An exploration of inter-pulse coupling in nanosecond pulsed high frequency discharge ignition

A parametric exploration of the dynamics of ignition using nanosecond pulsed high frequency discharges (NPHFD) in flowing mixtures of methane and air is conducted to determine the “inter-pulse coupling” effect of a burst of high frequency discharges in the pulse repetition frequency (PRF) range of 1-300 kHz. The impacts of PRF, number of pulses, equivalence ratio, discharge gap distance, and flow velocity are quantified in terms of ignition probability and minimum ignition power, and schlieren images of ignition kernel development are presented. Three regimes of inter-pulse coupling are found for different values of PRF: fully-coupled, partially-coupled, and decoupled. Each regime is characterized by distinct ignition probabilities and kernel structures. The fully-coupled regime occurs for the highest PRF and exhibits complete ignition of the kernel and the highest ignition probability. The partially-coupled regime occurs for intermediate PRF and exhibits only local ignition of portions of the kernel and has the lowest ignition probability. The decoupled regime occurs for the lowest PRF and exhibits multiple non-interacting ignition events with ignition probability being a linear function of the number of pulses. The effect of equivalence ratio is found to increase or decrease the ignition probability without altering the structure of inter-pulse coupling. The electrode gap distance determines the degree of heat and active species quenching to the electrode surfaces, and shifts the transition between the partially-coupled and fully-coupled regimes to higher PRFs as the gap distance is decreased. Flow velocity determines the degree of convective heat loss, with lower velocities increasing the ignition probability and altering the structure of inter-pulse coupling in a non-monotonic fashion.