Diffusivity and Surface Emissivity in Wood Drying
Abstract
Edge and end-coated sweetgum (Liquidambar styraciflua L.) and redwood (Sequoia sempervirens (D. Don.) Endl.) 6 X 10 cm samples, of thicknesses ranging from 0.5 to 3.0 cm in either the longitudinal or tangential directions, were dried, from either the water-soaked condition or from slightly below fiber saturation, to equilibrium in circulated air (300 ft/min) at 100 F and 75% relative humidity.
A constant-rate drying period was observed during the early stages of drying for the initially water-soaked samples but not for those initially below fiber saturation. The apparent diffusion coefficient D', calculated for the samples initially below fiber saturation on the assumption that surface resistance to drying was negligible, that is D' ≈ 0.2 a2/t0.5 (where a is the half-thickness and t0.5 is the half-drying time), increased with increasing wood thickness. The true diffusion coefficient D and the surface emission coefficient S (which is inversely proportional to the surface resistance to drying) were calculated from the linear relationships observed between t0.5/a2 (or 0.2/D') and 1/a, or between t0.5/a (or 0.2a/D') and a, using Newman's solution to the diffusion equation.
The diffusion coefficient D was higher for longitudinal than for tangential drying for both woods, and higher for sapwood than for heartwood of redwood. The surface emission coefficient S for redwood was found to be 60% greater than for sweetgum, presumably because redwood is less hygroscopic and also less dense than sweetgum.
References
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