Cavitation Effect On The S-shaped Characteristics Of Pump-turbine

In recent years,with the rapid development of economy and the continuous expansion of power grid,pumped storage power station has become the focus of attention because it can effectively solve the problem of peak and frequency modulation in power grid.The pump turbine tends to enter the unstable S-shaped region under the working condition of the turbine,the unit efficiency is seriously reduced in this area,and cavitation always occurs when the pump turbine enters the S-shaped region.Cavitation can disrupt the normal flow of liquids and increase the range of instability.In this paper,the cavitation S-shaped region of pump turbine is studied by numerical simulation,main research contents of this article are as follows:(1)Steady-state calculation is carried out for typical operating points in the Sshaped region,and the hydraulic loss and flow characteristics are analyzed.The research shows that the losses under different working conditions in the S-shaped region are mainly caused by the runner and the guide vane,and the losses are mainly caused by the vortex in the flow passage.Based on the analysis of flow characteristics,it is found that the formation of S-shape characteristics is related to the high speed circulation and flow separation at the runner inlet.When cavitation occurs,more vortices will form in the field,which increases the instability of the flow.The change of flow rate will affect the location and shape of the cavity.When the flow rate is large,the cavity volume is small;when the flow rate is lowl,the cavity volume increases and the flow passage is blocked.(2)The single-phase unsteady calculation is carried out for typical working conditions,and the formation reason and propagation rule of pressure pulsation in the field are analyzed.The results show that the flow of the turbine condition is stable,and there is no characteristic low frequency in the flow field,only high frequency due to the rotation of the runner.Under braking condition,the backflow area produces a low-frequency pressure pulsation in the flow field,and the position of the backflow zone changes constantly with the runner.Under the small flow reverse pump condition,a large number of vortex are generated in the flow passage,but their shape is basically unchanged.Therefore,the amplitude of pressure pulsation in the reverse pump condition is relatively low,and the low-frequency pressure pulsation in the flow field is also related to the backflow area.(3)The unsteady calculation of cavitation in typical conditions on the S-shaped region is carried out,and the pressure pulsation caused by cavitation is analyzed.The research shows that cavitation has little influence on the turbine condition turbine and only produces a low-frequency pressure pulsation with low amplitude in the flow field.The pressure pulsation is related to the periodic expansion and reduction of the cavitation.Under braking condition,the backflow area generated by bladeless area is still the most important factor affecting low-frequency pressure pulsation.When the pressure drops,the vortex at the outlet of the guide vanes increases obviously,and the generation of vortices affects the appearance of cavitation.By comparing the amplitude of pressure pulsation at different conditions,cavitation has a large influence on the draft tube under braking condition.The pressure pulsation of the draft tube is affected by the periodic generation and collapse of cavitation in the runner,and the amplitude of low-frequency pressure pulsation will increase significantly.