concordet, D; Toutain, P-L
2012-04-01
a) Key issues concerning Premix (Type A medicated articles) Bioequivalence evaluations: 1) This is a complex issue concerning both route of administration and formulation. 2) If the animal is not at the bunk/trough, the animal is not self-administering (eating medicated feed), thus there can be no drug absorption. b) Differing opinions among scientists and regulatory authorities/expert bodies regarding: 1) No harmonization on how to design, conduct, and interpret in vivo studies. 2) Applicability of biowaivers to Type A (premix) products. 3) Why are topdress and complete feed considered differently? Are they different formulations or different routes of administration? 4) Single dose vs. multi-dose studies. 5) What is the final formulation? c) What are the next steps: 1) Harmonize current bioequivalence guidelines through the VICH process. 2) Determine the applicability/non-applicability of the Biopharmaceutical Classification System (BCS). 3) Establish the Total Mixed Ration (i.e. formulation) effects. 4) Define the test subject (individual, pen, etc.).
Finite amplitude wave interaction with premixed laminar flames
NASA Astrophysics Data System (ADS)
Aslani, Mohamad; Regele, Jonathan D.
2014-11-01
The physics underlying combustion instability is an active area of research because of its detrimental impact in many combustion devices, such as turbines, jet engines, and liquid rocket engines. Pressure waves, ranging from acoustic waves to strong shocks, are potential sources of these disturbances. Literature on flame-disturbance interactions are primarily focused on either acoustics or strong shock wave interactions, with little information about the wide spectrum of behaviors that may exist between these two extremes. For example, the interaction between a flame and a finite amplitude compression wave is not well characterized. This phenomenon is difficult to study numerically due to the wide range of scales that need to be captured, requiring powerful and efficient numerical techniques. In this work, the interaction of a perturbed laminar premixed flame with a finite amplitude compression wave is investigated using the Parallel Adaptive Wavelet Collocation Method (PAWCM). This method optimally solves the fully compressible Navier-Stokes equations while capturing the essential scales. The results show that depending on the amplitude and duration of a finite amplitude disturbance, the interaction between these waves and premixed flames can produce a broad range of responses.
Influence of water content on hardening and handling of a premixed calcium phosphate cement.
Engstrand, Johanna; Aberg, Jonas; Engqvist, Håkan
2013-01-01
Handling of calcium phosphate cements is difficult, where problems often arise during mixing, transferring to syringes, and subsequent injection. Via the use of premixed cements the risk of handling complications is reduced. However, for premixed cements to work in a clinical situation the setting time needs to be improved. The objective of this study is to investigate the influence of the addition of water on the properties of premixed cement. Monetite-forming premixed cements with small amounts of added water (less than 6.8 wt.%) were prepared and the influence on injectability, working time, setting time and mechanical strength was evaluated. The results showed that the addition of small amounts of water had significant influence on the properties of the premixed cement. With the addition of just 1.7 wt.% water, the force needed to extrude the cement from a syringe was reduced from 107 (±15) N to 39 (±9) N, the compression strength was almost doubled, and the setting time decreased from 29 (±4) min to 19 (±2) min, while the working time remained 5 to 6h. This study demonstrates the importance of controlling the water content in premixed cement pastes and how water can be used to improve the properties of premixed cements.
Talbot, L.; Cheng, R.K.
1993-12-01
Turbulent combustion is the dominant process in heat and power generating systems. Its most significant aspect is to enhance the burning rate and volumetric power density. Turbulent mixing, however, also influences the chemical rates and has a direct effect on the formation of pollutants, flame ignition and extinction. Therefore, research and development of modern combustion systems for power generation, waste incineration and material synthesis must rely on a fundamental understanding of the physical effect of turbulence on combustion to develop theoretical models that can be used as design tools. The overall objective of this program is to investigate, primarily experimentally, the interaction and coupling between turbulence and combustion. These processes are complex and are characterized by scalar and velocity fluctuations with time and length scales spanning several orders of magnitude. They are also influenced by the so-called {open_quotes}field{close_quotes} effects associated with the characteristics of the flow and burner geometries. The authors` approach is to gain a fundamental understanding by investigating idealized laboratory flames. Laboratory flames are amenable to detailed interrogation by laser diagnostics and their flow geometries are chosen to simplify numerical modeling and simulations and to facilitate comparison between experiments and theory.
1991-10-01
and complexity of thermochemistry . Accordingly a practical viewpoint is required to meet near-term work required for use in advanced CFD codes...teachers the opportunity to learn/explore/ teach turbulence issues. While such a product could be an invaluable eductaional tool (university), it also
NASA Technical Reports Server (NTRS)
Bardina, Jorge E.
1995-01-01
The objective of this work is to develop, verify, and incorporate the baseline two-equation turbulence models which account for the effects of compressibility into the three-dimensional Reynolds averaged Navier-Stokes (RANS) code and to provide documented descriptions of the models and their numerical procedures so that they can be implemented into 3-D CFD codes for engineering applications.
Sen, Baris Ali; Menon, Suresh
2010-01-15
A large eddy simulation (LES) sub-grid model is developed based on the artificial neural network (ANN) approach to calculate the species instantaneous reaction rates for multi-step, multi-species chemical kinetics mechanisms. The proposed methodology depends on training the ANNs off-line on a thermo-chemical database representative of the actual composition and turbulence (but not the actual geometrical problem) of interest, and later using them to replace the stiff ODE solver (direct integration (DI)) to calculate the reaction rates in the sub-grid. The thermo-chemical database is tabulated with respect to the thermodynamic state vector without any reduction in the number of state variables. The thermo-chemistry is evolved by stand-alone linear eddy mixing (LEM) model simulations under both premixed and non-premixed conditions, where the unsteady interaction of turbulence with chemical kinetics is included as a part of the training database. The proposed methodology is tested in LES and in stand-alone LEM studies of three distinct test cases with different reduced mechanisms and conditions. LES of premixed flame-turbulence-vortex interaction provides direct comparison of the proposed ANN method against DI and ANNs trained on thermo-chemical database created using another type of tabulation method. It is shown that the ANN trained on the LEM database can capture the correct flame physics with accuracy comparable to DI, which cannot be achieved by ANN trained on a laminar premix flame database. A priori evaluation of the ANN generality within and outside its training domain is carried out using stand-alone LEM simulations as well. Results in general are satisfactory, and it is shown that the ANN provides considerable amount of memory saving and speed-up with reasonable and reliable accuracy. The speed-up is strongly affected by the stiffness of the reduced mechanism used for the computations, whereas the memory saving is considerable regardless. (author)
Galley, D.; Ducruix, S.; Lacas, F.; Veynante, D.
2011-01-15
A laboratory-scale swirling burner, presenting many similarities with gas turbines combustors, has been studied experimentally using planar laser induced fluorescence (PLIF) on OH radical and acetone vapor in order to characterize the flame stabilization process. These diagnostics show that the stabilization point rotates in the combustion chamber and that air and fuel mixing is not complete at the end of the mixing tube. Fuel mass fraction decays exponentially along the mixing tube axis and transverse profiles show a gaussian shape. However, radial pressure gradients tend to trap the fuel in the core of the vortex that propagates axially in the mixing tube. As the mixing tube vortex enters the combustion chamber, vortex breakdown occurs through a precessing vortex core (PVC). The axially propagating vortex shows a helicoidal trajectory in the combustion chamber which trace is observed with transverse acetone PLIF. As a consequence, the stabilizing point of the flame in the combustion chamber rotates with the PVC structure. This phenomenon has been observed in the present study with a high speed camera recording spontaneous emission of the flame. The stabilization point rotation frequency tends to increase with mass flow rates. It was also shown that the coupling between the PVC and the flame stabilization occurs via mixing, explaining one possible coupling mechanism between acoustic waves in the flow and the reaction rate. This path may also be envisaged for flashback, an issue that will be more completely treated in a near future. (author)
NASA Astrophysics Data System (ADS)
Okai, Keiichi; Zimmer, Laurent; Kurosawa, Yoji
Combined Laser Induced Ignition and Plasma Spectroscopy has a potential to give in situ measurement of fuel equivalence ratio in the laser ignition event at an exact time and location of plasma initiation. This method is an extension of a non-intuitive gaseous-composite measurement technique, i.e. Laser Induced Plasma Spectroscopy, to simultaneous ignition utilizing the same plasma source both for gaseous composite measurements and ignition. Calibration methods so far are not sufficient in terms of accuracy. The present study utilizes a new calibration technology without energy measurement under the ignition case. The example dealt in this study is a single injector combustor which provides a time-varying non-uniform fuel equivalence ratio under fuel lean cases. The result showed that the present calibration method is valid for the present measurement of the fuel equivalence ratio, and that the data is very useful to understand the ignition process of the configuration given.
NASA Astrophysics Data System (ADS)
Schmidt, Jacob; Kostka, Stanislav; Lynch, Amy; Ganguly, Biswa
2011-09-01
The effects of millisecond-wide, pulsed current-voltage-induced behavior in premixed laminar flames have been investigated through the simultaneous collection of particle image velocimetry (PIV) and chemiluminescence data with particular attention paid to the onset mechanisms. Disturbances caused by applied voltages of 2 kV over a 30-mm gap to a downward propagating, atmospheric pressure, premixed propane/air flame with a flow speed near 2 m/s and an equivalence ratio of 1.06 are investigated. The combined PIV and chemiluminescence-based experimental data show the observed disturbance originates only in or near the cathode fall region very close to the burner base. The data also suggest that the coupling mechanism responsible for the flame disturbance behavior is fluidic in nature, developing from the radial positive chemi-ion distribution and an ion-drift current-induced net body force that acts along the annular space discharge distribution in the reaction zone in or near the cathode fall. This net body force causes a reduction in flow speed above these near cathodic regions causing the base of the flame to laterally spread. Also, this effect seems to produce a velocity gradient leading to the transition of a laminar flame to turbulent combustion for higher applied current-voltage conditions as shown in previous work (Marcum and Ganguly in Combust Flame 143:27-36, 2005; Schmidt and Ganguly in 48th AIAA aerospace sciences meeting. Orlando, 2010).
NASA Astrophysics Data System (ADS)
Kühnen, Jakob; Hof, Björn
2015-11-01
We show that a simple modification of the velocity profile in a pipe can lead to a complete collapse of turbulence and the flow fully relaminarises. The annihilation of turbulence is achieved by a steady manipulation of the streamwise velocity component alone, greatly reducing control efforts. Several different control techniques are presented: one with a local modification of the flow profile by means of a stationary obstacle, one employing a nozzle injecting fluid through a small gap at the pipe wall and one with a moving wall, where a part of the pipe is shifted in the streamwise direction. All control techniques act on the flow such that the streamwise velocity profile becomes more flat and turbulence gradually grows faint and disappears. In a smooth straight pipe the flow remains laminar downstream of the control. Hence a reduction in skin friction by a factor of 8 and more can be accomplished. Stereoscopic PIV-measurements and movies of the development of the flow during relaminarisation are presented.
Premixed flame propagation in combustible particle cloud mixtures
NASA Technical Reports Server (NTRS)
Seshadri, K.; Yang, B.
1993-01-01
The structures of premixed flames propagating in combustible systems, containing uniformly distributed volatile fuel particles, in an oxidizing gas mixtures is analyzed. The experimental results show that steady flame propagation occurs even if the initial equivalence ratio of the combustible mixture based on the gaseous fuel available in the particles, phi(u) is substantially larger than unity. A model is developed to explain these experimental observations. In the model it is presumed that the fuel particles vaporize first to yield a gaseous fuel of known chemical composition which then reacts with oxygen in a one-step overall process. It is shown that the interplay of vaporization kinetics and oxidation process, can result in steady flame propagation in combustible mixtures where the value of phi(u) is substantially larger than unity. This prediction is in agreement with experimental observations.
Internal combustion engine using premixed combustion of stratified charges
Marriott, Craig D.; Reitz, Rolf D. (Madison, WI
2003-12-30
During a combustion cycle, a first stoichiometrically lean fuel charge is injected well prior to top dead center, preferably during the intake stroke. This first fuel charge is substantially mixed with the combustion chamber air during subsequent motion of the piston towards top dead center. A subsequent fuel charge is then injected prior to top dead center to create a stratified, locally richer mixture (but still leaner than stoichiometric) within the combustion chamber. The locally rich region within the combustion chamber has sufficient fuel density to autoignite, and its self-ignition serves to activate ignition for the lean mixture existing within the remainder of the combustion chamber. Because the mixture within the combustion chamber is overall premixed and relatively lean, NO.sub.x and soot production are significantly diminished.
Dynamics of premixed hydrogen/air flames in mesoscale channels
Pizza, Gianmarco; Frouzakis, Christos E.; Boulouchos, Konstantinos; Mantzaras, John; Tomboulides, Ananias G.
2008-10-15
Direct numerical simulation with detailed chemistry and transport is used to study the stabilization and dynamics of lean ({phi}=0.5) premixed hydrogen/air atmospheric pressure flames in mesoscale planar channels. Channel heights of h=2, 4, and 7 mm, and inflow velocities in the range 0.3{<=}U{sub IN}{<=}1100cm/ s are investigated. Six different burning modes are identified: mild combustion, ignition/extinction, closed steady symmetric flames, open steady symmetric flames, oscillating and, finally, asymmetric flames. Chaotic behavior of cellular flame structures is observed for certain values of U{sub IN}. Stability maps delineating the regions of the different flame types are finally constructed. (author)
Numerical simulation of premixed H2-air cellular tubular flames
NASA Astrophysics Data System (ADS)
Hall, Carl Alan; Wendell Pitz, Robert
2016-03-01
The detailed flame structure of laminar premixed cellular flames in the tubular domain is simulated in 2D using a fully-implicit primitive variable finite difference formulation that includes multicomponent transport and detailed chemical kinetics. Numerical results for H2/air flames are presented and compared against spatially resolved experimental measurements of temperature and chemical species including atomic H and OH. The experimental results compare well for flame structure and cell number, despite the numerical model under-predicting the peak temperature by 200 K. Numerical experiments were performed to assess the ability for cellular tubular flames to impact experimental and numerical investigations of practical flames. The cellular flame structure is found to provide a highly sensitive geometry that is useful for validating diffusive transport modelling approximations. This capability is exemplified through the development of a simple and accurate approximation for thermal diffusion (i.e. the Soret effect) that is suitable for practical combustion codes.
Flashback detection sensor for lean premix fuel nozzles
Thornton, Jimmy Dean; Richards, George Alan; Straub, Douglas L.; Liese, Eric Arnold; Trader, Jr., John Lee; Fasching, George Edward
2002-08-06
A sensor for detecting the flame occurring during a flashback condition in the fuel nozzle of a lean premix combustion system is presented. The sensor comprises an electrically isolated flashback detection electrode and a guard electrode, both of which generate electrical fields extending to the walls of the combustion chamber and to the walls of the fuel nozzle. The sensor is positioned on the fuel nozzle center body at a location proximate the entrance to the combustion chamber of the gas turbine combustion system. The sensor provides 360.degree. detection of a flashback inside the fuel nozzle, by detecting the current conducted by the flame within a time frame that will prevent damage to the gas turbine combustion system caused by the flashback condition.
Behaviour of a Premixed Flame Subjected to Acoustic Oscillations
Qureshi, Shafiq R.; Khan, Waqar A.; Prosser, Robert
2013-01-01
In this paper, a one dimensional premixed laminar methane flame is subjected to acoustic oscillations and studied. The purpose of this analysis is to investigate the effects of acoustic perturbations on the reaction rates of different species, with a view to their respective contribution to thermoacoustic instabilities. Acoustically transparent non reflecting boundary conditions are employed. The flame response has been studied with acoustic waves of different frequencies and amplitudes. The integral values of the reaction rates, the burning velocities and the heat release of the acoustically perturbed flame are compared with the unperturbed case. We found that the flame's sensitivity to acoustic perturbations is greatest when the wavelength is comparable to the flame thickness. Even in this case, the perturbations are stable with time. We conclude that acoustic fields acting on the chemistry do not contribute significantly to the emergence of large amplitude pressure oscillations. PMID:24376501
Pre-mixing apparatus for a turbine engine
Lacy, Benjamin Paul [Greer, SC; Varatharajan, Balachandar [Cincinnati, OH; Ziminsky, Willy Steve [Simpsonville, SC; Kraemer, Gilbert Otto [Greer, SC; Yilmaz, Ertan [Albany, NY; Melton, Patrick Benedict [Horse Shoe, NC; Zuo, Baifang [Simpsonville, SC; Stevenson, Christian Xavier [Inman, SC; Felling, David Kenton [Greenville, SC; Uhm, Jong Ho [Simpsonville, SC
2012-04-03
A pre-mixing apparatus for a turbine engine includes a main body having an inlet portion, an outlet portion and an exterior wall that collectively establish at least one fluid delivery plenum, and a plurality of fluid delivery tubes extending through at least a portion of the at least one fluid delivery plenum. Each of the plurality of fluid delivery tubes includes at least one fluid delivery opening fluidly connected to the at least one fluid delivery plenum. With this arrangement, a first fluid is selectively delivered to the at least one fluid delivery plenum, passed through the at least one fluid delivery opening and mixed with a second fluid flowing through the plurality of fluid delivery tubes prior to being combusted in a combustion chamber of a turbine engine.
Technology Transfer Automated Retrieval System (TEKTRAN)
Improvements to reduce chemical waste and environmental pollution for variable-rate sprayers used in orchards and ornamental nurseries require inline injection techniques. A microprocessor controlled premixing inline injection system implementing a ceramic piston chemical metering pump and two small...
Ksandopulo, G.I.
1995-08-25
This report describes the structure study of the premixed hydrocarbon-oxidizer Bunsen flames burning at the atmospheric pressure and also the ones with some inhibitors added. Studies were performed on hexane, propane, methane, acetylene, and hexene flames.
Design of turbulent tangential micro-mixers that mix liquids on the nanosecond time scale.
Mitic, Sandra; van Nieuwkasteele, Jan W; van den Berg, Albert; de Vries, Simon
2015-01-15
Unravelling (bio)chemical reaction mechanisms and macromolecular folding pathways on the (sub)microsecond time scale is limited by the time resolution of kinetic instruments for mixing reactants and observation of the progress of the reaction. To improve the mixing time resolution, turbulent four- and two-jet tangential micro-mixers were designed and characterized for their mixing and (unwanted) premixing performances employing acid-base reactions monitored by a pH-sensitive fluorescent dye. The mixing performances of the micro-mixers were determined after the mixing chamber in a free-flowing jet. The premixing behavior in the vortex chamber was assessed in an optically transparent glass-silicon replica of a previously well-characterized stainless-steel four-jet tangential micro-mixer. At the highest flow rates, complete mixing was achieved in 160ns with only approximately 9% premixing of the reactants. The mixing time of 160ns is at least 50 times shorter than estimated for other fast mixing devices. Key aspects to the design of ultrafast turbulent micro-mixers are discussed. The integration of these micro-mixers with an optical flow cell would enable the study of the very onset of chemical reactions in general and of enzyme catalytic reactions in particular.
