Experimental study on stability and rheological behaviour of hybrid Al2O3-TiO2 Therminol-55 nanofluids for concentrating solar collectors

In this paper, an experimental study on rheological behaviour and stability analysis of Al2O3-TiO2 Therminol-55 hybrid nanofluid for use in solar collectors has been done. Various hybrid nanofluid samples with varying concentration of 0.05 wt%, 0.075 wt%, 0.1 wt%, 0.25 wt%, 0.5 wt% were prepared using two step technique. Particle size distribution was experimentally determined using Smoluchowski approximation. Zeta potential (ZP) measurements were performed using univette and it was found that all hybrid nanofluid samples have ZP values in the range of 68.5 mV to 57.64 mV after 3 h of preparation, 64.3 mV to 48.36 mV after 72 h of preparation, 54.52 mV to 34.43 mV after 7 days of preparation. Flow behaviour was studied by varying shear rate from 10s(-1) to 500 s(-1) and it was found that all hybrid nanofluid samples possess Newtonian behaviour. With the increase in nanoparticle concentration, viscosity of nanofluid increases. Further dependence of viscosity on temperature is determined and it was found that increase in temperature causes decrease in viscosity and approaches to base fluid viscosity at higher temperature An increase in viscosity of 24% for 0.5 wt% and 22% for 0.05 wt% as compared to pure Therminol-55 at 20 degrees C was found which decreases to 15%-0.07% at 145 degrees C. The viscosity increments are insignificant at elevated temperatures as such candidature of Al2O3-TiO2 Therminol-55 hybrid nanofluid as heat transfer fluid is more strengthen and widens its scope of utility. (C) 2019 Elsevier B.V. All rights reserved.