# Knudsen number

The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale.

It is defined as:

[itex]\mathit{Kn} = \frac {\lambda}{L} = \frac {k_B T}{\sqrt{2}\pi\sigma^2 P L} [itex]

where

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