A THEORY OF DETERMINING MASS TRANSFERPARAMETERS FOR WOOD PARTICLE MATERIALS

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A theory of determining mass transfer parameters for wood particle materials

The transient mass transfer processes in the natural drying of wood particle materials were experimentally studied. A new theory to determine the mass transfer parameters in the materials was developed in terms of gradient transformation method (GTM). By making use of GTM. the water vapour diffusion coefficient and the surface emission coefficient of wood chip were experimentally determined both in air phase and in solid phase. It was found that the internal resistance to water vapour diffusion in the air phase of wood particle aggregates is around ten to the third power as large as that in common air The drag coefficient was given to quantify the effect. The phenomenon of undersurface diffusion in wood particle bed was quantitatively modelled. The dimensionless Fourier’s number and the Biot’s number for mass transfer were theoretically derived. The study showed that Biot’s number for the problem investigated was the ratio of the characteristic length of wood particle bed to the penetrating depth of the undersurface. An analytical solution of the nonlinear governing equation for water transport process in the aggregates of wood chip was obtained by introducing the variable coefficients measured in the study into the governing equation. The comparison between the analytical solution and the observed moisture content of wood chip showed that the deviation was less than ±7 %. The thermophysical properties of wood particle materials are little known at present. The knowledge provided in the paper will be aid in the handling, research or engineering application of wood chip, wood shavings etc.