Experiments in Nonisothermal Diffusion of Moisture in Wood

John F. Siau, Fucheng Bao, Stavros Avramidis


A series of nine experiments was performed with one surface of the wood specimen maintained at approximately 70 C and the other at 29C. The warm surface was exposed to relative humidities between 10% and 50%. One series of four experiments was conducted with the cool surfaces exposed to relative humidities between 34% and 42%, while those of the other series of five were exposed at 52% to 53%. In both series, a reversal of flux direction was observed as the relative humidity of the warm surface was increased. The results were analyzed using the general sorption data presented in the USDA Wood Handbook using two equations, the first of which is based on a gradient of activated moisture content and the second on a gradient of chemical potential that contains an additional term to account for the effect of the temperature gradient. Both equations predicted the observed reversal of flux direction, but the chemical potential equation generally provided the better fit to the experimental results.


Nonisothermal diffusion;activated moisture content;chemical potential;activation energy of diffusion

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