Interferences on flame kernel development by nano-second repetitively pulsed discharges (NRPD) in ignition processes of CH4-air mixture

Nanosecond repetitively pulsed discharge (NRPD) is a promising ignition technique, which is distinguished among methods that generate non-equilibrium plasma using a high reduced electric field and a strong discharge-induced flow. In this study, the NRPD ignition process was investigated using quiescent CH 4 -air ( ϕ = 0.75) mixtures and pure air. To investigate the effect of the NRPD-induced flow formed in high input energy per pulse, the discharge gap distance ( d gap ) and pulse repetition frequency (PRF) were set as variables. The measurement of the ignition probability ( P ig ) and simultaneous imaging of Schlieren and chemiluminescence provided knowledge about the ignition process and performance of NRPD ignition. The results showed a non-monotonic trend in P ig versus PRF, which can be attributed to flow induced by a strong discharge pulse. Additionally, a significant mismatch between the Schlieren-based kernel region and the residence of chemiluminescence was observed. This can be associated with a unique kernel formation and propagation process in which a flame kernel was initiated apart from the discharge gap. This process was prone to occur when the d gap was smaller than 1.4 mm and PRF was lower than 10 kHz. When a heat loss to the electrodes is high or PRF is low, the flame kernel could be partially quenched between the electrodes by the heat loss, resulting in displaced, vulnerable flame kernel initiation. In this process, flow induced by subsequent pulse could have a negative interference phenomenon, destroying these vulnerable flame kernels. Therefore, a PRF of 5 kHz, which does not offer sufficiently high chemical activeness but triggers the pulse-to-kernel interference phenomenon, had difficulty in stable ignition. However, a positive correlation was observed between flame kernel development and PRF in successful ignition cases. This finding clarifies that the initial flame kernel development, namely the initial flame kernel size does not necessarily correlate with ignition probability in NRPD ignition.