Ignition kernel development in a reactive flow for nanosecond-pulsed high-frequency and DC arc discharges

The development of energy efficient combustion systems is a critical technical objective in the engine and power generation industries. This pursuit often prompts using conditions at the limits of capability, dictating the need to expand the envelope of robust and reliable operation. Nanosecond-pulsed high-frequency discharges (NPHFD) have proven to be effective in extending ignition limits in both quiescent and flowing environments. However, limited research has been conducted comparing the kernel growth behavior in a flowing environment of a train of discrete nanosecond discharges to that of a conventional DC arc discharge. Therefore, this work focused on comparing these two types of discharges by matching the total energy deposited, total deposition time, and average power. For each system, three average power conditions (100, 180, and 400 W) were tested, each including five total discharge times (0.05, 0.15, 0.25, 0.50, and 1.00 ms) at two inter-electrode gap distances of 1 and 2 mm. The 1 mm gap distance results showed that the NPHFD system produced larger ignition kernel areas than the conventional system at the highest average power (therefore, highest pulsation frequency) setting. At all other average power conditions, the NPHFD system produced equivalent or smaller ignition kernels than the conventional system due to a lack of pulse-to-pulse coupling. At the 2 mm inter-electrode gap distance, the NPHFD ignition system produced larger kernels at all conditions except at the lowest average power and discharge duration setting. At the highest average power and longest discharge duration setting, the NPHFD kernel was nearly twice as large in area as the kernel produced by the conventional system at 5 ms after discharge. The larger kernel area created by the NPHFD system at the larger gap distance, higher average power, and longer total discharge duration conditions could be explained by the high level of pulse-to-pulse coupling and discharge-induced jetting behavior. The jetting behavior was a function of electrode polarity and mainly extended the reactive region of the kernel in the spanwise direction.