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. T he 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 diff erent inlet and outlet pressures and different mass flow rates. The model c an predict the viscous behaviour, parameters of flow rate and compression r atio. The predicted pattern was found to agree well with the measured patte rn as long as the regime is close to the Viscous regime. A discrepancy is t hen observed for the lower pressures close to the transition regime. This f irst study was conducted to validate the computation, to understand the lim itation 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 reserv ed.