The Flow Characteristics Investigation Of The S-Shaped Region For Pump-Turbine In Small Guide Vane Opening

Pumped-storage power station is the most effective regulator in current power grid.,and its operation unit the pump-turbine requires higher stability and flexibility.However,there exists tow unstable regions named S-Shaped characteristic region of the turbine mode and the hump region of the pump mode that have become the bottleneck for the development of pump-turbine and the key issues that affects the safety of generating units and power systems.This paper takes the pump-turbine model of a pumped storage power station as the research object,and carried out the numerical simulation study on the S-Shaped region with the guide vane opening is 7 degree.The main work and results are as follows:(1)Three dimensional modeling and grid division were carried out for the simulation calculation domain.The turbulence model of SST k-? was adopted and the boundary conditions were given according to the model test data.A number of operating conditions of the guide vane of 7 degree were selected for steady calculation.Then,the typical operating conditions were selected for unsteady calculation.The external characteristic curves obtained by the experiment test and simulation are compared to verify the feasibility of the numerical simulation method.(2)The flow field of pump turbine under different operating conditions is analyzed.By means of the pressure difference analysis and entropy production analysis,it is determined that different span-wise locations in runner shows that,under off-design conditions,the vortex structure forms on the hub side of the runner,the water ring forms in middle span-wise,and the back flow forms on shroud side.When the unstable flow structure appears in the runner,the high meridional and circumferential velocity areas appear at the inlet of the runner and larger range of low speed areas appear in the runner passages.The inflow in the runner passages is obstructed.When the water ring appears on the middle flow surface,the flow around the head of the runner is serious.(3)The methods of fast Fourier transform(FFT),peak-peak value of pressure pulsation and flow topology are used to analyze the pressure pulsation in the vaneless area and runner.The main frequency of pressure pulsation in the vaneless region is 20fn,which is mainly affected by dynamic and static interference.However,when it operates under partial operating conditions,a significant low frequency pulsation component appears.And the water ring has a positive influence on the high frequency component,and the vortex on the hub side has a negative influence on lower frequency.The distribution of peak-peak value shows that the pressure from inlet to outlet of runner is gradually reduced.The peak-to-peak pressure pulsation on the runner blade gradually decreases from the inlet to the outlet of the runner,and the pulsation peak-to-peak value of the suction surface is lower than that of the pressure surface.It shows that the flow structure on the pressure side is more complicated than that on the suction side,and this conclusion is proved by the flow topology analysis method.(4)The analysis for force characteristics of runner shows that as the flow rate decreases,the total force of the runner gradually decreases,and the amplitude of the radial force and axial force pulsation increases.After enters the runaway condition,the radial force component on the runner blade increases.The component of the axial force is relatively large on the hub and shroud sides,but in the opposite directions and canceling each other to reduce the force on the main shaft,while the components on the blade can be ignored basically.The load analysis of the runner blade shows that the load distribution at the inlet and outlet of the runner blade peaks,and the peak on pressure surface is lower than the suction surface.With the change of operating conditions,the pressure difference between the two sides of the blade decreases which means the function power of the runner decreases.