Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session AM: Reacting Flows I |
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Chair: Heinz Pitsch, Stanford University Room: 200B |
Sunday, November 22, 2009 8:00AM - 8:13AM |
AM.00001: Flamelet model for supersonic combustion Vincent Terrapon, Heinz Pitsch, Rene Pecnik The vast majority of computational work in supersonic turbulent combustion has so far relied on simplified/reduced mechanisms and the explicit transport of the involved species. Such approaches require then closure of the chemical source term in the species transport equation. An alternative approach is based on the flamelet concept which assumes that the chemical time scales are shorter than the turbulent time scales so that the flame can be approximated as one-dimensional. However, the implementation of the flamelet model is based on a low Mach number assumption, explaining the still very limited number of studies of high speed flows using this approach. Since supersonic speed and compressibility effects play an important role at supersonic speeds, the flamelet implementation has been reformulated where temperature is not any longer given by a chemistry table but computed from the total energy and the tabulated species mass fractions, thus, better accounting for compressibility effects. The model is applied to the combustor of the HyShot II vehicle and results are compared to experiment measurements and simulation data. [Preview Abstract] |
Sunday, November 22, 2009 8:13AM - 8:26AM |
AM.00002: LES/PDF Modeling of Soot Evolution in Turbulent Flames Venkatramanan Raman, Michael Mueller, Guillaume Blanquart, Heinz Pitsch Modeling soot evolution is turbulent flames is a complex problem due to the nonlinear interactions between the soot particles and the gas-phase turbulent combustion process. In this work, we develop a transported probability density function (PDF) approach for soot description in the context of large eddy simulation (LES) based combustion modeling. The soot number density is described using the bivariate VS (volume-surface) approach. The number density evolution equation is discretized using the hybrid method of moments technique, where the first four moments of the number density function are solved. In the transported PDF approach, the joint subfilter distribution of the gas-phase thermochemical scalars and the soot moments are evolved using a Lagrangian Monte-Carlo approach. The LES-PDF approach is validated using a piloted diffusion flame experiment. Results indicate that the PDF approach predicts delayed inception of soot particles and lower soot volume fraction as compared to the pure LES approach. While the soot volume fraction along the centerline of the jet is overpredicted by the simulation, the radial distribution is underpredicted as compared to the experiments. Further, the influence of mixing model on soot evolution is discussed. [Preview Abstract] |
Sunday, November 22, 2009 8:26AM - 8:39AM |
AM.00003: Simulation of an ethylene-air jet flame with soot and radiation modeling Jeffrey Doom, Joseph Oefelein Large eddy simulation of an ethylene-air diffusion flame and supporting direct numerical simulations are presented. A reduced mechanism recently developed by Wang et al. is used (22 species, 107 reactions) and a systematic study is performed which compares the reduced mechanism to the original full mechanism ({\it USC Mech Version II}: 111 species, 784 reactions). A series of calculations are then validated by comparing results with CHEMKIN, Lignell et al. ({\it Combust. Flame 2007}) and the premixed experiments from Bhargava \& Westmoreland ({\it Combust. Flame 1998}). The baseline soot model employed is from Leung et al ({\it Combust. Flame 1991}) and accounts for nucleation, growth, oxidation and coagulation. This model is coupled through source terms as a function of $C_2 H_2$, $C O$, $O_2$ and $H_2$. The first two moments are considered to account for the number density and soot mass per volume. Initially the radiation model assumes an optically thin medium in a manner consistent with Lignell et al. Results associated with the soot model will be presented along with comparisons with experimental data. [Preview Abstract] |
Sunday, November 22, 2009 8:39AM - 8:52AM |
