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

Highlights: • Stable ethylene and propylene glycol based TiO₂ nanofluids were prepared. • A device for measuring the thermal conductivity of dry nanopowders is presented. • Nanofluid thermal conductivity enhancements reach values up to 15.4%. • Nanofluids involving anatase nanocrystalline present slightly superior enhancements. • A parallel model using nanopowder measures gives good conductivity predictions for TiO₂-nanofluid. - Abstract: 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 nano-powders, 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, Peñas 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