Study On Gas Liquid Two Phase Leakage Flow Of Axial Clearance Of Impeller Of Liquid-ring Pump

Liquid ring pump is a kind of fluid machinery used to pump gas,which plays an irreplaceable role in low vacuum industries such as gas compression and transportation.Because of its advantages of large pumping capacity and isothermal compression,it has the advantages of safety and reliability in the transportation of flammable,explosive,toxic,condensable and other gases,and is widely used in coal,chemical,electric power,metallurgy and other industries.The liquid ring pump not only has complex gas-liquid two-phase flow structure in impeller and casing,but also has gas-liquid two-phase leakage flow from high-pressure exhaust area to low-pressure suction area,and from blade pressure side(PS)to suction side(SS)in the gap between impeller and pump casing.Leakage flow not only weakens the sealing ability of the water ring,but also interacts with the mainstream,resulting in leakage vortices,which increases the complexity of the flow.Therefore,the low efficiency of the liquid ring pump and the complex gas-liquid two-phase flow restrict the improvement of the performance of the liquid ring pump.In order to explore the mechanism of two-phase leakage flow in the axial clearance of impeller of liquid ring pump,the leakage flow in the clearance is studied by numerical method in this paper.The specific work includes the following aspects:1.Considering the axial clearance of impeller,the flow in 2BE-203 liquid ring pump was simulated numerically.The gas-liquid two phases in the impeller are basically completely separated,and a liquid ring of approximately equal thickness is formed in the pump case.The gas phase area increases gradually along the rotational direction from the angle of the beginning and end of the suction zone,and the area after entering the compression zone decreases gradually until the end angle of the exhaust.In the impeller passage,the gas-liquid two-phase interface between the adjacent two blades distributes in a "S" shape,due to the effect of pressure difference.The gas-liquid interface between the same blade pressure side and suction side is not in the same position.2.The flow field structure of gas-liquid two-phase flow in axial clearance is basically the same as that of gas-liquid two-phase flow in impeller.The gas-liquid two-phase flow is completely separated.Due to the lack of blade action,the liquid area in clearance section is larger than that in the middle section of impeller.The gas-liquid interface in the suction zone of the axial clearance is disordered,and a certain amount of droplets are formed in the gas phase,and a certain amount of bubbles are formed in the liquid phase,and some droplets move along the circumferential direction,and the bubbles rotate along the circumferential direction and diffuse into the liquid ring along the radial direction.A large number of droplets exist at the end of the suction zone in the axial clearance,and some droplets move along the wall of the suction zone to low pressure suction zone.The gas-liquid interface in the exhaust zone of clearance is relatively smooth,and there are some bubbles in the liquid ring outside the exhaust zone moving towards the hub along the rotating direction of the impeller.3.As the impeller rotates,the cross section(r=120mm)of the impeller passage gradually rotates clockwise,and the phase distribution in the cross section is non-uniform along the axial direction.The cross section gradually enters the liquid phase region of the liquid ring from the exhaust region,and the liquid phase first appears in the clearance,and then gradually diffuses to the entire impeller passage.With the impeller rotating,the cross section gradually enters the suction region and the gas phase first exits in the middle and lower parts of the blade suction side,then it gradually diffuses to the whole flow passage and the axial clearance.Within a certain time interval,the droplet position and the size of the droplet region have changed,which shows that the droplet has a certain displacement in the circumferential direction,but also moves a certain distance along the radial direction.In addition,the flow velocity in the clearance obviously is less behind the main flow area of the impeller,thus forming a flow leakage in the axial clearance,which reduces the efficiency of the pump.4.Due to the action of differential pressure,leakage flow flows from pressure side(PS)to suction side(SS).The flow direction of leakage flow is opposite to the mainstream.The corner vortex appears near the entrance of the pressure side of the gap,and leakage vortex appears behind the suction side.Leakage flow injects into the back of blade(SS)at a great speed,and continues to flow forward for a long distance,forming the wall jet.The interaction between the wall jet and the mainstream forms a shear layer,and its turbulent kinetic energy is very large.The wall jet is eventually involved in the leakage vortex with greater kinetic energy.The more the wall jet has momentum in the circumferential direction,the longer the shear layer is near the end wall of the pump.The wall jet eventually slows down as it interacts with the mainstream.At the interface between the wall jet and the main flow,the velocity in the circumferential direction decreases to zero.At this point,the flow begins to separate from the end wall,so the leakage vortices appear.Because the pressure difference between the pressure side and suction side is the fundamental cause of leakage flow,the greater the pressure difference is,the stronger the leakage vortices will be,and the greater the interference to the mainstream will be.Therefore,the distribution of pressure field in the pump has an important influence on the leakage flow structure.In the gap region,because the pressure changes in the suction region and compression region are more severe than those in the exhaust region and transition region,the vorticity mainly distributes around the suction region and compression region.