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

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 (0.9 mm inner diameter) for heat removal from a constant temperature source, with nanoparticle concentration (0.025-0.1 wt%) and volumetric flow rate (0.18-1.25 mL/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 0.05 wt%. 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.