Experimental study of the rheological behavior of TiO2-Al2O3/mineral oil hybrid nanofluids

Abstract

The characterization of the rheological properties of hybrid nanofluids plays an important role in determining their effectiveness as thermofluids for heat transfer applications. The present study experimentally determines how the particle concentration, nanoparticle composition, ratio of the hybrid nanoparticle mixture, and temperature affect the overall viscosity and the rheological characteristics of Al2O3-TiO2/MO hybrid nanofluids. Nanoparticles used for the experimentation are Al2O3 (5 nm) and TiO2 (5 nm) with paraffin oil (white, CAS No: 8020-83-5) as the base fluid. Nanofluids containing single and mixture of nanoparticles at 0.2–1.5 % volumetric concentrations are prepared using two-step method, and viscosity is measured using a Brookfield Dv2T rotational viscometer with a cone (CPA-40Z) and plate apparatus. Flow behavior indices for the Al2O3-TiO2/MO hybrid nanofluids tested tended to approach a value of one, which indicated a more Newtonian flow behavior when compared to the conventional Al2O3/MO and TiO2/MO nanofluids. While Brownian motion best describes the microstructural changes in the nanofluid that result in the observed non-Newtonian behavior of conventional nanofluids, it appears that the competing nonhydrodynamic forces from the constituent nanoparticles in hybrid nanofluids negate these effects and result in a more Newtonian, rheological behavior. It was also observed that the ratio of the hybrid mixture of constituent nanoparticle components did affect the rheological characteristics in an intuitive way, resulting in two observed relationships: 1) the hybrid mixture composition of 50% Al2O3/50% TiO2 tended to present a peak value relative to the other hybrid compositions and was exhibited primarily at particle volume fractions of 0.5% and 1.0% and at all temperatures, and 2) there was a general decrease in flow behavior index with increasing ratio of TiO2 in the Al2O3/TiO2 hybrid mixture and was exhibited primarily at particle volume fractions of 0.2% and 1.5% and at all temperatures less than 60 °C.