Fibre level modelling of free water behaviour during wood drying and wetting

Abstract

Most drying simulation models describe the moisture migration in wood as a diffusion process,including free water flow at contents greater than the fibre saturation point. This means that wood isseen as a homogeneous material lacking internal structure. However, especially in softwood narrowsections, bordered pits, divide the free water phase into rather distinct units. It is thus quite clear thatthe flow of free water is governed by capillary forces and not by diffusion. A model has been developedthat investigates how water filled units are emptied one by one in a drying process. Simulations withthe model explain some experimentally seen features that cannot be obtained using solely diffusiontype models.Water absorption by dried wood is generally assumed to be governed by capillary (surface tension)forces. An additional feature is that a considerable part of the bordered pits have been aspirated, i.e.closed, in the drying process and the number of possible flow paths is thus reduced. Thus the drivingforce for water flow is the capillary suction into the lumen. Modelling wood wetting by these principlesalso introduces some interesting specific features, such as a limit regarding the maximal achievablewater saturation.In summary it is found that specific behaviour seen on a real macroscopic level originates fromproperties at the microscopic, fibre level. This clearly indicates that experience from fibre level modelsshould be included in future drying and wetting simulation models. The work in this direction so far,has been promising.