Thermal Contact Resistance at a Metal Foam-solid Surface Interface

Accurate information on heat transfer and temperature distribution in metal foams is necessary for design and modeling of thermal-hydraulic systems incorporating metal foams. The analysis of this process requires determination of the effective thermal conductivity as well as the thermal contact resistance (TCR) associated with the interface between the metal foams and adjacent surfaces/layers. In the present study, a test bed that allows the separation of effective thermal conductivity and thermal contact resistance in metal foams is described. Measurements are performed in a vacuum under varying compressive loads using ERG Duocel aluminum foam samples with different porosities and pore densities. Also, a graphical method associated with a computer code is developed to demonstrate the distribution of contact spots and estimate the real contact area at the interface. Our results show that the porosity and the effective thermal conductivity remain unchanged with the variation of compression in the range of 0 to 2 MPa; but TCR decreases significantly with pressure due to an increase in the real contact area at the interface. Moreover, the ratio of real to nominal contact area varies between 0 to 0.013, depending upon the compressive force, porosity, and surface characteristics. INTRODUCTION Transport phenomena in porous media have been the focus of many industrial and academic investigations [1-4]. The majority of the studies reported in the literature deal with low porosity media such as granular materials and packed beds [1, 2]. Over the last decade, high porosity micro-structures such as open-cell metal foams have received more attention. Interest in these media stems from their relatively low cost, ultra-low density, high surface area to volume ratio, and their ability to mix the passing fluid. These features are highly desirable for a wide variety of applications including microelectronics cooling, aerospace technology, filtration, heating, and compact heat exchangers [3-7]. In majority of these applications, there is an interface between the foam and a solid surface which gives rise to an important phenomenon called thermal contact resistance (TCR) acting against heat transfer in metal foams. Due to high porosity and roughness of the free surface of metal foams, the actual contact area at the interface with a solid surface is very small; this emphasizes the significance of TCR in metal foamsolid surface interface. In some applications, metal foams are brazed to a metallic sheet which may create a perfect contact, but because of high porosity of the medium, TCR still exists due to constriction and spreading of the heat flow passing through the metal plate-foam interface. A review of the literature indicates that in all previous studies related to heat transfer in metal foams, e.g. [8-15], the TCR was neglected due to attachment to a metallic sheet or ‘bundled up’ with the effective thermal conductivity and only effective thermal conductivity values were reported. One fundamental issue with combining the two is that TCR is an interfacial phenomenon that is a function of mechanical load and surface characteristics and thermal conductivity of both interfacing surfaces, whereas thermal conductivity is a transport coefficient characterizing the bulk medium. Thermal conductivity and TCR should therefore be distinguished. Furthermore, the effect of compression on thermal conductivity and TCR has not been thoroughly investigated. The objective of this study is to measure the thermal conductivity and contact resistance of metal foams and estimate the size and distributions of contact spots (real contact area) at the interface. The experimental technique developed in this