Inhomogeneous Chamber States in Screw Spindle Vacuum Pumps

The identification of previously unrecognised inhomogeneous pressure distributions within the working chambers of screw spindle vacuum pumps provides new insights into their efficiency losses. These chamber inhomogeneities, which occur particular at low chamber pressures and high rotational speeds, lead to increased gap mass flow rates due to locally increased pressures and also reduce the filling efficiency by decreasing the intake of mass during the filling process.

A novel one-dimensional model is being developed to analyse these effects. Depending on the gas rarefaction, the geometry, and the gas-surface interaction, a closure problem arises, which is solved separately using analytical solutions and results from the direct simulation Monte Carlo (DSMC) method. The model is validated by experiments and three-dimensional CFD simulations. Additionally, transient CFD simulations are performed to investigate the chamber filling process with an expanding computational mesh.

Based on these findings, a dimensionless number is derived to quantify both the chamber inhomogeneity and the filling efficiency. This enables an efficient integration of the new model into a chamber model simulation software. A comparison of the simulation results with experimental data of a complete machine confirms the applicability of the model and leads to an excellent agreement.