Bio-Inspired Alula-Based Winglet Design for Enhanced Heat Transfer in High Temperature Fin-And-Tube Heat Exchangers: Article No. 128396

Fin-and-tube heat exchangers (FTHEs) are widely used for high-temperature flue-gas heat recovery, but their performance is often limited by wake regions and non-uniform fin-surface temperatures. This study proposes and numerically evaluates four bio-inspired longitudinal vortex generator (VG) configurations in a high-temperature FTHE with flue-gas inlet temperature ~1230 K: double-delta, curved double-delta, alula, and a new curved-alula geometry. The reference fin is not hydraulically plain; it already incorporates leading-edge separation columns and convex protrusions, so the alula-type winglets are assessed as downstream add-ons acting on a strongly disturbed flow. In a second step, perforations (one, two and three circular holes) are introduced into the curved-alula VGs to further tailor the flow field. Three-dimensional simulations with the Shear Stress Transpor (SST) k-w model, temperature-dependent flue-gas properties and conjugate conduction are carried out for gas-side Reynolds numbers Reg ~ 8.0 x 10^2 - 3.6 x 10^3 (mass flow rates 0.5 - 2.5 g/s), and the designs are compared in terms of surface heat flux, Nusselt number, friction factor and hydrothermal performance factor (HTPF). For this already-promoted fin, the additional downstream winglets provide moderate, incremental hydrothermal gains. At the highest Reynolds number, the best non-perforated design (curved-alula) increases surface heat flux from 1630.9 to 1794.7 kW/m2 (~ 10 % gain) and the Nusselt number from 227.6 to 242.6 (~ 7 % gain), while the friction factor rises from 0.26 to about 0.30, yielding HTPF values close to unity (~ 0.9 - 1.0). Introducing circular perforations into the curved-alula winglets acts mainly as a wake-bleeding refinement: the three-hole configuration provides a heat flux of 1824.7 kW/m2 and a pressure drop of 127.9 Pa, with HTPF in the range ~ 1.03 - 1.14 and a small (~ 1 - 3 %) improvement over the solid curved-alula design. Flow-field analysis shows that the perforated curved-alula VGs shrink tube-wake regions, thin the thermal boundary layer and homogenize the fin-surface temperature (outlet-gas temperature ~ 510 - 520 K and fin-surface temperature ~ 420 - 421 K for the three-hole case). An optimal flue-gas mass flow rate of ~ 1 g/s (Reg ~ 1.5 x 10^3) is identified, beyond which additional heat-transfer gains are offset by rapidly increasing pressure losses. Overall, the results highlight that initial fin geometry and VG placement are as important as VG shape: alula-based winglets are expected to yield larger relative gains on simpler flat-fin layouts or when positioned closer to the fin leading edge and tube array.