Scramjet cavity ignition using nanosecond-pulsed high-frequency discharges

Ignition in high-speed air-breathing combustors, such as scramjets, can be challenging because the high levels of turbulent fluctuations create unsteadiness in flow properties and fuel concentration at the point of energy deposition. Therefore, an ideal ignition device should deposit energy on a time scale that spans the fluctuations and/or ignites a wide range of fuel concentrations. To address this challenge, nanosecond-pulsed high-frequency discharges (NPHFD) over a range of pulsation frequencies (20-300 kHz) were used to ignite an ethylene fueled cavity in Mach-2 flow. Ignition tests were performed for a range of cavity fueling rates (equivalence ratios) by comparing the NPHFD with a traditional capacitive discharge (10-35 μs duration) using the same total energy deposition. Broadband chemiluminescence at 40 kHz sampling rates was used to interrogate the cavity ignition process. Little differences in ignitability were found using the NPHFD at 20 kHz and the capacitive discharge. For higher pulsation rates the ignitable range was broadened significantly, with 300 kHz igniting 55-105 SLPM cavity fueling rates, compared to only 75 SLPM for the capacitive discharge. Additionally, the cavity ignition process was more rapid using the NPHFD, with a 30% improvement in ignition time.