Laser-induced breakdown; Ignition; Diffusion flame; Schlieren imaging;INDUCED BREAKDOWN SPECTROSCOPY; AIR MIXTURES; ENERGY

An experimental investigation was performed to study the ignition and flame propagation behaviors of a methane diffusion jet flame (Re = 5500) when dual pulse laser-induced spark discharges were introduced in a mixing layer. Time intervals (dt) of 50 ns, 100 mu s, and 600 mu s between two laser pulses were tested, and the results were compared to a single pulse discharge case with the same total laser energy (60 mJ). The laser-induced breakdown was introduced to the mixing layer at 9.5 jet diameters downstream from the jet exit. The chemical delay time scale under the flow condition was approximately 350 mu s (t(cd)). The disturbances in density fields generated by laser-induced breakdowns and chemical reactions were visualized using a high-speed schlieren imaging technique. The visualization at stationary air showed that interactions between two laser-induced breakdowns increased the surface area of hot plumes, but the effects of the interactions diminished when the breakdowns were introduced in a non-reacting air jet. The effects of the flow on the hot plumes prevailed the interaction effects between two breakdowns in non-reacting air jet condition. However, when the dual pulse laser-induced spark discharges (dt = 600 mu s, dt/t(cd) = 1.71) were generated in a mixing layer of a methane jet, a rapid propagation of the flame was observed since the second breakdownenlarged the ignition kernel surfaces generated by the first breakdown. The results showed that using the dual pulse with intervals shorter than the electron lifetime scale (under 200 ns) or longer than the chemical delay time scale could be beneficial in enhancing ignition and flame propagation processes due to the increased energy deposition or hot surface area, respectively. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.