Flow behaviour of suspensions of functionalized graphene nanoplatelets in propylene glycol-water mixtures

The low thermal conductivity of the fluids usually used in heat exchange processes (water, oils, glycols…) has proved to be one of the limiting factors in improving their heat transfer performance. These traditional fluids enhance their thermal capabilities by dispersing nano-sized particles with high thermal conductivity, as many researches have revealed in the last decades. Although the thermal conductivity of nanofluids has centred the interest of the heat transfer community, the rheological behaviour of the resulting dispersions is very influential in the heat transfer process, pressure drops and pumping powers. In this work, different samples of functionalized graphene nanoplatelets dispersed into two different binary mixtures of propylene glycol and water at 10:90 wt% and 30:70 wt% have been analysed by using a DHR-2 rotational rheometer equipped with concentric cylinder geometry. Firstly, the flow curves for Krytox GPL102 oil, pure propylene glycol, and the two binary mixtures of propylene glycol and water used as base fluids were experimentally obtained. These values were used to check our experimental procedure finding a good agreement between them and those reported and well known in the literature, with an absolute average deviation of 2.0%. Then, the viscosity-shear rate curves in the temperature range from 278.15 to 323.15 K were obtained for the different graphene nanofluid sets. Furthermore, a new equation was proposed in this work to describe the viscosity dependence on both the temperature and the nanoparticles volume concentration of graphene nanoplatelet nanofluids with an absolute average deviation with respect to our experimental data lower than 2%. Additionally, oscillatory tests of the samples were performed observing pseudoplastic behaviour for the nanofluids at lower angular frequencies.