Cryopumping in the Near Free-Molecule Flow Region

  • E. S. J. Wang
  • J. A. CollinsJr.
  • J. D. Haygood
Conference paper
Part of the Advances in Cryogenic Engineering book series (ACRE, volume 8)


Fluid mechanics can be analytically separated into the three flow regions—continuum, transition and free-molecular--by use of the Knudsen number Kn (the ratio of the mean free path to a characteristic body dimension). The ratio expresses the degree of molecule-surface interaction. In spite of the fact that the Knudsen number can only be calculated from system dimensions within an order of magnitude, it is customary to arbitrarily separate the three regions according to the Knudsen number as follows[1]: for Kn less than 0.01, the flow is in the continuum region; for Kn between 0.01 and 10, the flow is said to be in the transition region; and for Kn greater than 10, free-molecular flow conditions prevail.


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Copyright information

© Springer Science+Business Media New York 1963

Authors and Affiliations

  • E. S. J. Wang
    • 1
  • J. A. CollinsJr.
    • 1
  • J. D. Haygood
    • 1
  1. 1.ARO, Inc.TullahomaUSA

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