Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 10.1.5 - Bubble Breakup in Euler-Lagrangian Simulations of Bubbly Flow
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 Material Information
Title: 10.1.5 - Bubble Breakup in Euler-Lagrangian Simulations of Bubbly Flow Bubbly Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Lau, Y.M.
Deen, N.G.
Kuipers, J.A.M.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: Euler-Lagrange simulations
bubbly flow
breakup
 Notes
Abstract: A basic model for bubble breakup was developed and implemented in an Euler-Lagrange model. In this approach, bubbles are tracked by solving Newton’s second law of motion. Bubble-bubble collisions are accounted for by means of a hard-sphere model developed by Hoomans et al. (1996), and coalescence is modeled with the model proposed by Darmana et al. (2001). Bubble breakup is based on comparing the bubble Weber number with a critical value, which depends on the properties of the surrounding liquid and the bubble shape. Simulations were performed for a square bubble column, which was studied earlier experimentally by Deen (2001), with superficial gas velocities of 5 mm=s. The results of the lower velocity showed reasonable good agreement compared with measurements for liquid and bubble velocities.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
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Bibliographic ID: UF00102023
Volume ID: VID00245
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: 1015-Lau-ICMF2010.pdf

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7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


Bubble Breakup in Euler-Lagrange Simulations of Bubbly Flow


Y.M. Lau, N.G. Deen* and J.A.M. Kuipers

Fundamentals of Chemical Reaction Engineering, IMPACT, University of Twente, Enschede, The Netherlands
Corresponding author: N.G.Deen@tnw.utwente.nl
Keywords: Euler-Lagrange simulations, bubbly flow, breakup




Abstract

A basic model for bubble breakup was developed and implemented in an Euler-Lagrange model. In this approach,
bubbles are tracked by solving Newton's second law of motion. Bubble-bubble collisions are accounted for by means
of a hard-sphere model developed by Hoomans et al. (1996), and coalescence is modeled with the model proposed by
Darmana et al. (2001). Bubble breakup is based on comparing the bubble Weber number with a critical value, which
depends on the properties of the surrounding liquid and the bubble shape. Simulations were performed for a square
bubble column, which was studied earlier experimentally by Deen (2001), with superficial gas velocities of 5 mmn/s.
The results of the lower velocity showed reasonable good agreement compared with measurements for liquid and
bubble velocities.


Nomenclature


Roman symbols
C coefficient (m)
D diameter (m)
E bubble aspect ratio (-)
E6 Eitvos number (-)
F force vector (N)
IPI interferometric particle imaging
M Morton number
PIV particle image velocimetry
Q rate of change of the distribution function (-)
R radius (m)
V volume (m3)
We Weber number (-)
m number of daughter bubbles (-)
n normal vector (-)
n number of bubbles (-)
g gravitational constant (nms1)
h film thickness (m)
p pressure (Nm 2)
t time (s)
u liquid velocity vector (nms 1)
v bubble velocity vector (nms 1)
x location vector (m)


Greek symbols
Q breakup frequency (s 1)
ID interphase transfer source term (Nm 3)
3 daughter size distribution (-)
e fraction (-)
p density (kgmn3)
a surface tension (Nm 1)
T stress tensor (Nm 2)
C critical Weber value correction (-)
Subscripts
D drag
G gravity
L lift
P pressure
S source term for number density
VM virtual mass
W wall
b bubble
break breakup
coal coalescence
coll collision
crit critical
1 liquid
ph phase change







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


Introduction


equations of motion are written as:


Due to the computational costs industrial bubbly flows
are usually modeled with an Euler-Euler model, also
known as the Two-Fluid Model (TFM). Here the liq-
uid and dispersed phase are both treated as continuous
phases with interfacial closures. However, typical two
fluid models are less suited to handle complex phenom-
ena such as coalescence and breakup, because only one
characteristic size of the dispersed phase is used. For
that reason a population balance equation (PBE) is con-
sidered for handling the variations in the size and shape
distributions of the dispersed phase statistically.
On the contrary, with an Euler-Lagrange model, the
so-called Discrete Bubble Model (DBM), the bubble
size distribution can be determined in a straightforward
manner, provided that coalescence and breakup mod-
els are incorporated. Darmana et al. (2001) already
considered coalescence, while Van den Hengel et al.
(2005) applied stochastic models for both coalescence
and breakup mechanisms. To improve the modeling of
the breakup of bubbles a fundamental breakup model is
developed and described in this paper. The model is sub-
sequently tested for the case of a square bubble column.


