Experimental Study on Heat Transfer and Thermo-Physical Properties of Covalently Functionalized Carbon Nanotubes Nanofluids in an Annular Heat Exchanger: A Green and Novel Synthesis

In order to eliminate the use of toxic acids in common carbon nanomaterial functionalization methods, a facile and eco-friendly procedure is introduced in this study to synthesize highly dispersed, covalently functionalized multiwalled carbon nanotubes (MWCNTs) for use in heat transfer fluids. The MWCNTs are treated covalently with clove buds in one pot using a free radical grafting reaction. The clove-treated MWCNTs (C-MWCNTs) are then dispersed in distilled water (DI water) at three different concentrations of C-MWCNTs (0.075, 0.125, and 0.175 wt %), resulting in C-MWCNT-DI water nanofluids. The effectiveness of the functionalization process is then verified using Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). UV vis spectroscopy is also used to examine the stability of the C-MWCNTs in the base fluid. The thermal conductivity, density, and dynamic viscosity of the C-MWCNT-DI water nanofluids are studied experimentally, and the results show that there is significant thermal conductivity enhancement for the C-MWCNT-DI water nanofluids, whereas there is only a slight increase in the viscosity and density of these nanofluids. Lastly, convective heat transfer experiments are carried out for the C-MWCNT-DI water nanofluids flowing through an annular heat exchanger at constant heat flux and fully developed turbulent flow conditions. The results show that there is a significant enhancement in the convective heat transfer coefficient and Nusselt number for the C-MWCNT-DI water nanofluids, whereas the increase in friction factor is almost negligible for these nanofluids. On the basis of the results, it can be concluded that the eco-friendly C-MWCNT-DI water nanofluids have strong potential for use as effective working fluids in various heat transfer applications.