Air Bubble Entrainment in Free-Surface Turbulent Shear Flows

378 Pages·1996·8.918 MB·English

by  Hubert Chanson (Auth.)

Abstract
In high velocity water flows, large quantities of air bubbles are entrained at the free-surfaces. Practical
applications are found in Chemical, Civil, Environmental, Mechanical, Mining and Nuclear Engineering. Air-
water flows are observed in small-scale as well as large-scale flow situations. E.g., thin circular jets used as
mixing devices in chemical plants (Q^ ~ 0.001 L/s, diameter ~ 1 mm), and spillway flows (Q^ > 10,000 m-^/s,
flow thickness over 10 m). In each case, however, the interactions between the entrained air bubbles and the
turbulence field are significant.
This monograph investigates the "air bubble entrainment in free-surface turbulent shear flows". It develops an
analysis of the air entrainment processes in free-surface flows. The air-water flows are investigated as
homogeneous mixtures with variable density. The variations of fluid density result from the non-uniform air
bubble distributions and the turbulent difftision process. Several types of air-water free-surface flows are studied
: plunging jet flows (Part II), open channel flows (Part III), and turbulent water jets discharging into air (Part
IV). Each configuration can be characterised as a high-velocity free-surface flow with turbulent shear layer and
large air bubble content. Experimental observations confirm the conceptual idea that the air-water mixture
behaves as a homogeneous compressible fluid.
The monograph presents numerous and recent experimental investigations with mean velocities up to 57 m/s
and mean air contents up to 70%. The analysis of experimental studies provides new information on the air-
water flow field : air bubble distributions, air-water velocity profiles, air bubble sizes and bubble-turbulence
interactions. The results show a strong similarity between all the flow patterns. In each case the distributions of
air concentration (i.e. void fraction) can be approximated by a simple advective diffusion theory. New analysis
is developed for each flow configuration and compared successfully with model and prototype data. The
velocity distributions in air-water flowsh ave the same shape as for monophase flows. However the presence of
air bubbles modifies some turbulence characteristics while the turbulence controls the mechanism of bubble
breakup.
The book presents new useful information for design engineers and research-and-development scientists who
need a better understanding of the fluid mechanics of air-water flows. Both qualitative and quantitative
information are provided. In some cases the limits of our knowledge are pointed out.
The book consists of five parts. Part I introduces the topic and its relevance, develops a dimensional analysis
and discusses the air-water gas transfer process. In each subsequent part, the distributions of air content and
air-water velocity are described. The results are grouped as : plunging jet flows (Part II), open channel flows
(Part III) and high-velocity water jets discharging into the atmosphere (Part IV). In Part V, an analogy between
the various types of air-water flows is developed.
In the appendices, tables of physical and chemical properties of fluids are provided in appendix A. The report
presents results expressed in SI Units. A table of unit conversions is given in appendix B. Estimates of bubble
rise velocity are discussed in appendix C. Appendix D develops sound celerity calculations in two-phase flows.
Appendices E, G, H and I present complete calculations of the air bubble diffusion process. Boundary layer
characteristics and jet trajectory calculations are detailed in appendices F and J respectively. Appendix K
xiv Abstract
defines bubble size distribution characteristic parameters. Observations by LEONARDO DA VINCI are
recounted in appendix L.
'Errare Humanum Est'. Appendix M presents a correction form. Readers who find an error or mistake are
welcome to record the error on the page and to send a copy to the author.
At the beginning of the book, the reader will find the table of contents, a list of symbols, a glossary and an
album of colourfiil photographs of'white waters'.
Resume
Les ecoulements liquides a grandes vitesses sont sujets, souvent, a un entrainement d'air important. Des
applications pratiques se retrouvent dans le Genie Chimique, le Genie Civil, I'Hydraulique des Ouvrages, le
Genie Mecanique, I'lndustrie Miniere et I'lndustrie Nucleaire. On observe des ecoulements eau-air pour des
gammes tres importantes d'echelles : par exemple, des jets circulaires tres fins utilises comme melangeurs en
chimie (Q^ - 0.001 L/s, diametre - 1 mm), ou les ecoulements dans les evacuateurs de crues (Q^ - 10000
m^/s, epaisseur d'eau superieure a 10 metres). Dans chaque cas, les bulles d'air, entrainees dans I'ecoulement,
interagissent avec I'ecoulement turbulent.
