In this paper, a numerical study of turbulent premixed flames of propane and air is presented. The experimental configuration investigated (Besson et al, 1999) is a planar channel that incorporates a sudden expansion in a form of two symmetrical backward-facing steps. Two lean mixtures of pro pane and air are injected in the upper and lower halves of the upstream part of the channel. separated by a plate and in the present work, the fuel-air ratios of these two streams are different. The numerical model includes a second-order closure for Reynolds stresses and uses a modified Bray-Moss-Libby theory to model a reactive turbulent flow with variable stoichiometry.