Large eddy simulation/probability density function modeling of a turbulent CH4/H2/N2 jet flame (original) (raw)
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Proceedings of the Combustion Institute, 2020
The use of the Eulerian Stochastic Fields (ESF) method to model the sub-grid turbulence-chemistry interaction (TCI) in the LES context can be computationally expensive if detailed chemistry mechanisms are involved. This work aims to assess whether it is possible to neglect the modelling of the TCI on sufficiently refined meshes while using finite rate chemistry, provided that at least 80 % of the turbulent kinetic energy scales are resolved. Turbulent non-premixed methane-air flames showing a moderate degree of local extinction are selected as benchmark. Results obtained for the Sandia flame E with and without transporting the ESF on three different meshes are discussed. Sensible deviations are visible on the fuel-rich side from section x/D = 30, by reducing the grid refinement. The influence of three finite rate chemistry solvers is further investigated on flame D, without the sub-grid scale chemistry model. All simulations are in good agreement with the experimental data and show a weak dependence on the chemistry involved. A trade-off assessment between computational time and accuracy is provided, in order to extend the validation to a more severe extinction regime.
Proceedings of the Combustion Institute, 2019
Tabulated flamelets are commonly used in turbulent combustion modeling due to their relatively low computational cost, which is attractive in industrial applications. However, these models require assumptions of tabulated chemistry and subgrid-scale models for control variable distributions, both of which may contribute to modeling errors. In the present work, large-eddy simulation (LES) with tabulated flamelets is employed to study a laboratory-scale high Karlovitz number stratified premixed jet flame that was investigated recently using direct numerical simulation (DNS). Particularly, the LES resolves properly the transported control variables at a near DNS level, mitigating the errors from subgrid-scale modeling of control variable distributions. Five different flamelet tables are tested in the current work, including the conditional mean from the DNS, counterflow stratified premixed 1D flames with and without differential diffusion, freely propagating premixed 1D flames, and 0D autoigniting plug-flow reactors. The LES results show that although the flamelet tables perform differently for the instantaneous distributions of the progress variable source term, their mean distributions are similar. The mean and rms (root mean square) radial profiles for axial velocity and temperature from the LES with different flamelet tables are in good agreement with those from the DNS; more evident discrepancies are observed for the CH 2 O mass fraction radial profiles. Finally, the flame * Corresponding author.
Modeling chemical flame structure and combustion dynamics in LES
Proceedings of the Combustion Institute, 2011
In turbulent premixed combustion, the instantaneous flame thickness is typically thinner that the grid size usually retained in Large Eddy Simulations (LES), requiring adapted models. Two alternatives to couple chemical databases with LES balance equations, the Thickened Flame (TFLES) and the Filtered Tabulated Chemistry (F-TACLES) models, are investigated here and compared in terms of chemical flame structure and dynamics. To avoid the uncertainties related to the modeling of sub-grid scale turbulence / flame interactions, this comparison is conducted in situations where the flame front is not wrinkled at sub-grid scale levels. The thinner quantity requiring an accurate discretization on the numerical grid mesh is the reaction rate of the thickened or filtered progress variable. The thermal flame structure is found to be considerably thicker in TFLES than when using F-TACLES. The simulation of a 2D unsteady Bunsen burner flame shows that the thermal thickness spreading strongly affects the flame dynamics giving a decisive advantage to F-TACLES compared to TFLES.
