Numerical investigation of three turbomolecular pump models in the free molecular flow range

Selecting a suitable model is important for the quick and accurate calculation of the pumping characteristics of a turbomolecular pump (TMP). Three different calculation models (2D, ideal 3D, and real 3D models) in the free molecular flow range were investigated in this study through the Monte Carlo method. Results show that when blade velocity ratio C ≤ 1, the simple ideal 3D model with paralleled blades is a better substitute for the complicated real 3D model compared with the 2D model. When C ≥ 2, maximum compression ratio Kmax of the TMP calculated with the 2D or ideal 3D models tends to saturate as C increases. However, the results computed with the real 3D model increase exponentially as C increases. Further investigation shows that in the rotating reference frame, molecules move toward the tip wall as a result of Coriolis and centrifugal acceleration, resulting in a significantly large Kmax and perfect pumping speed.