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Simultaneous heat and mass transfer in wet wood particles

University of British Columbia

A study is made of the simultaneous heat and mass transfer processes which occur within a finite wood cylinder of circular cross-section when it is convectively dried. Governing transport equations are developed allowing for the functional dependence of both thermal and moisture diffusivities on moisture content and temperature. Derivative type boundary conditions are included in the analysis. The equations are formulated using a cylindrical coordinate system because it is well suited to modelling wood's anisotropy. In the case considered, the axial coordinate direction is aligned with the wood-grain. Due to the coupling and non-linearity present in the transport and boundary equations, an implicit finite-difference solution scheme is formulated. The three-time-level scheme uses an equation splitting technique to simplify its solution on the computer. A mathematical wood-model, as available in the literature, is refined and used to determine moisture and thermal diffusivities, and mass transfer boundary conditions. Results from this wood-model apply to softwoods below the fiber-saturation moisture content. Combined diffusivities of liquid and vapour are calculated for the radial and axial directions and results for the radial direction are compared to those found from diffusion experiments. Desorptional isotherms are used in the mass transfer boundary condition equations to relate surface humidity and moisture content. The wood-model is used in the mass transfer equation to determine wood drying behaviour under isothermal conditions. Initial moisture contents are uniform and equal to the fiber-saturation value. Solutions are presented to give local and average moisture content as a function of time as well as moisture content profiles. The effect of drying temperature was investigated for one case of wood density and shrinkage.

Text

2010-02-19

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http://hdl.handle.net/2429/20515

Mechanical Engineering

University of British Columbia

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