In this study, influence of concentration, particle size and thermal conductivity of particle material on convective heat transfer of nanofluids is experimentally examined. Water-based nanofluids containing SiO2, micelle, polystyrene or Al2O3 particles are self-synthesized (apart from Al2O3) and measured with an annular tube heat exchanger. Concentrations of the nanofluids studied vary in the range of 0.1–2.2 vol-% and particle sizes between 8–58 nm. The heat transfer measurements cover both laminar and turbulent regimes with the Reynolds numbers varying in the range of 1000-11000. The measurements also include pressure losses in order to study the suitability of nanofluids for practical forced convection heat transfer applications. The fluids are thoroughly characterized: viscosities, thermal conductivities, densities, particle size distributions, shapes and zeta potentials are all determined experimentally.
In many previous studies, anomalous enhancement in convective heat transfer is observed based on comparison of the Nusselt numbers with equal Reynolds numbers. Also in this work, the nanofluids exhibit Nusselt numbers higher than water when compared on this basis. However, this comparison neglects the impact of differences in the Prandtl numbers. No difference is observed when the effect of Prandtl number is taken into account. All nanofluids studied performed as Gnielinski correlation predicts, and anomalous behavior was not observed. When compared by using equal pumping powers, the nanofluids show equal or poorer performance than water. Increase in the particle concentration lowers the heat transfer performance in all cases. However, the magnitude of this deteriorating effect is smaller for nanofluids with smaller particle size indicating that small particle size is beneficial for heat transfer of nanofluids. The thermal conductivity of the particle material does not have a notable impact on the convection heat transfer with the relatively low particle concentrations studied herein (≤ 1 vol-%).
Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016.