Heat and mass transfer model for wood including free water transport

Abstract

Knowledge about wood moisture conditions in a timber component is essential to predict its mechanical behavior. Not only stiffness and strength properties are highly dependent on wood moisture content but also diffusion coefficients, density, specific heat capacity and the thermal conductivity. Therefore, modern prediction tools, which are able to describe these effects, can benefit the development of new wood-based products. Especially, if they exhibit complex geometries and are made of materials with different moisture characteristics, as different and direction-dependent coefficients of expansion may lead to critical stresses. Transport mechanisms below the fiber saturation point were developed by [1-2]. Three coupled differential equations describe bound water, water vapor and energy conservation. Free water exists above the fiber saturation point with the corresponding transport mechanisms described in [3]. Values of the free water content can be much higher than those of bound water and water vapor. Thus, within the areas, where the switch from the transport mechanisms below the fiber saturation point to those above occur, high gradients can exist. To deal with these within the finite element method different procedures, like upstreaming and mass lumping [4], were used. A three-dimensional Abaqus User-Element Subroutine was developed to describe these coupled equations.