A Mass-Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying is often described in terms of different periods. Depending on the current moisture content (MC) and the structure of the wood, the driving forces behind the mass transport are essentially different. Above a certain level of MC, the fibres are partially saturated and the transport of liquid (free) water occurs mainly as a consequence of capillary action. On the other hand, below the fibre saturation point, bound water within the cell walls is conveyed by diffusion, and water vapour in the lumen moves under influence of pressures. In this contribution, a unified model is presented that takes into account the transport of the different water phases. Simulation of the drying of a Norway spruce sample from about 135% to 7% MC is carried out using the finite element method (FEM). The resulting mean MC is compared with experimental observations obtained from X-ray Computed Tomography showing reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation