Thermal and hydrodynamic characteristics of supercritical CO2 natural circulation in closed loops

Natural convective flow of supercritical fluids has become a hot topic in engineering applications. Natural circulation thermosyphon using supercritical/trans-critical CO2 can be a potential choice for effectively transportation of heat and mass without pumping devices. This paper presents a series of numerical investigations into the fundamental features in a supercritical/trans-critical CO2 based natural circulation loop. New heat transport model aiming at trans-critical thermosyphon heat transfer and stability is proposed with supercritical/trans-critical turbulence model incorporated. In this study, the fundamentals include the basic flow and heat transfer behavior of the above loop, the effect of heat source temperature on system stability, the effect of loop diameter on natural convection supercritical CO2 loop and its coupling effect with heat source temperature and the effect of constant changing heat input condition and system behavior evolution during unsteady input or failure conditions. The fundamental potentials of supercritical/trans-critical CO2 based natural convection system are confirmed. Basic supercritical CO2 closed loop flow and heat transfer behaviors are clarified. During this study, the CO2 loop stability map are also put forward and introduced as an important feature of supercritical CO2 system. Stability factors of natural convective trans-critical CO2 flow and its implications on real system control are also discussed in this paper.

► We model thermosyphon heat transfer and stability with super-/trans-critical turbulence model incorporated.
► Potentials of super-/trans-critical CO2 thermosyphon are confirmed.
► Three characteristics found: flow instability; high flow rate with density wave; heat transfer discrepancies.
► Major laws of system stability factors are different compared with traditional fluids.
► Traditional thermosyphon flow correlation has its limitations and deserves further development.