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.