Turbulent combustion modeling

A subgrid scale model for large eddy simulations of turbulent premixed combustion is developed and validated. The approach is based on the concept of artificially thickened flames, keeping constant the laminar flame speed sl0. This thickening is simply achieved by decreasing the pre-exponential factor of the chemical Arrhenius law whereas the molecular diffusion is enhanced. When the flame is thickened, the combustion–turbulence interaction is affected and must be modeled. This point is investigated here using direct numerical simulations of flame–vortex interactions and an efficiency function E is introduced to incorporate thickening effects in the subgrid scale model. The input parameters in E are related to the subgrid scale turbulence (velocity and length scales). An efficient approach, based on similarity assumptions, is developed to extract these quantities from the resolved velocity field. A specific operator is developed to exclude the dilatational part of the velocity field from the estimation of...

A thickened flame model for large eddy simulations of turbulent premixed combustion

Large eddy simulation (LES) is a promising tool for numerical simulations of reacting flows, especially when combustion instabilities are encountered. Nevertheless, a difficulty occurs in developing subgrid scale models for premixed combustion, because the flame front is generally too thin to be resolved on the computational grid. An approach based on the filtering of the balance equation for the progress variable c using a LES filter larger than the mesh size is proposed and investigated. Despite its similarities with the field description based on the G-equation, the advantage of this approach is that c and related quantities such as subgrid-scale flame surface density are physically well defined and may be easily extracted from direct numerical simulations (DNS) or experimental data to analyze and validate models. A three-dimensional DNS database where a laminar premixed flame interacts with a homogeneous and isotropic turbulent flowfield is used to investigate unresolved turbulent scalar transport and filtered reaction rate. The unresolved transport is found to exhibit a gradient or a countergradient feature, depending on the heat release parameter and the turbulence level. This finding is in aggreement with previous observations of the turbulent transport in Reynolds-averaged Navier-Stokes (RANS) equations. Nevertheless, the unresolved convective flux is lower than the resolved one. Accordingly, model uncertainties will have probably less dramatic consequences than in RANS because a part of the countergradient phenomenon will be incorporated into the motion of the resolved flow structures. The filtered reaction rate is closed, introducing a subgrid-scale flame surface density, Σ, modeled, in a first step, with an algebraic expression similar to the Bray-Moss-Libby (BML) formulation widely used in RANS context. This concept is very attractive because it could be refined using, for example, a dynamic algebraic formulation or a balance equation for Σ.

Direct numerical simulation analysis of flame surface density concept for large eddy simulation of turbulent premixed combustion

The development of the basic conceptual viewpoints, or paradigms, for turbulent combustion in gases over the last 50 years is reviewed. Significant progress has been made. Recent successes in the prediction of pollutant species and extinction/re-ignition phenomena in non-premixed flames are seen as the result of close interaction between experimentalists, theoreticians, and modellers. Premixed turbulent flames seem to be dependent on a much wider range of factors, and predictive capabilities are not so advanced. Implications for large eddy simulation (LES) and partially premixed combustion are outlined.

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Paradigms in turbulent combustion research

▪ Abstract Non-premixed turbulent combustion processes control most practical applications of combustion. Studying these mechanisms has been the objective of numerous theoretical and experimental works in the last century. In the past 10 years, direct numerical simulation (DNS) has emerged as a new methodology. It has become an essential tool to understand and model turbulent combustion. DNS numerically solves the set of equations describing turbulent flames by resolving all chemical and flow scales. Because formulated assumptions can be tested accurately, the resulting data provide unique information to build turbulent combustion models. This review first discusses the fundamental properties of laminar diffusion flames. It then presents various DNS results to illustrate the different problems that can be studied using this tool. These problems include validity of the modeling hypothesis, topology of flame surfaces, and ignition and extinction mechanisms. The review also discusses the different analysis t...

Direct numerical simulation of non-premixed turbulent flames

To characterize the dynamics and the physical properties of isoconcentration surfaces of a random reactive scalar field, an instantaneous isosurface quantity and its transport equation are introduced. For turbulent flows, the mean area of isoconcentration surface per unit volume is studied through its transport equation derived by using the probability density function (pdf) formalism. This approach allows the value of the reactive scalar used to define the level surface to be treated as an independent variable. It also leads to the definition of a surface density function. The developed formalism is applied to premixed turbulent combustion and a bridge is built between modeling approaches based on pdf, and others based on the flame surface concept.

E. Bidaux

Papers

Surface density function in premixed turbulent combustion modeling, similarities between probability density function and flame surface approaches