OH radical and temperature measurements during ignition of H2-air mixtures excited by a repetitively pulsed nanosecond discharge

Ignition delay time, time-resolved temperature, and time-resolved absolute OH concentration are measured in mildly preheated H-2-air mixtures (T = 473-500 K), excited by a repetitive nanosecond pulse discharge in a plasma flow reactor. The measurements are done in decaying plasma after the discharge pulse burst (pulse repetition rate 10-40 kHz). Ignition delay increases steeply as the number of pulses in the burst is reduced, exhibiting threshold-like behavior. Temperature and OH concentration during the ignition process are measured by two-line OH LIF, as functions of time delay after the discharge burst. Absolute calibration is done using a near-adiabatic flame. The results show that the temperature at the end of the discharge burst is 700-750 K, rising to 1500-1600 K within a few ms after the burst, indicative of ignition. During ignition, OH concentration increases by a factor of 20-50, from (0.5-1.1) x 10(14) cm(-3) at the end of the burst to peak value of (2.3-2.6) x 10(15) cm(-3). Kinetic modeling calculations show good agreement with the experimental results, demonstrating quantitative insight into kinetics of plasma-assisted ignition. Modeling calculations demonstrate that ignition is induced primarily by accumulation of H atoms and gradual temperature rise in the discharge, at temperatures significantly lower than autoignition temperature, by up to 200 K. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.