Fundamentals of
Multiphase Flows
Christopher E. Brennen
California Institute of Technology
Pasadena, California
Cambridge University Press 2005
ISBN 0521 848040
1
Preface
The subject of multiphase flows encompasses a vast field, a host of different
technological contexts, a wide spectrum of different scales, a broad range of
engineering disciplines and a multitude of different analytical approaches.
Not surprisingly, the number of books dealing with the subject is volumi-
nous. For the student or researcher in the field of multiphase flow this broad
spectrum presents a problem for the experimental or analytical methodolo-
gies that might be appropriate for his/her interests can be widely scattered
and difficult to find. The aim of the present text is to try to bring much
of this fundamental understanding together into one book and to present
a unifying approach to the fundamental ideas of multiphase flows. Conse-
quently the book summarizes those fundamental concepts with relevance to
a broad spectrum of multiphase flows. It does not pretend to present a com-
prehensive review of the details of any one multiphase flow or technological
context though reference to books providing such reviews is included where
appropriate. This book is targeted at graduate students and researchers at
the cutting edge of investigations into the fundamental nature of multiphase
flows; it is intended as a reference book for the basic methods used in the
treatment of multiphase flows.
I am deeply grateful to all my many friends and fellow researchers in the
field of multiphase flows whose ideas fill these pages. I am particularly in-
debted to my close colleagues, Allan Acosta, Ted Wu, Rolf Sabersky, Melany
Hunt, Tim Colonius and the late Milton Plesset, all of whom made my pro-
fessional life a real pleasure. This book grew out of many years of teaching
and research at the California Institute of Technology. It was my privilege to
have worked on multiphase flow problems with a group of marvelously tal-
ented students including Hojin Ahn, Robert Bernier, Abhijit Bhattacharyya,
David Braisted, Charles Campbell, Steven Ceccio, Luca d’Agostino, Fab-
rizio d’Auria, Mark Duttweiler, Ronald Franz, Douglas Hart, Steve Hostler,
2
Gustavo Joseph, Joseph Katz, Yan Kuhn de Chizelle, Sanjay Kumar, Harri
Kytomaa, Zhenhuan Liu, Beth McKenney, Sheung-Lip Ng, Tanh Nguyen,
Kiam Oey, James Pearce, Garrett Reisman, Y.-C. Wang, Carl Wassgren,
Roberto Zenit Camacho and Steve Hostler. To them I owe a special debt.
Also, to Cecilia Lin who devoted many selfless hours to the preparation of
the illustrations.
A substantial fraction of the introductory material in this book is taken
from my earlier book entitled “Cavitation and Bubble Dynamics” by
Christopher Earls Brennen,
c
1995 by Oxford University Press, Inc. It is
reproduced here by permission of Oxford University Press, Inc.
The original hardback edition of this book was dedicated to my mother,
Muriel M. Brennen, whose love and encouragement inspired me throughout
my life. The paperback edition is dedicated to another very special woman,
my wife Barbara, who gave me new life and love beyond measure.
Christopher Earls Brennen
California Institute of Technology
December
2008.
3
Contents
Preface
page
2
Contents
10
Nomenclature
11
1INTRODUCTIONTOMULTIPHASEFLOW
19
1.1
INTRODUCTION
19
1.1.1
Scope
19
1.1.2
Multiphase flow models
20
1.1.3
Multiphase flow notation
22
1.1.4
Size distribution functions
25
1.2
EQUATIONS OF MOTION
27
1.2.1
Averaging
27
1.2.2
Conservation of mass
28
1.2.3
Number continuity equation
30
1.2.4
Fick’s law
31
1.2.5
Equation of motion
31
1.2.6
Disperse phase momentum equation
35
1.2.7
Comments on disperse phase interaction
36
1.2.8
Equations for conservation of energy
37
1.2.9
Heat transfer between separated phases
41
1.3
INTERACTION WITH TURBULENCE
