An investigation on viscosity was conducted 2 weeks after the Al2O3-water nanofluids having dispersants were prepared at the volume concentration of 1-5%. The shear stress was observed with a non-Newtonian behavior. On further ultrasonic agitation treatment, the nanofluids resumed as a Newtonian fluids. The relative viscosity increases as the volume concentrations increases. At 5% volume concentration, an increment was about 60% in the re-ultrasonication nanofluids in comparison with the base fluid. The microstructure analysis indicates that a higher nanoparticle aggregation had been observed in the nanofluids before re-ultrasonication.
An experimental investigation was conducted on the effective thermal conductivity and viscosity of Al2O3 water-based nanofluids. The thermal conductivity was determined using the transient hot wire (THW) method. The results show that the thermal conductivity of Al2O3 nanofluids increases with increasing nanoparticle concentrations in a distinct linear trend. Adding (1 to 5) % of Al2O3 nanoparticles to water increases the effective thermal conductivity of Al2O3 nanofluids by (6 to 20) % at room temperature. There is a substantial increase in the enhancement of thermal conductivity as temperature increases. At 55 °C, this value increases to around 16 %. The thermal conductivity enhancement decreases from (30 to 10) % while the particle size increases from (10 to 35) nm, but enhancement increases when the particle size is above 35 nm. The increase in thermal conductivity is 27.5 % for a particle size of 150 nm. In the viscosity measurements, the effective viscosity increases up to 60 % as the volume concentration increases from 1 % to 5 % at 25 °C. As the temperature increases, the viscosity of Al2O3 nanofluids decreases exponentially. Experimental results also indicate that the viscosity of nanofluids is much higher when the nanoparticle size is smaller.
The effective thermal conductivity of Al2O3-water nanofluids has been measured using a transient hot wire method. Experimental results demonstrate that the thermal conductivity of Al2O3 nanofluids increases linearly with increasing nanoparticle concentration. Adding 5 vol % of Al2O3 nanoparticles in water increases the effective thermal conductivity of the nanofluids by 20%. Thermal conductivity of Al2O3 nanofluids increases with an increase of temperature. The enhancement is around 1.7% at 15 °C in comparison with around 16% at 55 °C in a 1 vol % nanofluid. The particle size is another important parameter for the effective thermal conductivity. The increase of thermal conductivity reduces from 30% to 10% as the particle sizes increase from 10 nm to 35 nm. The increase of the effective thermal conductivity starts as the particle size increases above 35 nm, reaching about 27.5% in the nanofluid with the particle size at 150 nm.
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