CFD Modeling of Wall Steam Condensation: Two-Phase Flow Approach versus Homogeneous Flow Approach
The present work is focused on the condensation heat transfer that plays a dominant role in many accident scenarios postulated to occur in the containment of nuclear reactors. The study compares a general multiphase approach implemented in NEPTUNE_CFD with a homogeneous model, of widespread use for engineering studies, implemented in Code_Saturne. The model implemented in NEPTUNE_CFD assumes that liquid droplets form along the wall within nucleation sites. Vapor condensation on droplets makes them grow. Once the droplet diameter reaches a critical value, gravitational forces compensate surface tension force and then droplets slide over the wall and form a liquid film. This approach allows taking into account simultaneously the mechanical drift between the droplet and the gas, the heat and mass transfer on droplets in the core of the flow and the condensation/evaporation phenomena on the walls. As concern the homogeneous approach, the motion of the liquid film due to the gravitational forces is neglected, as well as the volume occupied by the liquid. Both condensation models and compressible procedures are validated and compared to experimental data provided by the TOSQAN ISP47 experiment (IRSN Saclay). Computational results compare favorably with experimental data, particularly for the Helium and steam volume fractions.
Wall Condensation Modelling in Convective Flow
Modelling condensation of water vapour is important in a number of engineering applications, such as nuclear reactor containment, rocket engine nozzles and heat exchangers. The current study investigates the possibilities of modelling condensation induced by a cold surface in a flow at high velocity and temperature. A number of non-condensable gases are present in the flow. The possibilities of condensation modelling are investigated in ANSYS CFX and ANSYS Fluent, with focus on ANSYS CFX. A case study is done of a 2D flat plate, with water vapour and non-condensable gases at varying temperatures and velocities. The condensation models in ANSYS CFX are investigated for a few basic flow setups and the model deemed most appropriate for wall condensation is investigated in greater detail. The wall condensation model in CFX is investigated to a greater extend, and compares well with an analytical solution for laminar flow. The complexity of the flow is gradually increased to determine limitations and best practise settings for flows at high velocity and temperature. For isothermal walls and for a conjugated analysis, using a solid with a specified adiabatic wall temperature and heat flux coefficient to induce condensation, the wall condensation model works well for grids with a y+ above 1. For finer grids, convergence is found to be difficult to achieve. The choice of material properties for water vapour was found to play an important role in terms of stability. Real gas properties to define the water vapour material properties are deemed important to avoid unphysical results in terms of the temperature. The wall condensation model in ANSYS CFX is deemed to be an appropriate choice for future work with respect to validity and reduced complexity. If the wall condensation model in ANSYS CFX does not prove adequate, it is recommended to investigate an Euler-Euler multiphase model in ANSYS Fluent with condensation and the Eulerian wall film model enabled.
Improvement of HYCA3D code and experimental verification in rectangular geometry
Concerns about the local hydrogen behavior in a nuclear power plant (NPP) containment during severe accidents have increased with the 10CFR50.34(f) regulation after TMI accident. Consequently, investigations on the local hydrogen behaviors under severe accident conditions were required. An analytical model named HYCA3D was developed at Seoul National University (SNU) to predict the thermodynamics and the three dimensional behavior of a hydrogen/steam mixture, within a subdivided containment volume following hydrogen generation during a severe accident in NPPs. In this study, the HYCA3D code was improved with a steam condensation and spray model, and verified with hydrogen mixing experiments executed in a SNU rectangular mixing facility. Helium was used to simulate hydrogen in both the calculations and the experiments. The calculation results show good agreement with the experimental data.