Thermophysical properties of high porosity metal foams

Abstract

In this paper, we present a comprehensive analytical and experimental investigation for the determination of the effective thermal conductivity ( k e), permeability ( K) and inertial coefficient ( f) of high porosity metal foams. In the first part of the study, we provide an analysis for estimating the effective thermal conductivity ( k e). Commercially available metal foams form a complex array of interconnected fibers with an irregular lump of metal at the intersection of two fibers. In our theoretical model, we represent this structure by a model consisting of a two-dimensional array of hexagonal cells where the fibers form the sides of the hexagons. The lump is taken into account by considering a circular blob of metal at the intersection. The analysis shows that k e depends strongly on the porosity and the ratio of the cross-sections of the fiber and the intersection. However, it has no systematic dependence on pore density. Experimental data with aluminum and reticulated vitreous carbon (RVC) foams, using air and water as fluid media are used to validate the analytical predictions. The second part of our paper involves the determination of the permeability ( K) and inertial coefficient ( f) of these high porosity metal foams. Fluid flow experiments were conducted on a number of metal foam samples covering a wide range of porosities and pore densities in our in-house wind tunnel. The results show that K increases with pore diameter and porosity of the medium. The inertial coefficient, f, on the other hand, depends only on porosity. An analytical model is proposed to predict f based on the theory of flow over bluff bodies, and is found to be in excellent agreement with the experimental data. A modified permeability model is also presented in terms of the porosity, pore diameter and tortuosity of our metal foam samples, and is shown to be in reasonable agreement with measured data.