Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session F2: Reacting Flows: LESCFD Reacting

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Chair: Yuan Xuan, Pennsylvania State University Room: 402 
Monday, November 20, 2017 8:00AM  8:13AM 
F2.00001: Relationships between Physical and Fourier Space for LargeEddy Simulation of Premixed Turbulent Combustion: MultiDimensional Fourier Decomposition. Paulo Paes, James Brasseur, Yuan Xuan Large Eddy Simulation (LES) is a powerful formulation to model turbulent reacting flows with tradeoffs between complexity and resolution. LES assumes that all energydominated turbulence motions are wellresolved and forward cascadedominant so that modeled effects of SubFilterScale (SFS) motion are second order. However, the application of this scalebased decomposition to reacting turbulent flows is not straightforward since dynamically important kinetics within thin flame regions are mostly SFS. We aim to systematically refine understanding of the relationships between physical and scale space for LES of premixed turbulent combustion, beginning with reducedphysics simulations of the interactions between singlescale vortex arrays and laminar premixed flames. We apply Fourier scalebased decomposition where interpretation in inhomogeneous directions is unclear and where periodic extension of the finite domain in those directions produce boundary discontinuities. We present a strategy to remove the pollution to the signal from these discontinuities with minimal modification of the original signal by systemically isolating their spectral content. The procedure is applied to a 2D vortexflame interaction where we demonstrate 2D Fourierphysical space relationships in homogeneous and inhomogeneous directions. \textit{Supported by AFSOR.} [Preview Abstract] 
Monday, November 20, 2017 8:13AM  8:26AM 
F2.00002: Relationships between Physical and Fourier Space for LargeEddy Simulation of Premixed Turbulence Combustion: Transition from Weak to Strong Vortices. Yuan Xuan, Paulo Paes, James Brasseur Large Eddy Simulation (LES) is a powerful formulation for turbulent reacting flows with tradeoff between complexity and resolution. LES assumes that all energydominant motions are wellresolved and forward cascadedominant. But the application of this scalebased decomposition to reacting turbulent flows is not straightforward since dynamically important thin flame regions are mostly SFS. We aim to systematically refine understanding of the relationships between physical and scale space for LES of premixed turbulent combustion, beginning with reducedphysics simulations of the interaction between laminar premixed flame and singlescale vortex arrays with ``eddy'' strength systematically increased to create flame response from ``wrinkled'' to ``pinched''. We apply the Fourier description to these simulations using a newly developed procedure to remove the polluting content from discontinuities in inhomogeneous directions. Using waveletlike filters we identify four characteristic structural features concurrently in physical/Fourier space, with relative significance estimated from the variance contained in the spectrally filtered fields. Different variables display distinct dominant structural features that undergo systematic transition from weak to strong eddyflame interaction. [Preview Abstract] 
Monday, November 20, 2017 8:26AM  8:39AM 
F2.00003: Large eddy simulations of a reactive jet in hot vitiated crossflow: flame stabilisation mechanism Oliver Schulz, Emile Piccoli, Anne Felden, Gabriel Staffelbach, Nicolas Noiray This numerical study investigates the flame stabilization mechanism of a reactive jet in crossflow (RJICF). A premixed ethyleneair jet is injected into a hot vitiated crossflow at 1500K. Compressible 3d large eddy simulations (LES) were performed with the dynamic thickened flame (DTF) model and an analytically reduced chemistry (ARC) mechanism. Comparisons between LES and experiments are in good agreement for jet trajectories and velocities along the trajectories. LES capture the most common flameflow field interactions observed in the experiments. A detailed flame analysis utilizing chemical explosive mode analysis (CEMA) identifies autoignition as the dominant flame stabilisation mechanism on the windward side of the jet. Heat is released at an optimum mixture for fast ignition. This heat is transferred to richer mixtures, which are characterized by even smaller minimum autoignition times. This socalled most reactive mixture fraction determines the position of the windward flame. A 3d flame analysis shows autoignition regions that form close to structures resulting from the jet shear layer vortex shedding. These regions expand towards the flame tip and the side of the RJICF. Finally, they merge with the leeward propagating flame. [Preview Abstract] 
Monday, November 20, 2017 8:39AM  8:52AM 
F2.00004: Numerical simulation of turbulent burning velocity of CH$_{\mathrm{4}}$/H$_{\mathrm{2}}$/air flame using LES/FGM approach Masaya Muto, Hiroaki Nagai, Ryoichi Kurose, Fumiteru Akamatsu, Kei Inoue, Kenji Miyamoto The turbulent burning velocity, $s_{T}$, of hydrogen/methane/air mixture is numerically investigated by largeeddy simulation (LES) with flamelet generated manifold (FGM) method of turbulent jet flow. Volume ratio of the hydrogen/methane in the mixture, $\alpha$ is varied from 0 to 0.6 for the ambient pressure range of $P=$0.10.9 MPa. Equivalence ratio of the mixture is fixed to be unity. The results show that the ratio of $s_{T}$ to laminar burning velocity $s_{L}$, $s_{T}$/$s_{L}$ increases with increasing $\alpha$ and $P$ This is considered to be due to the facts that $s_{L}$ decreases with increasing $P$, and that the flame thickness decreases with increasing $\alpha$ and $P$, which causes to increase the surface area of flame sheet. [Preview Abstract] 
Monday, November 20, 2017 8:52AM  9:05AM 
F2.00005: ABSTRACT WITHDRAWN 
Monday, November 20, 2017 9:05AM  9:18AM 
