Numerical study on turbulent mixed convection heat transfer and pressure drop of nanofluid in vertical helically coiled tube heat exchanger

In this study, turbulent mixed convection heat transfer and pressure drop of Al2O3/water nanofluid in helically coiled tube heat exchangers were investigated. Thermo-physical properties of nanofluid and base fluid were considered temperature dependent and analysis for helical coils, with curvature ratios of 0.1, 0.0666 and 0.05, and nanoparticles with volume concentrations in the range of 0-2% in various Reynolds number were carried out. CFD analysis was done by 3D realizable ke turbulent model in ANSYS FLUENT 15. According to the results, it was shown that by increasing the coil curvature ratio and nanoparticles volume concentrations in a same Reynolds number, heat transfer coefficient and pressure drop of helical coils both increased. Using numerical data, a correlation for predicting outer Nusselt number in terms of Rayleigh number in helical coils were proposed. Thermal performance index were compared in various conditions and the maximum value was noticed for coil with a curvature ratio of 0.1 and nanoparticles volume concentrations of 2%. Results showed that using nanofluid and helical coils instead of base fluid and straight pipes, improves thermal performance of the heat exchangers.