Analyzing the ignition differences between conventional spark discharges and nanosecond-pulsed high-frequency discharges

Abstract

New methods of energy deposition using nanosecond-pulsed high-frequency discharges (NPHFD) have shown promise in igniting fuel-lean conditions. This work compared an NPHFD ignition system to a conventional, capacitive discharge system in a flowing environment under matching total energy and average power conditions. The results showed that, in every case, matching the average power and deposited energy between the two systems resulted in similar trends in kernel radius growth, time to minimum growth rate, and kernel radius at which minimum growth rate occurs. These results were used as a baseline for comparison for the lower average power NPHFD test conditions. Decreasing the power of the NPHFD system while maintaining the total energy deposited allowed kernel growth to be aided by flow advection, resulting in a ∼38% increase in the streamwise radius as compared to the baseline. The 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 respective values for the baseline condition. This behavior is due to the long duration of the discharge and the low energy density per unit volume in the fluid, respectively. The benefit 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. Conversely, for kernels 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.