Thermal Conductivities and Viscosities of Al₂O₃-Water Nanofluids With Low Volume Concentrations

Al₂O₃-water nanofluids containing low volume concentrations (0.1–0.5 vol. %) of Al₂O₃ nanoparticles with 40nm and 65nm average particle size were produced using a two-step method with ultrasonication and without any surfactant. The thermal conductivities and viscosities were evaluated by KD2-pro thermal property meter and rotational viscometer respectively at different temperature. Thermal conductivities measurements show that the thermal conductivities of Al₂O₃-water nanofluids are higher than water. The thermal conductivities with average particle size of 40nm and 65nm are respectively enhanced by 17.9% and 11.2% when approximately 0.5vol.% of Al₂O₃ nanoparticles are added. Furthermore, the experimental results show the thermal conductivities increased nearly linearly with the nanoparticle volume concentration increasing, and significantly increased with the temperature increasing. Comparison between the experiments and the theoretical models shows that the measured thermal conductivities are much higher than the values calculated from theoretical models, indicating new heat transport mechanisms included in nanofluids. In the contrast to thermal conductivities, the viscosities measurements show that the viscosities of the Al₂O₃-water nanofluids significantly decrease with increasing temperature, and increased nonlinearly with the nanoparticle volume concentration. As the volume concentration of nanoparticles is increased up to 0.5%, the viscosities of Al₂O₃-water nanofluids with average particle size of 40nm and 65nm are respectively increased nonlinearly up to 28.3% and 17.5%, which exceed the Einstein model predictions.