Research On The Effect Of Semi-open Impeller Tip Clearance On Centrifugal Pump Performance

The semi-open impeller centrifugal pump is often used in petrochemical,fire protection,aerospace field and other fields due to their advantages of simple processing,convenient replacement and cleaning.Because it has no front cover,there is a certain tip clearance between the pump casing and the blades.This structure results in the semi-open impeller centrifugal pump with lower efficiency than the same type of closed impeller centrifugal pump,and sensitive to the axial displacement of the rotor.From the perspective of the internal flow structure,the main factor causing the low efficiency of the semi-open impeller is the tip clearance leakage flow.It will not only hinder the main flow and increase the flow loss,but also increases the severity of the centrifugal pump cavitation.Therefore,the reasonable design of the tip clearance has become a key issue in the study of semi-open impellers.In this paper,a semi-open impeller centrifugal pump is taken as the research object.Through numerical calculations and experiments on the model pump,the internal flow of the blade tip clearance,axial force and cavitation performance of the centrifugal pump are studied.The main research contents are as follows:1.The effect of the tip clearance change on the flow structure of the inner flow field of the semi-open impeller centrifugal pumpFive schemes of blade tip clearance ?=1.3mm,1.1mm,0.9mm,0.7mm,0.5mm are designed.The effect of tip leakage flow on the flow structure of semi-open impeller under different tip clearances is studied.The calculation model adopts the SST k-ωturbulence model.The results show that with the decrease of the tip clearance,the leakage of the tip clearance decreases,and the head and efficiency of the centrifugal pump increase obviously,and the axial force increases.Using the Q criterion to discriminate the vortex in the blade tip clearance,it is found that as the clearance decreases,the strength of the main leakage vortex weakens,and the secondary leakage vortex gradually disappears.When the clearance is reduced to 0.7mm,the secondary leakage vortex disappears completely,and the strength of the main leakage vortex is the weakest.The blade pressure and velocity distribution are related to the tip clearance.The smaller the tip clearance,the larger the range of the high-pressure zone on the blade working surface near the blade outlet,which makes the pressure change at the outlet of the impeller more severe.At the same time,the amplitude of all speed on the blade increases,and the static head and dynamic head of the impeller increase,the growth rate of the impeller efficiency decreases.2.The effect of tip clearance change on cavitation performance of centrifugal pump with semi-open impellerThe effect of the tip clearance on the cavitation performance of the centrifugal pump under five schemes with ?=1.3mm,1.1mm,0.9mm,0.7mm,0.5mm is studied.The Zwart cavitation model is selected as the numerical calculation cavitation model,and the visualization experiment is used to verify it.The results show that the cavitation distribution area in the tip clearance is consistent with the cavitation distribution area in the impeller.Reducing the tip clearance can effectively suppress the cavitation caused by the tip clearance leakage vortex.With the decrease of the tip clearance,the cavitation performance of the centrifugal pump has increased significantly.But there is a certain range of tip reduction.When it is reduced to ?=0.5mm,the local leakage vortex cavitation will be strengthened again.?=0.7mm is the best value of blade tip clearance.3.Influence of the circumferential position of the balance hole on the cavitation performance of the centrifugal pumpThe influence of four schemes with the circumferential position angle of the balance hole θ=10°,15°,30° and 45° on the cavitation performance of the centrifugal pump was studied.Studies have shown that the change in the circumferential position of the balance hole has little effect on the external characteristics of the pump,but under cavitation conditions,as the position of the balance hole close to the blade back surface,the cavitation range in the impeller is reduced,and the cavitation of the tip clearance leakage vortex is suppressed.In contrast,the design of the balance hole is best near the back of the blade(θ=45°).