Measuring the liquid velocity and turbulence parameters in multiphase flows is a challenging task. In general, measurements based on optical methods are hindered by the presence of the gas phase. In the present work, it is shown that this leads to a sampling bias. Here, particle image velocimetry (PIV) is used to measure the liquid velocity and turbulence in a bubble column for different gas volume flow rates. As a result, passing bubbles lead to a significant sampling bias, which is evaluated by the mean liquid velocity and Reynolds stress tensor components. To overcome the sampling bias a window averaging procedure that waits a time depending on the locally distributed velocity information (hold processor) is derived. The procedure is demonstrated for an analytical test function. The PIV results obtained with the hold processor are reasonable for all values. By using the new procedure, reliable liquid velocity measurements in bubbly flows, which are vitally needed for CFD validation and modeling, are possible. In addition, the findings are general and can be applied to other flow situations and measuring techniques.

Der vorliegende Bericht gibt einen zusammenfassenden Überblick der im Vorhaben erreichten Ergebnisse. Ziel war die Qualifikation von CFD-Methoden für Zweiphasenströmungen mit Phasenüber¬gang. Dafür werden neuartige experimentelle Daten benötigt. Diese können an der TOPFLOW-Anlage des HZDR generiert werden, da die Anlage Experimente in für die Reaktorsicher-heits¬forschung relevanten Skalen und Parametern mit innovativen Messtechniken verbindet. Die experimentellen Arbeiten umfassen Untersuchungen zu Strömungen in vertikalen Rohren mit Hilfe der ultraschnellen Röntgentomographie, zu Strömungen mit und ohne Phasenübergang in einem Testbassin sowie zur Gegenstrombegrenzung in einem Heißstrangmodell. Diese werden im vorliegenden Bericht nur kurz dargestellt, da es zu allen 3 Versuchsserien ausführliche Dokumentationen in separaten Berichten gibt. Ein wichtiges Ergebnis der Arbeiten zur CFD-Qualifizierung ist der Erstellung des Baseline-Modellkonzepts sowie die Erstellung des Baseline-Modells für polydisperse Blasenströmungen. Damit wird ein wesentlicher Beitrag zur Erhöhung der Vorhersagefähigkeit von CFD-Codes auf Basis des Zwei- oder Mehr-Fluid-Modells erreicht. Das innovative Generalized Two-Phase Flow Konzept (GENTOP) zielt hingegen auf eine Erweiterung der Einsatzmöglichkeiten der Zweiphasen-CFD. In vielen Strömungen treten unterschiedlicher Morphologien der Phasen bzw. Strömungsformen parallel in einer Strömungsdomäne auf. Außerdem gibt es Übergänge zwischen diesen Morphologien. Mit dem GENTOP-Konzept wurde erstmals ein Rahmen geschaffen der die Simulation solcher Strömungen auf konsistente Art und Weise ermöglicht. Spezielle Modellentwicklungen erfolgten mit dem Ziel einer besseren Modellierung des Phasenübergangs.

