Liquid metal flows drive by gas bubbles in a static magnetic field [Elektronische Ressource] / Chaojie Zhang

Informations

• Publié par: technische_universitat_dresden
• Publié le: 01 janvier 2009
• Nombre de lectures: 31
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Liquid Metal Flows Driven by Gas Bubbles
in a Static Magnetic Field



Der Fakultät Maschinenwesen
der
Technischen Universität Dresden


zur
Erlangung des Grades
Doktoringenieur (Dr.-Ing.)
vorgelegte Dissertation



M. Eng. Chaojie Zhang
geb. am 14. Juli 1976

Tag der Einreichung: 09. April 2009 Preface
This dissertation investigates liquid metal flows driven by rising gas bubbles in a static
magnetic field, the direction of which is either vertical or horizontal, respectively. Using
ultrasoundDopplervelocimetry(UDV),wemeasurethevelocitiesofthegasandliquidphases
in model experiments based on the melt GaInSn. The results disclose different magnetic
damping influences on the flow depending on the direction of the magnetic field.
Chapter 1 consists of three parts: a short introduction to the research background; a
brief review on the fundamentals of magnetohydrodynamics (MHD) that are relevant for the
current work; as well as some descriptions of the model experiments using low-temperature
melt in the laboratory.
Chapter 2 reviews ultrasound Doppler methods for the measurements of fluid flow. We
focus on an ultrasound device DOP2000, whose capability is tested especially for the mea-
surements of bubble-driven flows.
Chapter 3 is concerned with the flow of a single bubble rising in a bulk of stagnant melt,
which is exposed to a vertical or a horizontal field. We measure the velocity of the bubble as
wellasthebubble-inducedliquidmotion,andcomparetheinfluenceofthemagneticdamping
as the field direction is changed.
Chapter 4 is focused on liquid metal flows driven by a bubble plume inside an insulating
vessel. We obtain velocity fields of the liquid phase, as well as the distributions of void
fraction, of the flow in a vertical and a horizontal magnetic field, respectively. The results
are compared and discussed.
Chapter 5 summarizes the current work and presents the main conclusions based on the
current results. The relevance of the current research work to the real industrial applications
is discussed.
iContents
Preface i
Contents ii
1 Introduction 1
1.1 Research background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Some fundamentals of magnetohydrodynamics . . . . . . . . . . . . . . . . . 2
1.3 Model experiments using low-temperature melts . . . . . . . . . . . . . . . . 6
2 Velocity measuring techniques for liquid metal flows 8
2.1 A literature review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1 Invasive methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.2 Non-invasive methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Fundamentals of ultrasound & Doppler effect . . . . . . . . . . . . . . . . . . 14
2.3 Ultrasound Doppler instruments . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.1 Continuous wave instrument . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.2 Pulse wave instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.3 UDV device: DOP2000 . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.4 Operation parameters in DOP2000 . . . . . . . . . . . . . . . . . . . 19
2.4 UDV application in fluid mechanics . . . . . . . . . . . . . . . . . . . . . . . . 21
2.4.1 UDV for transparent liquid . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.2 UDV for liquid metal . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.3 UDV for two-phase flow . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.5 Test problems for UDV in two-phase flow . . . . . . . . . . . . . . . . . . . . 25
2.5.1 Settling sphere experiment . . . . . . . . . . . . . . . . . . . . . . . . 25
2.5.2 Rising bubble experiment . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.5.3 Bubble chain flow experiment . . . . . . . . . . . . . . . . . . . . . . . 29
iiContents iii
3 Flow driven by a single bubble in a static magnetic field 34
3.1 Dimensionless parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2 Single bubble motion: a literature review . . . . . . . . . . . . . . . . . . . . 36
3.2.1 Bubble shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2.2 Drag coefficient & terminal velocity . . . . . . . . . . . . . . . . . . . 40
3.2.3 Bubble trajectory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2.4 Bubble motion in liquid metals . . . . . . . . . . . . . . . . . . . . . . 49
