An investigation on kernel growth variations between conventional spark discharges and nanosecond-pulsed high-frequency discharges

Depositing energy using nanosecond-pulsed high-frequency discharges (NPHFD) has been shown to lead to successful fuel-lean ignition. Despite these observations, questions remain regarding how the NPHFD ignition system will perform against the conventional ignition system on shorter timescales and in a flowing environment. This work provides a comparison between the NPHFD ignition system and a conventional, capacitive discharge system in a flowing environment where the total energy deposited, and average power is matched. The results show that matching these characteristics result in similar trends in radius growth, time to minimum growth rate, and radius at which minimum growth rate occurs between the two systems. In utilizing these results as a baseline, it was found that decreasing the average power of the NPHFD system while maintaining the total energy deposited resulted in a ~38% increase in streamwise radius due to advective effects. This larger kernel size comes at the expense of the kernel taking ~20% longer to transition to a self-propagating flame that occurs at a radius that is ~72% larger than the baseline condition. This behavior can be explained by the long duration of the discharge and the low energy density per unit volume in the fluid. Ultimately, the convenience of the larger kernel size comes at the cost of reliability. Therefore, in combustor conditions with strong external quenching physics, depositing the most energy in the shortest time will be optimal in preventing kernel extinction. Conversely, for kernels developing in the presence of mild turbulence, the average power can be decreased, and flow advection can be utilized to grow the kernel over a longer duration without risk of extinction.