An Experimental Investigation into Cluster Formation Towards Modelling Thermal Conductivity of Nanofluids

Abstract

The structural appearance and growth of nanoparticles in the form of nanoclusters have a considerable effect on the thermal conductivity enhancement of nanofluids. The growth of nanoclusters of Al2O3 and TiO2 nanoparticles (size 25-30 nm) in water (DI) (along with suitable surfactants) has been investigated. A comprehensive study on the size distribution of the particles while in suspension at different pH values at their respective zeta potentials along with the stability ratio of suspensions have been carried out. Experiments were performed to observe the effect of various surfactants on stability, nanocluster formation and on the thermal conductivity of Al2O3-H2O nanofluid, which was found to be getting improved significantly with SDS surfactant. The prolonged sonication was not adequate to break the clusters of Al2O3 nanoparticles into an average size of less than 163 nm, indicating the tendency of Al2O3 nanoparticles to remain in the form of clusters instead of individual nanoparticles of the initial size of 20 nm. The stability, thermal conductivity of Al2O3-H2O nanofluid and also the average nanocluster size is found to reduce to less than 200 nm from 1,602, 1,827, 1,069, and 922 nm by using sodium dodecyl sulfate (NaDS) (SDS) as an affecting surfactant out of several other surfactants. A consistent enhancement in the thermal conductivity (i.e., 0.70 W/mK over a period of 3 hours) was observed. The Box Behnken Design (BBD) under Response Surface Mythology (RSM) has been used to model the thermal conductivity of Al2O3-H2O nanofluid by taking volumetric concentration, temperature, and surfactant as the affecting parameters. The developed model is validated against the experimental data of thermal conductivity of Al2O3-H2O nanofluid and with the other existing models. The results confirmed that the model could predict the experimental results with a high accuracy level (R2 = 0.98).