Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 15.7.2 - Single vapor bubble behavior in a shear flow in microgravity
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 Material Information
Title: 15.7.2 - Single vapor bubble behavior in a shear flow in microgravity Boiling
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Yoshikawa, H.N.
Colin, C.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: flow boiling
microgravity
bubble dynamics
 Notes
Abstract: Behavior of single vapor bubbles on a wall sheared by a two-dimensional flow is investigated. Experiments are performed in a micro-gravity environmentwith a subcooled test liquid HFE-7000 at a low pressure ( 1-2 bar). Vapor bubbles are created by heating the liquid locally above the saturation temperature by a bubble generator on the wall designed for nucleation at an isolated site. These bubbles grow at this nucleation site under an imposed shear flow and depart downstream either along the wall or into the liquid. Geometric and kinematic features of the bubbles in the proximity of the nucleation site are measured by processing images obtained by optical observation. Different forces acting on the bubbles are calculated from these measurements to characterize the bubble departure from the view point of the force balance. The validity of the mechanistic approach to the bubble behavior is discussed.
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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Volume ID: VID00389
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Resource Identifier: 1572-Yoshikawa-ICMF2010.pdf

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


Single vapor bubble behavior in a shear flow in microgravity


H.N.Yoshikawa* and C. Colin*

Institute de M6canique des Fluides de Toulouse, All6e Prof. C. Soula, Toulouse, France
harunori@tunice.fr and colin@timft.fr
Keywords: Flow boiling, microgravity, bubble dynamics




Abstract

Behavior of single vapor bubbles on a wall sheared by a two-dimensional flow is investigated. Experiments are
performed in a micro-gravity environment with a subcooled test liquid HFE-7000 at a low pressure (~ 1-2 bar). Vapor
bubbles are created by heating the liquid locally above the saturation temperature by a bubble generator on the wall
designed for nucleation at an isolated site. These bubbles grow at this nucleation site under an imposed shear flow
and depart downstream either along the wall or into the liquid. Geometric and kinematic features of the bubbles in
the proximity of the nucleation site are measured by processing images obtained by optical observation. Different
forces acting on the bubbles are calculated from these measurements to characterize the bubble departure from the
view point of the force balance. The validity of the mechanistic approach to the bubble behavior is discussed.


Introduction

Nucleate boiling is an efficient mode of heat energy
transfer. Its application in orbital systems is waited for
removing heat from electronic devices with large power
consumption. Many studies have been devoted to the
heat transfer at a heating wall of different geometries
in the nucleate boiling regime on ground, see, e.g., Col-
lier and Thome (1996). However, because of the dom-
inant effect of the buoyancy due to the gravity in the
vapor bubble dynamics, these results cannot be extrap-
olated to the heat transfer in microgravity. In order to
predict the heat transfer in different gravity conditions,
the development of mechanistic models is very promis-
ing. Kurul and Podowski (1990), Basu et al. (2005) and
Yeoh et al. (2008)) presented these models, in which
three main contributions are identified in the total heat
flux from a heating wall to flowing liquid: the latent heat
during bubble vaporization, the unsteady heat diffusion
after a bubble departure associated with the restitution
of a thermal layer in the subcooled liquid adjacent to
the wall, and the convective heat transfer between nu-
cleation sites. The modeling of these different contribu-
tions depends on the bubble size (radius R,) at detach-
ment from the wall, the frequency of formation and the
density of active nucleation sites. Among these quanti-
ties, the bubble detachment size R, is very sensitive to
the gravitational environment. In order to predict R,
the bubble behavior in the proximity of the wall is of-
ten considered by force balance models as seen in Zeng


et al. (1993) and Duhar et al. (2009), while this approach
has never been validated in microgravity conditions.

Six forces can be distinguished in the dynamics of
a bubble. The hydrodynamic drag F, and lift F, rep-
resent the viscous and inertial effects of the flow sur-
rounding the bubble. They are along and perpendicu-
lar to the flow direction relative to the bubble motion,
respectively. The added-mass force F> depicts the in-
ertia of the liquid portion displaced by the bubble and
is associated with bubble growth and its unsteady mo-
tion. Besides these forces at the liquid-vapor interface,
the following two forces are exerted by the wall: the con-
tact pressure force F, and the capillary force F,. The
former is the reaction of the wall to the vapor pressure at
the bubble base on the wall and the latter is the attraction
from the wall due to the capillary tension at the bubble
foot. In normal gravity environment, the buoyancy FB
should be also included in the dynamics. These forces
are not completely known for bubbles on a wall (except
F). Their modeling is an essential part of the force
balance model and depends on the considered situation
of the bubbles (stationary at the nucleation site, sliding
on the wall or detached from the wall). The force bal-
ance model is based on the equilibrium of these forces:
F, + F, + F, + Fc + F, = 0. When the bubble de-
taches, the contact pressure and capillary forces vanish
very quickly and the added mass force strongly increases
due to bubble acceleration. It is very difficult to write
the force balance at the instant of detachment and it is











usual in the literature to consider that the bubble detach-
ment occurs when the balance of the forces modeled for
a considered situation is broken. The bubble detachment
radius R, is thus the largest value of the bubble radius
for which the balance of the forces for a bubble growing
on a wall is valid.

