Simulation of the Gas Flow in Siegbahn and Holweg Molecular Drag Pumps
A turbo molecular pump is used whenever a very low pressure must be maintained in a chamber. A turbo pump consists of multiple turbo stages where each stage contains a rotor and a stator.
Modern turbo pumps combine turbo stages and drag stages. A drag stage uses the principle of molecular drag to pump gas. There are two variants of drag stages: Siegbahn stages and Holweg stages both named after their inventors.
PI-DSMC can be used to simulate the flow in turbo stages, Siegbahn stages and Holweg stages.
The image below shows a cross section of a Siegbahn stage. The rotor is basically a plain disk while the stator contains spiral ribs forming channels between them. The rotating disk drives the gas through the channels. The spacing between rotor and stator has a significant impact on the performance of the stage.
The image below shows a stator with ribs and channels. Obviously, the performance of the stage depends on the shape of the channels.
Example - Siegbahn stage
PI-DSMC was used to simulate the 3D gas flow in a Siegbahn stage with a 90 degree rotational symmetry. In order to determine the compression, the pressure at the inlet and the outlet is determined from the simulation.
The image below shows the radial velocity profile. The flow speed is high near the rotor (top edge) and low near the stator.
Example - Holweg stage
The discussion above also applies to the Holweg stage. PI-DSMC was used to determine the compression and the pumping speed using a standard geometry.