CFD modelling to predict the counter-current flow limitations of the air/water counter-current two-phase flow in 1/3rd flat channel model of a hot-leg pressurized water reactor

The analytical simulation of the counter-current flow limitation phenomenon in a PWR is an essential element to understand safety-related issues in nuclear power plants. It is expected that the introduction of computational fluid dynamics (CFD) tools will enhance the accuracy of the simulation predictions compared to the established one-dimensional thermal hydraulic analyses. Nevertheless, the use of CFD for this complicated task is still a challenge today. Due to the need to understand the CCFL phenomenon in a PWR for reasons of safety and characterisation of normal operation, it is necessary to validate computer codes and to verify computational results using experimental data. Therefore it is also interest to prove the understanding of the general fluid dynamic mechanism leading CCFL and to identify the critical parameters affecting this phenomenon.

In order to improve the transient analysis of counter-current two-phase flows, experimental and numerical studies were conducted at Forschungszentrum Dresden-Rossendorf (FZD). A 1/3rd scale model of the hot leg PWR of a German Konvoy Pressurized Water Reactor with rectangular cross section was used. The experimental results on this topic were reported in previous reports [Deendarlianto et al. (2008) & Vallée et al. (2009)]. Selected an air-water CCFL experiment at 0.15 MPa and room temperature at FZD (experimental running number 30-09) was numerically modelled with three-dimensional two-fluid models of computer code CFX 12.0 (ANSYS CFX). The aim of this CFD simulation is to validate the prediction model of the CCFL with the existing multiphase flow models built in the commercial code ANSYS CFX. CFD simulation was performed using the multi-fluid Euler-Euler modeling approach or free surface model available in CFX. The calculation was carried out in fully transient manner using a gas/liquid inhomogeneous multiphase flow model coupled with a shear stress transport (SST) turbulence model. In the present numerical study, the drag coefficient was approach by using the Algebraic Interfacial Area Density (AIAD) model. The results indicated that quantitative agreement between calculation and experimental data was obtained for the occurrence of flooding point. Next, it was found also that a comparison with the high-speed video observations shows a good qualitative agreement.