Laser-Induced Plasma Ignition Experiments in a Scramjet Inlet

Abstract

An experimental investigation on the behavior of laser-induced plasma (LIP) ignition for scramjets has been conducted. Two different ignition methodologies are examined which are referred to as shear-layer-LIP-ignition and fuel-jet-LIP-ignition. A two-dimensional scramjet model with inlet injection, fueled with hydrogen gas, is used in the study. The experiments were conducted in the T-ADFA shock tunnel using a flow condition with a specific total enthalpy of 2.7 MJ/kg and a Mach 9 freestream. In the shear-layer-LIP-ignition experiment, LIP is formed in the shear-/mixing layers immediately downstream of four transversely orientated port hole injectors. In the fuel-jet-LIP-ignition experiment, LIP is formed inside the sonic throat of a single fuel injector. The influence of using fuel diluted with a plasma buffer gas (8% Ar, 92% H2) to extend plasma lifetimes is also investigated. The planar laser-induced fluorescence technique on the hydroxyl radical (OH-PLIF) is used to yield qualitative concentration images of the hydroxyl combustion species allowing us to compare the two different ignition methodologies in their effectiveness. Time-integrated schlieren images have been obtained and superimposed on the OH-PLIF intensity maps to determine the effect of the flow structure on the hydroxyl concentration. The broadband self-luminescence signal of the LIP in the early stages immediately after initiation is also recorded. Both ignition techniques are found to be effective in terms of post-LIP hydroxyl production. The fuel-jet-LIP-ignition technique allows lower laser energies to be used but requires a plasma buffer to compete with the shear-layer-LIP-ignition technique.