Numerical Simulation of 3D Flow in Turbomolecular Pump by Direct Simulation Monte Carlo Method

Abstract

A Turbomolecular pump (TMP) is one of the key apparatus to produce high and ultrahigh vacuum. It works mainly in the conditions of free molecular and transitional regimes, where the Navier-Stokes equations of continuum gas dynamics can not be correctly applied. In this study, the flow field in single blade row of one stage TMP is investigated by direct simulation Monte Carlo (DSMC) method with a 3D analysis in a rotating reference frame. Considering the Coriolis and centrifugal accelerations, the equations about the molecular velocities and position are deduced on this frame. The VSS model and NTC collision schemes are used to calculate the intermolecular collisions. The diffuse reflection is employed on the molecular reflection from the surfaces of boundary. The transmission probabilities are calculated and applied to analyze the relationship between the outlet pressure and the maximum pressure ratio. The pumping performances between H2 and N2 on the same blade speed and same blade speed ratio are compared and analyzed carefully. The maximum pumping efficiencies on the different blade angles are also calculated. Numerical results show good quantitative agreement with existing experiment data.