A facile, green fabrication of aqueous nanofluids containing hydrophilic functionalized carbon nanotubes toward improving heat transfer in a closed horizontal flow passage

In the present research, the synthesis of highly stable multiwalled carbon nanotubes in aqueous media is devel-oped using a one-pot, covalent and green functionalization technique to improve the heat transfer and hydrody-namic behavior of a horizontal stainless-steel tube subjected to a uniform heat flow at its outer surface. Instead of using corrosive inorganic acids, the free radical grafting of gallic acids is used in this procedure. In this work, GA-functionalized multiwalled carbon nanotubes water-based colloidal suspensions (nanofluids) were prepared at three different weight concentrations. Various characterization techniques comprising Fourier transform infrared spectroscopy, Raman spectroscopy, High resolution transmission Electron Microscopy, and zeta-potential mea-surements were performed and confirmed the success of MWCNT functionalization. The thermophysical proper-ties were evaluated experimentally and validated using empirical correlations available in the literature. To prove the colloidal suspension stability, ultraviolet-visible spectroscopy was used, and the results showed that nanofluid was stable for almost 60 days. When the GAMWCNT concentration was increased, there was a signif-icant improvement in the thermal conductivity. Moreover, the nanofluids' dynamic viscosity experienced a slight increment up to 7.30% when the GAMWCNTs were loaded relative to distilled water. Following the confirmation of the experimental setup's analytical correlations, tests for the colloidal GAMWCNT suspension flowing through a heated horizontal tube were carried out in a fully developed turbulent state. A significant enhancement in the convective heat transfer coefficient was obtained, with only minor growth in the relative pumping power by 33.05 and 1.19%, respectively. More importantly, the reported positive performance index indicator for all the Reynolds numbers of ranges shows the possibility of using the synthesized GAMWCNT aqueous suspensions as an alternative working fluid in heat transfer systems.(c) 2022 Published by Elsevier B.V.

Automated summary

In this study, a one-pot, covalent, and green functionalization technique was developed for the synthesis of highly stable multiwalled carbon nanotubes in aqueous media. The functionalized nanotubes exhibited improved thermal conductivity and dynamic viscosity. Experimental tests conducted in a heated horizontal tube demonstrated a significant enhancement in convective heat transfer coefficient with only minor growth in relative pumping power. The results suggest the potential use of the synthesized nanofluids as alternative working fluids in heat transfer systems.