42
1.3.1
Particles and turbulence
42
1.3.2
Effect on turbulence stability
46
1.4
COMMENTS ON THE EQUATIONS OF MOTION
47
1.4.1
Averaging
47
1.4.2
Averaging contributions to the mean motion
48
1.4.3
Averaging in pipe flows
50
1.4.4
Modeling with the combined phase equations
50
1.4.5
Mass, force and energy interaction terms
51
4
2 SINGLE PARTICLE MOTION
52
2.1
INTRODUCTION
52
2.2
FLOWS AROUND A SPHERE
53
2.2.1
At high Reynolds number
53
2.2.2
At low Reynolds number
56
2.2.3
Molecular effects
61
2.3
UNSTEADY EFFECTS
62
2.3.1
Unsteady particle motions
62
2.3.2
Effect of concentration on added mass
65
2.3.3
Unsteady potential flow
65
2.3.4
Unsteady Stokes flow
69
2.4
PARTICLE EQUATION OF MOTION
73
2.4.1
Equations of motion
73
2.4.2
Magnitude of relative motion
78
2.4.3
Effect of concentration on particle equation of motion 80
2.4.4
Effect of concentration on particle drag
81
3 BUBBLE OR DROPLET TRANSLATION
86
3.1
INTRODUCTION
86
3.2
DEFORMATION DUE TO TRANSLATION
86
3.2.1
Dimensional analysis
86
3.2.2
Bubble shapes and terminal velocities
88
3.3
MARANGONI EFFECTS
91
3.4
BJERKNES FORCES
95
3.5
GROWING BUBBLES
97
4 BUBBLE GROWTH AND COLLAPSE
100
4.1
INTRODUCTION
100
4.2
BUBBLE GROWTH AND COLLAPSE
100
4.2.1
Rayleigh-Plesset equation
100
4.2.2
Bubble contents
103
4.2.3
In the absence of thermal effects; bubble growth
106
4.2.4
In the absence of thermal effects; bubble collapse
109
4.2.5
Stability of vapor/gas
bubbles
110
4.3
THERMAL EFFECTS
113
4.3.1
Thermal effects on growth
113
4.3.2
Thermally controlled growth
115
4.3.3
Cavitation and boiling
118
4.3.4
Bubble growth by mass diffusion
118
4.4
OSCILLATING BUBBLES
120
4.4.1
Bubble natural frequencies
120
5
4.4.2
Nonlinear effects
124
4.4.3
Rectified mass diffusion
126
5 CAVITATION
128
5.1
INTRODUCTION
128
5.2
KEY FEATURES OF BUBBLE CAVITATION
128
5.2.1
Cavitation inception
128
5.2.2
Cavitation bubble collapse
131
5.2.3
Shape distortion during bubble collapse
133
5.2.4
Cavitation damage
136
5.3
CAVITATION BUBBLES
139
5.3.1
Observations of cavitating bubbles
139
5.3.2
Cavitation noise
142
5.3.3
Cavitation luminescence
149
6 BOILING AND CONDENSATION
150
6.1
INTRODUCTION
150
6.2
HORIZONTAL SURFACES
151
6.2.1
Pool boiling
151
6.2.2
Nucleate boiling
153
6.2.3
Film boiling
154
6.2.4
Leidenfrost effect
155
6.3
VERTICAL SURFACES
157
6.3.1
Film boiling
158
6.4
CONDENSATION
160
6.4.1
Film condensation
160
7 FLOW PATTERNS
163
7.1
INTRODUCTION
163
7.2
TOPOLOGIES OF MULTIPHASE FLOW
163
7.2.1
Multiphase flow patterns
163
7.2.2
Examples of flow regime maps
165
7.2.3
Slurry flow regimes
168
7.2.4
Vertical pipe flow
169
7.2.5
Flow pattern classifications
173
7.3
LIMITS OF DISPERSE FLOW REGIMES
174
7.3.1
Disperse phase separation and dispersion
174
7.3.2
Example: horizontal pipe flow
176
7.3.3
Particle size and particle fission
178
7.3.4
Examples of flow-determined bubble size
179
7.3.5
Bubbly or mist flow limits
181
7.3.6
Other bubbly flow limits
182
6
7.3.7
Other particle size effects
183
7.4
INHOMOGENEITY INSTABILITY
184
7.4.1
Stability of disperse mixtures
184
7.4.2
Inhomogeneity instability in vertical flows
187
7.5
LIMITS ON SEPARATED FLOW
191
7.5.1
Kelvin-Helmoltz instability
192
7.5.2
Stratified flow instability
194
7.5.3
Annular flow instability
194
8 INTERNAL FLOW ENERGY CONVERSION
196
8.1
INTRODUCTION
196
8.2
FRICTIONAL LOSS IN DISPERSE FLOW
196
8.2.1
Horizontal Flow
196
8.2.2
Homogeneous flow friction
199
8.2.3
Heterogeneous flow friction
201
8.2.4
Vertical flow
203
8.3
FRICTIONAL LOSS IN SEPARATED FLOW
205
8.3.1
Two component flow
205
8.3.2
Flow with phase change
211
8.4
ENERGY CONVERSION IN PUMPS AND TURBINES 215
8.4.1
Multiphase flows in pumps
215
9 HOMOGENEOUS FLOWS
220
9.1
INTRODUCTION
220
9.2
EQUATIONS OF HOMOGENEOUS FLOW
220
9.3
SONIC SPEED
221
9.3.1
Basic analysis
221
9.3.2
Sonic speeds at higher frequencies
225
9.3.3
