| Improving the operation stability of the axial flow pump is an important issue to be solved in the design and safe operation of the axial flow pump.Exploring the flow control method to improve the stable operation range of the axial flow pump is one of the keys.Based on the experience of vane-top jet control in compressor direction,an end-wall jet method for axial-flow pump is proposed,which aims at widening the stable operation range of axial-flow pump without reducing or improving the efficiency of axial-flow pump.In this paper,based on the in-depth study on the characteristics of axial flow pump leakage vortices,tip blockage and rotating stall mechanism,the corresponding end-wall jet scheme is designed by combining numerical simulation with experiment,and the parametric study is carried out by experimental design method.Finally,the stabilization mechanism of end-wall jet is revealed by full-channel unsteady numerical simulation.Relevant conclusions are as follows:(1)Research on the characteristics of leakage vortices at the top of blade,blockage at the top of blade and rotating stall mechanism of axial flow pump.Under the action of pressure difference on both sides of the blade,the axial flow pump blade top fluid injects into the suction side of the blade in the form of a jet to form a leakage flow,which is mixed with the mainstream to form a leakage vortex on the suction side.The tip leakage vortices are obviously unsteady.The blockage at the top of the blade of an axial flow pump is caused by the viscous flow at the top of the blade,the unstable inflow at the impeller inlet and the leakage vortex at the top.The breakdown of the leakage vortex at the top of the blade will further aggravate the blockage at the top and cause obvious unsteady blockage at the top.Under moderate stall condition,the leakage vortices at the vane top of the axial flow pump break up in the vane passage and form leading edge overflow.Partial leakage flow forms a secondary leakage flow across the top of the blade and flows to the leading edge of the blade in adjacent flow paths,resulting in a wide range of top blockage at the leading edge of the blade and forming a stall mass along the circumferential connection."Stall group" rotates at a speed lower than the speed of impeller,causing lowfrequency pressure pulsation at the blade top.Through unsteady calculation and experimental test,it is found that the rotating stall frequency of axial flow pump is about 22Hz,which is 75%of the speed of impeller.(2)End wall jet scheme and optimization design of axial flow pump.Based on the three variables of nozzle jet angle,circumferential coverage ratio and throat height,a parametric study was carried out by DOE method,and the design law of end-wall jet was analyzed by statistical method.It is found that with the increase of j et angle,the stable working range first increases and then decreases,and the optimum jet angle is 60 degrees.With the increase of circumferential coverage ratio,the stable working range first increases and then decreases.The optimum circumferential coverage ratio is 33.3%.The change of throat height has little influence on the stabilization effect of end-wall jet.(3)Stabilization mechanism of end-wall jet of axial flow pump.An unsteady numerical simulation study was carried out on the end-wall jet structure under the optimum parameters.Under this parameter,the improvement of head of axial flow pump in Hump area is 64.5%,and the improvement of efficiency at design operating point is 5.1%.It is found that the main reasons for end-wall jet stabilization are to restrain the breakdown of leakage vortices at the top of the vane and the blockage at the top of the vane.At the same time,under the action of end wall jet,the recovery of blockage at the top of blade of axial flow pump lags behind the recovery of leakage vortices at the top of blade,the unsteady flow at the top of blade decreases and the low frequency characteristic frequency at the top of original blade disappears. |
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