Investigation of Thermal and Hydraulic Performance in a Helical Two-Tube Heat Exchanger Equipped with Wire/Spring Turbulators

This study numerically investigated thermal enhancement in helical tube flows equipped with an embedded wire/spring turbulator. Five turbulator configurations (circular, square, and two different rectangular cross-sections) were examined for turbulent operation using water and three nanofluids (CuO–water, TiO₂–water, and Al₂O₃–water). The insertion of the turbulator reorganized the flow and intensified near-wall mixing, which led to consistently higher heat-transfer performance as evidenced by enhanced Nusselt number (Nu) levels. However, these thermal gains were accompanied by additional hydraulic losses, reflected in increased friction factor (f) and pressure-drop penalties. The results also revealed that Nu increases with increasing Dean number, indicating that curvature-driven secondary motions and turbulence augmentation act synergistically in promoting convection. In contrast, the hydrodynamic cost intensifies with flow strength, producing a net decrease in thermal efficiency, η, with Reynolds number for all cases. This decline occurs because the pressure-drop growth rate exceeds the incremental heat-transfer improvement. Among the examined fluids, nanofluids generally provided higher η than water, while Al₂O₃–water exhibited the most favorable behavior among the nanofluids. To jointly account for thermal and hydraulic effects, the thermal performance factor (TPF) was evaluated and found to decrease with Reynolds number across all fluids, implying that heat-transfer benefits become less dominant at higher inertia. The combined results highlight that selecting the appropriate turbulator cross-section and working fluid is essential for achieving an efficient thermal–hydraulic equilibrium.