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.
Pattern recognition algorithm for analysis of chugging direct contact condensation
Abstract Direct contact condensation of steam bubbles in a boiling water reactor suppression pool has long been studied utilizing video recording of experiments. The use of video recording enables observation of the behaviour of the bubble surface area and can assist in validation of computational fluid dynamics models. A direct contact condensation experiment of the suppression pool test facility PPOOLEX was recorded using high-speed cameras. The recorded video material was used for development of a pattern recognition and data analysis algorithm. 300 fps video of 48 s duration was cut into frames with a resolution of 768 px × 768 px . The side profile of the bubbles was identified and the volumes and surface areas of the bubbles were evaluated using a voxel-based method. The purpose of the algorithm was to determine the shape and size of steam bubbles during their formation, expansion, collapse and re-formation. The most probabilistic chugging frequencies were estimated. The bubble geometry data were also used to determine the velocity and acceleration of the phase interface, as condensation induced Rayleigh-Taylor instability develops on the bubble surface during the bubble collapse, as the heavy phase accelerates towards the light phase. Knowledge of the critical wave length is necessary for mesh spacing in CFD calculations. The algorithm appears to be promising. Some limitations exist and approximations need to be made due to the challenging video shooting conditions. The algorithm works well for cylindrical bubbles and provides important data on the dynamics of the phase interface necessary for numerical modelling of direct contact condensation.
Some implementations of projection methods for Navier-Stokes equations
Dans cet article on s'interesse a quelques opproximations spatiales des methodes de projections du type Chorin-Temam. On s'interesse plus particulierement a l'etape de projection, qui sert a imposer l'incompressibilite. Trois types d'approximations variationnelles sont etudiees. Dans la premiere on resout l'etape de projection sous la forme d'un probleme de Darcy avec des vitesses test satisfaisant une condition de Dirichlet (a premiere vue) paradoxale. Dans la seconde approche, le probleme est encore resolu sous sa forme div-grad (i.e. Darcy) mais les vitesses test satisfont cette fois-ci une condition a la frontiere portant uniquement sur la camposante normale. Dans la troisieme methode, l'etape de projection est resolue sous la forme d'une equation de Poisson avec une condition de Neumann. On montre que la premiere methode est legitime pour des approximations par elements finis, alors que la seconde a de l'interet aussi pour des approximations spectrales. La troisieme methode est probablement la plus aisee a mettre en oeuvre puisque qu'elle permet d'eviter l'inversion d'une matrice de masse qui est obligatoire pour les deux autres. Bien que les deux dernieres methodes n'imposent pas directement de compatibilite entre les espaces de vitesse et de pression, on montre qu'une telle condition est necessaire pour assurer la convergence de la methode. Finalement on montre quelques liens entre ces algorithmes et certaines techniques de preconditionnement de l'operateur d'Uzawa. AMS(MOS) subject classifications. 35A40. 35Q10. 65J15.