Flame kernel formation behaviors in close dual-point laser breakdown spark ignition for lean methane/air mixtures

Ignition behaviors of close dual-point laser breakdown spark ignition were investigated experimentally for methane/air mixtures at 0.1 and 1.0 MPa in a constant volume combustion vessel. Absorbed energy was measured from the difference between incident and transmitted laser rays using two joule meters as the ignition energy, and the behaviors of the initial flame kernel were observed with Schlieren photography using a high-speed video camera. First, the ignition behaviors of a single-point laser breakdown spark ignition were investigated. The results indicated that the effect of the focusing lens on the minimum ignition energy (MIE) was limited in terms of the absorbed energy. Although the MIE at 1.0 MPa was lower than that at 0.1 MPa near the stoichiometric equivalence relationship was reversed near the lean limit. For lean mixtures, local quenching of the initial flame kernel was clearly observed including third lobe region especially at 1.0 MPa. In the case of the close dual-point sparks for an equivalence ratio of 0.6, formation of a third lobe was suppressed. When the dual spark gap was large, two flame kernels were formed as observed in the case of the single spark. An optimal gap in which the absorbed energy was minimal for successful ignition existed depending on the pressure, although the magnitude of the associated energy was not so different from that in the case of the single spark. However, the growth rate of the initial flame kernel formed by the close dual sparks was considerably higher than that formed by the single spark, especially at 1.0 MPa. Enhancement of the flame kernel development due to the close dual spark was clearly observed.