The effect of nanosecond pulsed high frequency discharges on the temperature evolution of ignition kernels

Nanosecond pulsed high frequency discharges (NPHFD) have attracted interest as a means of enhancing ignition probability and ignition kernel growth rates. Previous studies have shown that increasing the pulse repetition frequency (PRF) of a NPHFD burst can result in enhanced probability of ignition relative to lower values of PRF. The cause of this enhanced ignition probability is not well understood, in part, because the temperature evolution of ignition kernels have not been measured. In this work, an inverse deconvolution technique was developed, evaluated, and used to determine the temporal evolution of the temperature and volume of ignition kernels initiated by NPHFD and single pulse discharges. Radiation intensity measurements of ignition kernels were compared to modeled values to determine path-averaged temperatures. Comparisons of values derived using the deconvolution technique and measured temperatures above a McKenna burner typically agreed within 4%. When the technique was applied to ignition kernels initiated by NPHFD over a range of PRF, it was observed that increasing the PRF of a discharge sequence increased the initial rate of change of the temperature as compared to lower PRF. Additionally, kernels generated with high PRF discharges achieved higher initial peak temperatures than those generated by lower PRF discharges. A relaxation in temperature was observed following the peak, which was followed by a subsequent temperature increase in all cases generated with PRF > 10 kHz. PRF above 5 kHz result in larger initial kernel volumes and volume rates of change compared to those observed for lower PRF values. This is attributed to pulse-topulse coupling during the energy deposition process. The observed relationship between kernel temperature, volume, and PRF help to explain ignition probability measurements reported previously. Findings from the effort illustrate how the rate of energy deposition through pulse-to-pulse coupling can significantly enhance ignition through elevated temperatures. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.