Die Visualisierung und Quantifizierung von Flüssigwasseransammlungen in Polymerelektrolytmembran-Brennstoffzellen konnte mittels Neutronenradiographie erreicht werden. Dank dieser neuartigen diagnostischen Methode konnte erstmals die Flüssigwasseransammlung in den porösen Gasdiffusionsschichten direkt nachgewiesen und quantifiziert werden. Die Kombination von Neutronenradiographie mit ortsaufgelösten Stromdichtemessungen bzw. lokaler Impedanzspektroskopie erlaubte die Korrelation des inhomogenen Flüssigwasseranfalls mit dem lokalen elektrochemischen Leistungsverhalten. Systematische Untersuchungen an Polymerelektrolyt- und Direkt-Methanol-Brennstoffzellen verdeutlichen sowohl den Einfluss von Betriebsbedingungen als auch die Auswirkung von Materialeigenschaften auf die Ausbildung zweiphasiger Strömungen.

Bei einer Vielzahl von natürlichen und technischen Strömungsvorgängen besteht das strömende Medium aus mehreren Phasen. Bei der Förderung derartiger Medien mit Kreiselpumpen führen bislang nicht genügend bekannte Vorgänge bei bestimmten Betriebsbedingungen zu einer starken Verringerung der Förderleistung und einem Anstieg des Leistungsbedarfs. Bei der Förderung von gasbeladenen Flüssigkeiten kann ein zu hoher Gasanteil zum vollständigen Zusammenbruch der Förderung führen. Zusätzlich führt die erosive Wirkung von Kavitation zu einer starken Minderung der Lebensdauer der Pumpe. Diese Arbeit beschreibt ein neuartiges Verfahren, das es erstmals gestattet, die Methode der Gamma-Tomographie zur Bestimmung der örtlichen Phasenverteilung innerhalb von rotierenden Pumpenläufern und ähnlichen Bauteilen anzuwenden. Dabei wird eine Zeitauflösung von ca. 100 µs erreicht, mit der die Gasverteilung wesentlich genauer als bisher erfaßt werden kann. Mit dem Tomographen wurden die Vorgänge innerhalb des Läufers einer Axialpumpe bei Förderung eines Luft-Wasser-Gemischs visualisiert. Es wird gezeigt, wie sich die Änderung äußerer Strömungsparameter und die Variation des Arbeitspunktes auf die räumliche Phasenverteilung innerhalb des Läufers auswirkt.

Bei einer Vielzahl von natürlichen und technischen Strömungsvorgängen besteht das strömende Medium aus mehreren Phasen. Bei der Förderung derartiger Medien mit Kreiselpumpen führen bislang nicht genügend bekannte Vorgänge bei bestimmten Betriebsbedingungen zu einer starken Verringerung der Förderleistung und einem Anstieg des Leistungsbedarfs. Bei der Förderung von gasbeladenen Flüssigkeiten kann ein zu hoher Gasanteil zum vollständigen Zusammenbruch der Förderung führen. Zusätzlich führt die erosive Wirkung von Kavitation zu einer starken Minderung der Lebensdauer der Pumpe. Diese Arbeit beschreibt ein neuartiges Verfahren, das es erstmals gestattet, die Methode der Gamma-Tomographie zur Bestimmung der örtlichen Phasenverteilung innerhalb von rotierenden Pumpenläufern und ähnlichen Bauteilen anzuwenden. Dabei wird eine Zeitauflösung von ca. 100 µs erreicht, mit der die Gasverteilung wesentlich genauer als bisher erfaßt werden kann. Mit dem Tomographen wurden die Vorgänge innerhalb des Läufers einer Axialpumpe bei Förderung eines Luft-Wasser-Gemischs visualisiert. Es wird gezeigt, wie sich die Änderung äußerer Strömungsparameter und die Variation des Arbeitspunktes auf die räumliche Phasenverteilung innerhalb des Läufers auswirkt.

