Scramjet cavity ignition using nanosecond-pulsed high-frequency discharges

Ignition of an ethylene-fueled cavity in Mach-2 flow was performed using nanosecond-pulsed high-frequency discharges (NPHFDs) over a range of pulse repetition frequencies (PRF = 1.5–300 kHz). The energy deposition rate and duration were controlled to be commensurate with the temporal and spatial fluctuations of velocity and fuel concentration within a cavity-based flame holder to enhance the ignition process. Comparison to a traditional capacitive discharge (CD) on an equal-energy basis showed that high-PRF NPHFD operation significantly broadened the ignition envelope: while the CD ignited near-stoichiometric mixtures only (equivalence ratio, Φ = 1.09) the NPHFD operating at PRF = 300 kHz resulted in ignition with Φ = 0.82–1.46. At lean conditions, the NPHFD could achieve comparable cavity ignition times with 5-times lower energy than the CD. Parametric exploration of NPHFD ignition showed that variation of both PRF and number of pulses (N) for the NPHFD were the most significant factors controlling ignition probability (P_ig) and ignition time (t_ig). PRF>5 kHz was required to achieve cavity ignition, which was the threshold for synergy between pulses to produce an ignition kernel. Higher PRF resulted in more pulse synergy within a burst of pulses, and ultimately higher P_ig. These results are similar to those from previous studies of ignition in subsonic flow and correlate well with length/time scales of a fluid volume passing through the inter-electrode gap region. Increased N, which is equivalent to longer discharge duration and higher total energy, increased P_ig for 5 kHz<PRF<50 kHz and decreased t_ig when PRF>50 kHz. Finally, when significant heat release did not occur within 2-3 cavity cycling times after energy deposition, the cavity was far less likely to ignite. The implication is that multiple parameters (i.e., minimum ignition energy, minimum ignition power, t_ig, and characteristic flow cycling time) must be simultaneously considered when developing an ignition method in a representative environment.