Ignition in High-Speed Flows

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.

CENTER-BASED GEOMETRY

Energy study

The effects of energy distribution on ignition during a burst of nanosecond-pulsed high-frequency discharges (NPHFD) are investigated in methane-air mixtures at a nominal flow velocity of 5.7 m/s, equivalence ratio of 0.61, and inter-electrode gap distance of 2 mm. These effects were considered in the context of three pulse-coupling regimes, occurring at different inter-pulse time conditions: fully-coupled (short inter-pulse time, high ignition probability), partially-coupled (intermediate inter-pulse time, low ignition probability), and decoupled (long inter-pulse time, variable ignition probability). High-speed schlieren imaging was used to analyze ignition probabilities and kernel growth characteristics. Two approaches were utilized to explore the effects of energy distribution. First, four nominal levels of energy per pulse were explored across a wide range of inter-pulse times using a fixed number of pulses. In the fully-coupled regime, energy per pulse did not show any influence over ignition probability or kernel growth rate. Outside of the fully-coupled regime, ignition probability drops to 0 for the lowest level of energy per pulse, whereas the highest level of energy per pulse has ignition probability of 1 for the entire range of inter-pulse times. For intermediate levels of energy per pulse, a partially-coupled regime characterized by significant drops in ignition probability is observed, with recovering ignition probability in the decoupled regime at longer inter-pulse time. Second, the energy per pulse, inter-pulse time, and the number of pulses are varied such that their individual effects could be isolated while maintaining constant total deposited energy or total discharge duration. In this exploration, inter-pulse time is found to be the driving parameter determining the inter-pulse coupling regimes and ignition probability. Lastly, single kernel analysis clarifies the effect of destructive kernel interactions in the partially-coupled regime, which, against intuition, results in increasing minimum ignition power for higher energy per pulse. In conclusion, high-frequency, low-energy discharge pulses can be the optimal ignition method for NPHFD ignition with high ignition probability and optimal energy efficiency.

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.

GAP DISTANCE STUDY

This study investigates the impact of inter-electrode gap distance on the ignition process using Nanosecond-pulsed high-frequency plasma discharges (NPHFD) in methane-air mixtures. The study categorizes the inter-pulse coupling into three regimes: fully coupled, partially coupled, and decoupled. In the fully coupled regime, $P_{I}$ reaches its maximum due to overlapping discharge radicals, while the partially coupled regime sees a decrease in $P_{I}$ due to destructive interactions between flame kernels. The decoupled regime shows less interaction, leading to a stable $P_{I}$ estimated statistically from a single pulse $P_{I}$ . The research also explores two hydrodynamic discharge regimes: the toroidal regime (gap distance $\leq$ 3.5 mm), characterized by significant local vorticity and a toroidal shape, and the diffusive regime (gap distance $>$ 3.5 mm), where the kernel appears to be more cylindrical. The study’s findings highlight that the toroidal regime exhibits a distinct kernel shape, significantly impacting ignition efficacy. Furthermore, the study’s results indicate that small d leads to a significant decrease in $P_{I}$ due to heat loss to the electrodes, while large d allows for better kernel growth and higher $P_{I}$ . The transition from fully coupled to partially coupled regimes occurs at shorter inter-pulse times for small gap distance. Additionally, in multi-pulse discharge scenarios, the $P_{I}$ curves distinctively mark the transition between the toroidal and diffusive regimes, emphasizing the importance of inter-pulse interference in ignition efficacy. Overall, the research provides a comprehensive understanding of the influence of discharge parameters on ignition mechanisms in NPHFD systems, contributing valuable insights for developing more efficient and reliable ignition systems in combustion applications.

WALL-BASED GEOMETRY

Velocity and temperature study

Ignition in high-speed systems poses distinct challenges due to the short gas residence time in the combustion chamber. The high velocity and high turbulence intensity, often lead to the rapid extinction of the ignition kernel, hindering successful ignition. The Nanosecond-Pulsed High-Frequency Discharge (NPHFD) ignition technique offers a promising solution, allowing precise adjustments of key parameters, such as pulse repetition frequency, number of pulses, and discharge voltage to control both the amount and duration of the deposited energy. This study aims to conduct a parametric investigation of wall-based NPHFD ignition in flowing ethylene–air mixtures under conditions designed to emulate a scramjet cavity environment. The ignition probability of the wall-based discharge geometry was studied as a function of varying pulse repetition frequency (1 – 200 kHz), temperature (T = 20 – 300 ◦C), velocity (U = 10 – 100 m/s), equivalence ratio (Φ = 0.2 – 0.6), and number of pulses (N = 25 – 100), providing a more comprehensive insight into flow and discharge behavior contributing to ignition than has ever 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. The results indicate that higher temperatures and an increased number of pulses significantly enhance the ignition probability, allowing for successful ignition at lower Φ and pulse repetition frequencies compared to ambient conditions. In contrast, higher velocities have an adverse effect, reducing ignition probability. These conflicting influences create ignition limits markedly different from those observed in previous studies, with ignition probability being examined under standard room conditions (T = 20◦C, P = 1 atm), and quiescent environments (U = 10 m/s), where limits are typically defined. A consistent trend was evident when comparing the ignition probability results from this study with the data obtained in a scramjet facility, where conditions inside the cavity (U = 36 – 147 m/s, T = 300◦C) were similar to those explored in a subsonic wind tunnel (U = 100 m/s, T = 300◦C); however, our findings showed lower values of ignition probability. The lower ignition probability could be explained by a difference in the values of Φ, with Φ = 0.535 in the wind tunnel, Φ = 0.82 in the scramjet. 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 the scramjet setup.

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.