Effect of ``Rougher-than-Rough'' Surfaces on Molecular Flow through Short Ducts

Abstract

Gaede reported lower molecular flow rates through some tubes than expected if he assumed the walls were 100% diffuse reflecting. He conjectured that back reflection from surface asperities might be responsible. We support Gaede's intuitive guess, having measured flow rates and applied the Monte Carlo calculational method to geometrical models. Our results show that: (1) surface finishes from ½ to 64 μ in. (0.013 to 1.6 μ) roughness yield flow rates from 5% above to 6% below the diffuse wall prediction; (2) specular reflection can account for the 5% higher value; (3) for cylinders with internal corrugations, fins and threads of groove depth from 0.1 to 15% of the minimum tube radius and included angle between 30° and 60°, conductance is from 15 to 20% lower than conductance assuming diffuse reflection from smooth walls. Experiment and calculation agree within ±4%.