Abstract:Using state-to-state model, quasi-one-dimensional nozzle flow of high-temperature nonequilibrium air is investigated numerically. The five chemical species mixture N ₂/O ₂/NO/N/O is considered with 61 bound vibrational levels for N ₂, 46 for O ₂, and 48 for NO. Each vibrational state is regarded as a pseudo species, which leads to a total of 157 species for the air mixture. The state-specific transition rate coefficients of some processes, which have no available data, are calculated based on the relaxation time and the rate coefficients of other similar processes. The flow simulation and analysis are made for reservoir temperature from 2000 to 8000 K and pressure from 1 to 20 MPa. The nozzle flow is essentially in equilibrium before the throat, but deviates from equilibrium shortly after the throat. The mass fraction of chemical species, populations of lower energy levels, and vibrational temperatures are frozen in the downstream not far away from the throat. The vibrational temperature of N ₂is freezing earlier and has a higher frozen value than that of NO and O ₂. The process of vibration–translation (VT) energy exchange is predominant for vibrational transition, the recombination reaction generates molecules preferably to middle vibrational levels. Throughout the nonequilibrium and frozen zone, the vibrational population distributions are far from Boltzmann distribution at vibrational temperature, and feature a large overpopulation of the high-lying vibrational levels. Increasing the reservoir pressure could reduce the nonequilibrium to a certain extent and delay the flow thermochemical freezing.