Knudsen-Diffusion

Knudsen diffusion ( by Martin Knudsen ) diffusion designated processes in which particles collide more frequently with the flow limitations as with other particles.

This means that the mean free path of the particles is greater than a geometric constraint. Knudsen diffusion thus occurs when the Knudsen number is much greater than one:

This is for the diffusion of thin gases ( Knudsen gases) in porous media the case.

Knudsen diffusion coefficient ( SI unit ) is dependent on the pore geometry but independent of the pressure and of the molar mass of a second gas in which the diffusion takes place. For a long, straight pore, the Knudsen diffusion coefficient is calculated as:

With

  • - Pore radius
  • - Universal gas constant
  • - Temperature
  • - Molar mass of the diffusing gas.

The Knudsen diffusion coefficient of a gas in a porous medium can be calculated from the value for a single straight pore calculate:

With

  • - Porosity of the medium, which stands for the diffusion available ( connected pores )
  • - tortuosity

Documents

  • PDF on Knudsen flow in porous media (111 kB)
  • Statistical Physics
  • Process engineering
Knudsen-Gas
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