Parametric Design Of Multistage Submersible Pump And Study On Suppression Of Secondary Separation Flow

The multistage submersible pump is widely used because of its strong adaptability and easy pressurization.The complex and changeable working environment has higher requirements for its operation stability.Therefore,the improvement of hydraulic performance of the multistage submersible pump is significance for energy saving and emission reduction.Diffuser is the main flow component of multistage submersible pump,the flow field is easy to induce the secondary flow separates vortexes at corner region during the working process.Therefore,the purpose of this paper is to explore the formation mechanism of the separation vortex in the corner region,and put forward the control method of the secondary flow separation vortex to improve the efficiency of pump.With the support of Jiangsu Natural Science Foundation（BK20190847）and China Postdoctoral Science Foundation（2019M661744）,this paper considers the Q80-20multistage submersible pump as the research object,using CFD simulation calculation and central composite design,and the diffuser was optimized by Stacking,Blade lean and Sweep,the diffuser efficiency,static pressure recovery coefficient and non-uniformity were selected as the optimization objectives.The main research work and contents of this paper are as follows:（1）The secondary flow separates vortexes in hub-suction surface corner region of the diffuser were captured by CFD,and the mechanisms of its formation are as follows:（1）In the inlet of diffuser,the curvature difference between shroud and hub surface induces the spanwise pressure difference,the spanwise pressure difference makes the fluid on hub surface to overcome the transverse pressure difference and flow to the pressure surface.（2）In the middle of diffuser,the fluid overcomes the adverse pressure gradient,and flows into the middle of diffuser.Therefore the low momentum fluid increases and deflects to the suction surface under the transverse pressure difference.The low momentum fluid accumulated on the suction surface cannot overcome the spanwise pressure difference from shroud to hub,but only flow back along the hub surface to form a secondary flow separation vortex.（2）On the basis of the blade inlet angle and outlet angle unchanged,the parametric equation is used to control wrap angle（?）of each flow surface,and then adjust the change law of the outlet edge to realize the parametric design of Stacking scheme.（1）The results of Stacking linear parametric design show that the better hydraulic performance can be obtained by decreasing the shroud wrap angle when the slope is negative.The reason is that the reduced wrap angle of hub weakens the transverse secondary flow in the middle of the diffuser,and at the same time increases the spanwise secondary flow from hub to shroud,thus the low-energy fluid in the corner region is reduced.Therefore,the diffuser performance increases unidirectionally with the decrease of slope.（2）The parabolic scheme is better than the linear scheme.Because the scouring of spanwise secondary flow at the inlet and the the transverse secondary flow in the middle are decreased.So the static pressure recovery coefficient is increased by 2%compared with the prototype pump,and the non-uniformity is reduced by 12%.（3）On the basis of Stacking scheme,the parametric design of Blade lean is carried out for the diffuser.The leading edge angleθof each flow surface of diffuser is controlled by the parametric equation,and then the circumferential change law of the inlet edge is adjusted.（1）The results of linear parametric design of Blade lean show that slope b₁is increased,the spanwise secondary flow at the inlet scour the inlet edge more seriously,the transverse pressure difference in the middle increases,and the low-energy fluid in the hub-suction surface corner region increases,so the diffuser efficiency and static pressure recovery coefficient decrease.（2）Blade lean parabolic scheme improves diffuser efficiency and static pressure recovery coefficient,but reduces the outflow uniformity.This is because the transverse pressure difference in the middle of diffuser increases,more fluid accumulates in the suction surface corner region,thus the flow uniformity is reduced.At the same time,the enhanced transverse pressure difference at the inlet offsets the scouring of hub by the spanwise secondary flow at inlet.Therefore,compared with the prototype pump,the static pressure recovery coefficient is increased by 2.64%,and the diffuser efficiency is increased by 0.32%.（4）Based on the Blade lean scheme,the parametric design of Sweep is carried out for the diffuser.It is assumed that the inlet and outlet points of blades on the hub surface are the initial position of flow direction,the positions of other flow surface profiles are controlled in the meridian plane by parametric equations,and then the distribution law of blade inlet and outlet edge is adjusted.（1）The results of Sweep linear parameter design show that the slope c₁is decreased,the spanwise pressure difference from shroud to hub of diffuser and the transverse pressure difference in the middle of diffuser decrease simultaneously,which jointly inhibit low energy fluid.Therefore,the efficiency of diffuser increases and the non-uniformity decreases.（2）The influence of the interaction between inlet and outlet direction changes on the hydraulic performance of diffuser was further investigated.The results show that the Sweep central composite design scheme further weakens the spanwise pressure difference at the inlet and the transverse pressure difference at the middle of diffuser,and meanwhile increases the spanwise pressure difference at the outlet.Therefore,the low energy fluid at the trailing edge is reduced,and the non-uniformity is reduced by 13.1%compared with original diffuser.However,as the adverse pressure gradient increases,the low energy fluid in the middle of diffuser increases,thus limiting the improvement of the diffuser efficiency.The research results of this paper have been applied in enterprise and achieved some results.