Comparison of moisture diffusion coefficients for pine, oak and linden wood

Abstract

This article presents experimental research on the moisture diffusivity in wood within the range of hygroscopic moisture. This research was carried out on samples of three types of trees: Scots pine, small-leaved linden and pedunculate oak. It included measurements of kinetics of moisture adsorption within the range of air relative humidity from 25% to 85%. For each type of wood, the experiment was carried out with unidirectional flow of moisture, in each of the material principal orthotropic directions, by examining the diffusion coefficient along the fibres, in the tangential direction and in the radial direction. Values of moisture diffusion coefficients and mass surface emission coefficient were found with the use of method of minimizing the objective function, that is, by fitting the computational adsorption kinetics curves to the experimental ones. At the same time, three cases of variations of the moisture diffusion coefficient together with moisture content of the material have been analysed each time: in the form of a constant, linear function and a quadratic function, assuming a constant value of mass surface emission coefficient on the absorbing surface of samples. The performed calculations allowed determining whether the moisture diffusion coefficients have extreme values within the analysed range of moisture content and what is the impact of various diffusion mechanisms on the whole process of transferring moisture in the considered cases.

Two-step diffusion in cellular hygroscopic (vascular plant-like) materials

“…We can also remark that the value found for D 1 corresponds to the typical ones found for the longitudinal diffusion coefficient of bound water in pine from standard macroscopic measurements ( 55 ). As far as we know, the value of the diffusion coefficient in the region containing both bound and free water has not been measured so far.…”

“…Obviously, this description, on the basis of two diffusion processes, each with a single (but different) diffusion coefficient, is a simplification as we can expect that the diffusion coefficient also somewhat varies with the water content in each region. Specifically, it is generally considered that the diffusion coefficient of bound water varies with the water content ( 55 ).…”

A Study on the Correlation between Wood Moisture and the Damping Behaviour of the Tonewood Spruce

“…Moisture diffusion coefficient decreasing together with the moisture of the material in the case of pine in the radial direction was found by Olek and Weres [53]. Perkowski et al [54] emphasise that the transport of moisture in wood is a very complicated process, which is a result of different diffusion mechanisms. Diffusion of water vapour (mostly in the cell lumens and also in cavities between microfibrils or in intercellular spaces), diffusion of bound water (inside walls and through the pits) and adsorption and desorption could be responsible for the value of the diffusion coefficient below the saturation point of fibres.…”

Model of Moisture Transport in Wood, Taking Into Account Diffusion and the Accompanying Adsorption of Water Vapour Through the Skeleton

“…where 0 is the water vapour density corresponding to the initial relative air humidity in which the wood was conditioned [kg/m3], is the water vapour density corresponding to the relative air humidity of the surrounding air [kg/m3], is the sample thickness [m]. For equation 2a uniform initial condition is adopted ( , = 0) = 0 (6) and it is assumed that there is a boundary condition on the boundary of the material with the same source of moisture sorption as inside it. In equation 60 is the mass concentration of bound water corresponding to the initial relative air humidity in pores [kg/kg].…”