Laser-Induced Plasma Ignition Experiments in a Direct-Connect Supersonic Combustor at Mach 3

A new direct-connect supersonic combustor has been designed and tested for the study of plasma-assisted combustion in the ACT-II supersonic combustion tunnel. Laser-induced plasma ignition was investigated in an ethylene-fueled cavity flame holder at Mach 3. Combustor design, geometry, and flow characteristics are discussed in the first part of the paper. The supersonic combustor includes a cavity flame holder, where ethylene fuel is injected, mixed with entrained air, and ignited. Twoinjector configurations are included, one horizontal and one vertical, which were designed in such a way as to enhance the cavity flow recirculation. Two different plasma ignition mechanisms were considered: 1) a laser-induced plasma located at the center of the cavity, and 2) a spark plug placed on the bottom surface of the cavity. The diagnostics tools tested to characterize the flow include time-resolved pressure measurements, high-speed schlieren visualization and OH* chemiluminescence visualization, and instantaneous planar laser-induced fluorescence of the CH radical. The second part of the paper discusses the results of the comparison between horizontal and vertical fuel injection. The larger backpressurization of the isolator and a more uniform OH* distribution within the cavity suggest a greater heat release and combustion efficiency when the horizontal jet is used.