Numerical simulations of vacuum pumps are a powerful approach for predicting performance characteristics and optimizing pump designs, eliminating the need for costly prototypes and experimental setups. Among the various vacuum pump technologies, turbomolecular pumps including disk-type (Siegbahn) molecular drag stages, stand out for their high efficiency in the transitional flow regime. This application study demonstrates how the Direct Simulation Monte Carlo (DSMC) method implemented in PICLas can be effectively used to simulate, analyze, and enhance the performance of disk-type molecular drag pump systems, delivering deeper insights into their operational behavior and optimization potential.
Mesh discretization & setup
The experimental configuration involves a single-stage disk-type (Siegbahn) molecular drag pump operating within the transitional flow regime. The system comprises a grooved rotor (Type-II) and plane stators positioned both above and below the rotor. Featuring ten grooves, the rotor allows the utilization of rotational symmetry within a 36° slice.
The simulation is conducted in the rotational frame of reference, with diatomic nitrogen as the test gas. The discretized geometry is presented above, and additional experimental and geometric details can be found in the work by Kwon et al. (2004). Boundary conditions include a fixed outlet pressure and a constant mass flow at the inlet. Complete diffuse reflection and full thermal accommodation is assumed as the gas-surface interaction. The simulation is carried out at various outlet pressures with different mass fluxes, specifically at 13.35 Pa, 26.65 Pa, and 40 Pa, combined with a mass flux of 0 sccm, 10 sccm, 30 sccm, and 60 sccm per pressure level. The rotational speed of the rotor are 24,000 rounds per minute. A simulation result is shown in terms of the surface pressure in the figure below.
Comparison with experimental measurements
Experimental measurements were conducted by Kwon et al. (2004) at two distinct locations: the inlet (p1,in) and the outlet (p2,out). The pressure readings obtained from the inlet are compared to the simulation results below and show good agreement. It should be noted that the accuracy of the simulation results strongly depends on the accommodation coefficient of the surface boundary, which is a function of the surface material, its finish temperature, and the gas species.
Finally, predictive simulations can be performed to investigate new concepts and gain detailed insights into the pumping mechanisms in the transitional flow regime. In addition to simulating individual helical-type (Holweck) and disk-type (Siegbahn) molecular drag pumps, it is possible to simulate multiple stages within a turbo-molecular pump.
More information about the experimental setup and measurements can be found here:
- Kwon, M. K., & Hwang, Y. K. (2004). A study on the pumping performance of the disk-type drag pumps for spiral channel in rarefied gas flow. Vacuum, 76(1), 63–71.
