<div>Figure 12 Comparison of theoretical thermal conductivity values for two
different nanofluids</div><div>The effect of particle size on thermal conductivity was quite evident in
all nanofluids, where the thermal conductivity increases with decrease
in size. The reason being the increase in specific surface area with
decrease in particle size. While there are few observations reported
where, thermal conductivity increases with increase in particles size.</div><div>Beck et al. [59] have reported that, thermal conductivity increases
with increase in particle size up to a diameter of 50nm for
Al<sub>2</sub>O<sub>3</sub> in both water and EG as base
fluids. They have attributed this decrease in enhancement to a decrease
in the thermal conductivity of the nanoparticles themselves as the
particle size becomes small enough to be affected by increased phonon
scattering. Li and Peterson [60] have also reported similar
observations for Al<sub>2</sub>O<sub>3</sub>/water nanofluids
for 36nm and 47nm particles sizes.</div><div>The main factor effecting the thermal conductivity is considered to be
the temperature of the nanofluid as most studies have demonstrated. As
there is increase in volume concentration, the thermal conductivity
increases which is observed by many researchers. However, the
enhancement tends to diminish at high concentration due to the
initiation of arrogation.</div><div>Based on current experimental investigations the nanofluids thermal
conductivity is greater than the base liquid which increase with
concentration and temperature. The thermal conductivity ratio increases
with volume fraction, but with different rates of increase for each
nanofluid. The thermal conductivity of nanofluid increase with decrease
in particle size.</div><div>The viscosity measurements were taken in the temperature range of
20-80<sup>o</sup>C and in the concentration range of 0.0-1.5%
for a particle size of 20nm. The viscosity of SiO<sub>2</sub>/60EGW
nanofluids are investigated experimentally and are shown plotted in<b>Figure 13</b> . As observed, the viscosity increases as the volume
concentration increase. However, the viscosity decrease exponentially as
the temperature increases similar to Azmi et al..On the other hand, the
experimental viscosity values are compared with equations (8) and (14)
for 60EGW base fluid and SiO<sub>2</sub>/60EGW nanofluids
respectively and a maximum deviation of 20% observed.</div>