Plasma assisted flame ignition of supersonic flows over a flat wall

A nanosecond pulsed plasma discharge located between two fuel jets is used to ignite and hold jet flames in supersonic crossflows, without the use of additional devices (e.g., cavities or backsteps) for flame holding. The fuel injection nozzles and discharge electrodes are mounted flush with the surface of the flat wall adjacent to the freestream flow. The nonequilibrium plasma is produced by repetitive pulses of 15 kV peak voltage, 10 ns pulse width and 50 kHz repetition rate. Sonic or subsonic fuel jets (hydrogen and ethylene) are injected into a pure oxygen freestream of Mach numbers Ma. = 1.7-2.4. The shockwave/flow structures induced by the fuel jets and the OH radical distribution resulting from combustion are characterized by Schlieren photography and planar laser induced fluorescence imaging, respectively. A configuration combining an upstream subsonic oblique jet and a downstream sonic transverse jet is shown to provide adequate flow conditions for jet flame ignition assisted by the plasma discharge. The experimental results are interpreted using a simple model by which the pulsed plasma serves as a source of reactive radicals added to a flammable gas mixture.