Experimental study of instability characteristics of a CO2 two-phase thermosyphon loop

Abstract

During the operation of a two-phase thermosyphon loop (TPTL), instability often occurs and is accompanied by fluctuations in temperature, pressure, and mass flow rate, which can lead to premature failure and damage to components. To avoid instability of the TPTL, the mechanism should be understood. In this study, the instability characteristics and occurrence conditions of a CO2 TPTL were experimentally investigated. A distribution map of the different types of instability is provided. Geyser boiling occurs mainly at low heat loads and filling ratios. The flow pattern in the downcomer experienced period change of “violent bubbly flow- annular flow- fluctuating annular flow- violent bubbly flow.” Natural circulation oscillation was more likely to occur at medium filling ratios. The flow pattern in the downcomer experienced a variation of “single liquid phase- churn flow- bubbly flow- churn flow-bubbly flow- single liquid phase.” Density wave oscillation mainly occurred when the filling ratio was as high as 60% and 70%, and an unstable churn flow was observed in the riser. Generally, the fluctuation period decreased as the heat load increased, and the fluctuation amplitude tended to decrease at higher filling ratios.