Performance and heat transfer measurements in asymmetrically-heated metal foam cooled by water

Abstract

Metal foams are attractive for heat transfer applications because of their large surface area to volume ratio, and their complex structure that promotes mixing and convection. Usually metal-foam applications involve symmetric heating. The literature shows that thermal asymmetry imposed on the boundaries substantially affects the heat transfer in porous media, including metal foam. It also shows that most, if not all, the published information in this regard is numerical and/or analytical and for fully-developed flow and heat transfer conditions. Typically, the Darcy flow model is employed. There seems to be lack of experimental data, especially in the more practical non-Darcy flow regimes. In this study, a metal-foam-filled rectangular channel subjected to constant heat flux of 12,787 W/m2 on one side was used to investigate convective heat transfer. The open-cell aluminum foam sample had 20 pores per inch and 91.8% porosity. The experimental work was conducted in the Reynolds number range 2.8–25 using water as a working fluid. Pressure drop measurements showed that the flow was in the Forchheimer regime. The permeability was obtained as 3.88 × 10−8 m2 and the Forchheimer coefficient was 0.085. The Fanning friction factor correlated with the permeability-based Reynolds number to the power − 0.5. The thermal entry length was determined for the lower flow velocities and reached 2.6 hydraulic diameters. Heat transfer results showed that the average Nusselt number correlated well the Reynolds number in a power law with the exponent of Reynolds number being 0.61. The Nusselt number reached a plateau at around Reynolds number equals 10. The Colburn j factor for the metal-foam heat sink correlated with the Reynolds number to the power −0.39, and was 407% higher than that of an empty channel. A way of isolating the effect of the random orientation of the ligaments of the foam was devised by comparing the performance of the foam to staggered pin fins having the same porosity and a diameter equal to the ligament of the foam. The orientation of the ligaments of the foam was responsible for about 33% average increase in heat transfer. The experimental data and correlation of obtained can aid numerical and modeling work concerning asymmetrically-heated metal-foam heat transfer.