In the present paper, we report vapor bubble behavior
on a heating surface in a shear flow observed in micro-
gravity (pG) experiments. Based on the observation,
different forces acting on the bubbles are calculated in
order to examine the mechanistic approach based on the
force balance. Only bubbles attached at their nucleation
site are considered. After presenting the experimental
setup and procedures, measured kinematic features of
bubbles are reported. The calculation of the forces is
given in the section for discussions with descriptions of
the adopted modeling of these forces.


Experimental

Setup A linear duct is used to establish a two-
dimensional flow. The duct has a rectangular cross sec-
tion 5 x 40 mm2, of which the aspect ratio is large for
the two-dimensionality of the flow at the central zone.
This duct is inserted in a hydraulic circuit where a gear
pump (IWAKI, MDG-R15C) circulates a test liquid. A
schematic illustration of the circuit is shown in fig. 1.
A refrigerant HFE-7000 (3M, Novec-7000) was chosen
as the test liquid because of its saturation temperature
convenient for experimentation. The physical and ther-
mal properties of the liquid are summarized in table 1.
The duct has 650 mm in length and a vapor bubble gen-
erator is flush-mounted at the axis of the lower wall,
which is also the wider wall of the duct (zx plane, see
fig. 1) at a distance 460 mm downstream from the in-
let. At this measurement section, transparent polycar-
bonate windows are embedded on the duct walls for op-
tical observation. The liquid temperature is measured
by thermocouples at inlet and just after the bubble gen-
erator. The liquid pressure is watched by an absolute
pressure transducer at the inlet. Typical values of the
temperature and the pressure of the liquid are 32 C and
1.4 x 105 Pa corresponding to a subcooling of 10 C.
The duct surface is covered by thermal insulation foam
to avoid heat loss to the ambience. The mass flow rate is
measured by a Coliolis flow meter (MICROMOTION,
R025S) which works independently from gravitational
environments. The hydraulic circuit is equipped of a vol-
ume varying chamber and a cooling facilities of Peltier
elements for decreasing the pressure and the tempera-
ture, respectively.
The bubble generator is a thin layer (~ 200 nm) of
gold sputtered on a glass substrate. A cavity of mouth


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

To yy R&Ti


High speed ies h Bubble generator
video camera


Q Coliolis
flow meter


S Volume varying
chamber -- zx


Peltier coolers
Gear pump

Figure 1: Schematic illustration of the experimental
setup



Table 1: Physical and thermal properties of HFE-7000
(at 25 C at an atmospheric pressure).


Density
Viscosity
Surface tension
Specific heat
Latent heat of vaporization
Saturation temperature


1400 kg/m3
0.32 x 10-6 m2/s
12.4 x 10-3 N/m
1300 J/kg.K
1.42 x 105 J/kg
34 C


size around 50 pm on the layer provides a nucleation
site and makes possible the single vapor bubble genera-
tion in subcooled liquid. The resistance is 20 2 at room
temperature. It is powered by a constant temperature
anemometer.

Analysis Growth and motion of bubbles at the bubble
generator are observed optically by a high speed cam-
era. A typical rate of image acquisition is 500 images
per second. The field of view is 2.4 mm x 2 mm for a
CCD sensor of 1200 x 1024 pixels. Thus the resolution
of the images are 508 pixels/mm. The optical axis of
the camera is set perpendicular to yz plane (see fig. 1)
so that projection of bubbles on the plane of the two
dimensional flow is observed. After experiments, cap-
tured images are processed to determine the best fitting
circle to the bubble projection and the positions and an-
gles of bubble's foot. For the definitions of the bubble's
geometric characteristics considered in the present paper
are illustrated in fig. 2. A typical result of the process-
ing is shown in fig. 3 (a), where determined best fitting
circle and contact angles are superposed on experimen-
tal image in pG. As seen in this example, the projection
of bubbles is well approximated by a circle so that the
circle radius R and the center position (y6, zc) represent
well, respectively, the size and the center of mass of the
bubble. The bubble volume can then be calculated by
Vb 47r/3 R3 and bubble translation can be followed
by observing the displacement of the center.


















Figure 2: Bubble geometry on a wall






(a) pG




(b) G (horizontal)


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


embedded there to avoid strong reflection of laser beam
at the wall surface. The duct is set horizontally with the
wider side being horizontal. Flashes of a laser sheet are
emitted vertically along the duct center plane (yz plane)
and images of seeded particles are recorded by a PCO
camera. Davis software is used to determine velocity
field by calculating the cross-correlation.


W (m/s)


(c) G (vertical)


Figure 3: Bubbles near the cavity of the bubble gen-
erator in different gravitational environments.
Liquid flow goes from the left to the right.


