Analysis of Critical Rate Processes for Ionization in Shock-Heated Air

Abstract

High-fidelity electronic state-resolved modeling and formal sensitivity analysis are used to study plasma chemical processes occurring behind strong shock waves in air. A collisional-radiative model is developed and assessed using measurements of electron number density from several shock tube experiments. A special emphasis is placed on the modeling of associative ionization and the role of reactant channels involving excited atoms. Explicit modeling of associative ionization involving excited atoms is found to only affect the post-shock electron number density profiles within a thin layer near the shock front. Sensitivity analysis is performed to identify critical rate parameters that strongly influence ionization predictions in two-temperature and collisional-radiative models. Reactions involving nitric oxide are found to dominate the sensitivity of electron number density predictions by the two-temperature model at shock velocities of 5 and 7 km/s, while charge exchange reactions dominate the sensitivity at 9 km/s. These chemical reactions, if more accurately characterized, can most significantly reduce the uncertainty in existing models of ionization in shock-heated air. In sensitivity analyses of the two-temperature and collisional-radiative models, the associative ionization to form NO+ is the dominant sensitive reaction during the post-shock region where most of the electron production occurs. Uncertainty in the prediction of electron number density by the collisional-radiative model is dominated by reactions involving N2(A) for most of the post-shock flowfield for all velocities tested. In particular, the sensitivity results indicate that modeling of excited state molecular dissociation should be assessed further.