Numerical Investigation of Flow Characteristics and Energy Loss Mechanisms of a Pump Turbine Under Sand-Laden Conditions

The fluctuating nature of renewable energy sources such as wind and solar power poses significant challenges to the stability of power grids, while pumped-storage hydropower, with its advantages in peak regulation and frequency control, has become an essential component of modern energy strategies. However, sediment in rivers adversely affects the operational efficiency and stability of PSH units, leading to accelerated wear and shortened service life. In this study, the low-pressure stage of a two-stage pump turbine was selected as the research object, and the Euler–Euler numerical method was employed to investigate the solid–liquid two-phase flow characteristics of the pump turbine in pump mode. The results show that, compared with the clear-water condition, the head and efficiency decrease by up to 7.9% and 15%, respectively, after the addition of sand particles. The average pressure within the flow-passage components increases, while the streamlines become more non-uniform, accompanied by the formation of vortices and backflow in the guide and return vanes. The total entropy generation increases with rising particle concentration but decreases with larger particle size. Among the components, the high-entropy regions are mainly located on the suction surface and trailing edges of the impeller blades, the inlet and blade surfaces of the guide vanes, and the inlet and trailing edges of the return vanes. Moreover, the pressure pulsation amplitudes at monitoring points in the vaneless region, guide vane–return vane interaction region, and leading edge of the return vane increase progressively with both particle size and concentration. The dominant and secondary frequencies at all monitoring points correspond to the blade-passing frequency (BPF) and its harmonics, indicating that rotor–stator interaction is the principal cause of pressure pulsations under pump operating conditions.