The Role of the Combustion Submodel for Large Eddy Simulation of Transient Premixed Flame-Vortex Interactions in Gas Explosions

In this paper, the sensitivity of large eddy simulation (LES) to the presence of the combustion submodel was investigated for transient interactions between premixed flame fronts and toroidal vortex structures generated at the wake of a circular orifice. To this end, LES computations were run, with and without the combustion submodel, for two orifice diameters: 40 mm and 20 mm. Nonuniform unstructured grids with a cell characteristic length varying in the range of 0.5-1 mm were used. In going from the 40-mm orifice to the 20-mm orifice, both the size and velocity of the vortex increase, leading to a different regime of interaction with the flame: the vortex only wrinkles the flame front in the 40-mm case (wrinkled regime) and also disrupts the continuity of the front, giving rise to the formation of separate reaction zones (i.e., flame pockets that leave the main front), in the 20-mm case (breakthrough regime). It has been found that the impact of the combustion submodel on LES predictions is strongly dependent on the regime of interaction. Results for the 40-mm orifice are substantially the same, regardless of the presence of the combustion submodel. Conversely, at the wake of the 20-mm orifice, the intensity of the flame-vortex interaction is such that the combustion submodel is strictly needed to reproduce both the qualitative (evolution of the pockets formed and their interaction with the main front) and quantitative (flame speed) characteristics of the flame propagation correctly.