Flow dynamics of laser-induced breakdown at a fuel–oxidizer interface and its effect on ignition

Abstract

Motivated by experimental observations, we analyze the ignition dynamics following a laser-induced breakdown near a distinct fuel–oxidizer interface using detailed simulations of the flow following a model energy deposition. Cases simulated with a detailed H2-O2 combustion model show the dependence of ignition time, and its success or failure, on the post-breakdown kernel geometry. For asymmetric kernels, ignition can occur by ejections of hot gas that propagate several times the kernel size. The extreme conditions induced near the laser focus lead to near-complete dissociation, allowing heat release by radical recombination to be a primary mechanism by which the ejected gas maintains ignition temperatures as it advects. For kernels close to ehe interface, a pronounced hydrodynamic interaction between the density gradient and expanding kernel results in ignition-suppressing flow that repels hot gas away so completely in some cases that ignition fails. Mechanisms for a wide range of parameters are studied with a four-species reduced combustion model, and we show that this ignition-adverse flow response, particularly salient in hydrogen–oxygen cases, is absent for heavier fuels. For disparate fuel and oxidizer molecular weights, the varied flow response due to the breakdown location is a primary determinant of outcome.