Study On Cavitation Characteristics And Suppression Method Of Multistage Centrifugal Pump As Hydraulic Turbine

China is a big energy consumption country,and there are problems such as unbalanced energy supply and demand and low utilization efficiency.Under the background of " carbon peak " and " carbon neutralization," China has accelerated the high-quality development of energy,promoted the optimization of energy structure,and promoted the efficient and rational use of energy.Pump as Turbine(PAT)is a way to improve energy efficiency.It can convert the pressure energy of high pressure liquid into rotating mechanical energy.It has the characteristics of simple structure,low price and easy maintenance.It is widely used in industrial waste energy recovery and hydropower generation.As a hydraulic turbine,the multistage centrifugal pump has a compact structure and small space occupation.It can obtain a wider energy recovery range by adjusting the impeller series,and has a broad application prospect in industrial production.However,the internal fluid flow state of the multistage centrifugal pump as a hydraulic turbine is a pressure gradient during the working process.As the pressure energy is continuously converted,a low pressure zone will appear with the strong rotation of the impeller,which is prone to cavitation.The generation and development of cavitation will reduce the hydraulic performance of the hydraulic turbine,induce vibration and noise,and seriously affect the normal operation of the hydraulic turbine.In this paper,a three-stage centrifugal pump is used as the research object.The cavitation characteristics and suppression methods of the hydraulic turbine are studied by numerical simulation.The main research contents are as follows :1.Study on cavitation characteristics of multistage centrifugal pump as hydraulic turbine.In order to study the internal unsteady flow characteristics of multistage centrifugal pump as hydraulic turbine under different cavitation degrees,the cavitation numerical simulation was carried out under the optimal working conditions,and the internal flow characteristics of the main flow components were analyzed.The results show that a small amount of cavitation is generated after the cavitation inception,which reduces the internal flow loss,increases the pressure head by 1.99% and the efficiency by0.17%.In severe cavitation state,the pressure head decreases by 49.25% and the efficiency decreases by 2.29%,which seriously affects the hydraulic performance of the turbine.There are airfoil cavitation and vortex cavitation in the last stage impeller.The distortion of the blade tail causes the local pressure of the suction surface to be too low,which induces airfoil cavitation.The main reason for the vortex cavitation phenomenon is the passage vortex generated by the excessive inlet flow angle of the impeller.Cavitation occurs first in the diffusion tube at the outlet of the tailrace chamber.The flow state in the chamber is affected by the uneven flow at the inlet of the tailrace chamber.The flow on the left side is more chaotic and the flow on the right side is more regular.2.Study on the influence of cavitation on pressure fluctuation characteristics and radial force of multistage centrifugal pump as hydraulic turbine.In order to study the pressure pulsation characteristics and radial force variation law of multistage centrifugal pump as hydraulic turbine under different cavitation degrees,the pressure pulsation variation law of the last stage impeller and guide vane is comprehensively analyzed by combining time frequency domain characteristics and range value.The results show that the main frequency of pressure pulsation at each monitoring point in the last stage impeller is low frequency,and the pressure pulsation intensity under different cavitation coefficients is negatively correlated with the radial distance of the positive guide vane.Under the critical cavitation state,the pressure fluctuation law changes greatly,the pressure fluctuation intensity increases significantly,and the fluctuation is more chaotic.The main frequency amplitude rises and shifts to the low frequency direction.Under the condition of severe cavitation,the tail of the suction surface of the impeller is affected by the self excited pressure oscillation caused by the change of cavitation shape and the interaction between the dynamic and static interference of the impeller guide vane,showing the characteristics of low amplitude pressure fluctuation.The generation and development of cavitation does not affect the main frequency of the monitoring points in the positive guide vane,and it is the blade frequency of the impeller under different cavitation coefficients.The number of radial force distribution regions of the last stage impeller is consistent with the number of blades.In the critical cavitation state,the radial force distribution is chaotic and loses its regularity.In the severe cavitation state,with the disappearance of vortex cavitation and the uniform distribution of cavitation near the outlet,the number of radial force distribution areas has the same regularity as the number of blades.The variation law of the radial force of the last stage guide vane is basically the same as that of the last stage impeller.3.Study on cavitation suppression of multistage centrifugal pump as hydraulic turbine.In order to study the influence and inhibition of different structures of the tail water chamber and the last stage impeller on the cavitation performance of the multistage centrifugal pump as a hydraulic turbine,three different structures of the tail water chamber and two different structures of the last stage impeller were matched respectively.The numerical calculation was carried out to explore the influence of different schemes on the cavitation performance of the hydraulic turbine and the inhibition of the optimal scheme on cavitation.The results show that the annular tail water chamber and the last stage impeller with short blades increase the efficiency of the original model by 0.30%,and slow down the development process of cavitation,so that the pressure head under severe cavitation is increased by 6.96%,which has a certain inhibitory effect on cavitation.The scheme optimizes the internal flow structure of the last stage impeller,enhances the flow stability,greatly reduces the vortex area,reduces the energy dissipation,improves the flow state at the inlet of the tailrace chamber,increases the uniformity of the inflow,reduces the area of the middle and low pressure area of the last stage impeller and the tailrace chamber,and effectively suppresses the cavitation phenomenon in the multistage centrifugal pump as the hydraulic turbine.