Thermal Conductivity Of Nanoparticles Research Articles

Measurements of the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

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.

Thermal Conductivity of Nanoparticle - Fluid Mixture

Effective thermal conductivity of mixtures of e uids and nanometer-size particles is measured by a steady-state parallel-plate method. The tested e uids contain two types of nanoparticles, Al 2O3 and CuO, dispersed in water, vacuum pump e uid, engine oil, and ethylene glycol. Experimental results show that the thermal conductivities of nanoparticle ‐e uid mixtures are higher than those of the base e uids. Using theoretical models of effective thermal conductivity of a mixture, we have demonstrated that the predicted thermal conductivities of nanoparticle ‐e uid mixtures are much lower than our measured data, indicating the dee ciency in the existing models when used for nanoparticle ‐e uid mixtures. Possible mechanisms contributing to enhancement of the thermal conductivity of the mixtures are discussed. A more comprehensive theory is needed to fully explain the behavior of nanoparticle ‐e uid mixtures. Nomenclature cp = specie c heat k = thermal conductivity L = thickness Pe = Peclet number P q = input power to heater 1 r = radius of particle S = cross-sectional area T = temperature U = velocity of particles relative to that of base e uids ® = ratio of thermal conductivity of particle to that of base liquid ¯ = .® i 1/=.® i 2/ ° = shear rate of e ow Ω = density A = volume fraction of particles in e uids Subscripts