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Knudsen Number (Kn) Equation and Calculator

Fluids Systems Design and Engineering

Knudsen Number (Kn) Equation and Calculator

The Knudsen number is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. The Knudsen number in a viscous flow phase < 0.01 and molecule-molecule collisions dominate gas behaviour which behaves as a fluid. In molecular flow Kn > 1 and molecule-surface collisions dominate..

Kn = λ / d



λ = Mean free path (mm)
d = Diameter of the opening that gas will pass through (mm)