Research On Influence Of Tip Clearance On Internal Flow And Cavitation Performance Of Axial Flow Pump

Axial flow pump is widely used in agricultural irrigation,south-to-north water transfer,water-jet propeller and other fields.Due to the need of installation,clearance must exist,and clearance leakage flow is an important factor affecting the head efficiency and cavitation performance of axial flow pump.In this paper,a 350ZQ-70 submersible axial flow pump with a specific speed of 700 is taken as the research model,and the internal flow of the axial flow pump under different clearance values are simulated and analyzed,in order to provide some experience for the design of high-performance axial flow pump.In this paper,according to the actual calculation resources,the RANS method is chosen.By comparing various turbulence models,the SST k-ω turbulence model with modified density is adopted,and the Z-G-B cavitation model is adopted to calculate the unsteady cavitation flow.Then,the influence of the clearance on the macroscopic performance and the microcosmic flow characteristics is analyzed.Finally,the cavitation control is explored based on previous studies.Through the study: with the increase of the clearance,the pump head and efficiency will decline,especially large clearance and small flow rate under the obvious reduction.When the clearance reaches 2 mm,the head curve appears hump,and the clearance flow instability becomes stronger.The change of clearance has the most obvious effect on the absolute pressure on the blade surface at a small flow rate.When the clearance reaches 2 mm,the high pressure area of the working face decreases in a large area,and the pressure gradient changes from radial distribution to chord distribution.With the increase of the clearance,the relatively low pressure area will appear at the inlet of the working face,while the absolute low pressure area at the back inlet will disappear.The flow rate can compensate the drop of head caused by the increase of clearance and integrate the leakage flow to a certain extent.For the working face,the pressure distribution at the hub is greatly affected by the flow rate,and the pressure distribution at the rim is greatly affected by the clearance.The large flow rate can compensate the pressure decrease at the inlet caused by the increase of clearance to some extent.For the reverse side,the effect of clearance on pressure distribution is secondary at large flow rate.At the root of the blade outlet,there is an eddy current area,where the hub shape changes from spherical to cylindrical,which will lead to load runout on the back side of the blade and has a non-negligible impact on the internal flow characteristics such as the velocity and pressure at the impeller outlet.Therefore,it should not be simplified in the three-dimensional modeling.At low flow rates,the increase of clearance mainly affects the axial surface velocity at the inlet side of the blade rim,while at design flow rates and large flow rates,the increase of clearance mainly affects the axial surface velocity at the outlet side of the blade rim.At the design flow rate,the flange clearance cavitation first appears in the middle of the blade,but not in the blade inlet.Because the position of cavitation starting point is related to the flow rate and pressure difference;With the increase of the flow rate,the starting point of cavitation is gradually away from the blade inlet,and the position of cavitation is determined by the pressure difference in the tip area of the blade.Flange clearance cavitation will affect the pressure distribution of adjacent blade working face.For the prototype pump,when the clearance increases from 1 mm to 1.5 mm,the power capacity of the impeller is mainly reduced,and the cavitation performance is close to that of the impeller.There is a critical clearance value between 1.5 and 2 mm,and when the clearance value exceeds this value,the cavitation stability decreases,leading to repeated load fluctuations on the back of the blade and induced pressure pulsation.For the prototype pump,within the range of 1-2mm,with the increase of clearance,the degree of cavitation at the rim increases continuously,but the degree of cavitation at the inlet edge decreases to a certain extent.According to the axial flow pump assembly structure,from the perspective of protecting the hub impeller body,taking into account the power capacity and operation stability,it is more appropriate to choose a clearance of 1.5 mm.For the research pump,adding a convex structure with a diameter of 2 mm and a length of 10 mm on the back edge of the blade with the middle chord failed to achieve an ideal effect.