Sonic speed with change of phase
227
9.4
BAROTROPIC RELATIONS
231
9.5
NOZZLE FLOWS
233
9.5.1
One dimensional analysis
233
9.5.2
Vapor/liquid nozzle flow
238
9.5.3
Condensation shocks
242
10 FLOWS WITH BUBBLE DYNAMICS
246
10.1 INTRODUCTION
246
10.2 BASIC EQUATIONS
247
10.3 ACOUSTICS OF BUBBLY MIXTURES
248
10.3.1 Analysis
248
10.3.2 Comparison with experiments
250
10.4 SHOCK WAVES IN BUBBLY FLOWS
253
7
10.4.1 Normal shock wave analysis
253
10.4.2 Shock wave structure
256
10.4.3 Oblique shock waves
259
10.5 FINITE BUBBLE CLOUDS
259
10.5.1 Natural modes of a spherical cloud of bubbles
259
10.5.2 Response of a spherical bubble cloud
264
11 FLOWS WITH GAS DYNAMICS
267
11.1 INTRODUCTION
267
11.2 EQUATIONS FOR A DUSTY GAS
268
11.2.1 Basic equations
268
11.2.2 Homogeneous flow with gas dynamics
269
11.2.3 Velocity and temperature relaxation
271
11.3 NORMAL SHOCK WAVE
272
11.4 ACOUSTIC DAMPING
275
11.5 LINEAR PERTURBATION ANALYSES
279
11.5.1 Stability of laminar flow
279
11.5.2 Flow over a wavy wall
280
11.6 SMALL SLIP PERTURBATION
282
12 SPRAYS
285
12.1 INTRODUCTION
285
12.2 TYPES OF SPRAY FORMATION
285
12.3 OCEAN SPRAY
286
12.4 SPRAY FORMATION
288
12.4.1 Spray formation by bubbling
288
12.4.2 Spray formation by wind shear
289
12.4.3 Spray formation by initially laminar jets
292
12.4.4 Spray formation by turbulent jets
293
12.5 SINGLE DROPLET MECHANICS
299
12.5.1 Single droplet evaporation
299
12.5.2 Single droplet combustion
301
12.6 SPRAY COMBUSTION
305
13 GRANULAR FLOWS
308
13.1 INTRODUCTION
308
13.2 PARTICLE INTERACTION MODELS
309
13.2.1 Computer simulations
311
13.3 FLOW REGIMES
312
13.3.1 Dimensional Analysis
312
13.3.2 Flow regime rheologies
313
13.3.3 Flow regime boundaries
316
8
13.4 SLOW GRANULAR FLOW
317
13.4.1 Equations of motion
317
13.4.2 Mohr-Coulomb models
317
13.4.3 Hopper flows
318
13.5 RAPID GRANULAR FLOW
320
13.5.1 Introduction
320
13.5.2 Example of rapid flow equations
322
13.5.3 Boundary conditions
325
13.5.4 Computer simulations
326
13.6 EFFECT OF INTERSTITIAL FLUID
326
13.6.1 Introduction
326
13.6.2 Particle collisions
327
13.6.3 Classes of interstitial fluid effects
329
14 DRIFT FLUX MODELS
331
14.1 INTRODUCTION
331
14.2 DRIFT FLUX METHOD
332
14.3 EXAMPLES OF DRIFT FLUX ANALYSES
333
14.3.1 Vertical pipe flow
333
14.3.2 Fluidized bed
336
14.3.3 Pool boiling crisis
338
14.4 CORRECTIONS FOR PIPE FLOWS
343
15 SYSTEM INSTABILITIES
344
15.1 INTRODUCTION
344
15.2 SYSTEM STRUCTURE
344
15.3 QUASISTATIC STABILITY
347
15.4 QUASISTATIC INSTABILITY EXAMPLES
349
15.4.1 Turbomachine surge
349
15.4.2 Ledinegg instability
349
15.4.3 Geyser instability
350
15.5 CONCENTRATION WAVES
351
15.6 DYNAMIC MULTIPHASE FLOW INSTABILITIES
353
15.6.1 Dynamic instabilities
353
15.6.2 Cavitation surge in cavitating pumps
354
15.6.3 Chugging and condensation oscillations
356
15.7 TRANSFER FUNCTIONS
359
15.7.1 Unsteady internal flow methods
359
15.7.2 Transfer functions
360
15.7.3 Uniform homogeneous flow
362
16 KINEMATIC WAVES
365
16.1 INTRODUCTION
365
9
16.2 TWO-COMPONENT KINEMATIC WAVES
366
16.2.1 Basic analysis
366
16.2.2 Kinematic wave speed at flooding
368
16.2.3 Kinematic waves in steady flows
369
16.3 TWO-COMPONENT KINEMATIC SHOCKS
370
16.3.1 Kinematic shock relations
370
16.3.2 Kinematic shock stability
372
16.3.3 Compressibility and phase change effects
374
16.4 EXAMPLES OF KINEMATIC WAVE ANALYSES
375
16.4.1 Batch sedimentation
375
16.4.2 Dynamics of cavitating pumps
378
16.5 TWO-DIMENSIONAL SHOCKS
383
Bibliography
385
Index
407
10
Nomenclature
Roman letters
a
Amplitude of wave-like disturbance
A
Cross-sectional area or cloud radius
A
Attenuation
b
Power law index
Ba
Bagnold number,
ρ
S
D
2
̇
γ/μ
L
c
Concentration
c
Speed of sound
c
κ
Phase velocity for wavenumber
κ
c
p
Specific heat at constant pressure
c
s
Specific heat of solid or liquid
c
v
Specific heat at constant volume
