Knudsen flow

Knudsen flow describes the movement of fluids with a Knudsen number near unity, that is, where the characteristic length in the flow space is approximately of the same order of magnitude as the mean free path. Depending on the source there is a range mentioned of for which Knudsen flow is obtained. It is named after Martin Knudsen. Other names for this flow regime are intermediate or slip flow, since it represents a transition state between free molecular flow and viscous flow. Thus the flow of fluids under Knudsen flow conditions is established both by molecular phenomena and by the viscosity.[1]

Separation processes

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.[2] 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.[3]

See also

References
  1. Roth, A. (1990). Vacuum Technology - third, updated and enlarged edition. Elsevier. pp. 62–64.
  2. Villani, S. (1976). Isotope Separation. Hinsdale, Ill.: American Nuclear Society.
  3. Kogan, A. (1998). "Direct solar thermal splitting of water and on-site separation of the products - II. Experimental feasibility study". International Journal of Hydrogen Energy. Great Britain: Elsevier Science Ltd. 23 (2): 89–98. doi:10.1016/S0360-3199(97)00038-4.
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