Discrete Bubble model

In the DBM each individual bubble is treated in a La-
grangian manner, while the liquid phase motion is de-
scribed on an Eulerian grid, taking into account the cou-
pling or interaction between the gas and the liquid phase.

Liquid phase hydrodynamics. The liquid phase is rep-
resented by the volume-averaged Navier-Stokes equa-
tions, which consist of the continuity and momentum
equations:


-(Cfii) +V cipu 0
at


09
-(e6P1U)+V-.e p quu = -etVP- Vt-e1T+ep1g+<
(2)
The presence of the bubbles is reflected by the liquid
phase volume fraction et and the total interphase mo-
mentum transfer i due to action of the interface forces
between the liquid and the bubbles. The liquid phase
flow is assumed to be Newtonian and a subgrid-scale
model by Vreman (2004) is employed for the turbulence.

Bubble dynamics. The bubbles are tracked by solving
the 2nd law of Newton for each individual bubble. In-
terfacial forces are taken into account by the net force
E F, experienced by each individual bubble. Then the


dxb dv
dt v P9dt


with Xb the location, v the velocity and Vb the volume
of the bubble.
The net force acting on each individual bubble is cal-
culated considering all the relevant forces (see table 1),
which are separate and uncoupled distributions originat-
ing from far field gravity, pressure, drag1, lift2, virtual
mass3 and wall-interaction4:

SF F F + F FD+FL FVM+ FW (4)


Collision/Coalescence. Collision and coalescence of
bubbles are treated with a model described by Darmana
et al. (2001), who combined the hard sphere collision
model by Hoomans et al. (1996) and the coalescence
model by Sommerfeld et al. (2003).
For a given bubble collision pair a and b, the film-
drainage time for coalescence to occur is calculated as:


tab pi 11
16o


h}
hf


where the ho is the initial film thickness and hf the final
film thickness just before film breakage. The equivalent
bubble radius for a system of two different sized bubbles
is obtained from:


a1 1b
R2 R


The contact time between two bubbles is calculated by
assuming that it is proportional to a deformation distance
divided by the normal component of the collision veloc-
ity:
t CcRab
tab v, v (7)

where Cc represents the deformation distance normal-
ized by the effective bubble radius.
The criteria for the occurrence of coalescence or col-
lision (and bounce off) is determined by the ratio of the
contact time and the film drainage time. When the con-
tact time is less than the film drainage time (tab < tab)
coalescence will not occur and the bubbles will bounce
off each other. In the other case (tab > tab) coales-
cence will commence and a new coalesced bubble is cal-
culated.
'Ishii & Zuber (1979)
2Auton (1983)
3Auton (1983)
4Tomiyama et al. (1995)







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


Table 1: Overview of forces acting on a bubble.
Force Closure
FG pggVb
Fp -VbVP
FD -Cv,. -IV UI(v U) CD = uEO6-5
FL -CLPVb(V u) x (V x u) CL 0.5
FVM -- (cvMflVbDt"u+ CVMPlVb(V u ) CVM = 0.5
S- d[ 1 1 i. f exp(-0.933Eo+ 0.179) 1< E6 < 5
-C 2 (L-y j -u) n O2y 0.007E6i+0.04 5 < E6 < 33


Breakup models in literature

Before deriving a breakup model for the DBM, breakup
models in literature are evaluated on the basis of suitable
implementation in the DBM.
Breakup has several causes: thermodynamic mech-
anisms, such as cavitation collapse, and fluid dy-
namic mechanisms, such as high relative veloci-
ties/acceleration, resonance and turbulence. Most
breakup models in literature, as described in reviews by
Lasheras et al. (2002) and Liao (2009), are statistical
models and consider breakup due to turbulence. These
statistical breakup models are only applicable in TFM,
where the continuous phase is described in the Eulerian
framework with the k E turbulence model and where
a PBE is solved for the dispersed phase. This PBE de-
scribes the temporal and spatial rate of change of a dis-
tribution function n(D, x, v, t), which is defined as the
probable number of particles (bubbles) with diameter D,
located at x, with a velocity v, at time t:

otn + Vx (vn) + Vv (Fn) = (8)
Sph + Scoll + Scoal + Sbreak

where the rates of change of n with time due to breakup,
coalescence and collision are denoted by Sbreak, Scoal
and Sco11. The force per unit mass acting on a particle
(bubble) is denoted by F, and the rate of change with
time of its diameter due to evaporation, condensation or
dissolution is given by Sph. The breakage kernel Sbreak
is usually modeled as:

