Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session G34: Stratified and Premixed Flames |
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Chair: Peter Hamlington, University of Colorado Room: 2024 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G34.00001: Analysis of lift-off height and structure of n-heptane tribrachial flames in laminar jet configuration Stefano Luca, Fabrizio Bisetti A set of lifted tribrachial n-heptane flames in a laminar jet configuration is simulated. The simulations are performed using finite rate chemistry and detailed transport, and aim at investigating the geometry and the structure of tribrachial flames. Varying the inlet velocity of the fuel, different stabilization heights are obtained, and the dependence on the inlet velocity is compared with experimental data. The results of the simulations show that when the stabilization height decreases, resulting in larger velocity and mixture fraction gradients at the tribrachial point, the tilt of the flame increases, while the heat release rate and radius of curvature decrease. A detailed analysis of the flame geometry, compared to unstretched premixed flames is performed, focusing on differential diffusion effects, flame stretch, and transport of heat and mass from the burnt gases to the flame front. Our analysis seems to indicate that for a flame that stabilizes further downstream positive stretch along the rich premixed wing leads to an increase in the rate of chemical reaction in the whole flame. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G34.00002: Structure of a laminar triple flame of a jet fuel surrogate Krithika Narayanaswamy, Perrine Pepiot Triple flames are found in jet flames and play an important role in the stabilization and thereby lift-off height of lifted jet flames. In this study, 2D laminar triple flames burning jet fuel are simulated using finite rate chemistry and detailed transport of species. The jet fuel is represented by using a surrogate mixture, comprised of n-dodecane, methyl-cyclohexane, and m-xylene. The chemical kinetics of this multi-component surrogate are described using a reduced model derived from a well-validated detailed mechanism. The structure of the simulated triple flames is explored by examining the reactivity of the different hydrocarbons in the multi-component fuel and the radical profiles. The heat release profiles of the lean and rich branches of the triple flame are compared to their unstretched 1D counterparts to identify similarities. Varying the composition of the components in the multi-component surrogate is found to result in little differences in the laminar flame speed predictions. The simulations are repeated for different fuel mixtures in order to investigate the effect of the surrogate fuel composition on the combustion process. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G34.00003: Direct numerical simulations of flow-chemistry interactions in statistically stationary turbulent premixed flames Hong G. Im, Paul G. Arias, Swetaprovo Chaudhuri, Chung K. Law The effects of Damkohler number and Karlovitz number on the flame dynamics of three-dimensional statistically planar turbulent premixed flames are investigated by direct numerical simulation incorporating detailed chemistry and transport for a hydrogen-air mixture. The mean inlet velocity was dynamically adjusted to ensure a stable flame within the computational domain, allowing the investigation of time-averaged quantities of interest. A particular interest was on understanding the effects of turbulence on the displacement speed of the flame relative to the local fluid flow. The results show that the displacement speed dynamics in response to turbulent eddies depends strongly on the specific choice of the iso-surfaces in the progress variable. As such, the statistical distribution of the flame speed versus the strain/curvature relations shows a significant sensitivity on the definition of the flame speed. Further analysis is conducted to examine the behavior of the alignment between the flame surface and the strain rate eigenvectors. The results for the reference conditions are compared against different parametric conditions in order to assess their effects on flame-flow interaction characteristics. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G34.00004: The turbulent flame speed for low-to-moderate turbulence intensities Moshe Matalon, Navin Fogla, Francesco Creta 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. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G34.00005: Impact of Karlovitz number on vortex evolution through a premixed flame Chris Bradley, Brock Bobbitt, Guillaume Blanquart As a canonical test case of premixed turbulent combustion, the vortex-flame interaction is investigated for the transformation of vorticity through the flame. This is analyzed as a function of the length and velocity scale of the vortex, which may be related to the Karlovitz number in premixed turbulent combustion. This analysis is performed using theoretical analysis of the vorticity equation and results from Direct Numerical Simulations. The vorticity is found to transform with different behavior due to the variable importance of viscous dissipation, dilatation, and baroclinic torque. The importance of these affects are shown to be based on the velocity and length scale of the vortex in relation to the velocity and length scale of the flame. The conditions under which these effects are dominant is outlined and confirmed through comparison of the theoretical and simulation results. