Transient chamber filling in rotary positive displacement vacuum pumps

Abstract

Chamber model simulation is a common approach to simulate rotary positive displacement vacuum pumps. Therefore the pump is abstracted into working chambers and connecting clearances, whereby the clearance leakages can be identified as the major loss mechanism in such machines. The clearance mass flow rates are calculated with respect to the thermodynamic states in the adjacent chambers, which are inhomogeneous for rarefied gases due to the movement of the rotors which causes a pressure gradient within the chamber. This effect increases with higher Knudsen numbers, because of the increasingly dominant friction. It is shown that inhomogeneous chamber states cause a non-complete chamber filling. As a result the mass-averaged pressure within the suction chamber is lower than the pressure in the suction port. Due to the non-constant chamber volume over time three-dimensional transient simulations with a Computational Fluid Dynamics (CFD) solver are performed in order to investigate the mass within a geometrically abstracted suction chamber. Based on a dimensionless number, a regression analysis is done to provide a quantitative estimation of this effect by means of analytical calculations. This is implemented in a chamber model simulation software and results of a test machine are compared to measurements and to previous simulations.