Bubbly Flow Physics for Applications in Aerated Hydroturbines and Underwater Transport A ...

272 Pages·2016·6.53 MB·English

Bubbly Flow Physics for Applications in Aerated Hydroturbines
and Underwater Transport

A DISSERTATION
SUBMITTED TO THE FACULTY OF
UNIVERSITY OF MINNESOTA
BY

Ashish Karn

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

Advisors: Jiarong Hong and Roger E A Arndt

May 2016
© Ashish Karn 2016
Acknowledgements
As I am concluding this dissertation, it is but natural to reminisce the chronology
of events that have led me here, in the first place. My deepest acknowledgment of
gratitude thus, for my beloved teacher who gave me the inspiration to pursue a doctoral
degree. Because of his inspiration and support, these years in the grad school were the
most enriching years of my life.
I would like to express my profuse appreciation and thankfulness to my advisors
Dr. Jiarong Hong and Dr. Roger Arndt for letting me join their groups. I could not have
completed this thesis without their help at every step, and I felt fortunate to have them as
advisors at every step of my PhD. I am thankful to them for patiently listening to all of
my puerile ideas and showing immense faith in me throughout. I would like to thank Dr.
Hong particularly for teaching me how to never give up and become responsible towards
my commitments. He has trained me at every step of my graduate career in every single
skill that I needed to be successful as a graduate student. I would also like to thank Dr.
Peter H. McMurry for his support in helping me pursue a graduate career at the U.
I am deeply thankful to all my colleagues at the University of Minnesota: Ellison
Kawakami, Garrett Monson, Chris Ellis and Chris Milliren for their guidance during the
first few years of my research. I would also like to thank my group members including
Rohan De, Roger Borgeson, Siyao Shao, David Sorenson, Brett Rosiejka, John Turner,
Lei Cao for helping me out in carrying out experiments at the water tunnel, at different
stages of my research. My thanks also to Mostafa, Kevin, Teja, Santosh for their
invaluable comments and assistance during the group meetings. A deep note of thanks to
my beloved friends Sanyam Mehta, Ayush Singhal, Shashank Goyal, Saket Gupta, Ankit
Mahajan, Raghav Karumur, Gokul Hariharan, Srijan Agarwal, Ankur Khare, Brijesh
Kumar, Pulkit Agarwal, Gyanender Singh and others for their valuable friendship and
brotherly association which made grad school a memorable and cherishable experience
for me.
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I would like to thank my committee members: Prof. Ellen Longmire, Dr. Lian
Shen and Dr. Cari Dutcher for their insightful comments during my PhD. Thanks also
goes to Dr. John S Gulliver for his support and valuable collaboration. Many thanks to
the staff members at SAFL, particularly Ben Erickson, Erik Steen and Jim Tucker for
their prompt and responsive support. I would also like to acknowledge the funding
support received from Graduate School under Doctoral Dissertation Fellowship, which
was instrumental in my graduate research and getting this dissertation completed in time.
Last but not the least, I would like to thank my parents and family members for all
the sacrifices they have done in letting me pursue my doctoral degree at the U.
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Abstract
Bubbly flows occur in a wide variety of situations and are frequently employed in
industry and other engineering applications. The current work deals with two such
examples of bubbly flows: speed enhancement and controllability of next-generation
high–speed underwater vehicles for naval defense applications and environment-friendly
power generation through aerating hydroturbines. A complete understanding of the
bubbly flow physics is imperative to leverage it for these different applications. These
investigations into bubbly flow physics are accomplished by studying the fundamental
fluid dynamics of bubbles at different size scales. A large bubble is used to envelop an
underwater vehicle causing tremendous reduction in flow resistance while the small
bubbles are employed for aeration applications in a hydroturbine. Our experiments have
provided critical insights into the design and development of operational strategies and
models for these novel technologies. For the dispersed bubbly flow regime, bubble size
characteristics in the wake of a ventilated turbine blade is measured using shadow
imaging and a newly developed image processing approach. Simultaneous mass transfer
measurements in the wake have shown an interrelationship with the bubble size and high
speed imaging of the bubbles reveal the physical mechanisms of bubble breakup and
coalescence and its effect upon the bubble sizes in the wake. In the supercavity bubble
regime, systematic studies are carried out to investigate the supercavity closure
mechanisms in detail, and a unified theory is proposed to predict different closure modes.
Further insights are provided into the interrelationship between supercavity closure,
ventilation demand and gas entrainment behaviors of supercavity. The effect of ocean
waves on the stability of supercavity bubbles and its closure are also investigated by
replicating the ocean waves in the high speed water tunnel. Specifically, a novel aspect of
the current research pertains to the visualization and quantification of the internal flows
inside supercavity bubbles and drops.

