Study On The Influence Of Cavitation On The Hump Characteristics Of Pump Turbine

Since the beginning of the new century,with the extensive use of wind,solar and nuclear energy in China,the requirements for the regulation capacity and stability of the power grid are increasing.The pumped storage device can adjust the peak-valley difference according to the electricity consumption and improve the operation stability of the power system..Pump turbine is the core of the pumped storage power station,the hump characteristics under the pump working condition is the main reason for restricting its development.In fluid machinery,widespread cavitation phenomenon is closely related to the change of the flow stability.Therefore,this paper mainly studies the coupling relationship between cavitation and hump characteristics of pump turbine by combining numerical simulation and experimental verification.The main research contents and conclusions of this paper are as follows:(1)For the qualitative analysis of the relationship between cavitation and hump characteristics,the CFX software is used,the SST k-? turbulence model and the Z-G-B cavitation model are selected to simulate the single-phase and variable cavitation steady state.It is found that cavitation mainly occurs near the suction surface of the runner blade near the inlet of the runner.The water vapor structure formed will gradually increase with the decrease of cavitation coefficient and extend into the flow passage,thus changing the flow structure of fluid and causing the increase of hydraulic loss.In addition,due to cavitation,the multi-phase calculation shows obvious hump characteristics,while the single-phase calculation is not obvious.This difference is mainly caused by the irregular change of hydraulic loss caused by the abnormal increase of cavitation in the hump characteristic working point.(2)In order to explore the effect of cavitation on hump characteristics from the perspective of internal characteristics,LES(large eddy simulation)was used to carry out single-phase and double-phase transient numerical simulation for peak-valley operating points in hump characteristic area.Fast Fourier transform was used to conduct frequency domain analysis of pressure pulsation data,and the source of main frequency pulsation was explored from the internal flow field.Research findings on the prediction of pressure pulsation.Both of them are accurate in predicting the location of pulse main frequency.The pressure pulsation amplitude of multi-phase cavitation calculation is higher.Because cavitation occurs on the suction surface of the rotor blade,it has a great influence on the main frequency and amplitude of pressure pulsation in the draft tube basin.In the wake basin,the frequency characteristics not found in the single-phase calculation were detected in the multiphase calculation.The main frequency in each basin comes from periodic oscillation of backflow,low-speed rotation of dean vortex,periodic collapse of cavitation and periodic rotation of low-speed vortex group.(3)In order to explore the influence of the existence of interstitial flow on cavitation,tthe steady-state numerical simulation was used to carry out multiphase numerical simulation for 10 operating points considering clearance flow.The study found that when considering the gap flow,the total flow loss increases,the head and efficiency decrease,and the hump characteristics are delayed.The occurrence of interstitial flow mainly affects the flow of runner.On the one hand,the sudden expansion and contraction structure in the gap flow causes an increase in flow loss.On the other hand,the existence of the gap flow reduces the degree of cavitation,and the flow in the runner channel is improved.