Molecular Flow
In contrast to viscous flow , molecular flow prevails when the mean free path of the gas particles is significantly greater than the diameter of the flow, i.e. H. if the Knudsen number is significantly greater than 1:
Molecular flow is predominant in the high and ultra-high vacuum range .
This clearly means that, due to the lack of particles in a vacuum chamber, there are almost no interactions with one another , but only collisions with the bounding walls. In simple sorption pumps , this property is used to bind particles by means of highly porous surfaces. With this passive system a weak high vacuum can be generated. Ion getter pumps are based on the same principle, but force the physisorption of free particles through ionization and electrical fields . This also makes it possible to achieve an ultra-high vacuum.
For estimations in space travel, a free molecular flow is calculated according to ECSS -E-10-04A if the Knudsen number is greater than 3. For small satellites this applies from an orbit height of 150 km, for large objects ( ISS ) 250 km.
References
- ↑ ECSS-E-10-04A (2000)