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.

In the paper, three dimensional unsteady CFD numerical method is established to simulate load rejection transient of Francis pump-turbine in pump mode. First, pumped storage power station’s whole flow system geometric model including diversion tunnel, surge tank, penstock, pump turbine unit and tailrace tunnel has been built and subdivided with prism and tetrahedral mesh. Then, three dimensional unsteady CFD simulation begins from the original steady pump condition, with the Realizable k–ε turbulent model. Through numerical calculation, the variation regularity of unit rotate speed, flow rate, torque, axial thrust and static pressure of measuring points with time are revealed during the load rejection transient. By comparison with experimental data, the changing regularity of transient dynamic parameters can be verified and their differences in the detail also can be reflected. Simultaneously, the phenomenon of water hammer has been captured by recording time-varying static pressure of measuring points. Furthermore, flow configuration in the passage undergoes very complex and unsteady change during load rejection transient.