Measurements and kinetic modeling of O2 vibrational Kinetics in O2-Ar mixtures partially dissociated by a Ns pulse discharge

Vibrational kinetics of O2 is studied during the O atom recombination in a 20% O2-Ar mixture, partially dissociated by a burst of ns discharge pulses in a heated plasma flow reactor. The discharge generates a diffuse volumetric plasma. The time-resolved temperature in the discharge afterglow is measured by Rayleigh scattering. Time-resolved O atom number density is measured by ps Two-Photon absorption Laser Induced Fluorescence (TALIF), calibrated in xenon. Timeresolved vibrational level populations of molecular oxygen, O2(v=8-20), are measured by ps Laser Induced Fluorescence (LIF) on the O2 Schumann-Runge bands, with absolute calibration by NO LIF. Time-resolved ozone number density is monitored by broadband UV absorption. The experimental results are compared with the predictions of a state-specific kinetic model, incorporating the dominant energy transfer processes and chemical reactions in a nonequilibrium O2-O-O3-Ar plasma, including electron impact ionization, dissociation, and electronic excitation; quenching of electronically excited species; vibration-translation (V-T) relaxation and vibrationvibration (V-V) energy transfer; and state-specific chemical reactions. The experimental results indicate a rapid initial decay of O2(v) populations generated by electron impact in the discharge, on a ~10 μs time scale, primarily due to the V-T relaxation by O atoms. This decay is followed by a much slower population reduction, on the time scale of several ms, much longer compared to that for V-T relaxation and V-V exchange. Both O atoms and the O2(v) populations decay on the same time scale, indicating that chemical reactions initiated by the O atom recombination result in the generation of vibrationally excited O2 molecules. On this time scale, the ozone number density remains nearly constant, indicating that the effect of photolytic dissociation of ozone by the pump laser pulse is negligible. These trends are reproduced by the kinetic model, which shows that the reaction of O atoms with ozone is the dominant pathway of O2(v) generation, even at elevated temperatures. Although the predicted relative O2(v) populations are close to the experimental results, their absolute number densities differ from the experimental data. This is likely due to uncertainties in the absolute calibration of LIF measurements and in the spectroscopic model used in the data reduction. The present work demonstrates the capability for the absolute, time-resolved measurements of vibrationally excited O2 in recombining gas flows, to quantify the energy partition in the recombination reactions such that control the surface heat flux in high-temperature chemically reacting boundary layer.