Numerical Study on the Relationship among Air Permeability, Pore Size and Pressure Assuming Molecular and Viscous Flows

The relationship among air permeability, pressure, and pore size from a viscous-to a molecular-flow region is not well understood. In this work, air permeability in a straight circular pipe was studied considering viscous and molecular flows. I learned that the air-permeability coefficient and intrinsic air-permeability coefficient exhibit contrasting pressure dependence: that of the air-permeability coefficient is larger in a larger pore, whereas that of the intrinsic air-permeability coefficient is larger in a smaller pore. I thus proposed a method to obtain the air-permeability coefficient at atmospheric pressure from that measured under vacuum or pressurised condition. From the Reynolds number study, turbulent flow study is unnecessary in air flow in concrete.