Influence of nanoparticle shape factor on convective heat transfer and energetic performance of water-based SiO2 and ZnO nanofluids

To appreciate the merits, in terms of energy, of two nanofluids and of two shapes of nanoparticles, an experimental study has been carried out on water-based SiO2 and ZnO nanofluids flowing inside a horizontal tube whose wall temperature is imposed. Pressure drop and heat transfer coefficients have been measured at two different inlet temperatures (20 °C, 50 °C) in heating and/or cooling conditions at various flow rates (200 < Re < 15,000). The Reynolds and Nusselt numbers have been determined by using thermal conductivity and viscosity measured in the same conditions as those in tests. The results obtained show a small improvement of Nusselt numbers of studied nanofluids compared to those of the base fluid. An energy Performance Evaluation Criterion (PEC) has been defined to compare heat transfer rate to pumping power. Only nanofluid with ZnO nanoparticles having a shape factor greater than 3 appears to reach a PEC as high as that of water.