Figure 12 Comparison of theoretical thermal conductivity values for two different nanofluids
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.
Beck et al. [59] have reported that, thermal conductivity increases with increase in particle size up to a diameter of 50nm for Al2O3 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 Al2O3/water nanofluids for 36nm and 47nm particles sizes.
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.
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.
The viscosity measurements were taken in the temperature range of 20-80oC and in the concentration range of 0.0-1.5% for a particle size of 20nm. The viscosity of SiO2/60EGW nanofluids are investigated experimentally and are shown plotted inFigure 13 . 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 SiO2/60EGW nanofluids respectively and a maximum deviation of 20% observed.