Characteristics of inter-pulse coupling regimes during nanosecond pulse discharge with elevated single-pulse energy in flowing mixtures

A two-dimensional plasma fluid model for the non-equilibrium nanosecond repetitively pulsed (NRP) plasma-assisted ignition/combustion (PAI/PAC) computation is established by solving two-term Boltzmann and fluid equations in OpenFOAM. The effects of inter-pulse coupling on ignition in a flowing CH4/Air mixture with low and high energy per pulse (Epp) are investigated with a fully analytic Jacobian for simulation speedup. Three inter-pulse coupling regimes are reproduced and examined with varied pulse repetition frequency (PRF) and Epp under a wide range of inlet flow velocities. The relationship between Da number (an analogous turbulent Damkohler number) and pulse interaction regime (PIR) number under, varied velocities and PRFs, is obtained to characterize different regimes. Under low Epp, the fully coupled cases with a high PRF of 100 and 80 kHz have a higher ignition performance, while the partially coupled (20 kHz) and decoupled (10 kHz) cases fail to ignite the 20 m/s flowing mixture. It is explained that more fresh fuel is injected into the discharge zone with PRF decreasing, and hydroxyl (OH) is transported out of the discharge zone before the subsequent discharge, leading to the weakened OH accumulation effect. By increasing Epp, self-sustained flames are established in both partially coupled and decoupled cases with large flame areas, elevated flame kernel growth rates, and low maximum temperatures compared with the fully coupled case. The comparison of spark equilibrium plasma and NRP nonequilibrium plasma shows that the elevated Epp dominates the ignition when the PRF decreases to the decoupled regime. Although there is a lack of OH accumulation effect, the kinetic impact of the collision of the electron and excited states with oxygen and fuel enhancing the formation of O and H atoms by NRP discharge contributes to the higher ignition ability. However, increasing Epp in the fully coupled regime has an insignificant effect on ignition for almost the same flame areas among NRP and spark discharges. In a nutshell, the combination of lower PRF and elevated Epp has a significant effect on flame initiation and propagation due to the longer discharge time and non-overlapping OH, while excessive OH accumulation for the higher PRF in the fully coupled regime is unfavorable for flame propagation.