Measuring the viscosity of Fe3O4-MWCNTs/EG hybrid nanofluid for evaluation of thermal efficiency: Newtonian and non-Newtonian behavior

In the present study, the rheological behavior of Fe3O4-MWCNTs/ethylene glycol hybrid nanofluid was investigated for different volumetric fractions (0.1, 0.25, 0.8, 1.25, and 1.8%) comprising equal amounts of suspended iron oxide (Fe3O4) nanoparticles and multi-walled carbon nanotubes (MWCNTs) within the temperature and shear ranges of 25 to 50 °C and 12.24 to 73.44 s−1, respectively. The results indicated that the nanofluid demonstrates a Newtonian behavior at volume fractions of 0.1, 0.25, 0.45, and 0.8%, whereas it demonstrates a non-Newtonian behavior at volume fractions of 1.25 and 1.8% similar to the power law model with an exponent lower than 1. Moreover, it was observed that viscosity is directly proportional to volume fraction, and inversely proportional to temperature. Then, the effect of nanoparticles and nanotubes on the thermal performance of ethylene glycol inside a pipe of heat exchanger was evaluated and the results revealed that an increase in the volume fraction at all temperatures leads to increased pressure drop and heat transfer coefficient. Finally, the results were used to develop an experimental equation as a function of temperature and volume fraction to estimate nanofluid viscosity in the simulations. The predicted values by the equation were in a very good agreement with the experimental results.