Large Eddy Simulation of premixed flames with multi-step global reaction mechanisms

Large Eddy Simulation is a powerful tool to simulate the unsteady lean premixed flames and to predict combustion instabilities. However the resolution of an LES grid is typically larger than the thickness of practical flames. In artificially thickened flame concept the mixing in the flame region is enhanced by explicitly increasing heat and mass diffusivity while decreasing the average reaction rates. The resulted flame is resolvable and propagates with the correct speed but the flame response to small turbulent structures is decreased which should be compensated. A novel flame thickening technique is applied here to implement a multi-step global reaction mechanism in an LES solver. This method exploits the intrinsic numerical diffusion of the upwind biased discretization schemes to implicitly enhance mixing in the flame region hence thickening the flame to sizes resolvable on an LES grid. Unlike the previous thickening method this technique does not increase the diffusion in the plane tangent to the flame front hence reduces the loss of the flame surface. Simulation of a lean premixed low swirl methane-air flame reveals that this new method tends to keep some of those small flame structures which are smeared out by the previous flame thickening methods.