Bubbly flows can be found in many applications in chemical, biological and power engineering. Reliable simulation tools of such flows that allow the design of new processes and optimization of existing one are therefore highly desirable. CFD-simulations applying the multi-fluid approach are very promising to provide such a design tool for complete facilities. In the multi-fluid approach, however, closure models have to be formulated to model the interaction between the continuous and dispersed phase. Due to the complex nature of bubbly flows, different phenomena have to be taken into account and for every phenomenon different closure models exist. Therefore, reliable predictions of unknown bubbly flows are not yet possible with the multi-fluid approach. A strategy to overcome this problem is to define a baseline model in which the closure models including the model constants are fixed so that the limitations of the modeling can be evaluated by validating it on different experiments. Afterwards, the shortcomings are identified so that the baseline model can be stepwise improved without losing the validity for the already validated cases. This development of a baseline model is done in the present work by validating the baseline model developed at the Helmholtz-Zentrum Dresden-Rossendorf mainly basing on experimental data for bubbly pipe flows to bubble columns, bubble plumes and airlift reactors that are relevant in chemical and biological engineering applications. In the present work, a large variety of such setups is used for validation. The buoyancy driven bubbly flows showed thereby a transient behavior on the scale of the facility. Since such large scales are characterized by the geometry of the facility, turbulence models cannot describe them. Therefore, the transient simulation of bubbly flows with two equation models based on the unsteady Reynolds-averaged Navier–Stokes equations is investigated. In combination with the before mentioned baseline model these transient simulations can reproduce many experimental setups without fitting any model. Nevertheless, shortcomings are identified that need to be further investigated to improve the baseline model. For a validation of models, experiments that describe as far as possible all relevant phenomena of bubbly flows are needed. Since such data are rare in the literature, CFD-grade experiments in an airlift reactor were conducted in the present work. Concepts to measure the bubble size distribution and liquid velocities are developed for this purpose. In particular, the liquid velocity measurements are difficult; a sampling bias that was not yet described in the literature is identified. To overcome this error, a hold processor is developed. The closure models are usually formulated based on single bubble experiments in simplified conditions. In particular, the lift force was not yet measured in low Morton number systems under turbulent conditions. A new experimental method is developed in the present work to determine the lift force coefficient in such flow conditions without the aid of moving parts so that the lift force can be measured in any chemical system easily.

Bubbly flows can be found in many applications in chemical, biological and power engineering. Reliable simulation tools of such flows that allow the design of new processes and optimization of existing one are therefore highly desirable. CFD-simulations applying the multi-fluid approach are very promising to provide such a design tool for complete facilities. In the multi-fluid approach, however, closure models have to be formulated to model the interaction between the continuous and dispersed phase. Due to the complex nature of bubbly flows, different phenomena have to be taken into account and for every phenomenon different closure models exist. Therefore, reliable predictions of unknown bubbly flows are not yet possible with the multi-fluid approach. A strategy to overcome this problem is to define a baseline model in which the closure models including the model constants are fixed so that the limitations of the modeling can be evaluated by validating it on different experiments. Afterwards, the shortcomings are identified so that the baseline model can be stepwise improved without losing the validity for the already validated cases. This development of a baseline model is done in the present work by validating the baseline model developed at the Helmholtz-Zentrum Dresden-Rossendorf mainly basing on experimental data for bubbly pipe flows to bubble columns, bubble plumes and airlift reactors that are relevant in chemical and biological engineering applications. In the present work, a large variety of such setups is used for validation. The buoyancy driven bubbly flows showed thereby a transient behavior on the scale of the facility. Since such large scales are characterized by the geometry of the facility, turbulence models cannot describe them. Therefore, the transient simulation of bubbly flows with two equation models based on the unsteady Reynolds-averaged Navier–Stokes equations is investigated. In combination with the before mentioned baseline model these transient simulations can reproduce many experimental setups without fitting any model. Nevertheless, shortcomings are identified that need to be further investigated to improve the baseline model. For a validation of models, experiments that describe as far as possible all relevant phenomena of bubbly flows are needed. Since such data are rare in the literature, CFD-grade experiments in an airlift reactor were conducted in the present work. Concepts to measure the bubble size distribution and liquid velocities are developed for this purpose. In particular, the liquid velocity measurements are difficult; a sampling bias that was not yet described in the literature is identified. To overcome this error, a hold processor is developed. The closure models are usually formulated based on single bubble experiments in simplified conditions. In particular, the lift force was not yet measured in low Morton number systems under turbulent conditions. A new experimental method is developed in the present work to determine the lift force coefficient in such flow conditions without the aid of moving parts so that the lift force can be measured in any chemical system easily.

