| The primary purpose of this study was to measure experimentally the uniaxial (tangential direction) equilibrium moisture profiles in moisture-sealed wood samples subjected to constant but different temperatures on opposite faces, and to compare these profiles with those predicted by each of several theoretical models. The test assembly consisted of eight end-matched wood laminae, each lamina measuring 0.2 cm (t) x 10 cm (r) x 10 cm (1). Opposite faces of each moisture-sealed assembly were exposed continuously for approximately five weeks to different but constant temperatures until the original uniform moisture content redistributed itself to a new constant but non-uniform moisture profile. At moisture equilibrium, the individual wood laminae were removed from the assembly and their moisture contents measured gravimetrically. Both the temperature and moisture profiles were plotted for each test. The experimental variables considered were wood species (yellow poplar and hard maple), initial moisture content (9%, 12%, and 15%), and temperature range (15 to 35 C, and 25 to 45 C).; At the steady state, a moisture content profile 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 Soret coefficient and the absolute magnitude of the calculated dM/dT ratio were found to vary with initial moisture content, temperature range, and species. These two quantities always increased with wood moisture content. They were also slightly higher at the higher temperature range. There was only a small difference between species, with yellow poplar giving somewhat higher mean values. The Soret coefficient was generally in the range of 0.03 to 0.05 1/K, except for the extremely high moisture contents near and above fiber saturation, where the calculated values approached 0.5 1/K.; The ratio dM/dT 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 provided the best agreement with the experimental values of dM/dT. The Siau model gave the next best prediction, followed by the Stanish model, with the Skaar-Siau model giving the poorest agreement with the experimental results. |
