Cavitation Characteristics and Hydrodynamic Radial Forces of a Reversible Pump–Turbine at Pump Mode

Abstract

As the most critical part of a hydropower station, reversible pump–turbines (RPTs) are facing the trend of high parameterization (e.g., high head and high rotation speed) with the rapid development of hydropower, which may cause more detrimental cavitation-induced hydrodynamic forces. Considering that cavitation occurs more easily under pump mode than turbine mode for the RPT, the characteristics and generation mechanism of the hydrodynamic radial force induced by cavitation of a RPT model at pump mode are investigated by computational fluid dynamics and experimental methods. Combined with the shear stress transport (SST) turbulence model, the three-dimensional (3D) unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved to calculate the cavitation flow of the RPT at various cavitation states. The blade loading and the pressure pulsations of the pressure/suction surfaces of the blade are monitored from the inlet to the outlet. Results show that with the development of cavitation, increasing cavitation bubbles covering the blade surfaces lead to the decrease of the blade loading, weaken the regularity of the pressure fluctuation, and increase the amplitude of the pressure pulsation, especially the suction surface. Furthermore, the characteristics of hydrodynamic radial forces under various cavitation states are discussed, and it is found that the values of the hydrodynamic radial forces become larger and the alternating hydrodynamic radial forces are more remarkable with the development of cavitation. Meanwhile, the symmetry of the radial force of the impeller presents a slight change due to the rotor–stator interaction between impeller and guide vanes and the symmetrical structure of the guide vanes. Then, to explore the generation mechanism of the hydrodynamic radial forces, the frequency spectrum of the pressure pulsation and hydrodynamic radial forces are analyzed comparatively. It is found that the hydrodynamic radial forces are affected by the rotor–stator interaction in the initial stage of cavitation, whereas the influence of the pressure distribution of suction surface of the blade increases with the development of cavitation.