hybrid enhancement of heat exchanger efficiency via geometric modifications and nanofluid-induced thermal property improvement

This study numerically investigates the combined heat transfer and fluid flow behavior in a double-pipe heat exchanger enhanced with spring-shaped turbulators and al₂o₃ nanofluids. four configurations are analyzed: circular turbulators (case a), elliptical turbulators (case b), circular turbulators with nanofluids (case c), and elliptical turbulators with nanofluids (case d), all compared against a smooth-pipe baseline. a validated 3d cfd (computational fluid dynamics) model implementing the realizable k-ε turbulence model (y+ ≈ 1) and second-order discretization evaluates key parameters, nusselt number, friction factor, and thermal performance across reynolds numbers (3,000 ≤ re ≤ 20,000). results demonstrate that elliptical turbulators (case b) outperform circular designs by 12–45% in heat transfer enhancement, with peak improvements (48.7%) at re = 3,000, attributed to stronger secondary flows. nanofluid addition further augments performance, particularly in laminar-transitional regimes, with case d achieving the highest thermal efficiency. however, enhancements diminish at higher re (>13,100) as natural turbulence dominates. pressure drops, penalties increase by up to 211.46 pa (case d at re = 20,000), highlighting a critical trade-off between heat transfer and hydraulic losses. 3d thermal contours reveal that elliptical turbulators create 25–30% larger high-efficiency zones, while nanofluids reduce radial temperature gradients by 35–40%. streamline analysis shows negligible global flow differences, confirming that improvements stem from localized near-wall effects. exergy analysis indicates 24% higher irreversibility for case d at low re, emphasizing thermodynamic trade-offs. for industrial applications, case d is recommended for low-flow systems (re < 6,200), while simpler designs suffice at higher re. this work provides actionable insights for optimizing heat exchanger performance through combined geometric and nanofluid enhancements, validated by rigorous numerical and experimental benchmarks.