Thermal conductivity, viscosity and stability of Al2O3-diathermic oil nanofluids for solar energy systems

Nanofluids have excellent potentiality in the field of heat transfer fluids and particularly for solar energy systems such as concentrated solar power plants. However they present many issues to be fixed in order to have a large diffusion. One of these is sedimentation. In this paper, stability, viscosity, FT-IR spectra, cluster size and thermal conductivity of Al2O3 – Therminol nanofluids have been investigated as heat transfer fluid in high temperature solar energy systems. Al2O3 – Therminol nanofluids have been prepared to investigate and to improve stability of the suspensions, varying temperature during mixing with magnetic stirrer, amount of surfactant and sonication time with ultrasonic vibrator. Stability of the nanofluid samples was investigated through backscattering technique and for cluster size analysis Dynamic Light Scattering (DLS) was used. Thermal conductivity of the sample was measured in order to evaluate not only the effect of both volume fraction and temperature, but also the influence of the surfactant (oleic acid). Stability of nanofluids depends on temperature during sample preparation and sedimentation phenomenon is inversely proportional to temperature during mixing with magnetic stirrer. Influence of concentration of surfactants was studied through preparation of samples having a solid phase particles concentration of 0.3 %vol, 0.7 %vol and 1.0 %vol, respectively. The presence of surfactants creates some bonds with nanoparticles, which mainly helps nanofluids long-term stability. On the other hand, the presence of surfactants inside the nanofluids does not influence their thermal conductivity. From DLS measurements, a dependence of cluster size on volume fraction was observed for all nanofluid samples. Experimental data show: viscosity increases by increasing volume concentration; nanofluids with and without surfactants show a non-Newtonian behavior and viscosity of nanofluids increases by increasing cluster size.