| Pump-turbine combines turbine and pump.In order to meet the requirements of power g rid,it needs to change frequently between design and non-design conditions,and the operation time in non-design conditions is longer than that in design conditions.There will be unstable flow area in the relation curve between flow rate and head of pump and turbine.In this area,one head corresponds to at least two flows.The flow in the channel becomes disordered,which causes the self-excited vibration of the unit and greatly interferes with the normal operation of the unit.This is what we c.all the hump area.Based on the incompressible assumption of fluid,the Navier-Stokes equation,continuity equation and Heart Stress Transport t-urbulence model of fluid are used to simulate a low specific speed pump turbine model.The flow characteristics and their causes in the hump region of pump operation are analyzed,and the influence of the upper crown clearance on the flow characteristics in the hump region is also discussed.Next:(1)The constant values of pump-turbine with different guide vane openings are calculated under the design flow rate.When the guide vane openings are greater than 80%of the design openings,it is found that the guide vane openings have less influence on the unit head,efficiency and head loss of the overflow component’s;when the guide vane openings are less than 80%of the design openings,the guide vane openings have less influence on the unit head,efficiency and head loss.FHead loss of overflow components has a greater impact.(2)The constanl value calculation of pumped-turbine with different flow rates at a certain opening is carried out.When analyzing the influence of flow rate on the flow characteristics of the unit,it is found that the efficiency decreases with the decrease of flow rate,and the head increases first and then decreases with the decrease of flow rate,forming the so-called hump zone.With the decrease of flow ratev￥ ·’the unstable flow first appears in the straight cone section of draft tube and forms a recirculation flow,at the same time,in the forming part of the runner outlet.When the unstable flow moves to the guide vane region,the flow in the volute is destroyed by the double helix motion.(3)Steady calculation of pump-turbine with different upper crown clearance values in the hump area is carried out.When analyzing the influence of clearance on the flow characteristics of units,it is found that the existence of clearance reduces the flow area in the hump area from 20%to 70%of the original design flow to 20%～45%of the design flow.The existence of clearance makes the distribution of flow velocity in draft tube more uniform,effectively reduces the outlet pressure of runner and greatly reduces it.The vorticity at the inlet of the guide vane makes the pressure distribution in the volute more uniform,and the pressure relationship between the inner and outer walls recovers somewhat.(4)The unsteady calculation of pump-turbine with a certain upper crown clearance value is carried out,and the pressure fluctuation characteristics under non-design working conditions are obtained by analyzing the time domain and frequency domain diagrams of pressure fluctuation at monitoring points.When the flow rate is large,the pressure fluctuation within each flow passage component changes little,and the main frequency of each monitoring point basically keeps the same.As the flow rate decreases,the pressure fluctuation increases sharply in the flow passage of the unit entering the hump area,and its main frequency begins.Change and inconsistency occur.On the basis of previous studies on hump area,this paper supplements its characteristics and generation mechanism,and provides guidance for subsequent studies.Then,it is found that the existence of upper crown clearance will reduce the flow rate in Hump area and alleviate the hump characteristics,which is helpful for the study of hump under the simultaneous action of gap flow and unstable flow.The motion state of the area laid the foundation. |
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