Flow modelling of a Holweck pump stage in the viscous regime

The gas flow through a vacuum component can be continuous, transitional or molecular depending on the pressure range and geometries involved. In this study, we investigate the exhaust stage of a ``hybrid turbomolecular{''} vacuum pump with a Holweck system. Continuous gas flows were simulated using a 3D computational fluid dynamic (CFD) code, which is commercially available. The 3D computational model uses as inputs: pressure, temperature of the gas, type of gas and geometry of the pump. The results of the computations are gas flow, local velocities, pressure, temperature, and velocity fields. The model was tested and validated on several Holweck pump geometries for different inlet and outlet pressures and different mass flow rates. The model can predict the viscous behaviour, parameters of flow rate and compression ratio. The predicted pattern was found to agree well with the measured pattern as long as the regime is close to the Viscous regime. A discrepancy is then observed for the lower pressures close to the transition regime. This first study was conducted to validate the computation, to understand the limitation of the code and know more about the flow in this type of geometry, and to optimize the design. (C) 2001 Elsevier Science Ltd. Al rights reserved.