Joint RANS/LES Approach to Premixed Flame Modelling in the Context of the TFC Combustion Model

Abstract

We present an original timesaving joint RANS/LES approach to simulate turbulent premixed combustion. It is intended mainly for industrial applications where LES may not be practical. It is based on successive RANS/LES numerical modelling, where turbulent characteristics determined from RANS simulations are used in LES equations for estimation of the subgrid chemical source and viscosity. This approach has been developed using our TFC premixed combustion model, which is based on a generalization of the Kolmogorov's ideas. We assume existence of small-scale statistically equilibrium structures not only of turbulence but also of the reaction zones. At the same time, non-equilibrium large-scale structures of reaction sheets and turbulent eddies are described statistically by model combustion and turbulence equations in RANS simulations or follow directly without modelling in LES. Assumption of small-scale equilibrium gives an opportunity to express the mean combustion rate (controlled by small-scale coupling of turbulence and chemistry) in the RANS and LES sub-problems in terms of integral or subgrid parameters of turbulence and the chemical time, i.e. the definition of the reaction rate is similar to that of the mean dissipation rate in turbulence models where it is expressed in terms of integral or subgrid turbulent parameters. Our approach therefore renders compatible the combustion and turbulent parts of the RANS and LES sub-problems and yields reasonable agreement between the RANS and averaged LES results. Combining RANS simulations of averaged fields with LES method (and especially coupled and acoustic codes) for simulation of corresponding nonstationary process (and unsteady combustion regimes) is a promising strategy for industrial applications. In this work we present results of simulations carried out employing the joint 306 Flow Turbulence Combust (2006) 77: 305-331 RANS/LES approach for three examples: High velocity premixed combustion in a channel, combustion in the shear flow behind an obstacle and the impinging flame (a premixed flame attached to an obstacle).

A Two-Fluid Conditional Averaging Paradigm for the Theory and Modeling of Turbulent Premixed Combustion

“…This figure shows the results of numerical simulations of the two 2D flames in the channel (close to the obstacle and free-standing impinging flames). The simulations were performed by Valirio Battaglia using the commercial code "Fluent" based on the TFC combustion model [17] (see [26] for more details).…”

“…It can be seen that the direction of the scalar flux in the flame close to the obstacle corresponds to gradient diffusion, while the scalar flux in the free-standing flame is countergradient except for the zone adjacent to the leading edge of the flame. In [26], we discussed the reason for the domination of turbulent diffusion in the head of the flame. From a theoretical viewpoint, these results are reasonable since they correlate with the profile of the pressure, in contrast to the results for the close-to-the-wall and free-standing impinging flames mentioned in Section 3.1 and obtained in [4] in the context of the Favre averaging framework.…”

RANS and LES Modeling of the GE10 Burner

“…The main idea of our joint RANS/LES approach [6] is to combine LES and RANS in a two-stage process. The first step consists of the RANS simulation which yields the averaged flow field; the second step entails LES using the mean turbulent dissipation rate ( , ) x t   obtained from RANS to estimate the subgrid turbulence u   and subgrid flame speed…”

“…To understand the non-stationary features of the flow, we performed a LES analysis of the burner. We used our Joint RANS/LES approach [6] that strongly shortens the LES analysis. The simulations were performed using the Fluent package, a finite volume code which gives the possibility to customize the models implemented.…”

“…We think that "LES instead of RANS" in industrial application is untimely and we proposed in [6] a joint RANS/LES approach where the mean fields are simulated by the RANS tool while the corresponding nonstationary fields are simulated by the LES one using for modeling some information from the preceding RANS simulation. The paper [6] contains numerical illustrations of this approach concerning mainly "academic" flames. We achieved agreement between RANS and LES subproblems by using in fact the same combustion models in both sub-problems.…”