Phase Changes of a Large-Heat-Capacity Fluid in Transcritical Expansion Flows

Summary

Substances of large heat capacity are able to undergo complete adiabatic liquid-vapour phase changes. The soundspeed discontinuities at the phase boundaries give rise to various real flow effects. The investigation of Laval nozzle flows assuming phase equilibrium shows discontinues choking at the liquid phase boundary. At the vapour phase boundary up to three shocks can occur simultaneously in the nozzle: one expansion shock and two compression shocks.

In the experiment the initial conditions in the blowdown reservoir are chosen such that the expansion adiabats intersect the two-phase region close to the critical point. The phase boundary reached first during the expansion appears as a pronounced nucleation front. At the second phase boundary the influence of the distribution of liquid and vapour on the transfer processes becomes apparent. During the expansion process on subcritical adiabats the remaining liquid forms relatively large droplets. The transition to the pure gas flow spreads over a large spatial region. On supercritical adiabats, however, the two-phase state is reached coming from the gas phase. The re-evaporation of the small condensation nuclei is completed in a distinct evaporation front.