Extinction conditions of a premixed flame in a channel
Alliche, Mounir; Haldenwang, Pierre; Chikh, Salah
2010-06-15
A local refinement method is used to numerically predict the propagation and extinction conditions of a premixed flame in a channel considering a thermodiffusive model. A local refinement method is employed because of the numerous length scales that characterize this phenomenon. The time integration is self adaptive and the solution is based on a multigrid method using a zonal mesh refinement in the flame reaction zone. The objective is to determine the conditions of extinction which are characterized by the flame structure and its properties. We are interested in the following properties: the curvature of the flame, its maximum temperature, its speed of propagation and the distance separating the flame from the wall. We analyze the influence of heat losses at the wall through the thermal conductivity of the wall and the nature of the fuel characterized by the Lewis number of the mixture. This investigation allows us to identify three propagation regimes according to heat losses at the wall and to the channel radius. The results show that there is an intermediate value of the radius for which the flame can bend and propagate provided that its curvature does not exceed a certain limit value. Indeed, small values of the radius will choke the flame and extinguish it. The extinction occurs if the flame curvature becomes too small. Furthermore, this study allows us to predict the limiting values of the heat loss coefficient at extinction as well as the critical value of the channel radius above which the premixed flame may propagate without extinction. A dead zone of length 2-4 times the flame thickness appears between the flame and the wall for a Lewis number (Le) between 0.8 and 2. For small values of Le, local extinctions are observed. (author)
PREFACE: Turbulent Mixing and Beyond Turbulent Mixing and Beyond
NASA Astrophysics Data System (ADS)
Abarzhi, Snezhana I.; Gauthier, Serge; Rosner, Robert
2008-10-01
The goals of the International Conference `Turbulent Mixing and Beyond' are to expose the generic problem of Turbulence and Turbulent Mixing in Unsteady Flows to a wide scientific community, to promote the development of new ideas in tackling the fundamental aspects of the problem, to assist in the application of novel approaches in a broad range of phenomena, where the non-canonical turbulent processes occur, and to have a potential impact on technology. The Conference provides the opportunity to bring together scientists from the areas which include, but are not limited to, high energy density physics, plasmas, fluid dynamics, turbulence, combustion, material science, geophysics, astrophysics, optics and telecommunications, applied mathematics, probability and statistics, and to have their attention focused on the long-standing formidable task. The Turbulent Mixing and Turbulence in Unsteady Flows, including multiphase flows, plays a key role in a wide variety of phenomena, ranging from astrophysical to nano-scales, under either high or low energy density conditions. Inertial confinement and magnetic fusion, light-matter interaction and non-equilibrium heat transfer, properties of materials under high strain rates, strong shocks, explosions, blast waves, supernovae and accretion disks, stellar non-Boussinesq and magneto-convection, planetary interiors and mantle-lithosphere tectonics, premixed and non-premixed combustion, oceanography, atmospheric flows, unsteady boundary layers, hypersonic and supersonic flows, are a few examples to list. A grip on unsteady turbulent processes is crucial for cutting-edge technology such as laser-micromachining and free-space optical telecommunications, and for industrial applications in aeronautics. Unsteady Turbulent Processes are anisotropic, non-local and multi-scale, and their fundamental scaling, spectral and invariant properties depart from the classical Kolmogorov scenario. The singular aspects and similarity of the
Nusselt number scaling in tokamak plasma turbulence
Takeda, K.; Benkadda, S.; Hamaguchi, S.; Wakatani, M.
2005-05-15
Anomalous heat transport caused by ion temperature gradient (ITG) driven turbulence in tokamak plasmas is evaluated from numerical simulations of the two-dimensional (2D) partial-differential equations of the ITG model and of a reduced 1D version derived from a quasilinear approximation. In the strongly turbulent state, intermittent bursts of thermal transport are observed in both cases. In the strongly turbulent regime, the reduced model as well as the direct numerical simulation show that the Nusselt number Nu (normalized heat flux) scales with the normalized ion pressure gradient K{sub i} as Nu{proportional_to}K{sub i}{sup 1/3}. Since the Rayleigh number for ITG turbulence is proportional to K{sub i}, the Nusselt number scaling for ITG turbulence is thus similar to the classical thermal transport scaling for Rayleigh-Benard convections in neutral fluids.
Wu, Fan; Ngothai, Yung; Wei, Jie; Liu, Changsheng; O'Neill, Brian; Wu, Yuequn
2012-04-01
By using a non-aqueous solution as the mixing liquid, the washout resistance of the calcium deficient apatite biocement (cd-AB) was significantly improved, over that of the conventional method of using cd-AB with water as the liquid phase. In this study, premixed and injectable cd-AB was prepared, which had the advantage of being stable in the syringe and hardens only after being delivered to the defect area. The cd-AB powder with a Ca/P ratio of 1.5 consists of a mixture of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA). A solution of polylactide (PLA) in N-methyl-2-pyrrolidone (NMP) was used as the liquid phase of the premixed cd-AB. The premixed cd-AB paste injected into an aqueous environment exhibited excellent washout resistance. The premixed cd-AB had longer setting time and lower compressive strength than conventional cd-AB. The hydration products of premixed cd-AB were a mixture of calcium deficient hydroxyapatite (cd-HA) and PLA. In vitro Tris-HCl immersion tests demonstrated that the premixed cd-AB could be degradable. The results revealed that the premixed cd-AB was cytocompatible and had no adverse effects on the attachment and proliferation of MG-63 osteoblast-like cells in vitro. The most distinct advantages of premixed and injectable PLA-modified cd-AB were its excellent washout resistance and in vitro degradability, suggesting that it may be a promising candidate for bone repair.
Stopper, U.; Aigner, M.; Ax, H.; Meier, W.; Sadanandan, R.; Stoehr, M.; Bonaldo, A.
2010-04-15
Several laser diagnostic measurement techniques have been applied to study the lean premixed natural gas/air flames of an industrial swirl burner. This was made possible by equipping the burner with an optical combustion chamber that was installed in the high-pressure test rig facility at the DLR Institute of Combustion Technology in Stuttgart. The burner was operated with preheated air at various operating conditions with pressures up to p = 6 bar and a maximum thermal power of P = 1 MW. The instantaneous planar flow field inside the combustor was studied with particle image velocimetry (PIV). Planar laser induced fluorescence (PLIF) of OH radicals on a single-shot basis was used to determine the shape and the location of the flame front as well as the spatial distribution of reaction products. 1D laser Raman spectroscopy was successfully applied for the measurement of the temperature and the concentration of major species under realistic gas turbine conditions. Results of the flow field analysis show the shape and the size of the main flow regimes: the inflow region, the inner and the outer recirculation zone. The highly turbulent flow field of the inner shear layer is found to be dominated by small and medium sized vortices. High RMS fluctuations of the flow velocity in the exhaust gas indicate the existence of a rotating exhaust gas swirl. From the PLIF images it is seen that the primary reactions happened in the shear layers between inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters. The results of the multiscalar Raman measurements show a strong variation of the local mixture fraction allowing conclusions to be drawn about the premix quality. Furthermore, mixing effects of unburnt fuel and air with fully reacted combustion products are studied giving insights into the processes of the turbulence-chemistry interaction. (author)
Magnetohydrodynamic Turbulence
NASA Astrophysics Data System (ADS)
Montgomery, David C.
2004-01-01
Magnetohydrodynamic (MHD) turbulence theory is modeled on neutral fluid (Navier-Stokes) turbulence theory, but with some important differences. There have been essentially no repeatable laboratory MHD experiments wherein the boundary conditions could be controlled or varied and a full set of diagnostics implemented. The equations of MHD are convincingly derivable only in the limit of small ratio of collision mean-free-paths to macroscopic length scales, an inequality that often goes the other way for magnetofluids of interest. Finally, accurate information on the MHD transport coefficients-and thus, the Reynolds-like numbers that order magnetofluid behavior-is largely lacking; indeed, the algebraic expressions used for such ingredients as the viscous stress tensor are often little more than wishful borrowing from fluid mechanics. The one accurate thing that has been done extensively and well is to solve the (strongly nonlinear) MHD equations numerically, usually in the presence of rectangular periodic boundary conditions, and then hope for the best when drawing inferences from the computations for those astrophysical and geophysical MHD systems for which some indisputably turbulent detailed data are available, such as the solar wind or solar prominences. This has led to what is perhaps the first field of physics for which computer simulations are regarded as more central to validating conclusions than is any kind of measurement. Things have evolved in this way due to a mixture of the inevitable and the bureaucratic, but that is the way it is, and those of us who want to work on the subject have to live with it. It is the only game in town, and theories that have promised more-often on the basis of some alleged ``instability''-have turned out to be illusory.
1983-09-01
turbulent non premixed ocmbustion", Recherche Adrospatiale, To be published. Comb. Sol . Tech., vol. 27, p. 69-73., Borghi, ., (1979) "Reactions chimiques ...34, i. of Appl. Mech. 32, 721-734. 0 0.2 0’ 0 0.8 Rodi, W., 1975, "A new method of analysing hot-wire sig- L nals in highly turbulent flow, and its...Lisboa (CTANFUTL). L several hours to analyse all the frequency results measured for a period of around three minutes and stored REFEIRECES - in the
NASA Astrophysics Data System (ADS)
Thomsen, Duane Douglas
1999-10-01
Laser-induced fluorescence (LIF) measurements of NO concentration in a variety of CH4/O2/N2 flames are used to evaluate the chemical kinetics of NO formation. The analysis begins with previous measurements in flat, laminar, premixed CH4/O2/N 2 flames stabilized on a water-cooled McKenna burner at pressures ranging from 1 to 14.6 atm, equivalence ratios from 0.5 to 1.6, and volumetric nitrogen/oxygen dilution ratios of 2.2, 3.1 and 3.76. These measured results are compared to predictions to determine the capabilities and limitations of the comprehensive kinetic mechanism developed by the Gas Research Institute (GRI), version 2.11. The model is shown to predict well the qualitative trends of NO formation in lean-premixed flames, while quantitatively underpredicting NO concentration by 30-50%. For rich flames, the model is unable to even qualitatively match the experimental results. These flames were found to be limited by low temperatures and an inability to separate the flame from the burner surface. In response to these limitations, a counterflow burner was designed for use in opposed premixed flame studies. A new LIF calibration technique was developed and applied to obtain quantitative measurements of NO concentration in laminar, counterflow premixed, CH 4/O2/N2 flames at pressures ranging from 1 to 5.1 atm, equivalence ratios of 0.6 to 1.5, and an N2/O2 dilution ratio of 3.76. The counterflow premixed flame measurements are combined with measurements in burner-stabilized premixed flames and counterflow diffusion flames to build a comprehensive database for analysis of the GRI kinetic mechanism. Pathways, quantitative reaction path and sensitivity analyses are applied to the GRI mechanism for these flame conditions. The prompt NO mechanism is found to severely underpredict the amount of NO formed in rich premixed and nitrogen-diluted diffusion flames. This underprediction is traced to uncertainties in the CH kinetics as well as in the nitrogen oxidation chemistry
Experimental investigation of stabilization mechanisms in turbulent, lifted jet diffusion flames
Su, L.K.; Sun, O.S.; Mungal, M.G.
2006-02-01
Simultaneous planar-laser induced fluorescence (PLIF) and particle image velocimetry (PIV) provide a comprehensive view of the molecular mixing and velocity fields in the stabilization region of turbulent, lifted jet diffusion flames. The Mie scattering medium for PIV is a glycerol-water fog, which evaporates at elevated temperatures and allows inference of the location of the high-temperature interface at the flame base. The jet Reynolds numbers vary from 4400 to 10,700. The mixing and velocity fields upstream of the flame base evolve consistently with nonreacting jet scaling. Conditional statistics of the fuel mole fraction at the instantaneous high-temperature interface show that the flame stabilization point does not generally correspond to the most upstream point on the interface (called here the leading point), because the mixture there is typically too lean to support combustion. Instead, the flame stabilization point lies toward the jet centerline relative to the leading point. Conditional axial velocity statistics indicate that the mean axial velocity at the flame front is {approx}1.8S{sub L}, where S{sub L} is the stoichiometric laminar flame speed. The data also permit determination of the scalar dissipation rates, {chi}, with the results indicating that {chi} values near the high-temperature interfaces do not typically exceed the quenching value. Thus, the flame stabilization process is more consistent with theories based on partial fuel-air premixing than with those dependent on diffusion flame quenching. We propose a description of flame stabilization that depends on the large-scale organization of the mixing field. (author)
NASA Technical Reports Server (NTRS)
Anderson, O. L.; Chiappetta, L. M.; Edwards, D. E.; Mcvey, J. B.
1982-01-01
A model for predicting the distribution of liquid fuel droplets and fuel vapor in premixing-prevaporizing fuel-air mixing passages of the direct injection type is reported. This model consists of three computer programs; a calculation of the two dimensional or axisymmetric air flow field neglecting the effects of fuel; a calculation of the three dimensional fuel droplet trajectories and evaporation rates in a known, moving air flow; a calculation of fuel vapor diffusing into a moving three dimensional air flow with source terms dependent on the droplet evaporation rates. The fuel droplets are treated as individual particle classes each satisfying Newton's law, a heat transfer, and a mass transfer equation. This fuel droplet model treats multicomponent fuels and incorporates the physics required for the treatment of elastic droplet collisions, droplet shattering, droplet coalescence and droplet wall interactions. The vapor diffusion calculation treats three dimensional, gas phase, turbulent diffusion processes. The analysis includes a model for the autoignition of the fuel air mixture based upon the rate of formation of an important intermediate chemical species during the preignition period.
Aspects of Turbulent / Non-Turbulent Interfaces
NASA Technical Reports Server (NTRS)
Bisset, D. K.; Hunt, J. C. R.; Rogers, M. M.; Koen, Dennis (Technical Monitor)
1999-01-01
A distinct boundary between turbulent and non-turbulent regions in a fluid of otherwise constant properties is found in many laboratory and engineering turbulent flows, including jets, mixing layers, boundary layers and wakes. Generally, the flow has mean shear in at least one direction within t he turbulent zone, but the non-turbulent zones have no shear (adjacent laminar shear is a different case, e.g. transition in a boundary layer). There may be purely passive differences between the turbulent and non-turbulent zones, e.g. small variations in temperature or scalar concentration, for which turbulent mixing is an important issue. The boundary has several major characteristics of interest for the present study. Firstly, the boundary advances into the non-turbulent fluid, or in other words, nonturbulent fluid is entrained. Secondly, the change in turbulence properties across the boundary is remarkably abrupt; strong turbulent motions come close to the nonturbulent fluid, promoting entrainment. Thirdly, the boundary is irregular with a continually changing convoluted shape, which produces statistical intermittency. Its shape is contorted at all scales of the turbulent motion.
Turbulence and turbulent mixing in natural fluids
NASA Astrophysics Data System (ADS)
Gibson, Carl H.
2010-12-01
Turbulence and turbulent mixing in natural fluids begin with big bang turbulence powered by spinning combustible combinations of Planck particles and Planck antiparticles. Particle prograde accretions on a spinning pair release 42% of the particle rest mass energy to produce more fuel for turbulent combustion. Negative viscous stresses and negative turbulence stresses work against gravity, extracting mass-energy and space-time from the vacuum. Turbulence mixes cooling temperatures until strong-force viscous stresses freeze out turbulent mixing patterns as the first fossil turbulence. Cosmic microwave background temperature anisotropies show big bang turbulence fossils along with fossils of weak plasma turbulence triggered as plasma photon-viscous forces permitting gravitational fragmentation on supercluster to galaxy mass scales. Turbulent morphologies and viscous-turbulent lengths appear as linear gas-protogalaxy-clusters in the Hubble ultra-deep field at z~7. Protogalaxies fragment into Jeans mass clumps of primordial-gas planets at decoupling: the dark matter of galaxies. Shortly after the plasma-to-gas transition, planet mergers produce stars that explode on overfeeding to fertilize and distribute the first life.
Chen, Jacqueline H.; Hawkes, Evatt R.
2004-08-01
Direct numerical simulation (DNS) with complex chemistry was used to study statistics of displacement and consumption speeds in turbulent lean premixed methane-air flames. The main focus of the study is an evaluation of the extent to which a turbulent flame in the thin reaction zones regime can be described by an ensemble of strained laminar flames. Conditional averages with respect to strain for displacement and consumption speeds are presented over a wide range of strain typically encountered in a turbulent flame, compared with previous studies that either made local pointwise comparisons or conditioned the data on small strain and curvature. The conditional averages for positive strains are compared with calculated data from two different canonical strained laminar configurations to determine which is the optimal representation of a laminar flame structure embedded in a turbulent flame: the reactant-to-product (R-to-P) configuration or the symmetric twin flame configuration. Displacement speed statistics are compared for the progress-variable isosurface of maximum reaction rate and an isosurface toward the fresh gases, which are relevant for both modeling and interpretation of experiment results. Displacement speeds in the inner reaction layer are found to agree very well with the laminar R-to-P calculations over a wide range of strain for higher Damkhler number conditions, well beyond the regime in which agreement was expected. For lower Damkhler numbers, a reduced response to strain is observed, consistent with previous studies and theoretical expectations. Compared with the inner layer, broader and shifted probability density functions (PDFs) of displacement speed were observed in the fresh gases, and the agreement with the R-to-P calculations deteriorated. Consumption speeds show a poorer agreement with strained laminar calculations, which is attributed to multidimensional effects and a more attenuated unsteady response to strain fluctuations; however, they
Hawkes, Evatt R.; Chen, Jacqueline H.
