N2 VIBRATIONAL TEMPERATURES AND OH NUMBER DENSITY MEASUREMENTS IN A NS PULSE DISCHARGE HYDROGEN-AIR PLASMAS

This work presents time-resolved measurements of nitrogen vibrational temperature, translational-rotational temperature, and absolute OH number density in lean hydrogen-air mixtures excited in a diffuse filament nanosecond pulse discharge, at a pressure of 100 Torr and high specific energy loading. The main objective of these measurements is to study a possible effect of nitrogen vibrational excitation on low-temperature kinetics of HO2 and OH radicals. N2 vibrational temperature and gas temperature in the discharge and the afterglow are measured by ns broadband Coherent Anti-Stokes Scattering (CARS). Hydroxyl radical number density is measured by Laser Induced Fluorescence (LIF) calibrated by Rayleigh scattering. The results show that the discharge generates strong vibrational nonequilibrium in air and H2-air mixtures for delay times after the discharge pulse of up to ~ 1 ms, with peak vibrational temperature of Tv $approx$ 2000 K at T $approx$ 500 K. Nitrogen vibrational temperature peaks $approx$ 200 $mu$s after the discharge pulse, before decreasing due to vibrational-translational relaxation by O atoms (on the time scale of a few hundred $mu$s) and diffusion (on ms time scale). OH number density increases gradually after the discharge pulse, peaking at t ~ 100-300 $mu$s and decaying on a longer time scale, until t ~ 1 ms. Both OH rise time and decay time decrease as H2 fraction in the mixture is increased from 1% to 5%. OH number density in a 1% H2-air mixture peaks at approximately the same time as vibrational temperature in air, suggesting that OH kinetics may be affected by N2 vibrational excitation. However, preliminary kinetic modeling calculations demonstrate that OH number density overshoot is controlled by known reactions of H and O radicals generated in the plasma, rather than by dissociation by HO2 radical in collisions with vibrationally excited N2 molecules, as has been suggested earlier. Additional measurements at higher specific energy loadings and kinetic modeling calculations are underway