Ab initio based model for high temperature nitrogen rovibrational excitation and dissociation
We present DMS calculations to investigate vibrational excitation and dissociation in molecular nitrogen, including both N 3 and N 4 processes and under strong nonequilibrium conditions. A preliminary constitutive model is also described and parameterized with the available DMS results. The analysis of vibrational energy distributions obtained with DMS shows that high vibrational energy states are immediately populated in the initial stages of the dissociation process, due to the very energetic nature of the molecular collisions. Throughout the excitation process, distributions remain highly nonequilibrium and generally have a strong bimodal character. Quite interestingly, the pre collision distributions of molecules undergoing dissociation rapidly converge toward the QSS distribution, even in the early portion of the excitation. This may suggest that, once high v states become sufficiently populated, the dissociation process is effectively initiated. Hence, we introduce the idea of a critical vibrational incubation temperature Tc,v to account for the excitation of the high energy tails of the system distribution f(v) If Tv Tc,v, little to no dissociation occurs, even at very high translational temperatures. Once high v states are sufficiently populated (Tv Tc,v) then dissociation proceeds at a rate controlled by a combination of Tt and Tv (and, in principle, Tr) A preliminary model for nitrogen vibrational excitation and dissociation is proposed here based on the analysis of the DMS results. The new model reproduces particularly well both energy and composition history, including the slower initial dissociation (incubation period) for the vibrational excitation and dissociation process simulated with DMS from several initial conditions. Furthermore, it also describes the QSS state, characterized by Tv Tt.