Sub-grid scale modeling of the equation of state for fully compressible combustion LES (original) (raw)
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Proceedings of the Combustion Institute, 2019
In large-eddy simulations (LES) of multicomponent and fully compressible flows, the spatially filtered pressure needs to be evaluated, i.e. the pressure averaged over a volume. The flow is non-homogeneous within this volume and the state relationship linking pressure, density, temperature and species mass fractions should not be applied directly to their values resolved on the LES mesh. In practice, the unresolved correlations between density, species and temperature are usually neglected to compute the filtered pressure from the resolved fields. Analyzing one-dimensional laminar and three-dimensional turbulent H 2 /O 2 space-filtered flames under lean and stoichiometric conditions, it is observed that a large part of the error introduced by the linearization of the equation of state can be counterbalanced by expressing the mean molar weight of the mixture with the Reynolds filtered species mass fractions, instead of the density-weighted (Favre) mass fractions. A sub-grid scale closure for the remaining part of the unknown correlation is also proposed, which relies on a scale similarity assumption. Finally, an approximate deconvolution/filtering procedure is discussed to estimate the Reynolds filtered mass fractions from the density-weighted mass fractions, which are the transported quantities in LES flow solvers.
Combustion and Flame, 2005
A consistent hybrid large-eddy simulation/filtered-density-function approach (LES-FDF) is formulated for variable-density low-Mach-number flows. The LES-FDF approach has been proposed as a suitable method for finite-rate-chemistry-based predictive modeling of turbulent reactive flows. Due to the large computational grid associated with LES, use of Lagrangian schemes is numerically expensive. In this work, a highly efficient parallel Lagrangian implementation is used for the simulation of a nonpremixed flame. This bluff-body-stabilized flame is characterized by complex flow fields that interact strongly with the combustion mechanism. A LES grid size of 1 million computational cells and roughly 15 million notional particles is used to simulate a time-accurate variable-density flow. The hybrid approach predicts the time-averaged velocity and root mean square (RMS) velocity components quite accurately. Species profiles including hydroxyl radical compare well with experimental data. Consistency and accuracy are established by comparing particle and Eulerian density, mixture fraction, and RMS mixture fraction fields. Scalar FDFs at select locations are shown to be well approximated by the presumed beta function used in typical combustion LES.
Proceedings of the Combustion Institute, 2007
A hybrid large-eddy simulation/filtered-density function (LES-FDF) methodology is formulated for simulating variable density turbulent reactive flows. An indirect feedback mechanism coupled with a consistency measure based on redundant density fields contained in the different solvers is used to construct a robust algorithm. Using this novel scheme, a partially premixed methane/air flame is simulated. To describe transport in composition space, a 16-species reduced chemistry mechanism is used along with the interaction-by-exchange with the mean (IEM) model. For the micro-mixing model, typically a constant ratio of scalar to mechanical timescale is assumed. This parameter can have substantial variations and can strongly influence the combustion process. Here, a dynamic timescale model is used to prescribe the mixing timescale , which eliminates the timescale ratio as a model constant. Two different flame configurations, namely, Sandia flames D and E are studied. Comparison of simulated radial profiles with experimental data show good agreement for both flames. The LES-FDF simulations accurately predict the increased extinction near the inlet and re-ignition further downstream. The conditional mean profiles show good agreement with experimental data for both flames.
A Combustion Regime-Based Model for Large Eddy Simulation
Energies
The aim of this work is to propose a unified (generalized) closure of the chemical source term in the context of Large Eddy Simulation able to cover all the regimes of turbulent premixed combustion. Turbulence/combustion scale interaction is firstly analyzed: a new perspective to look at commonly accepted combustion diagrams is provided based on the evidence that actual turbulent flames can experience locally several combustion regimes although global non-dimensional numbers would locate such flames in a single specific operating point of the standard combustion diagram. The deliverable is a LES subgrid scale model for turbulent premixed flames named Localized Turbulent Scales Model (LTSM). This is founded on the estimation of the local reacting volume fraction of a computational cell that is related to the local turbulent and laminar flame speeds and to the local flame thickness.
Large-Eddy Simulation of Turbulent Combustion
Annual Review of Fluid Mechanics, 2006
Large-eddy simulation (LES) of turbulent combustion is a relatively new research field. Much research has been carried out over the past years, but to realize the full predictive potential of combustion LES, many fundamental questions still have to be addressed, and common practices of LES of nonreacting flows revisited. The focus of the present review is to highlight the fundamental differences between Reynolds-averaged Navier-Stokes (RANS) and LES combustion models for nonpremixed and premixed turbulent combustion, to identify some of the open questions and modeling issues for LES, and to provide future perspectives.
