Research On Characteristics Of Tip Vortex And Energy Dissipation For An Axial-flow Pump

Due to the existence of tip clearance in axial flow pump,tip leakage flow will be generated in the impeller,and the interaction between the leakage flow and the main flow will form complex flows such as tip leakage vortex and separation vortex,which will seriously affect the performance of the pump under off-design conditions.Under cavitation conditions,these complex tip vortices may induce vortex cavitation,resulting in vibration and noise,affecting the stable operation of the pump.For scientific problems of tip vortex and energy dissipation in axial-flow pumps,we adopt the method of experimental observation,numerical simulation and theoretical analysis to study the characteristics of tip vortex and energy dissipation systematically in this paper.Then,a method of C groove at blade tip is proposed to control the tip vortex development under undesigned and cavitation conditions,and the mechanism of controlling the tip vortex and reducing the turbulence dissipation is revealed in the paper.The main research work and innovative results obtained are as follows:(1)The influence law of flow rate and tip clearance width on tip vortex and energy dissipation characteristics of axial flow pump is clarified,and the mechanism of turbulent dissipation in impeller and guide vane of axial flow pump is revealed.The results show that the impeller and guide vane are the main areas of high energy dissipation in axial flow pump,and the turbulent dissipation is the main part.The high turbulence dissipation in the impeller is mainly distributed on the suction surface at the inlet of the blade,the reflux area on the suction surface and the wake area at the outlet,which are caused by the flow separation at the leading edge of the blade,the reflux vortex on the suction surface and the wake flow.As the flow rate decreases,the attack angle between the fluid and the blade increases,generating more flow separation in the impeller,leading to tip leakage vortices increasing and extending further downstream along the blade suction surface,thus generating more turbulent dissipation in this area.As the tip clearance width increases,more mixing between the leakage flow and the main flow near the tip makes both tip separation vortexes and leakage vortexes increase,thus generating more turbulent dissipation around the tip clearance.(2)The influence law of net positive suction head(NPSH)and tip clearance width on tip cavitation flow and energy dissipation characteristics of an axial flow pump is clarified.The results show that with the decrease of NPSH,the attached cavitation and the leakage vortex cavitation increase,and the high turbulence dissipation area in the impeller mainly moves along the suction surface laterally and downstream.With the increase of tip clearance width,the tip cavitation attached on the blade tip and leakage vortex cavitation increase to some extent,so that the turbulence dissipation around tip clearance increases and moves downstream.The high turbulence dissipation area in the impeller is mainly distributed at the back of the cavity,which is caused by the unstable flow caused by the shedding of the tail of the cavity.(3)On the basis of the study of the influence law of the tip vortex and energy dissipation characteristics,a method of C groove at blade tip is proposed to control the tip vortex development under undesigned and cavitation conditions,and the mechanism of controlling the tip vortex and reducing the turbulence dissipation is revealed.The results show that,under the undesigned flow rate,the C groove reduces the tip separation vortex attached on the blade tip and hinders the fusion of tip separation of tip leakage vortex,thus inhibiting the development of the tip leakage vortex and tip separation vortex downstream,which makes the turbulent dissipation near the shroud reduced,finally improving the pump performance.Under cavitation condition,C groove hinders the fusion of the tip leakage vortex cavitation and tip clearance cavitation,which disperses the air bubbles,inhibiting the development of the tip leakage vortex downstream,and finally improving the pump performance.