Parametric Research and Theoretical Models for the Viscosity of Nanofluids

Improved performance of nanofluids (NFs) in various thermal transporting devices is strongly linked to their enhanced thermal properties, of which the thermal conductivity and viscosity are crucial. For the static and dynamic application of NFs in thermal systems, increasing the concentration of nanoparticles in NFs correspondingly enhances heat transfer and flow by increasing the coefficient of heat transfer. However, beyond a certain nanoparticle concentration, viscosity has a negative impact on the thermo–hydraulic performance, resulting in increased pump power, frictional loss, pressure drop, etc. The viscosity of NFs depends largely on several parameters, such as dispersion characteristics (sonication time, frequency, and amplitude), base fluid, nanoparticle concentration, nano size, surfactant, temperature, etc. Theory-based generalized models for predicting NF viscosity are lacking, which are thus of great importance in the design and operation of energy systems using NFs. This study presents brief documentation on the parameters that influence the viscosity of NFs. In addition, literature exploration of classical and contemporary models for the prediction of NF viscosity is carried out based on the underlying mechanisms, theories, and influencing parameters. The need to develop more detailed generalized models based on the physics of nano-suspensions for better convergence of experimental data is recommended.