A numerical study on a temporal mixing layer under transcritical conditions

Direct numerical simulation (DNS) of a cryogenic nitrogen/nitrogen gas temporally developing mixing layer in transcritical conditions was conducted to explore the influence of the pseudo vaporization at the pseudo-critical temperature. For comparison, a cryogenic nitrogen/nitrogen gas mixing layer in ideal-gas conditions was also simulated with the same initial profiles of density and velocity. Flow visualization and statistical analysis revealed that dominant eddies in development of turbulence were not directly affected by the pseudo vaporization, and were formed in the upper stream region of the mixing layer in the same manner as in the ideal-gas condition. Thus, the Reynolds stress and turbulent heat flux were similar between the two conditions. Accordingly, the mean and fluctuation profiles of streamwise velocity and specific enthalpy were also similar between the two conditions. Therefore, it is noted that the turbulent transport mechanisms of momentum and heat are little affected by the pseudo vaporization. In contrast, the density profiles were influenced by the pseudo vaporization. The cryogenic dense nitrogen was entrained deeply into the middle part of the mixing layer under the transcritical condition. This is because the nitrogen maintains a relatively high density until heated up to the pseudo-critical temperature. As a result, the mean profile of density in the transcritical condition was much higher than that in the ideal-gas condition through the mixing layers. Consequently, the transport mechanism of mass under the transcritical condition is considered to be influenced by the pseudo vaporization.