Investigation of high enthalpy thermochemical nonequilibrium flow over spheres

The hypersonic high enthalpy nitrogen flows over spheres are investigated by high-fidelity state-to-state (StS) modeling. The objective of the study is to understand the nonequilibrium behaviors in the shock layer, including the stagnation line features, surface heat transfer rate, and near-wall properties inside the thermal boundary layer. Two cases with the freestream total enthalpies of 16.5 and 15.5 MJ/kg are considered, and the numerical results are compared with the experimental data. The StS model yields an accurate prediction of the shock stand-off distance with the experiment rather than an underestimation by the traditional two-temperature model. Both the StS and two-temperature models provide general agreement of the stagnation point heat flux with the experiment. In comparison, the heat flux obtained by the StS model is lower than the two-temperature model. Note that our work finds distinctive behaviors of near-wall properties. The vibrational energy is not accommodated with the sphere surface and is in thermal nonequilibrium with the translational energy, with evidence showing that the vibrational temperature is much higher than the wall temperature and the translational temperature. The values of vibrational temperature in the immediate vicinity of the stagnation point are 9.3 and 10.0 times the wall temperature for the cases with total enthalpies of 16.5 and 15.5 MJ/kg, respectively. Moreover, the vibration temperature demonstrates a nonmonotonic variation trend with a local minimum, which can be explained by the nonequilibrium distributions of vibrational energy states due to vibrational-translational energy transfer and molecular recombination.