Investigation on the fluctuation of hydraulic exciting force on a pump-turbine runner during the load rejection process

Abstract

The flow-induced vibration is one of the important reasons causing the instability of pump-turbines during the load rejection process. However, the flow mechanism is not clear. In this study, the load rejection process of a pump-turbine was investigated, considering the clearance between the runner and stationary parts, through three-dimensional turbulent flow simulation with the dynamic mesh technology. Unsteady pressure boundary conditions were established at the inlet of the spiral-casing and outlet of the draft tube. The rotational speed of runner was calculated through coupling the rigid body motion with the water flow. The simulated rotational speed of runner shows a good consistency with the corresponding experimental data. Based on the numerical results, the fluctuation of the hydraulic thrust and hydraulic torque on the runner and their formation mechanism were analysed. The results show that there exists complex flow in the draft tube and in the vaneless space during the load rejection process. The complex flow leads to the severe fluctuation of hydraulic thrust and hydraulic torque on the runner with a low frequency, which is about 0.06∼0.4 times rated rotational frequency of the runner. Consequently, the hydraulic exciting source may induce intense vibration of the runner potentially.