A New One Dimensional Numerical Model for Water Transport in Wood

The main objective of this work is to obtain more accurate predictions of drying process of lumber through later numerical simulation models. The practical objective is to numerically analyze the combined non-linear heat and mass transfer for detailed time-dependent local moisture content and temperature distributions in a board. During this work, it has been tried to consider the conclusions and results of comparison of different wood drying models with benchmark experiments presented by Kamke and Vanek in IUFRO wood drying conference. The proposed model is not so complex that will need a lot of computational resources and CPU time for real applications with a long drying time such as two dimensional kiln drying of several stacks of boards. Also simple modeling is avoided. This will result in a more universal model in addition to better accuracy of simulations. In the governing equations different phases are considered including: dry wood, water vapor, free water and bound water. The present study uses the finite volume approach and Fully Implicit method for discretization, and TDMA method to solve the governing equations and sub-models implemented within them. The basic equations are validated for a one dimensional case by comparing the numerical results with published results. Here, a new model has been developed successfully for the diffusion process of bound water in the wood. Other relationships for some characteristics of wood and water phases and a mass flow boundary condition at the board surface are also proposed. This type of mass flow boundary condition takes into account the surface resistance to mass transfer in addition to air resistance represented by convective mass transfer coefficient obtained from analogy between heat and mass transfer. During the present work the better performance of the new diffusion model, submodels and relationships were verified with experiments and other numerical works. The good prediction of the slope of the average moisture content versus time by the proposed model during most of the drying period and particularly the final stage is a very important contribution. This will ensure that if drying is continued, the drying curve will still be in good agreement with experiments.