Development of three-dimensional simulation method for two-phase flow in square-pitch tube bundle in secondary side of steam generators based on porous drift-flux model

In designing a steam generator in nuclear power plant, it is necessary to evaluate the flow-induced vibration of a U-shaped tube bundle from the viewpoint of safety engineering. CFD technology is used for predicting velocity and void fraction distributions along the U-tubes to evaluate the force acting on the tube from the surrounding two-phase flow. This study validated a newly developed thermal-hydraulic simulation method based on the drift-flux model to simulate various operating conditions under flexible power operations, including partial load conditions. Constitutive equations for two-phase flow were implemented into the platform of a general-use commercial code, ANSYS fluent, through user-defined functions. Experimental data measured in the slice model U-tube bundle apparatus with boiling Freon two-phase flow, which was a simulant of steam-water under the prototypic pressure and thermal conditions, was utilized to validate the developed simulation method. The predicted void fraction and velocity distribution were compared with the experimental results. In the cold-side region, a low void fraction occurred in a partial load condition. It is considered to be caused by the liquid downward flow outside the bundle. It was confirmed that the developed simulation method could reproduce such phenomena in the secondary side of a steam generator.