Buoyancy influencing convective heat transfer characteristics of supercritical CO2 in a serpentine solar receiver tube at low Reynolds number

The tubular receiver is one of the most promising forms of supercritical CO2 (S-CO2) receiver. Due to the hightemperature and high-pressure working condition of S-CO2, the pressure resistance of solar receiver tube faces great challenges. The convective heat transfer performance of S-CO2 will be significantly improved by using the serpentine solar receiver tube instead of the traditional straight solar receiver tube in the S-CO2 tubular solar receiver, which will offset the negative effects of increased thermal resistance caused by increasing wall thickness to improve pressure resistance. Centrifugal buoyancy and gravitational buoyancy are important factors affecting the convective heat transfer characteristics of S-CO2 in serpentine tube which are numerically studied in this paper. The results show effect of buoyancy in the serpentine tube can significantly improve the convective heat transfer performance with a maximum value of 23% and reduce the entropy generation with a maximum value of 22% compared with straight tube. The convective heat transfer performance of S-CO2 for the downward flow is stronger than the upward flow due to the effect of gravitational buoyancy on the intensity of turbulence kinetic energy near the wall. The dominant position of centrifugal buoyancy and gravitational buoyancy affects the convective heat transfer characteristics of S-CO2 in serpentine tube. The synergistic/competitive relationship between centrifugal buoyancy and gravitational buoyancy will promote/suppress the change of the flow field which affects the convective heat transfer characteristics of S-CO2 in serpentine tube. The conclusions of this work can provide theoretical guidance for the design and optimization of tubular receiver using S-CO2 as heat transfer fluid.