Influence of the growth ring structure on the vapor transport in wood

Moisture transport in wood is complex due to its anisotropic material structure, resulting from the cells anatomy, growth rings where early- and latewood alternate and the presence of vessels, rays and pits. As a result, the moisture transport properties of wooden components are highly dependent on the direction (longitudinal, radial or tangential). Moreover, due to variations in cell geometry, and growth ring structure, the moisture transport properties, such as moisture capacity and vapor diffusion coefficient depend on location. The purpose of this paper is to develop a methodology to determine location dependent moisture transport properties and to study their influence on the global hygroscopic behavior of wood. In order to derive location dependent moisture transport properties, 2D micro structural images are obtained by Scanning Electron Microscopy of a full growth ring. The image is then thresholded to obtain a binary representation of the cell wall – cell lumen structure. The geometry of the cells is extracted and the cell borders are represented by lines. Based on the line configuration a mesh is generated. Finite element analyses are performed using a moving window methodology to obtain the position dependent material properties. A vapor pressure difference is imposed at two opposite sides, while the other sides remain vapor tight and the water vapor flux and water vapor diffusion coefficient are determined. These calculations are repeated for different relative humidities. The moisture capacity is determined based on the wall volume in the window. Different sizes of moving window are examined to find a suitable size of the representative elementary volume (area in 2D). In a second step, the location dependent moisture transport properties are used to analyze the water vapor transport at meso- and macroscale. The macroscopic moisture transport properties are then compared with measurements on wood. Further validation is performed by a adsorption/desorption test where the moisture distribution in the wood is measured by X-ray tomography.Presenters: name: Wolfgang Zillig affiliation: (Katholieke Universiteit Leuven, Laboratory of Building Physics) email: wolfgang.zillig@bwk.kuleuven.b