On gaseous microflows under isothermal conditions

Flow processes of gases and related heat and mass transfer properties in smallest channels or porous structures are of utmost interest for many technical application in engineering science. This so-called microflows occur for instance in geometrically defined microchannels which are parts of Micro/Nano Electro Mechanical Systems (MEMS/NEMS) or in porous catalysts or filters. In such applications the heat transfer or the yield in gas phase reactions is crucially dependent on the flow behavior. In microflows the fluid gas phase is in a state called rarefied, when the distance of solid boundaries is on the same order of magnitude as the gaseous mean free path. The ratio of mfp to a characteristic length is defined as the Knudsen number which is reciprocally proportional to the gaseous pressure. Hence, also gases in larger structures can be rarefied if the pressure is sufficiently low. For instance in vacuum applications or high altitude aerodynamics gaseous rarefaction has to be taken into account. Depending on the dimensionless quantified rarefaction it is commonly known that continuum fluid dynamics fail for modeling gas flows in very small geometries. Rarefied flows behave totally different as continuum flows. Hence research on gaseous microflows can contribute to the general understanding of rarefaction effects.