On pulse energy and energy distribution for ignition of flowing mixtures

Abstract

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. These effects were considered in the context of three pulse-coupling regimes, occurring at different inter-pulse time (IPT) conditions: fully-coupled (short IPT, high PI), partially-coupled (intermediate IPT, low PI), and decoupled (long IPT, variable PI). High-speed schlieren imaging was used to analyze ignition probabilities (PI) and kernel growth characteristics. Two approaches were utilized to explore the effects of energy distribution. First, four nominal levels of energy per pulse (Epp) were explored across a wide range of IPTs using a fixed number of pulses. In the fully-coupled regime, Epp did not show any influence over PI or kernel growth rate. Outside of the fully-coupled regime, PI drops to 0 for the lowest level of Epp, whereas the highest level of Epp has PI of 1 for the entire range of IPTs. For intermediate levels of Epp, a partially-coupled regime characterized by significant drops in PI is observed, with recovering PI in the decoupled regime at longer IPT. Second, the energy per pulse (Epp), inter-pulse time (IPT), and the number of pulses are varied such that their individual effects could be isolated while maintaining constant total deposited energy or total discharge duration. In this exploration, IPT is found to be the driving parameter determining the inter-pulse coupling regimes and PI. Lastly, single kernel analysis clarifies 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.