Abstract: [Objective] It is crucial for breaking through the supercritical carbon dioxide Brayton cycle power generation technology to clarify the heat transfer mechanism of supercritical fluid in heat exchangers. Compared to the straight pipe, the flow and heat transfer of supercritical carbon dioxide in the U-shaped bend still needs to be explored. [Methods] Numerical simulation method was used to study the convective heat transfer process of supercritical carbon dioxide inside a commonly used U-shaped bend in engineering in this work. The influences of gravity direction, geometric dimensions, thermal hydraulic conditions, and supercritical state on the flow and heat transfer of supercritical carbon dioxide are explored. [Results]The results show that as the diameter and curvature of the bent pipe increase, the convective heat transfer coefficient decreases and increases, respectively. Compared to the condition of parallel gravity, the maximum convective heat transfer coefficient is slightly high and the position shifts forward under the condition of counter gravity. As fluid mass flow rate and inlet temperature increase, the secondary turbulence phenomenon at the bend increases, while the pressure changes the turbulent vortex state. Compared to the inlet temperature, the convective heat transfer coefficient at the bend increases with the increase of mass flow rate and pressure but reduces as heat flux increases. [Conclusions] The above conclusions suggest that the convective heat transfer process of supercritical carbon dioxide in a U-shaped bend is more complex. Thus, more attention should be paid to the combined effects of tube structure, fluid property, and operating conditions on convective heat transfer for the heat exchanger design.

Key words: supercritical carbon dioxide, Brayton thermodynamic cycle, power generation system, heat exchanger, convective heat transfer, U-shaped tube, numerical simulation