Destructive inter-pulse coupling in nanosecond-pulsed high-frequency discharge ignition: Effect of hydrodynamic regimes

This study investigates the impact of inter-electrode gap distance (d) on the ignition process using Nanosecond-pulsed high-frequency plasma discharges (NPHFD) in methane-air mixtures. The study categorizes the inter-pulse coupling into three regimes: fully coupled, partially coupled, and decoupled. In the fully coupled regime, P_I reaches its maximum due to overlapping discharge radicals, while the partially coupled regime sees a decrease in P_I due to destructive interactions between flame kernels. The decoupled regime shows less interaction, leading to a stable P_I estimated statistically from a single pulse P_I. The research also explores two hydrodynamic discharge regimes: the toroidal regime (d ≤ 3.5 mm), characterized by significant local vorticity and a toroidal shape, and the diffusive regime (d > 3.5 mm), where the kernel appears to be more cylindrical. The study's findings highlight that the toroidal regime exhibits a distinct kernel shape, significantly impacting ignition efficacy. Furthermore, the study's results indicate that small d leads to a significant decrease in P_I due to heat loss to the electrodes, while large d allows for better kernel growth and higher P_I. The transition from fully coupled to partially coupled regimes occurs at shorter inter-pulse times for small d. Additionally, in multi-pulse discharge scenarios, the P_I curves distinctively mark the transition between the toroidal and diffusive regimes, emphasizing the importance of inter-pulse interference in ignition efficacy. Overall, the research provides a comprehensive understanding of the influence of discharge parameters on ignition mechanisms in NPHFD systems, contributing valuable insights for developing more efficient and reliable ignition systems in combustion applications.