Research On The Force Characteristics And Pressure Fluctuation Of Guide Vane Multistage Centrifugal Pump As Turbine

The continuous growth of energy demand is the primary problem facing the society,economy,technology and other aspects.The key of sustainable development is to make full use of renewable energy.In many industrial processes,such as petrochemical,coal chemical,seawater desalination,iron and steel metallurgy,a large amount of high-pressure liquid pressure energy is directly wasted,which can be recycled by using hydraulic turbine.Pump reversing as a hydraulic turbine device has been widely used because of its simple structure,small volume,low cost,reliable operation and convenient maintenance.At present,the research on centrifugal pump reversing as turbine is mainly focused on volute type single-stage centrifugal pump.By using the methods of theoretical derivation,numerical simulation and experimental research,aiming at the conversion relationship of flow,head and efficiency under the two best efficiency points when the centrifugal pump is used as the hydraulic turbine,the theory and method of optimal design of the impeller,the stability of the turbine condition and the force characteristics of the turbine.However,in the actual industrial process,there is a situation where the pressure difference is very high but the flow is relatively small,so it is necessary to select the multi-stage centrifugal pump as the turbine to meet the demand of recovery of high pressure and small flow residual pressure energy.In this paper,the guide vane type 4-stage centrifugal pump with specific speed n_s=62.5,design flow Q_d=85m³/h and single-stage head H=80m is taken as the research object.Aiming at the problems of force characteristic change and stability of guide vane multi-stage pump,the numerical simulation method is used to study the change of clearance leakage,the law of fluid movement in pump cavity,the calculation of axial force and the pressure fluctuation characteristics of main flow passage components under the two working conditions of pump and turbine.The main contents of this paper are as follows:1.The numerical simulation of the guide vane 4-stage centrifugal pump was carried out.The feasibility of the numerical simulation method was verified by comparing with the experimental results.Based on the steady numerical calculation of the guide vane 4-stage centrifugal pump under the pump condition and turbine condition,the external characteristic curve under the two conditions is obtained.The calculation of performance conversion coefficient based on the best efficiency points shows that there is a certain difference between the two conditions,compared with the performance conversion coefficient of single-stage turbine with the same specific speed,which shows that it is not feasible to directly use the performance conversion coefficient of single-stage turbine with the same specific speed to predict the performance of multi-stage turbine with guide vane.2.Compare the relative leakage of the clearance between the ring and the interstage under different flow conditions of the pump and the turbine,and get that the relative leakage of the clearance of the ring decreases with the increase of the flow;the relative leakage of the interstage in pump conditions decreases with the increase of the flow until the leakage flows change direction,and the relative leakage of the interstage in turbine conditions decreases with the increase of the flow until it finally tends to constant value.3.Based on the rotation angular velocity of the fluid in the front and rear cavities at different axial and radial positions under pump conditions of 0.75 Q_PB,1.0Q_PB,1.25Q_PB and turbine conditions of 0.75Q_TB,1.0Q_TB and 1.25Q_TB,the distribution diagram of the rotation angular velocity of the fluid in the front and rear cavities at all levels is drawn.According to the law of angular velocity distribution in the fluid core region of the cavity,the formula for calculating the rotational velocity of the fluid flow in the cavity is obtained,which is related to the flow rate and the radial position of the cavity.4.According to the numerical simulation results considering the influence of clearance leakage on the fluid movement of the pump cavity,the axial force of the model pump under the pump and turbine conditions is calculated.In a certain range of flow,the axial force in pump conditions decreases with the increase of flow,and the axial force in turbine conditions increases with the increase of flow.The axial force at the best efficiency point of turbine condition is greater than that at the best efficiency point of pump condition.The axial force of pump and turbine under the best condition is slightly larger than that calculated by the empirical formula,and the relative erro rs are 8.43%and 11.91%respectively.Therefore,the balance plate designed by the pump theory can still be used under the turbine condition.5.Based on the unsteady numerical simulation results of the simplified model of multistage pump under the pump and turbine conditions,the pressure fluctuation law of the monitoring points in the guide vanes and impellers of each stage is obtained.The pulsation intensity of each monitoring point is small under the working conditions of pump and turbine,and the model pump can still operate stably under the turbine condition.