Study On Flow Characteristics In "S" Zone Of Pump Turbine With Splitter Blades

Pumped storage power station is recognized as a reliable peak-shaving power source in the world,which can be used in peak-shaving and valley filling with low-carbon energy grid systems,such as wind power and nuclear power.The key unit of pumped storage power station is a reversible pump turbine.The reversible pump turbine has obvious S characteristic when it runs under the working condition of the turbine.The existence of S characteristic will seriously affect the stable operation of the unit,resulting in low success rate of start-up,difficulty of working condition conversion and grid connection,etc.It is difficult to play the role of pumped storage power station to the greatest extent.Based on the hydraulic model of a domestic pumped storage power station and the test report of the model test,the SST k-?turbulence model is used to simulate the braking and backwater pump conditions of a pump turbine with shunt vanes at 7.5 degree and 23.5 degree.The flow characteristics,pressure fluctuation and the force acting on the runner in S region were studied.The main contents of this paper are as follows:Five working conditions were selected to shape the S characteristic curve,and compared with the test report to discuss the reliability and feasibility of numerical simulation.The steady calculation results under design conditions were analyzed to explore the internal flow state of each component,and to study the influence of different openings on the internal flow state.The results show that the CFD numerical calculation can simulate and analyze the internal flow characteristics of the pump turbine with shunt blades well,and the internal flow state of the flow passage components is stable under the working conditions of the turbine,and the larger the opening of the guide vane,the smoother the flow is.The constant value simulation results of braking condition and reverse pump condition near zero flow rate at 7.5 degree and 23.5 degree guide vane opening are analyzed.The internal flow state of each flow passage component under different guide vane openings under off-design conditions was explored,and the influence of shunt vanes on the flow field in the runner region was analyzed by comparing with the existingresearch.The results show that under braking condition,unstable flow appears obviously in the whole flow passage,especially at small opening.Vortex structure caused by flow blockage appears in the volute part,slight flow separation occurs in the guide vane,large-scale vortices due to uneven pressure distribution occur in the runner passage,serious recirculation occurs in the draft tube,and negative flow passage occurs in the reverse pump condition.The negative pressure is higher in the pressure zone and under large opening.The flow of volute and draft tube is smooth.The guide vane and runner are seriously affected by the S characteristics.There are eddy structures and flow separation phenomena in the guide vane and runner area,and the flow impact mostly occurs in the upper and middle parts of the runner.By comparing with the existing research,it can be found that the existence of shunt blades can effectively reduce the generation of vortices in the blade passage when the unit is operating in the S characteristic region,thus improving the stability of the unit operation.According to the steady calculation,the S characteristic is more serious at small opening.Therefore,the unsteady numerical simulation is carried out for each working condition at 7.5 degree guide vane opening to explore the variation rule of flow field with time under turbine working condition,braking condition and reverse pump working condition.The pressure fluctuation and the force on the runner of the unit at 7.5 degree guide vane opening are studied by setting monitoring points.The results show that the internal flow state of a pump turbine with shunt vanes varies greatly with the runner under braking and reverse pump conditions,and the stagnant area in the volute and draft tube is relatively fixed,and the position and scale of most vortices in the channel are irregular.Through pressure fluctuation analysis,the number of blades of the runner has a great influence on the fluctuation period of each monitoring point,mainly in leaf frequency,and the closer to the runner,the greater the influence of the runner,the greater the change of the main frequency or the secondary main frequency of the runner will show the multiple of the leaf frequency.The pressure fluctuation in the volute region is mainly affected by the static and dynamic interference between the runner and guide vane,the main frequency isequal to the blade frequency,and the working condition of the reverse pump is mainly affected by its internal flow characteristics;the high frequency fluctuation in the guide vane region is mainly affected by the runner;the low frequency pressure fluctuation in the draft tube region is mainly caused by the vortex belt in the runner channel and is more affected by the runner.Small;In terms of force,the resultant force of radial force presents continuous periodic variation,and the number of peak and valley values corresponds to the number of blades;the runner under the condition of hydraulic turbine is subjected to larger radial force and smaller axial force,which indicates that the unstable operation of the unit under this condition is mainly affected by radial force.In the braking condition near zero flow rate,the radial force acting on the runner is small,but the periodicity is poor,the axial force value is large and the fluctuation range is obvious,so the internal flow characteristic is the worst.