C
Compliance
C
Damping coefficient
C
D
Drag coefficient
C
ij
Drag and lift coefficient matrix
C
L
Lift coefficient
C
p
Coefficient of pressure
C
pmin
Minimum coefficient of pressure
d
Diameter
d
j
Jet diameter
d
o
Hopper opening diameter
D
Particle, droplet or bubble diameter
D
Mass diffusivity
D
m
Volume (or mass) mean diameter
D
s
Sauter mean diameter
11
D
(
T
)
Determinant of the transfer matrix [
T
]
D
Thermal diffusivity
e
Specific internal energy
E
Rate of exchange of energy per unit volume
f
Frequency in
Hz
f
Friction factor
f
L
,f
V
Liquid and vapor thermodynamic quantities
F
i
Force vector
Fr
Froude number
F
Interactive force per unit volume
g
Acceleration due to gravity
g
L
,g
V
Liquid and vapor thermodynamic quantities
G
Ni
Mass flux of component
N
in direction
i
G
N
Mass flux of component
N
h
Specific enthalpy
h
Height
H
Height
H
Total head,
p
T
/ρg
He
Henry’s law constant
Hm
Haberman-Morton number, normally
gμ
4
/ρS
3
i, j, k, m, n
Indices
i
Square root of
−
1
I
Acoustic impulse
I
Rate of transfer of mass per unit volume
j
i
Total volumetric flux in direction
i
j
Ni
Volumetric flux of component
N
in direction
i
j
N
Volumetric flux of component
N
k
Polytropic constant
k
Thermal conductivity
k
Boltzmann’s constant
k
L
,k
V
Liquid and vapor quantities
K
Constant
K
∗
Cavitation compliance
Kc
Keulegan-Carpenter number
K
ij
Added mass coefficient matrix
K
n
,K
s
Elastic spring constants in normal and tangential directions
Kn
Knudsen number,
λ/
2
R
K
Frictional constants
Typical dimension
12
t
Turbulent length scale
L
Inertance
L
Latent heat of vaporization
m
Mass
̇
m
Mass flow rate
m
G
Mass of gas in bubble
m
p
Mass of particle
M
Mach number
M
∗
Mass flow gain factor
M
ij
Added mass matrix
M
Molecular weight
Ma
Martinelli parameter
n
Number of particles per unit volume
̇
n
Number of events per unit time
n
i
Unit vector in the
i
direction
N
(
R
)
,N
(
D
)
,N
(
v
) Particle size distribution functions
N
∗
Number of sites per unit area
Nu
Nusselt number
p
Pressure
p
T
Total pressure
p
a
Radiated acoustic pressure
p
G
Partial pressure of gas
p
s
Sound pressure level
P
Perimeter
Pe
Peclet number, usually
WR/α
C
Pr
Prandtl number,
ρνc
p
/k
q
General variable
q
i
Heat flux vector
Q
General variable
Q
Rate of heat transfer or release per unit mass
Q
Rate of heat addition per unit length of pipe
r, r
i
Radial coordinate and position vector
r
d
Impeller discharge radius
R
Bubble, particle or droplet radius
R
∗
k
Resistance of component,
k
R
B
Equivalent volumetric radius, (3
τ/
4
π
)
1
3
R
e
Equilibrium radius
Re
Reynolds number, usually 2
WR/ν
C
R
Gas constant
13
s
Coordinate measured along a streamline or pipe centerline
s
Laplace transform variable
s
Specific entropy
S
Surface tension
S
D
Surface of the disperse phase
St
Stokes number
Str
Strouhal number
t
Time
t
c
Binary collision time
t
u
Relaxation time for particle velocity
t
T
Relaxation time for particle temperature
T
Temp erature
T
Granular temperature
T
ij
Transfer matrix
u
i
Velocity vector
u
Ni
Velocity of component
N
in direction
i
u
r
,u
θ
Velocity components in polar coordinates
u
s
Shock velocity
u
∗
Friction velocity
U, U
i
Fluid velocity and velocity vector in absence of particle
U
∞
Velocity of upstream uniform flow
v
Volume of particle, droplet or bubble
V, V
i
Absolute velocity and velocity vector of particle
V
Volume
V
Control volume
̇
V
Volume flow rate
w
Dimensionless relative velocity,
W/W
∞
W, W
i
Relative velocity of particle and relative velocity vector
W
∞
Terminal velocity of particle
W
p
Typical phase separation velocity
W
t
Typical phase mixing velocity
We
Weber number, 2
ρW
2
R/S
W
Rate of work done per unit mass
x, y, z
Cartesian coordinates
x
i
Position vector
x
Mass fraction
X
Mass quality
z
Coordinate measured vertically upward
14
Greek letters
α
Volume fraction
β
Volume quality
