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 °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 long-term 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