Coupled heat transfer mechanisms and performance optimisation between fluids with dissimilar thermophysical properties in CO2 heat exchangers: A review

The CO2 power cycle has broad applications in waste heat recovery, nuclear power generation, and solar power generation, where heat exchangers serve as critical components that significantly influence system efficiency. A thorough investigation of the coupled heat transfer mechanisms between hot and cold fluids within CO2 heat exchangers is essential for improving cycle performance. However, most existing reviews primarily focus on the physical and heat transfer characteristics of CO2 itself, lacking a systematic overview of the thermophysical property differences, coupling heat transfer mechanisms, and performance optimisation strategies between CO2 and other working fluids across various application scenarios. To address this limitation, the present study systematically reviews the current research status and key challenges in the coupled heat transfer performance between CO2 and six representative working fluids. Existing evaluation indices and commonly used optimisation strategies—including channel structure optimisation, hybrid fluid addition, and operating parameter adjustment—are summarized. Finally, a one-dimensional model is developed to quantitatively assess the performance enhancement of six CO2 heat exchangers under different optimisation approaches. The results indicate that among the three methods, optimising channel structure and hydraulic diameter yields the most significant performance improvement. Notably, the optimal channel configuration varies considerably across different working fluid pairs. Among the additives, helium and methane show the greatest enhancement in the coupled heat transfer performance, whereas argon, krypton, and xenon lead to performance deterioration and markedly suppress the peak values of local coupled heat transfer coefficients. This review provides reference for the design and optimisation of CO2 heat exchangers.