Temperature dependence on thermal conductivity, particle aggregation and viscosity of aqueous and non-aqueous nanofluids

Abstract

We investigate the temperature-dependent thermal conductivity (k) of aqueous and nonaqueous stable nanofluids with average particles size of 8 nm stabilized with a monolayer of surfactant. Iron oxide (Fe₃O₄) nanoparticles are synthesized by a coprecipitation technique and are characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), dynamic light scattering (DLS), and theromogravimetric analysis (TGA). The particles are functionalized with suitable surfactants and dispersed in aqueous and nonaqueous base fluids. The thermal conductivity and viscosity measurements are carried out using a transient hot wire and a rotational rheometer, respectively. The thermal conductivity of aqueous nanofluids increases with temperature while it shows a decrease in nonaqueous nanofluids. Interestingly, the ratio of thermal conductivity of both nanofluids with respect to base fluids (k/k_f) remains constant with an increase in temperature, irrespective of the nature of the base fluid. This observation is in sharp contrast to microconvection theory predictions of an increase in thermal conductivity with a rise in temperature. These results unambiguously confirm the less dominant role of microconvection on thermal conductivity enhancement. Although the viscosity of nanofluids decreases with increases in temperature, the viscosity ratio with respect to base fluid remains constant. These results show that the viscosity and thermal conductivity of nanofluids simply tracks those properties of the base fluids. Measurement of particle size with temperature shows that the average particle size remains constant with temperature.