The key to success in separation of liquid mixtures is the efficient creation and utilization of vapour-liquid contact area. By packing the column with gas-liquid contact devices such as structured packing, the vapour-liquid contact area can be increased. However, the efficiency of these packed columns depends strongly on the local flow behaviour of the liquid and vapour phase inside the packing. The aim of this work was to develop three-dimensional CFD models to study the hydrodynamic behaviour on the corrugated sheets of packing. Different approaches are possible to simplify the problem and to extend it for more complex flow scenarios. In this work, three-dimensional CFD simulations were performed to study the complete fluid-dynamic behaviour. This was performed in two steps. As a first step, the developed model was validated with experimental studies using a simplified geometry i.e., an inclined plate. The three-dimensional Volume-of-Fluid (VOF) model was utilized to study the flow behaviour of the gas-liquid countercurrent flow. The influence of the liquid surface tension was taken into consideration using the Continuum Surface Force (CSF) model. The wetting characteristics of liquids with different viscosity (1 and 5 mPas) and contact angle (70° and 7°) were studied for different flow rates. Three different mixtures (water, water-glycerol (45 wt. %) and silicon-oil (DC5)) were considered. Initially, the rivulet width of experiments and simulations were compared and an error of 5 % maximum was determined. The results were also in good agreement with earlier studies. The percentage of wetting due to changes in flow rate, viscosity and contact angle was compared and discussed. For all tested systems, excellent agreement between the experiments and simulation studies was found. In addition, profiles of the velocity in the film at film flow conditions over a smooth inclined plate obtained from simulations were compared with experimental profiles obtained using a μPIV technique. A detailed sensitivity study was also performed in order to understand the changes in the velocity profiles due to small change in liquid flow rate, temperature and inclination angle. As a next step, the developed model was extended to geometries resembling real corrugated sheets of packing used in industrial applications. In earlier numerical studies of structured packing, geometries were simplified to enable easy meshing and faster computation. In this work, the geometries of corrugated sheets of packing were developed without any simplification and the flow behaviour was studied using the model validated in the first step. The flow behaviour on sheets with different geometrical modifications such as smooth and triangular crimp surfaces as well as perforations on the sheets were numerically studied and quantitatively compared with experimental studies for the three different fluid test systems. The agreement between the simulations and experiments was within an acceptable range for all system. The difference in the interfacial area between the corrugated sheets of a packing with and without perforation was analyzed and the prediction ability of different empirical correlations for the interfacial area available in literature was also compared and discussed. Furthermore, the numerical study was extended to understand the influence of the second corrugated sheet. Studying the flow behaviour between two sheets experimentally is very challenging, especially inside opaque packing. The model proved to be a very suitable tool to study the hold-up of the liquid between two sheets, the change in wetting behaviour due to small change in liquid inlet position. The results are also in good agreement with the earlier experimental studies, where researchers measured the liquid hold-up mainly in the region where two corrugated sheets touch each other. The three-dimensional CFD model was validated to study the flow behaviour on corrugated sheets of packing. The results from the simulations agree very well with findings from the experimental studies in terms of wetting and hold-up.

