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

The ignition and flame propagation behaviors of a CH₄ diffusion jet flame were studied when dual pulse laser-induced spark discharges were introduced in a mixing layer. Time intervals of 50 nsec 100 μsec and 600 μsec between two laser pulses were evaluated and the results were compared to a single pulse discharge case with the same total laser energy (60 mJ). 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 were generated in a mixing layer of a methane jet a rapid propagation of the flame was observed since the second breakdown enlarged the ignition kernel surfaces generated by the first breakdown. Using the dual pulse with intervals shorter than the electron lifetime scale 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. The shockwave and hot plume induced by the second laser pulse significantly interacted with the density gradient produced by heat release through the combustion.