Numerical investigation of the hump characteristic of a pump–turbine based on an improved cavitation model

► We proposed an improved cavitation model by which the simulation of hump characteristics can be realized. ► The differences between the results based on the single phase and cavitation model are analyzed. ► Pressure fluctuation at the wave trough of hump characteristic is determined by the rotational speed. ► The reason for forming the hump characteristics could be attribute to the cavitation at the suction side of the runner outlet. For the safe and stable operation of a pump–turbine at pump mode, the hump characteristics must be studied and the hump region should be avoided. 3-D (three dimensional), compressible, cavitating flows in a pump–turbine at pump mode were numerically studied using SST k–ω turbulence model and mixture model. The decrease of the kinematic eddy viscosity in the region of high volume fraction of water vapor was considered in the calculation. The flow and external characteristic in the hump region were analyzed. Results show that the hump characteristic of a pump–turbine might be related to the cavity flow in the pump–turbine. It is the appearance of the cavitation that reduces the head of the pump–turbine. The cavitation incipience is thought to occur when the pump–turbine runs at the peak head and the cavitation is worse at 80% discharge of the pump–turbine. The cavitation regions locate at the inlet of the suction side. Calculation results are in good agreement with experimental data. The pressure fluctuation at the wave trough of hump characteristic is determined by the rotational speed. Numerical study of hump characteristics can provide a basic understanding for the improvement of stable operation of a pump–turbine.