A two-dimensional orthotropic model for simulating wood drying processes

Wood is a naturally occurring resource that must be dried before it can be manufactured. Drying is important for a number of reasons that include the protection of the wood against biological damage and the reduction of the moisture content to final equilibrium levels. The complexities involved in modelling this drying process consist of analyzing the heat and mass transfer phenomena that arise in an anisotropic, nonhomogeneous, and hygroscopic porous medium. In this work, a two-dimensional orthotropic mathematical model is formulated and a numerical code based on a structured mesh cell centered control volume approach is implemented in order to allow a more comprehensive numerical investigation of the convective drying of wood to be undertaken. A comparison is made between two different numerical solution techniques; the first numerical method solves the system of equations by treating each equation in an uncoupled form, while the second scheme solves the entire system as a completely coupled set. The most efficient numerical algorithm was obtained when the system was solved using the coupled procedure. In order to examine the important differences of the overall kinetics for both low and high temperature drying, simulation results for three different cases of convective drying of wood are presented. These cases include the drying of wood below the boiling point at a relatively low temperature of 50 °C, a moderate temperature of 80 °C, and above the boiling point at the high temperature of 120 °C. The two-dimensional model highlights the following two very important facts: for an anisotropic medium, where the ratio between longitudinal and transverse permeabilities is of the order of 10³, the moisture migration occurs in the longitudinal sense which is the most permeable direction in the wood; and the behavior of the internal gaseous pressure can have a substantial impact on moisture migration.