Convective heat transfer performance of CuO–water nanofluids in U-shaped minitube: Potential for improved energy recovery

•Low concentration stable CuO–water nanofluid prepared without surfactant.•10% increase in thermal conductivity at 0.05wt% nanoparticle concentration.•Heat absorption from constant source temperature in U-shaped minitube tested.•Maximum enhancement in ‘h’ at optimum nanoparticle concentration.•34% enhancement in ‘h’ at low flow rates. Heat removal from a constant temperature heat source is relevant in energy recovery from thermal energy storage systems and catalytic reactors. Experiments were carried out to assess the heat transfer performance of CuO–water nanofluid flowing through a U-shaped minitube (0.9mm inner diameter) for heat removal from a constant temperature source, with nanoparticle concentration (0.025–0.1wt%) and volumetric flow rate (0.18–1.25mL/s) as the independent variables. The thermal conductivity and viscosity of CuO–water nanofluids increased linearly with nanoparticle concentration, with thermal conductivity enhancement higher than that of viscosity increase. The heat transfer rate, heat transfer coefficient and Nusselt number of CuO–water nanofluids were higher than that of water. The improved heat transfer performance of CuO–water nanofluids may be attributed to their improved thermal conductivity and particle migration effects. Our experiments reveal that the heat transfer performance of CuO–water nanofluids in U-shaped minitube is enhanced to a larger extent at the lower flow rates and at an optimum nanoparticle concentration of 0.05wt%. We believe that the existence of an optimum nanoparticle concentration may be attributed to the enhanced synergistic effect of higher thermal conductivity and particle migration.