Thermal and hydraulic analysis of magnetic hybrid nanofluids in twisted tape-enhanced parabolic solar collectors

This study investigates the thermohydraulic performance of Fe₃O₄-TiO₂/H₂O hybrid nanofluids in plain tube (PT) and twisted tube (TT) using the k-ω SST turbulence model for Reynolds numbers between 6,000 and 18,000. The CFD results showed good agreement with experimental data, with errors of 2.67–6.65 % for H₂O and 3.79–8.99 % for 0.4 vol% Al₂O₃-H₂O, confirming the solution's accuracy. Grid#1 (element size 1.4 mm) achieved the highest accuracy, with average errors as low as 2.83 % against Dittus-Boelter and 1.70 % against Blasius at 50 °C, confirming that finer grids significantly enhance prediction reliability. Heat transfer enhancement decreased with an increase in temperature, from 25.68 % at 20 °C to 11.23 % at 50 °C in PT, and from 17.84 % to 9.57 % in TT at 2 vol%. Pressure drop increased with nanoparticle concentration but declined with temperature, peaking at 320.84 % (PT) and 320.85 % (TT) at 20 °C, reducing to 200.84 % and 200.82 % at 50 °C, respectively. Thermal-hydraulic efficiency improved with Reynolds number and volume fraction but declined at higher temperatures, reaching a peak of 1.281 (4.3 %) in PT and 1.19271 (5.54 %) in TT at 20 °C and Re = 18,000. At 50 °C, efficiency gains were limited to 1.5 % (PT) and 2.77 % (TT). Overall, TTs increased pumping power by up to 3.4 times compared to PTs at Re = 18000 and 20 °C, and the use of nanofluids further raised pumping requirements by up to 8 times at 2 vol%.