Nanosecond Pulsed Plasma Discharge in High-speed Flow

Nanosecond Pulsed High-Frequency Discharge (NPHFD) plasma has presented advantages in the ignition for challenging applications, such as a scram-jet. It forms three inter-coupling regimes in flowing mixtures: the fully-coupled regime with short inter-pulse time (IPT) and high ignition probability (P_I), the partially coupled regime with intermediate IPT and low P_I, and the decoupled regime where IPT is long, and P_I becomes a function of the pulse number (N) and single pulse ignition probability. In this study, such regimes are investigated experimentally in a high-velocity fuel-air flow (10 – 100 m/s). The NPHFD plasma with approximately 5 – 6 ns full width at half maximum, minimum IPT of 5 ms, maximum discharge voltage of 20 kV, and maximum average power output of 500 W. The deposition energy, N and IPT are varied for parametric study over their effects on the ignition characteristics. Periodic elongations of the plasma arc are observed. The elongation has been shown to follow the residual hot ignition kernel formed from previous discharge pulses. The size and period of the elongation are a function of the inter-electrode gap distance, flow velocity, and gas composition. The deposition energy also changes with such elongation. These significantly influence the inter-coupling regimes, P_I, and ignition kernel characteristics. This study focuses on the effects of such elongation and the mechanism behind such a phenomenon.