Effect of aggregation on thermal conductivity and viscosity of nanofluids

Nanofluids have drawn large attention because they exhibit anomalous behaviour in their thermo physical properties. There has been an enormous innovation in heat transfer applications of these fluids especially to industrial sectors including transportation, power generation, cooling, thermal therapy for cancer treatment, etc. In the present work, we have studied the anomalous increase in the thermal conductivity and viscosity of nanofluids by taking clustering as one of the causes. It is assumed that the nanoparticles may aggregate on dispersion. Few of these nanoparticles may just touch each other, whereas others may do so along with interfacial layer developed around them (analogous to porous media). The variation in thermal conductivity has been studied with particle concentration, concentration of aggregates and thickness of interfacial layer. The concept of aggregation and equivalent volume fraction has also been used in Kreiger and Dougherty (K-D) model to study the viscosity of nanofluids. The obtained results for thermal conductivity agree well with the available experimental results when the effect of different types of clusters is taken into account. Viscosity increases with the increase in particle aggregate (ra) and is found to match well for ra = 3r at low concentration.