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 soot-turbulence-chemistry interactions. In particular, soot nucleation and growth happen in fuel-rich regions, and soot is then rapidly oxidized before being transported by turbulence into fuel-lean regions (in non-smoking 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 non-uniform distribution of soot in mixture fraction space. In this model, the sooting mode of the presumed subfilter PDF is locally activated only at fuel-rich 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 Reduced-Order Manifold Approaches for Simulating a Turbulent Lifted Jet Flame Bruce A Perry, Ruihong Chen, Michael E. Mueller Reduced-order manifold approaches use assumptions about combustion mode to constrain the thermochemical state space to a low-dimensional manifold generated from one-dimensional 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, reduced-order 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 mode-dependent 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 multi-modal turbulent flame structure. The analysis indicates that a more general modeling framework that retains the benefits of low-dimensionality 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 Badillo-Rios, 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 turbulence-chemistry 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. GRI-Mech 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: Low-Mach-Number Simulations of Diffusion Flames with the Chemical-Diffusive Model Joseph Chung, Xiao Zhang, Carolyn Kaplan, Elaine S Oran We describe the calibration and implementation of the chemical-diffusive 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 equivalence-ratio (ER) mixtures or mixtures with variable ER, but generally for premixed combustion. Now we have taken the variable-ER form of the CDM, incorporated it into a low-Mach-number solution of the Navier-Stokes equations (based on the BIC-FCT algorithm), and calibrated it for simulations of a diffusion flame. Computations of test problems, such as laminar and co-flow diffusion flames are demonstrated, culminating in a three-dimensional simulation of a fire whirl. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F02.00005: Mixing of confined reacting co-axial 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, large-eddy simulation (LES) is the practical approach whereas DNS is inaccessible. However, subgrid-scale (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 co-axial 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 |
Monday, November 19, 2018 9:18AM - 9:31AM |
F02.00007: Abstract Withdrawn Regime independent multi-scale models such as the linear-eddy mixing (LEM) model has been developed in the past as a subgrid model large-eddy simulations (LES). In this two-scale approach finite-rate kinetics and reaction-diffusion interactions are included without any filtering within the LES cells thereby enabling a prediction of the filtered reaction rate rather than modeling it on the LES level. In this study, the LEMLES methodology is extended to use well-known flamelet/progress variable (FPV) approach within a compressible (high pressure) LEM model. Simulations of premixed flame-turbulence interaction in a channel for various regimes (corrugated flamelet to broken reaction zones) for a range of pressure is carried out and results compared to the conventional LEMLES method and prior DNS data. The subgrid FPV model is also analyzed to assess the assumptions and closures used in conventional LES using the filtered FPV approach. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F02.00008: Examination of mixing and differential molecular diffusion in DNS of a high-Karlovitz number turbulent premixed jet flame Pei Zhang, Hemanth Kolla, Jacqueline H Chen, Haiou Wang, Evatt R. Hawkes, Haifeng Wang DNS of a high-Karlovitz number turbulent premixed jet flame has been reported recently (Wang et al., Proc. Combust. Inst., 2017, 36, 2045-2053). 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 sub-filter scale mixing and molecular diffusion in the context of large-eddy simulations (LES) and probability density function (PDF) method. First, a sub-filter scale mixing time scale is analyzed with respect to the filter size to examine the validity of a power-law scaling model for the mixing time scale in LES/PDF. The results show remarkable agreement with a simple power-law 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 non-reacting 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 area-weighted tangential strain rate in flames and propagating surfaces are quantitatively similar with positive means to increase the flame area, and the PDFs of the area-weighted 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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