Investigation of fluid flow and heat transfer in annulus conical tubes

In this study, the heat transfer and pressure drop characteristics of an annulus conical-tube heat exchanger were investigated numerically under both laminar and turbulent flow regimes. A three-dimensional (3D) computational fluid dynamics (CFD) model was developed and validated to provide additional insights into the thermal performance of the annulus conical-tube at a level of details not typically achievable through experiments. The commercial ANSYS 14.5 software package was employed to introduce the concept of an annulus conical coil and to evaluate its heat transfer enhancement compared with conventional annular helical and in-plane spiral coils. The realizable k–ε turbulence model was adopted to estimate the effects of turbulence accurately. The impacts of key design parameters-including the conical angle (0° for helical, 30°, 50°, and 70° for conical designs), annulus torsion (ζ = 0 for spiral, and ζ = 0.037, 0.057, and 0.091 for conical configurations), flow direction (innermost and outermost), CuO-water nanofluid concentration (1%, 3%, and 5%), and Reynolds number on the thermal and hydraulic performance of the annular conical coil were systematically examined. The simulations were performed for Re ≤ 7000 in the laminar regime and 9500 ≤ Re ≤ 30,000 in the turbulent regime. The results revealed that decreasing the conical angle from 70° to 30° increased the Nusselt number per unit length (Nu/L) by 2.37 and 2.45 times for the laminar and turbulent regions, respectively. Similarly, reducing the torsion from 0.091 to 0.037 significantly enhanced Nu/L by 96.9% and 100% for the laminar and turbulent regions, respectively. The innermost flow exhibited a 17.7% higher Nusselt number than the outermost flow. Using a CuO–water nanofluid with a 5% concentration improved the thermal characteristics by 53.2% and 35.8% in the laminar and turbulent regions, respectively. The thermo-hydraulic performance index (η) reached 2.98 and 1.12 at a pitch ratio (ζ) of 0.057 and a taper angle (φ) of 30° for the laminar and turbulent regions, respectively. Finally, new correlations for predicting the Nusselt number (Nu) and friction factor (f) in both laminar and turbulent flow regimes were proposed.