γ
Ratio of specific heats of gas
̇
γ
Shear rate
Γ
Rate of dissipation of energy per unit volume
δ
Boundary layer thickness
δ
d
Damping coefficient
δm
Fractional mass
δ
T
Thermal boundary layer thickness
δ
2
Momentum thickness of the boundary layer
δ
ij
Kronecker delta:
δ
ij
=1for
i
=
j
;
δ
ij
=0for
i
=
j
Fractional volume
Coefficient of restitution
Rate of dissipation of energy per unit mass
ζ
Attenuation or amplification rate
η
Bubble population per unit liquid volume
θ
Angular coordinate or direction of velocity vector
θ
Reduced frequency
θ
w
Hopper opening half-angle
κ
Wavenumber
κ
Bulk modulus of compressibility
κ
L
,κ
G
Shape constants
λ
Wavelength
λ
Mean free path
λ
Kolmogorov length scale
Λ
Integral length scale of the turbulence
μ
Dynamic viscosity
μ
∗
Coulombfrictioncoefficient
ν
Kinematic viscosity
ν
Mass-based stoichiometric coefficient
ξ
Particle loading
ρ
Density
σ
Cavitation number
σ
i
Inception cavitation number
σ
ij
Stress tensor
σ
D
ij
Deviatoric stress tensor
Σ(
T
)
Thermodynamic parameter
15
τ
Kolmogorov time scale
τ
i
Interfacial shear stress
τ
n
Normal stress
τ
s
Shear stress
τ
w
Wall shear stress
ψ
Stokes stream function
ψ
Head coefficient, Δ
p
T
/ρ
Ω
2
r
2
d
φ
Velocity potential
φ
Internal friction angle
φ
Flow coefficient,
j/
Ω
r
d
φ
2
L
,φ
2
G
,φ
2
L
0
Martinelli pressure gradient ratios
φ
Fractional perturbation in bubble radius
ω
Radian frequency
ω
a
Acoustic mode frequency
ω
i
Instability frequency
ω
n
Natural frequency
ω
m
Cloud natural frequencies
ω
m
Manometer frequency
ω
p
Peak frequency
Ω
Rotating frequency (radians/sec)
Subscripts
On any variable,
Q
:
Q
o
Initial value, upstream value or reservoir value
Q
1
,Q
2
,Q
3
Components of
Q
in three Cartesian directions
Q
1
,Q
2
Values upstream and downstream of a component or flow structure
Q
∞
Value far from the particle or bubble
Q
∗
Throat values
Q
A
Pertaining to a general phase or component,
A
Q
b
Pertaining to the bulk
Q
B
Pertaining to a general phase or component,
B
Q
B
Value in the bubble
Q
C
Pertaining to the continuous phase or component,
C
Q
c
Critical values and values at the critical point
Q
D
Pertaining to the disperse phase or component,
D
16
Q
e
Equilibrium value or value on the saturated liquid/vapor line
Q
e
Effective value or exit value
Q
G
Pertaining to the gas phase or component
Q
i
Components of vector
Q
Q
ij
Components of tensor
Q
Q
L
Pertaining to the liquid phase or component
Q
m
Maximum value of
Q
Q
N
Pertaining to a general phase or component,
N
Q
O
Pertaining to the oxidant
Q
r
Component in the
r
direction
Q
s
A surface, system or shock value
Q
S
Pertaining to the solid particles
Q
V
Pertaining to the vapor phase or component
Q
w
Value at the wall
Q
θ
Component in the
θ
direction
Superscripts and other qualifiers
On any variable,
Q
:
Q
,Q
,Q
∗
Used to differentiate quantities similar to
Q
̄
Q
Mean value of
Q
or complex conjugate of
Q
`
Q
Small perturbation in
Q
̃
Q
Complex amplitude of oscillating
Q
̇
Q
Time derivative of
Q
̈
Q
Second time derivative of
Q
ˆ
Q
(
s
)
Laplace transform of
Q
(
t
)
̆
Q
Coordinate with origin at image point
δQ
Small change in
Q
Re
{
Q
}
Real part of
Q
Im
{
Q
}
Imaginary part of
Q
17
NOTES
Notation
The reader is referred to section 1.1.3 for a more complete description of
the multiphase flow notation employed in this book. Note also that a few
symbols that are only used locally in the text have been omitted from the
above lists.
Units
In most of this book, the emphasis is placed on the nondimensional pa-
rameters that govern the phenomenon being discussed. However, there are
also circumstances in which we shall utilize dimensional thermodynamic and
transport properties. In such cases the International System of Units will be
employed using the basic units of mass (
kg
), length (
m
), time (
s
), and ab-
solute temperature (
K
).
