Spark and flame kernel interaction with dual-pulse laser-induced spark ignition in a lean premixed methane–air flow

The growth rate of flame kernels from dual-pulse laser-induced spark (DPLIS) ignition was compared to that from single-pulse laser-induced spark (SPLIS) ignition in a flowing premixed methane–air mixture. The flow speed ranged from 3.75 m/s to 12.5 m/s with equivalence ratios of 0.57–0.67. The pulse energy ranged from 10 mJ to 30 mJ, with each DPLIS pulse having half of the SPLIS pulse energy. High-speed schlieren imaging captured ignition and flame propagation. Two regimes were identified depending on the location of the first laser-induced spark (LIS) kernel: direct interaction and flame combination. The direct interaction regime occurred when the second LIS immediately combined with the first LIS. In the direct interaction regime, DPLIS ignition had faster flame kernel growth rates than SPLIS ignition when the second LIS occurred near the boundary of the first LIS expansion prior to the first LIS igniting the fuel–air mixture. The flame combination regime occurred when each LIS separately ignited the fuel–air mixture and then combined into a single flame kernel. DPLIS ignition had faster flame kernel growth rates in the flame combination regime than did SPLIS ignition when each DPLIS had fully developed into a self-sustaining flame.