The key to success in separation of liquid mixtures is the efficient creation and utilization of vapour-liquid contact area. By packing the column with gas-liquid contact devices such as structured packing, the vapour-liquid contact area can be increased. However, the efficiency of these packed columns depends strongly on the local flow behaviour of the liquid and vapour phase inside the packing. The aim of this work was to develop three-dimensional CFD models to study the hydrodynamic behaviour on the corrugated sheets of packing. Different approaches are possible to simplify the problem and to extend it for more complex flow scenarios. In this work, three-dimensional CFD simulations were performed to study the complete fluid-dynamic behaviour. This was performed in two steps. As a first step, the developed model was validated with experimental studies using a simplified geometry i.e., an inclined plate. The three-dimensional Volume-of-Fluid (VOF) model was utilized to study the flow behaviour of the gas-liquid countercurrent flow. The influence of the liquid surface tension was taken into consideration using the Continuum Surface Force (CSF) model. The wetting characteristics of liquids with different viscosity (1 and 5 mPas) and contact angle (70° and 7°) were studied for different flow rates. Three different mixtures (water, water-glycerol (45 wt. %) and silicon-oil (DC5)) were considered. Initially, the rivulet width of experiments and simulations were compared and an error of 5 % maximum was determined. The results were also in good agreement with earlier studies. The percentage of wetting due to changes in flow rate, viscosity and contact angle was compared and discussed. For all tested systems, excellent agreement between the experiments and simulation studies was found. In addition, profiles of the velocity in the film at film flow conditions over a smooth inclined plate obtained from simulations were compared with experimental profiles obtained using a μPIV technique. A detailed sensitivity study was also performed in order to understand the changes in the velocity profiles due to small change in liquid flow rate, temperature and inclination angle. As a next step, the developed model was extended to geometries resembling real corrugated sheets of packing used in industrial applications. In earlier numerical studies of structured packing, geometries were simplified to enable easy meshing and faster computation. In this work, the geometries of corrugated sheets of packing were developed without any simplification and the flow behaviour was studied using the model validated in the first step. The flow behaviour on sheets with different geometrical modifications such as smooth and triangular crimp surfaces as well as perforations on the sheets were numerically studied and quantitatively compared with experimental studies for the three different fluid test systems. The agreement between the simulations and experiments was within an acceptable range for all system. The difference in the interfacial area between the corrugated sheets of a packing with and without perforation was analyzed and the prediction ability of different empirical correlations for the interfacial area available in literature was also compared and discussed. Furthermore, the numerical study was extended to understand the influence of the second corrugated sheet. Studying the flow behaviour between two sheets experimentally is very challenging, especially inside opaque packing. The model proved to be a very suitable tool to study the hold-up of the liquid between two sheets, the change in wetting behaviour due to small change in liquid inlet position. The results are also in good agreement with the earlier experimental studies, where researchers measured the liquid hold-up mainly in the region where two corrugated sheets touch each other. The three-dimensional CFD model was validated to study the flow behaviour on corrugated sheets of packing. The results from the simulations agree very well with findings from the experimental studies in terms of wetting and hold-up.

Direct Contact Condensation between steam and water as well as bubble entrainment below the water surface play an important role in different accident scenarios for light water reactors. One example is the emergency core cooling water injection into a two-phase mixture. It has to be considered for example to evaluate potential pressurized thermal shock phenomena. This report documents experiments conducted in flat basin inside the TOPFLOW pressure chamber aiming on the generation of a database useful for CFD model development and validation. It comprises 3 different setups: condensation at a stratified flow of sub-cooled water, condensation at a sub-cooled water jet and a combination of both phenomena with steam bubble entrainment. The documentation includes all details on the experimental set up, on experimental conditions (experimental matrices), on the conduction of the experiments, on measuring techniques used and on data evaluation procedures. In addition, selected results are presented.

For the qualification and validation of two-phase CFD-models for medium and large-scale industrial applications dedicated experiments providing data with high temporal and spatial resolution are required. Fluid dynamic parameter like gas volume fraction, bubble size distribution, velocity or turbulent kinetic energy should be measured locally. Considering the fact, that the used measurement techniques should not affect the flow characteristics, radiation based tomographic methods are the favourite candidate for such measurements. Here the recently developed ultrafast X-ray tomography, is applied to measure the local and temporal gas volume fraction distribution in a vertical pipe. To obtain the required frame rate a rotating X-ray source by a massless electron beam and a static detector ring are used. Experiments on a vertical pipe are well suited for development and validation of closure models for two-phase flows. While vertical pipe flows are axially symmetrically, the boundary conditions are well defined. The evolution of the flow along the pipe can be investigated as well. This report documents the experiments done for co-current upwards and downwards air-water and steam-water flows as well as for counter-current air-water flows. The details of the setup, measuring technique and data evaluation are given. The report also includes a discussion on selected results obtained and on uncertainties.

For the qualification and validation of two-phase CFD-models for medium and large-scale industrial applications dedicated experiments providing data with high temporal and spatial resolution are required. Fluid dynamic parameter like gas volume fraction, bubble size distribution, velocity or turbulent kinetic energy should be measured locally. Considering the fact, that the used measurement techniques should not affect the flow characteristics, radiation based tomographic methods are the favourite candidate for such measurements. Here the recently developed ultrafast X-ray tomography, is applied to measure the local and temporal gas volume fraction distribution in a vertical pipe. To obtain the required frame rate a rotating X-ray source by a massless electron beam and a static detector ring are used. Experiments on a vertical pipe are well suited for development and validation of closure models for two-phase flows. While vertical pipe flows are axially symmetrically, the boundary conditions are well defined. The evolution of the flow along the pipe can be investigated as well. This report documents the experiments done for co-current upwards and downwards air-water and steam-water flows as well as for counter-current air-water flows. The details of the setup, measuring technique and data evaluation are given. The report also includes a discussion on selected results obtained and on uncertainties.