18
1
INTRODUCTION TO MULTIPHASE FLOW
1.1 INTRODUCTION
1.1.1 Scope
In the context of this book, the term
multiphase flow
is used to refer to
any fluid flow consisting of more than one phase or component. For brevity
and because they are covered in other texts, we exclude those circumstances
in which the components are well mixed above the molecular level. Conse-
quently, the flows considered here have some level of phase or component
separation at a scale well above the molecular level. This still leaves an
enormous spectrum of different multiphase flows. One could classify them
according to the state of the different phases or components and therefore
refer to gas/solids flows, or liquid/solids flows or gas/particle flows or bubbly
flows and so on; many texts exist that limit their attention in this way. Some
treatises are defined in terms of a specific type of fluid flow and deal with
low Reynolds number suspension flows, dusty gas dynamics and so on. Oth-
ers focus attention on a specific application such as slurry flows, cavitating
flows, aerosols, debris flows, fluidized beds and so on; again there are many
such texts. In this book we attempt to identify the basic fluid mechanical
phenomena and to illustrate those phenomena with examples from a broad
range of applications and types of flow.
Parenthetically, it is valuable to reflect on the diverse and ubiquitous chal-
lenges of multiphase flow. Virtually every processing technology must deal
with multiphase flow, from cavitating pumps and turbines to electropho-
tographic processes to papermaking to the pellet form of almost all raw
plastics. The amount of granular material, coal, grain, ore, etc. that is trans-
ported every year is enormous and, at many stages, that material is required
to flow. Clearly the ability to predict the fluid flow behavior of these pro-
cesses is central to the efficiency and effectiveness of those processes. For
19
example, the effective flow of toner is a major factor in the quality and speed
of electrophotographic printers. Multiphase flows are also a ubiquitous fea-
ture of our environment whether one considers rain, snow, fog, avalanches,
mud slides, sediment transport, debris flows, and countless other natural
phenomena to say nothing of what happens beyond our planet. Very critical
biological and medical flows are also multiphase, from blood flow to semen
to
the bends
to lithotripsy to laser surgery cavitation and so on. No single
list can adequately illustrate the diversity and ubiquity; consequently any
attempt at a comprehensive treatment of multiphase flows is flawed unless
it focuses on common phenomenological themes and avoids the temptation
to digress into lists of observations.
Two general topologies of multiphase flow can be usefully identified at
the outset, namely
disperse flows
and
separated flows
.By
disperse flows
we mean those consisting of finite particles, drops or bubbles (the disperse
phase) distributed in a connected volume of the continuous phase. On the
other hand
separated flows
consist of two or more continuous streams of
different fluids separated by interfaces.
1.1.2 Multiphase flow models
A persistent theme throughout the study of multiphase flows is the need to
model and predict the detailed behavior of those flows and the phenomena
that they manifest. There are three ways in which such models are explored:
(1) experimentally, through laboratory-sized models equipped with appro-
priate instrumentation, (2) theoretically, using mathematical equations and
models for the flow, and (3) computationally, using the power and size of
modern computers to address the complexity of the flow. Clearly there are
some applications in which full-scale laboratory models are possible. But,
in many instances, the laboratory model must have a very different scale
than the prototype and then a reliable theoretical or computational model
is essential for confident extrapolation to the scale of the prototype. There
are also cases in which a laboratory model is impossible for a wide variety
of reasons.
Consequently, the predictive capability and physical
understanding must
rely heavily on theoretical and/or computational models and here the com-
plexity of most multiphase flows presents a major hurdle. It may be possible
at some distant time in the future to code the Navier-Stokes equations for
each of the phases or components and to compute every detail of a multi-
phase flow, the motion of all the fluid around and inside every particle or
drop, the position of every interface. But the computer power and speed
20
required to do this is far beyond present capability for most of the flows
that are commonly experienced. When one or both of the phases becomes
turbulent (as often happens) the magnitude of the challenge becomes truly
astronomical. Therefore, simplifications are essential in realistic models of
most multiphase flows.
In disperse flows two types of models are prevalent,
trajectory models
and
two-fluid models
. In trajectory models, the motion of the disperse phase is
assessed by following either the motion of the actual particles or the motion
of larger, representative
particles
. The details of the flow around each of the
particles are subsumed into assumed drag, lift and moment forces acting on
and altering the trajectory of those particles. The thermal history of the
particles can also be tracked if it is appropriate to do so. Trajectory mod-
els have been very useful in studies of the rheology of granular flows (see
chapter 13) primarily because the effects of the interstitial fluid are small. In
the alternative approach,
two-fluid models
, the disperse phase is treated as
a second continuous phase intermingled and interacting with the continuous
phase. Effective conservation equations (of mass, momentum and energy) are
developed for the two fluid flows; these included interaction terms modeling
the exchange of mass, momentum and energy between the two flows. These
equations are then solved either theoretically or computationally. Thus, the
two-fluid models neglect the discrete nature of the disperse phase and ap-
proximate its effects upon the continuous phase. Inherent in this approach,
are averaging processes necessary to characterize the properties of the dis-
perse phase; these involve significant difficulties. The boundary conditions
appropriate in two-fluid models also pose difficult modeling issues.