AM.00004: Direct Quadrature Method of Moments for LES-based Modeling of Supersonic Combustion Pratik Donde, Heeseok Koo, Venkat Raman The LES/transported probability density function (PDF) model has been successfully used for predictive modeling of turbulent combustion in low-speed flows. The PDF approach evolves the joint-distribution of the gas-phase thermochemical composition and is ideally suited for supersonic flows, where conserved-scalar approaches are not valid due to the compressible nature of the flow. In low-speed flows, the high-dimensionality of the PDF transport equation is handled through the use of Monte-Carlo based stochastic methods. However, the presence of shocks and large density and pressure gradients pose significant challenges in the use of these stochastic methods for high-speed flows. In this work, we propose a direct quadrature method of moments (DQMOM) approach, which is a fully Eulerian method for solving the PDF transport equation. Here, the subfilter PDF is discretized in terms of a finite number of delta functions, each characterized by a weight and an abscissa. Eulerian transport equations for these quantities are similar in structure to scalar transport equations and can be solved using finite-volume/finite difference approaches. Here, the accuracy of the DQMOM approach and the numerical implementation of this method using shock-capturing schemes are discussed. [Preview Abstract] |
Sunday, November 22, 2009 8:52AM - 9:05AM |
AM.00005: Effects of Acoustic Excitation on Bluff-body Stabilized Premixed Reacting Flows Vaidyanathan Sankaran, Robert Erickson, Marios Soteriou Bluff body stabilized flames are used in numerous combustion applications to enable stable burning at high speeds. These confined flames are susceptible to acoustic excitation arising due to the confinement that can lead to thermoacoustic instabilities which are detrimental to the operability of the combustion device. In this study, we formulate a computational approach for the simulation of this phenomenon that is based on the one way coupling of an acoustic solution to a low Mach number but dilatational reacting flowfield. The latter is simulated with a purely Lagrangian and grid free approach that captures the rotational flowfield using the discrete Vortex Method and the reacting field by a kinematical solution of the G-equation. Earlier studies using this flow simulation approach have shown that the unsteady interactions, such as the transition from the asymmetric Von-Karman shedding to the more symmetric shedding structure present when reaction occurs can be captured accurately. Flame response to longitudinal acoustic waves is simulated and results are to be discussed in the context of transfer functions of heat release response to acoustic velocity excitation. Dominant mechanisms by which the flame responds to acoustics will also be identified. Finally, results are to be contrasted to those from analytical models that are in use in thermoacoustic studies today. [Preview Abstract] |
Sunday, November 22, 2009 9:05AM - 9:18AM |
AM.00006: The Interaction of High-Speed Turbulence with Flames Alexei Poludnenko, Elaine Oran Interaction of flames with turbulence occurs in systems ranging from chemical flames on Earth to thermonuclear burning fronts in supernovae. We present results of a systematic study of the dynamics and properties of turbulent flames formed under the action of high-speed turbulence in stoichiometric hydrogen-air mixture. Numerical simulations were performed using the massively parallel reactive-flow code Athena-RFX. Here we discuss (1) global properties of the turbulent flame in this regime (flame width, speed, etc.); (2) the internal structure of the flame brush; and (3) the internal structure of the flamelets folded inside the flame brush. We demonstrate that, in the case of hydrogen, turbulence does not affect the internal flame structure essentially for all subsonic turbulent intensities. We address the relative role of large-scale and small-scale motions on global and local properties of the turbulent flame. We also consider the processes that determine the turbulent burning velocity and identify two distinct regimes of flame evolution. Finally, we discuss the effects of non-equilibrium non-Kolmogorov turbulence on the turbulent flame properties. [Preview Abstract] |
Sunday, November 22, 2009 9:18AM - 9:31AM |
