Influence of the clearance flow on the load rejection process in a pump-turbine

The clearance flow between the runner and the stationary parts greatly effects the efficiency and the hydraulic thrust of pump-turbines. Especially in the load rejection process, the clearance flow of the runner is more important to accurately predict the hydraulic torque on the runner and the variation of the rotational speed. However, the clearance flow was generally neglected in the most previous studies. To investigate the influence of the clearance flow on the simulation of load rejection process, three dimensional simulations were carried out in this paper with and without considering the clearance flow of the runner. To simulate transient characteristic of the boundary conditions, the unsteady total pressure and static pressure boundary conditions were determined at the spiral casing inlet and draft tube outlet using the user defined function. The method of coupling the rigid body motion with the water flow in the pump-turbine was adopted to predict the variation of the rotational speed. Dynamic mesh technology was used to simulate the dynamic closing process of the guide vanes. Based on the available experimental data, the numerical simulation results with and without considering the clearance flow of the runner were compared and analyzed. The results indicate that the accurate simulations of the fluctuating performance characteristics and the unsteady vortex flows in the pump-turbine are significantly influenced by the clearance flow between the runner and the stationary parts during the load rejection transient process. Moreover, the closer to the small discharge conditions, the influence of the clearance flow is more severe. Therefore, the shunt and impact action of the clearance flow to unsteady main flow in pump-turbines should be considered during the simulation of the load rejection transient process.