Ignition and flame propagation enhancement by dual-pulsed laser-induced breakdown

Ignition probability and flame growth rates of single and dual-pulse laser-induced breakdown were experimentally investigated in a turbulent premixed methane-air flow to examine enhancement by dual-pulse at fuel-lean conditions. The ignition and flame propagation was visualized with high-speed schlieren imaging. Successful ignition was confirmed by CO₂ filtered infrared imaging. Experiments were performed in a 3.8 cm by 3.8 cm square cross-section wind tunnel with a bulk velocity 10 m/s, equivalence ratios of 0.45 to 0.6 and total laser energy of 25 mJ per pulse. Ignition probability enhancement by dual-pulsed laser-induced breakdown with time intervals in the tens of nanosecond ranges occurred in premixed flows with equivalence ratios of 0.45 to 0.5 where pulse-to-pulse energy coupling enlarged the spark area and increased the available ignition energy. Dual-pulsed laser-induced breakdown with time intervals between pulses of hundreds of microseconds had increased flame growth rates in premixed flows with equivalence ratios 0.5 to 0.6 where conditions were sufficient for the first breakdown to ignite the mixture before arrival of the second breakdown. The hot plume induced by the second breakdown interacted with the ignition kernel induced by the first breakdown increasing the flame surface area and the flame growth rate.