Study On Vortex Evolution Mechanism And Instability Characteristics Of Tip Leakage Flow In Axial-Flow Pump

As the core power equipment in national strategic projects such as the South-to-North Water Diversion Project,ship water jet propulsion,and nuclear power plants,the stability of the axial flow pump is very important.The tip leakage flow is a typical flow phenomenon,accompanied by abundant cavitation and vortex motion forms,which seriously affects the safe and stable operation of the axial flow pump.Mastering its evolution mechanism and instability characteristics has important guiding significance for the hydraulic optimization and stable operation strategy of axial flow pumps.In this paper,the leakage vortex at the tip of the axial flow pump is taken as the main research object,and the flow rate,the cavitation number,and the width of the tip clearance are used as variables to study.The NACA0009 hydrofoil is selected as the auxiliary research object,and the applicability of basic theories such as turbulence model and vortex identification is compared and verified.The research work and main conclusions of this paper are as follows:(1)Numerical calculation theoretical analysis and calculation verificationThe external characteristics and cavitation high-speed photography experiments were carried out on the axial flow pump.The SST-CC turbulence model and the ZGB cavitation model were used for numerical simulation verification.Different vortex identification methods were compared and analyzed.The leakage vortex flow was decomposed by Liutex method,and the composition of the vortex direction categories was clarified.(2)Formation mechanism and geometric influence of tip leakage vortexThe formation mechanism of the tip leakage vortex(TLV)of the hydrofoil and the axial flow pump is similar,while the axial flow pump has the active convective shear of the axial main flow,so the shear vortex strength is higher.The gap area of the axial-flow pump can be divided into leakage jet zone and shear zone.The shear zone is the main location where the leakage shear vortex is formed.The TLV concentrated vortex originates from the leakage vortex on the tip side of the middle leading edge,where is the main producing area of the TLV.The blade tip geometry has a significant effect on the TLV.The fillet of the clearance pressure edge can significantly inhibit the formation of the tip separation vortex(TSV).Increasing the gap width within a certain range will make the leakage vortex thicker and extend farther.(3)Influence mechanism of cavitation on the tip leakage vortex flowCavitation can change the vortex structure and suppress the leakage vortex strength;the larger the cavitation number,the higher the proportion of TLV vortex cavitation in the total cavitation volume in the impeller domain,and vice versa;cavitation significantly changes the physical quantity of vortex dynamics.Cavitation significantly changes the distribution of vortex dynamics physical quantities,delays the "stretching and bending term",and significantly affects the "baroclinic moment term" and "expansion term".In the standard flow condition,the leakage vortex rotates stably with the rotating shaft,and the TLV presents a "swaying" shape above the suction surface,so the pressure fluctuation near the centerline of the vortex is large,and the pressure fluctuation at the tip of the cavitation is significant.However,under the condition of low flow rate,the instability and shedding of the incoming vortex from the upper-stage blade changes the stability of the cavitation cavity on the suction surface.As a result,the cavitation shedding occurs,which has a significant impact on the stability of the tip leakage vortex flow and the pressure fluctuation field.(4)Evolution mechanism of vortices transform in tip leakage flowUnder standard flow conditions,the leakage flow is mainly in the form of circumferential vortices such as TLV and TSV.At a small cavitation number,a "circumferential low velocity zone" is formed at the end of the cavitation above the mid-rear suction surface of the blade.The leakage shear vortex gradually accumulates at this location,forming the secondary tip leakage vortex(S-TLV).Under small flow conditions,the S-TLV formed at the middle and rear of the blade will be transformed into a perpendicular cavitation vortex(PCV)due to the increased angle between the leakage flow and the circumferential direction.Interestingly,the phenomenon of mutual entrainment,collision and vortex breaking occurred between PCV and TLV.The fluid above the suction surface of the clearance area is divided into two parts: the fluid on the tip side and the fluid on the hub side.Due to the "circumferential velocity drop" of the fluid on the tip side,the fluid on the hub side is relatively "overrun" in the circumferential direction.The resulting circumferential shear produces a vertical upward torque on the TLV.The weak vortex of the TLV will form a spiral vortex structure(SWV)under the action of this torque.In addition,another mechanism of PCV formation is that TLV is partially shed from the main vortex structure due to external influences such as cavitation shedding,and is converted into a PCV structure containing an axial vortices under the action of vertical torque.Under the condition of low flow rate,vortices in tip leakage flow changes from the circumferential direction to the axial direction,and finally the tip leakage flow coexists in the form of the circumferential and axial vortices.(5)Unstable characteristics of leakage flowThe leakage vortex flow can be divided into strong vortex flow(SWF)and weak vortex flow(WVF).When the leakage vortex flows from the leading edge to the trailing edge of the blade,SVF gradually transforms into WVF.Under different working conditions,the circumferential velocity of the tip side fluid in the clearance area is lower than that of the inner side fluid.This circumferential flow inhomogeneity is widespread,and the vertical torque formed has an important impact on the leakage vortex stability.Under the action of the vertical torque,the WVF was unstable first,which appears in the form of spiral vortex fragmentation.When the "severe cavitation shedding condition" occurs,the SVF does not have time to transform into the WVF state,and vortex crushing occurs under the impact of cavitation collapse.These are two forms of tip leakage flow vortex breakdown in axial flow pumps,and are also an important mechanism for the instability of tip leakage flow.The reason for the phenomenon of cavitation shedding is mainly due to the strong disturbance caused by the impact of the incoming vortex on the cavitation bubbles on the suction surface.Some cavities then fall off from the main body in advance,and this process presents an unstable periodicity.The decrease of the cavitation number increases the cavities distribution and increases the back pressure gradient.Thus,the instability phenomenon under the condition of small flow is aggravated.The flow index plays a "qualitative change" effect on the stability of the leakage vortex,while the cavitation index plays a role of enhancing or weakening.The unstable characteristics of the tip leakage vortex flow are characterized by entropy production and pressure fluctuation.The incoming vortex constitutes a strong disturbance to the flow field above the suction surface,which improves the intensity of pressure fluctuation and the entropy production.Within a certain range,the increase of the gap width and the cavitation number will heighten the proportion of high-frequency components in the pressure fluctuation.