Study On The Influence Of Tip Clearance Leakage Flow On Stall Characteristics Of Semi-open Centrifugal Pump

With the rapid development of industrial technology,the operating conditions of the medium-low specific speed centrifugal pump gradually expand to a small flow rate.Therefore,the problem of widening the stable operating conditions of the centrifugal pump needs to be solved.The flow separation in the centrifugal impeller is easy to evolve into a stall vortex during small flow operation,and the stall vortex expands to the entire flow path,causing stall in the impeller;especially in a semi-open centrifugal impeller,the unsteadiness of the tip leakage vortex accelerates the formation and development of the stall.The purpose of this paper is to study the correlation mechanism between the tip leakage flow and stall inception in the impeller.Based on this,the flow characteristics inside the impeller under different casing treatment methods are analyzed.The medium-low specific speed semi-open centrifugal pump is used as the research object.The unsteady tip leakage flow and its correlation with stall initiation are studied by numerical simulation.Meanwhile,the circumferential groove,axial straight groove and axial chute groove of the casing treatments are processed,and the improvement mechanism of stall and the influence of the three casing treatment methods on the flow inside the impeller are deeply analyzed.This paper first analyzes the steady and unsteady characteristics of the tip leakage.The steady-state characteristics show that the leakage characteristics mainly depend on the streamlines released by the gap height below 75%of the leading edge of the blade.As the flow rate decreases,the leakage vortex rupture position moves up.The unsteady results show that the unsteady pulsation of the tip load is an important cause of the instability of the tip leakage vortex;the tip separation vortex generated by the blade leading edge overflow is the main component of the unsteady characteristics of the tip leakage flow.Secondly,the correlation between the stall inception and the tip leakage vortex at three tip clearances are analyzed.At TC1 the blocking effect caused by the leading edge corner separation is stronger than that of the tip leakage vortex.The corner separation rapidly forms a stall vortex at the near hub and develops into a full-leaf high stall.At TC1.5.the tip separation vortex is mainly formed by the overflow of the leading edge.The overflow of leading edge and the break down of tip leakage vortex cause the blocking of the leading edge.When the leading edge overflow and the trailing edge backflow simultaneously stall occur.At TC2.5,the leading edge overflow increased the blockage at the tip and evolved into a part span stall.Therefore,as the gap increases,the leakage flow gradually plays a leading role in the formation of the stall.Finally,the flow characteristics of the three casing treatment methods at the stall point of the impeller are analyzed.The calculation results show that the backflow vortex at the tip region is treated into a rotating vortex at the tip of the groove,and the leading edge blocking caused by the tip leakage vortex is decreased.When the casing is treated in the axial straight groove,the separation vortex near the tip moves into the axial straight groove,and the two vortex in the straight groove rotate in the opposite direction.The axial straight groove treatment can effectively improve the flow features of the tip region.But there are still blockages in other flow channels.The casing treatment axial chute groove causes the backflow vortex at the leading edge to be decomposed and moved into the axial chute groove,forming three vortices with different sizes,which can effectively decrease the backflow and blockage of the tip region,but it cannot completely eliminate the blockage.Therefore,among the three casing treatment methods,the circumferential groove treatment is most effective in decreasing the blocking effect at the stall point.This paper clarifies the unsteadiness of the tip leakage flow and its action mechanism in the stall inception of the impeller,and compares the flow laws of the casing treatment methods in the stall condition.The research on the related mechanism of stall and its control methods has important reference value for improving the operational stability of semi-open impeller centrifugal pumps,and provides a further research basis for the study of subsequent casing treatment methods.