Numerical Study of the Water/Air Countercurrent Flow Limitation in a Scaled PWR Hot Leg

Abstract

In the present work, a numerical study of countercurrent flow limitation (CCFL) with air-water as working fluids was carried out using Computational Fluid Dynamics (CFD). The CFD code Star-CCM+ was used, presenting itself a robust and reliable tool for this type of problem. The used geometry was a scaled (1:14) PWR hot leg, with small deviations from the experimental facility built in the Centro de Desenvolvimento da Tecnologia Nuclear (CDTN). The aim of this work was to study numerically the behavior of the system in a potential Loss of Coolant Accident (LOCA) coming from the cold leg of the reactor, which would cause an interruption in the core cooling capabilities. Experimental data from the CDTN experimental facility were used as input for Star-CCM+, which imply in a fixed water mass flow and raising air mass flow. The raising in air mass flow was performed by giving ramp increments through simulation time, providing numerical stability and reproducing the experimental conditions. A numerical tactic was employed regarding the air density variation in the experiment. As a result, the degree of freedom of the simulation were decreased, leading to an enhancement in the numerical stability. As a first assessment, only one mesh was used with a fixed time step, allowing the evaluation of the numerical stability using global Convective Courant Number. The numerical result was compared with experimental ones, showing consistency with the actual physical behavior of this kind of system.