Modeling of the morphology and of the heat transfer in open-cell foams

Abstract

High porosity open-cell foams are nowadays widely used in a large number of systems. Their thermo-mechanical characteristics are very useful when the efficiency in the heat removing is necessary and so they are used from the power electronic systems to the solar concentration or thermo-chemical applications. The mentioned applications generally imply high temperatures and then, the radiative heat transfer plays a significant role in the heat transfer process that, therefore, must be suitably accounted for. The aim of this work is to improve existing models and develop accurate and user friendly new radiative heat transfer models. It has required a refined morphological modeling of inherently disordered reticulated material, too. The morphology of the actual open-cell foams has been reviewed. Useful correlations among the principal morphological parameters of the foams have been proposed according to the basic unit geometry assumed. Also some new correlations for the prediction of the interfacial surface area per unit volume as a function of the windows diameter and the porosity have been derived. The morphological models herein proposed are compared with experimental data found in the literature and with the existing correlations which are considered and discussed in the thesis. The results are good, particularly, when reference is made to the dependence of the struts cross-section on the porosity. Finally, the radiation heat transfer in open cells foams has been modeled: first by a simplified analytical-numerical method based on a cubic cell, that modified an analytical model taken from the literature; then by an analytical-numerical based on a tetrakaidecahedric cell. The configuration factors have been evaluated both by numerical ray-tracing Monte Carlo method and analytically with a suitable spherical approximation of the tetrakaidecahedron. The models agree well with the experimental results, and work better than a model found in literature and based on a simple cubic representation of the foam unit cell.