Experiments on Steady-State Nonisothermal Moisture Movement in Wood
Keywords:Nonequilibrium thermodynamics, nonisothermal, thermal diffusion, moisture transport, Soret effect
The study was conducted to measure experimentally the steady-state uniaxial (tangential direction) moisture content and temperature profiles in moisture-sealed wood samples whose opposite faces were subjected to constant but different temperatures, and to compare these profiles with those predicted by several theoretical models. The experimental variables considered were wood species, initial moisture content, and temperature range.
Opposite faces of the moisture-sealed assembly were exposed continuously for approximately five weeks to different but constant temperatures until the original uniform moisture content redistributed itself. At the steady state, a moisture content gradient opposite to the temperature gradient was established. The temperature gradient was constant in all cases, with the moisture content profile increasing almost exponentially with decreasing temperature. The absolute value of the ratio of the moisture content gradient to the temperature gradient (dM/dT)s was found to increase with wood moisture content and was observed to be higher at the higher temperature range. There was only a small difference in the -(dM/dT)s between the two species studied.
The ratio -(dM/dT)s was analyzed in terms of five different theoretical models, two of which are based on nonequilibrium thermodynamics (NET) and three on classical thermodynamics. The two NET models (basic NET and Nelson models) provided the best agreement with the experimental values. The Siau model gave the next best prediction, followed by the Stanish model, and lastly by the Skaar-Siau model.
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