Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session H6: CFD: Reactive Flows II |
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Chair: Seung Hyun Kim, Ohio State University Room: 105 |
Monday, November 23, 2015 10:35AM - 10:48AM |
H6.00001: A New Method for Large Eddy Simulation of Turbulent Premixed Combustion Seung Hyun Kim A new method for large eddy simulation (LES) of turbulent premixed combustion is presented. The method is based on the front propagation formulation (FPF) of filtered reaction rates. In premixed combustion LES where a filter scale is typically taken as grid spacing, the spurious propagation of filtered flame fronts can occur due to under-resolved reaction zones. The FPF method avoids this spurious propagation by discretely preserving the total reaction rates on computational grids. The method not only recovers the flamelet limit when turbulence is not strong enough to perturb inner structures of the flame fronts, but also allows for the broadening of filtered flame fronts by turbulence. The FPF method is applied to LES of laboratory flames. The analysis and validation of the method will be presented. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H6.00002: Analysis of entropy generation in turbulent reacting flows using large eddy simulation Mehdi Safari The entropy transport equation is considered in large eddy simulation (LES) of turbulent reacting flows. This equation includes irreversible losses by entropy production due to viscous dissipation, heat conduction, mass diffusion and chemical reaction, all of which appear as unclosed terms. The closure is provided by entropy filtered density function (En-FDF) which includes the effect of chemical reaction in a closed form. An exact transport equation is developed for the En-FDF. The transport equation for En-FDF is modeled by a set of stochastic differential equations. The modeled En-FDF transport equation is solved by a Lagrangian Monte Carlo method. The methodology is applied to turbulent nonpremixed jet flames to analyze local entropy generation effects. Various modes of entropy generation are obtained and analyzed. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H6.00003: Subfilter scalar variance models for LES of premixed turbulent flames Guillaume Blanquart, Simon Lapointe, Tomas Tussie The subfilter scalar variance plays an important role in large eddy simulations of turbulent reacting flows. It is not available in the simulations and needs to be modeled. Subfilter scalar variance models often take the form of a constant coefficient multiplying the square of the filter width and the square of the gradient of the filtered progress variable (referred to as the mixing model). Variance models are first studied \textit{a priori} using results from constant density DNS of scalar mixing in isotropic turbulence. Scalar variance models based on a generalized Taylor expansion are accurate for small filter widths but errors arise in the inertial subrange. Results suggest that a constant coefficient computed from an assumed Kolmogorov spectrum is often sufficient to predict the subfilter scalar variance in homogeneous isotropic turbulence. The analysis is then extended to variable density reacting flows using DNS of turbulent $n$-heptane/air premixed flames at varying Karlovitz numbers. Results from homogeneous isotropic turbulence still hold when taking into account the change in the Kolmogorov length scale across the flame. The optimal coefficient in the mixing model varies between the two limits of small filter widths and assumed Kolmogorov spectrum. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H6.00004: Computational Study of the Effect of Compositionally Inhomogeneous Fuel Streams on Turbulent Jet Flames Michael E. Mueller, Bruce A. Perry, Assaad R. Masri A new piloted turbulent jet burner has been developed at The University of Sydney to investigate how inhomogeneous partially premixed inlet conditions affect flame structure and stability characteristics [S. Meares, A.R. Masri, Combust. Flame 161 (2014) 484-495]. Compositional inhomogeneity at the inlet is achieved by recessing a central tube that separates the fuel stream and a surrounding annular air flow to allow for a controlled amount of mixing before the gases reach the nozzle exit. In this work, Large Eddy Simulation of the burner is performed using a conventional nonpremixed flamelet/progress variable model. The geometry is divided into three separately computed domains: fully developed pipe/annulus flow, pipe flow in the region of fuel/air mixing upstream of the nozzle, and the turbulent flame. The results for two recess distances of the central tube (inhomogeneous fuel inlet and effectively homogeneous fuel inlet) are compared to recent experimental measurements. Discrepancies between the simulation and experiment show that premixed combustion is dominant only for the inhomogeneous case at the base of the flame. Sensitivitiese to grid resolution in both the upstream mixing domain and the turbulent flame domain as well as pilot conditions are assessed. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H6.00005: Flamelet modeling of differential molecular diffusion in CO/H2 and ethylene DNS flames Chao Han, David Lignell, Evatt Hawkes, Jacqueline Chen, Haifeng Wang A class of consistent differential diffusion models suitable for flamelet modeling of turbulent non-premixed combustion is developed recently. In this work, these differential diffusion models are further validated in two DNS temporally evolving jet flames with two different fuels, Syngas and ethylene. The dependence of differential diffusion on the Reynolds number, which is missing is previous models, is incorporated into the new models, and is based on a limiting analysis of the behaviors of differential molecular diffusion at the limits of small and large Reynolds numbers. The performances of the models are thoroughly examined in the two DNS flames. The effect is studied of the Reynolds number, Damkohler number, and Lewis number on the differential molecular diffusion in the temporally evolving jet flames. