Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F02: Turbulent Combustion II: Combustion Modeling 
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Chair: Xinyu Zhao, University of Conneticut Room: Georgia World Congress Center B203 
Monday, November 19, 2018 8:00AM  8:13AM 
F02.00001: Evaluation of a conditional presumed subfilter PDF model in LES of turbulent nonpremixed sooting flames Suo Yang, Michael Mueller In turbulent nonpremixed flames, soot evolution is strongly affected by sootturbulencechemistry interactions. In particular, soot nucleation and growth happen in fuelrich regions, and soot is then rapidly oxidized before being transported by turbulence into fuellean regions (in nonsmoking flames). Additionally, different soot evolution processes are dominant over different mixture fraction ranges. Therefore, a presumed subfilter PDF conditioned on mixture fraction is proposed to account for this nonuniform distribution of soot in mixture fraction space. In this model, the sooting mode of the presumed subfilter PDF is locally activated only at fuelrich mixture fraction values where growth is locally faster than the oxidation. This model is implemented within a Large Eddy Simulation (LES) framework and applied to a series of turbulent nonpremixed sooting flames. Compared to the previous unconditional presumed subfilter PDF model, the soot volume fraction values are significantly increased and in significantly better agreement with the experimental measurements. 
Monday, November 19, 2018 8:13AM  8:26AM 
F02.00002: Comparison of ReducedOrder Manifold Approaches for Simulating a Turbulent Lifted Jet Flame Bruce A Perry, Ruihong Chen, Michael E. Mueller Reducedorder manifold approaches use assumptions about combustion mode to constrain the thermochemical state space to a lowdimensional manifold generated from onedimensional component problems relevant for the assumed combustion mode. These models significantly reduce the computational cost associated with the simulation of turbulent combustion systems. In this work, reducedorder manifold models assuming the nonpremixed, premixed, and autoignition modes are all applied in Large Eddy Simulations of the Cabra flame, a lifted flame formed by a rich methane/air jet with a vitiated coflow generated by upstream lean premixed combustion of a hydrogen/air mixture. Due to the elevated temperature in the coflow, autoignition contributes the stabilization of the globally nonpremixed lifted flame, which challenges modedependent modeling approaches. Comparison of the predictions using these different approaches to the experimental data indicates that no single mode is able to adequately capture the multimodal turbulent flame structure. The analysis indicates that a more general modeling framework that retains the benefits of lowdimensionality while relaxing the assumptions on combustion mode would lead to significantly improved predictions. 
Monday, November 19, 2018 8:26AM  8:39AM 
F02.00003: Effect of Alternative Kinetic Mechanisms on Turbulent Combustion in a Shear Coaxial Injector Salvador BadilloRios, Ann Karagozian The use of full detailed kinetics in turbulent combustion simulations is impractical given the associated large computational cost. Studies have shown that turbulent flames may involve key reaction pathways that are significantly different from those in laminar flames, thus affecting the ability of reduced kinetic models to reasonably capture turbulencechemistry interactions and related flow field behavior. The present study examines the effects of alternative kinetic models on turbulent combustion processes as a means of determining the conditions (if any) under which certain reaction pathways are altered and to aid in the development of more accurate reduced kinetic models. Utilizing the General Equation and Mesh Solver (GEMS) code, 2D axisymmetric parametric studies and simulations for a single element shear coaxial rocket injector are performed. GRIMech 3.0 and several reduced kinetic models are used to study the combustion of gaseous methane and oxygen, with a focus on global effects of the kinetics on flow and reaction dynamics. Results show differences in peak temperatures and flame anchoring among the models, in addition to differing grid and time resolution requirements. 
Monday, November 19, 2018 8:39AM  8:52AM 
F02.00004: LowMachNumber Simulations of Diffusion Flames with the ChemicalDiffusive Model Joseph Chung, Xiao Zhang, Carolyn Kaplan, Elaine S Oran We describe the calibration and implementation of the chemicaldiffusive model (CDM) for the simulation of diffusion flames. The CDM uses the relatively simple functional form of an Arrhenius rate along with diffusion parameters, energy, and a progress variable to control the conversion of reactants to products and the rate of chemical energy release. The constants for the model are determined by an optimization procedure. Input into this procedure is obtained from detailed chemical models or experimental data. Prior CDM applications computed properties of flames and detonations for single equivalenceratio (ER) mixtures or mixtures with variable ER, but generally for premixed combustion. Now we have taken the variableER form of the CDM, incorporated it into a lowMachnumber solution of the NavierStokes equations (based on the BICFCT algorithm), and calibrated it for simulations of a diffusion flame. Computations of test problems, such as laminar and coflow diffusion flames are demonstrated, culminating in a threedimensional simulation of a fire whirl. 
