Contributed by the Heat Transfer Division for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received by the Heat Transfer Division May 23, 2000; revision received November 13, 2000. Associate Editor: D. Poulikakos.

The Knudsen number (Kn) is defined as the ratio of the mean free path of a gas, λ, to a dimension of an object which interacts with the gas, x,  
Kn=λx.
(1)
According to Rohsenow and Choi 1, when fluid velocities are small relative to the speed of sound, the dynamics of a gas can be loosely classified into four regimes, the boundaries of which are determined by characteristic Knudsen numbers.

When Kn<0.01, the mean free path of a gas is small compared with the dimensions of the system, and the gas may be approximated as a continuous medium. This is known...

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