Abstract The present study investigated the heat transfer and flow characteristics of supercritical CO2 in U-ducts by numerical simulations. Analyses focused on the effect of the turn geometry and the gravity direction on the flow field and temperature field of supercritical CO2. Results show that, as the radius of the turn geometry increases from 0.5D to 3.0D, the flow and temperature distribution after the turn region changes a lot, due to the flow separation and reattachment. In the turn region for the vertical upward flow, the gravity plays a dominant role in the change of flow field, and a secondary flow structure is induced by gravity, which is completely opposite to the Dean Vortex in conventional fluid flow. The angle between the centrifugal force and gravity direction have a significant influence on the migration and deposition process of the low-temperature and high-density fluid when supercritical CO2 passing through the turn region, making the flow field and temperature distribution after the turn different completely.