Rheological behaviour of functionalized graphene nanoplatelet nanofluids based on water and propylene glycol:water mixtures

Energy efficiency of systems stands out as a critical parameter to achieve sustainability. Heat transfer processes spend an important part of overall energy consumption, so the enhancement of its efficiency is an important research field. Nanofluids have received increasing attention to reach this goal because of its initial conception as thermal-improved working fluids. Nevertheless, the increase in thermal conductivity produced by the dispersion of nanoparticles should not be the only focus. In forced convection processes, viscosity plays an important role in achieving an improvement of the exchange of heat. In addition, the energy consumption needed to pump the new fluid is directly related to its viscosity. In this work, the rheological behaviour of different loaded functionalized graphene nanoplatelet dispersions (nanoparticle mass concentrations, wt%, of 0.25, 0.50, 0.75 and 1.0) in three based fluids (water, propylene glycol:water mixture at 30:70 wt% and propylene glycol:water mixture at 50:50 wt%) has been investigated by using a rotational rheometer. A double cone geometry was employed together with a cover specially appropriated to help thermostatization and avoid evaporation at high temperatures. The viscosity curves in the shear rate range from 10 to 1000 s(-1), with 10 points per decade, and in the temperature range from 283.15 to 353.15 K, with 10 K step, were determined for the three base fluids and the twelve different nanofluids. The dynamic viscosities values of the base fluids were compared with those available in the literature, absolute average deviations being lower than the experimental uncertainty. The dynamic viscosities of the nanofluids were obtained for the Newtonian ranges and their dependences with temperature and nanoadditive concentration were analysed, as well as the influence of the base fluid on these trends. Furthermore, the fitting parameters of an equation that models temperature and nanoparticle concentration dependences are provided, which allow to describe viscosity data for each of the three sets of nanofluids with deviations lower than 1.8%. Finally, linear viscoelastic oscillatory experiments were performed for those samples that showed evidences of Non-Newtonian behaviour in the non-linear tests. These oscillatory tests were carried out in the deformation range from 0.1 to 1000% at constant frequency, 1 Hz, and at 283.15 and 293.15 K. Unlike the base fluids and other nanofluids sets, the nanofluids of the analysed propylene glycol:water 50:50 nanofluid set at 283.15 K and 293.15 K present a clear shear thinning (pseudoplastic) non-Newtonian behaviour, the Newtonian plateaus being easily identified as the concentration rises in the lowest deformations range.