Kunkelmann, Christian (2011)
Numerical Modeling and Investigation of Boiling Phenomena.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication
Abstract
The subject of the present thesis is the numerical modeling and investigation of boiling phenomena. The heat transfer during boiling is highly efficient and therefore used for many applications in power generation, process engineering and cooling of high performance electronics. The precise knowledge of particular boiling processes, their relevant parameters and limitations is of utmost importance for an optimized application. Therefore, the fundamentals of boiling heat transfer have been intensively studied in the last decades and are still subject of many ongoing research activities all over the world. In spite of this effort, many aspects of boiling heat transfer are still not completely understood. The difficulty is mainly due to the small length and time scales. In addition to highly resolved experiments, the numerical modeling of boiling heat transfer has been established in fundamental research during the last years. However, most of the existing models and methods are limited with respect to their applicability. Thus, 3D simulations of boiling in complex geometries cannot be handled by the existing methods. However, the use of complex heater geometries is one of the possibilities to fulfill the demand for more efficient heat transfer units and its numerical investigation is therefore desirable. Within the framework of the present thesis, a numerical model was developed which enables the simulation of boiling processes in arbitrarily complex geometries at a high level of accuracy. The model is based on the Volume-of-Fluid solver of the Computational Fluid Dynamics software OpenFOAM and resolves all relevant length and time scales. The latter particularly applies for the microscopic, but highly relevant, region at the 3-phase contact line where the liquid-vapor interface meets the wall. The present thesis contains a detailed description of the model and comprehensive information about its validation. Furthermore, several simulations of different boiling phenomena are presented. The simulation results are discussed and compared to mostly experimental data available in literature. Simulations were accomplished for nucleate boiling of single bubbles and merging bubbles, flow boiling in a near-wall shear flow, boiling in a structured micro-channel and film boiling of droplets (Leidenfrost phenomenon). Good agreement to existing data is achieved. Further, the simulation results enable a detailed analysis and a more comprehensive understanding of the transfer mechanisms. Hereby, the knowledge gained during highly resolved experiments can be extended significantly. The formation of an enclosed droplet within a merged bubble which was observed but not understood experimentally is an excellent example. The detailed analysis of the simulation results enable a clarification of the causes for the formation of the droplet and lead to a gain in knowledge which would not have been possible in the experiment. In summary, the present thesis includes the development, implementation and validation of a boiling model as well as a wide range of simulations on different boiling phenomena. The latter clearly demonstrate the potential of the numerical investigation of boiling phenomena in fundamental research and in the design of small boiling devices.
Item Type: |
Ph.D. Thesis
|
Erschienen: |
2011 |
Creators: |
Kunkelmann, Christian |
Type of entry: |
Primary publication |
Title: |
Numerical Modeling and Investigation of Boiling Phenomena |
Language: |
English |
Referees: |
Stephan, Dr.-Ing. Peter ; Jakirlic, Dr.-Ing. Suad |
Date: |
30 May 2011 |
Refereed: |
12 August 2011 |
URL / URN: |
urn:nbn:de:tuda-tuprints-27319 |
Abstract: |
The subject of the present thesis is the numerical modeling and investigation of boiling phenomena. The heat transfer during boiling is highly efficient and therefore used for many applications in power generation, process engineering and cooling of high performance electronics. The precise knowledge of particular boiling processes, their relevant parameters and limitations is of utmost importance for an optimized application. Therefore, the fundamentals of boiling heat transfer have been intensively studied in the last decades and are still subject of many ongoing research activities all over the world. In spite of this effort, many aspects of boiling heat transfer are still not completely understood. The difficulty is mainly due to the small length and time scales. In addition to highly resolved experiments, the numerical modeling of boiling heat transfer has been established in fundamental research during the last years. However, most of the existing models and methods are limited with respect to their applicability. Thus, 3D simulations of boiling in complex geometries cannot be handled by the existing methods. However, the use of complex heater geometries is one of the possibilities to fulfill the demand for more efficient heat transfer units and its numerical investigation is therefore desirable. Within the framework of the present thesis, a numerical model was developed which enables the simulation of boiling processes in arbitrarily complex geometries at a high level of accuracy. The model is based on the Volume-of-Fluid solver of the Computational Fluid Dynamics software OpenFOAM and resolves all relevant length and time scales. The latter particularly applies for the microscopic, but highly relevant, region at the 3-phase contact line where the liquid-vapor interface meets the wall. The present thesis contains a detailed description of the model and comprehensive information about its validation. Furthermore, several simulations of different boiling phenomena are presented. The simulation results are discussed and compared to mostly experimental data available in literature. Simulations were accomplished for nucleate boiling of