Knudsen flow describes the movement of fluids with a high Knudsen number, that is, where the characteristic length in the flow space is of the same or smaller order of magnitude as the mean free path.
Separation processes using Knudsen flow
For a gas passing through small holes in a thin wall in the Knudsen-flow regime, the number of molecules that pass through a hole is proportional to the pressure of the gas and inversely proportional to its molecular mass. It is therefore possible to effect a partial separation of a mixture of gases if the components have different molecular masses. The technique is used to separate isotopic mixtures, such as uranium, using gaseous diffusion through porous membranes. It has also been successfully demonstrated for use in hydrogen production, as a technique for separating hydrogen from the gaseous product mixture created when water is heated at high temperatures using solar or other energy sources.