Collisional-radiative modeling of shock-heated nitrogen mixtures

A three-temperature collisional-radiative model for shock-heated nitrogen-argon mixtures is developed to facilitate the study of nonequilibrium electronic excitation and ionization behind strong shock waves. Model predictions accurately reproduce measurements of N-2 dissociation for mixtures of 2%-10% N-2 in argon, with some discrepancies observed for 20% N-2 mixtures. Potential causes of the discrepancies are discussed. Net dissociation in mixtures containing 20% N-2 is significantly impacted by the dissociation of N-2(A), the first excited electronic state of N-2, indicating that molecular electronic excitation can affect net dissociation in shock-heated nitrogen flows. The collisional-radiative model successfully predicts the three-stage behavior and induction time observed in concentration measurements of atomic nitrogen in its fourth excited state, the 3 s P-4 level, behind reflected shocks. Mechanisms for the observed behavior are discussed, which deviate from those inferred using a simpler kinetic model. Excited state number density predictions are strongly influenced by the modeling of radiation self-absorption and the inclusion of the measured non-ideal pressure rise. At higher N-2 concentrations, the measured data indicate increased efficiency of atomic nitrogen electronic excitation in collisions with N as compared to collisions with N-2 and Ar. A global sensitivity analysis of the excited state predictions is then performed, identifying the processes in the kinetic model that most sensitively influence the predicted excited state time history and further clarifying the dominant mechanisms affecting the experimental observables.