A numerical study is performed to assess the influence of thermochemical nonequilibrium on the transport coefficients used in Computational Fluid Dynamics (CFD) simulations of hypersonic external flowfields. An assessment is made of transport coefficients, from simplified methods of Blottner curve fits and Variable Hard Sphere (VHS) model, to the more detailed state kinetic models for hypersonic flows ranging for Mach numbers 7 and above. The role of state kinetics and simplified nonequilibrium models in strong nonequilibrium conditions is delineated for the CFD prediction of surface heat flux. The state to state kinetics modeling of vibration-vibration (V-V) energy exchanges alter the flow expansion as well an increase in the population distribution of the middle and upper energy states, thus affecting surface heat flux predictions. Current usage in the community of the simplified models of Blottner curve fits and the VHS model for the calculations of transport coefficients were shown to be adequate only for select gas mixtures and flow conditions. A justification is provided for a more general state kinetic approach of calculating transport coefficients from the Chapman-Enskog solution of the Wang-Chang Uhlenbeck equation, valid for a wide range of flow conditions encountered in high temperature air mixture of hypersonic flows.
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