A numerical modelling study of transport phenomena in wood drying




Esfahanian, Ahmad Hashemi

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This thesis presents a numerical simulation study for the heat and mass transfer in- and out-side individual boards of a stack during kiln drying of wood and on the effect of side gaps between the boards. The objective is to optimize the drying process for efficiency and high quality products. A literature survey in the area is presented. The importance of the correct link between the transport processes in wood, and heat/mass transfer and fluid flow in the surrounding drying air is emphasized. Objectives, motivations and needs for the present study are also presented. This is followed by a detailed account of the governing equations, description of models, physical properties, and discretization and solution procedures used in the present study. A sample stack of planks has been used to evaluate the performance of various turbulence models and upwinding schemes of the CFX software developed for predicting the transport parameters in air. Given a typical stack set up for drying 105 x 105 mm western hemlock lumber, the effects of side gaps on surface coefficients are studied for different air velocities. An optimum gap size for maximum heat/mass transfer is suggested. The model developed for heat and mass transfer inside the wood is validated for a one dimensional case by comparing the numerical results with published results. The improved performance using a newly proposed relationship for the diffusivity of bound water has been demonstrated. Also a new relationship for the mass transfer boundary condition at the surfaces was proposed to incorporate the effect of the surface resistance. Relative effects of model unknowns in predicting the average moisture content and board center temperatures are discussed. A Fortran program was developed to solve the two-dimensional coupled heat and mass transfer equations inside the wood during the drying process. The model considers the changes in air temperature and humidity due to heat and mass transfer to and from the boards. The iterative SOR (Successive Over Relaxation) method was modified to increase accuracy and stability. Predictions for average and local moisture content are in good agreement with experiments. The effect of side gap on the drying process inside the wood was also examined. A plot of standard deviation of each board versus the board average moisture content is suggested for the judgement about the uniformity of the products. Results indicate that without using the extra gap size there exists a considerable difference between the maximum and minimum final average moisture content of the boards in each row. The first and last boards are usually over-dried. By using the previously suggested gap size the maximum difference of the final average moisture content is almost half the case without the extra gap. Average diffusion and surface coefficients are extracted from the experimental data of drying a stack without side gap. A software tool has been developed to solve the simple unsteady one-dimensional diffusion problem. Results are compared with experiments. The introduced method can be used to obtain the average diffusion and surface coefficients.



Lumber, Drying, Transport theory