Implementation of the heat and mass transfer models for BT and post-BT regions in three-field two-fluid CFD

Abe, Satoshi  ; Obi, Yoshio*; Satou, Akira ; Okagaki, Yuria  ; Shibamoto, Yasuteru 

A modeling of heat transfers with boiling transition (BT) and that after the occurrence of BT, called post-BT, is one of the key technical issues to estimate the duration of surface dryout and the peak cladding temperature during DBA (Design Basis Accident) and BDBA (Beyond Design Basis Accident) in light water reactors. Recently, CFD (Computational Fluid Dynamics) has emerged as a powerful tool for representing the heat transfer mechanism. Our main purpose is to obtain in-depth physical insight into the BT and post-BT phenomena by combining experiment and CFD simulation research. This paper introduces our developing activity for an integrated three-field two-fluid CFD methodology based on the Eulerian-Eulerian approach toward the accurate prediction of the dryout behavior from annular-mist to mist flow regimes. We implemented following interaction terms and functions into OpenFOAM ver.7, an opensource code developed by OpenFOAM foundation, as (1) Interaction terms between liquid film and droplets due to deposition and entrainment, (2) Interaction terms on the liquid interface between the liquid film and the gas phase on friction and heat conduction, (3) Heat transfer from the heated wall to the liquid film, (4) Dryout occurrence judgement and the switching function on the boundary condition. The dryout occurrence judgment is based on a correlation on critical film thickness, which is originally applied into the MARS (Multi-dimensional Analysis of Reactor Safety) code. A trial calculation with the developed solver called two Phase Three Field Euler Foam was performed to check the solver operation. The CFD could simulate temperature increase behavior due to the dryout occurrence, whereas here were still challenges in reproducing the transition from mist flow to single-phase vapor flow.