Analysis of the compression characteristics of vapor jet injected into high vacuum

Abstract

The working principle of diffusion pump, a kind of vacuum pumps, is to use the high momentum of vapor jet which is injected into high vacuum. Since flow regime inside the diffusion pump consists of continuum, transition and free molecular, a continuum approach is not valid to calculate the flow field. In this paper, the compression characteristics of a vapor jet into high vacuum were simulated using the direct simulation Monte Carlo (DSMC) method. The hard sphere model is used as a molecular model and the no time counter (NTC) scheme is employed as a collision sampling technique. Steady state flow characteristics such as velocity, temperature, density distributions of the pumped gas and oil vapor are calculated by simulating about hundreds of thousand molecules. The main result is that the pumping speed of a single stage pump can be described based on the transmission probabilities which have nearly constant values irrespective of pressures at the inlet and outlet. Using these features, compression characteristics of diffusion pump can also be described by transmission probability concept as in the case of other high vacuum pump.