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The saturated water separated by the steam separator in a natural circulation steam generator may carry a small amount of saturated steam into the downcomer. The steam contacts subcooled water and condenses directly in the downcomer causing the variations in the pressure and steam quality and likely affecting the stability of the water cycle in the secondary loop. It is not conducive to core heat extraction and thus affects nuclear safety. The mathematical model of the downcomer was established in this study based on the internal structures of a natural circulation steam generator. The volume-of-fluid (VOF) and large-eddy-simulation (LES) models were used for analysis on FLUENT software (ANSYS, Pittsburgh, PA. USA) platform. The influence of direct contact condensation of top-down flowing steam on the flow properties in the downcomer of the steam generator under high pressure was studied. The trend of the temperature, pressure, and the void fraction were obtained by combining these models with the condensation model. Further, a one-dimensional calculation program based on the differential drop was also developed to assess the flow field in the downcomer. The calculation results are in good agreement with the experimental results which indicated that, when affected by the saturated steam carried downward, the flow temperature close to the exit of the downcomer rises slightly due to the absorption of the heat released by the steam condensation. Furthermore, the density corrected by the pressure-drop is more reliable than that corrected by the temperature. After the velocity in the downcomer has increased to a certain value, the sensitivity of steam quality to the subcooling degree in the downcomer begins to decline. The results in this paper can be used to perform stability analyses and to design steam generators. The results of research are helpful to the stability analysis and the design of a steam generator, and to improve the accuracy of the measurement of the steam generator operating parameters, thus enhancing the safety of Pressurize Water Reactor operating system.

Effect of Saturated Steam Carried Downward on the Flow Properties in the Downcomer of Steam Generator