Gas Flow at Vacuum Conditions
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Gas Flow at Vacuum Conditions book
Gas flow is one of the most vital processes that take place in vacuum systems. This chapter discusses fundamental theories on the gas flow under vacuum conditions. It considers two major gas flow regimes with typical characteristics: the continuum gas flow and the molecular flow. At subatmospheric pressures, when the mean free path of molecules is significantly smaller than the characteristic dimension, diameter of a tube with a circular cross section, flowing gas can radially be divided into coaxial cylindrical shells with the thicknesses of mean free path. The turbulent flow regime generally develops at higher pressures, gas flow around obstacles, and rough surfaces of duct walls. In the viscous laminar regime, a gas flowing in a cylindrical tube can be divided into coaxial gas shells with the thickness of a mean free path. Molecular conductance of long cylindrical tubes with invariable cross sections can be derived using a similar model as that described at the deduction of laminar conductance.