Molecular Flow

Abstract

The mean free path (mfp) of molecules increases when the gas pressure is reduced, and at low enough pressure the (mfp) is so large that the molecules begin to bounce from wall to wall of the flow channel rather than collide with each other. When this happens the character of the flow changes. Thus, different flow regimes are encountered depending on the value of the ratio

$${\rm{Kn}} = {{(mfp)} \over d} = {{\rm{mean\;free\;path\;of\;molecules}} \over {\rm{diameter\;of\;flow\;channel}}}$$

where Kn is called the Knudsen number.