In contrast, separated flows present many fewer issues. In theory one must
solve the single phase fluid flow equations in the two streams, coupling them
through appropriate kinematic and dynamic conditions at the interface. Free
streamline theory (see, for example, Birkhoff and Zarantonello 1957, Tulin
1964, Woods 1961, Wu 1972) is an example of a su
ccessful implementation
of such a strategy though the interface conditions used in that context are
particularly simple.
In the first part of this book, the basic tools for both trajectory and
two-fluid models are developed and discussed. In the remainder of this first
chapter, a basic notation for multiphase flow is developed and this leads
naturally into a description of the mass, momentum and energy equations
applicable to multiphase flows, and, in particular, in two-fluid models. In
chapters 2, 3 and 4, we examine the dynamics of individual particles, drops
and bubbles. In chapter 7 we address the different topologies of multiphase
21
flows and, in the subsequent chapters, we examine phenomena in which
particle
interactions and the particle-fluid interactions modify the flow.
1.1.3 Multiphase flow notation
The notation that will be used is close to the standard described by Wallis
(1969). It has however been slightly modified to permit more ready adop-
tion to the Cartesian tensor form. In particular the subscripts that can be
attached to a property will consist of a group of uppercase subscripts fol-
lowed by lowercase subscripts. The lower case subscripts (
i
,
ij
,etc.)are
used in the conventional manner to denote vector or tensor components. A
single uppercase subscript (
N
) will refer to the property of a specific phase
or component. In some contexts generic subscripts
N
=
A, B
will be used
for generality. However, other letters such as
N
=
C
(continuous phase),
N
=
D
(disperse phase),
N
=
L
(liquid),
N
=
G
(gas),
N
=
V
(vapor) or
N
=
S
(solid) will be used for clarity in other contexts. Finally two upper-
case subscripts will imply the difference between the two properties for the
two single uppercase subscripts.
Specific properties frequently used are as follows.
Volumetric fluxes
(vol-
ume flow per unit area) of individual components will be denoted by
j
Ai
,j
Bi
(
i
= 1, 2 or 3 in three dimensional flow). These are sometimes referred to as
superficial component velocities. The
total volumetric flux
,
j
i
is then given
by
j
i
=
j
Ai
+
j
Bi
+
...
=
N
j
Ni
(1.1)
Mass fluxes
are similarly denoted by
G
Ai
,G
Bi
or
G
i
. Thus if the densities
of individual components are denoted by
ρ
A
,ρ
B
it follows that
G
Ai
=
ρ
A
j
Ai
;
G
Bi
=
ρ
B
j
Bi
;
G
i
=
N
ρ
N
j
Ni
(1.2)
Velocities of the specific phases are denoted by
u
Ai
,u
Bi
or, in general, by
u
Ni
. The relative velocity between the two phases
A
and
B
will be denoted
by
u
ABi
such that
u
Ai
−
u
Bi
=
u
ABi
(1.3)
The volume fraction of a component or phase is denoted by
α
N
and, in
the case of two components or phases,
A
and
B
, it follows that
α
B
=1
−
α
A
. Though this is clearly a well defined property for any finite volume in
the flow, there are some substantial problems associated with assigning a
22
value to an infinitesimal volume or point in the flow. Provided these can
be resolved, it follows that the volumetric flux of a component,
N
,andits
velocity are related by
j
Ni
=
α
N
u
Ni
(1.4)
and that
j
i
=
α
A
u
Ai
+
α
B
u
Bi
+
...
=
N
α
N
u
Ni
(1.5)
Two other fractional properties are only relevant in the context of one-
dimensional flows. The
volumetric quality
,
β
N
, is the ratio of the volumetric
flux of the component,
N
, to the total volumetric flux, i.e.
β
N
=
j
N
/j
(1.6)
where the index
i
has been dropped from
j
N
and
j
because
β
is only used in
the context of one-dimensional flows and the
j
N
,
j
refer to cross-sectionally
averaged quantities.
The
mass fraction
,
x
A
, of a phase or component,
A
, is simply given by
ρ
A
α
A
/ρ
(see equation 1.8 for
ρ
). On the other hand the
mass quality
,
X
A
,
is often referred to simply as
the quality
and is the ratio of the mass flux of
component,
A
, to the total mass flux, or
X
A
=
G
A
G
=
ρ
A
j
A
N
ρ
N
j
N
(1.7)
Furthermore, when only two components or phases are present it is often
redundant to use subscripts on the volume fraction and the qualities since
α
A
=1
−
α
B
,β
A
=1
−
β
B
and
X
A
=1
−X
B
. Thus unsubscripted quanti-
ties
α
,
β
and
X
will often be used in these circumstances.
