Nanosecond Pulsed Plasma Discharges in High-speed Flow: Plasma Morphology and Inter-coupling Effects

Nanosecond Pulsed High-Frequency Discharges (NPHFD) present a significant potential for applications in high-speed flows, where ignition poses substantial challenges. This study examines the effects of flow velocity (U = 10 – 80 m/s), pulse number (10 & 100), inter-pulse time (τ = 0.5 – 1000 ms), and plasma shape on ignition probability (P_I) and flame kernel development. Key findings highlight plasma elongation, wherein hot and ionized gas from preceding discharges facilitates subsequent discharge elongation. As the flame kernel moves downstream with the bulk flow, "snap back" events are observed when discharged plasma prefers the fresh gas between electrodes instead of elongating. The cumulative effects of τ, U, discharge voltage, and inter-electrode gap distance influence the frequency and maximum length of the elongation. This phenomenon enhances the understanding of pulse-kernel interactions, promoting successful ignitions under previously unfavorable conditions by mitigating the destructive pulse-kernel interactions that reduced P_I. The study also observed significant declines of P_I with increasing U. These findings offer novel insights for optimizing plasma-assisted ignition processes in high-velocity environments.