The use of open cell metal foams in heat exchangers : possibilities and limitations

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.

Open cell metal foam is under consideration for use in all kinds of heat transfer applications: electronics cooling, automotive heat exchangers, heat pumps, geothermal energy... In order to design heat exchanger with this material, the foams need to be thermo-hydraulically characterized. Because of their interesting combination of properties (large specific surface area, tortuous flow paths, high strength.) a large amount of research has been done over the past decade. This paper will present a summary of this past research and indicate fields for further development. These are mainly due to issues with the complex foam geometry and how to characterize it adequately. Recently an advanced geometric model was developed at the Ghent University based on detailed micro- computed tomography scans of foam samples. Using this model it has been shown that for the hydraulic characterization the data spread can be reduced, by selecting the proper characteristic length scales. This is not only porosity; strut and cell dimension should also be included. Hydraulic analysis shows that applications with high fluid velocity are not intuitively favorable for foams. Low Reynolds-number forced convection and natural convection problems appear more interesting. However, by adapting the heat exchanger design to account for the pressure loss (e.g. foamed fins or thin foam layers), the advantages of foam can still be used at higher velocities. For thermal characterization little information is available. Experiments at the Ghent University, based on unsteady analysis have shown to give good results. These experiments have to be coupled with advanced numerical models of foams. Using these models, correlations for the combined conductive and convective heat transfer can be derived, which are needed to design and further optimize heat exchangers. Open-cell metal foams clearly are a promising material for heat transfer technology. In order to see the first large scale use, specific applications will have to be selected and the heat exchanger designs will have to be optimized. Some examples are presented in the paper.