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H6.00006: A Two Mixture Fraction Flamelet Model for Large Eddy Simulation of Turbulent Jet Flames with Inhomogeneous Inlets Bruce A. Perry, Michael E. Mueller, Assaad R. Masri A revised flamelet/progress variable (FPV) model in which two mixture fractions are defined has been developed to address the known limitation that single mixture fraction FPV models require there to be a single, compositionally uniform fuel stream. The revised model is applied in a large eddy simulation of a new turbulent jet burner with inhomogeneous partially premixed inlet conditions that was developed at the University of Sydney [S. Meares, A.R. Masri, Combust. Flame 161 (2014) 484-495]. Compositional inhomogeneity at the inlet is achieved by recessing a central tube that separates the fuel stream and a surrounding annular air flow to provide controlled mixing upstream of the nozzle. The first mixture fraction characterizes the mixing between the jet and surrounding air. The second mixture fraction tracks mixing of fuel and air upstream of the nozzle and defines the fuel side boundary condition for solution of the 1D flamelet equation in terms of the first mixture fraction. The predictions using both the single mixture fraction and the two mixture fraction FPV models are compared to recent experimental results. It is shown that use of a single mixture fraction is insufficient to accurately capture the structure of the flame with inhomogeneous inlet conditions. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H6.00007: Numerical Study of Flame Stabilization Mechanism in a Premixed Burner with LES Non-adiabatic Flamelet Approach Yihao Tang, Malik Hassanaly, Venkat Raman In the development of highly efficient gas turbine combustion system, using high-hydrogen-content fuels is a new solution that limits pollutant emissions but also triggers flame stabilization issues. One promising concept to handle such instabilities within a large range of operating conditions is the FLOX$^{\textregistered}$ burner. A noticeable feature of the FLOX$^{\textregistered}$ burner is that it discharges high momentum jets without swirl, and flame stabilization is achieved in the shear layer around the jets. Experimental investigations have concluded that low velocity zones were absent and the flashback propensity was effectively decreased. It is proposed to study the stabilization mechanism to understand what physical phenomena are decisive in the process. In a preliminary numerical study, an adiabatic flamelet table was used along with LES simulations. Although the flow field's main features were captured, the simulation had issues in accurately predicting some important thermochemical quantities, including near wall quenching effects and OH mass fraction distribution. This work focuses on the effect of the adiabatic hypothesis on the flame stabilization mechanism. A non-adiabatic flamelet model is implemented and the impact on the stabilization mechanism is being quantified. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H6.00008: Comparison of Flamelet Models with the Transported Mass Fraction Approach for Supersonic Combustion Wenhai Li, Ken Alabi, Foluso Ladeinde In this study, two fully compressible RANS, LES, and combined RANS/LES flow solvers -- AEROFLO and VULCAN, both of which were originally developed by the United States Department of Defense but have since been significantly enhanced and commercialized by our organization, are used to investigate the accuracy of flamelet-based approach when employed to model supersonic combustion. The flamelet results from both codes are assessed relative to solutions obtained by solving the transport equations for the mass fractions -- which is also supported by one of the codes, and making familiar assumptions about the closure of the reaction rate. The studies are carried out in the flamelet regime, and the numerical procedures are based on high-order schemes, which are also used to solve the level-set and mixture fraction transport equations used to study, respectively, premixed and non-premixed combustion. The effects of supersonic Mach numbers on the results are discussed. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H6.00009: Modeling studies of a turbulent pulsed jet flame using LES/PDF Pei Zhang, Haifeng Wang The combustion field in a pulsed turbulent piloted jet flame is studied using an advanced large eddy simulation (LES) / probability density function (PDF) method. Measurement data with a joint OH-PLIF/OH* chemiluminescence/LDV system are available including the temporal series of the axial velocity and planar OH images. A time-dependent inflow condition is specified based on the measurement data. A direct comparison of the mean and rms velocities from the calculations and from the measurement shows a satisfactory prediction of the flow fields by using the employed modeling methods. The predicted OH mass fractions are compared qualitatively with the measured OH images at selected temporal and spatial locations. The comparison shows a good agreement. Conditional quantities and flame index are extracted from the simulations to examine the bimodal and multi-regime combustion dynamics in the flame. [Preview Abstract] |
Monday, November 23, 2015 12:32PM - 12:45PM |
H6.00010: Interaction of Thermodiffusive Instabilities and Turbulence in Lean Hydrogen/Air Mixtures using Tabulated Chemistry Jason Schlup, Guillaume Blanquart The combustion of lean hydrogen mixtures is prone to thermodiffusive instabilities due to the strongly non-unity fuel Lewis number. Simulations of the combustion process can aid in designing new burners to reduce operating risks associated with thermodiffusive instabilities; however, direct numerical simulations of large scale burners with detailed chemistry mechanisms are prohibitively expensive. The significant simulation time requires that computational costs decrease by using reduced order chemistry and turbulence modeling. In this work, a chemistry table, created with one-dimensional flames, is used to reduce the simulation cost. Direct numerical simulations of turbulent combustion with lean hydrogen/air mixtures are performed. Both statistically planar and spherically expanding flames are considered, and the turbulence level varies from laminar to fully turbulent flow conditions. The chosen equivalence ratio displays thermodiffusive instabilities in the wrinkled flame front. The influence of turbulence intensity on the flame instabilities are explored, and the results are compared to previous studies to determine the adequacy of the tabulated chemistry method for this set of simulation parameters. [Preview Abstract] |
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