The exhaust air pressurization process of a pump-turbine during the shift from this condition is explored to address the issue of instability that is easy to generate in pumped storage units during phase transition from power generation to power generation. Unsteady multiphase flows in a prototype pump-turbine are simulated using the SST k-w model and VOF model. The simulations demonstrate the transient variation trends in the unit's active power

exhaust rate

vaneless zone pressure

and the relationship of the evolution of internal flows in the pump-turbine versus its external characteristics. They also reveal the dynamic distribution of air and liquid phases in the unit in exhaust pressurization process. The study demonstrates that in this process

the unit's active power absorption from the grid and the exhaust rate decreases gradually after their initial rapid decreases

its pressurization effect is evident

and its pressure pulsation frequencies in the vaneless zone are higher. In the vaneless zone

a high-speed water ring forms gradually driven by the inflow coming from tail water

so that air-liquid movements are chaotic and the air is prone to flowing backward into the draft tube

which has a significant impact on the unit's stability. The findings in this study are useful for improving the stability and safety of pumped storage units.