Mechanistic insights into the unstable flow of pump-turbine unit at varied heads during pump mode start-up

Frequent start-stop operations of pumped storage power stations subject pump-turbine units to high-amplitude water hammer pressures at small guide vane openings, thereby compromising operational stability and service life. This study numerically simulates the pump mode start-up process of a prototype pump-turbine under three heads, with a focus on runner forces and internal flow dynamics. Results show that the x-direction component of runner hydraulic thrust exhibits minimal head dependence, whereas the y-component decreases by 28.2 % at 220.05 m relative to 175.4 m. At 220.05 m head, the force magnitude increases significantly, and rotor eccentricity intensifies. During small guide vane openings, reverse flow develops in the vaneless space between stay and guide vanes, driven by reverse blockage and forward blockage. Additionally, significant flow maldistribution persists in stay vane passages throughout the start-up process. The absolute radial hydraulic thrust on guide vanes decreases with increasing guide vane opening and head, achieving a maximum reduction of 75.4 %. These findings establish a dual-blockage theoretical framework for understanding transient instabilities, providing critical guidance for stability optimization in high head pumped-storage power plant designs.