Enhancing the thermal performance of double-pipe heat exchangers using wire coil inserts: A focused review

Double-pipe heat exchangers are widely used in industrial applications due to their simplicity and efficiency. To further enhance thermal performance, passive techniques such as wire coil inserts have been extensively studied. This first-of-its-kind review explores the enhancement of thermal performance in double-pipe heat exchangers using wire coil inserts. It examines various geometrical parameters, placements (in both inner and outer tubes), and working fluids. Results indicate that wire coil inserts can increase the Nusselt number by up to 400 %, with thermal performance factors reaching 4, depending on operating conditions, insertion method, and incorporating other techniques such as nanofluids and active methods. Despite the enhancement, friction factors also increased by 2–7 times, highlighting the need to balance between heat transfer improvement and pressure loss. Insets in the inner tube generally outperformed outer tube inserts, particularly at lower pitch-to-diameter ratios. Numerical simulations using the SST k-ω turbulence model showed strong agreement with experimental data, with prediction errors under ±10 %. The review critically compares enhancement techniques, evaluates computational fluid dynamics modeling approaches, and identifies gaps in hybrid insert applications use and outer tube design. This work serves as a practical reference for optimizing insert configurations and simulation strategies in advanced heat exchanger design.