Thermal-hydraulic performance enhancement of fin-and-tube heat exchangers using carbon nanotube coatings under dry and wet conditions

Carbon nanotube coatings have been widely adopted in various heat transfer fields to improve their heat transfer performance. However, comprehensive studies on the thermal–hydraulic performance of fin-and-tube heat exchangers with carbon nanotube coatings are limited. In this study, the heat transfer characteristics of fin-and-tube heat exchangers with uncoated, hydrophilic, hydrophobic, and carbon nanotube coatings were measured under dry and wet conditions. The Colburn j-factor, friction factor, and thermal–hydraulic performance (jf^–1/3) of fin-and-tube heat exchangers with various coatings were analyzed and compared under dry and wet conditions. The carbon nanotube-coated heat exchanger showed the best performance in terms of heat transfer and frictional pressure drop under both dry and wet conditions among the tested fin-and-tube heat exchangers. This is because the unique microscopic surface structure of the carbon nanotube coating increased the heat transfer coefficient, whereas the superlubricity of the carbon nanotube coating decreased the frictional pressure drop. The behavior of liquid droplets on the surfaces of various fin-and-tube heat exchangers was investigated in detail under dry and wet conditions. In addition, the thermal–hydraulic performance (jf^–1/3) of the carbon nanotube-coated heat exchangers was 8.9% and 6.1% on average higher than that of the uncoated heat exchanger under dry and wet conditions, respectively. The results of this study can provide helpful information for developing fin-and-tube heat exchangers with carbon nanotube coatings to achieve substantial energy savings.