Direct contact condensation of steam injected into water is a special mode of condensation where condensation occurs on the interface between steam and water. This type of condensation forms an essential part of various industrial applications and correct prediction and modelling of the condensation behaviour is crucial to obtain an optimised design of such devices. While present prediction models for direct contact condensation are valid for a limited range of flow conditions only, the work presented in this thesis provides improved models for direct contact condensation. The models are developed in the form of diagrams and include: a condensation regime diagram, for predicting the condensation behaviour, a steam plume length diagram, for predicting the penetration distance of steam into water, and a heat transfer coefficient diagram. These models are derived using a wide range of data and therefore provide more accurate predictions compared with alternative models available in literature. In contrast to present models, the derived models presented in this work are constructed using an additional physical parameter to describe the process. The diagrams are validated against independent experiments and demonstrate close agreement. Furthermore, the predictions from the condensation regime diagram and steam plume length diagram are self-consistent. The models developed in this study are capable of predicting condensation behaviour for a wide range of initial conditions and can be used in conjunction with computational fluid dynamics techniques for direct contact condensation.

Verlässliche Vorhersagen zum Verlauf von Störfallszenarien in Reaktorsystemen sind mit CFD-Modellen möglich, wenn diese anhand von Experimenten entwickelt und validiert sind. Motiviert durch die Vorgänge, die bei einem Thermoshock-Szenario unter Druck im Primärkreis eines Druckwasserreaktors entstehen, wurden im Rahmen dieser Arbeit Experimente zur Direktkontaktkondensation von Dampf an unterkühltem Wasser bei hohen Drücken untersucht. Der beschriebene Versuchsaufbau erlaubt es, in einer Anlage, die drei Phänomene geschichtete Strömung, Strahl und Blasenmitriss zu untersuchen. Eine umfassende Instrumentierung ermöglichte es, besonders viele Informationen aus den Experimenten zu erfassen. Verschiedene bildgebende Messverfahren erlauben einen besonderen Einblick in die Strömung ohne Rückkopplung ins Fluid zu haben. Einzelne Mess- und Auswertemethoden wurden extra für die Untersuchung entwickelt und beschrieben. Vor Allem die Messergebnisse der Strahlexperimente mit Kondensation sind umfangreich und neuartig. Die starke Turbulenz im Inneren der untersuchten Strahlen führen zu den höchsten Kondensationsraten in den vorliegenden Experimenten. Hier wurden die Strahldurchmesser-Verläufe für verschiedene Randbedingungen verglichen, um zu zeigen, wie stark die Kondensation an Strahlen vom Umgebungsdruck abhängt. Die Gas-Mitriss-Experimente sind die ersten dokumentierten Versuche dieser Art. Sie zeigen, dass das mitgerissene Gas bei Eintrittsunterkühlungen oberhalb von 10 K sofort an der Eintrittsstelle kondensiert. Es kommt nicht zur Bildung von Blasen oder zum Mitriss nach unten. Vielmehr ist ausschließlich ein negativer Meniskus zu erkennen, der eine von den Randbedingungen abhängige Geometrie hat. Je geringer die Eintritts-Unterkühlung und je höher die Strahlgeschwindigkeit ist, desto tiefer dringt der Gas-Meniskus in die Wasservorlage ein. Die Menge an mitgerissenem Gas ist auch bei hohen Geschwindigkeiten klein gegenüber der Kondensationsmenge am Strahl. Die Experimente wurden im Wesentlichen darauf ausgelegt, Daten für den Vergleich mit CFD-Simulationen zu liefern. Vor allem der Einfluss des Umgebungsdruckes auf die Strahlgeometrie und die Kondensationsrate sollte weiter untersucht und in Simulationen abgebildet werden.:1. Motivation 2. Stand von Wissenschaft und Technik 2.1. Kondensation in geschichteter Strömung 2.2. Geometrie von Freistrahlen 2.3. Kondensation am Freistrahl 2.4. Blasenmitriss 2.5. Blasenmitriss bei gleichzeitiger Kondensation 2.6. Modellierung 2.7. Anwendung der Erkenntnisse auf den Hypothetischen Störfall 3. Versuchsanlage 3.1. Messtechnik 3.1.1. Schnelle Temperaturmesstechnik 3.1.2. Hochgeschwindigkeitskamera 3.1.3. Infrarotkamera 3.1.4. Temperatur- und Druckmesslanzen 3.2. Abgeleitete Größen 3.3. Messung der Kondensationsrate 3.4. Geschwindigkeits- und Turbulenzmessung 4. Experimente und Ergebnisse 4.1. Geschichtete Strömung 4.2. Freistrahl 4.3. Gasmitriss 5. Zusammenfassung und Ausblick / Reliable predictions on the behaviour of accident scenarios in reactor systems are possible with CFD models if they have been developed and validated on the basis of experiments. Motivated by the processes that occur in a Pressurized Thermal Shock scenario in the primary circuit of a Pressurized Water Reactor, experiments on the Direct Contact Condensation of steam on subcooled water were investigated at high pressures. The described experimental setup allows to study all three phenomena: stratified flow, jet and bubble entrainment. Comprehensive instrumentation made it possible to gather a considerable amount of information from the experiments. Various imaging techniques allow a particular insight in the flow without feedback into the fluid. Some