Ignition in High-Speed Flows

Ignition in flowing environments is crucial for combustion engine design, impacting performance, efficiency, and emissions. Despite its importance, this area is underexplored, with foundational research by Ballal and Lefebvre in the 1970s on minimum ignition energy (MIE) and recent advancements in nanosecond-pulsed high-frequency discharge (NPHFD) plasmas demonstrating improved control and reduced power requirements. Professor Lefkowitz has shown through extensive research that high pulse repetition frequencies (PRF) enhance ignition kernel growth, with factors like flow velocity, electrode gap distance, and energy per pulse being key to optimizing ignition. NPHFD ignition offers precise control and energy savings, with future research needed to refine models and improve ignition strategies, ultimately enhancing engine performance and reducing emissions.

Abtract

Combustion in high-speed systems presents unique challenges, primarily due to the extremely short residence time of the gas in the combustion chamber. Combined with a highly turbulent environment, this often leads to rapid quenching of the ignition kernel, thus preventing successful ignition events. The nanosecond-pulsed high-frequency discharge (NPHFD) ignition technique has emerged as a promising solution for addressing these challenges. NPHFD stands out for its ability to finely tune critical 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 using the NPHFD method. The primary objectives are to examine P_IG under high-speed and high-turbulence conditions similar to those in a scramjet cavity and to determine the minimum ignition energy and the minimum ignition power. The fuel-air mixture is ignited by a burst of high-frequency discharges within a PRF ranging between 1-200 kHz. At the present stage, two equivalence ratios of 0.45 and 0.5 were chosen, and P_IG was studied as a function of PRF. Additionally, two different temperature setpoints, T₀= 293K and T₁= 373K, were selected, and their influence on the P_IG was explored. The kernel was captured using a high-speed Schlieren imaging system and a high-speed IR camera equipped with a CO₂ filter. Results show that the higher the PRF, the easier and more robust the ignition, and a regime called “fully coupled” could be defined, where P_IG = 1. The lower the PRF, the harder it is to ignite until no ignition is observed at 1 kHz. For the two temperature levels studied, it was found that higher temperatures achieved an increase in P_IG, accompanied by a broadened fully coupled regime. Through this analysis, a deeper understanding of the NPHFD ignition process and its characteristics has been obtained. The impacts of PRF, equivalence ratio, temperature variation, and flow velocity will be expanded up to P_IG, to better understand which set of conditions helps to achieve the highest P_IG overall.

For the NPHFD, three inter-pulse coupling regimes have been discovered:

Fully coupled regime: The inter-pulse time is short, and ignition probability is the highest with negligible local quenching of ignition kernels.
Partially coupled regime: Intermediate inter-pulse time leads to destructive interaction between kernels, amplifying local quenching and causing a drop in ignition probability.
Decoupled regime: Ignition probability becomes a function of the single pulse ignition probability and the number of repetitions, as the kernels created by the sparse pulses do not interact.

Recent research by our group at the Technion – Israel Institute of Technology explores the impact of energy distribution, inter-electrode gap distance, and local hydrodynamics on ignition in methane-air mixtures using NPHFD. These studies are the first to reveal the mechanism behind increasing local quenching in the partially coupled regime due to destructive pulse-kernel interference, where the high expansion of the plasma discharge can penetrate into the flame kernel formed by the previous pulse. These findings underscore the importance of understanding inter-pulse coupling and hydrodynamic effects in optimizing ignition processes, paving the way for advanced combustion technologies.

Published studies:

Shen, S., Laso, I., Rozin, N., & Lefkowitz, J. K. (2023). On pulse energy and energy distribution for ignition of flowing mixtures. Proceedings of the Combustion Institute, 39(4), 5487-5498.
Shen, S., Rempe, E., Senior-Tybora, W., & Lefkowitz, J. K. (2024). Destructive inter-pulse coupling in nanosecond-pulsed high-frequency discharge ignition: Effect of hydrodynamic regimes. Proceedings of the Combustion Institute, 40(1-4), 105445.