Thermal conductivity of dry anatase and rutile nano-powders and ethylene and propylene glycol-based TiO2 nanofluids

Thermal conductivity behaviour was studied for two TiO2 nano-powders with different nanocrystalline structures, viz. anatase and rutile, as well as nanofluids formulated as dispersions of these two oxides up to volume concentrations of 8.5% in two different glycols, viz. ethylene and propylene glycol. Because it is known that titanium dioxide can exhibit three different crystalline structures, the dry nano-powders were analysed using X-ray Diffraction to determine the nanocrystalline structure of the powders. Two different techniques were employed in the thermal conductivity study of the materials. Dry nanopowders, with and without compaction, were analysed at room temperature by using a device based on the guarded heat flow meter method. Nanofluids and base fluids were studied with a transient hot wire technique over the temperature range from (283.15 to 343.15) K. The base fluid propylene glycol was measured by using both techniques in order to verify the good agreement between both sets of results. The experimental measurements presented in this work were compared with other literature data for TiO2 nanofluids in order to understand the thermal conductivity enhancement as a function of nanoparticle concentration. Different theoretical or semi-theoretical approaches such as Maxwell, Penas et al., Yu-Choi were evaluated comparing with our experimental values. A parallel model was used to predict thermal conductivities employing experimental values for dry nanopowder. (C) 2014 Elsevier Ltd. All rights reserved.