Scalar transport modeling in large eddy simulation of turbulent premixed flames

Scalar transport in turbulent premixed flames is considered from the viewpoint of subgrid-scale (SGS)modeling for large eddy simulation (LES). It is well known from experiments and from modeling for Reynolds-averaged Navier-Stokes (RANS) simulations of turbulent premixed flames that scalar transport frequently occurs in directions that run counter to the mean scalar gradient. Such countergradient transport is known to be promoted by high rates of heat release and opposed by high local turbulence intensities. The objective of the present work is to devise SGS models for countergradient transport that are both physically realistic and computationally inexpensive. To this end, modeling is conducted using physical arguments supported by data obtained from direct numerical simulation (DNS). Following a brief review of previous SGS scalar transport modeling, a simple algebraic closure is proposed based on the Bray-Moss-Libby formulation originally developed for RANS simulations. Both gradient and countergradient transport are explicitly accounted for, and a decomposition of the total scalar transport into contributions from inflame and non-reacting components is shown to be helpful in identifying the underlying physics. Filtered results obtained from two different sets of DNS data are compared a priori with the proposed model. One set of DNS data displays strong countergradient transport, while the other set contains local regions of gradient transport alongside regions of countergradient transport. Good agreement is found between model and DNS in both cases, and some interesting observations are made concerning the origins of countergradient transport and the ability of LES to capture the phenomenon.