Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session H26: Reactive Flows V: Numerical Approaches to Turbulent Combustion |
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Chair: Venkat Raman, University of Texas at Austin Room: 31B |
Monday, November 19, 2012 10:30AM - 10:43AM |
H26.00001: A novel methodology for simulating low Mach number combustion Ammar Abdilghanie, James Riley, Oscar Flores, Robert Moser The velocity-vorticity formulation for the solution of unsteady three-dimensional incompressible flows (Kim, Moin and Moser JFM 1987) is extended for compressible flows under the low Mach number approximation for combustion applications. A key advantage of the methodology is the elimination of the pressure from the momentum equations and, as a result, the errors and complications of pressure boundary conditions associated with pressure-splitting algorithms. The added efficiency of the method for horizontally homogeneous flows makes it computationally very attractive. The hydrodynamic part of the algorithm comprises two evolution equations for two dependent variables and two Poisson-type equations that, by construction, ensure mass conservation. Sixth-order accurate compact scheme is used for spatial discretization in the vertical and third order implicit-explicit Runge-Kutta is used for time advancement. Open boundary conditions are used at the top boundary and free-slip no-flux conditions are employed at the bottom. Simulations of compressible Taylor Green vortex flow are briefly discussed and future research directions are summarized. [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H26.00002: Vorticity dynamics in variable density flows Peter Hamlington, Alexei Poludnenko, Elaine Oran The dynamics of vorticity in incompressible flows has been the subject of considerable research, but remains relatively poorly understood in variable density flows. Such flows include reacting and supersonic flows where the behavior of the vorticity is central to understanding the interactions between turbulence, shock waves, and flames. The variations in density across shocks and flames are often anisotropic, and here we discuss the differing effects of isotropic and anisotropic density changes on the vorticity. We focus in particular on flames and shocks, which represent different ends of the variable density spectrum; flames can create a rapid anisotropic expansion of the fluid while shocks produce a rapid anisotropic compression. These density changes result in anisotropic vorticity suppression across flames and anisotropic vorticity generation across shocks. In reacting flows, we discuss the effects of anisotropic suppression on intermittency, turbulence-flame interactions, and flame properties. We also propose a decomposition of the strain rate that allows the relative effects of turbulent and flame straining to be understood. We then compare vorticity-shock interactions with the reacting flow case and outline directions for future research. [Preview Abstract] |
Monday, November 19, 2012 10:56AM - 11:09AM |
H26.00003: Discontinuous Galerkin Method for Combustion Yu Lv, Matthias Ihme Over recent years, the discontinuous Galerkin (DG) method has found increased interest in application to advection-dominated hyperbolic flows. However, extending DG to combustion introduces several challenges that are associated with the treatment of diffusion-dominated multi-species transport, non-uniform thermal properties, and the consideration of complex and stiff reaction chemistry. By considering all of these aspects, a DG-formulation for multi-species combustion has been developed. After presenting the numerical method and demonstrating higher-order convergence properties, this formulation is applied to relevant combustion problems, involving multi-species mixing, deflagration, and detonation-systems. Results are compared against solutions from conventional finite-volume/finite-difference-schemes, and potential benefits of the DG-method for combustion are discussed. [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H26.00004: Empirical low-dimensional manifolds in composition space Yue Yang, Stephen B. Pope, Jacqueline H. Chen To reduce the computational cost of turbulent combustion simulations with a detailed chemical mechanism, it is useful to find a low-dimensional manifold in composition space that can approximate the full system dynamics. Most previous low-dimensional manifolds in turbulent combustion are based on the governing conservation equations or thermochemistry and their application involves certain assumptions. On the other hand, empirical low-dimensional manifolds (ELDMs) are constructed based on samples of the compositions observed in experiments or in direct numerical simulation (DNS). Plane and curved ELDMs can be obtained using principal component analysis (PCA) and multivariate adaptive spline regression (MARS), respectively. Both PCA and MARS are applied to the DNS datasets of a non-premixed CO/H2 temporally evolving jet flame (Hawkes et al., 2007) and an ethylene lifted jet flame (Yoo et al., 2011). We observe that it requires very high dimensions to represent the species mass fractions accurately by a plane ELDM, while better accuracy can be achieved by curved ELDMs with lower dimensions. In addition, the effect of differential diffusion on ELDMs is examined in large-eddy simulations with PDF modeling. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H26.00005: An Adjoint Approach for Determining Sensitivity of Combustion Simulations to Model Parameters Kalen Braman, Venkat Raman Simulations of turbulent combustion typically involve numerous models including those for the gas phase chemistry, turbulence, and combustion. Such models typically involve a host of parameters, and simulation results are generally sensitive to these parameters. Here, an adjoint-based approach is developed to determine the sensitivity of emissions at the combustion exit to these model parameters. First, adjoint equations for the turbulent combustion system in the context of the Reynolds-averaged Navier-Stokes (RANS) approach are derived. Then, a novel numerical scheme for solving these equations is introduced. The methodology is verified by comparing against a forward sensitivity computation. Finally, the sensitivity of emissions to model parameters is determined in a canonical jet flame configuration. