Rheology of water-ethanol carbon nanofluids and their effectiveness in convective heat transfer

For a nanofluid to succeed as working fluid in a convective application, it is mandatory to study its thermophysical properties and heat transfer effectiveness. In this context, the work presented here examines the rheology of two water-ethanol carbon nanofluids in the interval 10–40°C and their thermal performance in laminar and turbulent convection. The nanofluids are composed of carbon nanospheres dispersed in a 1:1 mixture of water and ethanol at 1.0% and 2.0% volume concentrations. Emphasis was placed on ethanol for its high efficacy as dispersant and ability to permit the nanofluids operation at low temperatures. Viscosity, shear stress, and shear rate measurements were done in a cone/plate rheometer. Heat transfer effectiveness in laminar flow was assessed through the viscosity/conductivity enhancements ratio ( C μ / C k < 4 ). For turbulent flow, it was done via numerical simulations and the performance evaluation criterion ( PEC > 1 ). Results showed Newtonian behavior for both the basefluid and the two nanofluids, confirmed by flow index values repeatedly close to 1. Viscosity and shear stress decreased with temperature and increased with concentration. Maximum viscosity enhancements of 5.7% and 11.3% were observed at 40°C for each nanofluid. Regarding the heat transfer, both nanofluids outperformed the basefluid. In laminar flow, C μ / C k remained lower than 4 at all cases. Similarly, the turbulent flow criterion PEC > 1 was always met, yet higher potential was seen for the 2.0% nanofluid. The largest Nusselt number and pressure drop increments achieved by the 1.0% nanofluid were 24.5% and 8.3%, respectively, while those of the 2.0% nanofluid were 57.9% and 13.5%, correspondingly.