Numerical studies of flame acceleration and onset of detonation in homogenous and inhomogeneous mixture

Numerical investigations have been conducted for flame acceleration and transition to detonation in a horizontal obstructed channel with 60 percent blockage ratio filled with hydrogen/air mixture. Both homogeneous and inhomogeneous hydrogen/air mixtures have been considered. The later has a vertical concentration gradient. The density-based solver within the OpenFOAM CFD toolbox developed by the present authors [1] is used. High-resolution grids are facilitated by using adaptive mesh refinement technique, which leads to 30 grid points per half-reaction length (HRL) in the finest region near the flame and shock fronts. The forward and backwards jets which represent Richtmyer-Meshkov (RM) instability, were found to impact on the shock front, resulting in the appearance of a secondary triple point on the initial Mach stem on the flame front. Moreover, since both the forward and backwards jet propagates in the shear layer, some small vortices can be found on the surface of the secondary shear layer, which represents the Kelvin-Helmholtz (KH) instability. Additionally, it has been found that the inhomogeneous (non-uniform) mixtures cause higher shock and flame velocities compared to the homogeneous mixtures concentration. Also, for both homogenous and inhomogeneous mixtures with 30% hydrogen concentration, the onset of detonation occurs within the obstructed channel section, but the homogeneous mixtures show slightly faster flame acceleration and earlier onset.