Heat transfer characteristics and optimization strategies in supercritical fluid heat exchangers with non-uniform thermal boundaries: A systematic review

The dual optimization of heat transfer and fluid flow constitutes subjects of paramount interest in heat exchange equipment. Non-uniform thermal boundaries are common in supercritical heat exchangers, interacting with drastic changes in properties to make the heat transfer process very complex. This paper first reviews the forms of boundary conditions in supercritical heat exchangers, including plate heat exchangers, shell-and-tube heat exchangers, printed circuit heat exchangers and some emerging heat exchangers. It attempts to translate structural impacts into non-uniform thermal boundary conditions. Then, based on single-tube analysis, the effects of axially and circumferentially non-uniform heat flux on the flow and heat transfer characteristics of supercritical fluids are examined. To reduce wall temperature and improve heat transfer performance, some methods for optimizing structures are introduced. Finally, referencing constructal theory, some optimization strategies and evaluation methods from laminar heat exchangers, which are similarly sensitive to thermal boundary conditions, are introduced into the supercritical domain. This establishes the spatiotemporal matching theory of supercritical substance flow and energy flow, providing reference and guidance for the intelligent design and safe, efficient operation of supercritical heat exchangers. Additionally, some reasonable suggestions for future research are given, including study and utilization of the supercritical thermal entrance effect and the exploration of optimal heating strategies.