Effect of pulse repetition rate on filamentary discharge assisted low-temperature ignition in methane-air flows

Nanosecond repetitively pulsed (NRP) discharges are an efficient and fast source of radicals and heat and can ignite fuel-air mixtures in extreme conditions. We present pulse-to-pulse coupling in NRP discharges at 1 to 30 kHz pulse repetition rate (PRR) and its influence on ignition of methane-air flows. Contrary to NRP discharges in pin-to-pin configurations, our DBD electrode configuration allows pulse-to-pulse coupling in the entire studied PRR range while maintaining plasma in a non-equilibrium regime. We use three diagnostics tools for plasma and combustion parameters characterization: 1) High-speed intensified imaging, 2) phase-locked OES, and 3) electrical characterization. The effect of the number of pulses per burst and the PRR on ignition have been evaluated. We found that 2 to 5 pulses per burst are required to ignite the mixture depending on energy coupling into plasma per pulse. Moreover, the PRR affects the ignition kernel growth rate, but the final kernel size only depends on the number of pulses per burst. We estimate pulse-to-pulse plasma gas temperature and effective E/N using OES. We observe an increase in plasma pulse-to-pulse gas heating with increasing PRR even though energy coupling per pulse is similar which we attributed to filament contraction.