High-speed observations of cryogenic single and coaxial jets under subcritical and transcritical conditions

Cryogenic single round and coaxial jets were observed under subcritical and transcritical conditions using a high-speed camera to explore the effects of the single-phase-like and two-phase-like properties of transcritical fluids on the unsteady behavior and flow structures of the jets. Cryogenic nitrogen and gaseous nitrogen were used as injectants. Cryogenic nitrogen was clearly visualized in all cases as the dark core. In the cases of single round jets, the large-scale characteristics of the jet behavior observed, such as the width of the dark core and the length scale of wave development in the mixing layers, were similar to those observed under subcritical conditions. Furthermore, dense nitrogen in the mixing layer entrained into the ambient environment and stretched in a manner similar to that observed under subcritical conditions. However, in contrast to the droplet formation observed under subcritical conditions, the entrained dense nitrogen diffused in a manner similar to turbulent mixing in a variable-density single-phase mixing layer. This means that the microscale behaviors that occur under transcritical conditions, which are comparable to turbulent eddies, are similar to those that occur in variable-density single-phase jets. In the cases of coaxial jets, the turbulent-eddy-scale phenomena observed were also similar to those observed for variable-density single-phase jets. However, in contrast to the phenomena observed for single round jets, the larger-scale behavior of the dark core was different from that observed under subcritical conditions. Two significant features were observed only under transcritical conditions. The first feature was the formation of shear-layer instability waves near the injector exit. The second feature was the shedding of dense nitrogen lumps from the end of the dark core at nearly uniform intervals. Because similar behavior was not observed under subcritical conditions, the single-phase-like properties of transcritical fluids are considered to have induced these features. The present correlation between the dark core length and the outer-to-inner jet momentum flux ratio was found to be in good agreement with the empirical correlation for variable-density single-phase coaxial jets.