Abstract: Transport properties of high-temperature gases significantly influence the nonequilibrium flowfields and aero-thermodynamic characteristics of high-speed aircraft. A numerical simulation method for nonequilibrium flowfields has been established, incorporating four transport mixing laws: Chapman, Yos, Armaly, and Wilke. Among these, the Wilke mixing law stands out for its computational efficiency and widespread application. Validation was performed using equilibrium air and the FIRE II nonequilibrium flow case. The influence of different mixing laws was investigated on the FIRE II flowfield, shear stress, and heat flux. The key factors affecting mixing law accuracy were identified, leading to the development of a modified Wilke mixing rule with enhanced precision. The study demonstrates that: 1) The mixing law considerably affects shear stress and heat flux predictions under high-speed, high-temperature and moderately ionized flight conditions of FIRE II. 2) The Wilke mixing law yields noticeably lower wall viscosity and thermal conductivity, directly leading to significant underestimation of both shear stress and heat flux and inducing increases in wall tangential velocity and temperature gradients. However, these gradient increases are insufficient to counteract the dominant effect of the reduced transport properties. 3) The discrepancy between the Wilke mixing law and Chapman method stems mainly from the order-of-magnitude difference between collision integrals involving ionized species and other interaction types. Consequently, the transport properties of ionized species should comprehensively account for collisions both among ionized species and between ionized and non-ionized species. 4) The proposed modification to the Wilke mixing law preserves its computational efficiency and simplicity while markedly improving accuracy under certain ionization levels. Predictions of transport properties, heat flux, and shear stress with the modified method show excellent agreement with those obtained from the Chapman method.

Key words: high-temperature gas, nonequilibrium, coefficient of viscosity, thermal conductivity, transport model, mixing law, numerical simulation