Runner clearance flow pattern and pressure pulsation in the S-shaped region of high-head pumped storage unit

The clearance flow pattern and pressure pulsation inside the labyrinth ring become highly complex during operation in the S-shaped area. However, the source of pressure pulsation in the clearance flow-path and its relationship with the main flow-path are not clear. Therefore, this study simulated the three-dimensional transient flow in a prototype pumped storage unit (PSU). By analyzing frequency spectra of pressure pulsations and the vortex evolution in the upper crown clearance, this study explored the transient characteristics mechanism of clearance flow-paths in the S-shaped area. The results reveal that pressure pulsations within the clearance not only originate from conventional sources such as rotor–stator interactions and rotating stall in the main flow-path but also exhibit a previously unidentified mechanism. Notably, self-excited vibrations induced by unstable vortex shedding near the labyrinth were observed to generate significant pressure fluctuations, which probably induce hydrodynamic resonance. At the same time, it was found from the axial hydraulic thrust analysis that the value of the axial force under the reverse-pump condition is huge, which easily causes force imbalance on the rotor and seriously affects the stable operation of PSUs. Thus, this study serves as an academic reference for ensuring the stability of PSUs.