Monte Carlo simulation of gas free molecular flow in turbo molecular pump’s inlet tube

ABSTRACT

Due to the beaming effect of inlet tube and reflection effect of turbine cover in the turbo molecular pump, the positional and angular distribution of incident gas molecules (IGM) on the turbine blades would no longer be uniformly distributed. Based on the test particle Monte Caro method, a new self-defining procedure is proposed and it can be more accurate to describe the status of IGM into turbine blades. The results show that the influence of rotor speed on the transmission probability is indistinctive. Meanwhile, the incident density between the tip and root of the blades is distinct. The positional beaming effect and the angular beaming effect are enhanced with the lengthening of the tube and less molecule diverge towards the tube wall. The positive transmission probability of first-stage turbine blades using our new method is different from those obeying the cosine distribution under the same number of incident molecules. The calculation results of new method verified by another procedure are reasonable and accurate.

KEYWORDS:

• Monte Carlo simulation
• turbo molecular pump
• inlet tube
• free molecular flow