Ultra-stable nanofluid containing Functionalized-Carbon Dots for heat transfer enhancement in Water/Ethylene glycol systems: Experimental and DFT studies

A facile hydrothermal method was applied to synthesize functionalized-carbon dot nanoparticles. The analysis revealed a low crystallinity with amorphous nature for particles with a size below 17 nm, which were functionalized with oxygen (17.9%) and nitrogen (12.2%). A nanofluid was formed by dispersing the nanoparticles in a mixture of water and ethylene glycol. The zeta potential measurement confirmed the stability of the nanofluid (-61.5 mV). Viscosity and density measurements revealed that the suspended nanoparticles did not noticeably increase the viscosity (maximum 8%) and density (maximum 1.2%). The thermal conductivity increased as temperature and nanoparticle concentration increased, and a maximum enhancement of 21% was obtained at 45 degrees C and 0.5 Wt%. Then, the convection heat transfer was investigated in the turbulent regime. The results showed a remarkable enhancement of the convective heat transfer coefficient (34%) at the Reynolds number of 15529 and 0.5 Wt%. Finally, the density functional theory (DFT) method was applied to interpret the longterm stability of the nanofluid. These results showed that the surface functional groups play a prominent role in the stability of the nanofluids. The calculations indicate that the bonding between the functionalized nanoparticles and the solvent fluid occurs through hydrogen bonds and electrostatic dipolar interactions. (C) 2021 The Authors. Published by Elsevier Ltd.