Interplay between magnetophoresis and diffusion in magnetic fluid seals for vacuum devices and their lifespan

The paper presents the results of numerical simulation of the problem of diffusion and magnetophoresis rivalry in a magnetic fluid seal (MFS) for vacuum devices. It is taken into account that the diffusion coefficient and viscosity of the fluid (i.e., friction on the surface of the sealed shaft) depend on the concentration of particles. A modified expression for the mobility of particles in a magnetic fluid is used. The result of the rivalry of the studied processes determines the lifespan of the device in the standby mode.

It is revealed that with a sufficiently large magnetic field under the rectangular pole of the MFS, the magnetic fluid loses its fluidity and the device fails. In smaller fields and with large gaps between the pole and the shaft, the maximum concentration and maximum viscosity are not so high, and the seal lifespan in the parking mode is unlimited. The effect of the fluid properties and the design parameters of the MFS pole on the lifespan of the device in large magnetic fields is found. The optimum gap between the pole and the shaft is 0.15–0.2 mm with a pole tip width of 0.3 mm. In the case of a vacuum oil-based magnetic fluid, the seal lifespan in parking mode is about one and a half years.