Le titre original de I'ouvrage est : 'entrainement de bulles d'air dans les ecoulements turbulents cisailles en
presence d'une surface libre'. Le present document decrit les processus d'entrainement d'air dans les
ecoulements turbulents cisailles a surface libre. On considere chaque type d'ecoulement comme un melange
homogene diphasique avec une densite variable. Les variations de densite sont causees par les distributions
non-uniformes des bulles d'air et le processus de diffusion turbulente des bulles d'air. Plusieurs types
d'ecoulements sont consideres : les jets plongeant (Section II), les ecoulements en canaux a surface libre
(Section III), et les jets d'eau debitant dans I'atmosphere (Section IV). Chaque type d'ecoulements se comporte
comme un ecoulement a grandes vitesses avec une surface libre d'echange avec I'atmosphere, en presence d'une
couche de cisaillement, et avec des taux de vide tres importants. Les donnees experimentales confirment
I'hypothese d'un ecoulement diphasique homogene.
Cette etude s'appuie sur un nombre considerable de donnees experimentales, avec des vitesses moyennes
comprises entre 1 et 57 m/s, et des taux de vide moyens jusqua 70%. Les resultats experimentaux fournissent de
nouvelles informations sur les caracteristiques des ecoulements diphasiques : distributions du taux de vide,
profils des vitesses moyennes, distributions de tallies de bulles d'air, et interactions entre les bulles d'air et la
turbulence. On observe, en particulier, une similarite caracteristique entre tons les ecoulement etudies : dans
chaque cas, les distributions de taux de vide peuvent etre predites par des modeles simples de diffusion
turbulente. De nouveaux resultats analyticaux sont presentes pour chaque ecoulement, et ils sont compares avec
des resultats experimentaux. Les profiles de vitesses moyennes ont la meme forme que pour les ecoulement
monophasiques. Mais les bulles d'air interagissent avec la turbulence, tandis que la turbulence controle le
mecanisme de cassures des bulles.
Cette monographic regroupe de nombreuses informations pratiques pour les Ingenieurs de Recherche et
Developpement, souhaitant acquerir une meilleure comprehension des ecoulement diphasiques a surface libre.
On presente, a la fois, des informations qualitatives et quantitatives. Dans certains cas, on indique clairement le
manque de connaissances sur certains phenomenes.
Le livre comprend cinq parties. Dans la section I, on discute differents types d'entrainement d'air, et on
develope une analyse adimensionnelle. Puis on etudie les mecanismes d'entrainement et de diffusion des bulles
d'air pour chaque type d'ecoulement. Les resultats sont regroupes en Section II (jets plongeants). Section III
(ecoulements en canaux a surface libre) et Section IV (jets libres). Dans la section V, une analogic entre les
ecoulements diphasiques eau-air est developpee.
xvi Resume
En appendices, le lecteur trouvera des tableaux de proprietes physiques et chimiques (App. A), une table de
conversions d'unites (App. B), et des resultats pour estimer la vitesse moyenne d'ascension d'une bulle d'air
dans un fluide (App. C). La celerite du son en milieu diphasique est detaillee en appendice D. Les appendices
E, G, H et I presentent les calculs complets de diffusion turbulente de buUes d'air. Les appendices F et J
detaillent les calculs de couche limite et de trajectoire de jet libre. L'appendice K decrit les caracteristiques
principales des distributions de tailles de buUes d'air. On trouvera des extraits des notes de Leonard de VINCI
dans I'appendix L.
'Errare Humanum Est' (I'erreur est humaine) : le lecteur, trouvant des erreurs ou omissions, pourra les envoyer
a I'auteur en utilisant la derniere page du livre (App. M).
Au debut du livre, on trouvera la table des matieres, une liste de notation, un glossaire et un album of
photographies couleurs de l"eau blanche'.
Album of 'white water' photographs
Figs. I and II - Waterfalls in the rain
forest, Gold Coast Hinterland,
Australia (courtesy of
Dr R.RANKIN).
Fig. Ill - Cascading waters
in a Chinese garden,
Sydney, Australia
(photograph by the author).
Fig. IV - White effects in DarUng Harbour, Sydney, Australia (photograph by the author).
Multiple jets discharging nearly horizontally create a 'white water' effect at the free-surface of the pool.