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H26.00006: Relating filtered and unfiltered quantities in large eddy simulation of turbulent combustion Venkatramanan Raman, Colin Heye Large eddy simulation (LES) is currently recognized as a valuable tool for modeling turbulent combustion. Although significant advances have been made in the development of reliable models in LES, there remains considerable ambiguity regarding the fundamental definition of the methodology and its use for predicting measurable flow quantities. The root cause of these problems is the filtering operation used to obtain the LES governing equations. Here, we discuss the relation between filtered and unfiltered quantities through a probabilistic framework. Basic transformation rules to relate combustion-related quantities to mixture fraction or other such scalars are presented. Using a direct numerical simulation (DNS) of homogeneous isotropic turbulence, the importance of a missing modeling step is discussed. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H26.00007: PDF investigations of turbulent non-premixed jet flames with thin reaction zones Haifeng Wang, Stephen Pope PDF (probability density function) modeling studies are carried out for the Sydney piloted jet flames. These Sydney flames feature much thinner reaction zones in the mixture fraction space compared to those in the well-studied Sandia piloted jet flames. The performance of the different turbulent combustion models in the Sydney flames with thin reaction zones has not been examined extensively before, and this work aims at evaluating the capability of the PDF method to represent the thin turbulent flame structures in the Sydney piloted flames. Parametric and sensitivity PDF studies are performed with respect to the different models and model parameters. A global error parameter is defined to quantify the departure of the simulation results from the experimental data, and is used to assess the performance of the different set of models and model parameters. [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H26.00008: Modeling local extinction in turbulent combustion using an embedding method Robert Knaus, Carlos Pantano Local regions of extinction in diffusion flames, called ``flame holes," can reduce the efficiency of combustion and increase the production of certain pollutants. At sufficiently high speeds, a flame may also be lifted from the rim of the burner to a downstream location that may be stable. These two phenomena share a common underlying mechanism of propagation related to edge-flame dynamics where chemistry and fluid mechanics are equally important. We present a formulation that describes the formation, propagation, and growth of flames holes on the stoichiometric surface using edge flame dynamics. The boundary separating the flame from the quenched region is modeled using a progress variable defined on the moving stoichiometric surface that is embedded in the three-dimensional space using an extension algorithm. This Cartesian problem is solved using a high-order finite-volume WENO method extended to this nonconservative problem. This algorithm can track the dynamics of flame holes in a turbulent reacting-shear layer and model flame liftoff without requiring full chemistry calculations. [Preview Abstract] |
Monday, November 19, 2012 12:14PM - 12:27PM |
H26.00009: Numerical simulation of turbulent stratified flame propagation in a closed vessel Catherine Gruselle, Ghislain Lartigue, Perrine Pepiot, Vincent Moureau, Yves D'Angelo Reducing pollutants emissions while keeping a high combustion efficiency and a low fuel consumption is an important challenge for both gas turbine (GT) and internal combustion engines (ICE). To fulfill these new constraints, stratified combustion may constitute an efficient strategy. A tabulated chemistry approach based on FPI combined to a low-Mach number method is applied in the analysis of a turbulent propane-air flame with equivalence ratio (ER) stratification, which has been studied experimentally by Balusamy [S. Balusamy, Ph.D Thesis, INSA-Rouen (2010)]. Flame topology, along with flame velocity statistics, are well reproduced in the simulation, even if time-history effects are not accounted for in the tabulated approach. However, these effects may become significant when exhaust gas recirculation (EGR) is introduced. To better quantify them, both ER and EGR-stratified two-dimensional flames are simulated using finite-rate chemistry and a semi-detailed mechanism for propane oxidation. The numerical implementation is first investigated in terms of efficiency and accuracy, with a focus on splitting errors. The resulting flames are then analyzed to investigate potential extensions of the FPI technique to EGR stratification. [Preview Abstract] |
Monday, November 19, 2012 12:27PM - 12:40PM |
H26.00010: Evaluation of a Consistent LES/PDF Method Using a Series of Experimental Spray Flames Colin Heye, Venkat Raman A consistent method for the evolution of the joint-scalar probability density function (PDF) transport equation is proposed for application to large eddy simulation (LES) of turbulent reacting flows containing evaporating spray droplets. PDF transport equations provide the benefit of including the chemical source term in closed form, however, additional terms describing LES subfilter mixing must be modeled. The recent availability of detailed experimental measurements provide model validation data for a wide range of evaporation rates and combustion regimes, as is well-known to occur in spray flames. In this work, the experimental data will used to investigate the impact of droplet mass loading and evaporation rates on the subfilter scalar PDF shape in comparison with conventional flamelet models. In addition, existing model term closures in the PDF transport equations are evaluated with a focus on their validity in the presence of regime changes. [Preview Abstract] |
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