Ab initio based model for high temperature nitrogen rovibrational excitation and dissociation

We present preliminary DMS calculations to investigate vibrational excitation and dissociation in molecular nitrogen, including both N3 and N4 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 T_v,c to accounts for the excitation of the high energy tails of the system distribution f(v). If T_v < T_v,c, little to no dissociation occurs no matter what the translational temperature is. Once high-v states are sufficiently populated (T_v ≃ T_v,c), then dissociation proceeds at a rate controlled by a combination of T_t and T_v (and, in principle, T_r). The prelinimary model for nitrogen vibrational excitation and dissociation is based on the analysis of the DMS results. In particular, the model is tested against the results obtained for the highly, but stringent, nonequilibrium case with T_t = 30, 000 K and T_r(0) = T_v(0) = 3, 000 K. As such, the results presented here are only intended to be demonstrative. As other DMS data become available, we plan to further refine the model and make it reliably applicable over the relevant temperature range for hypersonic applications. The new model reproduces particularly well the composition history, including the slower initial dissociation (incubation period). Furthermore, it also describes the QSS state, characterized by T_v < T_t.