Experimental investigation of near-critical CO2 heat transfer performance in a closed natural circulation loop

The natural circulation system based on supercritical CO2 has attracted attention due to its simple struc-ture and safety compared with the forced circulation system. This work presented an experimental in-vestigation of the heat transfer characteristics of CO2 based on natural circulation. A closed loop natural circulation experimental system was built, which was specially designed for near-critical pressure ex-periments. The effects of filling ratio, coolant temperature, heat transfer rate and system pressure were carefully discussed. Experimental results illustrated the loop had excellent heat transfer capability at sub -critical pressure, but the heating section showed heat transfer deterioration at supercritical pressures. The heat transfer behaviors were analyzed. From subcritical pressure to supercritical pressure in the loop, the heat transfer mechanism transforms from two-phase transition to supercritical single-phase convection. Gas-like layer and insulating layer will form near the wall in the heating section, resulting in a large thermal resistance at the wall for heat to enter the core flow. (C) 2022 Elsevier Ltd. All rights reserved.