2006-01-01
Direct numerical simulation (DNS) with complex chemistry was used to study statistics of displacement and consumption speeds in turbulent lean premixed methane-air flames. The main focus of the study is an evaluation of the extent to which a turbulent flame in the thin reaction zones regime can be described by an ensemble of strained laminar flames. Conditional averages with respect to strain for displacement and consumption speeds are presented over a wide range of strain typically encountered in a turbulent flame, compared with previous studies that either made local pointwise comparisons or conditioned the data on small strain and curvature. The conditional averages for positive strains are compared with calculated data from two different canonical strained laminar configurations to determine which is the optimal representation of a laminar flame structure embedded in a turbulent flame: the reactant-to-product (R-to-P) configuration or the symmetric twin flame configuration. Displacement speed statistics are compared for the progress-variable isosurface of maximum reaction rate and an isosurface toward the fresh gases, which are relevant for both modeling and interpretation of experiment results. Displacement speeds in the inner reaction layer are found to agree very well with the laminar R-to-P calculations over a wide range of strain for higher Damkohler number conditions, well beyond the regime in which agreement was expected. For lower Damkohler numbers, a reduced response to strain is observed, consistent with previous studies and theoretical expectations. Compared with the inner layer, broader and shifted probability density functions (PDFs) of displacement speed were observed in the fresh gases, and the agreement with the R-to-P calculations deteriorated. Consumption speeds show a poorer agreement with strained laminar calculations, which is attributed to multidimensional effects and a more attenuated unsteady response to strain fluctuations; however
NASA Astrophysics Data System (ADS)
Poludnenko, Alexei
2016-11-01
Turbulent reacting flows are pervasive both in our daily lives on Earth and in the Universe. They power modern society being at the heart of many energy generation and propulsion systems, such as gas turbines, internal combustion and jet engines. On astronomical scales, thermonuclear turbulent flames are the driver of some of the most powerful explosions in the Universe, knows as Type Ia supernovae. Despite this ubiquity in Nature, turbulent reacting flows still pose a number of fundamental questions often exhibiting surprising and unexpected behavior. In this talk, we will discuss several such phenomena observed in direct numerical simulations of high-speed, premixed, turbulent flames. We show that turbulent flames in certain regimes are intrinsically unstable even in the absence of the surrounding combustor walls or obstacles, which can support the thermoacoustic feedback. Such instability can fundamentally change the structure and dynamics of the turbulent cascade, resulting in a significant (and anisotropic) redistribution of kinetic energy from small to large scales. In particular, three effects are observed. 1) The turbulent burning velocity can develop pulsations with significant peak-to-peak amplitudes. 2) Unstable burning can result in pressure build-up and the formation of pressure waves or shocks when the flame speed approaches or exceeds the speed of a Chapman-Jouguet deflagration. 3) Coupling of pressure and density gradients across the flame can lead to the anisotropic generation of turbulence inside the flame volume and flame acceleration. We extend our earlier analysis, which relied on a simplified single-step reaction model, by demonstrating existence of these effects in realistic chemical flames (hydrogen and methane) and in thermonuclear flames in degenerate, relativistic plasmas found in stellar interiors. Finally, we discuss the implications of these results for subgrid-scale LES combustion models. This work was supported by the Air Force
The interaction of high-speed turbulence with flames: Turbulent flame speed
Poludnenko, A.Y.; Oran, E.S.
2011-02-15
Direct numerical simulations of the interaction of a premixed flame with driven, subsonic, homogeneous, isotropic, Kolmogorov-type turbulence in an unconfined system are used to study the mechanisms determining the turbulent flame speed, S{sub T}, in the thin reaction zone regime. High intensity turbulence is considered with the r.m.s. velocity 35 times the laminar flame speed, S{sub L}, resulting in the Damkoehler number Da=0.05. The simulations were performed with Athena-RFX, a massively parallel, fully compressible, high-order, dimensionally unsplit, reactive-flow code. A simplified reaction-diffusion model, based on the one-step Arrhenius kinetics, represents a stoichiometric H{sub 2}-air mixture under the assumption of the Lewis number Le=1. Global properties and the internal structure of the flame were analyzed in an earlier paper, which showed that this system represents turbulent combustion in the thin reaction zone regime. This paper demonstrates that: (1) The flame brush has a complex internal structure, in which the isosurfaces of higher fuel mass fractions are folded on progressively smaller scales. (2) Global properties of the turbulent flame are best represented by the structure of the region of peak reaction rate, which defines the flame surface. (3) In the thin reaction zone regime, S{sub T} is predominantly determined by the increase of the flame surface area, A{sub T}, caused by turbulence. (4) The observed increase of S{sub T} relative to S{sub L} exceeds the corresponding increase of A{sub T} relative to the surface area of the planar laminar flame, on average, by {approx}14%, varying from only a few percent to as high as {approx}30%. (5) This exaggerated response is the result of tight flame packing by turbulence, which causes frequent flame collisions and formation of regions of high flame curvature >or similar 1/{delta}{sub L}, or ''cusps,'' where {delta}{sub L} is the thermal width of the laminar flame. (6) The local flame speed in the cusps
Cheng, R.K.; Bedat, B.; Yegian, D.T.
1999-07-01
The field effects of buoyancy on laminar and turbulent premixed v-flames have been studied by the use of laser Doppler velocimetry to measure the velocity statistics in +1g, -1g and {micro}g flames. The experimental conditions covered mean velocity, Uo, of 0.4 to 2 m/s, methane/air equivalence ratio, f, of 0.62 to 0.75. The Reynolds numbers, from 625 to 3130 and the Richardson number from 0.05 to 1.34. The results show that a change from favorable (+1g) to unfavorable (-1g) mean pressure gradient in the plume create stagnating flows in the far field whose influences on the mean and fluctuating velocities persist in the near field even at the highest Re we have investigated. The use of Richardson number < 0.1 as a criterion for momentum dominance is not sufficient to prescribe an upper limit for these buoyancy effects. In {micro}g, the flows within the plumes are non-accelerating and parallel. Therefore, velocity gradients and hence mean strain rates in the plumes of laboratory flames are direct consequences of buoyancy. Furthermore, the rms fluctuations in the plumes of {micro}g flames are lower and more isotropic than in the laboratory flames to show that the unstable plumes in laboratory flames also induce velocity fluctuations. The phenomena influenced by buoyancy i.e. degree of flame wrinkling, flow acceleration, flow distribution, and turbulence production, can be subtle due to their close coupling with other flame flow interaction processes. But they cannot be ignored in fundamental studies or else the conclusions and insights would be ambiguous and not very meaningful.
Short communication: Preference for flavored concentrate premixes by dairy cows.
Harper, M T; Oh, J; Giallongo, F; Lopes, J C; Weeks, H L; Faugeron, J; Hristov, A N
2016-08-01
Flavor preferences may be used to stimulate feed intake in dairy cows, which may improve use of robotic milking systems and increase feed intake of sick cows. A cafeteria-design experiment was used to determine if dairy cows have flavor preferences. Sixteen lactating Holstein cows averaging 197±32d in milk, 1.9±0.8 lactations, 27.8±4.2kg/d of dry matter intake, and 41.5±7.4kg/d of milk yield were involved in the experiment. Cows were offered 7 flavored concentrate premixes (FCP) and 1 control premix. The FCP flavors were anise, fenugreek, honey, orange, thyme, molasses, and vanilla; the absence of flavor, neutral, acted as a control. The inclusion rate of the flavors in FCP was 250 to 300g/t on an as-is basis. Cows were not adapted to the flavors before the experiment. Cows were housed in a tiestall barn and offered, on each day, 4 different FCP (1kg each) in plastic bins placed in front of each cow. The experiment lasted 6 consecutive days. Each FCP was presented to each cow once every 2d, 2h after the morning feeding. Flavors and position of the bins in front of the cows were randomized. As a result, each flavor was presented to each cow 3 times during the experiment, at 3 different bin locations. Each cow had access to the FCP for 5min from the time they started eating. Eating time and amount eaten were recorded. The vanilla and fenugreek FCP were consumed the most, at 408 and 371g/5-min offering, respectively, whereas the orange and anise FCP were consumed the least, at 264 and 239g/5-min offering, respectively. Similarly, cows spent the most time eating the vanilla and fenugreek FCP at 99 and 75 s/offering, respectively, and the least amount of time eating the orange and anise FCP at 49 and 50 s/offering, respectively. We detected an effect of bin position: the 2 center FCP were consumed more than the outer 2 FCP. Flavor had no effect on consumption rate. In conclusion, relative to the control, concentrate intake was not affected by flavor, but dairy cows
Suarez, Lorena; Rodriguez, Maria Galiana; Sanchez, Cecilia; Dieguez, Marta; Riestra, Maria; Casal, Florentino; Delgado, Elias; Menendez, Edelmiro; Umpierrez, Guillermo E.
2015-01-01
OBJECTIVE Premixed insulin is a commonly prescribed formulation for the outpatient management of patients with type 2 diabetes. The safety and efficacy of premixed insulin formulations in the hospital setting is not known. RESEARCH DESIGN AND METHODS In a prospective, open-label trial, we randomized general medicine and surgery patients to receive a basal-bolus regimen with glargine once daily and glulisine before meals (n = 33) or premixed human insulin (30% regular insulin and 70% NPH insulin) twice daily (n = 39). Major outcomes included differences in daily blood glucose (BG) levels and frequency of hypoglycemic events (<70 mg/dL) between treatment groups. RESULTS At the first prespecified interim analysis, the study was stopped early because of an increased frequency of hypoglycemia >50% in patients treated with premixed human insulin. A total of 64% of patients treated with premixed insulin experienced one or more episodes of hypoglycemia compared with 24% in the basal-bolus group (P < 0.001). There were no differences in mean daily BG level after the first day of insulin treatment (175 ± 32 vs. 179 ± 43 mg/dL, P = 0.64) between groups. A BG target between 80 and 180 mg/dL before meals was achieved in 55.9% of BG readings in the basal-bolus group and 54.3% of BG readings in the premixed insulin group (P = 0.23). There was no difference in the length of hospital stay or mortality between treatment groups. CONCLUSIONS Inpatient treatment with premixed human insulin resulted in similar glycemic control but in significantly higher frequency of hypoglycemia compared with treatment with basal-bolus insulin regimen in hospitalized patients with diabetes. PMID:26459273
NASA Astrophysics Data System (ADS)
Danby, Sean James
The understanding of complex premixed combustion reactions is paramount to the development of new concepts and devices used to increase the overall usefulness and capabilities of current technology. The complex interactions which occur within any modern practical combustion device were studied by isolating a single turbulent scale of the turbulence-chemistry interaction. Methane-air flame equivalence ratios (φ = 0.64, 0.90, and 1.13) were chosen to observe the mild affects of thermo-diffusive stability on the methane-air flame. Nitrogen was used as a diluent to retard the flame speeds of the φ = 0.90, and 1.13 mixtures so that the undisturbed outwardly propagating spherical flame kernel propagation rates, drf/dt, were approximately equal. Five primary propane equivalence ratios were utilized for investigation: φ = 0.69, 0.87, 1.08, 1.32, and 1.49. The choice of equivalence ratio was strategically made so that the φ = 0.69/1.49 and φ = 0.87/1.32 mixtures have the same undiluted flame propagation rate, drf/dt. Therefore, in the undiluted case, there are three flame speeds (in laboratory coordinates, not to be confused with burning velocity) represented by these mixtures. Three vortices were selected to be used in this investigation. The vortex rotational velocities were measured to be 77 cm/s, 266 cm/s and 398 cm/s for the "weak", "medium" and "strong" vortices, respectively. Ignition of the flame occurred in two ways: (1) spark-ignition or (2) laser ignition using an Nd:YAG laser at its second harmonic (lambda = 532 nm) in order to quantify the effect of electrode interference. Accompanying high-speed chemiluminescence imaging measurements, instantaneous pressure measurements were obtained to give a more detailed understanding of the effect of vortex strength on the overall flame speed and heat release rate over an extended time scale and to explore the use of a simple measurement to describe turbulent mixing. Further local flame-vortex interface analysis was
Turbulence modeling of gas-solid suspension flows
NASA Technical Reports Server (NTRS)
Chen, C. P.
1988-01-01
The purpose here is to discuss and review advances in two-phase turbulent modeling techniques and their applications in various gas-solid suspension flow situations. In addition to the turbulence closures, heat transfer effect, particle dispersion and wall effects are partially covered.
A generalized four-fifth law for compressible turbulence
NASA Astrophysics Data System (ADS)
Aluie, Hussein
2016-11-01
Kolmogorov's 4/5-th law is a celebrated exact result of incompressible turbulence, and is key to the formulation of his 1941 phenomenology. We will present its generalization to compressible turbulence. Partial support was provided by NSF Grant OCE-1259794, US Department of Energy (US DOE) Grant DE-SC0014318, and the LANL LDRD program through Project Number 20150568ER.
Statistical turbulence theory and turbulence phenomenology
NASA Technical Reports Server (NTRS)
Herring, J. R.
1973-01-01
The application of deductive turbulence theory for validity determination of turbulence phenomenology at the level of second-order, single-point moments is considered. Particular emphasis is placed on the phenomenological formula relating the dissipation to the turbulence energy and the Rotta-type formula for the return to isotropy. Methods which deal directly with most or all the scales of motion explicitly are reviewed briefly. The statistical theory of turbulence is presented as an expansion about randomness. Two concepts are involved: (1) a modeling of the turbulence as nearly multipoint Gaussian, and (2) a simultaneous introduction of a generalized eddy viscosity operator.
NASA Astrophysics Data System (ADS)
Venaille, Antoine; Nadeau, Louis-Philippe; Vallis, Geoffrey
2014-12-01
We investigate the non-linear equilibration of a two-layer quasi-geostrophic flow in a channel with an initial eastward baroclinically unstable jet in the upper layer, paying particular attention to the role of bottom friction. In the limit of low bottom friction, classical theory of geostrophic turbulence predicts an inverse cascade of kinetic energy in the horizontal with condensation at the domain scale and barotropization in the vertical. By contrast, in the limit of large bottom friction, the flow is dominated by ribbons of high kinetic energy in the upper layer. These ribbons correspond to meandering jets separating regions of homogenized potential vorticity. We interpret these results by taking advantage of the peculiar conservation laws satisfied by this system: the dynamics can be recast in such a way that the initial eastward jet in the upper layer appears as an initial source of potential vorticity levels in the upper layer. The initial baroclinic instability leads to a turbulent flow that stirs this potential vorticity field while conserving the global distribution of potential vorticity levels. Statistical mechanical theory of the 1 1/2 layer quasi-geostrophic model predicts the formation of two regions of homogenized potential vorticity separated by a minimal interface. We explain that cascade phenomenology leads to the same result. We then show that the dynamics of the ribbons results from a competition between a tendency to reach the equilibrium state and baroclinic instability that induces meanders of the interface. These meanders intermittently break and induce potential vorticity mixing, but the interface remains sharp throughout the flow evolution. We show that for some parameter regimes, the ribbons act as a mixing barrier which prevents relaxation toward equilibrium, favouring the emergence of multiple zonal (eastward) jets.
The effect of turbulence on pollutant formation in a tubular burner
NASA Technical Reports Server (NTRS)
Robinson, G. F.; Kovitz, A. A.
1975-01-01
The role of turbulence in pollutant formation was investigated both experimentally and analytically with the objective of understanding the important mechanisms of a fairly basic level. Preliminary measurements of the cold-flow turbulence characteristics downstream of three perforated plate flame holders, of varying solidity ratio, were made. In the combustion experiment, measurements of the concentration distributions of CO, NO(x), HC, and CO2, downstream of the same flame holders, were made. The combustor configuration was tubular and constant-area, and the fuel was premixed, prevaporized propane. The experimental results were modeled by a one-dimensional analysis in which transport phenomena were neglected, and more successfully, by a quasi-one-dimensional analysis containing recirculation zones and turbulent transport effects.
On flame kernel formation and propagation in premixed gases
Eisazadeh-Far, Kian; Metghalchi, Hameed; Parsinejad, Farzan; Keck, James C.
2010-12-15
Flame kernel formation and propagation in premixed gases have been studied experimentally and theoretically. The experiments have been carried out at constant pressure and temperature in a constant volume vessel located in a high speed shadowgraph system. The formation and propagation of the hot plasma kernel has been simulated for inert gas mixtures using a thermodynamic model. The effects of various parameters including the discharge energy, radiation losses, initial temperature and initial volume of the plasma have been studied in detail. The experiments have been extended to flame kernel formation and propagation of methane/air mixtures. The effect of energy terms including spark energy, chemical energy and energy losses on flame kernel formation and propagation have been investigated. The inputs for this model are the initial conditions of the mixture and experimental data for flame radii. It is concluded that these are the most important parameters effecting plasma kernel growth. The results of laminar burning speeds have been compared with previously published results and are in good agreement. (author)
Rayleigh-Taylor Instability in non-premixed reacting flames.
NASA Astrophysics Data System (ADS)
Attal, Nitesh; Ramaprabhu, Praveen
2015-11-01
The Rayleigh-Taylor instability (RTI) occurs at a perturbed interface between fluids of different densities when a light fluid pushes a heavier fluid. The mixing driven by RTI affects several physical phenomena, such as Inertial Confinement Fusion, Supernovae detonation, centrifugal combustors and liquid rocket engines. The RTI in such flows is often coupled with chemical/nuclear reactions that may form complex density stratifications in the form of flames or ablative layers. We investigate such a non-premixed fuel-air interface subject to a constant acceleration and developing under the influence of chemical reactions using high-resolution, Navier-Stokes simulations. The H2 fuel is diluted with N2 to vary the density difference across the interface in thermal equilibrium (at 1000K). The intervening layer between fuel and air is subject to exothermic combustion reactions to form a flame. Following combustion, initially unstable fuel-air interfaces at an Atwood number (At) <0.5, transform into stable (fuel-flame) and unstable (flame-air) interfaces. We report on interfaces (At = 0.2 and 0.6) with single wavelength, sinusoidal perturbations and a broadband spectrum of multimode perturbations.
Stationary premixed flames in spherical and cylindrical geometries
NASA Technical Reports Server (NTRS)
Ronney, P. D.; Whaling, K. N.; Abbud-Madrid, A.; Gatto, J. L.; Pisowiscz, V. L.
1994-01-01
Stationary source-free spherical flames ('flame balls') in premixed combustible gases were studied by employing low-gravity (micro-g) environments in a drop tower and an aircraft flying parabolic trajectories to diminish the impact of buoyancy-induced convective flow. Flame balls were found in all mixture families tested when: (1) the Lewis number Le of the deficient reactant was sufficiently low; and (2) the compositions were sufficiently close to the flammability limits. Probably as a consequence of the reduction in buoyant convection, the flammability limits at micro-g were significantly more dilute than those at Earth gravity; for example, 3.35% H2 vs 4.0% H2 in lean H2-air mixtures. By comparison with analytical and computational models, it is inferred that the phenomenon is probably related to diffusive-thermal effects in low-Le mixtures in conjunction with flame-front curvature and radiative heat losses from the combustion products. The chemical reaction mechanism appears to play no qualitative role. In the aircraft experiments, the gravity levels (approximately equal 10(exp -2)g(sub 0)) were found to cause noticeable motion of flame balls due to buoyancy, which in turn influenced the behavior of flame balls. At these g levels, a new type of transient, nearly cylindrical flame structure, termed 'flame strings,' was observed.
