Periodic flow excursion in an open two-phase natural circulation loop Original Research Article

Flow excursion instability in an open two-phase natural circulation loop was studied both experimentally and analytically. At high inlet subcooling, stable mode (continuous circulation) does not exist; instead, three different circulation modes such as periodic circulation (A), multimode circulation, and periodic circulation (B) appear at different heat flux conditions. The multimode circulation is represented by the combination of periodic circulations A and B and it is termed here as “periodic flow excursion”. The decrease in the inlet restriction and/or the increase in the exit restriction broadened the region of the flow excursion, and these aspects were summarized on the instability map of the heat flux vs. inlet subcooling. In addition, with the one-dimensional two-phase homogeneous equilibrium assumption, the region of flow excursion was obtained analytically, which fairly agrees with the experimental results.