Visible dynamic changes in the mechanism of water evaporation surface formation during wood drying

Internal moisture distribution seriously affects timber shrinkage relations during drying, and it is also an essential parameter for testing the validity of a three-dimensional heat and mass transfer model. Here, we present an experimental study of 3D changes in moisture content at different stages during constant temperature and humidity drying of two wood species, covering visible moisture movement evaporation surface, 2D and 3D moisture variations, moisture values in the dry and wet zones, and color changes. We show that the evaporating surface was a wet front, its influence on the wood 3D structure is in the tangential > radial > longitudinal direction, and the color difference ranges from 10.6 to 17.6 when the wet front is present. Moisture values for poplar and pine at the disappearance of the wet front ranged from 14.2–19.7% to 20.4–22.5%, respectively. In mass transfer during wood drying, wood dimensional simplification and homogenization in the drying model should not be used, and the moisture content in each wood thickness layer should be evaluated in a diversified manner.