Implications of a flame surface density approach to large eddy simulation of premixed turbulent combustion

Large eddy simulation (LES) is now widely regarded as an improvement on existing computational fluid dynamics (CFD) techniques in addressing classes of combustion problems where traditional CFD approaches have experienced some difficulty [1-3]. This is particularly true in situations where there is significant unsteadiness that is characterized by large-scale flow-flame interactions. The flame surface density (FSD) approach to the modeling of premixed turbulent combustion is well established in the context of Reynolds-averaged simulations, and has shown potential as a technique for LES [4]. In this paper, results are presented by using the flame surface density model of Hawkes and Cant [4] in a flame propagation test case that further demonstrates the feasibility of the approach. Firstly, the response of the model to variations in turbulence intensity is examined, and an assessment of the relative importance of the source terms in the balance equation for FSD is made. Secondly, it is shown how LES can exploit the effects of large-scale coherent structures in the prediction of FSD through an analysis of the resolved strain source term. Lastly, the model behavior under variations of the filter size is examined. It is an essential but difficult test for FSD models for LES that the results are independent of the filter size. It is shown that the FSD responds to variations in filter size as expected. An increase in filter size results in a decrease in resolved wrinkling, but an increase in sub-grid wrinkling. The net propagation rate of the turbulent flame is shown to be largely independent of the chosen filter size. (C) 2001 by The Combustion Institute.