Analysis of free molecular flow in a tube of finite length using the time-transient Monte Carlo method.

Abstract

Results by modification of the time transient method are presented, in which the superficial average velocity (streaming velocity) can be obtained in addition to molecular number density. The molecular number flux was thus obtained by the product of molecular number density and superficial average velocity.The results of analysis of molecular flow in a circular tube were newly studied. The study revealed that there was a distortion of molecular number density distribution affected by outer storage. It became clear that the jumps in molecular number density and superficial average velocity at inlet and outlet openings were induced by discontinuity of macroscopic transfer rate (molecular number flux) between the tube and the outer space.For a finite length tube, using Knudsen’s simple equation for a tube of infinite length, the transfer rate, i.e. the conductance, can be easily calculated by applying the slip distance.