Heat transfer enhancement in double-pipe heat exchangers remains critical for industrial energy efficiency, yet swirl flow decay—a phenomenon where rotational momentum diminishes axially—presents a key challenge in maintaining optimal thermal performance. This study investigates swirl decay dynamics and its implications for heat transfer in annular flows using three-dimensional computational fluid dynamics (CFD) simulations in ANSYS Fluent, employing k-ε RNG-swirl modified turbulence model and finite volume discretization. The swirl decay was induced by inlet annular swirlers with vane angles of 30º, 45º and 60º. Three objectives guide the work: (1) quantifying the impact of decaying swirl on annular heat transfer, (2) developing a universal model correlating swirl number decay rate with friction factor under heated/unheated conditions, and (3) assessing the thermal benefits of sustaining a minimum swirl intensity. Simulations reveal that axial and tangential velocity profiles exhibit a radial outward bias (shift away from the inner pipe), amplified by larger swirl angles but reduced by higher Reynolds numbers. Swirl decay induces oscillating velocity peaks, particularly in regions of weakened rotational momentum, while thermal effects in heated flows accelerate decay and alter recirculation zones, notably extending the central vortex beyond the swirler. Local Nusselt number peaks near the swirler exit, attributed to thermal boundary layer thinning, inform a novel correlation linking swirl decay rate (quantified via the swirl number, a dimensionless measure of rotational momentum) to friction factor, valid across both adiabatic and heated cases. To enable practical design, the Decay Percentage, a new parameter defining allowable swirl loss over the exchanger length, is introduced, facilitating optimised configurations that balance compactness (2%–69% size reduction) and efficiency (139%–242% heat transfer gain). These findings advance passive enhancement strategies for heat exchangers in energy systems, offering a framework for performance tuning through controlled swirl decay.
Mohannad Eslam Ahmed Eid Shahin (Sat,) studied this question.