Moisture Transport in Porous Media. Modelling Based on Kirchhoff Potentials

Sammanfattning: The work presented in this thesis is focused on the development of the new concept to describe isothermal moisture transport using Kirchhoff potentials. The first four papers deal with moisture transport in porous media using Kirchhoff potentials. The last three papers deal with wood. Here the theory is extended to the case of anisotropic moisture transport. The seven papers have the following contents: I The theoretical, numerical and computational basis for isothermal moisture transport in building materials using Kirchhoff potentials is described. It is shown that Kirchhoff potentials simplify the whole process from measurement via mathematical and numerical modelling to numerical calculations. II A fast transient technique for determining the diffusion coefficients in hygroscopic materials is presented. A sample of the material is subjected to a staircase function of steps of relative humidity and the moisture uptake is measured by continuous weighing. III The paper presents a method to calculate the moisture penetration depth and the medium moisture level in a material subjected to a periodic relative humidity variation at the boundary. The moisture flow process may be highly non-linear. IV A method to determine the sorption coefficient in a semi-infinite region is presented. A Boltzmann solution for piece-wise linear transport data is used. For the special case of linear data in two-intervals, explicit formulas to get the sorption coefficient are given. V A theory based on the use of Kirchhoff potentials to calculate moisture flow in wood is presented. Anisotropy is allowed for using different flow coefficients in the different directions, in both sapwood and heartwood. VI The paper presents a calculation procedure and a two-dimensional PC-model especially designed for drying of wood. The model accounts for different moisture flow coefficients in radial and tangential directions for both sapwood and heartwood. Calculation results from the model are compared with independently measured two-dimensional moisture distributions. The agreement was good. VII Measurements of moisture content distributions were performed during drying of Scots pine (Pinus sylvestris). The measurements were done in heartwood and sapwood separately and in all three directions to grain separately A new method evaluate flow coefficients from transient measurements is presented.