Numerical Simulations on Countercurrent Gas-Liquid Flow in a PWR Hot Leg with Air-Water Flow in a 1/15th Scale-Model

To clarify the countercurrent flow in a PWR hot leg under reflux condensation, numerical simulations of countercurrent air-water flow for a 1/15th scale model of the PWR hot leg were conducted using the two-fluid model implemented in CFD software. In this paper, the effect of expansion of the inclined pipe, which is the actual plant geometry, was evaluated. When increasing the air velocity, CCFL characteristics and the mechanism of flow pattern transition had significant differences between the case with and without expansion of the inclined pipe. CCFL characteristics were mitigated in the case with expansion. The effect of computational grid size was also discussed. When the supplied water velocity was small, the predicted flow pattern transition point agreed well with the measured data by increasing the number of cells. On the other hand, when the air velocity was decreasing, there were no significant differences in each case.