Numerical Simulation And Experiments Of The Flow Instability In Mixed-Flow Pump

The mixed flow pump has the advantages of large flow rate,good anti-cavitation performance and wide range of high efficiency,so it is widely used in the South-to-North Water Diversion,power industry and water supply of urban.However,the decline of pump performance and flow instability caused by the stall,tip leakage flow and tip leakage vortex are serious problems of South-to-North Water Diversion project.This paper was supported by national natural science foundation”investigation transient cavitation form and induced hydraulic oscillation characteristics of mixed flow pump in accelerated flow condition(51579118)”.The large eddy simulation and pressure fluctuation experiment investigated instability flow in mixed flow pump.The influence of tip gap size and gap pressure difference on tip leakage flow was analyzed,the relationship between the leakage and the swirl strength of tip leakage vortex,the influence of the tip leakage flow on inlet velocity of the impeller.And then understanding the reasons of decline of performance in mixed flow pump and flow instability characteristics.Therefore,a method of suppressing tip leakage flow is proposed.The works are as follows:(1)The effect of tip gap sizes on tip leaf leakageSelected four kinds of tip clearance(0mm,0.25 mm,1mm,2mm)model pump to simulate and analyze the distribution of the pressure difference in the gap area,the leakage on the seven characteristic surfaces,the distribution of axial velocity at inlet of blades and the tip leakage vortex swirl strength.With the increase of the tip clearance the leakage is gradually increased and the leakage from the impeller inlet to outlet at the same tip clearance is increased at first and then decreased.On the one hand,the area of the seven characteristic surfaces is increased at first and then decreased,and more importantly,the distribution of the pressure difference in the gap area is increased at first and then decreased.At the same time,the swirl strength of the tip leakage vortex is increased as the tip clearance increased so the leakage and flow loss of the pump will increase.(2)The effect of grooves on the performance and flow stability of mixed flow pumpsTo reduce tip leakage and increase performance of mixed flow pumps,1mm tip clearance model pump will be selected to research the effect of the groove angle,numbers and deeps on pump performance.The simulation result shows that the minimum goove angle(case 1)in the four grooved angles has the most significant effect in reducing the tip leakage and tip leakage vortex swirl strength.flow direction of the leaf leakage flow is control by the case1,2 and 3,The leaf inlet flow field is not affected by the leakage flow,So that the velocity distribution of blade inlet is not affected by the tip leakage flow,which effectively alleviates the stall of the impeller.The simulate result of different number of grooves show that with the increase of grooves the tip leakage is gradually in 0.4Qopt and 0.8Qopt conditions.But the number of grooves has little effect on tip leakage vortex swirl strength and the distribution of axial velocity at adjacent blade inlet.So the case1?case5 and case6 is reducing the amount of tip leakage to affect the performance of the mixed flow pump.The simulate result of different deeps of grooves show that with the increase of grooves the tip leakage gradually decreased in 0.6Qopt to 1.0Qopt condition,but increased in the 0.4Qoptt condition.As the groove depth decreases,the control of the tip leakage flow direction is reduced,resulting in back flow increased at blade B inlet.However,the case5,case7 and case8 has similar tip leakage vortex swirl strength and the distribution of axial velocity,which indicating that the decrease of performance of case7 and 8 is main caused by increasing in leakage losses.(3)Numerical simulation and experimental pressure fluctuation analysisThe simulation and experimental results show that pressure fluctuation of monitoring point at blade inlet,blade outlet and guide vane inlet are regular in 0.4Qopt to 1.0Qopt.There are four peaks and valleys in one impeller rotation cycle.However,the pressure fluctuation of monitoring point located in guide vane outlet is irregular in 0.4Qopt,with the increase of flow rate,there are five peaks and valleys in one impeller rotation cycle.The main frequencies of pressure fluctuation at inlet of blade and guide vane measuring points in 0.4Qopt is blade passing frequency,but there are some differences between the simulated and experimental results at t blade outlet and guide vane outlet monitoring point.With the increase of flow rate the simulation results are similar with the experimental results,the main frequencies of pressure fluctuation at inlet of blade,outlet of blade and inlet of guide vane measuring points is blade passing frequency,while t the main frequencies of pressure fluctuation at outlet of guide vane is 5 times axis passing frequency.