Figure 15 Comparison of viscosity enhancement of SiO2/60EGW and SiO2/40EGW nanofluids
Based on the research available from the literature, the factors influencing the viscosity are nanoparticle size, volume concentration and temperature whereas the material doesn’t seem to be effecting much. The viscosity of nanofluids are increasing with particles sizes at higher concentrations as observed Nguyen et al. [63], where he has reported that there is no big change in viscosity for a varying sizes of 36nm and 47nm for the nanoparticles at 4% concentration. But, if the volume concentration is increased, then the viscosity of the nanofluid seems to be increasing with the particle size.
He et al. [64] have also reported similar observations with TiO2/water nanofluids which shows that viscosity is increasing with particle size. However, contradicting observations were made by Namburu et al. [33] indicating that, viscosity of nanofluids decreasing with particles size for SiO2/60EGW nanoparticles which have supported by results of Pastoriza-Gallego et al. [65] and Anoop et al. [66] for CuO/water and Al2O3/water nanofluids respectively. While analysis of Prasher et al. [67] was different from others, where he has showed that viscosity of nanofluid is irrespective of the particle size.
In case of influence of temperature on the viscosity, it is quite evident that, the temperature is the most critical and influential parameter as recommended by whole nanofluid research community. The overall research indicates a pretty common observation of downward trend in viscosity with an increase in temperature. As the temperature increases, the intermolecular attraction between the nanoparticles and their base fluids weakens [68]. Hence, the viscosity of nanofluids decreases with the increase in temperature.
Based on the research it can be concluded that, viscosity increases with concentration of the nanofluid. Viscosity of the nanofluid decrease with increase in temperature. An increase in viscosity with decrease in particle size is reported in the literature.
The enhancement ratio is plotted for the SiO2nanoparticles in both the base fluids for the estimation of optimum heat transfer at a concentration in Figure 16 . Enhancement ratio can be defined as ratio of viscosity enhancement to thermal conductivity enhancement as given in correlation (20). According to Garg et al. [69], the enhancement in heat transfer under turbulent flow is a maximum when the value of ER ≤ 5. As shown the figure 16, the optimum heat transfer enhancement can be obtained at 1.0% and 1.4% volume concentrations for a given temperature of 70oC in 60EGW and 40EGW based nanofluids respectively.
\(ER=\frac{\left(\frac{\mu_{\text{nf}}}{\mu_{\text{bf}}}-1\right)}{\left(\frac{k_{\text{nf}}}{k_{\text{bf}}}-1\right)}\)(16)
Hence, the optimum parameters for obtaining enhancements will be 1.0% and 1.4% for 60EGW and 40EGW based nanofluids at 70oC temperature. The predicted values of thermal conductivity are compared, Al2O3 nanoparticles show some better values than SiO2 nanoparticles in both the base fluids such as 60EGW and 40EGW. When compared in SiO2nanofluids, the influence of 40EGW base fluid has better impact on SiO2 nanofluids. Though change of material doesn’t have any influence on viscosity, 40EGW based nanofluids have shown higher values than 60EGW based nanofluids.