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 dis-charges (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 ap-proaches 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 maintain-ing 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.& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.