Research On The Flow Structure Of Internal Jet And Its Effects On The Cavitation Performance In A Centrifugal Pump

The phenomenon of submerged internal jet(internal jet for short)near the impeller inlet of the canned motor pump with the internal cooling circulation will reduce pump cavitation performance and affect the steady operation of the pump.In this paper,based on the numerical simulation and experiment methods,internal jet flow field structure and unsteady pressure pulsation characteristics are studied in a centrifugal pump with the internal jet.The cavitation performance of the pump is predicted.The morphology and distribution of the cavitation bubbles are revealed.The measure to improve the cavitation performance of the canned motor pump is explored,and finally the modified canned motor pump is tested.The main work and results of this paper are as follows:1.Three-dimensional model and mesh generation are carried out for the flow passage of the centrifugal pump with the internal jet,and predict the performance of the pump.Compared with the centrifugal pump without the internal jet,the head of the pump with the internal jet decreases,the shaft power increases and the efficiency decreases.The average axial velocity and flow rate of the internal jet decrease with the increase of the pump flow rate.The larger the internal jet hole diameter,the better the axial force balance effect.2.Based on the steady numerical calculation,the basic flow field characteristics of the internal jet in the pump are explored.According to the axial velocity distribution,the flow field in the inlet section of the pump is divided into four zones:the internal jet core zone,the internal jet boundary layer zone,the affected mainstream zone and the unaffected mainstream zone.When the internal jet exists,the static pressure value at the suction side of the impeller inlet decreases,and a local low pressure region appears near the axis.The vorticity in different zones of the inlet section presents a"double-peak"distribution and decreases with the development of the internal jet.The effect of the internal jet on the turbulence intensity in the inlet section is related to the internal jet hole diameter.The larger the hole diameter,the stronger the influence.3.The pressure pulsation in the inlet section shows the periodic variation characteristics,and there are obvious differences at different monitoring points.There are three main excitation frequencies in the pressure pulsation signal of the inlet section,including the blade frequency,the frequency generated by the influence of shaft hole and the low frequency caused by the periodic change of the internal jet.The low frequency signal is affected by the axial velocity field of the internal jet and is closely related to the working condition and the internal jet diameter hole.4.The influence of different internal jet hole diameters on the cavitation in the impeller passage is explored,the reasons of cavitation occurring in the internal jet itself are revealed and cavitation flow in the internal jet flow under different working conditions are compared.Cavitation in advance occurs in impeller passage because of the internal jet.The larger the internal jet diameter hole,the stronger the influence on the cavitation development of the passage.The larger axial velocity of the internal jet cause the local low pressure area in the inlet section,and the cavitation of the internal jet is more likely to appear.The cavitation of the internal jet at small flow rate appears earlier than that at large flow rate.When the NPSH_a decreases,the cavitation of the internal jet at small flow rate presents the vortex band structure and eventually transform into a more violent cavitation cloud structure.With the increase of flow rate,the development of the cavitation of internal jet is slow and the cavitation of internal jet only develop into vortex band structure.5.The cavitation characteristics of the canned motor pump with internal cooling circulation are studied,and a modified canned pump model with diversion pipe is proposed.Vortex band structure appears in the inlet section at different conditions when NPSH_a decreases in the unmodified canned motor pump,and the vortex band structure at small flow rate will eventually develop into annular cavitation cloud structure,and the vortex band structure remains unchanged at large flow rate.The cavitation performance of the modified pump with diversion pipe is improved remarkably.In the presence of diversion pipe,the development of cavitation in the impeller passage is inhibited,and only small cavitation bubbles appear near the internal jet hole.So the cavitation performance is better than that of the unmodified canned motor pump.The modified canned motor pump is further tested.On the premise that the head meets the requirements,the critical NPSH of the pump is reduced to 16.7%of that of the unmodified canned motor pump,and the cavitation performance is significantly improved.