Laser-Induced Plasma Ignition Studies for Scramjet Propulsion

Abstract

An experimental investigation of the characteristics of laser-induced plasma (LIP) ignition for scramjet propulsion is presented. This work demonstrates for the first time, that LIP can be used to promote the formation of hydroxyl radicals (OH) in a hydrogen fuelled, hypersonic scramjet. The experimental campaign was conducted in the free-piston shock tunnel T-ADFA at the Australian Defence Force Academy. A rectangular-duct scramjet model with inlet injection, is used in the study. Two ignition methodologies are investigated. A Q-switched laser is used to generate LIP either in the shear layers (referred to in this document as shear-layer LIP ignition) at the injector site or inside the sonic throat of a single fuel injector (fuel-jet LIP ignition). Time-integrated chemiluminescence imaging of OH was used to investigate autoignition and radical-farming characteristics. The laser-induced fluorescence technique on the nitric oxide molecule (NO PLIF) is used for flow visualisation as well as two-line thermometry measurements. The temporal evolution of the LIP ignition process is visualised using the OH PLIF technique. The influence of using hydrogen fuel diluted with argon to serve as a plasma buffer, extending plasma lifetimes, is also investigated. The broadband self-luminosity signal of the LIP in the early stages immediately after laser initiation is recorded and used in combination with blast wave theory to obtain several conditions through which the LIP can be approximated in future computational fluid dynamic simulations of the LIP ignition process. The evolution of laser-induced plasmas in hydrogen, argon and air has been investigated as a function of pressure and laser energy using time-resolved schlieren visualisations in a gas cell. Time-resolved plasma spectroscopic temperature measurements have been conducted to investigate the effect of LIP confinement in a cylindrical reflector.