Molecular transition and slip flows in the pumping channels of drag pumps

Numerical and experimental investigations are performed for the molecular t ransition and slip flows in pumping channels of a helical-type drag pump (H TDP) and disk-type drag pump (DTDP). The flow occuring in the pumping chann el develops from the molecular transition to slip flow traveling downstream . Two different numerical methods are used in this analysis: the first one is a continuum approach in solving the Navier-Stokes equations with slip bo undary conditions, and the second one is a stochastic particle approach thr ough the use of the direct-simulation Monte Carlo method. From the present study of a HTDP, the numerical results of predicting the performance, obtai ned by both methods, agree well with the experimental data for the range of Knudsen number Kn less than or equal to 0.01 (i.e., the slip-how regime). But the results from the second method only agree with the experimental dat a for Kn>0.01 (i.e., the molecular-transition regime). From the analysis fo r a DTDP, we found that an actual pumping passage is strictly limited to a narrow region because of the significant backstreaming of molecules from th e outlet. (C) 2000 American Vacuum Society. [S0734-2101(00)03603-4].