Synergistic effects of spiral tape inserts and magnetic hybrid nanofluids on heat transfer and entropy generation in transition flow regime

Abstract The transition flow regime (Re ≈ 2,300–3,000) is critically important in thermal engineering due to its sensitivity to flow disturbances and potential for optimized heat transfer. However, inefficiencies in this regime often limit the performance of conventional heat exchangers. This study investigates the enhancement of thermal and thermodynamic performance in circular tubes using spiral tape inserts (STIs) in conjunction with magnetic hybrid nanofluids composed of $${\text{Fe}}_{3}{\text{O}}_{4}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mtext>Fe</mml:mtext> <mml:mn>3</mml:mn> </mml:msub> <mml:msub> <mml:mtext>O</mml:mtext> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> -MgO/water at concentrations ranging from 0.00625 to 0.1 vol%. Experiments were conducted under a constant heat flux boundary condition to assess the combined effects of passive (STI) and active (nanofluid) techniques. Key performance indicators included the Nusselt number (Nu), friction factor, thermal performance factor (η), and entropy generation rate. Results show that the STI–nanofluid configuration significantly improves convective heat transfer, with up to ~ 460% increase in Nu at 0.025 vol% concentration. The thermal performance factor exhibited a nearly fourfold improvement compared to a plain tube baseline. Although friction factor increased by 8–10 times, it contributed less than 10% to the overall performance penalty. Notably, thermal entropy generation decreased by more than 50%, indicating a substantial gain in second-law efficiency. This work demonstrates that integrating STIs with magnetic hybrid nanofluids offers a promising and scalable strategy for enhancing both energy efficiency and thermal effectiveness in systems operating within the transition flow regime. These findings contribute to the development of next-generation heat exchangers and advanced thermal systems.