Characterization of successive laser induced plasma ignition in an ethylene fuelled model scramjet engine

Ignition of ethylene fuel by means of single and successive laser induced plasma has been conducted in a supersonic direct-connected test facility with incoming flow of Ma2.52, and total temperature of 1486K. A wall cavity was designed as the flameholding device. Ethylene was injected 10mm upstream of the leading edge of the cavity flameholder through a Φ2.0mm orifice. The fuel injection pressure was maintained at 3MPa and the corresponding global equivalence ratio was 0.17. Successive laser induced plasma was obtained by focusing a dual-cavity Nd: YAG laser beam with a wavelength of λ =532nm using plane-convex lens. The time interval between the two successive laser pulses and the incident laser energy can be varied. The ignition position was chosen at the place 5mm above the cavity wall and 45mm downstream of the cavity leading edge in the centerline of model test section. The flame kernel generation and its development were observed and recorded by a high-speed camera. The results indicated that the successive laser induced plasma ignition with a time interval of 15ns can achieve successful ignition with less total incident laser energy, which was superior to the single laser induced plasma ignition. Higher incident laser beam energy resulted in increase of the initial flame kernel size and shortening of the ignition time. The time interval between the two successive laser pulses should be less than 50μs in the present study to produce a condition that the two induced flame kernels can merge together and propagate into a stable combustion flame.