Optimization Design And Unsteady Characteristics Study Of Mixed Flow Pump Impeller Based On Circulation Distribution

Due to its good comprehensive performance,the guide vane mixed flow pump has been widely used in industrial production,urban sewage treatment and large regional water transfer projects.In recent years,with the start of the Diversion from Yangtze River to Huaihe River and the second phase of South-North Water Diversion Project,the performance of mixed flow pump has also pushed forward towerd higher requirements.Therefore,it is important to study the design optimization of mixed flow pumps.At present,the design methods of mixed flow pump mainly include traditional design based on geometric parameters and inverse design based on hydrodynamic parameters.The inverse design method has the advantages of fewer design parameters,closer connection between parameters and hydraulic performance,and higher possibility of obtaining innovative solutions.However,there are few studies on the optimization of mixed-flow pumps based on inverse design,and even in these studies,in order to facilitate the control of the impeller head,the impeller outlet circulation is mostly set as a constant distribution.From the inverse design theory and related research,it is known that the impeller outlet circulation distribution has a great impact on the performance of mixed-flow pumps.Therefore,it is of great significance to study the influence of the circulation distribution on the optimal design of the mixed-flow pump based on the inverse design.This dissertion is funded by the National Key Research and Development Program of China"Multi-objective hydraulic design method of vane pump and key technology of gas-liquid two-phase pressurized transportation（2018YFB0606103）"and the Wuhan Science and Technology Plan"Research,development and industrialization of key technologies for large-scale high efficiency water transfer pump system（2018060403011350）".Based on the inverse design method,a mixed-flow pump impeller with 511 specific speed was designed and optimized with constant,linear and non-linear circulation distribution.The influence of circulation distribution on the energy characteristics,unsteady characteristics,entropy production characteristics and tip leakage flow characteristics of the optimized results were studied.The main work and innovative achievements of this paper are as follows:（1）By using the parabolic equation to describe the circulation distribution at impeller outlet,with r V_h and r V_s as control parameters,combined with the coupled calculation of continuity equation,energy equation and velocity gradient equation,the parameterization of the circulation distribution at impeller outlet under constant,linear and non-linear distribution is completed.Based on the characteristics of the circulation method,the optimization strategy of the impeller of the guide vane type mixed flow pump is analyzed.On this basis,the r V_h,r V_s and streamline loading control parameters are used as design parameters,the efficiency at 0.8Q_des,1.0Q_des and 1.2Q_des are used as the optimization objectives,as well as the head at 1.0Q_des,L₂₇(3¹³)standard orthogonal table was used to carry out parameter analysis on a vane-type mixed pump model with 511 specific speed under the above three circulation distributions.It was found that the maximum difference in head was essentially constant in each experimental design for constant,linear and non-linear distributions of the circulation.r V_h and r V_s not only have a large influence on the performance of the mixed-flow pump,but also on the response relationship between the other parameters and the performance of the mixed-flow pump.The variation range and maximum value of the efficiency at each working condition under the nonlinear distribution of circulation are increased compared to the constant and linear distribution of circulation.（2）Taking the significance factors in the results of orthogonal test analysis as the design parameters,the head at design condition variation less than 3%as the constraint,and the weighted efficiency at 0.8Q_des,1.0Q_des and 1.2Q_des as the optimization objectives,the mixed-flow pump model was optimized by combination using the experimental design,approximate model and optimization algorithm under constant,linear and nonlinear distribution of the circulation,respectively,and the internal flow patterns of the optimized results F0（original model）,F1（optimized results of constant circulation distribution）,F2（optimized results of linear circulation distribution）and F3（optimized results of nonlinear circulation distribution）were compared and analyzed.It was found that the circulation distribution has a great influence on the energy characteristics and the blade loading distribution.In addition,when a nonlinear circulation distribution is used in the optimization design and a larger value of circulation is given at the hub,the axial velocity and total pressure near the hub can be effectively increased,thus preventing the boundary layer de-flow caused by the accumulation of low-momentum fluid,and further improve the optimization results.（3）The reliability of the optimized design was verified by building a test bench and conducting experimental measurements of the energy characteristics of the F3.The trends of pressure pulsation intensity with flow rate at different locations were investigated by analyzing the standard deviation of the measured pressure signals at the P1（impeller inlet）,P2（between the impeller outlet and the guide vane inlet）and at the P3（guide vane outlet）.The time domain and frequency domain characteristics of the pressure pulsation signals at different conditions and different locations were studied by performing fast Fourier transform on the measured pressure signals.The trend of the amplitude of each frequency domain with time under different working conditions at different locations was studied by performing continuous wavelet transform on the measured pressure signals.It was found that the optimization system used in this paper is reliable.The pressure pulsation intensity at P1 and P3 decreases first and then increases with the increase of flow rate,while the pressure pulsation intensity at P2 basically does not change with the change of flow rate.The main frequency at P1 and P2 is 1 times the blade frequency,and the amplitude does not change with the change of time,while the main frequency at P3 changes with the change of the flow,and the amplitude also changes rapidly with the change of time.（4）On the basis of verifying the accuracy of numerical simulation by experiments,the time-averaged velocity,time-averaged pressure,pulsating velocity and pulsating pressure in F1,F2 and F3 are compared and analyzed by unsteady numerical simulation,and the influence of circulation distribution on the internal unsteady characteristics of the optimization results of the mixed-flow pump is studied.It was found that compared with the constant distribution of circulation,the use of linear and nonlinear distribution of circulation in the optimization design is beneficial to the reduction of the internal pulsation velocity value of the optimization result,thereby reducing the turbulent energy loss.In addition,the use of nonlinear circulation distribution in the optimization design can also significantly reduce the pulsating pressure value inside the optimization result,thereby improving the operation stability of the mixed-flow pump.（5）Based on the theory of entropy production,the spatial distribution characteristics of time-averaged entropy production,wall entropy production,pulsation entropy production and total entropy production in F1,F2 and F3 were quantified and visualized,and the influence of circulation distribution on the internal energy loss characteristics of mixed-flow pump optimization results was studied.It was found that the entropy production loss in the downstream components of the impeller is affected by the flow pattern at the impeller outlet,and it has a great contribution to the total entropy production loss of the mixed-flow pump.However,the use of linear and nonlinear circulation distribution in the optimal design is beneficial to suppress the backflow near the hub at impeller outlet,and improve the attack angle near the shroud,thereby reducing the loss of entropy production in the impeller and its downstream components.（6）According to the assembly characteristics of mixed-flow pump impeller blades,the tip clearances with dimensions of 0.2mm,0.4mm,0.6mm and 0.8mm are set at the shroud of F1,F2 and F3,respectively.The influence of the circulation distribution under different tip clearance sizes on the energy characteristics of the mixed-flow pump is studied.Based on the diagnosis theory of vortex dynamics,the influence of the circulation distribution on the impeller’s work performance under different tip clearance sizes is studied.It was found that the use of nonlinear circulation distribution in the optimization design is beneficial to the reduction of the total pressure flow reduction on the blade pressure surface near the shroud caused by the tip leakage flow,and the suppression of the flow separation on the blade pressure surface caused by the entrainment of the tip leakage flow and the main flow,thereby reducing the drop of head and efficiency caused by the increase of the tip clearance size.