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Table of Contents
Acknowledgments…………………………………………………………………………i
Abstract…………………………………………………………………………………...iii
Table of Contents………………………………………………………………………....iv
List of Tables……………………………………………………………………….…...viii
List of Figures…………………………………………………………………………….ix

Chapter 1 Introduction..................................................................................................... 1
1.1. Multiphase flows ....................................................................................................... 1
1.2. Cavitation and Supercavitation ................................................................................. 3
1.2.1. Fundamentals of Cavitation ................................................................................ 3
1.2.2. Supercavitation: A limiting case of cavitation .................................................... 5
1.2.3. Supercavitation: A limiting case of bubbly flow ................................................ 8
1.3. Research Motivation: Underwater ventilated flows ................................................. 9
1.4. Thesis Outline ......................................................................................................... 15
Part A: Small Dispersed Bubble Flow Regime............................................................ 20
Chapter 2. Development of an Integrative Image Measurement Technique for Dense
Bubbly Flows ................................................................................................................... 21
2.1. Bubbly flows and characterization techniques ....................................................... 22
2.2. Description of Experimental Setup ......................................................................... 24
2.3. Overview of the image-processing task .................................................................. 28
2.4. Proposed image-processing approach ..................................................................... 32
2.4.1. Summary of the algorithm ................................................................................ 32
2.4.2. Extended H-Minima Transform ........................................................................ 33
2.4.3. Cluster Processing ............................................................................................. 34
2.4.4. Bubble size and shape measurements ............................................................... 37
2.5. Results and Validation ............................................................................................ 38
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2.5.1. Validation through synthetic bubble images ..................................................... 38
2.5.1.1. Synthetic bubble generation procedure…………...……………………….38
2.5.1.2. Single bubble measurement………….………………………………...….40
2.5.1.3. Polydisperse bubble size distribution measurement………….…...…...….42
2.5.2. Experimental validation of the image-processing approach ............................. 46
2.6. Conclusions ............................................................................................................. 50
Chapter 3. Characterization of the turbulent bubbly wake of ventilated hydrofoil 53
3.1. Relevance ................................................................................................................ 54
3.2. Experimental Setup and Conditions........................................................................ 56
3.3. Results for a reference case..................................................................................... 59
3.3.1. Validation of image processing approach at multiple locations ....................... 59
3.3.2. Bubble size and shape distribution in the wake ................................................ 61
3.3.3. Void fraction measurements. ............................................................................ 65
3.3.4. Bubble Velocity Measurements in the wake .................................................... 66
3.4. Effect of variation in angle of attack of hydrofoil .................................................. 68
3.5. Effect of Variation in Reynolds Number ................................................................ 74
3.6. Effect of variation in air entrainment coefficient .................................................... 78
3.7. Effect of air injection mechanism ........................................................................... 82
3.8. Bubble size prediction............................................................................................. 86
3.9. Conclusions ............................................................................................................. 88
Chapter 4. Investigations into breakup and coalescence mechanisms in the hydrofoil
bubbly wake ..................................................................................................................... 91
4.1. Relevance ................................................................................................................ 92
4.2. Experimental Setup and Conditions........................................................................ 94
4.3. Results and Discussion ........................................................................................... 95
4.3.1. Mean bubble statistics ....................................................................................... 95
4.3.2. Bubble size distributions at the reference location ........................................... 98
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4.3.3. Stable bubble size in the turbulent hydrofoil wake ......................................... 100
4.3.4. Variation of mean bubble statistics in the wake ............................................. 105
4.4. The Coalescence-dominant regime ....................................................................... 107
4.5. The breakup-dominant regime .............................................................................. 113
4.6. Conclusions ........................................................................................................... 116
Chapter 5. Mass transfer studies across ventilated hydrofoils: A step towards
hydroturbine aeration. ................................................................................................. 120
5.1. Background and literature review ......................................................................... 121
5.2. Experimental Setup and Conditions...................................................................... 126
5.3. Mass Transfer Model ............................................................................................ 128
5.4. Results and Discussion ......................................................................................... 132
5.4.1.Results on Bubble Size .................................................................................... 132
5.4.2. Results on Dissolved Oxygen Uptake............................................................. 138
5.5. Conclusions ........................................................................................................... 144