F2.00006: Role of Unsteady Effects in Radiation Heat Losses in Turbulent Nonpremixed Flames A. Cody Nunno, Jeffry K. Lew, Michael E. Mueller Accounting for heat losses due to radiation in turbulent nonpremixed flames is critical for predicting pollutants such as nitrogen oxides and soot. In reducedorder manifold approaches (“flamelet” models), the effects of heat losses require the computation of thermochemical states with reduced enthalpy. In this work, the role of unsteady effects in radiation heat losses is assessed by examining two methods for computing thermochemical states at reduced enthalpy. In the first method, unsteady “flamelet” equations including a radiation heat loss source term are solved at constant scalar dissipation rate, initialized with adiabatic solutions of the steady “flamelet” equations. In the second method, only steady “flamelet” equations with a radiation heat loss source term are solved but with a variable coefficient on the source term range from zero to unity. Both approaches are applied to the Sandia D flame. A priori analysis of the two methods indicates that the two methods are equivalent at larger scalar dissipation rates and/or smaller heat losses. A posteriori comparisons of Large Eddy Simulation (LES) with experimental measurements will be used to determine the relative effects of the two approaches on predictions of temperature, major species, and pollutants. [Preview Abstract] 
Monday, November 20, 2017 9:18AM  9:31AM 
F2.00007: Modeling Soot Formation in HighPressure Turbulent Spray Flames Daniel Haworth, Sebastian FerreyroFernandez Most soot models are based on physical understanding derived from experiments at atmospheric or moderately elevated pressures, compared to the pressures that are of interest in engines and other applications. The emphasis in model development has been on kinetic processes rather than on turbulent hydrodynamics and mixing, but there is evidence that transport and mixing become relatively more ratecontrolling with increasing pressure. Here simulations are performed of transient highpressure turbulent spray flames under enginerelevant conditions. An unsteady RANS formulation is adopted, with various gasphase chemical mechanisms and soot models, and a transported composition probability density function method to account for unresolved turbulent fluctuations in composition and temperature. Computed total soot mass and soot spatial distributions are highly sensitive to the modeling of unresolved turbulent fluctuations. To achieve agreement between model and experiment and to capture the highly intermittent nature of soot in the turbulent flame, it is necessary to accurately represent mixing and the low diffusivity of soot particles. The results suggest that mixing is at least as important as kinetics in controlling soot formation and evolution in highpressure turbulent flames. [Preview Abstract] 
Monday, November 20, 2017 9:31AM  9:44AM 
F2.00008: Evaluation of a strainsensitive transport model in LES of turbulent nonpremixed sooting flames Jeffry K. Lew, Suo Yang, Michael E. Mueller Direct Numerical Simulations (DNS) of turbulent nonpremixed jet flames have revealed that Polycyclic Aromatic Hydrocarbons (PAH) are confined to spatially intermittent regions of low scalar dissipation rate due to their slow formation chemistry. The length scales of these regions are on the order of the Kolmogorov scale or smaller, where molecular diffusion effects dominate over turbulent transport effects irrespective of the largescale turbulent Reynolds number. A strainsensitive transport model has been developed to identify such species whose slow chemistry, relative to local mixing rates, confines them to these small length scales. In a conventional nonpremixed ``flamelet'' approach, these species are then modeled with their molecular Lewis numbers, while remaining species are modeled with an effective unity Lewis number. \textit{A priori} analysis indicates that this strainsensitive transport model significantly affects PAH yield in nonpremixed flames with essentially no impact on temperature and major species. The model is applied with Large Eddy Simulation (LES) to a series of turbulent nonpremixed sooting jet flames and validated via comparisons with experimental measurements of soot volume fraction. [Preview Abstract] 
Monday, November 20, 2017 9:44AM  9:57AM 
F2.00009: Method of moments comparison for soot population modeling in turbulent combustion Shao Teng Chong, Hong Im, Venkat Raman Representation of soot population is an important component in the efficient computational prediction of particulate emissions. However, there are a number of momentsbased techniques with varying numerical complexity. In the past, development of such methods has been principally carried out on canonical laminar and 0D flows. However, their applications in realistic solvers developed for turbulent combustion may face challenges from turbulence closure to selection of moment sets. In this work, the accuracy and relative computational expense of a few common soot method of moments are tested in canonical turbulent flames for different configurations. Large eddy simulation (LES) will be used as the turbulence modeling framework. In gridfiltered LES, the interaction of numerical and modeling errors is a firstorder problem that can undermine the accuracy of soot predictions. In the past, special momentsbased methods for solvers that transport high frequency content fluid with ability to reconstruct particle size distribution have been developed. Here, a similar analysis will be carried out for the momentbased soot modeling approaches above. Specifically, realizability of moments methods with nonlinear advection schemes will be discussed. [Preview Abstract] 
Monday, November 20, 2017 9:57AM  10:10AM 
F2.00010: A revised NOMANI model for NO prediction using improved mechanism indicator Daehyun Han, Seongwon Kang The main objective of this study is to predict NO emission efficiently and accurately using a revised NOMANI model in laminar and turbulent flames. Although the prompt and thermal NO mechanisms have very different reaction time scales, many practical combustion models assume fast chemistry and have a limitation in predicting formation of thermal NO. The NOMANI model by Pecquery et al. (2014) is based on the flameletprogress variable approach and suggests a new tabulation with NO composition as an axis instead of the progress variable. In the present study, various mechanism indicators other than the original progress variable are analyzed to represent local mechanism of NO production more accurately. In order to develop improved weight parameters for nonpremixed flames and postflame regions, we consider the species involved in different NO formation mechanisms. We analyze the proposed approach using DNS of laminar premixed and nonpremixed flames. As validation cases, LES studies are performed for turbulent flames, which leads to improved results. [Preview Abstract] 
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