The condensation rate has to be high in the safety pressure suppression pool systems ofBoiling Water Reactors (BWR) in order to fulfill their safety function. The phenomenadue to such a high direct contact condensation (DCC) rate turn out to be very challengingto be analysed either with experiments or numerical simulations. In this thesis, thesuppression pool experiments carried out in the POOLEX facility of Lappeenranta Universityof Technology were simulated. Two different condensation modes were modelledby using the 2-phase CFD codes NEPTUNE CFD and TransAT. The DCC models appliedwere the typical ones to be used for separated flows in channels, and their applicability tothe rapidly condensing flow in the condensation pool context had not been tested earlier.A low Reynolds number case was the first to be simulated. The POOLEX experimentSTB-31 was operated near the conditions between the ’quasi-steady oscillatory interfacecondensation’ mode and the ’condensation within the blowdown pipe’ mode. The condensationmodels of Lakehal et al. and Coste & Lavi´eville predicted the condensationrate quite accurately, while the other tested ones overestimated it. It was possible to getthe direct phase change solution to settle near to the measured values, but a very highresolution of calculation grid was needed.Secondly, a high Reynolds number case corresponding to the ’chugging’ mode was simulated.The POOLEX experiment STB-28 was chosen, because various standard and highspeedvideo samples of bubbles were recorded during it. In order to extract numericalinformation from the video material, a pattern recognition procedure was programmed.The bubble size distributions and the frequencies of chugging were calculated with thisprocedure. With the statistical data of the bubble sizes and temporal data of the bubble/jetappearance, it was possible to compare the condensation rates between the experimentand the CFD simulations.In the chugging simulations, a spherically curvilinear calculation grid at the blowdownpipe exit improved the convergence and decreased the required cell count. The compressibleflow solver with complete steam-tables was beneficial for the numerical success ofthe simulations. The Hughes-Duffey model and, to some extent, the Coste & Lavi´evillemodel produced realistic chugging behavior. The initial level of the steam/water interfacewas an important factor to determine the initiation of the chugging. If the interfacewas initialized with a water level high enough inside the blowdown pipe, the vigorouspenetration of a water plug into the pool created a turbulent wake which invoked thechugging that was self-sustaining. A 3D simulation with a suitable DCC model producedqualitatively very realistic shapes of the chugging bubbles and jets. The comparative FFTanalysis of the bubble size data and the pool bottom pressure data gave useful informationto distinguish the eigenmodes of chugging, bubbling, and pool structure oscillations.
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