Monday, November 19, 2018 8:52AM  9:05AM 
F02.00005: Mixing of confined reacting coaxial jets with disparate viscosity Mustafa Usta, Vincent Lee, Dennis E. Oztekin, Gokul Pathikonda, Michael Cameron Reza Ahmad, Devesh Ranjan, Cyrus K Aidun, Irfan Khan Mixing of miscible liquids with disparate viscosity is important in a number of industrial processes including reacting flows. When the reaction rate is much faster than the rate of mixing (Da >>1), the reaction becomes mixing limited. The relevant scale, Batchelor, for the reaction is much smaller than the turbulent dissipative scale for liquids (Sc ≈ 1000). Therefore, largeeddy simulation (LES) is the practical approach whereas DNS is inaccessible. However, subgridscale (SGS) modeling becomes challenging since there is large viscosity difference between the liquids and the mixture involves reacting fluids. In this study, we focus on mixing of two miscible liquids of different viscosity in a coaxial jet mixer. The experimental setup includes PIV and PLIF to resolve the velocity field and the mixture fraction. The computational approach is based on LES and the results with regular Smagorinsky, dynamic and dynamic mixed SGS models will be presented with comparison to experiments for the viscosity ratio of one. The experimental setup and the computational results for viscosity ratio of up to 1000; and the challenges in the SGS modeling of the reacting flows will be presented. 
Monday, November 19, 2018 9:05AM  9:18AM 
F02.00006: Assessment of the Thickened Flame Model for Large Eddy Simulations of Turbulent Premixed Flames Near Extinction Conditions Peiyu Zhang, Bifen Wu, Xinyu Zhao Artificially thickened flame (ATF) models are widely adopted as closure models for turbulencechemistry interactions in large eddy simulations (LES). Most of the existing ATF modeling studies involve one or twostep chemical mechanisms, targeting at strongly burning turbulent premixed flames. Increasing numbers of studies begin to focus on combining ATF models with reduced or detailed chemical mechanisms recently, to account for finiterate chemistry effects such as autoignition or local extinction in turbulent flames. In this study, a hierarchy of laminar and turbulent flames are employed to investigate the effect of the thickening factors near extinction conditions. The suitability of several flame sensors for the detection of the flame fronts is also assessed, and new flame sensors based on minor species that are only available in multistep chemistry are proposed and evaluated a priori and a posteriori. Preliminary results show that a formylradical based flame sensor is able to recover the flame speed under both laminar and turbulent conditions. Sufficiently small thickening factors are required to capture the response of flames to local strain rates. 
Monday, November 19, 2018 9:18AM  9:31AM 
F02.00007: Abstract Withdrawn

Monday, November 19, 2018 9:31AM  9:44AM 
F02.00008: Examination of mixing and differential molecular diffusion in DNS of a highKarlovitz number turbulent premixed jet flame Pei Zhang, Hemanth Kolla, Jacqueline H Chen, Haiou Wang, Evatt R. Hawkes, Haifeng Wang DNS of a highKarlovitz number turbulent premixed jet flame has been reported recently (Wang et al., Proc. Combust. Inst., 2017, 36, 20452053). The DNS flame features an intense interaction between the turbulence and flame structures in the broken reaction zone regime as suggested by the DNS dimensionless parameters. In this work, we analyze the DNS results to gain insights into the effect of subfilter scale mixing and molecular diffusion in the context of largeeddy simulations (LES) and probability density function (PDF) method. First, a subfilter scale mixing time scale is analyzed with respect to the filter size to examine the validity of a powerlaw scaling model for the mixing time scale in LES/PDF. The results show remarkable agreement with a simple powerlaw scaling when the filter size is sufficiently large. Second, the conditional diffusion velocities in the composition space are explored by using the DNS data for the purpose of understanding the unclosed term in the PDF method. Third, the effect of differential molecular diffusion in the DNS flame is examined and quantified. All these results are expected to have implications for LES/PDF modeling of turbulent premixed combustion under extreme conditions. 
Monday, November 19, 2018 9:44AM  9:57AM 
F02.00009: Modeling of the turbulent burning velocity using Lagrangian statistics of propagating surfaces Jiaping You, Yue Yang We develop a model for estimating the turbulent burning velocity based on Lagrangian statistics of propagating surfaces. An ensemble of propagating surface elements with a constant displacement speed is initially arranged on a plane in nonreacting homogeneous isotropic turbulence (HIT) to model the propagation of a planar premixed flame front. The turbulent burning velocity is estimated by the area ratio of the global propagating surface at a truncation time when the statistical geometry of propagating surfaces reaches a stationary state. This model is validated by the direct numerical simulation (DNS) of hydrogen/air turbulent premixed flames propagating in stationary HIT with detailed chemistry. We demonstrate that the modelled turbulent burning velocity agrees well with DNS at small and moderate Karlovitz numbers. The probability density functions (PDFs) of the areaweighted tangential strain rate in flames and propagating surfaces are quantitatively similar with positive means to increase the flame area, and the PDFs of the areaweighted mean curvature in propagating surfaces are wider than those in flames. In addition, the computational cost of the proposed model is much lower than the corresponding combustion DNS. 
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