On Pulse Energy and Energy Distribution for Ignition of Flowing Mixtures

The effects of energy distribution on ignition during a burst of nanosecond-pulsed high-frequency discharges (NPHFD) are investigated in methane-air mixtures at a nominal flow velocity of 5.7 m/s, equivalence ratio of 0.61 and inter-electrode gap distance of 2 mm. Different levels of energy per pulse (Epp) have been tested across a wide range of inter-pulse times (IPT), resulting in variation of the partially-coupled regime (a set of conditions with low ignition probability). Epp, IPT, and number of pulses (N) are varied such that the individual effects could be isolated from total deposited energy or total discharge duration. These effects were considered in the context of three pulse-coupling regimes: fully-coupled (short IPT, high PI), partially-coupled (intermediate IPT, low PI), and decoupled (low IPT, variable PI). In the fully-coupled regime, for a fixed number of pulses, Epp did not show any influence on PI or kernel growth rate. Outside of the fully-coupled regime, ignition probability drops to 0 at low Epp, whereas at high Epp, PI is 1 for the entire range of IPT. For intermedium Epp, a partially-coupled regime is observed, with recovering probability in the decoupled regime at longer IPT. Furthermore, single kernel analysis discovers the effect of destructive kernel interactions in the partially-coupled regime, which, against intuition, results in increasing minimum ignition power for higher Epp. In conclusion, high frequency low energy discharge pulses can be the optimal ignition method for NPHFD ignition with high PI and optimal energy efficiency.