Heat transfer performance of supercritical CO2 in a vertical U-tube

Supercritical carbon dioxide (sCO2) is taken as an excellent working medium for efficient and compact energy conversion devices. However, heat transfer performance (HTP) of sCO2 in a vertical U-tube is intricate owing to dramatic variation of its temperature-dependent thermophysical properties. In this study, flow and heat transfer performance (FHTP) of sCO2 in a vertical U-tube is studied with an improved shear stress transfer model. Additionally, the impacts of operational parameters (heat flux q, mass flux G and pressure P), flow directions and bending diameters (dbend) of U-tube on FHTP are investigated. In both directions, the differences in local pressure drop caused by buoyancy effect and thermal expansion are analyzed deeply. The results demonstrate that the bend section can disturb flow and vortexes persist in downstream, thus improve the HTP. However, an apparent heat transfer deterioration zone can be observed in the ascending section. Under both directions, the average heat transfer coefficient (havg) raises with the growing G and P and decreasing q and dbend, while the total pressure drop (ΔPt) rises with the raising G and q as well as decreasing P. Moreover, the U-tube with down-up direction obtains higher havg and lower ΔPt than up-down.