Thermal Conductivity and Rheological Studies for Graphene-Al<sub>2</sub>O<sub>3</sub> / Ethylene Glycol-Water Hybrid Nanofluid at Low Temperatures

Abstract

In this study, the effect of nanoparticle concentration and temperature on the thermophysical properties of graphene-Al2O3/ethylene glycol-water hybrid nanofluid at low temperatures was experimentally investigated. The nanoparticles were characterized by XRD, SEM, TEM and BET methods. The dynamic viscosity and thermal conductivity of the hybrid nanofluids were experimentally determined at different volume concentrations of nanoparticles (0.05%, 0.1%, 0.5%, 1%, 1.5%, 2%, and 2.5%) and at temperatures ranging from 263 to 303 K. The experimental results showed that the incorporation of nanoparticles into the base fluid significantly increased the viscosity. The hybrid nanofluid with solid volume fractions less than 0.5% exhibited Newtonian behavior, while those with higher solid volume fractions (0.5% - 2.5%) exhibited shear-thinning non-Newtonian behavior and followed the power law model. The hybrid nanofluids showed surprising behavior at sub-zero temperatures and low concentration, which can be attributed to oleic acid and graphene nanoplatelets. The thermal conductivity of the hybrid nanofluids increased with increasing temperature and volumetric concentration of the nanoparticles. The ratios of thermal conductivity were increased by about 44.02% and 4.07% at 303 K and 263 K, respectively, with a solid concentration of 2.5 vol%. Based on the experimental results, a reliable correlation was presented to evaluate the thermal conductivity of graphene-Al2O3/ethylene glycol-water hybrid nanofluid at different temperatures and nanoparticle concentrations.