Ignition enhancement by dual-pulse laser-induced spark ignition in a lean premixed methane-air flow

Ignition of fuel-lean premixed methane-air flow was investigated using single-pulse laser-induced spark (SPLIS) and dual-pulse laser-induced spark (DPLIS). Ignition enhancement was quantified with ignition probability in flow speeds and equivalence ratios of 3.75m/s-12.5m/s and 0.53-0.57, respectively. The range of total laser pulse energy per ignition event was 20-30 mJ. For DPLIS, the energy per pulse was half of the total energy and the time interval between pulses varied from 10ns to 100ns where pulse-to-pulse coupling increased the spark surface area. Broadband chemiluminescence within the visible wavelength region was used to determine ignition of the combustible mixture. The ignition probability increased with velocity for both SPLIS and DPLIS as more fuel-oxidizer mixture was exposed to the ignition source. For a fixed total energy, ignition probability was higher for DPLIS than SPLIS particularly as the equivalence ratio was increased. Depending on the flow velocity, a change in the total energy determined whether SPLIS or DPLIS had a higher ignition probability at a given flow velocity. The study showed that using DPLIS to ignite fuel-lean combustible mixtures can enhance ignition compared to SPLIS particularly at low total laser energy and high-speed flow conditions.