Analysis of fluid flow through porous media based on x-ray micro-tomographic reconstructions

Abstract

This thesis deals with creeping fluid flow through fibrous porous materials. Permeability through a porous medium is a measure of the ability of the material to transmit fluids. For testing and demonstration purposes the permeability values of a few fibrous heterogeneous materials, namely synthetic non-woven felt, wet pressing felt, cardboard, newsprint and hardwood paper, were measured experimentally. Reconstructions of the same materials under similar compression states were captured by x-ray micro-tomography. The actual microscopic pore structure thus obtained was utilised in the numerical lattice-Boltzmann analysis for solving the fluid flow permeability of the materials. Agreement between the experimental and numerical results was good. In addition, numerical permeability results for certain geometries were compared to the values obtained by finite-difference and finite-element methods. All numerical results were found to be in close agreement. The resolution of the tomographic reconstructions is limited causing discretisation uncertainty in the numerical analysis. The reliability of the numerical results was estimated by performing simulations for high and low resolution lattices. These results were further compared to the corresponding analytical solutions. The discretisation uncertainty was found not to be essentially larger than the typical total uncertainty of the experimental results. The advantage of the combination of x-ray tomography and numerical methods arises in the cases where direct experimental measurements are not feasible. An example of such an application, reported in this thesis, is to analyse the permeability of individual, distinct layers in a layer-structured material in order to determine the contribution of this layer to the overall permeability. In the final part of the work, tomographic reconstructions of porous heterogeneous materials were combined to a void space segmentation analysis. The utilisation of the method was demonstrated by running the analyses for a series of tomographic reconstructions of liquid packaging boards. The methods facilitate e.g. the analysis between the structural characteristics of pore structure and their relation to imbibition process. Demonstration analyses indicate the void space segmentation analysis to give valuable information on correlations between the structure of porous heterogeneous materials and their fluid flow properties.