ON THE FLOW OF GASES AND WATER VAPOR THROUGH WOOD

Abstract

The quantities of water vapor passing through wood have been measured under the conditions of hydrodynamic flow and of kinetic diffusion. Air and oxygen pass through wood under hydrodynamic flow conditions in accordance with the Knudsen–Poiseuille law. Water vapor passes through wood under hydrodynamic flow conditions in much larger quantities than those predicted by the law. The rate of passage of water vapor under diffusion conditions follows the Fick diffusion law but with a diffusion coefficient that increases with relative humidity. The distinct features of the diffusion and hydrodynamic flow processes are pointed out. Porous, hygroscopic media permit the transfer of water vapor either by diffusion or by hydrodynamic flow, as determined by two distinct sets of conditions, while compact, hygroscopic media permit the transfer of water vapor by diffusion only, regardless of the external conditions.