Forced-convective heat-transfer coefficient and pressure drop of water-based nanofluids in a horizontal pipe

In this paper the heat transfer performance of Al₂O₃, SiO₂ and multi-walled CNTs (MWCNTs) in a closed loop were investigated. Heat transfer coefficient and pressure drop were measured in a horizontal thermal-insulated test-section. Special care was taken in the loop calibration and the estimation of measurement uncertainties. The results show that the Gnielinski correlation can be used to predict the turbulent heat transfer coefficient as long as the proper experimental values for the thermophysical properties of each nanofluid are used. Also, the Colebrook-White correlation for the friction factor showed good agreement with the experimental results for pressure drop. The nanofluids showed an increased heat transfer coefficient with respect to that of water on a constant Reynolds number basis, but a reduced performance when compared on a constant pumping power basis. Note that some authors treat CNTs as nanofluids because one of their dimensions is in the nanoscale, but strictly speaking this dimension is not the one that is actually contributing to the changes in the thermal properties of the mixture. We include them in our analysis to demonstrate that this kind of suspension also follows conventional correlations.