Measurements of the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

Abstract

This paper reports on the measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids by using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al2O3 thin film. Using distilled water and toluene as the standard liquids, the length and radius of the hot wire and the thickness of Al2O3 film are calibrated before and after the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al2O3/water, ZrO2/water, TiO2/water and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperatures are clarified. The average diameters of Al2O3, ZrO2, TiO2 and CuO particles are 20, 20, 40 and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1 % for the thermal conductivity and 5 % for the thermal diffusivity. The measured results show that the effective thermal conductivities of the nanofluids with the spherical nanoparticles have no anomalous enhancements and can be predicted by the model equation of Hamilton and Crosser in a lower volume fraction range. Furthermore, the measured values are lower than those predicted in a higher volume fraction range because the settling of some particles occurred in this range.