Characteristic of rich-mixture ignition kernel driven by nanosecond repetitive pulsed discharge

Plasma-assisted ignition is a promising technology to improve engine performance. Nanosecond repetitive pulsed discharge is widely used in plasma-assisted ignition owing to its chemical activations and thermal and hydrodynamic expansions. However, the influence of ultrafast heating and hydrodynamic effects on the development of the rich-mixture ignition kernel is largely unknown. The present study aims to illustrate these effects using electrical and schlieren measurement. The number and the frequency of discharge pulses are exactly controlled to establish the relationship among the discharge energy, frequency, and rich-mixture ignition-kernel characteristics. The evolution of the ignition kernel in the early stage is mainly dominated by the discharge energy and frequency, i.e., a greater energy and a higher frequency yield a larger ignition kernel. Moreover, the influence of both the energy and frequency on the ignition kernel gradually disappears as the ignition kernel develops. According to the experimental data and theoretical analysis, the calculated laminar burning velocity is 0.319 m/s with a Markstein length of 13.43±0.11 cm when the voltage is 5.9 k V, the frequency is 3 k Hz, and the equivalence ratio is 1.3. This result indicates that the rich-mixture flame is stable in the early stage of ignition.