3.3 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.4 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.4.1 Flow without a magnetic field . . . . . . . . . . . . . . . . . . . . . . 51
3.4.2 Flow in a vertical magnetic field . . . . . . . . . . . . . . . . . . . . . 56
3.4.3 Flow in a horizontal magnetic field . . . . . . . . . . . . . . . . . . . . 62
3.5 Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4 Flow driven by a bubble plume in a static magnetic field 70
4.1 The influence of a DC field: a literature review . . . . . . . . . . . . . . . . . 70
4.1.1 MHD two-phase flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.1.2 Jet flow in a static magnetic field . . . . . . . . . . . . . . . . . . . . . 71
4.1.3 Convective flow damped by a static magnetic field . . . . . . . . . . . 73
4.1.4 Convective flow enhanced by a static magnetic field . . . . . . . . . . 76
4.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.3 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.3.1 Flow in a longitudinal magnetic field . . . . . . . . . . . . . . . . . . . 78
4.3.2 Flow in a transverse magnetic field . . . . . . . . . . . . . . . . . . . . 82
4.4 Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5 Summary 103
Appendices 106
Acknowledgements 110
Bibliography 112Chapter 1
Introduction
This chapter presents a brief review of the research background, some related MHD funda-
mentalsconcerningastaticmagneticfield,andtheapproachestoconductmodelexperiments
in laboratory using low-temperature melt. Specifically, we focus on the influence of a static
magnetic field on several two-phase flows encountered in metallurgical engineering. It is im-
portant to comprehend the flow phenomena and their physical mechanisms before we can
control such flows reliably and effectively in real applications. Next, we briefly look through
some fundamentals of magnetohydrodynamics, which will serve as the basis for the under-
standing in the present work. Finally, the necessities and advantages of laboratory model
experiments are discussed. The use of low-temperature melt simplifies experiments greatly
and makes it much easier to observe the influence of a magnetic field on the flow.
1.1 Research background
Two-phase flows consisting of gas and liquid metals are usually encountered in metallurgical
engineering. Inasteel-makingprocess, forinstance, two-phaseflowsareindispensableinsev-
eralstages, seeforexampleThomas(2003a,b)andthereferencesthereinforacomprehensive
review.
As a refining technique, inert gas bubbles are usually injected into a bulk of molten melt
inside a ladle. The rising bubbles drive the surrounding fluid into motion and so enhance the
mixing inside the melt. As a result, the melt can be refined because of a more homogeneous
distribution of the physical and chemical properties. The details of the dispersed bubble
motion, the distribution of the void fraction, as well as the velocity field of the liquid phase
are important information for the optimization of the refining process.
Examples of two-phase flows can be found in other stages of the steel-making process
too. In a continuous casting process, for instance, fresh melt is introduced into a bottomless
mould through a submerged entry nozzle (SEN). Usually, inert gas is added to the melt in
ordertoavoidthecloggingofthenozzle. Asaresult,atwo-phasejetflowisformedinsidethe
mould. The flow pattern is important for the casting process, because the strong shear stress
1Chapter 1. Introduction 2
in the flow can easily destroy the solidified strand close to the mould wall. This becomes
especially dangerous at high casting speeds. Therefore, a reliable flow control is needed in
order to avoid such phenomena.
Electromagnetic fields are attractive tools to control liquid metal flows at high temper-
atures, because the induced Lorentz force acts on the fluid in a contactless way. Various
fields generated by alternating current (AC) or direct current (DC) can be applied; see re-
views given by Sneyd (1993), Moreau (1990), Davidson (2001) and Toh et al. (2006). It is
well-known that a rotating magnetic field (RMF) or a traveling magnetic field (TMF) can
be used as an electromagnetic stirrer to enhance the mixing in the melts. In comparison,
a DC magnetic field often works as an “electromagnetic brake”, which usually suppresses
fluid motion. Many investigations are devoted to the application of an electromagnetic field
in a continuous casting process, see Taniguchi (2006). Several numerical simulations were
conducted to predict the flow in the mold region under the influence of external magnetic
fields; see for example Oka