Theoretical And Experimental Study On Magnetic Fluid Rotary Seal For Sealing Liquid

Magnetic fluid is a novel functional material which shows both liquidity and paramagnetism.Magnetic fluid seal is non-contact liquid seal with a series features such as zero leakage,low friction,long life and high reliability,which has been successfully applied in many fields,such as sealing gas and vacuum,etc.However,in a liquid environment,the operation of magnetic fluid seal involves complex physical processes,so the seal performance is difficult to guarantee.At present,the failure mechanism of magnetic fluid seal for sealing liquid is not clear,and there is no magnetic fluid seal structure which can effectively seal liquid.Based on the above background,in this paper,two kinds of rotary seal structure of direct contact and gas isolating type are studied by theory,simulation and experiment to solve the problem of the technology of sealing liquid with magnetic fluid.In theory aspect of direct contact magnetic fluid seal,the Kelvin-Helmholtz instability of the interface between the magnetic fluid and the sealed liquid is studied.To improve the performance of magnetic fluid seal under the condition of direct contact of magnetic fluid and the sealed liquid,two structures of direct contact magnetic fluid seal are designed:one is the structure with small gap;the other is the structure with a shield and small gap.In simulation aspect,the magnetic field of different seal gap is simulated by using the finite element software Ansys.On the basis of the above simulation,the theoretical maximum of critical pressure of magnetic fluid static seal is obtained.The simulation results show that the magnetic fluid seal structure with small gap can obviously improve the seal performance of the magnetic fluid seal.In order to investigate the damage process of the interface between the magnetic fluid and the sealed liquid,the fluid dynamics of the sealed liquid and magnetic fluid are simulated by the software Fluent.Phase distribution and velocity distribution of the district of the sealed liquid and magnetic fluid are obtained.In experiment aspect of direct contact magnetic fluid seal,the interfacial stability experiment of two kinds of fluid and the experiment of seal performance are carried out.The results of the interface stability experiment show that the stability time of the interface is shortened with the increase of the spindle speed,which verifies the conclusion of the theoretical study.In the experiment of seal performance,a test rig of direct contact magnetic fluid seal is set up and the seal performance tests of two direct contact magnetic fluid seals for sealing liquid medium are carried out on the experimental platform.The experimental results show that the magnetic fluid seal structure with small seal gap can obviously enhance the critical pressure and seal life of magnetic fluid seal;After adding the shield,the seal life is further prolonged.When the seal gap is 0.05mm and the thickness of the shield is within 3mm,the seal life increases with the increase of shield thickness.When the thickness of the shield is greater than 3mm,the seal life of the shield with different thicknesses is basically unchanged.Under the conditions of the sealing gap being 0.05mm,the thickness of the shield being 10mm and the rotating speed being 2000r/min,the sealing structure does not leak for 120 hours in a continuous operation.However,when the rotating shaft speed is higher,the seal life of the seal structure adding the shield is still low.In order to enhance the performance of magnetic fluid seal for liquid medium fundamentally,on the basis of the original magnetic fluid seal,the structure of magnetic fluid seal with gas isolating device is studied which can prevent the direct contact of magnetic fluid and the sealed liquid.Gas chamber is arranged between magnetic fluid and the sealed liquid in the model.Magnetic fluid and the sealed liquid are separated by compressed gas.The problem is transformed from sealing liquid medium into sealing gas medium.In simulation aspect,the magnetic fluid seal with gas isolating device is modeled to simulate the fluid dynamics of the sealed liquid and compressed gas.The simulation comes in two stages involving gas charging process and gas pressure retention process.Phase distribution,pressure distribution and velocity distribution of the district of the sealed liquid and compressed gas are obtained in the two stages.The simulation results show that when the gap of compressed gas channel is not more than 0.3mm,the interface of compressed gas and the sealed liquid being in a stable state,the structure of magnetic fluid seal with gas isolating device has good seal performance.In addition,the shaft speed has no significant influence on the stability of the interface of compressed gas and the sealed liquid.In experiment aspect,a test rig of magnetic fluid seal with gas isolating device is set up.Critical pressure test and seal life test for sealing liquid medium are carried out on the experimental platform.Experimental results show that the performance of the structure of magnetic fluid seal with gas isolating device for sealing liquid is not affected by shaft speed.Its critical pressure is the same as that of the magnetic fluid seal for sealing gas,and the seal life is obviously prolonged which each is more than 120h.The seal performance of the structure of magnetic fluid seal with gas isolating device for sealing liquid is remarkably promoted.In order to adapt to the change of the pressure of the sealed liquid,the structure of magnetic fluid seal with gas isolating device adding closed loop control system is proposed.The control system is used to ensure the stable operation of the structure of magnetic fluid seal with gas isolating device.