CFD simulation on the operation characteristics of CO2 two-phase thermosyphon loop

Abstract

The working fluid flow and vapor–liquid distribution in a CO2 two-phase thermosyphon loop (TPTL) can hardly be properly understood through experiment. In the present study, a two-dimensional computational fluid dynamics model of a CO2 TPTL was established and verified. The prestart, oscillatory, and stable operation states of the CO2 TPTL were reproduced and analyzed by simulation. The operating characteristics under different filling ratios were studied through inner parameters distribution and the heat transfer performance was compared. In the prestart operation stage, superheating was observed in the riser, and vapor–liquid reverse flow occurred in the downcomer, thus indicating an irregular flow in the loop. In the oscillatory operation stage, the operating parameters and flow pattern exhibited periodic variations. In the stable operation stage, the TPTL was operating in an adequate condition until it reached its heat transfer limit. When the filling ratio was too high or too low, premature single-phase heat exchange occurred in the TPTL. Superheating or subcooling in the TPTL led to high loop thermal resistance. Under a medium filling ratio, the thermal resistance was the lowest, and heat transfer limit was the highest. The heat transfer limit of the TPTL was approximately 1150 W.