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

Large Eddy Simulation is a powerful tool to simulate the unsteady lean premixedflames and to predict combustion instabilities. However the resolution of an LES grid is typicallylarger than the thickness of practical flames. In artificially thickened flame concept the mixing inthe flame region is enhanced by explicitly increasing heat and mass diffusivity while decreasingthe average reaction rates. The resulted flame is resolvable and propagates with the correct speedbut 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 reactionmechanism in an LES solver. This method exploits the intrinsic numerical diffusion of theupwind biased discretization schemes to implicitly enhance mixing in the flame region hencethickening the flame to sizes resolvable on an LES grid. Unlike the previous thickening methodthis technique does not increase the diffusion in the plane tangent to the flame front hencereduces the loss of the flame surface. Simulation of a lean premixed low swirl methane-air flamereveals that this new method tends to keep some of those small flame structures which aresmeared out by the previous flame thickening methods