Comparison of convective heat transfer of turbulent nanofluid flow through helical and conical coiled tubes

Application of nanofluid and coiled tubes are two passive methods for increasing the heat transfer. In the present study, the turbulent flows of water and nanofluid in coiled tubes heat exchanger were numerically studied. CuO-water nanofluid containing 1 vol% copper oxide nanoparticles was used and single-phase approach was considered for nanofluid flow. The effect of different geometrical parameters on Nusselt number was also investigated for both helical and conical coiled tubes. Water properties were defined as temperature dependent function, and constant wall temperature was employed as wall boundary condition. Simulation results were validated by available experimental and numerical data of heat transfer coefficient and pressure drop inside the helically coiled tube. The results show that for the helically coiled tubes, Nusselt number increased by tube diameter enhancement, while the increase of pitch circle diameter resulted in its decline. However, the variation of helical pitch exhibited a slight effect on the heat transfer. Moreover, for conical coiled tubes, increase helical pitch and cone angle reduced Nusselt number; while this parameter increased as the tube diameter increases. Addition of 1% copper oxide to water led to 8% heat transfer augmentation in the helically coiled tube. Also, using CuO-water nanofluid, the performance index was enhanced by 3.5 and 5% for helical and conical coiled tubes, respectively.