抄録
The purpose of this paper was to measure the change of moisture diffusion coefficient with drying and then to investigate the relationship between moisture content and diffusion coefficient. In this research, discussions were limited to the isothermal moisture movement, and also one dimensional diffusion in the direction of the board thickness (tangential direction) of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco.). During conventional kiln-drying, the inner moisture distribution change of the board can be non-destructively detected with the medical X-ray CT Scanner. Therefore, it can been continuously measured at the same part of the same section. This method was developed in this study to obtain more accurate moisture content in wood from CT images. As the noise of images was reduced by employing alternative techniques, the standard error achieved finally was 0.8% MC in the hygroscopic range. The diffusion coefficients were calculated directly from the moisture distribution change, by an appropriating differential equation for non-steady diffusion, as the driving force is MC.
This research gave two interesting results. One is that, on the boundary layer between the lumber and the surrounding air, the moisture sorption did not rearch to an equilibrium state. Its MC was higher than EMC, which is being used generally. The maximum difference between them in the first stage of drying, was 6% MC, and in the last stage, it became 3-5% MC at least.
Another result was that the diffusion coefficient appeared constant independent of MC. However, the computer simulation of moisture distribution showed that a more appropriate correlation seemed to be “the diffusion coefficient is dependent on MC, a cubic function with the maximum value of about 15% MC”.