Part B: Supercavity Bubble Regime ........................................................................... 146
Chapter 6. An experimental investigation into supercavity closure mechanisms .. 147
6.1. Background and literature review ......................................................................... 151
6.2. Experimental Methods .......................................................................................... 153
6.3. Experimental Observations ................................................................................... 157
6.3.1. Observations of different closure modes ........................................................ 157
6.3.2. Blockage Effects ............................................................................................. 165
6.4. Hypothesis and explanation of observations......................................................... 173
6.4.1. Variation of Closure with Fr or CQs ............................................................... 178
6.4.2. Variation of supercavity closure with blockage ratios .................................... 178
6.5. Summary and Discussion ...................................................................................... 180
6.5.1. Closure of a ventilated supercavity in a free surface facility .......................... 181
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6.5.2. On the closure of a natural supercavity ........................................................... 182
6.5.3. On the occurrence of pulsating cavity closure ................................................ 184
6.5.4. Dependence of supercavity behavior upon internal cavity flow ..................... 184
Chapter 7. Dependence of supercavity closure upon flow unsteadiness .................. 189
7.1. Relevance of unsteady flows in the study of supercavitation ............................... 190
7.2. Setup and Conditions for unsteady state experiments .......................................... 191
7.3. Closure transitions under unsteady flows ............................................................. 194
7.4. Conclusions ........................................................................................................... 201
Chapter 8. Exploring the interrelationship between supercavity closure, ventilation
hysteresis and demand .................................................................................................. 203
8.1. Critical gas entrainment rates during ventilation hysteresis ................................. 204
8.2. Experimental setup and procedure ........................................................................ 206
8.3. Gas entrainment dependence under steady states ................................................. 208
8.4. Unsteady state gas entrainment characteristics ..................................................... 214
8.5. Conclusions ........................................................................................................... 218
Chapter 9. Ongoing and future work .......................................................................... 221
9.1. Relevance of ongoing work in control and operation of underwater vehicles ..... 222
9.2. Prior Literature ...................................................................................................... 226
9.3. Proposed Research Approach ............................................................................... 229
9.3.1. Qualitative flow information inside a supercavity .......................................... 230
9.3.2. Quantitative flow information inside a supercavity ........................................ 232
9.4. Development of an image distortion-correction approach .................................... 233
9.5. Preliminary work done and ongoing research ...................................................... 236
9.5. Scope for future work ........................................................................................... 238
References ...................................................................................................................... 241

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List of Tables
Table 3.1: Bubble size for both LEV and TEV Foils at all the experimental conditions (at x = 377
mm)………………………………………………………………………………..…….………..83
Table 3.2: The parameters obtained by the experimental data to the fitting equation………...….87
Table 4.1: Pressure in the test-section and the air temperature at each experimental condition…95
Table 4.2: Bubble size characteristics obtained at different experimental conditions at the
reference location. N is the total number of bubbles at each downstream location in the entire
dataset of 2000 images……………………………………………………………………………96
Table 4.3: Fitting parameters used to fit bubble size distribution with a lognormal distribution at
different experimental conditions. C ’ is defined in equation 5……….........................................99
n
Table 4.4: Variation of d at different downstream locations in the hydrofoil wake for selected
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test-cases………………………………………………………………………...………………106
Table 5.1: The variation of different representative diameters for the reference case. d and d in
2 3
equations 3 and 5-9 are equivalent to d and d , respectively in equation
20 30
5.11..……………………………………………………………………………….…………….134
Table 5.2: Description of gas transfer experiments conducted in the present study with all the
relevant experimental conditions. d is obtained from imaging experiments. The β values are
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determined from a fit of the model to experiments. Time to approach saturation, t is for gas
sat
transfer experiments…………….………………………….........................................................136
Table 6.1: The acronyms and the names of different closure modes observed in our experiments.
Note that FC listed here is considered as a state of supercavity not a closure
mode……………………………………………………………………………………….…….159
Table 6.2: Critical ventilation rates at different blockage ratios………………………..……….166
Table 6.3: Variation of estimated ΔP ̃ with cavity states for B = 9 %.........…………...……….177

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