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

Abstract The interplay of humidity and non-equilibrium, transient plasma was studied via ignition experiments in a C 2 H 4 –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 H 2 O–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 ∼5 × 10 14 /cm 3 and then decayed below 1 × 10 14 /cm 3 after ∼100 µs. O 3 , which is long lived, peaked at a concentration of 1.4 × 10 15 /cm 3 . An increase in humidity from X H 2 O  ≈ 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 O 3 with increased humidity, there was no strong commensurate effect on ignition and flame propagation in C 2 H 4 –air mixtures: Peak pressure rise rate was at its maximum value at X H 2 O  = 1% but was only 25% less at X H 2 O  = 5%.