Numerical investigation of heat transfer performance in a segmentally baffled shell-and-tube heat exchanger using Di- and tri-hybrid nanofluids: a comparative analysis of round and twisted tube arrangements

The growing global energy demand requires compact heat exchangers with high thermal performance and low pumping-power requirements. Shell-and-tube heat exchangers (STHXs) are widely used in industrial thermal systems, while baffles, twisted tubes, and hybrid nanofluids (HNFs) are known to enhance heat-transfer performance individually. However, the combined thermo-hydraulic behaviour of di- and tri-hybrid nanofluids in identical baffled round- and twisted-tube STHX configurations remains insufficiently explored. To address this gap, a 3D CFD model was developed in ANSYS Fluent 2020 R1 for a six-baffle STHX using water, five water-based di-hybrid nanofluids, and an Al ₂O₃-CuO-ZnO tri-hybrid nanofluid with 5% individual nanoparticle volume concentration, represented by an idealized homogeneous effective-property model. Primary simulations were conducted for mass flow rates of 0.5–2.0 kg/s with a twisted tube twist ratio (TR) of 6.75. Based on the results, the Al ₂O₃-CuO-ZnO tri-hybrid nanofluid in the twisted-tube configuration showed the highest predicted thermo-hydraulic performance, with a 7.85% improvement in shell outlet temperature, a 39.3% reduction in pressure drop and the highest PEC (1.109) among the primary TR = 6.75 working-fluid cases relative to the water-based round-tube baseline. Twist-ratio (TR) sensitivity analysis over TR = 2.25–11.25 showed systematically improved thermo-hydraulic performance with decreasing TR, with the highest investigated PEC of 1.216 at TR = 2.25. Flow-field analysis indicates that the twisted tube enhances secondary flow, turbulent mixing, vorticity, and velocity–temperature-gradient synergy, explaining the improved heat-transfer behaviour. The reduced pressure drop of the tri-hybrid twisted-tube reflects the combined effects of nanofluid properties, fixed mass-flow-rate conditions, and the geometry of the tube. Within the investigated seven-tube STHX and uniformly dispersed single-phase assumption, the study provides a CFD-based screening framework for identifying promising tube-geometry and nanofluid combinations for future multiphase, stability, and experimental assessment.