A Review on Passive Heat Transfer Enhancement Techniques Applied to Supercritical Thermal Systems

Supercritical heat exchangers play a critical role in advanced thermal management and energy conversion systems, where performance is essential for improving overall system efficiency. Compared with active techniques, passive heat transfer enhancement (HTE) methods that rely solely on geometric and fluid modifications without additional energy input offer higher system reliability and broader engineering applicability. However, existing studies are highly fragmented and lack a systematic examination of the enhancement mechanisms and design implications. This paper systematically reviews recent advances in passive HTE strategies for supercritical fluids (SCFs), including channel configuration, internal disturbance structures, surface modification, and working-fluid tailoring. The underlying mechanisms involved are analysed and the effects on heat transfer, flow resistance and overall performance are discussed. The analysis indicates that the effectiveness of most passive enhancement strategies ultimately originates from their ability to strengthen momentum and energy transport in the near-wall region. Simple geometries such as variable diameter tubes provide limited improvement despite their structural simplicity, whereas more complex configurations generally can achieve stronger enhancement but are accompanied by increased pressure drop penalties, requiring careful evaluation of the overall thermal-hydraulic performance. In contrast, fluid-based approaches using nanofluids or supercritical mixtures regulate thermophysical properties, with performance strongly dependent on nanoparticle type, particle concentration, dispersion stability, and mixture composition. These findings provide theoretical insights and design guidelines for the development of next-generation high-performance supercritical heat exchangers.