Study of nanosecond pulsed high frequency discharge ignition in a flowing methane/air mixture

Nanosecond pulsed high frequency discharges are applied to a flowing mixture of methane and air to examine the effects of the plasma characteristics on ignition. The energy deposition rate and the total energy depostition were varied in a mixture with fixed equivalence ratio (φ = 0.6), flow velocity (U = 10 m/s), and electrode geometry (pin-to-pin, 2 mm gap). The probability of ignition and the ignition kernel growth rate were monitored, and the influence of “inter-pulse coupling” was determined. Results indicate that ignition events may fall into one of three regimes: fully-coupled, partially-coupled, and decoupled. It was found that the fully-coupled regime, occurring at the highest pulse repetition frequencies (> 10 kHz), has the highest probability of ignition. In addition, for fixed total energy deposition, the lowest pulse repetition frequency within the fully-coupled regime was found to produce the fastest kernel growth rate. It was found that the effect of increased energy deposition via the application of additional discharge pulses always led to higher igntition probability and faster kernel growth rate, but may not achieve 100% ignition probability in the partially-coupled regime.