Heat Transfer Enhancement of Tube Bundle with Symmetrically Inclined Annular Fins for Waste Heat Recovery

Enhancing convective heat transfer efficiency in waste heat recovery applications is critical for improved energy utilization. This study conducts a convective heat transfer optimization of a tube bundle for waste heat recovery of flue gas based on an exergy destruction minimization method. The results indicate that the multi-longitudinal vortex flow is the optimal flow field for heat transfer in a tube bundle. To achieve this flow field, a novel tube bundle equipped with symmetrically inclined annular fins has been proposed and the thermal–hydraulic performance has been numerically investigated. The effects of key geometric parameters, including fin inclination angle (θ = 30°, 35°, 40°, 45°, 50°) and fin diameters (D = 62, 68, 74 mm), were systematically analyzed under varying inlet velocities (8–16 m/s) and heat flux densities (23,000–49,000 W/m2) at inlet temperatures of 527 K and 557 K. Results demonstrate that both the convective heat transfer coefficient (h) and tube bundle power consumption (Pw) increase with rising fin diameters and inclination angle. At a constant D, h and Pw exhibit a positive correlation with θ. Crucially, compared to a traditional smooth-tube bundle, the optimal annular fin configuration (θ = 45°, D = 74 mm) achieved a significant enhancement in the convective heat transfer coefficient of 22.76% to 31.22%. This improvement is attributed to intensified vortex generation near the fins, particularly above and below them at higher angles, despite a reduction in vortex count. These findings provide valuable insights for the design of high-efficiency finned tube heat exchangers for flue gas waste heat recovery.