Die Visualisierung und Quantifizierung von Flüssigwasseransammlungen in Polymerelektrolytmembran-Brennstoffzellen konnte mittels Neutronenradiographie erreicht werden. Dank dieser neuartigen diagnostischen Methode konnte erstmals die Flüssigwasseransammlung in den porösen Gasdiffusionsschichten direkt nachgewiesen und quantifiziert werden. Die Kombination von Neutronenradiographie mit ortsaufgelösten Stromdichtemessungen bzw. lokaler Impedanzspektroskopie erlaubte die Korrelation des inhomogenen Flüssigwasseranfalls mit dem lokalen elektrochemischen Leistungsverhalten. Systematische Untersuchungen an Polymerelektrolyt- und Direkt-Methanol-Brennstoffzellen verdeutlichen sowohl den Einfluss von Betriebsbedingungen als auch die Auswirkung von Materialeigenschaften auf die Ausbildung zweiphasiger Strömungen.

Es wurde ein neues Zweifluidmodell entwickelt, um Nichtgleichgewichtseffekte in kritischen und nahkritischen Ein-komponenten-Zweiphasenströmungen von anfänglich unter-kühlten oder siedenden Fluiden durch Rohre und Düsen zu untersuchen. Das vorgeschlagene Sechs-Gleichungsmodell besteht aus den phasenbezogenen Erhaltungsgleichungen für Masse und Impuls, der Bilanzgleichung für die thermische Energie der flüssigen Phase sowie einer zusätzlichen Transport-gleichung für die volumetrische Blasenanzahl. Zur Lösung des Systems aus partiellen Differentialgleichungen wird ein semi-implizites Finite-Differenzen-Zeitschrittverfahren angewendet. Die Schließung des Gleichungssystems wird durch Einbindung thermodynamischer Beziehungen und konstitutiver Gleichungen, die den strömungsformabhängigen Impuls-, Wärme- und Stofftransport beschreiben, erreicht. Für Strömungssysteme mit spontaner Entspannungsverdampfung aus dem rein flüssigen Zustand (Flashing) werden verschiedene Keimbildungsmodelle eingesetzt, die den Anfangszustand der verzögerten Dampfbildung beschreiben. Auf diese Weise werden thermodynamische Nichtgleichgewichtszustände als Folge von Zuständen mit für die Aktivierung von Keimstellen benötigtem Energieüberschuß, eingeschränkt vorhandener Phasengrenzfläche sowie begrenzter Wärmeübertragung zwischen den Phasen betrachtet. Abweichungen vom fluid-dynamischen Gleichgewicht (Phasenschlupf) ergeben sich aufgrund unterschiedlicher Trägheitseigenschaften und verschieden stark ausgeprägter mechanischer Kopplung zwischen den Phasen. Die mit diesem Modell erhaltenen numerischen Ergebnisse stimmen gut mit experimentellen Werten für Zweiphasen-strömungen mit unterschiedlichen Eintrittsbedingungen und Kanalgeometrien überein. / A new two-fluid flow model has been developed in order to examine non-equilibrium effects in critical and near-critical one-component two-phase flows of initially subcooled or saturated fluids through pipes and nozzles. The six-equation model proposed consists of the phasic conservation equations of mass and momentum, the liquid thermal energy, and of an additional transport equation for the bubble number density. To solve for the unknowns of the system of partial differential equations, a semi-implicit finite difference time-marching method is utilized. The closure of the set of equations is accomplished by thermodynamic relationships and additional constitutive equations describing momentum transport, interphase heat, and mass transfer which account for different flow regimes. For fluid flow systems undergoing a sudden change of phase from the pure liquid state (flashing), distinct nucleation models are included to describe the initial state of delayed vapor generation. In this way thermal non-equilibrium states are considered to be the consequence of excessive energy states required to activate nucleation sites, of restricted interfacial area and limited heat transfer between the phases. Deviation from fluid-dynamic equilibrium (phasic slip) results from different inertial properties and from distinct strength of mechanical coupling between the phases. The numerical results obtained with this model agree quite well with experimental data for two-phase flows with various inlet conditions and channel geometries.