The dimensionless ratio Kn = λ/d, mean free path divided by a characteristic length such as the bore of a tube. Below about 0.01 the gas is in viscous flow, above about 0.5 it is in molecular flow, and in between the flow is transitional. Engineers use it to decide which conductance formula applies and whether a line will be the bottleneck.
Knudsen number in that 1-inch line is over one once we're below a millitorr, so use the molecular conductance formula.
Also heard as
- Kn
Related terms
Mean free path
Average distance a gas molecule travels before it collides with another molecule.
Viscous flow
Flow regime in which molecules collide mostly with each other, so the gas moves as a continuous fluid.
Transitional flow
Intermediate flow regime between viscous and molecular, where wall and intermolecular collisions both matter.
Molecular flow
Flow regime in which molecules bounce between the walls almost without meeting each other.
Conductance
How easily a pipe, orifice or valve lets gas through, expressed in litres per second.