An experimental investigation of the effects of chemical and ionizational nonequilibrium in recombining atmospheric pressure air plasmas

Results from experimental and numerical investigations of the mechanism of ionizational nonequilibrium in recombining plasmas of air, nitrogen/argon, and air/argon are presented. Measurements of electron and excited state concentrations in plasmas produced with a 50 kW RF torch operating at atmospheric pressure are compared with numerical simulations performed with three reaction mechanisms widely used for air plasma kinetics. It is shown that as electron recombination in molecular plasmas occurs primarily through inherently fast two-body dissociative recombination ionizational nonequilibrium is ultimately caused by slow three body neutral recombination. The neutral three-body recombination reactions currently utilized in the aerospace community are typically deduced from application of the principle of detailed balance to the “thermal” dissociation rates measured in shock tubes. Such measurements, however, are known to suffer from shock-induced departures from Boltzmann distributions of the populations of vibrational levels. There has been relatively little success in correcting the measured dissociation rates for this type of behavior.; The rates of the controlling NO and N₂ thermal dissociation rates are assessed through a comparison between the predictions of the kinetic models and the measured equilibrium and nonequilibrium concentrations of electrons, of the A, C, and D states of NO, and of the B and C state of N₂. The air and argon/air experiments presented here are used to assess the rates of NO thermal dissociation from the literature. Results of a recently developed novel collisional radiative model, for which the experimental results of this work served as both a guide for development and as a benchmark, are also presented. The substantiated collisional radiative model is used to deduce the thermal rate of N₂ dissociation by N atom impact from the nonequilibrium nitrogen/argon experiments by explicitly accounting for the observed vibrational nonequilibrium ($kfN2,N$ = 4.2 × 1024 T−1.76 e −113200/T cm3/mole s, 4700 < T < 7200 K).