Structural study of non-premixed tubular hydrocarbon flames
Hu, Shengteng; Pitz, Robert W.
2009-01-15
Tubular non-premixed flames are formed by a uniquely designed opposed tubular burner. Structural measurements of hydrocarbon flames are conducted using the laser-induced Raman scattering technique. Temperature and major species concentrations are recorded for flames produced by 30% CH{sub 4}/N{sub 2} and 15% C{sub 3}H{sub 8}/N{sub 2} burning against air. Numerical simulations of these flames with detailed chemistry show good agreement between the measured and simulated results. By comparing the numerical results of the tubular curved flames to those of the opposed-jet planar flames, it is shown that flame curvature towards the fuel stream strongly effects the temperature ({+-}80 K) of flames with low fuel Lewis number (15% H{sub 2}/N{sub 2}, Le{sub f} = 0.41). The effect of curvature on flames with high (15% C{sub 3}H{sub 8}/N{sub 2}, Le{sub f} = 1.51) and near-unity (30% CH{sub 4}/N{sub 2}, Le{sub f}{approx_equal}1) fuel Lewis numbers is much less. (author)
Unstrained and strained flamelets for LES of premixed combustion
NASA Astrophysics Data System (ADS)
Langella, Ivan; Swaminathan, Nedunchezhian
2016-05-01
The unstrained and strained flamelet closures for filtered reaction rate in large eddy simulation (LES) of premixed flames are studied. The required sub-grid scale (SGS) PDF in these closures is presumed using the Beta function. The relative performances of these closures are assessed by comparing numerical results from large eddy simulations of piloted Bunsen flames of stoichiometric methane-air mixture with experimental measurements. The strained flamelets closure is observed to underestimate the burn rate and thus the reactive scalars mass fractions are under-predicted with an over-prediction of fuel mass fraction compared with the unstrained flamelet closure. The physical reasons for this relative behaviour are discussed. The results of unstrained flamelet closure compare well with experimental data. The SGS variance of the progress variable required for the presumed PDF is obtained by solving its transport equation. An order of magnitude analysis of this equation suggests that the commonly used algebraic model obtained by balancing source and sink in this transport equation does not hold. This algebraic model is shown to underestimate the SGS variance substantially and the implications of this variance model for the filtered reaction rate closures are highlighted.
Development of a lean premixed burner for hydrogen utilization
Keller, J.O.
1996-10-01
The long-term mandate of the hydrogen program is to develop the technologies needed to establish a hydrogen economy. Although a hydrogen fueled automobile has been established as a demonstration project, there are at least three other end use sectors that are recognized by the H{sub 2} program and that are addressed by this project. These end uses are: (1) power generation from stationary turbines, (2) generation of process heat or steam, and (3) commercial and residential direct use applications. Eliminating carbon from the fuel will remove carbon containing species from the emissions, however, NO{sub x} resulting from thermal NO production cannot be ignored. Thermal NO production is minimized by reducing the peak combustion temperature and the residence time at the peak temperature. NO can be reduced to extremely low levels (a few ppm) by operating sufficiently lean to reduce the peak combustion temperatures below 1700 to 1800 K. The objectives for this project are to: (1) develop an environmentally benign and safe burner operating on hydrogen in a lean premixed mode, (2) provide a facility in which fundamental investigations can be performed to support other programs.
Numerical simulation of premixed flame propagation in a closed tube
NASA Astrophysics Data System (ADS)
Kuzuu, Kazuto; Ishii, Katsuya; Kuwahara, Kunio
1996-08-01
Premixed flame propagation of methane-air mixture in a closed tube is estimated through a direct numerical simulation of the three-dimensional unsteady Navier-Stokes equations coupled with chemical reaction. In order to deal with a combusting flow, an extended version of the MAC method, which can be applied to a compressible flow with strong density variation, is employed as a numerical method. The chemical reaction is assumed to be an irreversible single step reaction between methane and oxygen. The chemical species are CH 4, O 2, N 2, CO 2, and H 2O. In this simulation, we reproduce a formation of a tulip flame in a closed tube during the flame propagation. Furthermore we estimate not only a two-dimensional shape but also a three-dimensional structure of the flame and flame-induced vortices, which cannot be observed in the experiments. The agreement between the calculated results and the experimental data is satisfactory, and we compare the phenomenon near the side wall with the one in the corner of the tube.
Spectral output from a premixed chain reaction cw HF laser
Stanton, A.C.; Bien, F.
1980-07-01
Spectral measurements of the output from a purely chemical chain reaction cw HF laser are reported. The laser is a subsonic H/sub 2/-F/sub 2/ flame, with supersonic premixing and spatially uniform initiation by a stationary normal shock. Initial chemical production of fluorine atoms is by the bimolecular reaction of F/sub 2/ with NO. Spectral measurements of the laser output near the initiating shock indicate lasing transitions in the P branches of the v=3 ..-->.. v=2, v=2 ..-->.. v=1, and v=1 ..-->.. v=0 HF bands. Further downstream, the upper vibrational levels are strongly deactivated, and lasing occurs only in the v=1 ..-->.. v=0 band. Laser emission in the v=2 ..-->.. v=1 band reappears at reduced NO flow rates, suggesting efficient deactivation of HF (v) by NO, possibly through multiquantum V-V exchange. An approximate rate of 5 x 10/sup -13plus-or-minus0.5/ cm/sup 3//sec for deactivation of HF (v=2) by NO is inferred.
Turbulent Flame Propagation Characteristics of High Hydrogen Content Fuels
Seitzman, Jerry; Lieuwen, Timothy
2014-09-30
This final report describes the results of an effort to better understand turbulent flame propagation, especially at conditions relevant to gas turbines employing fuels with syngas or hydrogen mixtures. Turbulent flame speeds were measured for a variety of hydrogen/carbon monoxide (H2/CO) and hydrogen/methane (H2/CH4) fuel mixtures with air as the oxidizer. The measurements include global consumption speeds (ST,GC) acquired in a turbulent jet flame at pressures of 1-10 atm and local displacement speeds (ST,LD) acquired in a low-swirl burner at atmospheric pressure. The results verify the importance of fuel composition in determining turbulent flame speeds. For example, different fuel-air mixtures having the same unstretched laminar flame speed (SL,0) but different fuel compositions resulted in significantly different ST,GC for the same turbulence levels (u'). This demonstrates the weakness of turbulent flame speed correlations based simply on u'/SL,0. The results were analyzed using a steady-steady leading points concept to explain the sensitivity of turbulent burning rates to fuel (and oxidizer) composition. Leading point theories suggest that the premixed turbulent flame speed is controlled by the flame front characteristics at the flame brush leading edge, or, in other words, by the flamelets that advance farthest into the unburned mixture (the so-called leading points). For negative Markstein length mixtures, this is assumed to be close to the maximum stretched laminar flame speed (SL,max) for the given fuel-oxidizer mixture. For the ST,GC measurements, the data at a given pressure were well-correlated with an SL,max scaling. However the variation with pressure was not captured, which may be due to non-quasi-steady effects that are not included in the current model. For the ST,LD data, the leading points model again faithfully captured the variation of turbulent flame speed over a wide range of fuel-compositions and turbulence intensities. These results provide
Swozzle based burner tube premixer including inlet air conditioner for low emissions combustion
Tuthill, Richard Sterling; Bechtel, II, William Theodore; Benoit, Jeffrey Arthur; Black, Stephen Hugh; Bland, Robert James; DeLeonardo, Guy Wayne; Meyer, Stefan Martin; Taura, Joseph Charles; Battaglioli, John Luigi
2002-01-01
A burner for use in a combustion system of a heavy-duty industrial gas turbine includes a fuel/air premixer having an air inlet, a fuel inlet, and an annular mixing passage. The fuel/air premixer mixes fuel and air into a uniform mixture for injection into a combustor reaction zone. The burner also includes an inlet flow conditioner disposed at the air inlet of the fuel/air premixer for controlling a radial and circumferential distribution of incoming air. The pattern of perforations in the inlet flow conditioner is designed such that a uniform air flow distribution is produced at the swirler inlet annulus in both the radial and circumference directions. The premixer includes a swozzle assembly having a series of preferably air foil shaped turning vanes that impart swirl to the airflow entering via the inlet flow conditioner. Each air foil contains internal fuel flow passages that introduce natural gas fuel into the air stream via fuel metering holes that pass through the walls of the air foil shaped turning vanes. By injecting fuel in this manner, an aerodynamically clean flow field is maintained throughout the premixer. By injecting fuel via two separate passages, the fuel/air mixture strength distribution can be controlled in the radial direction to obtain optimum radial concentration profiles for control of emissions, lean blow outs, and combustion driven dynamic pressure activity as machine and combustor load are varied.
Prediction of free shear flows: A comparison of the performance of six turbulence models
NASA Technical Reports Server (NTRS)
Launder, B. E.; Morse, A.; Rodi, W.; Spalding, D. B.
1973-01-01
The performance is evaluated of three distinct classes of turbulence model. These classes are: (1) Turbulent-viscosity models in which the length scale of turbulence is found by way of algebraic formulas, (2) turbulent-viscosity models in which the length scale of turbulence is found from a partial differential equation of transport, and (3) models in which the shear stress itself is the dependent variable of a partial differential conservation equation. Two models were examined in each class; thus, six different models were tested. A complete mathematical statement of these models is provided and a brief commentary on the models is included.
Jaojaruek, Kitipong; Jarungthammachote, Sompop; Gratuito, Maria Kathrina B; Wongsuwan, Hataitep; Homhual, Suwan
2011-04-01
This study conducted experiments on three different downdraft gasification approaches: single stage, conventional two-stage, and an innovative two-stage air and premixed air/gas supply approach. The innovative two-stage approach has two nozzle locations, one for air supply at combustion zone and the other located at the pyrolysis zone for supplying the premixed gas (air and producer gas). The producer gas is partially bypassed to mix with air and supplied to burn at the pyrolysis zone. The result shows that producer gas quality generated by the innovative two-stage approach improved as compared to conventional two-stage. The higher heating value (HHV) increased from 5.4 to 6.5 MJ/Nm(3). Tar content in producer gas reduced to less than 45 mg/Nm(3). With this approach, gas can be fed directly to an internal combustion engine. Furthermore, the gasification thermal efficiency also improved by approximately 14%. The approach gave double benefits on gas qualities and energy savings.
Exhaust emissions from a premixing, prevaporizing flame tube using liquid jet A fuel
NASA Technical Reports Server (NTRS)
Marek, C. J.; Papathakos, L. C.
1976-01-01
Emissions of nitrogen oxides, carbon monoxide, and unburned hydrocarbons were measured in a burner where liquid Jet A fuel was sprayed into the heated air stream and vaporized upstream of a perforated plate flameholder. The burner was tested at inlet air temperatures at 640, 800, and 833 K, an inlet pressure of 5.6 X 100,000 N/m squared, a reference velocity of 25 m/sec, and equivalence ratios from lean blowout to 0.7. Nitrogen oxide levels of below 1.0 g NO2/kg fuel were obtained at combustion efficiencies greater than 99 percent. The measured emission levels for the liquid fuel agreed well with previously reported premixed gaseous propane data and agreed with well stirred reactor predictions. Autoignition of the premixed fuel air mixture was a problem at inlet temperatures above 650 K with 104 msec premixing time.
Study of premixing phase of steam explosion with JASMINE code in ALPHA program
Moriyama, Kiyofumi; Yamano, Norihiro; Maruyama, Yu; Kudo, Tamotsu; Sugimoto, Jun
1996-08-01
Premixing phase of steam explosion has been studied in ALPHA Program at Japan Atomic Energy Research Institute (JAERI). An analytical model to simulate the premixing phase, JASMINE (JAERI Simulator for Multiphase Interaction and Explosion), has been developed based on a multi-dimensional multi-phase thermal hydraulics code MISTRAL (by Fuji Research Institute Co.). The original code was extended to simulate the physics in the premixing phenomena. The first stage of the code validation was performed by analyzing two mixing experiments with solid particles and water: the isothermal experiment by Gilbertson et al. (1992) and the hot particle experiment by Angelini et al. (1993) (MAGICO). The code predicted reasonably well the experiments. Effectiveness of the TVD scheme employed in the code was also demonstrated.
Vortex combustor for low NOx emissions when burning lean premixed high hydrogen content fuel
Steele, Robert C.; Edmonds, Ryan G.; Williams, Joseph T.; Baldwin, Stephen P.
2009-10-20
A trapped vortex combustor. The trapped vortex combustor is configured for receiving a lean premixed gaseous fuel and oxidant stream, where the fuel includes hydrogen gas. The trapped vortex combustor is configured to receive the lean premixed fuel and oxidant stream at a velocity which significantly exceeds combustion flame speed in a selected lean premixed fuel and oxidant mixture. The combustor is configured to operate at relatively high bulk fluid velocities while maintaining stable combustion, and low NOx emissions. The combustor is useful in gas turbines in a process of burning synfuels, as it offers the opportunity to avoid use of diluent gas to reduce combustion temperatures. The combustor also offers the possibility of avoiding the use of selected catalytic reaction units for removal of oxides of nitrogen from combustion gases exiting a gas turbine.
PREMIX: PRivacy-preserving EstiMation of Individual admiXture
Chen, Feng; Dow, Michelle; Ding, Sijie; Lu, Yao; Jiang, Xiaoqian; Tang, Hua; Wang, Shuang
2016-01-01
In this paper we proposed a framework: PRivacy-preserving EstiMation of Individual admiXture (PREMIX) using Intel software guard extensions (SGX). SGX is a suite of software and hardware architectures to enable efficient and secure computation over confidential data. PREMIX enables multiple sites to securely collaborate on estimating individual admixture within a secure enclave inside Intel SGX. We implemented a feature selection module to identify most discriminative Single Nucleotide Polymorphism (SNP) based on informativeness and an Expectation Maximization (EM)-based Maximum Likelihood estimator to identify the individual admixture. Experimental results based on both simulation and 1000 genome data demonstrated the efficiency and accuracy of the proposed framework. PREMIX ensures a high level of security as all operations on sensitive genomic data are conducted within a secure enclave using SGX. PMID:28269933
Gravitational Influences on Flame Propagation Through Non-Uniform Premixed Gas Systems
NASA Technical Reports Server (NTRS)
Miller, Fletcher J.; Easton, John; Ross, Howard D.; Marchese, Anthony
1999-01-01
Flame propagation through non-uniformly premixed gases occurs in several common combustion situations. As summarized in a previous conference paper, non-uniform premixed gas combustion has received scant attention compared to the more usual limiting cases of diffusion or uniformly premixed flames. It is the goal of this research to further our knowledge of layered combustion, in which a fuel concentration gradient exists normal to the direction of flame spread, in particular by focusing on the role that gravity plays. Gravity can affect flame propagation in at least three ways: through a hydrostatic pressure gradient, by altering the initial distribution of fuel vapor, and through buoyantly induced flows once ignition has occurred. An understanding of the phenomena involved is important to fire safety, especially aboard spacecraft since no microgravity data exist. The data obtained will also be useful to verify theoretical models of this problem, which are easier to implement if buoyancy is neglected.
Effect of premixing quality on oxides of nitrogen in gas turbine combustors foi HC
NASA Technical Reports Server (NTRS)
Roffe, G.; Ferri, A.
1976-01-01
Experiments were conducted to determine the effectiveness of several premixing prevaporizing gas turbine combustor designs in reducing formation of oxides of nitrogen at the supersonic cruise condition. An atomized spray from a single injector mounted on the axis of the mixer tube produced a high initial concentration of fuel near the axis and only moderate premixed conditions entering the combustor. A fuel spray produced by 12 flush-mounted normal injection orifices in the mixer tube wall produced a good initial despersion of fuel and resulted in nearly complete premixing. Oxides of nitrogen emission levels of the order of 0.2 g NO2/kg fuel were obtained at 99 percent combustion efficiency at an equivalence ratio of 0.4. Overall total pressure drop was less than 3 percent through the 1-meter combustor module.
PREMIX: PRivacy-preserving EstiMation of Individual admiXture.
Chen, Feng; Dow, Michelle; Ding, Sijie; Lu, Yao; Jiang, Xiaoqian; Tang, Hua; Wang, Shuang
2016-01-01
In this paper we proposed a framework: PRivacy-preserving EstiMation of Individual admiXture (PREMIX) using Intel software guard extensions (SGX). SGX is a suite of software and hardware architectures to enable efficient and secure computation over confidential data. PREMIX enables multiple sites to securely collaborate on estimating individual admixture within a secure enclave inside Intel SGX. We implemented a feature selection module to identify most discriminative Single Nucleotide Polymorphism (SNP) based on informativeness and an Expectation Maximization (EM)-based Maximum Likelihood estimator to identify the individual admixture. Experimental results based on both simulation and 1000 genome data demonstrated the efficiency and accuracy of the proposed framework. PREMIX ensures a high level of security as all operations on sensitive genomic data are conducted within a secure enclave using SGX.
Vortex combustor for low NOX emissions when burning lean premixed high hydrogen content fuel
Steele, Robert C; Edmonds, Ryan G; Williams, Joseph T; Baldwin, Stephen P
2012-11-20
A trapped vortex combustor. The trapped vortex combustor is configured for receiving a lean premixed gaseous fuel and oxidant stream, where the fuel includes hydrogen gas. The trapped vortex combustor is configured to receive the lean premixed fuel and oxidant stream at a velocity which significantly exceeds combustion flame speed in a selected lean premixed fuel and oxidant mixture. The combustor is configured to operate at relatively high bulk fluid velocities while maintaining stable combustion, and low NOx emissions. The combustor is useful in gas turbines in a process of burning synfuels, as it offers the opportunity to avoid use of diluent gas to reduce combustion temperatures. The combustor also offers the possibility of avoiding the use of selected catalytic reaction units for removal of oxides of nitrogen from combustion gases exiting a gas turbine.
Multidimensional Potential Burgers Turbulence
NASA Astrophysics Data System (ADS)
Boritchev, Alexandre
2016-03-01
We consider the multidimensional generalised stochastic Burgers equation in the space-periodic setting: partial {u}/partial t+(nabla f({u}) \\cdot nabla) {u}-ν Δ {u}= nabla η, quad t ≥ 0, {x} in{T}^d=({R}/ {Z})^d, under the assumption that u is a gradient. Here f is strongly convex and satisfies a growth condition, ν is small and positive, while η is a random forcing term, smooth in space and white in time. For solutions u of this equation, we study Sobolev norms of u averaged in time and in ensemble: each of these norms behaves as a given negative power of ν. These results yield sharp upper and lower bounds for natural analogues of quantities characterising the hydrodynamical turbulence, namely the averages of the increments and of the energy spectrum. These quantities behave as a power of the norm of the relevant parameter, which is respectively the separation ℓ in the physical space and the wavenumber k in the Fourier space. Our bounds do not depend on the initial condition and hold uniformly in {ν}. We generalise the results obtained for the one-dimensional case in [10], confirming the physical predictions in [4, 30]. Note that the form of the estimates does not depend on the dimension: the powers of {ν, |{{k}}|, ℓ} are the same in the one- and the multi-dimensional setting.