AM.00007: Transport by molecular diffusion in LES of a turbulent diffusion flame Konstantin Kemenov, Sharadha Viswanathan, Haifeng Wang, Stephen Pope Molecular diffusion effects in LES of a piloted methane-air (Sandia D) flame are investigated on a series of grids with progressively increased resolution towards the DNS limit. The role of molecular diffusivity in effecting spatial transport is studied by drawing a comparison with the turbulent diffusivity and analyzing their statistics conditioned on temperature. Statistical results demonstrate that the molecular diffusivity in the near-field almost always exceeds the turbulent diffusivity, except at low temperatures (less than 500K). Thus, by altering the jet near-field, molecular transport plays an important role in the further downstream jet development. Molecular diffusivity continues to dominate in the centerline region throughout the flow field. Overall, the results suggest the strong necessity to represent molecular transport accurately in LES studies of turbulent reacting flows. [Preview Abstract] |
Sunday, November 22, 2009 9:31AM - 9:44AM |
AM.00008: A LES/FDF/PMC-based Detailed Model for Luminous Turbulent Flames Ankur Gupta, Daniel Haworth, Michael Modest A comprehensive model is presented for luminous turbulent flames with full consideration of turbulence/chemistry interactions (TCI), turbulence/radiation interactions (TRI), and detailed gas- phase chemistry and soot. A large-eddy simulation/composition filtered mass density function (LES/C-FDF) formulation is adopted that accounts exactly for the influence of subfilter-scale turbulent fluctuations on chemical source terms and radiative emission. A consistent Lagrangian Monte Carlo particle/Eulerian mesh method is used to solve the modeled C-FDF equation. A second Monte Carlo particle method (photon Monte Carlo - PMC) is used to solve the radiative transfer equation; the radiation model includes spectral radiation properties and absorption. Soot is modeled using a method of moments. The model is validated using experimental data for luminous turbulent nonpremixed jet flames. The model then is exercised to isolate and quantity different contributions to TRI. [Preview Abstract] |
Sunday, November 22, 2009 9:44AM - 9:57AM |
AM.00009: Flow field structure near the reaction zone in turbulent nonpremixed jet flames Mirko Gamba, Noel T. Clemens, Ofodike A. Ezekoye Quasi-instantaneous pseudo-volumes of the 3D velocity field in the far field of turbulent nonpremixed jet flames are constructed from cinematographic kilohertz-rate stereoscopic PIV applying Taylor's hypothesis. Jet flames at jet exit Reynolds numbers of 8,000-15,000 were considered. The approach enable computation of all nine velocity gradients and the 3D kinematic quantities. 10 Hz OH PLIF imaging was also included to mark the reaction zone. Three-dimensional rendering of regions of intense vorticity and energy dissipation reveals their sheet-like nature and their tendency to exist near the OH layers. Contrary to nonreacting jets, this feature is believed to be a due to the stabilizing effect of heat release and the laminar shear caused by the flame. Single-point statistics of the velocity gradients indicate anisotropy in the flow with strong gradients predominantly in the radial direction. However, the 1D energy spectrum and single-point statistics of the principal strain and strain-vorticity alignment follow the known trends from incompressible turbulence. [Preview Abstract] |
Sunday, November 22, 2009 9:57AM - 10:10AM |
AM.00010: Effects of Swirl on Strongly-Pulsed Turbulent Diffusion Flames Y.-H. Liao, J.C. Hermanson The dynamics of large-scale structures in strongly-pulsed, swirling, turbulent jet diffusion flames were examined experimentally. The combustor used a combination of axial and tangentially-injected air to produce a range of swirl numbers. Gaseous ethylene fuel was injected through a 2 mm diameter nozzle on the combustor centerline with a jet-on Reynolds number of 5000. The flames were fully-modulated, with the fuel flow completely shut off between pulses. High-speed imaging of the flame luminosity was employed to examine the flame dimensions and the celerity of the large-scale flame structures. The flames were found to be approximately 15-20{\%} shorter when swirl was imposed, depending on the injection time. The more compact flames in swirl appear to be due to the presence of recirculation inside the flames. For longer injection times, the celerity of the flame structures generally decreases as the swirl intensity increases. This is evidently due to the reversed velocity in the recirculation zone. For shorter injection times, the flame celerity has an increasing trend with increased swirl intensity due to flames being closer to the fuel nozzle at burnout. [Preview Abstract] |
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