Influence of porosity and pore density on the thermal and hydraulic performance of metal foam heat exchangers

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

Metal foam shows a great potential for heat transfer applications. In this work the influence of the volumetric porosity, the pore density and the foam material (aluminum or copper) on the heat transfer and pressure drop characteristics are investigated. Two-dimensional simulations are performed using a porous medium approach: the Darcy-Forchheimer-Brinkman flow model is combined with the two-equation energy model. Round tube heat exchanger with a staggered tube layout are considered. Simulations are performed for inlet velocities between 1.2 m/s and 3.2 m/s. The validation experiment shows a good match between the simulations and the measurements, proving the quality of the simulations. It is found that the friction factor is mainly determined by the porosity, while the Colburn j-factor is mainly determined by the pore density. For a given pumping power the heat exchanger volume increases with decreasing PPI value to perform the same heat duty. For the same PPI value and a fixed pumping power, the heat exchanger volume increases with increasing porosity. For the same fan power, the heat transfer rate for copper foam is up to 20% higher compared to foam made of the aluminum alloy AlSi7Mg0.3. Comparison to a bare tube bundle shows that the heat transfer rate of a foamed heat exchanger is up to 6 times larger for the same fan power. This article illustrates that optimization is required to design heat exchangers which fully benefit from the unique advantages of open-cell metal foam.