The Molecular Flow Module provides tools specifically designed to assist with the modeling of vacuum systems. Historically such tools have not been available because most academic research on rarefied gas flows has been focused on aerospace applications. Gases at low pressures cannot be modeled with conventional fluid dynamics tools because kinetic effects become important as the mean free path of the gas molecules becomes comparable to the length scale of the flow. For gases the ratio of the molecular mean free path to the flow geometry size is given by the Knudsen number (Kn=λ/l). Rarefied gas flows occur when the mean free path, λ, of the molecules becomes comparable with the length scale of the flow, l. There are four flow regimes depending on the value of the Knudsen number (Ref. 1):

These flow regimes are shown in Figure 2-1.

In the continuum flow regime the Navier-Stokes equations are applicable. Gases flowing in the slip flow regime show continuum behavior except in a thin layer, the Knudsen layer, close to the surfaces of the containing geometry. The effect of the Knudsen layer can be modeled using special boundary conditions for the Navier-Stokes equations. The Molecular Flow Module is designed to address kinetic gas flows (Knudsen numbers greater than 0.1) and includes the Transitional Flow and Free Molecular Flow interfaces, which are described in more detail next.