Effects of Fuel Mixture Properties on Nanosecond Pulsed High Frequency Discharge Ignition
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2019, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. The effect of fuel mixture properties on nanosecond pulsed high frequency discharge (NPHFD) ignition is parametrically investigated. Methane, air, and helium mixtures were used to investigate the impacts of pulse repetition frequency (PRF) and Lewis number in a vertical wind tunnel facility at a nominal flow velocity of 10 m/s. High-frame-rate schlieren imaging and infrared imaging were used to analyze the ignition probability and kernel growth characteristics with varying NPHFD parameters. Of particular interest is the inter-coupling between nanosecond pulses, as observed by Lefkowitz and Ombrello1. This partially-coupled regime is not well understood and is a primary focus for the present work. Two primary mixtures were investigated to isolate Lewis number effects while adiabatic flame temperature: =0.60 CH4-Air, Le=0.98 and =0.65 CH4-Air, 10% He diluent, Le=1.20. From this study a clear reduction in ignition probability is observed for the partially-coupled regime near 250 s inter-pulse time, which agrees with past studies. With increased Lewis number, the destructive impact is enhanced, with a further reduction in ignition probability. High-speed schlieren imaging showed significantly stunted growth of the effected gas volume with increased Lewis number in the partially-coupled regime. Broadband infrared imaging agrees with these trends with much lower signal seen with higher Lewis number. This ongoing study will lead to a more complete understanding of the complex interaction between the energy deposition-induced flowfield, thermal expansion and heat release from the ignition, and the coupling of subsequent pulses. Ultimately, better understanding of the underlying physical mechanisms will lead to reliable ignition using NPHFD for high-speed combustion applications.
