Flow and heat transfer characteristics of supercritical CO2 in serpentine tubes under semi-circumferential heating conditions

The tubular solar receiver is one of the most promising candidates for direct supercritical carbon dioxide (S-CO₂) solar receivers, which can improve the pressure-bearing performance by increasing the wall thickness of the tube. However, increasing wall thickness induces larger conductive thermal resistance, leading to higher wall temperature and lower thermal efficiency of the receiver. The serpentine tube can enhance the convective heat transfer, mitigating the negative impacts of the increased wall thickness. Nonetheless, the flow and heat transfer characteristics of S-CO₂ in serpentine tubes under semi-circumferential heating conditions require further investigation. In this work, the flow and heat transfer characteristics of S-CO₂ in a serpentine tube under semi-circumferential heating conditions were numerically investigated. The results showed that in high Reynolds number flows, the radial resultant buoyancy force, dominated by centrifugal buoyancy, induces secondary flow in the tube, improving the field synergy of the cross-section inside the tube. Under the influence of secondary flow, the high-temperature S-CO₂ accumulates on the heated side of the concave area and on the insulated side of the convex area, causing local heat transfer deterioration in such areas, but the secondary flow enhances the overall heat transfer performance of the serpentine tube. Compared to circumferential heating, the heat transfer coefficient with semi-circumferential heating is enhanced in the concave areas while deteriorated in the convex areas.