Effective thermal conductivity and thermal diffusivity of nanofluids containing spherical and cylindrical nanoparticles

This paper reports on measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids 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 and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Au/toluene, Al2O3/water, and carbon nanofiber (CNF)/water nanofluids are measured and the effects of the volume fraction and thermal conductivity of the nanoparticles and temperature are clarified. The average diameters of Au and Al2O3 spherical particles are 1.65 and 20 nm, respectively. The average length and diameter of CNFs are 10 mu m and 150 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 demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancements and can be predicted accurately by the model equation of Hamilton and Crosser [Ind. Eng. Chem. Fundam. 1, 187 (1962)] for the spherical nanoparticles and by the unit-cell model equation of Yamada and Ota [Waerme-Stoffuebertrag. 13, 27 (1980)] for carbon nanofibers. (c) 2006 American Institute of Physics.