Potential capabilities of Reynolds stress turbulence model in the COMMIX-RSM code
NASA Technical Reports Server (NTRS)
Chang, F. C.; Bottoni, M.
1994-01-01
A Reynolds stress turbulence model has been implemented in the COMMIX code, together with transport equations describing turbulent heat fluxes, variance of temperature fluctuations, and dissipation of turbulence kinetic energy. The model has been verified partially by simulating homogeneous turbulent shear flow, and stable and unstable stratified shear flows with strong buoyancy-suppressing or enhancing turbulence. This article outlines the model, explains the verifications performed thus far, and discusses potential applications of the COMMIX-RSM code in several domains, including, but not limited to, analysis of thermal striping in engineering systems, simulation of turbulence in combustors, and predictions of bubbly and particulate flows.
The propagation of premixed flames in closed tubes
NASA Astrophysics Data System (ADS)
Matalon, Moshe; Metzener, Philippe
1997-04-01
A nonlinear evolution equation that describes the propagation of a premixed flame in a closed tube has been derived from the general conservation equations. What distinguishes it from other similar equations is a memory term whose origin is in the vorticity production at the flame front. The two important parameters in this equation are the tube's aspect ratio and the Markstein parameter. A linear stability analysis indicates that when the Markstein parameter [alpha] is above a critical value [alpha]c the planar flame is the stable equilibrium solution. For [alpha] below [alpha]c the planar flame is no longer stable and there is a band of growing modes. Numerical solutions of the full nonlinear equation confirm this conclusion. Starting with random initial conditions the results indicate that, after a short transient, a at flame develops when [alpha]>[alpha]c and it remains flat until it reaches the end of the tube. When [alpha]<[alpha]c, on the other hand, stable curved flames may develop down the tube. Depending on the initial conditions the flame assumes either a cellular structure, characterized by a finite number of cells convex towards the unburned gas, or a tulip shape characterized by a sharp indentation at the centre of the tube pointing toward the burned gases. In particular, if the initial conditions are chosen so as to simulate the elongated finger-like flame that evolves from an ignition source, a tulip flame evolves downstream. In accord with experimental observations the tulip shape forms only after the flame has travelled a certain distance down the tube, it does not form in short tubes and its formation depends on the mixture composition. While the initial deformation of the flame front is a direct result of the hydrodynamic instability, the actual formation of the tulip flame results from the vortical motion created in the burned gas which is a consequence of the vorticity produced at the flame front.
Structure and Mixing of a Turbulent Meandering Plume Part 2: Turbulent Mixing and Eddy-Diffusivity
NASA Astrophysics Data System (ADS)
Webster, D. R.; Young, D. L.; Larsson, A. I.
2016-11-01
Turbulent mixing in a meandering non-buoyant chemical plume is far less understood than in a straight plume - partially due to the difficulty separating the plume meander fluctuations from the turbulent fluctuations. In this study we present high resolution measurements of the covariance of the turbulent fluctuations of velocity and concentration in a phase-locked meandering plume, acquired by combining simultaneous PTV velocity and LIF concentration measurements. The effectiveness of the eddy-diffusivity model for predicting the turbulent flux is assessed. Analysis of the data reveals that the spatial distribution of the turbulent flux is governed by the large-scale alternating-sign vortices that induce the plume meander. Further, regions of high turbulent flux are co-located with areas of large phase-averaged concentration gradients. As a result, the eddy-diffusivity framework models the turbulent flux effectively. As expected from turbulent mixing theory, the eddy-diffusivity coefficient plateaus at a constant value once the plume width reaches the size of the largest eddies (i.e., the scale of the water depth in this open channel flow). However, when the plume width is less than the water depth the eddy-diffusivity coefficient scales with the plume width to the 3/4 power. This differs from the theoretical 4/3 scaling that results from the assumption of an inertial subrange. The extent of the inertial subrange is extremely limited in the current moderate-Re open channel flow.
NASA Astrophysics Data System (ADS)
Hernandez Perez, Francisco E.; Lee, Bok Jik; Im, Hong G.; Fancello, Alessio; Donini, Andrea; van Oijen, Jeroen A.; de Goey, L. Philip H.
2016-11-01
Large eddy simulations (LES) of a turbulent premixed jet flame in a confined chamber are performed using the flamelet-generated manifold technique for tabulation of chemical kinetics and the OpenFOAM framework for computational fluid dynamics. The configuration is characterized by an off-center nozzle having an inner diameter of 10 mm, feeding a lean methane-air mixture with an equivalence ratio of 0.71 and mean velocity of 90 m/s, at 573 K and atmospheric pressure. Conductive heat loss is accounted for in the manifold via burner-stabilized flamelets and the subgrid-scale (SGS) turbulence-chemistry interaction is modeled via presumed filtered density functions. The effects of heat loss inclusion as well as SGS modeling for both the SGS stresses and SGS variance of progress variable on the numerical predictions are all systematically investigated. Comparisons between numerical results and measured data show a considerable improvement in the prediction of temperature when heat losses are incorporated into the manifold, as compared to the adiabatic one. In addition, further improvements in the LES predictions are achieved by employing SGS models based on transport equations.
Analyzing and Tracking Burning Structures in Lean Premixed Hydrogen Flames
Bremer, Peer-Timo; Weber, Gunther; Pascucci, Valerio; Day, Marc; Bell, John
2009-06-01
This paper presents topology-based methods to robustly extract, analyze, and track features defined as subsets of isosurfaces. First, we demonstrate how features identified by thresholding isosurfaces can be defined in terms of the Morse complex. Second, we present a specialized hierarchy that encodes the feature segmentation independent of the threshold while still providing a flexible multi-resolution representation. Third, for a given parameter selection we create detailed tracking graphs representing the complete evolution of all features in a combustion simulation over several hundred time steps. Finally, we discuss a user interface that correlates the tracking information with interactive rendering of the segmented isosurfaces enabling an in-depth analysis of the temporal behavior. We demonstrate our approach by analyzing three numerical simulations of lean hydrogen flames subject to different levels of turbulence. Due to their unstable nature, lean flames burn in cells separated by locally extinguished regions. The number, area, and evolution over time of these cells provide important insights into the impact of turbulence on the combustion process. Utilizing the hierarchy we can perform an extensive parameter study without re-processing the data for each set of parameters. The resulting statistics enable scientist to select appropriate parameters and provide insight into the sensitivity of the results wrt. to the choice of parameters. Our method allows for the first time to quantitatively correlate the turbulence of the burning process with the distribution of burning regions, properly segmented and selected. In particular, our analysis shows that counter-intuitively stronger turbulence leads to larger cell structures, which burn more intensely than expected. This behavior suggests that flames could be stabilized under much leaner conditions than previously anticipated.
Assessment of dynamic closure for premixed combustion large eddy simulation
NASA Astrophysics Data System (ADS)
Langella, Ivan; Swaminathan, Nedunchezhian; Gao, Yuan; Chakraborty, Nilanjan
2015-09-01
Turbulent piloted Bunsen flames of stoichiometric methane-air mixtures are computed using the large eddy simulation (LES) paradigm involving an algebraic closure for the filtered reaction rate. This closure involves the filtered scalar dissipation rate of a reaction progress variable. The model for this dissipation rate involves a parameter βc representing the flame front curvature effects induced by turbulence, chemical reactions, molecular dissipation, and their interactions at the sub-grid level, suggesting that this parameter may vary with filter width or be a scale-dependent. Thus, it would be ideal to evaluate this parameter dynamically by LES. A procedure for this evaluation is discussed and assessed using direct numerical simulation (DNS) data and LES calculations. The probability density functions of βc obtained from the DNS and LES calculations are very similar when the turbulent Reynolds number is sufficiently large and when the filter width normalised by the laminar flame thermal thickness is larger than unity. Results obtained using a constant (static) value for this parameter are also used for comparative evaluation. Detailed discussion presented in this paper suggests that the dynamic procedure works well and physical insights and reasonings are provided to explain the observed behaviour.
On the modelling of non-reactive and reactive turbulent combustor flows
NASA Technical Reports Server (NTRS)
Nikjooy, Mohammad; So, Ronald M. C.
1987-01-01
A study of non-reactive and reactive axisymmetric combustor flows with and without swirl is presented. Closure of the Reynolds equations is achieved by three models: kappa-epsilon, algebraic stress and Reynolds stress closure. Performance of two locally nonequilibrium and one equilibrium algebraic stress models is analyzed assuming four pressure strain models. A comparison is also made of the performance of a high and a low Reynolds number model for combustor flow calculations using Reynolds stress closures. Effects of diffusion and pressure-strain models on these closures are also investigated. Two models for the scalar transport are presented. One employs the second-moment closure which solves the transport equations for the scalar fluxes, while the other solves the algebraic equations for the scalar fluxes. In addition, two cases of non-premixed and one case of premixed combustion are considered. Fast- and finite-rate chemistry models are applied to non-premixed combustion. Both show promise for application in gas turbine combustors. However, finite rate chemistry models need to be examined to establish a suitable coupling of the heat release effects on turbulence field and rate constants.
Turbulent Combustion in Aluminum-air Clouds for Different Scale Explosion Fields
NASA Astrophysics Data System (ADS)
Kuhl, Allen; Balakrishnan, Kaushik; Bell, John; Beckner, Vincent
2015-06-01
We have studied turbulent combustion effects in explosions, and proposed heterogeneous continuum models for the turbulent combustion fields. Also we have proposed an induction-time model for the ignition of Al particle clouds, based on Arrhenius fits to the shock tube data of Boiko. Here we explore scaling issues associated with Al particle combustion in such explosions. This is a non-premixed combustion system; the global burning rate is controlled by rate of turbulent mixing of fuel (Al particles) with air. For similitude reasons, the turbulent mixing rates should scale with the explosion length and time scales. However, the induction time for ignition of Al particles depends on an Arrhenius function, which is independent of such scales. To study this, we have performed numerical simulations of turbulent combustion in unconfined Al-SDF (shock-dispersed-fuel) explosion fields at different scales. Three different charge masses were assumed: 1-g, 1-kg and 1-T Al-powder charges. We found that there are two combustion regimes: an ignition regime--where the burning rate decays a power law function of time, and a turbulent combustion regime--where the burning rate decays exponentially with time.
Numerical simulation of a laboratory-scale turbulent V-flame
Bell, J. B.; Day, M. S.; Shepherd, I. G.; Johnson, M. R.; Cheng, R. K.; Grcar, J. F.; Beckner, V. E.; Lijewski, M. J.
2005-01-01
We present a three-dimensional, time-dependent simulation of a laboratory-scale rod-stabilized premixed turbulent V-flame. The experimental parameters correspond to a turbulent Reynolds number, Ret = 40, and to a Damköhler number, Da = 6. The simulations are performed using an adaptive time-dependent low-Mach-number model with detailed chemical kinetics and a mixture model for differential species diffusion. The algorithm is based on a second-order projection formulation and does not require an explicit subgrid model for turbulence or turbulence/chemistry interaction. Adaptive mesh refinement is used to dynamically resolve the flame and turbulent structures. Here, we briefly discuss the numerical procedure and present detailed comparisons with experimental measurements showing that the computation is able to accurately capture the basic flame morphology and associated mean velocity field. Finally, we discuss key issues that arise in performing these types of simulations and the implications of these issues for using computation to form a bridge between turbulent flame experiments and basic combustion chemistry. PMID:16006519
Time-dependent Computational Studies of Premixed Flames in Microgravity
NASA Technical Reports Server (NTRS)
Kailasanath, K.; Patnaik, Gopal; Oran, Elaine S.
1993-01-01
This report describes the research performed at the Center for Reactive Flow and Dynamical Systems in the Laboratory for Computational Physics and Fluid Dynamics, at the Naval Research Laboratory, in support of NASA Microgravity Science and Applications Program. The primary focus of this research is on investigating fundamental questions concerning the propagation and extinction of premixed flames in earth gravity and in microgravity environments. Our approach is to use detailed time-dependent, multispecies, numerical models as tools to simulate flames in different gravity environments. The models include a detailed chemical kinetics mechanism consisting of elementary reactions among the eight reactive species involved in hydrogen combustion, coupled to algorithms for convection, thermal conduction, viscosity, molecular and thermal diffusion, and external forces. The external force, gravity, can be put in any direction relative to flame propagation and can have a range of values. Recently more advanced wall boundary conditions such as isothermal and no-slip have been added to the model. This enables the simulation of flames propagating in more practical systems than before. We have used the numerical simulations to investigate the effects of heat losses and buoyancy forces on the structure and stability of flames, to help resolve fundamental questions on the existence of flammability limits when there are no external losses or buoyancy forces in the system, to understand the interaction between the various processes leading to flame instabilities and extinguishment, and to study the dynamics of cell formation and splitting. Our studies have been able to bring out the differences between upward- and downward-propagating flames and predict the zero-gravity behavior of these flames. The simulations have also highlighted the dominant role of wall heat losses in the case of downward-propagating flames. The simulations have been able to qualitatively predict the
Non-premixed acoustically perturbed swirling flame dynamics
Idahosa, Uyi; Saha, Abhishek; Xu, Chengying; Basu, Saptarshi
2010-09-15
An investigation into the response of non-premixed swirling flames to acoustic perturbations at various frequencies (f{sub p}=0-315 Hz) and swirl intensities (S=0.09 and 0.34) is carried out. Perturbations are generated using a loudspeaker at the base of an atmospheric co-flow burner with resulting velocity oscillation amplitudes vertical stroke u'/U{sub avg} vertical stroke in the 0.03-0.30 range. The dependence of flame dynamics on the relative richness of the flame is investigated by studying various constant fuel flow rate flame configurations. Flame heat release rate is quantitatively measured using a photomultiplier with a 430 nm bandpass filter for observing CH* chemiluminescence which is simultaneously imaged with a phase-locked CCD camera. The flame response is observed to exhibit a low-pass filter characteristic with minimal flame response beyond pulsing frequencies of 200 Hz. Flames at lower fuel flow rates are observed to remain attached to the central fuel pipe at all acoustic pulsing frequencies. PIV imaging of the associated isothermal fields show the amplification in flame aspect ratio is caused by the narrowing of the inner recirculation zone (IRZ). Good correlation is observed between the estimated flame surface area and the heat release rate signature at higher swirl intensity flame configurations. A flame response index analogous to the Rayleigh criterion in non-forced flames is used to assess the potential for a strong flame response at specific perturbation configurations and is found to be a good predictor of highly responsive modes. Phase conditioned analysis of the flame dynamics yield additional criteria in highly responsive modes to include the effective amplitude of velocity oscillations induced by the acoustic pulsing. In addition, highly responsive modes were characterized by velocity to heat release rate phase differences in the {+-}{pi}/2 range. A final observed characteristic in highly responsive flames is a Strouhal number between
Influence of particle size on hardening and handling of a premixed calcium phosphate cement.
Aberg, Jonas; Engstrand, Johanna; Engqvist, Håkan
2013-04-01
Premixed calcium phosphate cements (pCPC) have been developed to circumvent problems related to mixing and transfer of cements in the operating room. In addition, by using pCPC the short working times generally associated with conventional water-mixed cements are avoided. In this work, the influence of particle size on handling and hardening characteristics of a premixed monetite cement has been assessed. The cements were evaluated with respect to their injectability, setting time and compressive strength. It was found that cements with smaller particle sizes were more difficult to inject and had higher compressive strength. Regarding setting time, no clear trend could be discerned. The addition of granules made the cements easier to inject, but setting time was prolonged and lower strengths were obtained. The main findings of this work demonstrate that particle size can be used to control handling and physical properties of premixed cements and that previous knowledge from water-based CPC, regarding effects of particle size, is not directly applicable to premixed CPC.
Experimental and detailed chemical kinetic modeling work has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, n-butane_{¯}oxygen_{¯}argon burner s...
The Statistical Mechanics of Ideal Homogeneous Turbulence
NASA Technical Reports Server (NTRS)
Shebalin, John V.
2002-01-01
Plasmas, such as those found in the space environment or in plasma confinement devices, are often modeled as electrically conducting fluids. When fluids and plasmas are energetically stirred, regions of highly nonlinear, chaotic behavior known as turbulence arise. Understanding the fundamental nature of turbulence is a long-standing theoretical challenge. The present work describes a statistical theory concerning a certain class of nonlinear, finite dimensional, dynamical models of turbulence. These models arise when the partial differential equations describing incompressible, ideal (i.e., nondissipative) homogeneous fluid and magnetofluid (i.e., plasma) turbulence are Fourier transformed into a very large set of ordinary differential equations. These equations define a divergenceless flow in a high-dimensional phase space, which allows for the existence of a Liouville theorem, guaranteeing a distribution function based on constants of the motion (integral invariants). The novelty of these particular dynamical systems is that there are integral invariants other than the energy, and that some of these invariants behave like pseudoscalars under two of the discrete symmetry transformations of physics, parity, and charge conjugation. In this work the 'rugged invariants' of ideal homogeneous turbulence are shown to be the only significant scalar and pseudoscalar invariants. The discovery that pseudoscalar invariants cause symmetries of the original equations to be dynamically broken and induce a nonergodic structure on the associated phase space is the primary result presented here. Applicability of this result to dissipative turbulence is also discussed.
Turbulent kinetic energy equation and free mixing
NASA Technical Reports Server (NTRS)
Morel, T.; Torda, T. P.; Bradshaw, P.
1973-01-01
Calculation of free shear flows was carried out to investigate the usefulness of several concepts which were previously successfully applied to wall flows. The method belongs to the class of differential approaches. The turbulence is taken into account by the introduction of one additional partial differential equation, the transport equation for the turbulent shear stress. The structure of turbulence is modeled after Bradshaw et al. This model was used successfully in boundary layers and its applicability to other flows is demonstrated. The work reported differs substantially from that of an earlier attempt to use this approach for calculation of free flows. The most important difference is that the region around the center line is treated by invoking the interaction hypothesis (concerning the structure of turbulence in the regions separated by the velocity extrema). The compressibility effects on shear layer spreading at low and moderate Mach numbers were investigated. In the absence of detailed experiments in free flows, the evidence from boundary layers that at low Mach numbers the structure of turbulence is unaffected by the compressibility was relied on. The present model was tested over a range of self-preserving and developing flows including pressure gradients using identical empirical input. The dependence of the structure of turbulence on the spreading rate of the shear layer was established.
