Thermal–hydraulic performance of high-angle twisted tapes with MWCNT–Fe3O4 hybrid nanofluids under turbulent flow

Abstract

The thermo-hydraulic performance of plain tube (PT) and twisted-tape (TT) inserts having different swirl angles (30°, 45°, 60°, and 90°) using base fluid (DW) and hybrid MWCNTs–Fe 3 O 4 nanofluids was numerically solved in this study. The physical problem was solved at various fluid-flow velocities (Re = 5000–17,000) and inlet temperatures (T in  = 293.15–323.15 K). The two geometries were solved using the SST k-ω turbulence model and validated against the Dittus–Boelter and Blasius correlations, with four grid sizes. The numerical results demonstrate that TT inserts significantly enhance convective heat transfer, yielding Nu improvements of 45–87% relative to PT, while incurring pressure-drop penalties (ΔP) of 115–140%. The TT-90° configuration consistently achieved the highest thermo-hydraulic performance (η TH ), with η TH  = 1.3975 at Re = 5000, corresponding to a 39.8% net improvement under equal pumping power. The hybrid nanofluid alone provided reasonable heat-transfer augmentation (≈ 0.3–0.8%) relative to DW but introduced additional hydraulic resistance (≈ 13–21%). Flow-structure analysis revealed intensified secondary vortices and swirl-induced mixing as the primary mechanisms governing heat-transfer enhancement. Overall, the integration of MWCNTs–Fe 3 O 4 /DW and high-angle TT offers superior thermo-hydraulic performance, particularly at lower fluid flow velocity.