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 mixing timescale model for TPDF simulations of turbulent premixed flames
Combustion and Flame
Transported probability density function (TPDF) methods are an attractive modeling approach for turbulent flames as chemical reactions appear in closed form. However, molecular micro-mixing needs to be modeled and this modeling is considered a primary challenge for TPDF methods. In the present study, a new algebraic mixing rate model for TPDF simulations of turbulent premixed flames is proposed, which is a key ingredient in commonly used molecular mixing models. The new model aims to properly account for the transition in reactive scalar mixing rate behavior from the limit of turbulence-dominated mixing to molecular mixing behavior in flamelets. An a priori assessment of the new model is performed using direct numerical simulation (DNS) data of a lean premixed hydrogen-air jet flame. The new model accurately captures the mixing timescale behavior in the DNS and is found to be a significant improvement over the commonly used constant mechanical-to-scalar mixing timescale ratio model. An a posteriori TPDF study is then performed using the same DNS data as a numerical test bed. The DNS provides the initial conditions and time-varying input quantities, including the mean velocity, turbulent diffusion coefficient, and modeled scalar mixing rate for the TPDF simulations, thus allowing an exclusive focus on the mixing model. The new mixing timescale model is compared with the constant mechanical-to-scalar mixing timescale ratio coupled with the Euclidean Minimum Spanning Tree (EMST) mixing model, as well as a laminar flamelet closure by Pope and Anand (S.B. Pope, M.S. Anand, Proc. Combust. Inst. 20 (1984) 403-410). It is found that the laminar flamelet closure is unable to properly capture the mixing behavior in the thin reaction zones regime while the constant mechanical-to-scalar mixing timescale model under-predicts the flame speed. The EMST model coupled with the new mixing timescale model provides the best prediction of the flame structure and flame propagation among the models tested, as the dynamics of reactive scalar mixing across different flame regimes are appropriately accounted for.
Computational study of lean premixed turbulent flames using RANSPDF and LESPDF methods
Combustion Theory and Modelling, 2013
A computational study is performed on a series of four piloted, lean, premixed turbulent jet flames. These flames use the Sydney Piloted Premixed Jet Burner (PPJB), and with jet velocities of 50, 100, 150 and 200 m/s are denoted PM1-50, PM1-100, PM1-150 and PM1-200, respectively. Calculations are performed using the RANS-PDF and LES-PDF methodologies, with different treatments of molecular diffusion, with detailed chemistry and flamelet-based chemistry modelling, and using different imposed boundary conditions. The sensitivities of the calculations to these different aspects of the modelling are compared and discussed. Comparisons are made to experimental data and to previously-performed calculations. It is found that, given suitable boundary conditions and treatment of molecular diffusion, excellent agreement between the calculations and experimental measurements of the mean and variance fields can be achieved for PM1-50 and PM1-100. The application of a recently developed implementation of molecular diffusion results in a large improvement in the computed variance fields in the LES-PDF calculations. The inclusion of differential diffusion in the LES-PDF calculations provides insight on the behaviour in the near-field region of the jet, but its effects are found to be confined to this region and to the species CO, OH and H 2. A major discrepancy observed in many previous calculations of these flames is an overprediction of reaction progress in PM1-150 and PM1-200, and this discrepancy is also observed in the LES-PDF calculations; however, a parametric study of the LES-PDF mixing model reveals that, with a sufficiently large mixing frequency, calculations of these two flames are capable of yielding improved reaction progress in good qualitative agreement with the mean and RMS scalar measurements up to an x/D of 30. Lastly, the merits of each computational methodology are discussed in light of their computational costs.
A Hybrid Rans/PDF Approach for Modelling Hot and Diluted Flames
Results of numerical modelling of turbulent non-premixed Methane/Hydrogen (1/1 by volume) flames issuing from a jet in hot coflow (JHC) are presented. The JHC burner is designed to emulate Moderate and Intense Low oxygen Dilution (MILD) combustion regime. A comparison between the measured and computed temperature and species profiles is presented for three flames with different oxygen content in the hot coflow (oxygen mass-fraction of 3%, 6%, and 9%). The composition PDF approach with the EMST mixing model coupled with a finite-volume flow solver was used in the modelling. The performance of three welldocumented chemical kinetics mechanisms (Smooke, ARM, and GRI3.0) is examined.