Large Eddy Simulation of Premixed Combustion: Sensitivity to Subgrid Scale Velocity Modeling
Combustion Science and Technology
An algebraic reaction rate closure involving filtered scalar dissipation rate of reaction progress variable is studied. The filtered scalar dissipation rate closure requires a model for sub-grid scale velocity, u 0 Δ , which is estimated using four algebraic models and transported subgrid scale kinetic energy. A priori analyses using direct numerical simulation (DNS) data show that the filtered dissipation rate, and thus the reaction rate closure, has some sensitivity to the u 0 Δ model. The sensitivity of various statistics obtained from large eddy simulation (LES) of three piloted Bunsen flames of stoichiometric methaneair mixture to the modeling of u 0 Δ is observed to be weaker compared to that for the DNS analysis. Moreover, analysis using transported sub-grid scale kinetic energy does not indicate a necessity to include flame-generated turbulence in the modeling of u 0 Δ for the Bunsen flames in the thin reaction zones regime. The measured and computed flame brush structures are compared and studied and the algebraic closure for the filtered reaction rate is found to be quite good.
Mixing Models for Large-Eddy Simulation of Nonpremixed Turbulent Combustion
Journal of Fluids Engineering, 2001
The application of mixture fraction based models to large-eddy simulations (LES) of nonpremixed turbulent combustion requires information about mixing at length scales not resolved on the LES grid. For instance, the large-eddy laminar flamelet model (LELFM) takes the subgrid-scale variance and the filtered dissipation rate of the mixture fraction as inputs. Since chemical reaction rates in nonpremixed turbulence are largely governed by the mixing rate, accurate mixing models are required if mixture fraction methods are to be successfully used to predict species concentrations in large-eddy simulations. In this paper, several models for the SGS scalar variance and the filtered scalar dissipation rate are systematically evaluated a priori using benchmark data from a DNS in homogeneous, isotropic, isothermal turbulence. The mixing models are also evaluated a posteriori by applying them to actual LES data of the same flow. Predictions from the models that depend on an assumed form for t...
Combustion Theory and Modelling, 2006
Reactive flow simulations using large-eddy simulations (LES) require modelling of sub-filter fluctuations. Although conserved scalars like mixture fraction can be represented using a beta-function, the reactive scalar probability density function (PDF) does not follow an universal shape. A one-point one-time joint composition PDF transport equation can be used to describe the evolution of the scalar PDF. The high-dimensional nature of this PDF transport equation requires the use of a statistical ensemble of notional particles and is directly coupled to the LES flow solver. However, the large grid sizes used in LES simulations will make such Lagrangian simulations computationally intractable. Here we propose the use of a Eulerian version of the transported-PDF scheme for simulating turbulent reactive flows. The direct quadrature method of moments (DQMOM) uses scalar-type equations with appropriate source terms to evolve the sub-filter PDF in terms of a finite number of delta-functions. Each delta-peak is characterized by a location and weight that are obtained from individual transport equations. To illustrate the feasibility of the scheme, we compare the model against a particle-based Lagrangian scheme and a presumed PDF model for the evolution of the mixture fraction PDF. All these models are applied to an experimental bluff-body flame and the simulated scalar and flow fields are compared with experimental data. The DQMOM model results show good agreement with the experimental data as well as the other sub-filter models used.
Progress-variable approach for large-eddy simulation of non-premixed turbulent combustion
Journal of Fluid Mechanics, 2004
A new approach to chemistry modelling for large-eddy simulation of turbulent reacting flows is developed. Instead of solving transport equations for all of the numerous species in a typical chemical mechanism and modelling the unclosed chemical source terms, the present study adopts an indirect mapping approach, whereby all of the detailed chemical processes are mapped to a reduced system of tracking scalars. Here, only two such scalars are considered: a mixture fraction variable, which tracks the mixing of fuel and oxidizer, and a progress variable, which tracks the global extent of reaction of the local mixture. The mapping functions, which describe all of the detailed chemical processes with respect to the tracking variables, are determined by solving quasi-steady diffusion-reaction equations with complex chemical kinetics and multicomponent mass diffusion. The performance of the new model is compared to fast-chemistry and steady-flamelet models for predicting velocity, species concentration, and temperature fields in a methane-fuelled coaxial jet combustor for which experimental data are available. The progress-variable approach is able to capture the unsteady, lifted flame dynamics observed in the experiment, and to obtain good agreement with the experimental data, while the fast-chemistry and steady-flamelet models both predict an attached flame.
Numerical Heat Transfer, Part A: Applications
The influences of differential diffusion of heat and mass on the Favre-filtered scalar dissipation rate (SDR) transport have been analyzed and modeled using a priori analysis of Direct Numerical Simulations (DNS) data of freely propagating statistically planar turbulent premixed flames with different values of global Lewis number, Le. The DNS data has been explicitly filtered using a Gaussian filter to obtain the unclosed terms of the Favre-filtered SDR transport equation, arising from turbulent transport (T 1), density variation due to heat release (T 2), strain rate contribution due to the alignment of scalar and velocity gradients (T 3), correlation between the gradients of reaction rate and reaction progress variable (T 4), molecular dissipation of SDR (À D 2), and diffusivity gradients f(D). The statistical behaviors of these terms and their scaling estimates reported in a recent analysis have been utilized here to propose models for these unclosed terms in the context of Large Eddy Simulations (LES) and the performances of these models have been assessed using the values obtained from explicitly filtered DNS data. These newly proposed models are found to satisfactorily predict both the qualitative and quantitative behaviors of these unclosed terms for a range of filter widths � for all Le cases considered here.