Effects of Plasma Discharges on the Ignition of a Laminar Nonpremixed Jet Flame

This paper experimentally investigates the effect of plasma discharges on the ignition of a laminar nonpremixed methane jet flame in a stream of co-flow air. The Reynolds number of the jet flame, based on the nominal jet velocity and the nozzle diameter, is approximately Re _j = 2000. The plasma discharge, a corona type, is produced between two tungsten wires with a diameter of 0.5 mm and a gap of 15 mm. Results show that the application of plasma discharge in a near-nozzle region is capable of ignition of the flame studied here, even though most of the ignition locations are in the lean side of the stoichiometric methane mass fraction. In general, the ignition probability (i.e., flame ignition is successfully achieved) is relatively larger as the discharge is closer to the stoichiometric methane mass fraction. In addition, the ignition probability is seen to increase with the discharge energy density. Results of the most probable ignition time, defined as the mode of the probability density function of the required flame ignition time, are consistent with those of the ignition probability. The most probable ignition time is seen to decrease with the increased ignition probability. Overall, the shorter ignition time occurs when the discharge is closer to the stoichiometric methane mass fraction or when the discharge energy density becomes larger. The estimated reduced electric field suggests the importance of the electron-impact dissociation induced by plasma discharges to the flame ignition.