The variation of element ratio in the numerical solution of hypersonic nonequilibrium flow is discussed. The modification method to adjust the species mass fraction for the purpose of element ratio preservation is analyzed through numerical practice and the study of the equivalent equation of the adjustment, with the conclusion that the simple modification method may cause unphysical solution. Considering that obvious variation of the element mass fraction appears mainly near the shock wave, an improved modification method is proposed, which is related to the magnitude of the gradients of species mass fraction. The application to numerical simulation of hypersonic nonequilibrium flow over wide range of flight Mach number and altitude shows that the improved modification method can preserve the element ratio in the flowfield, as well as avoid the numerical error that may occur in the simple modification method.
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