Fig. V - Tountain of fame' at Movie World, Gold coast, Australia
(photograph by the author).
Fig. VI - Goomoolahra Falls,
Springbrook, Australia
(photograph by the author).
Fig, VII - Free-surface aeration
down a chute spillway
(photograph from the collection
of late Professor G.R. McKAY,
Australia).
Flood discharging over the
Little Nerang dam spillway
(Austraha) before completion.
Fig. VIII - Flow aeration at
the downstream end of a
chute spillway (courtesy of
Hydro-Electric Commission
Tasmania).
View from upstream of the
'white waters' taking off
from the flip bucket, Reece
dam (Australia). Flow from
bottom to top.
.^ -^tn \ ^.
Acknowledgements
The author wants to thank particularly Professor CJ. APELT, University of Queensland, who supported this
project since its beginning and reviewed the manuscript. Professor I.R. WOOD, University of Canterbury, who
initiated the author's interest on the topic and Dr J.M. MICHEL, Institut de Mecanique de Grenoble, who
advised generously the writer for many years. He thanks also Dr G.M. EVANS, University of Newcastle, for his
comments.
The help and assistance of the following students and colleagues must be acknowledged :
Ms J. PATTERSON and QIAO G.L., MM. D. BAXTER, T. BRATTBERG, A. FEITZ, M. MENDEL and D.
SARTOR, Dr P.D. CUMMINGS;
MM. G. ILLIDGE and J. CRACKNELL.
The author wishes to express his gratitude to the following people who made available some photographs of
interest:
Mr H.O. ANWAR, Oxfordshire, United Kingdom,
Companhia Paranaense de Energia (COPEL), Brazil,
Dr P.D. CUMMINGS, Brisbane, Australia,
Professor J. W. HOYT, San Diego State University, USA,
The Hydro-Electric Commission (H.E.C.) of Tasmania, Australia,
Dr J.M. MICHEL, LEGI-IMG, France,
Professor N.L. de S. PINTO, Curitiba, Brazil,
Dr R. RANKIN, Rankin Publishers, Brisbane, Australia,
Professor I.R. WOOD, University of Canterbury, New Zealand.
The author thanks also the following people in providing some information and for the assistance :
Mr H.O. ANWAR, Oxfordshire, United Kingdom; Professor C.J. APELT, University of Queensland, Australia;
Ms CHOU Ya-Hui, Hsin-Yuan, Taiwan R.O.C.; Dr G.M. EVANS, University of Newcastle, Australia;
Professor J.W. HOYT, San Diego State University, USA; Mr S. LI, Hydro-Electric Commission of Tasmania,
Australia; Dr J.M. MICHEL, LEGI-IMG, France; Dr B.S. THANDAVESWARA, Indian Institute of
Technology, Madras; Professor I.R. WOOD, University of Canterbury, New Zealand.
The author acknowledges the support of the Department of Civil Engineering at the University of
Queensland which allowed him to use several experimental facilities.
At last, the author indicates that the monograph was started during a Special Study Program at the National
Cheng Kung University (Taiwan R.O.C.). And he acknowledges the support of the Department of Hydraulics
and Ocean Engineering, National Cheng Kung University (Taiwan R.O.C.) and the assistance of Professor LEE
Jaw-Fang (NCKU) during his stay in Tainan.
About the author
The author received a degree of Ingenieur Hydraulicien' from the Ecole Nationale Superieure d'Hydraulique
et de Mecanique de Grenoble in 1983 and a degree of 'Ingenieur Genie Atomique' from the 'Institut National
des Sciences et Techniques Nucleaires' in 1984. He worked for the industry in France as a R&D engineer at the
Atomic Energy Commission from 1984 to 1986, and as a computer professional in fluid mechanics for
Thomson-CSF between 1989 and 1990. From 1986 to 1988, he studied at the University of Canterbury (New
Zealand) as part of a Ph.D. project.
The author is a senior lecturer in fluid mechanics, hydraulics and environmental engineering at the
University of Queensland since 1990. His research interests include design of hydraulic structures,
experimental investigations of two-phase flowsa nd water quality modelling in coastal and hydraulic structures.
He is the author of "Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways" (Pergamon, 1995)
and his publication record includes over 60 international refereed papers. The author has been active also as
consultant for both governmental agencies and private organisations.