The effect of humidity on hydroxyl and ozone production by nanosecond discharges

The interplay of humidity and non-equilibrium, transient plasma was studied via ignition experiments in a C2H4-air mixture, concentration measurements in humid air, and detailed simulations. Hydroxyl (OH) and ozone (O-3) produced via non-equilibrium plasma were characterized in a flowing H2O-air mixture at atmospheric pressure with varying the levels of humidity using planar laser-induced fluorescence (PLIF) and UV absorption, respectively. The OH, which was created in the discharge streamers, peaked at a concentration of similar to 5x 10(14)/cm(3) and then decayed below 1 x 10(14)/cm(3) after similar to 100 mu s. O-3, which is long lived, peaked at a concentration of 1.4 x 10(15)/cm(3). An increase in humidity from X-H2O approximate to 0.2% to 1% resulted in a monotonic increase in the concentration of OH and a 67% decrease in that of O-3. Zero-dimensional Boltzmann modeling of non-equilibrium plasma discharges in humid air showed qualitative agreement with these results and points to the decrease in o concentration (with increasing humidity) as the reason for the decreased O-3 concentration. In spite the dramatic decline in X-O3 with increased humidity, there was no strong commensurate effect on ignition and flame propagation in C2H4-air mixtures: Peak pressure rise rate was at its maximum value at X-H2O = 1% but was only 25% less at X-H2O = 5%. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.