Direct Numerical Simulations of Premixed Turbulent Flames with Reduced Chemistry: Validation and Flamelet Analysis (original) (raw)
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In the recently introduced flamelet-generated manifold (FGM) method the ideas of the manifold and the flamelet approach are combined: a manifold is constructed using one-dimensional (1D) flamelets. In this paper the effect of flame stretch on the accuracy of the FGM method is investigated. In order to isolate the effect of flame stretch, premixed methane/air counterflow flames are simulated. In the case of unit Lewis numbers, a 1D manifold is sufficient to model the main effects of flame stretch. A manifold with two progress variables reproduces the results computed using detailed kinetics almost exactly. When non-unit Lewis numbers are used, the enthalpy and element composition of the burnt mixture change, which may influence the mass burning rate significantly. If these composition changes are included in the manifold using one additional controlling variable, the results agree well with detailed computations.
Modelling of Premixed Laminar Flames using Flamelet-Generated Manifolds
Combustion Science and Technology, 2000
In order to reduce the computational cost of flame simulations, several methods have been developed during the last decades, which simplify the description of the reaction kinetics. Most of these methods are based on partial-equilibrium and steady-state assumptions, assuming that most chemical processes have a much smaller time scale than the flow time scale. These assumptions, however, give poor approximations in the 'colder' regions of a flame, where transport processes are also important. The method presented here, can be considered as a combination of two approaches to simplify flame calculations, i.e. a flamelet and a manifold approach. The method, to which we will refer as the Flamelet-Generated Manifold (FGM) method, shares the idea with flamelet approaches that a multi-dimensional flame may be considered as an ensemble of one-dimensional flames. The implementation, however, is typical for manifold methods: a low-dimensional manifold in composition space is constructed, and the thermo-chemical variables are stored in a database which can be used in subsequent flame simulations. In the FGM method a manifold is constructed using one-dimensional flamelets. Like in other manifold methods, the dimension of the manifold can be increased to satisfy a desired accuracy. Although the method can be applied to different kinds of flames, only laminar premixed flames are considered here. Since the major parts of convection and diffusion processes are present in one-dimensional flamelets, the FGM is more accurate in the 'colder' zones of premixed flames than methods based on local chemical equilibria. Therefore, less controlling variables are sufficient to represent the combustion process. Test results of one and two-dimensional premixed methane/air flames show that detailed computations are reproduced very well with a FGM consisting of only one progress variable apart from the enthalpy to account for energy losses.
Modelling of Turbulent Premixed Flames using Strained–Flamelets
A model for the mean reaction rate in turbulent premixed flames, that accounts for effects of strain rate on flame structure is presented. This model is based on the strained asymmetric counterflow unburnt-toburnt laminar flame and its characteristics. The conditional scalar dissipation rate is chosen to parametrise the strained-flamelet structure since it is dictated by the convection-diffusion-reaction balance. RANS calculations of laboratory scale flames using this approach give good comparisons with experimental results and suggest that commonly used unstrained-flamelet formulation overpredicts the mean reaction rate leading to smaller flame brush compared to experiment.
Analysis of the flamelet concept in the numerical simulation of laminar partially premixed flames
Combustion and Flame, 2008
The aim of this work is to analyze the application of flamelet models based on the mixture fraction variable and its dissipation rate to the numerical simulation of partially premixed flames. Although the main application of these models is the computation of turbulent flames, this work focuses on the performance of flamelet concept in laminar flame simulations removing, in this way, turbulence closure interactions. A well-known coflow methane/air laminar flame is selected. Five levels of premixing are taken into account from an equivalence ratio Φ = ∞ (nonpremixed) to Φ = 2.464. Results obtained using the flamelet approaches are compared to data obtained from the detailed solution of the complete transport equations using primitive variables. Numerical simulations of a counterflow flame are also presented to support the discussion of the results. Special emphasis is given to the analysis of the scalar dissipation rate modeling.
