Effect of Nanoparticle Size, Morphology and Concentration on Specific Heat Capacity and Thermal Conductivity of Nanofluids

The effect of particle size, particle morphology and volume fraction of nanoparticles on the temperature dependent specific heat capacity of metal oxide nanofluids is investigated using differential scanning calorimeter. The stable colloidal suspensions of kerosene based magnetite (Fe3O4), polyalphaolefin (PAO) based alumina (Al2O3) spheres and alumina nanorods are used in the present studies. The nanoparticle concentrations and size of Fe3O4 nanoparticles are varied from 5 to 25 wt% and 3.6 to 8.6 nm, respectively. The results show that the specific heat capacity decreases with increase in volume fraction and particle size in kerosene based Fe3O4 nanofluids but enhances in the case of PAO based Al2O3 nanofluids. These results suggest that PAO molecules strongly modify the interfacial thermal characteristics of Al2O3 nanoparticles that in turn increases the heat capacity of PAO based Al2O3 nanofluids. For kerosene based nanofluids, the C-p data was in reasonable agreement with theoretical model for specific heat, which is derived by assuming thermal equilibrium between the particles and the surrounding fluid (Model II) using classical and statistical mechanics but showed large deviation from the mixing model (Model I). Our study shows that the C-p decreases with increase in the aspect ratio of nanoparticles due to reduced surface atomic contributions. We also compare the effect of particle size and surface morphologies on the thermal conductivity enhancement of nanofluids.