Study on pressure fluctuation characteristics and internal unsteady flow features of pump as turbine system under full operating conditions

The pump as turbine (PAT), serving as the core energy conversion component under reverse power generation in pumping stations, has internal unsteady flow and pressure fluctuations that are critical to the safe and stable operation of the station. In this study, combined model experiments and numerical simulations were employed to systematically investigate PAT pressure fluctuations and unsteady flow features under both pump and turbine modes. Continuous wavelet transform (CWT) and entropy production theory were introduced to analyse complex flow behaviour, establishing the relationships among pressure fluctuations, flow characteristics, and energy dissipation at key locations. The results reveal that in both modes, pressure fluctuations within the impeller exhibit periodic fluctuations at the shaft frequency (fSF), blade passing frequency (fBPF), and their corresponding harmonics. The rotor–stator interaction induces frequency components that are multiples of the guide vane number (7fSF), particularly evident at the impeller outlet in pump mode and at the impeller inlet in turbine mode. As flow rate increases, harmonic components become more pronounced. Pressure fluctuations in both the inlet and outlet channels are dominated by low-frequency components below 10 Hz with high amplitudes. Entropy production analysis indicates that the impeller serves as the primary energy dissipation region. The coexistence of high turbulent kinetic energy and eddy viscosity zones within the impeller indicates the presence of turbulent fluctuations, which leads to unsteady pressure fluctuations and account for 35%–75% of the total entropy production. These findings provide theoretical insights for addressing resonance-related issues in the engineering application of PAT systems.