Experimental study of trans-critical and supercritical CO2 natural circulation flow in a closed loop

This experimental study deals with the basic stability behaviors of CO2 natural circulation loop (NCL) in the critical region. Supercritical natural circulation loop system was carefully established and tested, in which loop strong natural circulation can be achieved due to densities differences of the heater and cooler. Compared with normal fluid based NCLs, trans-critical and supercritical fluid based NCLs can sustain much higher flow rate and heat transfer capacity. In the current study, effects of operation parameters (charging mass, heating power and coolant temperature) on stability and flow transitions are investigated. Three types of trans-critical flow condition in high pressure NCL are identified and analyzed in this study. It is found that the two-phase flow or single-phase gas flow at sub-critical region may lead to instability. However, for supercritical condition in the current tests, all cases show single-directional steady behavior. The stability evolutions along with the variations of operating conditions are also analyzed for each case. The results for basic near-critical fluid NCL behaviors and rules of parameter dependencies can be useful for understanding of related systems.