Ignition and flame propagation by dual-pulsed laser-induced breakdowns

Experimental investigations of laser-induced spark ignition and flame propagation by successive laser pulses were performed in a stoichiometric methane-air mixture at 4 m/s bulk flow and a methane diffusion jet flame (Re = 5500). Time intervals (dt) of 50 ns and 600 μs between two laser pulses were tested, and the results were compared to a single pulse discharge case with the same total laser energy. High-speed schlieren and CH∗ chemiluminescence imaging techniques were used to visualize the ignition and flame propagation. In a premixed flame, the flame propagation speed was enhanced for successive laser-induced spark ignition with a time interval of 50 ns due to the increased energy deposition to the discharge. Successive laser-induced spark ignition with a time interval of 600 μs induced strong pulse-to-pulse coupling that increases the area of the propagating flame, improving flame propagation speed. In a diffusion flame, a rapid propagation of the flame was observed when the dual pulse laser-induced spark discharges (dt = 600 μs) are generated in a mixing layer of a methane jet since the second breakdown enlarges the ignition kernel surfaces generated by the first breakdown.