Increasing the thermal performance of double-pipe heat exchangers by active methods: A comprehensive review

This review investigates recent advancements in active heat transfer enhancement techniques for double-pipe heat exchangers, which are widely used in industrial and energy systems due to their simple design, ease of maintenance, and operational flexibility. A typical double-pipe heat exchanger consists of one pipe inside another, enabling efficient heat exchange between two fluids. While passive methods for improving thermal performance in double-pipe heat exchangers offer limited enhancement, active techniques, such as mechanical vibration, tube rotation, fluid injection, electromagnetic fields, and acoustic excitation, have demonstrated greater effectiveness by disturbing the thermal boundary layer and inducing enhanced turbulence, leading to significantly improved heat transfer rates. This review classifies active techniques into mechanical, electromagnetic, acoustic, and thermal categories, providing detailed insights into their mechanisms, working fluids, flow regimes, and combined applications with passive elements. Experimental and numerical studies are compared based on the thermal enhancement factor, revealing that electromagnetic techniques, particularly magnetic turbulators, achieved the highest thermal enhancement factor values (above 4), especially in laminar flows. Mechanical vibration and tube rotation also demonstrated strong enhancement, with promising results when integrated with nanofluids or twisted tapes. Despite trade-offs in pressure drop and energy input, active methods show high potential for targeted heat transfer improvement in double-pipe heat exchangers, especially under controlled flow conditions. The findings serve as a benchmark for selecting optimal enhancement strategies in advanced thermal systems.