Introduction to quantum turbulence
Barenghi, Carlo F.; Skrbek, Ladislav; Sreenivasan, Katepalli R.
2014-01-01
The term quantum turbulence denotes the turbulent motion of quantum fluids, systems such as superfluid helium and atomic Bose–Einstein condensates, which are characterized by quantized vorticity, superfluidity, and, at finite temperatures, two-fluid behavior. This article introduces their basic properties, describes types and regimes of turbulence that have been observed, and highlights similarities and differences between quantum turbulence and classical turbulence in ordinary fluids. Our aim is also to link together the articles of this special issue and to provide a perspective of the future development of a subject that contains aspects of fluid mechanics, atomic physics, condensed matter, and low-temperature physics. PMID:24704870
Modeling Compressed Turbulence
Israel, Daniel M.
2012-07-13
From ICE to ICF, the effect of mean compression or expansion is important for predicting the state of the turbulence. When developing combustion models, we would like to know the mix state of the reacting species. This involves density and concentration fluctuations. To date, research has focused on the effect of compression on the turbulent kinetic energy. The current work provides constraints to help development and calibration for models of species mixing effects in compressed turbulence. The Cambon, et al., re-scaling has been extended to buoyancy driven turbulence, including the fluctuating density, concentration, and temperature equations. The new scalings give us helpful constraints for developing and validating RANS turbulence models.
Experimental study of turbulent flame kernel propagation
Mansour, Mohy; Peters, Norbert; Schrader, Lars-Uve
2008-07-15
Flame kernels in spark ignited combustion systems dominate the flame propagation and combustion stability and performance. They are likely controlled by the spark energy, flow field and mixing field. The aim of the present work is to experimentally investigate the structure and propagation of the flame kernel in turbulent premixed methane flow using advanced laser-based techniques. The spark is generated using pulsed Nd:YAG laser with 20 mJ pulse energy in order to avoid the effect of the electrodes on the flame kernel structure and the variation of spark energy from shot-to-shot. Four flames have been investigated at equivalence ratios, {phi}{sub j}, of 0.8 and 1.0 and jet velocities, U{sub j}, of 6 and 12 m/s. A combined two-dimensional Rayleigh and LIPF-OH technique has been applied. The flame kernel structure has been collected at several time intervals from the laser ignition between 10 {mu}s and 2 ms. The data show that the flame kernel structure starts with spherical shape and changes gradually to peanut-like, then to mushroom-like and finally disturbed by the turbulence. The mushroom-like structure lasts longer in the stoichiometric and slower jet velocity. The growth rate of the average flame kernel radius is divided into two linear relations; the first one during the first 100 {mu}s is almost three times faster than that at the later stage between 100 and 2000 {mu}s. The flame propagation is slightly faster in leaner flames. The trends of the flame propagation, flame radius, flame cross-sectional area and mean flame temperature are related to the jet velocity and equivalence ratio. The relations obtained in the present work allow the prediction of any of these parameters at different conditions. (author)
Development of a Premixed Combustion Capability for Scramjet Combustion Experiments
NASA Technical Reports Server (NTRS)
Rockwell, Robert D.; Goyne, Christopher P.; Rice, Brian E.; Chelliah, Harsha; McDaniel, James C.; Edwards, Jack R.; Cantu, Luca M. L.; Gallo, Emanuela C. A.; Cutler, Andrew D.; Danehy, Paul M.
2015-01-01
Hypersonic air-breathing engines rely on scramjet combustion processes, which involve high speed, compressible, and highly turbulent flows. The combustion environment and the turbulent flames at the heart of these engines are difficult to simulate and study in the laboratory under well controlled conditions. Typically, wind-tunnel testing is performed that more closely approximates engine testing rather than a careful investigation of the underlying physics that drives the combustion process. The experiments described in this paper, along with companion data sets being developed separately, aim to isolate the chemical kinetic effects from the fuel-air mixing process in a dual-mode scramjet combustion environment. A unique fuel injection approach is taken that produces a nearly uniform fuel-air mixture at the entrance to the combustor. This approach relies on the precombustion shock train upstream of the dual-mode scramjet combustor. A stable ethylene flame anchored on a cavity flameholder with a uniformly mixed combustor inflow has been achieved in these experiments allowing numerous companion studies involving coherent anti-Stokes Raman scattering (CARS), particle image velocimetry (PIV), and planar laser induced fluorescence (PLIF) to be performed.
Turbulent Spots Inside the Turbulent Boundary Layer
NASA Astrophysics Data System (ADS)
Skarda, Jinhie; Wu, Xiaohua; Moin, Parviz; Lozano-Duran, Adrian; Wallace, James; Hickey, Jean-Pierre
2016-11-01
We present evidence that the buffer region of the canonical turbulent boundary layer is populated by locally generated turbulent spots, which cause strong indentations on the near-wall low-momentum streaks. This evidence is obtained from a spatially-developing direct numerical simulation carrying the inlet Blasius boundary layer through a bypass transition to the turbulent boundary layer state over a moderate Reynolds number range. The turbulent spots are structurally analogous to their transitional counter-parts but without any direct causality connection. High-pass filtered time-history records are used to calculate the period of turbulent spot detection and this period is compared to the boundary layer bursting period reported in hot-wire experiments. The sensitivity of the results to parameters such as the high pass filter frequency and the amplitude discriminator level is examined. The characteristics of these turbulent spots are also quantified using a spatial connectivity based conditional sampling technique. This evidence seems to be at odds with the notion that the buffer region is dominated by quasi-streamwise vortices, and contributes to the potential unification of the studies on near-wall turbulent boundary layer dynamics.
Aeroacoustics of Turbulent High-Speed Jets
NASA Technical Reports Server (NTRS)
Rao, Ram Mohan; Lundgren, Thomas S.
1996-01-01
Aeroacoustic noise generation in a supersonic round jet is studied to understand in particular the effect of turbulence structure on the noise without numerically compromising the turbulence itself. This means that direct numerical simulations (DNS's) are needed. In order to use DNS at high enough Reynolds numbers to get sufficient turbulence structure we have decided to solve the temporal jet problem, using periodicity in the direction of the jet axis. Physically this means that turbulent structures in the jet are repeated in successive downstream cells instead of being gradually modified downstream into a jet plume. Therefore in order to answer some questions about the turbulence we will partially compromise the overall structure of the jet. The first section of chapter 1 describes some work on the linear stability of a supersonic round jet and the implications of this for the jet noise problem. In the second section we present preliminary work done using a TVD numerical scheme on a CM5. This work is only two-dimensional (plane) but shows very interesting results, including weak shock waves. However this is a nonviscous computation and the method resolves the shocks by adding extra numerical dissipation where the gradients are large. One wonders whether the extra dissipation would influence small turbulent structures like small intense vortices. The second chapter is an extensive discussion of preliminary numerical work using the spectral method to solve the compressible Navier-Stokes equations to study turbulent jet flows. The method uses Fourier expansions in the azimuthal and streamwise direction and a 1-D B-spline basis representation in the radial direction. The B-spline basis is locally supported and this ensures block diagonal matrix equations which are solved in O(N) steps. A very accurate highly resolved DNS of a turbulent jet flow is expected.
Turbulence, Turbulence Control, and Drag Reduction.
1987-08-01
Onsager (1945) and Weizs~cker (1948). has made remarkable strides in advancing our understanding of turbulent flows. It is this description of turbulent...tujrbuilce Inl thle lar to thle Intermiittenit trans’ition to turbulence lus.t ,iedipen itlik. N\\N.tern onI the other. O pen0 * ~ ~ h 1 ~ kdinition10 po...Some Studies of Non-Simple Pipe Flows K R SREENIVASAN 2.AR’ .\\ variety o phenooena occrs ’.5’, ,sTecla’., f we stray,’ away from straight circ- lar i es a
Spark ignited turbulent flame kernel growth. Annual report, January--December, 1992
Santavicca, D.A.
1994-06-01
Cyclic combustion variations in spark-ignition engines limit the use of dilute charge strategies for achieving low NO{sub x} emissions and improved fuel economy. Results from an experimental study of the effect of incomplete fuel-air mixing (ifam) on spark-ignited flame kernel growth in turbulent propane-air mixtures are presented. The experiments were conducted in a turbulent flow system that allows for independent variation of flow parameters, ignition system parameters, and the degree of fuel-air mixing. Measurements were made at 1 atm and 300 K conditions. Five cases were studied; a premixed and four incompletely mixed cases with 6%, 13%, 24% and 33% RMS (root-mean-square) fluctuations in the fuel/air equivalence ratio. High speed laser shadowgraphy at 4,000 frames-per-second was used to record flame kernel growth following spark ignition, from which the equivalent flame kernel radius as a function of time was determined. The effect of ifam was evaluated in terms of the flame kernel growth rate, cyclic variations in the flame kernel growth, and the rate of misfire. The results show that fluctuations in local mixture strength due to ifam cause the flame kernel surface to become wrinkled and distorted; and that the amount of wrinkling increases as the degree of ifam. Ifam was also found to result in a significant increase in cyclic variations in the flame kernel growth. The average flame kernel growth rates for the premixed and the incompletely mixed cases were found to be within the experimental uncertainty except for the 33%-RMS-fluctuation case where the growth rate is significantly lower. The premixed and 6%-RMS-fluctuation cases had a 0% misfire rate. The misfire rates were 1% and 2% for the 13%-RMS-fluctuation and 24%-RMS-fluctuation cases, respectively; however, it drastically increased to 23% in the 33%-RMS-fluctuation case.
Experimental study of combustion in a turbulent free shear layer formed at a rearward facing step
NASA Technical Reports Server (NTRS)
Pitz, R. W.; Daily, J. W.
1981-01-01
A premixed propane-air flame is stabilized in a turbulent free shear layer formed at a rearward facing step. The mean and rms averages of the turbulent velocity flow field are determined by LDV for both reacting (equivalence ratio 0.57) and nonreacting flows (Reynolds number 15,000-37,000 based on step height). The effect of combustion is to shift the layer toward the recirculation zone and reduce the flame spread. For reacting flow, the growth rate is unchanged except very near the step. The probability density function of the velocity is bimodial near the origin of the reacting layer and single-peaked but often skewed elsewhere. Large-scale structures dominate the reacting shear layer. Measurements of their passing frequency from LDV are consistent with high-speed Schlieren movies of the reacting layer and indicate that the coalescence rate of the eddies in the shear layer is reduced by combustion.
Simulations of diffusion-reaction equations with implications to turbulent combustion modeling
NASA Technical Reports Server (NTRS)
Girimaji, Sharath S.
1993-01-01
An enhanced diffusion-reaction reaction system (DRS) is proposed as a statistical model for the evolution of multiple scalars undergoing mixing and reaction in an isotropic turbulence field. The DRS model is close enough to the scalar equations in a reacting flow that other statistical models of turbulent mixing that decouple the velocity field from scalar mixing and reaction (e.g. mapping closure model, assumed-pdf models) cannot distinguish the model equations from the original equations. Numerical simulations of DRS are performed for three scalars evolving from non-premixed initial conditions. A simple one-step reversible reaction is considered. The data from the simulations are used (1) to study the effect of chemical conversion on the evolution of scalar statistics, and (2) to evaluate other models (mapping-closure model, assumed multivariate beta-pdf model).
PREFACE Turbulent Mixing and Beyond
NASA Astrophysics Data System (ADS)
Abarzhi, Snezhana I.; Gauthier, Serge; Niemela, Joseph J.
2010-12-01
The goals of the International Conference 'Turbulent Mixing and Beyond', TMB-2009, are to expose the generic problem of non-equilibrium turbulent processes to a broad scientific community, to promote the development of new ideas in tackling the fundamental aspects of the problem, to assist in the application of novel approaches in a broad range of phenomena, where the turbulent processes occur, and to have a potential impact on technology. The Conference provides the opportunity to bring together researchers from different areas, which include but are not limited to fluid dynamics, plasmas, high energy density physics, astrophysics, material science, combustion, atmospheric and Earth sciences, nonlinear and statistical physics, applied mathematics, probability and statistics, data processing and computations, optics and telecommunications, and to have their attention focused on the long-standing formidable task of non-equilibrium processes. Non-equilibrium turbulent processes play a key role in a broad variety of phenomena spanning astrophysical to atomistic scales and high or low energy density regimes. Inertial confinement and magnetic fusion, light-matter interaction and non-equilibrium heat transfer, strong shocks and explosions, material transformation under high strain rate, supernovae and accretion disks, stellar non-Boussinesq and magneto-convection, planetary interiors and mantle-lithosphere tectonics, premixed and non-premixed combustion, non-canonical wall-bounded flows, hypersonic and supersonic boundary layers, dynamics of atmosphere and oceanography, are just a few examples. A grip on non-equilibrium turbulent processes is crucial for cutting-edge technology such as laser micro-machining, nano-electronics, free-space optical telecommunications, and for industrial applications in the areas of aeronautics and aerodynamics. Non-equilibrium turbulent processes are anisotropic, non-local, multi-scale and multi-phase, and often are driven by shocks or
NASA Astrophysics Data System (ADS)
Gopalan, Balaji
In part I, high speed in-line digital holographic cinematography is used for studying turbulent diffusion of slightly buoyant 0.5-1.2 mm diameter diesel droplets (specific gravity of 0.85) and 50 mum diameter neutral density particles. Experiments are performed in a 50x50x70 mm3 sample volume in a controlled, nearly isotropic turbulence facility, which is characterized by 2-D PIV. An automated tracking program has been used for measuring velocity time history of more than 17000 droplets and 15000 particles. The PDF's of droplet velocity fluctuations are close to Gaussian for all turbulent intensities ( u'i ). The mean rise velocity of droplets is enhanced or suppressed, compared to quiescent rise velocity (Uq), depending on Stokes number at lower turbulence levels, but becomes unconditionally enhanced at higher turbulence levels. The horizontal droplet velocity rms exceeds the fluid velocity rms for most of the data, while the vertical ones are higher than the fluid only at the highest turbulence level. The scaled droplet horizontal diffusion coefficient is higher than the vertical one, for 1 < u'i /Uq < 5, consistent with trends of the droplet velocity fluctuations. Conversely, the scaled droplet horizontal diffusion timescale is smaller than the vertical one due to crossing trajectories effect. The droplet diffusion coefficients scaled by the product of turbulence intensity and an integral length scale is a monotonically increasing function of u'i /Uq. Part II of this work explains the formation of micron sized droplets in turbulent flows from crude oil droplets pre-mixed with dispersants. Experimental visualization shows that this breakup starts with the formation of very long and quite stable, single or multiple micro threads that trail behind millimeter sized droplets. These threads form in regions with localized increase in concentration of surfactant, which in turn depends on the flow around the droplet. The resulting reduction of local surface tension
Non-premixed conditions in the flameholding recirculation region behind a step in supersonic flow
NASA Astrophysics Data System (ADS)
Thakur, Amit
Flameholding in supersonic flow depends on local conditions in the recirculation region, and on mass transfer into and out of this region. Large gradients in local gas composition and temperature exist in the recirculation region. Hence, stability parameter correlations developed for premixed flames cannot be used to determine blowout stability limits for non-premixed flames encountered in practical devices. In the present study, mixture samples were extracted at different locations in the recirculation region and the shear layer formed behind a rearward-facing step in supersonic flow, and analyzed by mass spectrometry to determine the species concentration distribution in the region. The point-wise mass spectrometer measurements were complemented by acetone planar laser-induced fluorescence (PLIF) measurements to get a planar distribution of fuel mole fraction in the recirculation region. Non-reacting flow tests and combustion experiments were performed by varying various fuel related parameters such as injection location, injection pressure and fuel type. Fuel injection upstream of the step was not effective in supplying enough fuel to the recirculation region and did not sustain the flame in combustion experiments. Fuel injection at the step base was effective in sustaining the flame. For base injection, the local fuel mole fraction in the recirculation region determined from experiments was an order of magnitude higher than the global fuel mole fraction based on total moles of air flowing through the test section and total fuel injected in the test section. This suggests substantial difference in flame stability curve for non-premixed conditions in the scramjet engine compared to premixed flow. For base injection, fuel remained in the recirculation region even at higher injection pressure. Due to slower diffusion rate, the heavier fuel had higher local mole fraction in the recirculation region compared to lighter fuel for a unit global fuel mole fraction
NASA Astrophysics Data System (ADS)
Trisjono, Philipp; Kang, Seongwon; Pitsch, Heinz
2016-12-01
The main objective of this study is to present an accurate and consistent numerical framework for turbulent reacting flows based on a high-order finite difference (HOFD) scheme. It was shown previously by Desjardins et al. (2008) [4] that a centered finite difference scheme discretely conserving the kinetic energy and an upwind-biased scheme for the scalar transport can be combined into a useful scheme for turbulent reacting flows. With a high-order spatial accuracy, however, an inconsistency among discretization schemes for different conservation laws is identified, which can disturb a scalar field spuriously under non-uniform density distribution. Various theoretical and numerical analyses are performed on the sources of the unphysical error. From this, the derivative of the mass-conserving velocity and the local Péclet number are identified as the primary factors affecting the error. As a solution, an HOFD stencil for the mass conservation is reformulated into a flux-based form that can be used consistently with an upwind-biased scheme for the scalar transport. The effectiveness of the proposed formulation is verified using two-dimensional laminar flows such as a scalar transport problem and a laminar premixed flame, where unphysical oscillations in the scalar fields are removed. The applicability of the proposed scheme is demonstrated in an LES of a turbulent stratified premixed flame.
Presumed PDF Modeling of Early Flame Propagation in Moderate to Intense Turbulence Environments
NASA Technical Reports Server (NTRS)
Carmen, Christina; Feikema, Douglas A.
2003-01-01
The present paper describes the results obtained from a one-dimensional time dependent numerical technique that simulates early flame propagation in a moderate to intense turbulent environment. Attention is focused on the development of a spark-ignited, premixed, lean methane/air mixture with the unsteady spherical flame propagating in homogeneous and isotropic turbulence. A Monte-Carlo particle tracking method, based upon the method of fractional steps, is utilized to simulate the phenomena represented by a probability density function (PDF) transport equation. Gaussian distributions of fluctuating velocity and fuel concentration are prescribed. Attention is focused on three primary parameters that influence the initial flame kernel growth: the detailed ignition system characteristics, the mixture composition, and the nature of the flow field. The computational results of moderate and intense isotropic turbulence suggests that flames within the distributed reaction zone are not as vulnerable, as traditionally believed, to the adverse effects of increased turbulence intensity. It is also shown that the magnitude of the flame front thickness significantly impacts the turbulent consumption flame speed. Flame conditions studied have fuel equivalence ratio s in the range phi = 0.6 to 0.9 at standard temperature and pressure.
