Comparison between metal foam and finned tube heat exchangers for HVAC applications

Abstract

Nowadays the most commonly used heat exchangers in residential air conditioning applications for heat exchange between air and refrigerant are of the round tube and fin type. Recent investigations have revealed that combining round tubes with open cell metal foam can lead to heat exchangers which outperform their finned counterparts. The thermal contact resistance is one of the issues which is very critical in order to compete with the finned heat exchangers. Using flat tubes allows for a larger base surface for the same flow area available to the medium in the tubes. Since a larger contact area between the foam and the tubes leads to a reduced thermal contact resistance, flat tubes and metal foam are expected to be a good combination. In this work a comparison is made between commonly used louvered fin and round tube heat exchangers and metal foam enhanced flat tube heat exchangers. The comparison is made using computational fluid dynamics (CFD) and a two-dimensional volume averaged model for the metal foam. The influence of the contact resistance, pore density, external porosity, tube spacing and tube width are investigated. The most important parameters and their optimal values are identified. It is revealed that the contact resistance and surface efficiency are absolutely critical in order for the metal foam heat exchangers to achieve good performance.