![]() In the following, the most recent well-known models for estimating the viscosity of nanofluids will be summarized. However, in fact, there are many factors affecting viscosity of nanofluids, such as temperature, pH, volume fraction, particles’ size, particle size distribution, electrical double layer (EDL), zeta potential, base fluid type, aspect ratio of particles, packing coefficient, particles’ agglomeration, nanolayers, and magnetic properties for ferromagnetic type of nanoparticles. Some studies on nanofluids’ rheological behavior were devoted to understanding whether these fluids are Newtonian or non-Newtonian toward finding their viscosity based on the relation between the shear stress and the shear rate. Therefore, a great deal of effort has been made in the last two decades for developing reliable models to predict the viscosity of nanofluids. Among these properties, viscosity has a vital role in all nanofluids’ transport phenomena. Because of the very complex nature of such fluids, the prediction of their thermophysical properties has become a challenging problem for research. Nanofluids have found many applications in science and industry. These factors are the viscosity of the base fluid, nanoparticles’ volume fraction, nanoparticles’ size, the temperature of the system, some molecular properties, and zeta potential. An accurate model was developed based on the Buckingham Pi theorem to fit all factors affecting viscosity in a dimensionless form. Furthermore, experiments on different-sized ferrous oxide-based nanofluids revealed inverse relation between the size of nanoparticles and viscosity. Both types of nanofluids showed increasing viscosity with increasing nanoparticle loading and decreasing viscosity with increasing temperature. All viscosity measurements were conducted using a capillary viscometer at temperatures ranging between 25 and 65☌. Zeta potential measurement, which was performed to check the stability of the nanofluids, showed stable suspensions. This study focused on measuring the viscosity and analyzing the behavior of two types of nanofluids: ferrous oxide-deionized (DI) water nanofluids and graphene-DI water nanofluids at different temperatures and volume fractions.
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