Non-equilibrium condensation and secondary vortex dynamics of moist air in the high-speed turbo-expander

The spontaneous condensation of water vapor during rapid expansion of moist air induces strong thermodynamic non-equilibrium effects and influences the vortex structures in the high-speed turbine passage. In this study, a numerical simulation is performed on the high-speed turbine flow passage using an Euler–Euler multiphase framework coupled with classical nucleation theory and droplet growth models, and experimental validation is conducted on a reverse Brayton cycle test rig. The relative deviations between the simulation and experimental results are 2.57% for isentropic efficiency and 4.17% for outlet humidity, respectively. Comparative analysis with dry air conditions reveals that phase-change effects induce a substantial temperature rise of 18.8 K at the outlet relative to the dry air case, accompanied by a maximum liquid mass fraction reaching 0.77%. Entropy generation analysis indicates that the impeller passage is the primary source of irreversible losses, accounting for over 41% of the total. The secondary vortices induced by condensation near the suction surface and the associated local pressure fluctuations are identified as the main loss contributors. The humidity-induced increase in entropy generation follows a power-law relationship, with a critical relative humidity threshold of approximately 30%. Beyond this threshold, the entropy generation increases superlinearly with relative humidity, with an exponent of 1.803. The efficiency loss is linearly correlated with the entropy generation increase. These findings reveal the coupling mechanism between non-equilibrium condensation and vortex dynamics in high-speed turbines.