As a low-cost scheme for small-scale hydropower generation, pump as turbines (PATs) are used at different hydrosites around the world. Nevertheless, a big number of recently conducted studies on PAT performance have mainly focused on the centrifugal type, despite the fact that the axial-flow type has a comparatively large flow capacity, thus disposing of higher power density. Therefore, this article seeks to investigate the flow dynamics of an axial-flow PAT and associated energy loss characteristics, under both pump and turbine operating modes. It adopts the numerical simulation method and uses entropy production theory to propose an energy loss intensity model in the cylindrical coordinate system, which quantitatively gives the spatial variation pattern for energy losses in pump and turbine operating modes. In addition, the correlation between energy loss and flow instability is deeply analyzed, where the energy characteristics in pump and turbine modes are quantitatively evaluated. It is shown that the energy loss within the impeller and the guide vane flow fields, for both operating modes, is mainly because of the turbulent entropy production. The proportion of direct entropy production and wall entropy production is found to be relatively small. The velocity gradient, flow vorticity, turbulence intensity, and energy losses within the flow passages of the axial-flow PAT have been closely related. However, owing to the difference in PAT operating modes, there is a significant difference in the location of energy losses. The unstable flow phenomena, such as the impact at the blade inlet, flow deviation at the blade outlet, flow separation, back-flow, and vortex, are the main reasons for entropy production. This study serves as a reference for the design, optimization, and application of axial-flow PATs.

The transient characteristic of the power-off process is investigated due to its close relation to hydraulic facilities’ safety in a pumped storage hydropower (PSH). In this paper, power-off transient characteristics of a PSH station in pump mode was studied using a three-dimensional (3D) unsteady numerical method based on a single-phase and volume of fluid (SP-VOF) coupled model. The computational domain covered the entire flow system, including reservoirs, diversion tunnel, surge tank, pump-turbine unit, and tailrace tunnel. The fast changing flow fields and dynamic characteristic parameters, such as unit flow rate, runner rotate speed, pumping lift, and static pressure at measuring points were simulated, and agreed well with experimental results. During the power-off transient process, the PSH station underwent pump mode, braking mode, and turbine mode, with the dynamic characteristics and inner flow configurations changing significantly. Intense pressure fluctuation occurred in the region between the runner and guide vanes, and its frequency and amplitude were closely related to the runner’s rotation speed and pressure gradient, respectively. While the reversed flow rate of the PSH unit reached maximum, some parameters, such as static pressure, torque, and pumping lift would suddenly jump significantly, due to the water hammer effect. The moment these marked jumps occurred was commonly considered as the most dangerous moment during the power-off transient process, due to the blade passages being clogged by vortexes, and chaos pressure distribution on the blade surfaces. The results of this study confirm that 3D SP-VOF hybrid simulation is an effective method to reveal the hydraulic mechanism of the PSH transient process.