Analysis and Comparison of the Models for the Rarefied Gas Effect on Gas Lubrication

Ultrathin gas film lubrication has been widely used in recent years, such as dry gas seal face gas lubrication, gas lubrication in the hard disk drive, etc. The rarefied gas effect must be considered when the gas film thickness is very thin. In order to analyze, compare, and select the appropriate rarefied effect model on gas lubrication, a comparative analysis has been carried out on the influence laws of the Poiseuille flow rate and the ratio of the effective viscosity coefficients and the dynamic viscosity, μeff/μ, with inverse Knudsen number, D ,or Knudsen number, Kn, as to different models. The results show that when inverse Knudsen number increases or Knudsen number decreases, the Poiseuille flow rate and the ratio of the effective viscosity coefficients and the dynamic viscosity,μeff/μ, of different models approach to each other. However, there are significant differences as to different models when the Knudsen number is large, and only several models from Hwang, Veijola, Peng etc agree with Fukui’s model.

Simulation of Gas Flow Over a Micro Cylinder

Gas flow over a micro cylinder is simulated using both a compressible Navier-Stokes solver and a hybrid continuum/particle approach. The micro cylinder flow has low Reynolds number because of the small length scale and the low speed, which also indicates that the rarefied gas effect exists in the flow. A cylinder having a diameter of 20 microns is simulated under several flow conditions where the Reynolds number ranges from 2 to 50 and the Mach number varies from 0.1 to 0.8. It is found that the low Reynolds number flow can be compressible even when the Mach number is less than 0.3, and the drag coefficient of the cylinder increases when the Reynolds number decreases. The compressible effect will increase the pressure drag coefficient although the friction coefficient remains nearly unchanged. The rarefied gas effect will reduce both the friction and pressure drag coefficients, and the vortex in the flow may be shrunk or even disappear.