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The research work presented here intends to contribute to the overall research effort towards nanofluids engineering and characterization. To accomplish the latter, multiwalled carbon nanotubes (MWCNTs) are added to an ethylene glycol (EG) based fluid. Different aspects concerning the nanofluids preparation and its thermal characterization will be addressed. The study considers and exploits the relative influence of CNTs concentration on EG based fluids, on the suspension effective thermal conductivity and viscosity. In order to guarantee a high-quality dispersion it was performed a chemical treatment on the MWCNTs followed by ultrasonication mixing. Furthermore, the ultrasonication mixing-time is optimized through the UV-vis spectrophotometer to ensure proper colloidal stability. The thermal conductivity is measured via transient hot-wire within a specified temperature range. Viscosity is assessed through a controlled stress rheometer. The results obtained clearly indicate an enhancement in thermal conductivity consistent with carbon nanotube loading. The same trend is observed for the viscosity, which decreases with temperature rise and its effect is nullified at higher shear rates.

SUMMARYThe intriguing behaviour of carbon nanotube suspensions shows that thermal conduction cannot be described by conventional approaches. These results led the researchers on the percolation and the interfacial layer resistance (also known as Kapitza resistance) as the main mechanisms governing the effective thermal conductivity enhancement for these nanoparticles suspensions. A numerical simulation on the behaviour of these suspensions, when subjected to a Brownian force field, was conducted to characterize the main factors affecting the dynamic interactions and percolation structures formation. To this end, three different numerical models based on continuum mechanics were developed. The obtained results suggest that the size, shape and aspect ratio of the nanoparticles are main factors controlling the dynamic network formation. On the other hand, the influence of the Brownian motion and the structural flexibility of the nanotubes seem to have a rather negligible effect on the results. The numerical model developed and proposed here may assist in understanding and correlating the experimental thermal conductivity data of nanofluids, contributing to demystify some of the intriguing behaviours reported.