Der vorliegende Bericht gibt einen zusammenfassenden Überblick der im Vorhaben erreichten Ergebnisse. Ziel war die Qualifikation von CFD-Methoden für Zweiphasenströmungen mit Phasenüber¬gang. Dafür werden neuartige experimentelle Daten benötigt. Diese können an der TOPFLOW-Anlage des HZDR generiert werden, da die Anlage Experimente in für die Reaktorsicher-heits¬forschung relevanten Skalen und Parametern mit innovativen Messtechniken verbindet. Die experimentellen Arbeiten umfassen Untersuchungen zu Strömungen in vertikalen Rohren mit Hilfe der ultraschnellen Röntgentomographie, zu Strömungen mit und ohne Phasenübergang in einem Testbassin sowie zur Gegenstrombegrenzung in einem Heißstrangmodell. Diese werden im vorliegenden Bericht nur kurz dargestellt, da es zu allen 3 Versuchsserien ausführliche Dokumentationen in separaten Berichten gibt. Ein wichtiges Ergebnis der Arbeiten zur CFD-Qualifizierung ist der Erstellung des Baseline-Modellkonzepts sowie die Erstellung des Baseline-Modells für polydisperse Blasenströmungen. Damit wird ein wesentlicher Beitrag zur Erhöhung der Vorhersagefähigkeit von CFD-Codes auf Basis des Zwei- oder Mehr-Fluid-Modells erreicht. Das innovative Generalized Two-Phase Flow Konzept (GENTOP) zielt hingegen auf eine Erweiterung der Einsatzmöglichkeiten der Zweiphasen-CFD. In vielen Strömungen treten unterschiedlicher Morphologien der Phasen bzw. Strömungsformen parallel in einer Strömungsdomäne auf. Außerdem gibt es Übergänge zwischen diesen Morphologien. Mit dem GENTOP-Konzept wurde erstmals ein Rahmen geschaffen der die Simulation solcher Strömungen auf konsistente Art und Weise ermöglicht. Spezielle Modellentwicklungen erfolgten mit dem Ziel einer besseren Modellierung des Phasenübergangs.

In this paper we present detailed Euler-Euler Large Eddy Simulations (LES) of dispersed bubbly flow in a rectangular bubble column. The motivation of this study is to investigate potential of this approach for the prediction of bubbly flows, in terms of mean quantities. The set of physical models describing the momentum exchange between the phases was chosen according to previous experiences of the authors. Experimental data, Euler-Lagrange LES and unsteady Euler-Euler Reynolds-Averaged Navier-Stokes model are used for comparison. It was found that the presented modelling combination provides good agreement with experimental data for the mean flow and liquid velocity fluctuations. The energy spectrum made from the resolved velocity from Euler-Euler LES is presented and discussed.

In this paper we present detailed Euler-Euler Large Eddy Simulations (LES) of dispersed bubbly flow in a rectangular bubble column. The motivation of this study is to investigate potential of this approach for the prediction of bubbly flows, in terms of mean quantities. The set of physical models describing the momentum exchange between the phases was chosen according to previous experiences of the authors. Experimental data, Euler-Lagrange LES and unsteady Euler-Euler Reynolds-Averaged Navier-Stokes model are used for comparison. It was found that the presented modelling combination provides good agreement with experimental data for the mean flow and liquid velocity fluctuations. The energy spectrum made from the resolved velocity from Euler-Euler LES is presented and discussed.

Measuring the liquid velocity and turbulence parameters in multiphase flows is a challenging task. In general, measurements based on optical methods are hindered by the presence of the gas phase. In the present work, it is shown that this leads to a sampling bias. Here, particle image velocimetry (PIV) is used to measure the liquid velocity and turbulence in a bubble column for different gas volume flow rates. As a result, passing bubbles lead to a significant sampling bias, which is evaluated by the mean liquid velocity and Reynolds stress tensor components. To overcome the sampling bias a window averaging procedure that waits a time depending on the locally distributed velocity information (hold processor) is derived. The procedure is demonstrated for an analytical test function. The PIV results obtained with the hold processor are reasonable for all values. By using the new procedure, reliable liquid velocity measurements in bubbly flows, which are vitally needed for CFD validation and modeling, are possible. In addition, the findings are general and can be applied to other flow situations and measuring techniques.