Sbreak (D)


j m(Do)",(D, Don)(Do)n(D,t)dD

(D)n(D,t) (9)

where m(Do) represents the mean number of bubbles
resulting after the breakage of a mother bubble hav-
ing the diameter Do, 3(D, Do) is the distribution of
the daughter bubbles formed after the fragmentation of


a mother bubble with diameter Do and (2(D) is the
breakup rate of bubbles with diameter D.
The breakup rate in the breakup kernel of a PBE is
expressed as the breakup fraction multiplied by the fre-
quency. The breakup fraction is usually modeled in
terms of a so-called activation complex, e.g. in the
Prince & Blanch (1990) model the activation complex
is a function of the ratio between surface energy and the
mean turbulent kinetic energy. The frequency is deter-
mined by the collision rate between the bubbles and the
turbulent eddies.
For a complete breakup model, one must include
an expression for the size distribution 3(D, Do) of the
daughter bubbles resulting from breakup of a mother
bubble of size Do in addition to a model for (2(D). This
term can be formulated with: statistical models, phe-
nomenological models based on the change in surface
energy of a breaking bubble and hybrid models which
are based on a combination of both. In all cases, both
the shape of the daughter probability density function
(pdf) and the number of daughter bubbles formed by a
breakup event, m(Do), must be determined. The latter
is done by either assuming a given number of daughter
bubbles a priori, or by deriving an empirical relation for
the number of daughter bubbles from available experi-
mental data.


Breakup model for the DBM

From the breakup models in literature none is really suit-
able for implementation in the DBM, because all models
are derived for the PBE and breakup is determined statis-
tically rather than deterministically. Here a fundamental
breakup model is derived for the DBM with the same
structure as the ones from literature: a breakup criteria
and a daughter bubble distribution.

Breakup criteria. Hinze (1955) is one of the first to
establish a breakup theory by introducing a dimension-
less ratio between the force which causes deformation
and the surface tension which tends to restore the bubble












Table 2: A few experimentally derived critical Weber
values in literature.
Flow conditions Critical Weber value

Turbulent pipeflow 1.1 (t)
Microgravity conditions 5
Balancing drag and buoyancy 0.25
Turbulent upward jet 2.5


sphericity. In turbulent flows the deformation is induced
by inertia and the dimensionless number is known as the
Weber number:

We = p 2d (10)

where 6u2 is the mean square velocity difference over
a characteristic distance. Assuming that only velocity
fluctuations over a distance close to the bubble diameter
are capable of causing large deformations, the character-
istic distance is taken equal to the bubble diameter.
The theory implies that there exists a critical Weber
value above which the bubble breaks up. Various exper-
imental investigations (see table 2) have been conducted
in search for this critical value, e.g. pipeflow5, micro-
gravity6, drag-buoyancy balance7, upward jet8, etc.
For the DBM a critical value is derived from the force
balance between external stresses from the continuous
phase, which attempt to destroy the bubble, and the sur-
face stress of the bubble plus the viscous stress of the
fluid inside it, which restore its form. Since the surface
stress (surface energy) is dependent on the surface area
of the bubble, this critical value incorporates a correction
factor, which is a function of the bubble aspect ratio. The
critical Weber value is as follows:


We = 12 ((E)


(1+ 2E1 6075) -02213
3E10717 )
and from Clift et al. (1978):


1
1 + 0.1631. "


(E6 < 40, M < 10 6)


Daughter bubble distribution. Binary breakup of bub-
bles is assumed and the breakup fraction, that deter-
mines the volumes of the daughter bubbles is derived
5Hesketh et al. (1991)
6Risso (2000)
7Senhaji (1993)
8Sevik & Park (1973)


7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


from a random number distribution, for which an U-
shaped beta probability density function (see Figure 1)
is chosen. This function assumes that the probability of
equal volume breakup is smaller than breakup in a small
and a large daughter bubble. As for the locations, the
larger daughter is placed at the same location as the par-
ent bubble, with the smaller daughter bubble randomly
placed around it (see figure 2).


10
9
8
7
6
LL
o 5
0-
4
3
2
1
0
0 0.2 0.4 0.6 0.8
Relative daughter bubble volume


Figure 1: Beta probability density function of the
daughter bubble size distribution.


