Assessment of Classical Impulsive Models of Dissociation in Thermochemical Nonequilibrium

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In strong shock waves in rarefied gases, chemical processes such as dissociation and exchange reactions occur in conditions of thermodynamic nonequilibrium, and the rate coefficients are functions of both translational–rotational and vibrational temperatures. In this work, we reexamine and validate the physical model for nonequilibrium dissociation rates proposed earlier by Macheret et al. The model is based on the classical impulsive approximation and enables analytical formulas for reaction thresholds, probabilities, and multitemperature rate coefficients. The recent quasi-classical trajectory calculations for , , , , and are used to evaluate the Macheret–Fridman dissociation model. The full version of the model that incorporates the reduction of dissociation energy due to rotation of the dissociating molecule as well as the appearance of the centrifugal barrier is found to be in good agreement with quasi-classical trajectory data, whereas the simplified version of the model that ignores the rotational effect significantly underpredicts the dissociation rates in vibrationally cold conditions. The good agreement with quasi-classical trajectory data validates the classical impulsive approximation and implies that details of the potential energy surface are not important for dissociation at high temperatures. Preliminary one-dimensional computational fluid dynamics calculations with the validated dissociation model were conducted for conditions of the shock-tube experiments in pure oxygen by Ibraguimova et al. Generally reasonable agreement was found between the calculations and the experimental data; however, some underprediction of the peak vibrational temperature suggests that some improvement in the model would be desirable.