Comparison between metallic-foam and finned-tube water-to-air heat exchangers

The thermal performance of two water-to-air heat exchanger prototypes, with the air-side conventional finned surface replaced by metal foams, is investigated. Two different metal-foam packaging techniques (S-type, K-type) are tested. The heat exchangers are inserted in the same fan-coil case, equipped with a 3-speed fan, to compare their performance under the same operative conditions. A 10 PPI aluminum foam with 96% porosity shows similar pressure drops and air flow rates with respect to the traditional finned fan-coil. Nonetheless, a strong penalty in terms of heat transfer rate is observed with respect to the conventional finned surface, due to the worse tubes-foam thermal contact and to the 60% lower surface-to-volume ratio of the metal-foam. Due to the presence of by-pass corridors for air, the performance of the K-type heat exchanger is worse than that of the S-type. Also the fin efficiency of the metal foam prototypes is lower than that of the traditional heat exchanger. The experimental data highlight that metal foams cannot compete with conventional fins if the tubes-foam thermal contact is not as good as that of finned surfaces.