Model for hysteretic moisture behaviour of wood

Sorption hysteresis of wood (spruce) is modeled using a modified Preisach-Mayergoyz (PM) approach taking into account independently water film adsorption and capillary condensation. The PM model is based on the independent domain theory (Everett 1964), which subdivides the material into domains or sorption sites that fill/unfill independently of each other. Different reasons for the filling and unfilling of a domain at different vapour pressure exist. First, for cylindrical pores, capillary condensation results in a cylindrical interface between the adsorbed film and the vapour phase, while evaporation/desorption is associated with the formation of a hemispherical meniscus. Due to a difference in film thickness, hysteresis exists. Further, in porous materials forming a complex network, so-called ink bottle pores show a pore entry radius smaller than the radius of the pore chamber resulting in hysteresis. A change in moisture content in the porous material is the outcome of the behavior of an assemblage of different domains. An efficient way to represent these domains is to map the domains on the so-called the PM space (Preisach-Mayergoyz), where they are characterized by a PM distribution. Integration in the PM space over the filled domains leads then to prediction of the according moisture content. However, the identification of the PM distribution is an ill-posed problem. In this paper, the PM model is modified by considering capillary condensation hysteretic and film forming process non-hysteretic. To simplify the identification process, the PM distribution is formulated in the integrated PM domain (IPM). A new description of the IPM distribution function is introduced, which is determined from main adsorption and primary desorption curves. The parameter identification is discussed and it is found that main adsorption and primary desorption data are necessary. It is found that although film adsorption is modeled non hysteretic, the moisture content due to film forming becomes hysteretic due to the hysteretic behaviour of the capillary condensation process. The model is used to analyze the moisture content variations of a specimen of wood exposed to different relative humidity conditions. Starting from equilibrium at 54%RH, the conditions are increased to 79.5%RH for 13 days and then decreased to 54%RH for 15 days, followed by daily cycles between these two values. The water vapour diffusion coefficient is considered to depend on the moisture content. The model demonstrates a good agreement with experimental data for the longitudinal, radial and tangential directions.Presenters: name: Dominique Derome affiliation: (Concordia University) email: derome@alcor.concordia.c