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G34.00006: Bivariate chemistry model for the simulation of high Karlovitz premixed turbulent flames in the absence of differential diffusion Bruno Savard, Guillaume Blanquart In recent work (Savard et al., Proc. Comb. Inst. (2014)), it was shown that the structure, i.e. the dependence of the species mass fractions on temperature, of a high Karlovitz n-heptane/air turbulent premixed flame was similar to that of an unstretched one-dimensional flame in the absence of differential diffusion. The mean profiles of the species chemical source terms conditional to temperature were also found to be very close to those of a corresponding unstretched one-dimensional flame. However, while minimal deviation from the one-dimensional solution was found in the flame structure, large relative fluctuations around the mean (close to the one-dimensional solution) were found in the species source terms. Accordingly, conventional tabulated chemistry with only one progress variable transported is shown to adequately represent the flame structure and the mean species source terms, but not the source terms fluctuations. In this work, a bivariate chemistry model (i.e. a model based on two variables) that captures these species source terms fluctuations is presented. The model is first developed for unity Lewis number flames, but an extension to flames with differential diffusion is discussed. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G34.00007: Differing effects of viscosity and density changes on turbulence in high Karlovitz premixed flames Brock Bobbitt, Guillaume Blanquart The change in turbulence characteristics through a premixed flame is an important phenomenon in premixed turbulent combustion as the characteristics behind the preheat zone are the relevant quantities for the effects on the reaction zone. Both the fluid properties of viscosity and density are altered through the preheat zone which induce changes to the turbulence. As the viscosity can increase by a factor of 30 and density can decrease by a factor of 6 in many hydrocarbon/air flames, these effects are significant. This work focuses on the individual effects of each of these fluid properties to better understand the coupling of the turbulence and flame. This is analyzed through high Karlovitz number Direct Numerical Simulations of a \textit{n}-heptane/air flame and varying the relative importance of density and viscosity. The viscosity change relates to viscous dissipation while the density change relates to dilation and baroclinic torque in the vorticity equation. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G34.00008: 3D DNS of Turbulent Premixed Flame with over 50 Species and 300 Elementary Reactions Masayasu Shimura, Basmil Yenerdag, Yoshitsugu Naka, Yuzuru Nada, Mamoru Tanahashi Three-dimensional direct numerical simulation of methane-air premixed planar flame propagating in homogenous isotropic turbulence is conducted to investigate local flame structure in thin reaction zones. Detailed kinetic mechanism, GRI-Mech 3.0 which includes 53 species and 325 elementary reactions, is used to represent methane-air reaction, and temperature dependences of transport and thermal properties are considered. For a better understanding of the local flame structure in thin reaction zones regime, distributions of mass fractions of major species, heat release rate, temperature and turbulent structures are investigated. Characteristic flame structures, such as radical fingering and multi-layered-like flame structures, are observed. The most expected maximum heat release rate in flame elements is lower than that of laminar flame with same mixture. To clarify mechanism of the decrease in local heat release rate, effects of strain rates tangential to flame front on local heat release rate are investigated. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G34.00009: Spectral Kinetic Energy Transfer Through a Premixed Flame Brush Colin A.Z. Towery, Alexei Y. Poludnenko, Peter E. Hamlington 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. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G34.00010: High spatial resolution PIV and CH-PLIF measurements of a Shear Layer Stabilized Flame Christopher Foley, Ianko Chterev, Jerry Seitzman, Tim Lieuwen In practical combustors, flames stabilize in thin shear layers with very high strain rates, which alter the flame burning rate - either enhancing or diminishing reaction rates, and even leading to extinction. Therefore, the bulk velocity that provides stable operation in these combustors is limited, presumably due to the associated maximum stretch rate that the flame is able to withstand. The focus of this work is to develop a deeper understanding of the interaction between flow and flame for a shear layer stabilized, premixed flame. This study consists of planar, high resolution, simultaneous PIV and CH-PLIF measurements, in a 8 x 6 mm plane with 0.11 mm and 0.16 mm PIV vector and CH-PLIF image resolution, respectively, of the flame stabilization region in a swirling jet. The hydrodynamic strain induced stretch rate along the high CH concentration layer of the flame front is calculated from these measurements. In addition, this study elucidates the unsteady behavior of the flame in the thin shear layer. The measured flame stretch is highly spatially and temporally dependent, and dominated by contributions from normal and shear strain terms of axial velocity. Although normal strain is much greater than shear, the near horizontal flame orientation results in neither strain term dominating flame stretch. Furthermore, the flame angle changes the sign of the shear strain contributions as observed experimentally, an important implication for reduced order modeling approaches. [Preview Abstract] |
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