A Review of Hybrid Passive Heat Transfer Enhancement Using Nanofluids and Inserts in Compact Heat Exchangers

The growing demand for compact and energy-efficient thermal systems has intensified research on advanced heat transfer enhancement methods. Among passive strategies, inserts such as twisted tapes and wire coils have been extensively studied for their ability to induce turbulence and improve thermal performance, while nanofluids engineered by dispersing high-conductivity nanoparticles in conventional base fluids have shown promise in enhancing thermal conductivity. However, both techniques face individual limitations: nanofluids alone do not significantly modify flow dynamics, whereas inserts often lead to higher pressure drops. Recent studies indicate that combining nanofluids with inserts can create a synergistic effect, resulting in superior heat transfer enhancement compared to either method applied independently. This review presents a preliminary investigation into the application of nanofluid-insert hybrid passive techniques for the design of compact and high-performance heat exchangers. Emphasis is placed on their combined influence on the heat transfer coefficient, Nusselt number, pressure drop, and thermohydraulic performance. The paper further discusses experimental and computational findings, comparative analyses, and design considerations for balancing enhancement with energy penalties. The review highlights the potential of nanofluid and insert combinations to advance next-generation heat exchangers that are efficient, compact, and sustainable, while identifying existing research gaps and opportunities for further optimization.