Dual-pulse laser-induced spark ignition and flame propagation of a methane diffusion jet flame

Abstract

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