Non-Fickian moisture diffusion in thermally modified beech wood analyzed by the inverse method

• A clear assessment of the reliability of parameters used to model non-Fickian diffusion by the inverse method.
• A significant reduction in water diffusivity in thermally modified wood.
• A significant increase of the non-Fickian effect on the diffusion in wood after thermal modification.
• The time constant of molecular relaxation is of the order of 100 h.

Non-Fickian diffusion in native and thermally modified wood was analyzed by the inverse method. A low quality of the identified diffusivity values was found for the diffusivity which was either constant or varying with water content. This was explained by the non-Fickian behavior. It was especially distinct for thermally modified wood for which an increased delay in obtaining the hygroscopic equilibrium was clearly shown. Such a delay was explained by time required for molecular reorganization to produce new sorption sites. This phenomenon was accounted to improve the physical model by modifying the convective boundary condition. A relaxation mechanism was used for this purpose with an adequate time constant.