Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D26: Reacting Flows II: DNS/LES/RANS |
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Chair: Richard Miller, Clemson University Room: 321 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D26.00001: Subgrid-scale mixing of temperature perturbations from flamelet in turbulent partially premixed flames Shuaishuai Liu, Chenning Tong Recent studies have shown that the subgrid-scale (SGS) mixture fraction and temperature in turbulent partially premixed flames have different structures for different SGS scalar variance. For large SGS variance the molecular transport and chemical reaction are tightly coupled while mixing models are greatly based on non-reactive scalars. To account for this coupling effect we use a method proposed by Bilger and Pope [1, 2] to decompose the temperature (a reactive scalar) into a flamelet part and the perturbations from it. The molecular transport of the former is in close form while the latter in unclosed. The diffusion and dissipation of the temperature perturbations are analyzed using high-resolution line images obtained in turbulent partially premixed (Sandia) flames. The results show that for flame regions that are nearly fully burning, the SGS mixing of the temperature perturbations is similar to that of a non-reactive scalar. \\[4pt] [1] R.W. Bilger. Combust. Sci. Tech., 22 (1980) 251-261.\\[0pt] [2] S.B. Pope. Prog. Energy Combust. Sci., 11 (1985) 119-192. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D26.00002: Analysis of Subgrid-Scale Backscatter in Turbulent Reacting Flows Jeffrey O'Brien, Javier Urzay, Matthias Ihme, Parviz Moin, Amirreza Saghafian In Large-Eddy Simulations of turbulent flows, subgrid-scale (SGS) backscatter of kinetic energy can cause numerical instabilities and the physical mechanism of backscatter is not well understood. While some effort has been made to analyze the phenomenon in inert flows, the behavior of backscatter in reacting flows has been largely unexamined. In this study, Direct Numerical Simulations of inert and reacting supersonic, temporal, hydrogen-air mixing layers are analyzed to assess the effects of compressibility and combustion on SGS backscatter. As in inert, incompressible flows, it is found that a large fraction of the flow domain experiences backscatter at any given time. However, unlike in earlier incompressible studies, the intensity of the backscatter is considerably weaker than that of the forwardscatter such that net backscatter is not observed when averaging in a homogenous direction. In addition, a relationship between the SGS dissipation and eddy viscosity is derived to quantify effects of compressibility. Six allowed combinations of these effects are identified, and their relative frequency is evaluated for both reacting and non-reacting flows. It is found that backscatter occurs preferentially in regions with positive eddy viscosity and local expansion. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D26.00003: A Moments-Based Method for Turbulent Combustion Based on Principal Components: A priori and a posteriori validation Hessam Mirgolbabaei, Tarek Echekki Moment-based methods have been widely used in turbulent combustion modeling. These methods are based on the reconstruction of thermo-chemical scalars' statistics from a set of transported moments. This study is concerned with the development of a systematic strategy to construct representative moments using principal component analysis (PCA) and non-linear variants of this method, mainly Kernel PCA and PCA based on a bottleneck artificial neural network. In order to transport the principal components, transport terms must be evaluated and tabulated for a reference problem that spans the desired composition space of the problem of interest. A formulation for these transport terms is developed and validated. \textit{A priori} and \textit{a posteriori} results are presented to validate the proposed moment-based approach and illustrate the computational saving resulting from its implementation. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D26.00004: Modeling of Unsteady Heat Transfer in Flame-Wall Interaction Hao Wu, Matthias Ihme An extension of the flamelet/progress variable model is developed to include wall-heat loss effects due to convective heat-transfer. The model introduces a source term in the unsteady flamelet equations, which is modeled based on a modified temperature boundary condition of the counter-flow diffusion flame configuration. The thermochemical composition of the resulting non-adiabatic flamelet structure forms a three-dimensional manifold, which is parameterized in terms of mixture fraction, temperature, and scalar dissipation rate. The performance of the model is evaluated in an a priori study of a H$_{2}$/O$_{2}$ diffusion flame that is stabilized at an inert isothermal wall. Comparisons with DNS-data show that the developed non-adiabatic flamelet model accurately represents conditional and unconditional results for temperature, chemical composition, and wall heat transfer. Following this a priori investigation, the model is applied in LES of a coaxial H$_{2}$/O$_{2}$ rocket injector, and simulation results from this a posteriori analysis will be compared with experimental data. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D26.00005: Unsteady effects on Polycyclic Aromatic Hydrocarbons in a turbulent jet flame Yuan Xuan, Guillaume Blanquart Large Eddy Simulations (LES) have been performed on an ethylene/air piloted turbulent sooting jet flame. The current work focuses on capturing the interaction between turbulent transport and the formation of soot precursors. Particular attention is paid to the formation and transport of Polycyclic Aromatic Hydrocarbons (PAH), for their importance in the nucleation process of soot. Given the large time scale related to PAH formation, these species exhibit substantial unsteady effects when subjected to turbulent perturbations. Therefore, transport equations need to be solved for these species along with the Navier-Stokes equations. The chemical source terms are closed using a recently developed linear relaxation model. All other species are assumed to be in steady state and can be evaluated using chemistry tabulation. The mean and variance of velocity components, temperature, and species mass fractions are compared to experimental measurements. Joint probability density functions of soot volume fraction and temperature are particularly analysed. Additional LES results using full chemistry tabulation for all species including PAH are compared to the previous LES results using the PAH relaxation model, to investigate the importance and influence of turbulence-chemistry interaction. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D26.00006: Large Eddy Simulation of ignition in an annular multi-injector combustor Ronan Vicquelin, Maxime Philip, Matthieu Boileau, Thomas Schmitt, Jean-Fran\c{c}ois Bourgoin, Daniel Durox, S\'ebastien Candel The present work deals with validating the LES methodology for transient ignition simulations, and in particular elucidating the mechanisms that control the light round sequence in a laboratory annular combustor, representative of many practical industrial systems. The simulation benefits from the unique experimental database built at EM2C on a fully transparent annular chamber equipped with 16 premixed swirled injectors. The F-TACLES combustion model is used for its ability to properly represent the flame propagation. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D26.00007: Large-eddy simulations of real-fluid effects in rocket engine combustors Peter C. Ma, Jean-Pierre Hickey, Matthias Ihme This study is concerned with the LES-modeling of real-fluid effects in rocket combustors. The non-ideal fluid behavior is modeled using the Peng-Robinson equation of state, and high-pressure effects on the thermo-viscous transport properties are also considered. An efficient and robust algorithm is developed to evaluate the thermodynamic state-vector. The highly non-linear coupling of the primitive thermodynamic variables in regions near the critical point requires special consideration to avoid spurious numerical oscillations. To avoid these non-physical oscillations, a second-order essentially non-oscillatory (ENO) scheme is applied in regions that are identified by a density-based sensor. The resulting algorithm is applied in LES to a coaxial rocket-injector, and super- and transcritical operating conditions are considered. Simulation results and comparisons with experimental data will be presented, and the influence of boundary conditions on the mixing characteristics will be discussed. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D26.00008: Direct Numerical Simulation Study of Nonequilibrium Effects on Mixing and Combustion in Supersonic Jets Heeseok Koo, Venkat Raman, Philip L. Varghese Thermochemical nonequilibrium could be significant in scramjet engines due to intense shock-based compression in the pre-combustion isolator region. In particular, vibrational nonequilibrium could adversely affect ignition time and mixing efficiency. To understand the role of nonequilibrium in such flows, direct numerical simulation (DNS) of supersonic flows with vibrational excitation are studied. A linear time-scale model is used to describe the vibrational relaxation of excited species. Essentially, nonequilibrium alters the flow by changing the physical properties that are related to the translational temperature. Such changes introduce nonlinear effect on the scalar mixing process. Further, the redistribution of energy amongst the internal states affects chemical rates. An analysis of the impact of nonequilibrium on combustion is provided. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D26.00009: Quasi-dual-mode behavior in the combustor of the HyShot scramjet Johan Larsson, Ronan Vicquelin, Julien Bodart, Ivan Bermejo-Moreno, Stuart Laurence The flow in the HyShot scramjet combustor is studied using wall-modeled LES with a flamelet combustion model. The focus is on the qualitative changes in the flow structure as the fuel/air equivalence ratio (ER) is increased. For low ERs, an essentially linear pressure-rise is found, consistent with fully supersonic combustion. For higher ERs, a qualitatively different yet stable flow develops, with a stronger shock-train towards the end of the combustor. This shock-train is analogous to what occurs in the isolator in dual-mode operation, but is lodged within the region of heat release, with a position that depends on the ER; this is consistent with recent experimental data obtained by Laurence et al at the German Aerospace Center (DLR). [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D26.00010: A Priori Analysis of Flamelet-based Modeling for a Dual-Mode Scramjet Combustor Jesse Quinlan, James McDaniel, Tomasz Drozda, Guilhem Lacaze, Joseph Oefelein A priori analysis of a dual-mode scramjet combustor is performed using a Reynolds-Averaged Navier-Stokes simulation dataset utilizing finite-rate kinetics to investigate the applicability of flamelet-based combustion models. The HIFiRE Direct Connect Rig flow path is simulated in a dual-mode configuration for the combustion of a JP-7 fuel surrogate using an 18-step chemical mechanism. Simulation results are validated using experimental time-averaged wall pressure measurements. Analysis of the flame structure and combustion mode suggests that combustion regions are predominately of a non-premixed nature and of a high Damkohler number, thereby suggesting the applicability of non-premixed flamelet-based modeling techniques. Regions of premixed combustion were detected but with significantly lower heat-release contribution when compared to the non-premixed combustion regions. Representative flamelet boundary conditions are estimated using an analysis of probability density functions for temperature and pressure. The effects of compressibility on the flame structure and corresponding flamelets are investigated. Insights and future work regarding development of non-premixed flamelet-based models for high-speed compressible flows are discussed. [Preview Abstract] |
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