Effects of inclination angle and operation parameters on supercritical CO2 natural circulation loop

Highlights: • Supercritical natural circulation experimental loop system is established and tested. • Stability evolutes from unstable to stable condition with the increase of heat input near-critical NCL. • Horizontal loop is found to have highest thermal resistance while vertical loop is preferred for heat transfer. • Effect of inclination on the flow state is not significant when the inclination angle is greater than 60°. - Abstract: Supercritical fluid based natural circulation flow has been proposed in many energy conversion systems such as solar collector, waste heat recovery, nuclear cooling, etc. The current study is focused on the effect of inclination angle and operation parameters on supercritical CO2 based natural circulation loops. In this study, two-dimensional numerical simulation has been carried out with turbulent model incorporated. Then, a supercritical natural circulation loop system has been established, experimented and also compared with previous studies. In the numerical simulation, the flow condition is found closely related to the heat influx and inclination angle: Under high heat flux conditions the system show stable flow, while relative low heat flux conditions show complex unstable behaviors. Also it is found that when the inclination angle is greater than 60°, the effect of inclination on the flow state is not significant. In the experiments, supercritical cases are found stable under the current operation range. The steady operation pressure inside the NCL changes greatly due to the sensitive fluid properties and the effect of inclination on real systems show similar rend with numerical results. Judging from both the numerical and experimental results, larger inclination angles will have higher circulation rate and better heat transfer performance. New correlation for supercritical CO2 NCL flows is also experimentally proposed in this study