Heat transfer enhancement in a shell and double‐coil heat exchanger using twisted tapes and hybrid nanofluids

Double coils in shell‐and‐coil heat exchangers improve heat transfer over single coils by enhancing fluid flow and turbulence, and by increasing tube length in limited spaces. The spiral design also offers self‐cleaning capabilities and uniform flow distribution, maximizing surface area utilization and minimizing fouling. This study evaluated the performance of a shell‐and‐double‐coil heat exchanger (SDCHE) with three internal helical twisted‐tape designs (30‐turn, 20‐turn, and 10‐turn) as a turbulator, and two water‐based hybrid nanofluids, and compared it with that of a single‐spiral coil. In the second part of this work, numerical simulations were conducted to compare the thermo‐hydraulic performance of an advanced SWCNT‐MWCNT/water hybrid nanofluid (at = 0.3%) with that of the CuO‐MgO/water mixture (at = 0.3%) and pure water. The Performance Evaluation Criterion (PEC) is a vital dimensionless metric that compares a system's heat‐transfer enhancement against its increased frictional pressure drop, ultimately determining whether a modified design's thermal benefits justify the additional pumping power required to operate it. Initially, four configurations (with and without fins) were compared using water, showing significant increases in flow mixing and heat transfer with internal helical twisted‐tape. The 30‐turn configuration exhibited the highest heat transfer rate and performance improvement. At Reynolds number Re = 2000, the 30‐turn, 20‐turn, and 10‐turn cases yielded PECs of about 2.71, 2.41, and 2.02, respectively, with the simple case serving as the reference model ( PEC = 1). Subsequently, PEC values for Water/SWCNT‐MWCNT and Water/Cuo‐Mgo were 2.84 and 2.65, respectively.