Numerical study of the effect of turbulence on rate of reactions in the MILD combustion regime
NASA Astrophysics Data System (ADS)
Mardani, Amir; Tabejamaat, Sadegh; Baig Mohammadi, Mohammadreza
2011-12-01
In this paper, the importance of fluctuations in flow field parameters is studied under MILD combustion conditions. In this way, a turbulent non-premixed CH4+H2 jet flame issuing into a hot and deficient co-flow air is modeled using the RANS Axisymmetric equations. The modeling is carried out using the EDC model to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. Results illustrate that although the fluctuations in temperature field are small and the reaction zone volume are large in the MILD regime, the fluctuations in temperature and species concentrations are still effective on the flow field. Also, inappropriate dealing with the turbulence effect on chemistry leads to errors in prediction of temperature up to 15% in the present flame. By decreasing of O2 concentration of hot co-flow air, the effect of fluctuations in flow field parameters on flame characteristics are still significant and its effect on species reaction rates does not decrease. On the other hand, although decreasing of jet inlet Reynolds number at constant inlet turbulence intensity addresses to smaller fluctuations in flow filed, it does not lead to lower the effect of turbulence on species distribution and temperature field under MILD combustion conditions.
Mao, Jing; Liu, Yan; Zhou, Bin; Yao, Liyun
2008-08-01
In this study, calcium phosphate cements (CPC) were prepared by mixing cement powders of tetracalcium phosphate (TTCP) with a cement liquid of phosphate acid saline solution. Tetracycline (TTC)-CPC, chitosan-CPC and chitosan-TTC-CPC were investigated with different premixed schedule. It was demonstrate that both TTC and chitosan worked on the phase transition and crystal characteristics. TTCP mixed with phosphate acid saline solution had similar features of Fourier transform-infrared spectrometry (FT-IR) no matter it was mixed with chitosan or TTC or both. TTC premixed with cement liquid or powder had significant different features of FT-IR and 876 cm(-1) seemed to be a special peak for TTC when TTC was premixed with cement liquid. This was also supported by XRD analysis, which showed that TTC premixed with cement liquid improved phase transition of TTCP to OCP. Chitosan, as organic additive, regulates the regular crystal formation and inhibits the phase transition of TTCP to OCP, except when it is mingled with cement liquid premixed with TTC in field scanning electron microscope. It was concluded that the premixed schedule influences the crystal formation and phase transition, which may be associated with its biocompatibility and bioactivities in vivo.
Turbulence generation by waves
Kaftori, D.; Nan, X.S.; Banerjee, S.
1995-12-31
The interaction between two-dimensional mechanically generated waves, and a turbulent stream was investigated experimentally in a horizontal channel, using a 3-D LDA synchronized with a surface position measuring device and a micro-bubble tracers flow visualization with high speed video. Results show that although the wave induced orbital motion reached all the way to the wall, the characteristics of the turbulence wall structures and the turbulence intensity close to the wall were not altered. Nor was the streaky nature of the wall layer. On the other hand, the mean velocity profile became more uniform and the mean friction velocity was increased. Close to the free surface, the turbulence intensity was substantially increased as well. Even in predominantly laminar flows, the introduction of 2-D waves causes three dimensional turbulence. The turbulence enhancement is found to be proportional to the wave strength.
Kerstein, A.R.
1996-12-31
One-Dimensional Turbulence is a new turbulence modeling strategy involving an unsteady simulation implemented in one spatial dimension. In one dimension, fine scale viscous and molecular-diffusive processes can be resolved affordably in simulations at high turbulence intensity. The mechanistic distinction between advective and molecular processes is thereby preserved, in contrast to turbulence models presently employed. A stochastic process consisting of mapping {open_quote}events{close_quote} applied to a one-dimensional velocity profile represents turbulent advection. The local event rate for given eddy size is proportional to the velocity difference across the eddy. These properties cause an imposed shear to induce an eddy cascade analogous in many respects to the eddy cascade in turbulent flow. Many scaling and fluctuation properties of self-preserving flows, and of passive scalars introduced into these flows, are reproduced.
Weak turbulence theory for reactive instability
Yoon, P. H.
2010-11-15
In the present paper, the customary weak turbulence theory is generalized to include reactive instabilities. For the sake of simplicity, the formalism assumes electrostatic perturbation propagating in one-dimensional uniform unmagnetized plasmas. By weak turbulence theory it is meant as the perturbative nonlinear theory based upon Vlasov equation, truncated at the second (or up to third) order nonlinearity and ensemble averaged. By reactive instability it is meant as the plasma instability whose growth rate is not necessarily exceedingly small. The traditional weak turbulence theory found in the literature is applicable only to weakly growing plasma instabilities whose real frequency {omega}{sub k} can be determined from the real part of the dispersion relation, Re {epsilon}(k,{omega}{sub k})=0, while the growth rate may be determined by the Landau formula, {gamma}{sub k}=-Im {epsilon}(k,{omega}{sub k})[{partial_derivative} Re {epsilon}(k,{omega}{sub k})/{partial_derivative}{omega}{sub k}]{sup -1}. This implies the assumption that |{gamma}{sub k}|<<{omega}{sub k}. On the other hand, for reactive instabilities for which {gamma}{sub k}/{omega}{sub k} is not necessarily small, the real frequency and growth/damping rate must be determined from the complex roots of the dispersion relation, {epsilon}(k,{omega}{sub k}+i{gamma}{sub k})=0. The present paper extends the textbook weak turbulence theory to deal with such a situation.
NASA Technical Reports Server (NTRS)
Canuto, V. M.
1990-01-01
Some of the astrophysical scenarios in which turbulence plays an important role are discussed in view of the comparative advantages of currently available models of turbulence phenomena; attention is given to a specific model that has undergone continuous refinement since 1985. The desideratum in this inquiry is a turbulence model which incorporates the best features of an a priori deterministic model, as well as a redundant set of results from full numerical simulations for a wide variety of turbulent flows; there should also be a simplification of the former, and an enlargement of the complexities of the latter.
Tactical missile turbulence problems
NASA Technical Reports Server (NTRS)
Dickson, Richard E.
1987-01-01
Of particular interest is atmospheric turbulence in the atmospheric boundary layer, since this affects both the launch and terminal phase of flight, and the total flight for direct fire systems. Brief discussions are presented on rocket artillery boost wind problems, mean wind correction, turbulent boost wind correction, the Dynamically Aimed Free Flight Rocket (DAFFR) wind filter, the DAFFR test, and rocket wake turbulence problems. It is concluded that many of the turbulence problems of rockets and missiles are common to those of aircraft, such as structural loading and control system design. However, these problems have not been solved at this time.
NASA Astrophysics Data System (ADS)
Jejjala, Vishnu; Minic, Djordje; Ng, Y. Jack; Tze, Chia-Hsiung
We propose a string theory of turbulence that explains the Kolmogorov scaling in 3+1 dimensions and the Kraichnan and Kolmogorov scalings in 2+1 dimensions. This string theory of turbulence should be understood in light of the AdS/CFT dictionary. Our argument is crucially based on the use of Migdal's loop variables and the self-consistent solutions of Migdal's loop equations for turbulence. In particular, there is an area law for turbulence in 2+1 dimensions related to the Kraichnan scaling.
Chaos Synchronization in Navier-Stokes Turbulence
NASA Astrophysics Data System (ADS)
Lalescu, Cristian; Meneveau, Charles; Eyink, Gregory
2013-03-01
Chaos synchronization (CS) has been studied for some time now (Pecora & Carroll 1990), for systems with only a few degrees of freedom as well as for systems described by partial differential equations (Boccaletti et al 2002). CS in general is said to be present in coupled dynamical systems when a specific property of each system has the same time evolution for all, even though the evolution itself is chaotic. The Navier-Stokes (NS) equations describe the velocity for a wide range of fluids, and their solutions are usually called turbulent if fluctuation amplitudes decrease as a power of their wavenumber. There have been some studies of CS for continuous systems (Kocarev et al 1997), but CS for NS turbulence seems not to have been investigated so far. We focus on the synchronization of the small scales of a turbulent flow for which the time history of large scales is prescribed. Our DNS results show that high-wavenumbers in turbulence are fully slaved to modes with wavenumbers up to a critical fraction of the Kolmogorov dissipation wavenumber. The motivation for our work is to study deeply sub-Kolmogorov scales in fully developed turbulence (Schumacher 2007), which we found to be recoverable even at very high Reynolds number from simulations with moderate resolutions. This work is supported by the National Science Foundation's CDI-II program, project CMMI-0941530
On the Interaction of a Premixed Flame with an Acoustic Disturbance
NASA Technical Reports Server (NTRS)
Hood, Caroline; Frendi, Abdelkader
2005-01-01
The main objective of this research is to analyze the effect of acoustic disturbances on a premixed flame and determine their role in the onset of combustion instabilities. Computations for the one-dimensional, unsteady combustion of a lean premixed methane-air mixture are performed. An acoustic excitation is introduced in the chamber and interacts with the flame front. Our results indicate that as the amplitude of the acoustic excitation is increased, the flame front position fluctuates rapidly. This phenomenon is even more intense when the frequency of the acoustic disturbance matches the fundamental frequency of the chamber. Our results suggest that the interactions between the flame and the acoustic excitation may result in flame extinguishment. In addition various passive control devices are tested and we found that the Helmholtz resonator with rounded inlet corners is the most efficient.
An investigation of diamond film deposition in a premixed oxyacetylene flame
NASA Astrophysics Data System (ADS)
Cappelli, Mark A.; Paul, P. H.
1989-09-01
Polycrystalline diamond film synthesis has been demonstrated using a wide variety of enhanced chemical vapor deposition (CVD) techniques. The method of choice depends on the end application of the deposited film or coating. Diamond film has been deposited in a single-nozzle pre-mixed oxy-acetylene flame. Results of runs of varying duration suggest that diamond is deposited via the transport of hydrocarbon fragments produced at the secondary flame front. Planar laser induced photodissociation fluorescence suggests that this region is rich in C2H species. Emission studies also suggest that the post primary flame zone presents a source of C2 radicals which may account for the observed graphite and diamond-like carbon deposited on the substrate exposed to this region of the flame. The results on the pre-mixed flame suggest that it would be possible and more convenient to attempt large area deposition using a multi-nozzle diffusion flame.
PREMIXED FLAME PROPAGATION AND MORPHOLOGY IN A CONSTANT VOLUME COMBUSTION CHAMBER
Hariharan, A; Wichman, IS
2014-06-04
This work presents an experimental and numerical investigation of premixed flame propagation in a constant volume rectangular channel with an aspect ratio of six (6) that serves as a combustion chamber. Ignition is followed by an accelerating cusped finger-shaped flame-front. A deceleration of the flame is followed by the formation of a "tulip"-shaped flame-front. Eventually, the flame is extinguished when it collides with the cold wall on the opposite channel end. Numerical computations are performed to understand the influence of pressure waves, instabilities, and flow field effects causing changes to the flame structure and morphology. The transient 2D numerical simulation results are compared with transient 3D experimental results. Issues discussed are the appearance of oscillatory motions along the flame front and the influences of gravity on flame structure. An explanation is provided for the formation of the "tulip" shape of the premixed flame front.
Management of Type 2 diabetes in Ramadan: Low-ratio premix insulin working group practical advice
Hassanein, Mohamed; Belhadj, Mohamed; Abdallah, Khalifa; Bhattacharya, Arpan D.; Singh, Awadhesh K.; Tayeb, Khaled; Al-Arouj, Monira; Elghweiry, Awad; Iraqi, Hinde; Nazeer, Mohamed; Jamoussi, Henda; Mnif, Mouna; Al-Madani, Abdulrazzaq; Al-Ali, Hossam; Ligthelm, Robert
2014-01-01
The challenge of insulin use during Ramadan could be minimized, if people with diabetes are metabolically stable and are provided with structured education for at least 2–3 months pre-Ramadan. Although, American diabetes association (ADA) recommendations 2010 and South Asian Consensus Guideline 2012 deal with management of diabetes in Ramadan and changes in insulin dosage, no specific guidance on widely prescribed low-ratio premix insulin is currently available. Hence, the working group for insulin therapy in Ramadan, after collective analysis, evaluation, and opinion from clinical practice, have formulated a practical advice to empower physicians with pre-Ramadan preparation, dose adjustment, and treatment algorithm for self-titration of low-ratio premix insulin. PMID:25364673
Emissions of oxides of nitrogen from an experimental premixed-hydrogen burner
NASA Technical Reports Server (NTRS)
Anderson, D. N.
1976-01-01
Flame-tube experiments using premixed hydrogen and air were conducted to determine the emissions of oxides of nitrogen (NOx) resulting from ultralean combustion. Measurements of NOx emissions and combustion efficiency were made for inlet mixture temperatures of 600 and 700 K, pressures of 3.8 x 10 to the 5th power and 5.2 x 10 to the 5th power N/m squared, reference velocities of 15 to 18 m/sec, and equivalence ratios of 0.2 to 0.4. At the 700 K inlet mixture temperature, NOx emissions were 0.06 ppmv, and combustion efficiency was 98 percent at an equivalence ratio of 0.24. The use of a high-blockage (92-percent blockage) flameholder made it possible to conduct tests without upstream burning in the premixing duct for mixtures with equivalence ratios less than 0.4. For richer mixtures upstream burning did occur and prevented further testing.
Degree of vaporization using an airblast type fuel injector for a premixed-prevaporized combustor
NASA Technical Reports Server (NTRS)
Tacina, R. R.
1978-01-01
Vaporization data are presented which could be useful in designing premixed-prevaporized fuel preparation systems for gas turbine combustors. Lean, premixed-prevaporized combustion systems are being developed because they operate with low flame temperatures and, therefore, produce low levels of nitrogen oxides. Parametric tests of the effect of inlet air temperature, length (residence time), reference velocity, pressure and fuel-air ratio on the degree of vaporization are reported. Jet A and Diesel no. 2 fuel were tested. A formula is provided which shows the effect of inlet air temperature, residence time, reference velocity, and pressure on the degree of vaporization for a constant fuel-air ratio of 0.020. The results of the effect of inlet air temperature on the degree of vaporization using Jet A and Diesel no. 2 are nearly identical.
NASA Astrophysics Data System (ADS)
Chen, Dongliang; Sun, Jinhua; Chen, Sining; Liu, Yi; Chu, Guanquan
2007-01-01
In order to explore the flame propagation characteristics and tulip flame formation mechanism of premixed methane/air mixture in horizontal rectangular ducts, the techniques of Schlieren and high-speed video camera are used to study the flame behaviors of the premixed gases in a closed duct and opened one respectively, and the propagation characteristics in both cases and the formation mechanism of the tulip flame are analyzed. The results show that, the propagation flame in a closed duct is prior to form a tulip flame structure than that in an opened duct, and the tulip flame structure formation in a closed duct is related to the flame propagation velocity decrease. The sharp decrease of the flame propagation velocity is one of the reasons to the tulip flame formation, and the decrease of the flame propagation velocity is due to the decrease of the burned product flow velocity mainly.
Extinction and near-extinction instability of non-premixed tubular flames
Hu, Shengteng; Pitz, Robert W.; Yu, Wang
2009-01-15
Tubular non-premixed flames are formed by an opposed tubular burner, a new tool to study the effects of curvature on extinction and flame instability of non-premixed flames. Extinction of the opposed tubular flames generated by burning diluted H{sub 2}, CH{sub 4} or C{sub 3}H{sub 8} with air is investigated for both concave and convex curvature. To examine the effects of curvature on extinction, the critical fuel dilution ratios at extinction are measured at various stretch rates, initial mixture strengths and flame curvature for fuels diluted in N{sub 2}, He, Ar or CO{sub 2}. In addition, the onset conditions of the cellular instability are mapped as a function of stretch rates, initial mixture strengths, and flame curvature. For fuel mixtures with Lewis numbers much less than unity, such as H{sub 2}/N{sub 2}, concave flame curvature towards the fuel suppresses cellular instabilities. (author)
Relative stability of selenites and selenates in feed premixes as a function of water activity.
Eisenberg, Sylvan
2007-01-01
Sodium selenite is more hygroscopic than sodium selenate. It is, therefore, more likely to dissolve when dispersed in feeds of relatively high water activity. When dissolved, it may form selenious acid and disperse as a vapor. This is easily demonstrated by mounting a filter paper wetted with a reagent such as ascorbic acid over the subject feed, but not in contact with it. The paper turns brown as elemental selenium is formed from reduction of the vapor. Analysis of the paper ensures that the brown is indeed selenium. Though premixes are generally low enough in moisture content to ensure stability of the selenites, this is not true of many feeds. The water activities of a number of feeds, feed premixes, and feed ingredients have been determined instrumentally and compared to those of saturated solutions of sodium selenite and sodium selenate. There is no question that the selenite often dissolves with the potential to react and, in so doing, loses its nutritional function.
Marques, Carla S.T.; Barreta, Luiz G.; Sbampato, Maria E.; dos Santos, Alberto M.
2010-11-15
In this study, nitric oxide laser-saturated fluorescence (LSF) measurements were acquired from premixed ethanol flames at atmospheric pressure in a burner. NO-LSF experimental profiles for fuel-rich premixed ethanol flames ({phi} = 1.34 and {phi} = 1.66) were determined through the excitation/detection scheme of the Q{sub 2}(26.5) rotational line in the A{sup 2}{sigma}{sup +} - X{sup 2}{pi} (0,0) vibronic band and {gamma}(0,1) emission band. A calibration procedure by NO doping into the flame was applied to establish the NO concentration profiles in these flames. Chemiluminescent emission measurements in the (0, 0) vibronic emission bands of the OH{sup *} (A{sup 2}{sigma}{sup +} - X{sup 2}{pi}) and CH{sup *}(A{sup 2}{delta} - X{sup 2}{pi}) radicals were also obtained with high spatial and spectral resolution for fuel-rich premixed ethanol flames to correlate them with NO concentrations. Experimental chemiluminescence profiles and the ratios of the integrated areas under emission spectra (A{sub CH*}/A{sub CH*}(max.) and A{sub CH*}/A{sub OH*}) were determined. The relationships between chemiluminescence and NO concentrations were established along the premixed ethanol flames. There was a strong connection between CH{sup *} radical chemiluminescence and NO formation and the prompt-NO was identified as the governing mechanism for NO production. The results suggest the optimum ratio of the chemiluminescence of two radicals (A{sub CH*}/A{sub OH*}) for NO diagnostic purposes. (author)
Self-organized Spiral and Circular Waves in Premixed Gas Flames
NASA Technical Reports Server (NTRS)
Pealman, Howard G.; Ronney, Paul D.
