Heat And Mass Transfer In Wood During Convective Heating Drying

Understanding the law of heat and mass transfer in wood during convective drying contributes to perfect drying schedule, save more energy and improve the quality of drying wood. Larch(Larix gmelinii Rupr.) was used as experimental material and the basic law of heat and mass transfer in wood during convective drying was thus studied in this research.Three dimensional temperature distributions together with their change were explored through experiment, and the influence of length, width, thickness of wood on the overall heat transfer was discussed, respectively. Results indicate that heat transfer was mainly determined by two factors, thickness and width of wood, and the length of wood could be neglected due to the ratio of wood length to its thickness being tremendous in practical drying cases. Surface temperature of wood increased at once as the environmental temperature increased, and temperature gradient across the thickness of wood was apparent initially but tended to be smooth quickly as long as the environmental temperature stayed constant. Temperature difference in wood was small in most time during convective drying, therefore the non-isothermal diffusive effect was temporary and could be neglected. Coupled heat and mass effect was relatively apparent in the position near the wide side, and the temperature gradient was from surface to the center across the width of wood. Consequently the influence of width on the overall heat transfer should be taken into account in general cases except that the ratio of width to thickness is tremendous or the wide side sealed effectively.Fick's one-dimensional second law was used to describe the moisture transfer across the thickness in wood during convective drying assuming that the permeating of free water in wood equals to diffusing in form. Based on the obtaining of time and space distribution of moisture content, the moisture diffusion coefficient in wood was calculated from finite difference inversed method. Results indicated that moisture diffusion coefficient decreased as the decreasing wood overall moisture content, but shown no significant dependence on the moisture content of the given position in wood. Moisture diffusion coefficient increased as the increasing wood temperature, and the influence was much more apparent than that of overall moisture content. Moisture diffusion coefficient was still affected by the property of wood. The relationship of moisture diffusion coefficient between overall moisture content were fitted at different temperature, and the correlation coefficients were relatively high ranging from 0.9936 to 0.9999.Based on Whitaker's volume-averaging theory and the phase transformation was taken into account, the mathematical model of heat and mass transfer was established during wood convective drying. In the model, the permeating of free water in wood equaled to diffusing in form, and the diffusion of vapor in the areas, where moisture content was above fiber saturation point, was neglected, free water (in those areas moisture content was above fiber saturation point) and bound water (in those areas moisture content was below fiber saturation point) was in equilibrium with vapor in different way under the small atmosphere in wood, respectively. Control equations were then written out their apparent finite difference formats, corresponding program was written in Matlab and the model was solved by computing numerical solution. An experiment was conducted to verify the accuracy of above heat and mass transfer model. Results shown that for layer moisture content near the surface, experimental values were dropped faster than predicted ones, but for those near the center, experimental values dropped little slower than predicted ones; for temperature distribution, predicted values had an agreement with experimental ones.