Simulation of Laser-Induced-Plasma Ignition in a Hypersonic Crossflow

Abstract

A numerical study of a laser-induced-plasma ignition system in a scramjet-like geometry is conducted. Simulations are based on experiments in which hydrogen is injected into hypersonic air-crossflow, compressed by a 9 • ramp from a freestream Mach number of 8 to below hydrogen autoignition conditions. Ignition of the jet is conducted using a laser pulse, modelled in the simulations as a small region of high temperature and pressure corresponding to the energy input measured in the experiments. To capture the highly unsteady nature of the fuel-air interface and its coupling with the dynamics of the expanding laser spark, Wall-Modelled Large Eddy Simulations are used to resolve the majority of turbulent motion. Detailed non-equilibrium thermochemistry models are included in the calculations. Simulations have revealed that the cloud of hydroxyl radicals detected in the experiments is created by the blast wave which forms from the expanding plasma kernel merging with the jet's bow shock. This event raises the temperature and pressure in the upstream mixing layer and spontaneous ignition of the fuel is observed. Shortly after this ignition, combustion ceases due to hostile conditions for flame anchoring. Downstream of the jet, the simulations suggest that the plasma kernel remains can initiate some radical formation in the wake region, but no stable flame forms due to nominally low pressures in the experimental condition studied.