It is clear that a multiphase mixture has certain
mixture
properties of
which the most readily evaluated is the
mixture
density denoted by
ρ
and
given by
ρ
=
N
α
N
ρ
N
(1.8)
On the other hand the specific enthalpy,
h
, and specific entropy,
s
,being
defined as per unit mass rather than per unit volume are weighted according
to
ρh
=
N
ρ
N
α
N
h
N
;
ρs
=
N
ρ
N
α
N
s
N
(1.9)
23
Other properties such as the
mixture
viscosity or thermal conductivity can-
not be reliably obtained from such simple weighted means.
Aside from the relative velocities between phases that were described ear-
lier, there are two other measures of relative motion that are frequently
used. The
drift velocity
of a component is defined as the velocity of that
component in a frame of reference moving at a velocity equal to the total
volumetric flux,
j
i
, and is therefore given by,
u
NJi
,where
u
NJi
=
u
Ni
−
j
i
(1.10)
Even more frequent use will be made of the
drift flux
of a component which
is defined as the volumetric flux of a component in the frame of reference
moving at
j
i
. Denoted by
j
NJi
this is given by
j
NJi
=
j
Ni
−
α
N
j
i
=
α
N
(
u
Ni
−
j
i
)=
α
N
u
NJi
(1.11)
It is particularly important to notice that the sum of all the drift fluxes must
be zero since from equation 1.11
N
j
NJi
=
N
j
Ni
−
j
i
N
α
N
=
j
i
−
j
i
= 0
(1.12)
When only two phases or components,
A
and
B
, are present it follows that
j
AJi
=
−
j
BJi
and hence it is convenient to denote both of these drift fluxes
by the vector
j
ABi
where
j
ABi
=
j
AJi
=
−
j
BJi
(1.13)
Moreover it follows from 1.11 that
j
ABi
=
α
A
α
B
u
ABi
=
α
A
(1
−
α
A
)
u
ABi
(1.14)
and hence the drift flux,
j
ABi
and the relative velocity,
u
ABi
,aresimply
related.
Finally, it is clear that certain basic relations follow from the above def-
initions and it is convenient to identify these here for later use. First the
relations between the volume and mass qualities that follow from equations
1.6 and 1.7 only involve ratios of the densities of the components:
X
A
=
β
A
/
N
ρ
N
ρ
A
β
N
;
β
A
=
X
A
/
N
ρ
A
ρ
N
X
N
(1.15)
On the other hand the relation between the volume fraction and the volume
quality necessarily involves some measure of the relative motion between
the phases (or components). The following useful results for two-phase (or
24
two-component) one-dimensional flows can readily be obtained from 1.11
and 1.6
β
N
=
α
N
+
j
NJ
j
;
β
A
=
α
A
+
j
AB
j
;
β
B
=
α
B
−
j
AB
j
(1.16)
which demonstrate the importance of the drift flux as a measure of the
relative motion.
1.1.4 Size distribution functions
In many multiphase flow contexts we shall make the simplifying assumption
that all the disperse phase particles (bubbles, droplets or solid particles)
have the same size. However in many natural and technological processes it
is necessary to consider the distribution of particle size. One fundamental
measure of this is the size distribution function,
N
(
v
), defined such that
the number of particles in a unit volume of the multiphase mixture with
volume between
v
and
v
+
dv
is
N
(
v
)
dv
. For convenience, it is often assumed
that the particles size can be represented by a single linear dimension (for
example, the diameter,
D
,orradius,
R
, in the case of spherical particles) so
that alternative size distribution functions,
N
(
D
)or
N
(
R
), may be used.
Examples of size distribution functions based on radius are shown in figures
1.1 and 1.2.
Often such information is presented in the form of cumulative number
distributions. For example the cumulative distribution,
N
∗
(
v
∗
), defined as
N
∗
(
v
∗
)=
v
∗
0
N
(
v
)
dv
(1.17)
is the total number of particles of volume less than
v
∗
. Examples of cumu-
lative distributions (in this case for coal slurries) are shown in figure 1.3.
In these disperse flows, the evaluation of global quantities or characteris-
tics of the disperse phase will clearly require integration over the full range
of particle sizes using the size distribution function. For example, the volume
fraction of the disperse phase,
α
D
,isgivenby
α
D
=
∞
0
vN
(
v
)
dv
=
π
6
∞
0
D
3
N
(
D
)
dD
(1.18)
where the last expression clearly applies to spherical particles. Other prop-
erties of the disperse phase or of the interactions between the disperse and
continuous phases can involve other moments of the size distribution func-
tion (see, for example, Friedlander 1977). This leads to a series of mean
25