The influence of different flow regimes on heat transfer performance and exergy loss of Al₂O₃/DI-water and CuO/DI-water nanofluids

The convective heat transfer performance and energy efficiency of Al₂O₃/DI-water and CuO/DI-water nanofluids flowing through a straight vertical tube was experimentally studied for laminar, transitional and turbulent flow regimes. A circulating rig was built to conduct the experiments at constant heat flux and various particle concentrations of 0.20, 0.30 and 0.50 wt% for both of the nanofluids. Specifically, the influence of transitional flow on the heat transfer coefficient, friction factor and exergy loss of the nanofluids were analysed. An improvement in the convective heat transfer coefficient for both Al₂O₃ and CuO nanofluids was found when compared to DI-water at all flow conditions. The maximum enhancement of 25% in heat transfer coefficient was observed for the 0.50 wt% CuO nanofluid. At laminar flow conditions, the pumping power was similar for all the working fluids, however, it was more pronounced under the transitional and turbulent flow regimes with an average of 3.9% increment in pumping power for CuO nanofluids. Besides, the highest energy efficiency was found to be 84% with 12.8% and 3.45% average reduction in exergy loss for 0.50 wt% of CuO/DI-water nanofluid in laminar and turbulent flow conditions, respectively. New correlations are also proposed based on the experimental results, which can predict the Nusselt number for both nanofluids in laminar and turbulent flow regimes with high accuracy. It was concluded that the copper oxide nanofluid might be a good candidate for heat transfer applications because of their superior heat transfer performance in comparison to other classes of nanofluids as well as DI-water. Therefore, these types of nanofluids can be used to improve the heat transfer in many industrial sectors with more effective way.