Passive Techniques for Heat Transfer Enhancement in Selected Industrial Heat Exchangers: A Comprehensive Review of Geometric Modifications and Nanofluids

Passive heat transfer enhancement remains the most economical route to compact, energy-efficient heat exchangers because it requires no external power and can be retrofitted into existing hardware. This paper reviews passive enhancement techniques applied to five widely used heat exchanger configurations: plate heat exchangers (PHEs), corrugated-plate heat exchangers, double-pipe heat exchangers, printed-circuit heat exchangers (PCHEs) and flat-plate heat exchangers. This comprehensive review draws on 210 references from the peer-reviewed literature, published mainly over the past two decades, with respect to geometric modifications (twisted tapes, wire coils, corrugations, embossing and shaped fins), nanofluid working media and their combined use. The evidence is synthesised through the thermal performance factor (TPF) so that heat transfer gains are always weighed against the accompanying pressure penalty. Across the collected data, twisted-tape inserts raise the Nusselt number by 20–120% in turbulent flow, with friction factor increases of 30–200% and TPF values typically between 1.0 and 1.5; compound and nanofluid-assisted tapes push the TPF above 2, and in the laminar regime it can reach 4–5, while wire coils outperform twisted tapes in turbulent flow. Nanofluids improve thermal conductivity by 1–52% at volume fractions up to about 8%, but viscosity penalties of 3–210% and long-term stability continue to limit commercial uptake. Corrugated surfaces yield Nusselt-number gains of 10–61% through secondary flow and boundary-layer disruption, while in PCHEs the transition from zigzag to S-shaped and aerofoil fins reduces the pressure drop by up to a factor of 20 at comparable thermal duty. A cross-cutting comparison identifies the best-performing technique for each configuration and exposes four persistent gaps: the scarcity of like-for-like comparisons across heat exchanger types, the limited evidence on long-term nanofluid stability and cost, the under-representation of PCHE and flat-plate passive studies, and the early stage of data-driven optimisation of enhancement geometries. The review provides a consolidated, quantitative basis for selecting enhancement strategies and for directing future work.