Improved thermal performance of heat exchanger with TiO₂ nanoparticles coated on the surfaces

The sol-gel process was used to fabricate titanium dioxide (TiO₂) films, which were deposited on aluminum substrate and baked at five different temperatures. Microstructures of the films were observed by using a scanning electron microscope. It was found that the higher the baking temperature (T_B) was, the more non-continuous layer and the thinner of nanoparticles deposited on the surface, and the maximum area of film was obtained at T_B = 250 °C. The maximum heat transfer coefficient h occured at T_B = 250 °C among the six heat exchangers with the same Reynolds number (Re) and relative air humidity (RH). But when T_B > 250 °C, the h decreases with increasing T_B. The increment of the h for nanocoating surfaces is higher than that of non-coated surface with the increasing of RH. The h has higher value at T_B = 250 °C than the others with the same RH. However, the h with RH = 55% and RH = 70% doesn't appear to be much different from each other at T_B = 250 °C. The h has lower value at T_B = 600 °C than that of non-coated surface with the same RH, which is influenced by the thermal contact resistance between the nanocoating and substrate.