Accurate basis of comparison for convective heat transfer in nanofluids

Thermal conductivity and viscosity of alumina (Al2O3), zirconia (ZrO2), and titania (TiO2) nanofluids (NFs) were measured at 20 °C. All the NF systems were water based and contained 9 wt.% solid particles. Additionally, the heat transfer coefficients for these NFs were measured in a straight tube of 1.5 m length and 3.7 mm inner diameter. Based on the results, it can be stated that classical correlations, such as Shah and Gnielinski, for laminar and turbulent flow respectively, can be employed to predict convective heat transfer coefficients in NFs, if the accurate thermophysical properties are used in the calculations. Convective heat transfer coefficients for NFs were also compared with those of the base fluids using two different bases for the comparison, with contradictory results: while compared at equal Reynolds number, the heat transfer coefficients increased by 8–51%, whereas compared at equal pumping power the heat transfer coefficients decreased by 17–63%. As NFs have higher viscosity than the base fluids, equal Reynolds number requires higher volumetric flow, hence higher pumping power for the NFs. It is therefore strongly suggested that heat transfer results should be compared at equal pumping power and not at equal Reynolds number.