In free molecular flow, gas–gas collisions are negligible over the trajectories relevant to the observable. Molecules still encounter walls, whose scattering of momentum and energy must be modeled. In a long narrow tube, exceeding the diameter does not by itself prove that every complete transmission path is collision-free: repeated wall reflections can create much longer paths. The approximation must be checked against those paths as well as the width. [ref]
https://farside.ph.utexas.edu/teaching/355/Surveyhtml/node223.html. Fitzpatrick, Molecular Flow, wall encounters in a long tube
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Free molecular flow refers to a regime of gas flow where the mean free path of gas molecules is much larger than the characteristic dimensions of the system through which the gas is flowing. In this condition, gas molecules travel between collisions without interacting with other molecules, often behaving as if they are in a vacuum. ### Key Characteristics of Free Molecular Flow: 1. **Mean Free Path**: This is the average distance a molecule travels between successive collisions.