Dynamic heat transfer characteristics of printed circuit precooler in supercritical CO₂ Brayton cycle

The dynamic characteristics of heat exchangers are critical in supercritical CO₂ Brayton cycle, especially in the precooler where significant thermophysical property variations occur. However, few studies have comprehensively analyzed the dynamic heat transfer characteristics in precooler, particularly with respect to the detailed flow and thermal field evolution. A three-dimensional numerical model was developed to study the dynamic performance of a printed circuit precooler, with an emphasis on the impact of thermophysical properties. The study examines responses to abrupt changes in CO₂ and water inlet temperature, as well as mass flux variations. Results show that the precooler exhibits rapid response characteristics with mild fluctuations in heat transfer parameters and flow structure. Notably, the large specific heat near the critical point results in minimal variation in CO₂ outlet temperature despite abrupt increases in CO₂ inlet temperature, thereby enhancing system stability. Conversely, a decrease in CO₂ mass flux causes a transition from forced convection to mixed flow, with strengthened buoyancy effects and decreased entropy production. These variations lead to significant changes in CO₂ outlet temperature, posing potential challenges to system stability under CO₂ mass flux step change conditions. The heat transfer shows a relatively lower response rate to parameter changes of water compared to CO₂. Although variations in water parameters have a negligible effect on CO₂ heat transfer, they influence the overall heat transfer coefficient. The dynamic heat transfer characteristics of the precooler presented in this paper address research gaps left by 1D numerical and experimental studies, which have provided limited information on flow and thermal fields during dynamic processes. These insights are essential for understanding the dynamic behavior of the entire system.