An experimental investigation on rheological properties and heat transfer performance of MWCNT-water nanofluid flow inside vertical tubes

The contradictory results in literature regarding water-based carbon nanofluids for the turbulent regime are addressed in this paper. Convective heat transfer and friction factor of MWCNT-water nanofluids inside a vertical circular tube are investigated experimentally. The tested tubes are under constant heat flux boundary condition and the flow regime inside the tubes is turbulent. Distilled water and nanofluids with different particle weight concentrations of 0.05%, 0.1%, and 0.2% are utilized as the working fluids. Such tube and flow conditions are considered to specifically answer the question “whether water-based carbon nanofluids increase the heat transfer rate or friction factor in the turbulent regime". The required thermo-physical properties of the MWCNT-water nanofluids for calculating the convective heat transfer coefficient and pressure drop are measured. Observations clearly show that the heat transfer coefficient of nanofluid is higher than that of the base fluid and increases with the particle concentrations. Moreover, the measurements show that the pressure drop of nanofluid is slightly higher than that of the base fluid and does not increase significantly with increasing the nanoparticles volume fraction. Also, two correlations are proposed using the experimental data to predict the Nusselt number and friction factor of the nanofluid flow inside vertical tubes. Finally, the heat transfer performance analysis of MWCNT-water nanofluids show that the thermal performance factor for all cases are greater than unity which indicate that this nanofluid enhances the heat transfer without huge penalty in pumping power.