Study On The Flow Law Of The Tip Clearance Of The Centrifugal Pump With Semi-open Impeller And Its Pump Performance

With the development of fluid machinery to high efficiency and energy saving,semi-open impeller centrifugal pump is widely used in municipal,petrochemical,aerospace and other fields because of its advantages such as good non blocking,high strength and large limit peripheral speed.For the semi-open centrifugal pump,in order to avoid the high-speed rotating impeller and the inner wall of the pump cover from scratch or collision,it is necessary to maintain a certain clearance between the impeller and the pump cover-the tip clearance.Scholars at home and abroad have made some research results on the influence of tip clearance on the performance of turbomachinery,but they mainly focus on axial-flow pump and centrifugal compressor.There are few reports on how to select the tip clearance of semi-open impeller centrifugal pump,how to determine the optimal axial clearance and the flow law of leakage flow in the tip clearance.In this paper,a single-stage single suction semi-open impeller centrifugal pump provided by a domestic factory is taken as the research object,and the leakage flow law of the tip clearance of the semi-open centrifugal pump and its influence on the performance and internal flow of the pump are studied by numerical simulation.The main work and research contents of this paper are as follows:1.The influence of changing the tip clearance on the external characteristics of semi-open impeller centrifugal pump.The ratio of the blade tip clearance t to the impeller blade outlet width b is used to define the relative clearance ?.The semi-open impeller centrifugal pumps with relative clearances of 3.8%,7.7%,11.5%and 15.4%were selected as the research objects,and their external characteristics were analyzed.The relationship with the size of the tip clearance is found:For this model pump,when the optimal relative clearance ?=7.7%(ie,tip clearance t=0.5 mm)is selected,this pump has a wide high-efficiency interval and a high efficiency at 1.0Qd.Model pumps with different relative tip clearances from design conditions to large flow conditions have large differences in efficiency at the same operating point.For model pumps with relative tip clearances of 3.8%and 15.4%,the efficiency difference is up to 10%at 1.6 Qd.For the model pump with relative clearance ?=3.8%,the head and efficiency at the design operating point are higher than other model pumps with relative tip clearances.However,under high flow conditions,the head and efficiency decline quickly,the high efficiency area is narrow,and the stability is poor.2.Compare and analyze the performance difference between semi-open impeller and closed impeller centrifugal pump with the same parameters.The external characteristics of the semi-open impeller centrifugal pump with the best tip clearance determined and the closed-impeller centrifugal pump with the same geometric parameters were compared,and it was obtained that the lift and efficiency of the closed impeller centrifugal pump at each operating point were higher than semi-open impeller centrifugal pump.Under design conditions,the closed-impeller centrifugal pump is 6.86m higher than the semi-open impeller centrifugal pump,and the efficiency is 13.08%higher.Under design conditions,the head ratio and efficiency ratio of the semi-open impeller and closed-impeller are 0.95 and 0.835,respectively.The head ratio is 0.797?0.965 and the efficiency ratio is 0.819?0.861 in the full flow range.3.Analysis of tip clearance flow and axial flow in impeller under design condition.Due to the existence of the tip clearance,there are backflow,leakage flow and other unstable flows in the axial section of the impeller,and obvious backflow vortices are generated in the entire axial section.With the increase of the relative tip clearance value,the backflow vortex in the middle section of the impeller flow channel is obviously aggravated.With the increase of the relative tip clearance,the increase of the leakage amount causes the high-pressure fluid accumulated in the pressure surface of the blade to leak through the tip clearance to the suction side of the blade and form a tip leakage vortex in the flow path of the suction side of the blade.The size of the tip leakage vortex increases as the tip clearance increases.4.Vorticity distribution and its variation.The pressure difference between the pressure side of the semi-open impeller blade and suction side of the blade causes the fluid near the blade tip to form a primary tip leakage vortex(PTLV)under the action of the pressure difference,and the blade tip clearance jet interacts with the main flow to form a secondary tip leakage vortex(STLV).The primary tip leakage vortex and the secondary tip leakage vortex attract each other,wind up and form a "tubular" vortex belt on the suction side of the impeller blade.The"tubular" vortex belt develops towards the blade outlet,ruptures and eventually evolves into a Dispersed tip leakage vortex along the blade flow direction,which is dissipated at the blade outlet,exacerbating the flow loss at the impeller outlet.5.Analysis of the influencing factors of tip clearance leakage flow.The factors affecting the tip leakage flow of the semi-open impeller centrifugal pump are blade load and wheel cover scraping.The ratio of the blade load to the wheel cover scraping is "scraping rate" R,and the range of the scraping rate R along the blade flow direction under different relative tip clearances is 0.40 to 1.5,indicating that the two influencing factors of blade load and wheel cover scraping can not be ignored;The R value decreases from the inlet 1.5 to the outlet 0.4 along the blade flow direction,indicating that the promotion effect of the wheel cover scraping on the leakage flow gradually increases along the blade flow direction from the blade inlet to the outlet6.The influence of the tip clearance on the flow in the impeller of the semi open impeller centrifugal pump and its variation lawUnder the same relative tip value,as the flow rate increases,the pressure at the same radial position of the impeller gradually decreases,and the local high-pressure area at the outlet of the impeller gradually decreases.With the increase of the relative tip value,the area of the high turbulent kinetic energy area in the inlet flow path of the blade gradually increases;Under high-flow conditions,the pressure distribution in the tip layer is small,and the pressure distribution is gentle under the flow conditions,and the local high-pressure area is less than the small-flow conditions;When the model pump is under the condition of large flow rate,the distribution of streamlines in the middle section of the impeller is generally smooth,and there are few return vortices and flow separation vortices.