single bubbles and merging bubbles, flow boiling in a near-wall shear flow, boiling in a structured micro-channel and film boiling of droplets (Leidenfrost phenomenon). Good agreement to existing data is achieved. Further, the simulation results enable a detailed analysis and a more comprehensive understanding of the transfer mechanisms. Hereby, the knowledge gained during highly resolved experiments can be extended significantly. The formation of an enclosed droplet within a merged bubble which was observed but not understood experimentally is an excellent example. The detailed analysis of the simulation results enable a clarification of the causes for the formation of the droplet and lead to a gain in knowledge which would not have been possible in the experiment. In summary, the present thesis includes the development, implementation and validation of a boiling model as well as a wide range of simulations on different boiling phenomena. The latter clearly demonstrate the potential of the numerical investigation of boiling phenomena in fundamental research and in the design of small boiling devices. |
Alternative Abstract: |
Alternative abstract | Language |
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Das Thema der vorliegenden Arbeit ist die numerische Modellierung und Untersuchung von Siedephänomenen. Der Wärmetransport beim Sieden ist äußerst effizient und kommt daher in vielen Anwendungen im Bereich der Kraftwerks- und der Verfahrenstechnik aber auch bei der Kühlung von Hochleistungselektronik zum Einsatz. Die genaue Kenntnis über bestimmte Siedevorgänge sowie deren relevante Parameter und Grenzen ist unerlässlich für den optimalen Einsatz in einer technischen Anwendung. Aus diesem Grund sind die physikalischen Grundlagen des Siedens in den vergangenen Jahrzehnten intensiv erforscht worden und auch aktuell noch immer Gegenstand vieler Forschungsvorhaben in aller Welt. Trotz dieser Bemühungen sind viele Aspekte des Siedevorgangs noch immer nur unvollständig verstanden. Vor allem die sehr kleinen Zeit- und Längenskalen stellen hierbei Schwierigkeiten dar. Neben hochauflösenden experimentellen Untersuchungen, hat sich in den letzten Jahren auch die numerische Modellierung von Siedephänomenen in der Grundlagenforschung etabliert. Allerdings sind viele der bestehenden Modelle und Methoden hinsichtlich ihrer Anwendbarkeit sehr stark eingeschränkt, insbesondere hinsichtlich der dreidimensionalen Simulation von Siedevorgängen in komplexer Geometrie. Der Einsatz von komplexer Heizwandgeometrie ist jedoch eine vielversprechende Möglichkeit um die Nachfrage nach immer effizienteren Wärmeübertragern zu erfüllen und sollte daher auch numerisch untersucht werden können. In der vorliegenden Arbeit wurde ein numerisches Modell entwickelt, mit dem Siedevorgänge in beliebig komplexer Geometrie mit hoher Genauigkeit simuliert werden können. Das Modell basiert auf der Volume-of-Fluid Methode der Computational Fluid Dynamics Software OpenFOAM und löst alle für das Sieden relevanten Zeit- und Längenskalen auf. Letzteres gilt insbesondere für den mikroskopisch kleinen, aber für das Blasensieden äußerst relevanten, Bereich der 3-Phasen-Kontaktlinie, in dem die Dampf-Flüssig-Phasengrenze auf die Wand trifft. Die vorliegende Dissertationsschrift enthält eine detaillierte Beschreibung des entwickelten Modells sowie ausführliche Informationen zu dessen Validierung. Darüber hinaus werden Simulationen von verschiedenen Siedephänomenen vorgestellt, deren Ergebnisse diskutiert und mit bereits veröffentlichten, vorranging experimentellen Daten verglichen. Die durchgeführten Simulationen umfassen Blasensieden von Einzelblasen und koaleszierenden Blasen, Strömungssieden in einer wandnahen Scherströmung, Sieden in einem strukturierten Mikrokanal sowie Filmsieden von Tropfen (Leidenfrost-Phänomen). Es konnte eine gute Übereinstimmung zu bereits veröffentlichten Ergebnissen erzielt werden. Darüber hinaus ermöglichen die hochaufgelösten Simulationen eine detaillierte Analyse der komplexen Transportvorgänge und damit einen Erkenntnisgewinn, welcher die experimentell gewonnenen Erfahrungen und Erkenntnisse deutlich erweitert. Ein Beispiel hierzu ist die experimentell belegte aber bislang nicht verstandene Entstehung eines eingeschlossenen Tropfens innerhalb einer koaleszierten Blase. Die detaillierte Analyse der Simulationsergebnisse ermöglicht die Klärung der Ursachen für die Tropfenentstehung und führt somit zu einem Erkenntnisgewinn, der rein durch Experimente nicht möglich gewesen wäre. Zusammenfassend beinhaltet die vorliegende Arbeit die Entwicklung, Implementierung und Validierung des Siedemodells sowie ein breites Spektrum an Simulationen zu unterschiedlichen Siedephänomenen. Letztere zeigen sehr deutlich das Potential der numerischen Untersuchung von Siedephänomenen im Bereich der Grundlagenforschung und bei der Auslegung von kleinen Siedeapparaten. | German |
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Uncontrolled Keywords: |
Sieden, numerische Simulation, Zweiphasenströmung, Kontaktlinie |
Alternative keywords: |
Alternative keywords | Language |
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Boiling, numerical Simulation, two phase flow, contact line | English |
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Classification DDC: |
500 Science and mathematics > 530 Physics 600 Technology, medicine, applied sciences > 620 Engineering and machine engineering |
Divisions: |
16 Department of Mechanical Engineering |
Date Deposited: |
10 Oct 2011 09:58 |
Last Modified: |
05 Mar 2013 09:55 |
PPN: |
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Referees: |
Stephan, Dr.-Ing. Peter ; Jakirlic, Dr.-Ing. Suad |
Refereed / Verteidigung / mdl. Prüfung: |
12 August 2011 |
Alternative keywords: |
Alternative keywords | Language |
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Boiling, numerical Simulation, two phase flow, contact line | English |
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