of the measurement and evaluation methods were specifically developed for the investigation and have been described. Especially the measurement results of the jet experiments with condensation are comprehensive and unique. The strong turbulence inside the examined jets results in the highest condensation rates in these experiments. Here, the jet diameter profiles were compared for different boundary conditions in order to show that condensation in jets is strongly influenced by ambient pressure. The gas entrainment experiments are the first documented experiments of their kind. They show that the entrained gas condenses immediately at the point of entrainment with inlet subcooling above 10 K. There is no formation of bubbles or entrainment downwards. Only a negative meniscus is visible, which has a geometry dependent on the boundary conditions. The lower the inlet subcooling and the higher the jet velocity, the deeper the gas meniscus penetrates into the water layer. The amount of entrained gas is small in comparison to the amount of condensation at the jet even at high velocities. The experiments were essentially designed to provide data for comparison with CFD simulations. In particular, the influence of the ambient pressure on the beam geometry and the condensation rate should be further investigated and reproduced in simulations.:1. Motivation 2. Stand von Wissenschaft und Technik 2.1. Kondensation in geschichteter Strömung 2.2. Geometrie von Freistrahlen 2.3. Kondensation am Freistrahl 2.4. Blasenmitriss 2.5. Blasenmitriss bei gleichzeitiger Kondensation 2.6. Modellierung 2.7. Anwendung der Erkenntnisse auf den Hypothetischen Störfall 3. Versuchsanlage 3.1. Messtechnik 3.1.1. Schnelle Temperaturmesstechnik 3.1.2. Hochgeschwindigkeitskamera 3.1.3. Infrarotkamera 3.1.4. Temperatur- und Druckmesslanzen 3.2. Abgeleitete Größen 3.3. Messung der Kondensationsrate 3.4. Geschwindigkeits- und Turbulenzmessung 4. Experimente und Ergebnisse 4.1. Geschichtete Strömung 4.2. Freistrahl 4.3. Gasmitriss 5. Zusammenfassung und Ausblick

In the event of hypothetical accident scenarios in PWR, emergency strategies have to be mapped out, in order to guarantee the reliable removal of decay heat from the reactor core, also in case of component breakdown. One essential passive heat removal mechanism is the reflux condensation cooling mode. This mode can appear for instance during a small break loss-of-coolant-accident (LOCA) or because of loss of residual heat removal (RHR) system during mid loop operation at plant outage after the reactor shutdown. In the scenario of a loss-of-coolant-accident (LOCA), which is caused by the leakage at any location in the primary circuit, it is considered that the reactor will be depressurized and vaporization will take place, thereby creating steam in the PWR primary side. Should this lead to ¿reflux condensation¿, which may be a favorable event progression, the generated steam will flow to the steam generator through the hot leg. This steam will condense in the steam generator and the condensate will flow back through the hot leg to the reactor, resulting in counter-current steam/water flow. In some scenarios, the success of core cooling depends on the behaviour of this counter-current flow. Over several decades, a number of experimental and theoretical studies of counter-current gas¿liquid two-phase flow have been carried out to understand the fundamental aspect of the flooding mechanism and to prove practical knowledge for the safety design of nuclear reactors. Starting from the pioneering paper of Wallis (1961), extensive CCFL data have been accumulated from experimental studies dealing with a diverse array of conditions A one-dimensional two field model was developed in order to predict the counter-current steam and liquid flow that results under certain conditions in the cold leg of a PWR when a SBLOCA (small break loss of coolant accident) in the hot leg is produced. The counter-current model that has been developed can predict the pressure, temperature, velocity profiles for both phases, also by taking into account the HPI injection system in the cold leg under a counter-current flow scenario in the cold leg. This computer code predicts this scenario by solving the mass, momentum and energy conservation equations for the liquid and for the steam separately, and linking them by using the interfacial and at the steam wall condensation and heat transfer, and the interfacial friction as the closure relations. The convective terms which appear in the discretization of the mass and energy conservation equations, were evaluated using the ULTIMATE-SOU (second order upwinding) method. For the momentum equation convective terms the ULTIMATE-QUICKEST method was used. The steam-water counter-current developed code has been validated using some experimental data extracted from some previously published articles about the direct condensation phenomenon for stratified two-phase flow and experimental data from the LAOKOON experimental facility at the Technical University of Munich. / Romero Hamers, A. (2014). STUDY OF THE THERMAL STRATIFICATION IN PWR REACTORS AND THE PTS (PRESSURIZED THERMAL SHOCK) PHENOMENON [Tesis doctoral]. Editorial Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/36536 / Alfresco