Flamelet Analysis of Turbulent Combustion
Lecture Notes in Computer Science, 2005
Three-dimensional direct numerical simulations are performed of turbulent combustion of initially spherical flame kernels. The chemistry is described by a progress variable which is attached to a flamelet library. The influence of flame stretch and curvature on the local mass burning rate is studied and compared to an analytical model. It is found that there is a good agreement between the simulations and the model. Then approximations to the model are evaluated.
Simulation of Spherically Expanding Turbulent Premixed Flames
Combustion Science and Technology, 2013
Statistically spherical expanding turbulent premixed flames are computed using unsteady Reynolds-Averaged Navier-Stokes (URANS) approach. Mean reaction rate is closed using strained and unstrained flamelet models and an algebraic model. The flamelets are parametrised using the scalar dissipation rate in the strained flamelet model. It is shown that this model is able to capture the measured growth rate of methane-air turbulent flame ball, which is free of thermo-diffusive instability. The spherical flames are observed to accelerate continuously. The flame brush thickness grows in time and the role of turbulent diffusion on this growth seems secondary compared to the convection due to the fluid velocity induced by the chemical reaction. The spherical flames have larger turbulent flame speed, the leading edge displacement speed s t , compared to the planar flames for a given turbulence and thermo-chemical condition. The computational results suggest s t /s 0 L ∼ Re n t with 0.57 ≤ n ≤ 0.58, where Re t is the turbulence Reynolds number and s 0 L is the unstrained planar laminar flame speed, for both spherical and planar flames.
Two recent developments in numerical simulation of premixed and partially premixed turbulent flames
Comptes Rendus Mécanique, 2006
A subgrid scale closure for Large Eddy Simulation of premixed turbulent combustion (FSD-PDF) is proposed. It combines the Flame Surface Density (FSD) approach with a presumed Probability Density Function (PDF) of the progress variable that is used in flamelet chemistry tabulation. The FSD is useful to introduce in the presumed PDF the influence of the spatially filtered thin reaction zone evolving within the subgrid. This is achieved via the exact relation between the PDF and the FSD. In a second part, Direct Numerical Simulation of partially premixed combustion in a swirling flow is reported. The results are used to analyze the structure of the leading edge flame that ensures flame stabilization. To cite this article:
A priori analysis of turbulent flamelet combustion in a premixed spherical flame kernel
Three-dimensional direct numerical simulations are performed of turbulent combustion of initially spherical flame kernels. The chemistry is described by a progress variable which is attached to a flamelet library. In order to progress towards turbulent closure of such a system the validity of the flamelet approach must be studied. Then the turbulent closure of the equation for the progress variable should be investigated. The influence of flame stretch and curvature on the local mass burning rate is studied and compared to an analytical model. Then first investigations of the relative importance of the unresolved turbulent flux compared to the source term of the progress variable are presented. * Corresponding author: r.j.m.bastiaans@tue.nl Associated Web site: http://www.combustion.tue.nl Proceedings of the European Combustion Meeting 2005 area, affecting the consumption rate as well. In the turbulent case, stretch rates vary significantly in space and time. An expression for the stretch rate is derived directly from its mass-based definition in [8],
Journal of Thermal Science and Technology, 2015
A new multi-dimensional flamelet generated manifolds approach for approximating partially premixed flame structure Abstract A new multi-dimensional flamelet generated manifolds(MFM) approach is proposed based on solving multi-dimensional flamelet equation set in mixture fraction Z and normalized flamelet progress variable c coordinate, to capture both non-premixed and premixed combustion characteristics in partially premixed flames. Local scalar dissipation rates appeared as coefficients in multi-dimensional flamelet equation set have been modeled based on the analysis of local combustion regime during multi-dimensional flamelet calculation. Simulation results of counter-flow laminar partially premixed flames suggests that this new MFM approach can reproduce both non-premixed and premixed flame structure accurately, with computational efforts greatly reduced compared to former MFM approach.