WOOD DRYING MODELLING BASED ON THE WATER POTENTIAL CONCEPT: HYSTERESIS EFFECTS

The boundary absorption curve of the moisture content - water potential relationship of aspen sapwood was determined at 20°C. In order to quantify the impact of the hysteresis of the relationship on the results obtained with a wood drying model based on water potential, the results obtained were merged to the boundary desorption curve and hypothetical scanning curves were used to model the forced convection drying of aspen sapwood.

A strong hysteresis was found between the boundary desorption and absorption curves, the moisture content being higher in desorption than in absorption at a given water potential. The effect of the hysteresis on the simulation results was found to be significant. The results show that differences in the predicted drying time as high as 25% can occur depending on the moisture content - water potential curve used. This demonstrates that the sorption history of wood should be considered in the development of wood drying models.     

MODELING VACUUM-CONTACT DRYING OF WOOD: THE WATER POTENTIAL APPROACH

ABSTRACT

A two-dimensional mathematical model for vacuum-contact drying of wood is presented. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady state conservation equation of dry air. Most of the model parameters were determined during independent experiments. The set of equations is then solved in a coupled form using the finite element method. The validation of the model is performed using experimental results obtained during vacuum-contact drying of sugar maple sapwood. The experimental and calculated data are in good agreement. Nevertheless, some discrepancies are observed which can be attributed to the boundary conditions used and to the fact that heat transfer by convection was neglected.     

NONLINEAR FINITE ELEMENT ANALYSIS OF COUPLED HEAT AND MASS TRANSFER PROBLEMS WITH AN APPLICATION TO TIMBER DRYING*

A finite element formulation and solution of a set of nonlinear coupled heat and mass transfer equations for porous capillary media is presented. The model considers temperature and moisture dependent material properties and can accomodate diffusion of moisture as either a liquid or a vapor. Application was made to drying of timber and predicted results agreed well with the experimental data.     

2D MODELLING OF DRYING OF POROUS MEDIA: INFLUENCE OF EDGE EFFECTS AT THE INTERFACE.

TThis paper is essentially devoted to the study of the interfacial coefficient problem and to a characterization of the falling rate period.By means of a 2D numerical simulation, it is shown that the falling rate period is characterized by strong humidity heterogeneities at the interface. These heterogeneities affect significantly the interfacial mass transfer coefficient to be used in the traditional ID approach of drying. In this paper. the heterogeneities are due to the coupling between external and internal transfer in a boundary layer flow leading edge region.0ther phenomena that may also cause the occurence of humidity heterogeneities at the interface are discussed.