/


Figure 2: Illustration of the binary breakup process of
a parent bubble in two daughter distribution, with the
larger bubble at the original location and the smaller
bubble randomly placed around it.



Results and discussion


Liquid and bubble velocities. Simulations were per-
formed for the experimental setup of Deen (2001),
who performed PIV measurements in a square bubble-
column with a perforated bottom-plate and a superficial
gas velocity of 5 mm/s. The inlet bubble diameter was
set to 1.5 mm and a snapshot of the DBM simulation is
illustrated in figure 3, where various data (gas-fraction,
bubble radii, locations and velocities) are plotted. To
evaluate the DBM results, liquid and bubble velocities of
experiment and simulation are compared at three differ-
ent heights (0.15m, 0.25m and 0.35m) within the bub-
ble column. The averaged vertical liquid velocities are
plotted in figure 4, along with its standard deviation in










7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010


Sasfroction
0.44

0.37

0.29

0.22

0.15

0.07

0.00


radius (m)
5.2e-03 ..;

4.4e-03

3.6e-03

2.8e-03

2.0e-03

1.2e-03

3.7e-04


r-I ..r, (m/s)
:0 39

': J49

0.359

0.269

0.180

0.090

0.000


Figure 3: Simulation results for a superficial gas velocity of 5 mm/s: (a) gas fraction; (b) bubble locations and radii;

and (c) liquid velocity flow field.


(a) height = 0.15m (b) height = 0.25m (c) height = 0.35m


Figure 4: Time-averaged vertical liquid velocities at different heights.


+ + + + + + +
018+ +
016
014 +
012
S 01
.8 oo
006
004
002 operment +
simulaton -


+ + + ++++










percent +
simulation -


02 04 06 08 1 0 02 04 06 08 1 0 02 04 06
XD XD XD

(a) height = 0.15m (b) height = 0.25m (c) height = 0.35m


Figure 5: Standard deviation of the vertical liquid velocities at different heights.


+ + +
+ + + +


005







7th International Conference on Multiphase Flow,
ICMF 2010, Tampa, FL, May 30 -June 4, 2010



mula t35 ----------n-


(a) height = 0.15m (b) height = 0.25m (c) height = 0.35m

Figure 6: Time-averaged vertical bubble velocities at different heights.


18000

16000

14000

12000

10000

8000

6000

4000

2000

0


experiment
simulation


0.002 0.004 0.006 0.008 0.01 0.012


diameter[m]

Figure 7: Comparison between bubble size distributions from experiment and simulation at a superficial gas velocity
of 5 mm/s.


figure 5 and the averaged vertical bubble velocities in
figure 6. The averaged liquid velocities at the bottom of
the column show good agreement with the experiment,
while in the higher region of the column the values are
underpredicted. This also holds for the standard devia-
tion, but differs from the averaged vertical bubble veloc-
ities. Here there is overprediction in the lower region,
while in the higher region the velocities at the centerline
show good agreement. A reason for the differences be-
tween simulation and experiments could be due to the
applied closure relations, which are strictly only valid
for single bubbles. Using closure relations derived for
heterogeneous bubbly flows could yield a better predic-
tion of the liquid and bubble velocities.



Bubble size distribution. Hansen (2009) used the in-
terferometric particle imaging technique (IPI) to mea-


sure in the same setup with similar settings the bubble
size distribution. The experimental and simulation re-
sults are plotted in figure 7. The experimental data con-
tains more smaller bubbles than the simulations, imply-
ing that the simulation might require a better definition
of the boundary condition (inlet bubble diameter). Be-
sides this difference, the bubble size distribution of the
simulation does show the same qualitative trend with the
experiment.

Conclusions

A fundamental breakup model has been successfully for-
mulated and implemented. The obtained simulation re-
sults shows that the DBM extended with a coalescence
and a breakup model can be used to study the bubble size
distribution. For the case of a low superficial gas veloc-
ity the trend of the bubble size distribution agrees well


expsr,%; __


34 06











with the experiments. Velocities of liquid and bubble
show some agreement. However, simulations with im-
proved closure relations are still needed for better eval-
uation. For cases of high superficial gas velocity more
investigations are still required.


Acknowledgements

This project is part of the Industrial Partnership Program
"Fundamentals of Heterogeneous Bubbly Flow", which
is funded by FOM, AkzoNobel, Corus, DSM and Shell.


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