Research On The Influence Of Tip Clearance On Unsteady Flow And Vibration Characteristics In Axial Flow Pump

Axial flow pump is one of the key components in the power system of large ships.Due to its special structure,there must be a gap between the impeller and the pipeline wall,which usually does not exceed 0.1% of the impeller diameter.Although the size of the gap is very small,the impact of the small vortex structure inside it(the scale ratio of the impeller flow field to the gap flow field vortex structure exceeds 1000)on the main flow field of the impeller cannot be ignored.In addition,the impact of superimposed impeller rotation stall makes the hydraulic characteristics inside the pump more complex.This article calculates the rotational stall state of the pump under different blade tip clearances using a modified DDES turbulence model,and finds that the modified DDES turbulence model is in good agreement with the experimental results of pressure pulsation.In addition,this article uses the DDES turbulence model for analysis under different blade tip clearances,and preliminary conclusions are drawn as follows:The phenomenon of rotational stall in axial flow pumps is caused by the uneven distribution of velocity in the inner diameter of the impeller.However,as the gap size increases,the non-uniformity of radial velocity decreases,which to some extent alleviates the hydraulic loss of axial flow pumps.In addition,when an axial flow pump operates under optimal operating conditions,the increase in gap size will increase the tip leakage flow intensity,and the increase in gap size will promote the energy dissipation of the pump.Rotating stall will result in high amplitude and high frequency of pressure fluctuations in axial flow pumps.When the size of the tip clearance is increased,the frequency and high amplitude under stall conditions will be suppressed.However,under optimal operating conditions,the increase in tip clearance size will promote the frequency and amplitude of pressure fluctuations.In addition,this paper introduces the vorticity transport equation to conduct in-depth analysis of the structure of the tip leakage vortex,and preliminarily obtains that the amplitude of pressure fluctuations is affected by the compression-expansion term,and the frequency is affected by the tensile term and viscous dissipation term.The rotating stall characteristic will intensify the deformation and vibration of the blade tip.At the same time,increasing the gap size can reduce the deformation of the blade tip under the rotating stall condition.In addition,the equivalent stress decreases as the gap size increases.However,under optimal operating conditions,on the contrary,when the gap size increases,the blade deformation will significantly increase.The performance indicators of axial flow pumps with different blade tip clearances under different inflow conditions are completely different.Therefore,from a comprehensive performance perspective,it is reasonable to control the size of the blade tip clearance at 0.05% D.