1994-01-01
A diffusive-thermal high Lewis number (Le) gas-phase oscillator has been observed in premixed flames using a lean mixture of butane and oxygen diluted with helium (Le approx. 3.0). This reactive-diffusive system exhibits both propagating radial pulsations and rotating spiral waves perhaps,analogous to those observed in other excitable media such as the Belousov-Zhabotinsky reaction.
Self-organized spiral and circular waves in premixed gas flames
NASA Technical Reports Server (NTRS)
Pearlman, Howard G.; Ronney, Paul D.
1994-01-01
A diffusive-thermal high Lewis number (Le) gas-phase oscillator has been observed in premixed flames using a lean mixture of butane and oxygen diluted with helium (Le approximately equals 3.0). This reactive-diffusive system exhibits both propagating radial pulsations and rotating spiral waves perhaps analogous to those observed in other excitable media such as the Belousov-Zhabotinsky reaction.
Sarathy, S M; Pitz, W J; Westbrook, C K; Mehl, M; Yasunaga, K; Curran, H J; Tsujimura, T; Osswald, P; Kohse-Hoinghaus, K
2010-12-12
Butanol is a fuel that has been proposed as a bio-derived alternative to conventional petroleum derived fuels. The structural isomer in traditional 'bio-butanol' fuel is n-butanol, but newer conversion technologies produce iso-butanol as a fuel. In order to better understand the combustion chemistry of bio-butanol, this study presents a comprehensive chemical kinetic model for all the four isomers of butanol (e.g., 1-, 2-, iso- and tert-butanol). The proposed model includes detailed high temperature and low temperature reaction pathways. In this study, the primary experimental validation target for the model is premixed flat low-pressure flame species profiles obtained using molecular beam mass spectrometry (MBMS). The model is also validated against previously published data for premixed flame velocity and n-butanol rapid compression machine and shock tube ignition delay. The agreement with these data sets is reasonably good. The dominant reaction pathways at the various pressures and temperatures studied are elucidated. At low temperature conditions, we found that the reaction of alphahydroxybutyl with O{sub 2} was important in controlling the reactivity of the system, and for correctly predicting C{sub 4} aldehyde profiles in low pressure premixed flames. Enol-keto isomerization reactions assisted by HO{sub 2} were also found to be important in converting enols to aldehydes and ketones in the low pressure premixed flames. In the paper, we describe how the structural features of the four different butanol isomers lead to differences in the combustion properties of each isomer.
On the structure, propagation, and stabilization of laminar premixed flames. Final report
Law, Chung K.
1999-07-01
The primary objective of the funded program was to qualitatively understand and quantitatively determine the structure and dynamics of laminar premixed flames. The investigation was conducted using laser-based experimentation, computational simulation with detailed chemistry and transport, and activation energy asymptotic analysis. Highlights of accomplishments were discussed in the annual reports submitted to the program monitor for this project. Details are reported in the thirty journal publications cited in the journal article list which is the major component of this final report.
Soot Formation in Laminar Premixed Methane/Oxygen Flames at Atmospheric Pressure. Appendix H
NASA Technical Reports Server (NTRS)
Xu, F.; Lin, K.-C.; Faeth, G. M.; Urban, D. L. (Technical Monitor); Yuan, Z.-G. (Technical Monitor)
2001-01-01
Flame structure and soot formation were studied within soot-containing laminar premixed methanefoxygen flames at atmospheric pressure. The following measurements were made: soot volume fractions by laser extinction, soot temperatures by multiline emission, gas temperatures (where soot was absent) by corrected fine-wire thermocouples, soot structure by thermophoretic sampling and transmission electron microscope (TEM), major gas species concentrations by sampling and gas chromatography, and gas velocities by laser velocimetry. Present measurements of gas species concentrations were in reasonably good agreement with earlier measurements due to Ramer et al. as well as predictions based on the detailed mechanisms of Frenklach and co-workers and Leung and Lindstedt; the predictions also suggest that H atom concentrations are in local thermodynamic equilibrium throughout the soot formation region. Using this information, it was found that measured soot surface growth rates could be correlated successfully by predictions based on the hydrogenabstraction/carbon-addition (HACA) mechanisms of both Frenklach and co-workers and Colket and Hall, extending an earlier assessment of these mechanisms for premixed ethylene/air flames to conditions having larger H/C ratios and acetylene concentrations. Measured primary soot particle nucleation rates were somewhat lower than the earlier observations for laminar premixed ethylene/air flames and were significantly lower than corresponding rates in laminar diffusion flames, for reasons that still must be explained.
Soot Formation in Laminar Premixed Methane/Oxygen Flames at Atmospheric Pressure
NASA Technical Reports Server (NTRS)
Xu, F.; Lin, K.-C.; Faeth, G. M.
1998-01-01
Flame structure and soot formation were studied within soot-containing laminar premixed mc1hane/oxygen flames at atmospheric pressure. The following measurements were made: soot volume fractions by laser extinction, soot temperatures by multiline emission, gas temperatures (where soot was absent) by corrected fine-wire thermocouples, soot structure by thermophoretic sampling and transmission electron microscope (TEM), major gas species concentrations by sampling and gas chromatography, and gas velocities by laser velocimetry. Present measurements of gas species concentrations were in reasonably good agreement with earlier measurements due to Ramer et al. as well as predictions based on the detailed mechanisms of Frenklach and co-workers and Leung and Lindstedt: the predictions also suggest that H atom concentrations are in local thermodynamic equilibrium throughout the soot formation region. Using this information, it was found that measured soot surface growth rates could be correlated successfully by predictions based on the hydrogen-abstraction/carbon-addition (HACA) mechanisms of both Frenklach and co-workers and Colket and Hall, extending an earlier assessment of these mechanisms for premixed ethylene/air flames to conditions having larger H/C ratios and acetylene concentrations. Measured primary soot particle nucleation rates were somewhat lower than the earlier observations for laminar premixed ethylene/air flames and were significantly lower than corresponding rates in laminar diffusion flames. for reasons that still must be explained.
Persson, Cecilia; Engqvist, Håkan
2011-01-01
Calcium silicate-based materials (also called MTA) are increasingly being used in endodontic applications. However, the handling properties of MTA are not optimal when it comes to injectability and cohesion. Premixing the cements using glycerol avoids these issues. However, there is a lack of data on the effect of common cement variables on important properties of premixed cements for endodontic applications. In this study, the effects of liquid-to-powder ratio, amount of radiopacifier and amount of calcium sulfate (added to control the setting time) were screened using a statistical model. In the second part of the study, the liquid-to-powder ratio was optimized for cements containing three different amounts of radiopacifier. Finally, the effect of using glycerol rather than water was evaluated in terms of radiopacity. The setting time was found to increase with the amount of radiopacifier when the liquid-to-powder ratio was fixed. This was likely due to the higher density of the radiopacifier in comparison to the calcium silicate, which gave a higher liquid-to-powder ratio in terms of volume. Using glycerol rather than water to mix the cements led to a decrease in radiopacity of the cement. In conclusion, we were able to produce premixed calcium silicate cements with acceptable properties for use in endodontic applications.
Bluff-body stabilized flame dynamics of lean premixed syngas combustion
NASA Astrophysics Data System (ADS)
Im, Hong G.; Kim, Yu Jeong; Lee, Bok Jik; Kaust Team
2015-11-01
Recently, syngas combustion has been actively investigated for the potential application to integrated gasification combined cycle (IGCC) systems. While lean premixed combustion is attractive for both reduced emission and enhanced efficiency, flame instability becomes often an issue. Bluff-bodies have been adopted as effective flame holders for practical application of premixed flames. In the present study, high-fidelity direct numerical simulations are conducted to investigate the dynamics of lean premixed syngas flames stabilized on a bluff-body, in particular at the near blow-off regime of the flame. A two-dimensional domain of 4 mm height and 20 mm length with a flame holder of a 1 mm-by-1 mm square geometry is used. For a syngas mixture with the equivalence ratio of 0.5 and the CO:H2 ratio of 1, several distinct flame modes are identified as the inflow velocity approaches to the blowoff limit. The sequences of extinction pathway and combustion characteristics are discussed.
Gravitational Influences on Flame Propagation Through Non-Uniform, Premixed Gas Systems
NASA Technical Reports Server (NTRS)
Miller, Fletcher J.; Easton, John; Marchese, Anthony; Hovermann, Fred
2003-01-01
Flame propagation through non-uniformly premixed (or layered) gases has importance both in useful combustion systems and in unintentional fires. As summarized recently and in previous Microgravity Workshop papers, non-uniform premixed gas combustion receives scant attention compared to the more usual limiting cases of diffusion or uniformly premixed flames, especially regarding the role gravity plays. This paper summarizes our recent findings on gravitational effects on layered combustion along a floor, in which the fuel concentration gradient exists normal to the direction of flame spread. In an effort to understand the mechanism by which the flames spread faster in microgravity (and much faster, in laboratory coordinates, than the laminar burning velocity for uniform mixtures), we have begun making pressure measurements across the spreading flame front that are described here. Earlier researchers, testing in 1g, claimed that hydrostatic pressure differences could account for the rapid spread rates. Additionally, we present the development of a new apparatus to study flame spread in free (i.e., far from walls), non-homogeneous fuel layers formed in a flow tunnel behind an airfoil that has been tested in normal gravity.
Gravitational Influences on Flame Propagation through Non-Uniform, Premixed Gas Systems
NASA Technical Reports Server (NTRS)
Miller, Fletcher J.; Easton, John; Ross, Howard D.; Marchese, Anthony; Perry, David; Kulis, Michael
2001-01-01
Flame propagation through non-uniformly premixed (or layered) gases has importance both in useful combustion systems and in unintentional fires. As summarized previously, non-uniform premixed gas combustion receives scant attention compared to the more usual limiting cases of diffusion or uniformly premixed flames, especially regarding the role gravity plays. This paper summarizes our progress on furthering the knowledge of layered combustion, in which a fuel concentration gradient exists normal to the direction of flame spread. We present experimental and numerical results for flame spread through propanol-air layers formed near the flash point temperature (25 C) or near the stoichiometric temperature (33 C). Both the model and experimental results show that the removal of gravity results in a faster spreading flame, by as much as 80% depending on conditions. This is exactly the opposite effect as that predicted by an earlier model reported. We also found that having a gallery lid results in faster flame spread, an effect more pronounced at normal gravity, demonstrating the importance of enclosure geometry. Also reported here is the beginning of our spectroscopic measurements of fuel vapor.
Optimization of instant dalia dessert pre-mix production by using response surface methodology.
Jha, Alok; Shalini, B N; Patel, Ashok Ambalal; Singh, Mithilesh; Rasane, Prasad
2015-02-01
Dalia, a wheat-based, particulate containing dairy dessert is popularly consumed as a breakfast food and is also considered as a health food. Though popular throughout Northern parts of the country, its limited shelf-life even under refrigeration imposes severe restrictions on its organized manufacture and marketing. In order to promote dalia dessert as a marketable product, in the present study, a process was developed for manufacture of instant dalia pre-mix, as a dry product with long shelf-life, which could be attractively packaged and easily reconstituted for consumption. During the investigation, the effect of different levels of milk solids and wheat solids was studied on dalia pre-mix quality by employing a central composite rotatable design (CCRD). The suggested formulation had 17.82 % milk solids and 2.87 % wheat solids. This formulation was found to be most appropriate for manufacture of instant dalia pre-mix with predicted sensory scores (Max. 100) of 85.35, 41.98 and 67.27 for mouthfeel, consistency and flavor, respectively; the viscosity of the product was 941.0 cp.
How ''flat'' is the rich premixed flame produced by your McKenna burner?
Migliorini, F.; De Iuliis, S.; Cignoli, F.; Zizak, G.
2008-05-15
McKenna burners are widely used in the combustion community for producing ''flat'' premixed flames. These flames are considered as standards for the development and calibration of optical techniques. Rich premixed flames produced by McKenna burners are frequently investigated in order to understand soot formation processes both by optical and by sampling techniques. Measurements are normally performed along the axis of the flames, with a uniform distribution of temperature and species concentration assumed in the radial direction. In this work it is shown that the soot radial profiles of rich premixed ethylene-air flames produced by a McKenna burner with a stainless steel porous plug may be far from being ''flat.'' Soot is mainly distributed in an annular region and nonsoot fluorescing species are present in the core of the flames. This surprising result was verified under several working conditions. Furthermore, flames cannot be considered axial-symmetric but present a skewed soot distribution. Another McKenna burner with a bronze porous disk was used to produce flames of the same equivalence ratio and flows. These flames show a completely different soot radial profile, closer to the claimed flat distribution. These results cast doubts about the conclusions drawn in several studies on soot formation performed with a stainless steel McKenna burner. (author)
Prediction of electron and ion concentrations in low-pressure premixed acetylene and ethylene flames
NASA Astrophysics Data System (ADS)
Cancian, J.; Bennett, B. A. V.; Colket, M. B.; Smooke, M. D.
2013-04-01
Flame stabilisation and extinction in a number of different flows can be affected by application of electric fields. Electrons and ions are present in flames, and because of charge separation, weak electric fields can also be generated even when there is no externally applied electric field. In this work, a numerical model incorporating ambipolar diffusion and plasma kinetics has been developed to predict gas temperature, species, and ion and electron concentrations in laminar premixed flames without applied electric fields. This goal has been achieved by combining the existing CHEMKIN-based PREMIX code with a recently developed methodology for the solution of electron temperature and transport properties that uses a plasma kinetics model and a Boltzmann equation solver. A chemical reaction set has been compiled from seven sources and includes chemiionisation, ion-molecule, and dissociative-recombination reactions. The numerical results from the modified PREMIX code (such as peak number densities of positive ions) display good agreement with previously published experimental data for fuel-rich, non-sooting, low-pressure acetylene and ethylene flames without applied electric fields.
On the Structure and Stabilization Mechanisms of Planar and Cylindrical Premixed Flames
NASA Technical Reports Server (NTRS)
Eng, James A.; Zhu, Delin; Law, Chung K.
1993-01-01
The configurational simplicity of the stationary one-dimensional flames renders them intrinsically attractive for fundamental flame structure studies. The possibility and fidelity of studies of such flames on earth, however, have been severely restricted by the unidirectional nature of the gravity vector. To demonstrate these complications, let us first consider the premixed flame. Here a stationary, one-dimensional flame can be established by using the flat-flame burner. We next consider nonpremixed flames. First it may be noted that in an unbounded gravity-free environment, the only stationary one-dimensional flame is the spherical flame. Indeed, this is a major motivation for the study of microgravity droplet combustion, in which the gas-phase processes can be approximated to be quasi-steady because of the significant disparity between the gas and liquid densities for subcritical combustion. In view of the above considerations, an experimental and theoretical program on cylindrical and spherical premixed and nonpremixed flames in microgravity has been initiated. For premixed flames, we are interested in: (1) assessing the heat loss versus flow divergence as the dominant stabilization mechanism; (2) determining the laminar flame speed by using this configuration; and (3) understanding the development of flamefront instability and the effects of the flame curvature on the burning intensity.
Zhao, Yan; Tong, Chenning; Ma, Lin
2010-04-01
The study of turbulent combustion calls for new diagnostics that can measure multidimensional mixture fraction under a wide range of flame conditions. A laser diagnostic technique based on photodissociation spectroscopy (PDS) is proposed to address this need. This paper describes the concept of the PDS-based diagnostic, reports its experimental demonstration in a non-premixed jet flame, and assesses its performance and applicable range. This new technique is centered around the creative use of photodissociation (PD) for flow visualization. A carefully chosen PD precursor is seeded into the flow of interest to measure mixture fraction. The precursor is chosen such that (1) both the precursor itself and the products formed from the precursor (if it reacts) can be completely and rapidly photodissociated; thus, the concentration of one of the photofragments forms a conserved scalar and can be used to infer the mixture fraction, and (2) the target photofragment offers friendly spectroscopic properties (e.g., strong laser-induced fluorescence signals and/or simple signal interpretation) so multidimensional imaging can be readily obtained. Molecular iodine (I(2)) was identified as a precursor satisfying both requirements and was seeded into a carbon monoxide (CO)-air jet flame for single-shot two-dimensional imaging of mixture fraction. This demonstration illustrates the potential of the PDS-based technique to overcome the limitations of existing techniques and to provide multidimensional measurements of mixture fraction in a variety of reactive flows.
2012-04-01
afterburners to reduce screech in order to avoid deterioration and failure of the engine. The low frequency dynamic instability (i.e., 50 – 120 Hz) is...requirements for high-speed applications. In a combustor system, a typical afterburner operation is described as follows. The liquid fuel is...exit through a converging/diverging nozzle with extensive film cooling and a variable throat area located downstream of the afterburner exit. The
2011-03-01
have been systematically investigated in Sandia’s Engine Combustion Simulation Lab. Recently, interest in the Single-Fuel Concept for the U.S...emission. A detailed chemical kinetic mechanism for ethylene combustion , including fuel-rich chemistry and benzene formation steps, was compiled...Spray Combustion ...............................................................................13 4.6 Large Eddy Simulation
2006-10-01
Thermophoresis is the tendency for a particle to drift “down” the temperature gradient. Sung et al. [48] found that the thermophoretic drift velocity...with flamefront strain) are mini- mally affected by thermophoresis since the tempera- ture gradient is small. While deriving a contribution to the
NASA Technical Reports Server (NTRS)
Montgomery, David
1988-01-01
Three areas of study in MHD turbulence are considered. These are the turbulent relaxation of the toroidal Z pinch, density fluctuations in MHD fluids, and MHD cellular automata. A Boolean computer game that updates a cellular representation in parallel and that has macroscopic averages converging to solutions of the two-dimensional MHD equations is discussed.
Direct Numerical Simulations of Turbulent Autoigniting Hydrogen Jets
NASA Astrophysics Data System (ADS)
Asaithambi, Rajapandiyan
flame base. Mass-fraction of the hydroperoxyl radical, HO2, peaks in magnitude upstream of the flame's stabilization point indicating autoignition. A flame structure similar to a triple-flame, with a lean premixed flame and a rich premixed flame flanking a thick diffusion flame is identified by the flame index. Radicals formed in the shear layer ahead of ignition and oxygen from the coflow do not get fully consumed by the flame and are transported along the edges of the flame brush into the core of the jet. Ignition delays from a well-stirred reactor model and an autoigniting diffusion flame model are able predict the lift-off height of the turbulent flame. The local entrainment rate was observed to increase with axial distance until the flame stabilization point and then decrease downstream. Data from probes placed along the flame reveals a highly turbulent flow field with variable composition at a given location. In general however, it is observed that the turbulent kinetic energy (TKE) is very high in cold fuel rich mixtures and is lowest in hot fuel lean mixtures. Autoignition occurs at the most-reactive hot and lean mixture fractions where the TKE is the lowest.