Enhanced OH Chemiluminescent Emission from Transient Plasma Ignited Methane-Air Mixtures Relative to Spark Ignition

Summary form only given. Ignition phenomena in gaseous CH₄-air mixtures from transient plasma and spark discharges were studied using optical techniques. It was found that transient plasmas, composed of pulsed discharges persisting for times shorter than the plasma formative phase in gas mixtures, produced shorter ignition delay times - a critical factor in applications such as pulsed detonation engines. 1. The transient plasma is generated via 66 kV, 70 ns FWHM pulses with pulse energies of the order 800 mJ/pulse. The free radical OH is present in many chain propagation and branching reactions, therefore is indicative of the intensities of chemical reaction processes before and after ignition. CH₄-air combustion tests were performed at 1 atm, at equivalence ratios of phi =1 and phi =1.2, and chemiluminescent emission of OH at 308 nm was recorded. Emission intensity after ignition, induced by transient plasma, was typically a factor of 5-8 greater than by capacitive spark discharge. In addition, before ignition a large OH signature was present immediately after the application of the transient plasma, whereas there was none detected when using a spark. Associated pressure curves were also compared: the transient plasma case had larger peak pressures, with shorter ignition delays consistent with previous data. 2. Additionally a PicoStar ICCD was used to obtain images of streamer propagation, channel formation, and spark formation. The imaging of the streamers and the strong OH signal indicates that transient plasma in the tested geometry yielded a volumetric ignition process populating the discharge volume with reactive free radicals. Physics and chemistry issues resulting from the combination of these effects, volumetric ignition and direct production of free radicals, will be discussed.