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Molecular Flow Through a Microcapillary

Application ID: 10014

Computing molecular flows in arbitrary geometries produces complex integral equations that are very difficult to compute analytically. Analytic solutions are, therefore, only available for simple geometries. One of the earliest problems solved was that of gas flow through tubes of arbitrary length, which was first treated correctly by Clausing. Later, the integral expressions he derived were computed more accurately by Cole. These authors derived values for the transmission probability of molecules incident on a tube of arbitrary length, which is independent of the pressure, provided that the Knudsen number is much greater than one (in the molecular flow regime).

This model uses the Free Molecular Flow interface to compute the transmission probability for the molecular flow of molecules through a microcapillary with different length / radius ratios. The results are compared with the accurate solution as computed by Cole.

This model is included as an example in the following products:

Molecular Flow Module

The combination of COMSOL® products required to model your application depends on several factors and may include boundary conditions, material properties, physics interfaces, and part libraries. Particular functionality may be common to several products. To determine the right combination of products for your modeling needs, review the Specification Chart and make use of a free evaluation license. The COMSOL Sales and Support teams are available for answering any questions you may have regarding this.