Nanosecond-Pulsed High-Frequency Discharge Ignition at Scramjet-relevant Conditions: Effects of Velocity and Temperature

Ignition in high-speed systems presents unique challenges attributed to the short residence time of the gas in the combustion chamber, which, combined with high turbulence intensity, often leads to rapid quenching of the ignition kernel and prevents successful ignition. The nanosecond-pulsed high-frequency discharge (NPHFD) ignition technique has emerged as a promising solution to address these limitations. NPHFD stands out for its ability to fine-tune ignition parameters such as pulse repetition frequency (PRF), number of pulses (N), and discharge voltage, which allows for control over the quantity and duration of energy deposition. This study aims to conduct a parametric exploration of the dynamics of wall-based ignition in flowing mixtures of ethylene (C₂H₄) and air with NPHFD employment. The primary objectives are to examine ignition probability (P_IG) under high-speed and high-temperature flow, emulating scramjet cavity conditions, and to evaluate how it compares to full-scale scramjet combustor results at matched conditions. PIG of the wall-based system was studied as a function of varying PRF, temperature (T), velocity (U), equivalence ratio (Φ), and N, providing a more comprehensive insight into flow and discharge behavior contributing to ignition than has been explored in any previous work. The kernel was captured using a high-speed schlieren imaging system and a high-speed infrared camera equipped with a CO₂ filter. Results display that elevated T and N variation affects P_IG drastically, resulting in successful ignition occurring at lower Φ and PRF than at ambient temperature; however, increasing velocity presents an opposite effect causing P_IG to decrease. These competing effects result in ignition limits significantly different than low temperature and quiescent environments in which the limits are typically measured. Through this analysis, a deeper understanding of the NPHFD ignition process and its characteristics can be obtained allowing to understand how the wall-based configuration can match scramjet setup.