Numerical modeling of hydrogen diffusion jet flame: the role of the combustion model
The prediction of turbulent reactive flows largely depends on the proper choice of a combustion model. The closure of the highly non linear production term in the species conservation equation is one of the most challenging aspects when modeling turbulent combustion with RANS/FANS approaches. The main value of the species reaction rates cannot be obtained by simply substituting the mean variables into the Arrhenius equations, and the effects of turbulence on temperature and species concentration fields must be taken into account. Several approaches have been proposed in the literature to provide a proper modelling of mean reaction rates. The Eddy Dissipation Model is based on the infinitely fast chemistry hypothesis and assumes that the reaction rate is controlled by turbulent mixing . A generalized formulation of the Eddy Dissipation Model, i.e. EDM/FR, has been proposed in order to take into account finite rate chemistry effects, however, this model is not suited to treat detailed combustion mechanisms and fails with a number of reactions larger than three or four. The Eddy Dissipation Concept (EDC) is an extension of EDM which allows detailed kinetics to be included in the calculations . The model assumes that chemical reactions occur within the smallest turbulent structures, called fine structures. These are treated as perfectly stirred reactors (PSR) which exchange mass with the surrounding fluid. The overall reaction rate in each PSR is controlled by chemical kinetics. The properties of the fine structures are derived from a step-wise energy cascade model and expressed with quantities related to the main flow, such as the turbulent kinetic energy, k, and the turbulent dissipation rate, _. Two formulations of EDC have been developed, for fast and finite rate chemistry respectively [2]. Both combustion models described above are based on the reaction rate approach. A different approach is based on the primitive variable and originates a group of combustion models. In its simplest formulation, this approach allows the decoupling of chemistry and flow field calculation. The chemical reaction rates are computed off-line and stored in lookup tables, accessible by the flow solver. The instantaneous variables are expressed in terms of mixture fraction and enthalpy and, then, integrated over an assumed _-PDF probability density function to get the mean scalar variables. In the Flamelet approach, the flame is modelled as an ensemble of thin and steady laminar flames, strained by the turbulent motion. According to this model, two scalar equations, for the mixture fraction and its variance, have to be solved independently of the number of species involved in the chemical mechanism. The influence of the outer flow field on the inner reaction zone is described by the scalar dissipation rate. This represents a reciprocal residence time within the flame sheets which is increased by stretch effects of turbulence motion and reduced by diffusion. At a critical value of the dissipation rate the flame shows a threshold behaviour and extinguishes. The mean values of scalar properties are computed from the instantaneous values which are integrated over a probability density function with a presumed _-PDF shape. The integration is not carried out during the CFD calculation but is part of the flamelet library generation, in order to reduce the computational cost. The flamelet libraries are created with a mixture fraction ranging between zero and unity and for discrete
Combustion and Flame, 1999
A computational study of a nonisenthalpic premixed turbulent jet flame is described. The flame burns a homogeneously premixed stoichiometric methane-air mixture injected into a coflow of air. The enthalpy (chemical ϩ sensible) varies because of mixing between the jet fluid and the coflow. The performance of the Bray-Moss (BM) model and three flame surface density (FSD) models is evaluated by comparing the predictions of mean velocity and temperature profiles with recent experimental data. The reaction progress variable approach, which is established for isenthalpic flames, is extended to the present nonisenthalpic flames by including mean and mean square mixture fraction equations. The joint probability density function (PDF) of the reaction progress variable and the mixture fraction is modeled in terms of two statistically independent PDFs. The time-averaged reaction rate term is modeled using the BM and the FSD models. The effects of mixing with the coflow air were found to be unimportant in the evaluation of the flame speed required for modeling the mean reaction rate term. All models yielded reasonable predictions of mean velocity. Predictions of time-averaged temperatures agree better with the thin filament pyrometry data than those of Favre-averaged temperatures. The BM and MB models provided the best agreement with the mean temperature data but the other FSD models with slight tuning of the constants could provide similar agreement as well. The results show that a simple extension of the FSD models is promising for the treatment of nonisenthalpic flames. It appears that the differences in the conceptual framework of the FSD models disappear in their implementation using basically the same turbulence properties of kinetic energy and dissipation rates.