The flow transition characteristics of supercritical CO₂ based closed natural circulation loop (NCL) system

Supercritical fluid based natural circulation loop (NCL) systems have been proposed and used in many energy conversion systems in recent years. Supercritical fluid systems can contribute to new generation system design as well as thermo-hydraulic theories of functional fluids. In this study, the flow transition characteristics of supercritical CO₂ based NCL system are discussed both numerically and experimentally. Numerical models with variable heat input conditions are simulated, which are focused on the flow transitions during the input changes. It is found that the supercritical NCL flow is very sensitive to heat influx changes. The numerical results are also discussed together with experimental results on the general trends in this study. Experimental results show that besides several kinds of transition flows, the heating control and cooling control can be both effective during the continuous operations of a supercritical loop system. The heater/cooler effect and parameter analysis are also conducted, which contributes to the design of active control of supercritical NCL system.