A Novel Sealing Method Using Nano-micron Magnetic Powders and its Leakage Rate Analysis

Abstract

Magnetic fluid sealing is a high-performance sealing technique compared to traditional sealing methods, but is limited to a few working conditions. Here we present a novel sealing method using nano-micron magnetic powders as sealing medium. Nano and micron-sized magnetic particles of different materials and size distribution were prepared and mixed. Packing process of magnetic particles was simulated by DEM. Leakage rate was calculated by Darcy’s Law. Finally, the static sealing experiment showed a high pressure resistance and a rather low leakage rate of this novel sealing method.1. IntroductionSealing with magnetic fluids has been a commonly used high-performance sealing technique with its unique advantages like low friction torque, no leakage and long lifetime. However, it is not capable of resisting high pressure difference or working under extreme temperatures. Magnetic powders are mixture of nano- and micron-sized particles which can be magnetized by an external magnetic field. This research prepared nano-micron magnetic powders of great magnetic and rheological properties. Then the packing process of magnetic powders was simulated and leakage rate was calculated. A static sealing device was designed to test the sealing behavior of this novel sealing method.2. Methods2.1 Preparation of magnetic powdersChemical precipitation was used to prepared magnetic nanoparticles. Experimental conditions were adjusted to acquire nanoparticles of different materials, sizes and magnetic intensity. Micron particles were prepared by high-energy ball milling method. Surfactants were coated on particles to control the interparticle forces.2.2 Simulation of packing processInterparticle forces of magnetic particles are complex and so is the packing process of magnetic powders. In this research, packing process of nano-micron magnetic powders was simulated by DEM using Hertzian-Mindlin model with JKR. Fraction, sizes, magnetization and surfactants of powder preparation, leads to changes of interparticle forces, especially cohesive and magnetic forces. Magnetic particles follow the basic equation:midvi(t)/dt = mig + Fid(t) + Fic(t) + Fim(t)vi(t) is the ith particle's velocity at time t; Fid(t), Fic(t), Fim(t) are the total drag force, net contact force and magnetic force of the ith particle, respectively.2.3 Calculation of leakage rate by Darcy's LawDarcy's law is commonly used to describe the flow of a fluid through a porous medium. We established a microscopic model to estimate the permeability for calculation of leakage rate of the sealing device. The permeability κ is calculated by the equation:κ = μÛh / △pμ is the fluid viscosity; Û is the outlet velocity; h is the computational zone height; △p is the pressure difference.2.4 Design and experiments of static sealing deviceA static sealing device was designed, where magnetic field distribution was simulated to maximize the magnetic flux density in the sealing gap. For each magnetic powder example, sealing capacity and leakage rate were measured.2.5 ResultsThe simulation results showed that the permeability decreased rapidly with the decrease of particle sizes, and appropriate mixture of nano- and micron-sized particles further decreased the permeability. For the powders prepared, the permeability could be as low as 5.09E-18 m-2. Considering the size of the sealing device, the calculated leakage rate was 2.22E-9 m3/s. Experiments indicated a sealing capacity ≥ 0.5 MPa and leakage rate ≤ 1E-6 Pa×m3/(s×cm).3. DiscussionBy selecting magnetic materials, mixture of particles of different sizes and coating particles with surfactants, nano-micron magnetic powders can possess great magnetic and rheological properties. Proper size distribution and interparticle forces lead to a denser packing of magnetic powders. Further, a denser packing leads to a rather low leakage rate, which is verified by both simulation and sealing experiments. A novel sealing method using nano-micron magnetic powders shows distinctive property and may be a promising sealing technique. **