Shock focusing and detonation initiation at a flame front

Multidimensional numerical simulations were performed to study the interaction of focused shock waves and a flame front leading to detonation initiation. The fully compressible Navier-Stokes equations, coupled with a chemical-diffusive model for energy release and conversion of fuel to product in a stoichiometric hydrogen-air mixture, were solved using a third-order method on a dynamically adapting mesh. Preliminary simulations of deflagration-to-detonation transition (DDT) in an obstructed channel, when compared to previous experiments, point to a DDT scenario where detonation initiation arises from multishock focusing at a flame front. A detailed examination of an idealized problem showed two mechanisms of detonation formation: (1) direct detonation initiation triggered at the collision spot by focusing shocks at the flame front, and (2) focusing of relatively weak shocks leading to a delayed transition to detonation through the reactivity-gradient mechanism. Comparisons between the detailed analysis of shock-focusing and experimentally observed DDT phenomena suggests that shock focusing plays an important role in the occurrence of DDT for this problem. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.