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.
ON THE UNEXPECTEDLY LARGE EFFECT OF THE RE-VAPORIZATION OF THE CONDENSATE LIQUID FILM IN TWO TESTS IN THE PANDA FACILITY REVEALED BY SIMULATIONS WITH THE GOTHIC CODE
The recently concluded OECD SETH project included experiments on basic flows and gas transport and mixing driven by jets and plumes in the multi-compartment geometry (two large, connected vessels) of the PANDA facility, which have been simulated with the advanced containment code GOTHIC. The two experiments considered here featured injection of saturated steam in one vessel and venting from the second vessel, both being initially filled with pure air. In these two tests steam was injected at different positions. In both tests, condensation produced a liquid film on the walls in the region above the injection. For these tests the analyses with GOTHIC provided strong indications that the key phenomenon for the simulation was the re-vaporization of the condensate film, and more precisely the amount of run-down liquid which flows into the lower part of the vessel, its velocity and its re-vaporization in that region. The unexpectedly large role played by the liquid film suggested a review of the modeling issues associated with the simulation of run-down condensate. It was concluded that important gaps exist in the knowledge of several fundamental processes, which should be considered in future theoretical and experimental work related to containment thermal-hydraulics.
Investigations of dielectric gratings using electrodynamic models based on volume integral equations
Eigenmodes of planar and cylindrical dielectric gratings (DGs) and electromagnetic wave scattering from multiplayer DGs are investigated. The DG characteristics are calculated using numerical-analytical solution of integral equations (IEs).