In this study, the effects of entropy layer on the boundary layer over hypersonic blunt cones for a thermochemical equilibrium gas are investigated using the direct numerical simulation method. The flow and stability characteristics of the entropy and boundary layers are presented considering the chemical reactions. It is found that the entropy layer has little influence on the inner layer inside the boundary layer. The inner layer thickness increases when chemical equilibrium is considered, which stems from the enhanced viscosity protection near the wall surface. At the leading edge of the blunt cone, due to the effects of the equilibrium gas, the temperature in the boundary and entropy layers decreases and the boundary-layer-edge parameters significantly change. The entropy layer gradually vanishes along the downstream direction, and the effects of chemical reactions are concentrated in the boundary layer. The entropy swallowing point of the equilibrium gas does not significantly differ from that of the perfect gas. For all the analyzed cases, chemical reactions stabilize the entropy-layer instability modes. The instability region and frequency range of the equilibrium gas decrease, and the growth rate of the most unstable perturbation of the equilibrium gas is considerably smaller than that of the perfect gas. Moreover, the results show that the equilibrium gas has a minor influence on the similar process of the blunt cone boundary layer tending to the sharp cone.

The transport process of high-temperature air is vital in aerospace fields and has attracted increased attention in recent years. In this paper, an adequate study of factors affecting transport coefficients for high-temperature air is conducted. The results of a different-species model at different pressures and temperatures show that the 9-species air model is applicable to calculate the viscosity and translational thermal conductivity coefficients before significant ionization occurs. Based on the Chapman-Enskog method, simplified mixing rules for calculating viscosity and translational thermal conductivity coefficients of high-temperature air are developed by omitting unimportant matrix elements and assuming the reduced collision integral ratio to a reasonable constant value. The diagonal elements’ magnitude of the transport matrix indicates that collision related to the electrons has a little impact on viscosity but has a great influence on translational thermal conductivity. New simplified mixing rules can accurately calculate the viscosity and translational thermal conductivity coefficients of high-temperature air when dissociation or weak ionization occurs. The improved mixing rules are obviously more accurate than the Wilke mixing rule.