Effects of pulsation frequency and energy deposition on ignition using nanosecond repetitively pulsed discharges

The effects of pulse repetition frequency (PRF) and energy deposition was investigated using nanosecond repetitively pulsed discharges in a constant volume combustion chamber. Quiescent lean propane-air mixtures (equivalence ratio of 0.7) at 0.2 MPa were ignited using fixed total energy deposition and number of pulses across the range of PRF from 2 to 90 kHz. Individual pulse energies were measured by electrical probes, and the ignition time was quantified through the use of high-frame-rate intensified imaging of the chemiluminescence. For fixed inter-electrode gap and constant total energy, the ignition time was minimized at a certain pulsation frequency, with higher PRF being detrimental and resulting in longer ignition delay times. Decreasing the total energy deposited in the mixture shifted the minimum ignition time to higher PRF. On the contrary, increasing the gap distance at constant total energy allowed for the same minimum ignition time to be achieved at significantly lower PRF. The minimum ignition delay times for fixed inter-electrode gap distance were similar for a given amount of mean energy deposited per unit time. The effect of energy deposition, PRF, and inter-electrode gap was attributed to the competition between characteristic recirculation time from the discharge-induced flowfield and the inter-pulse time.