Characterization of CO2 fluid crossing critical region flow and heat transfer in a vertical loop. Part ΙI: mode analysis

Supercritical circulation loop flow and heat transfer is crucial for the application design of nuclear power and also for solar thermal conversion systems. In this study, the transcritical flow and heat transfer behavior of CO₂ flowing upward in a vertical circular loop has been numerically and experimentally investigated. In Part I of this study, basic experimental system verification and trend analysis have been reported. In Part II, five heat transfer modes have been categorized and the effects the transcritical interface, radial physical properties, and heat transfer mechanisms have been explained. The analysis of the interface evolution along with heat transfer behaviors in this study revealed that the thinner gas-like layer (d/R < 0.1) undergoes radial expansion and competes with the liquid-like layer, intensifying heat transfer along the flow direction. When the subcritical-to-supercritical transition area reaches the core of the circular tube, the distance between the interface and the wall continuously increases, leading to a thicker gas-like layer (d/R > 0.1) and consequently decrease the heat transfer capacity. From Mode I to Mode IV, and to Mode V, with the increase of inlet temperature or the increase of boundary heat input intensity, the “cross pseudo-critical interface” will move and interacts with core flow, then heat transfer can be categorized, to be several different modes and contribute to normal heat transfer, enhanced heat transfer and/or deteriorated heat transfer.