Gravitational environment Experiments were per-
formed in a micro-gravity environment (pG) realized
by parabolic flights of an aircraft during the 50th ESA
Parabolic Flight Campaign in May 2009. The duct is set
to be aligned in the perpendicular direction to the plane
of aircraft's parabola in order to minimize the influence
of Coliolis force associated with the curvature of the air-
craft's trajectory. At each parabola, microgravity state
lasted 22 s with typical quality 10 2g (g: gravitational
acceleration). The temperature in the cabin is kept ap-
proximatively 20 C by air conditioning. Experiments
in normal gravity (G) environment were also carried out
in laboratory with setting the duct either horizontally
or vertically. In the horizontal configuration, the bub-
ble generator faces upward so that generated bubbles are
subjected to vertical buoyancy detaching them from the
wall. In the vertical configuration, the flow goes upward.
The ambient temperature was around 30 C.

PIV measurements Besides the experiments for ob-
serving vapor bubble behavior, measurements of the ve-
locity field at the measurement section of the duct were
performed by the particle image velocimetry technique
without bubble generation The bubble generator was
removed from the duct and a policarbonate window is


Figure 4: Velocity profile at different flow rate


Results

Flow Experimental velocity profiles of the flow on the
yz plane are shown in fig. 4 for different flow rates Q.
For small Q (0.0030 and 0.0178 kg/s), the Saint-Venant
(1855) solution for steady laminar flow is also shown in
broken line. Turbulent flow profiles of Reichardt (1951)
type are plotted with solid line for larger Q (0, 0333,
0.0639 and 0.0933 kg/s):
W = w* [K log(1 +Ky )

c( 1 eY e- C 033y+/x) (1)

with y+ =w'y/v, where* is the friction velocity. The
constant of von Karman is = 0.41. The other constants
are set as X 11 and c = 8.67. As seen in the figure,
the laminar and turbulent theory predict well, respec-
tively, low and high rate flows. (The laminar-turbulent
transition between 0.0178 and 0.0333 kg/s correspond
to a threshold around 3000 in the Reynolds number
based on the hydraulic diameter of the duct.) In the
present paper, reported bubble behavior is obtained with
Q 0.0333 kg/s. In the later presented force calcula-
tions, the profile (1) will be used. (The friction velocity
for this flow rate is 0.00568 m/s.)

Bubble growth The incipience of bubble nucleation oc-
curs rapidly at the cavity mouth of the bubble genera-
tor. Within 1 ms, a semi-spherical incipience is formed













and develops into spherical shape after another short pe-
riod of the same order. The nucleated bubble grows with
the time as shown in fig. 5 where the time evolution
of the bubble radius R is shown for a pG experiment.
The bubble grows until its departure from the nucleation
site. A fitting curve for R during the growth has a time-
dependency R oc t1/3, which is slower than the diffu-
sion control growth proprotional to t1/2. This might be
a result of the recondensation of vapor at the bubble top
where luminous plume indicating heat transfer is typi-
cally observed in liquid flow (see fig. 3 (a)). The fitting
curve will be used later for calculating different forces.
At the departure, the bubble also detaches from the wall.
The bubble foot is broken at the departure within a very
short period (~ 1 ms). Typically, no sliding motion with
attached foot on the wall was observed in the present pG
experiments. After the departure, the bubble decreases
in size through recondensation in subcooled liquid. In
fig. 5, the bubble foot size (the radius of the contact area
on the wall) is also shown. It increases until the depar-
ture with a small rate at the beginning (t < 130 ms) and a
larger rate at the end (130 < t < 190 ms). The change in
the increase rate is coincident with the increase in the up-
stream contact angle a that will be shown later in fig. 7.
The fitting lines for rf data shown in the figure will also
be used in the force calculations.
In the inset of the figure, the evolutions of bubble size
in normal gravity G are shown for experiments with dif-
ferent duct configurations (horizontal and vertical). The
flow rate is the same as in the pG experiment. With
both duct configurations, the bubble sizes are found to
be much smaller than in the pG experiment. The bub-
bles depart from the nucleation site, sliding on the wall
with attached foot until the detachment from the wall.
The moments of the departure and the detachment are
indicated in the figure, while, because their departure is
observed immediately after their incipience at the nu-
cleation site, it is difficult to distinguish the departure
moment.


Bubble motion Figure 6 shows the displacement of the
bubble center (ye, z,) in different scales for these coordi-
nates. In pG experiments, the position z, along the wall
does not vary much until the departure. At the nucle-
ation site, the bubble is drawn downstream a little by the
flow. After detachment, the bubble is advected and with
a velocity slightly increasing in time. The coordinate y,
increases as the bubble grows. Its variation merges ap-
proximatively with that of the bubble radius R before
and even after the detachment. For G experiments, the
displacement of the bubble is shown in the inset of the
figure. In both duct configurations, monotonic increases
are found in the z, behavior. The bubbles are accelerated
along the wall at constant rates. No qualitative change


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


5
E0164 Dptur ht R +f
E 012
o01 o G(horizontal) G (horizontal)
2 oo A G (vertical) G (vertical)
0 0 Detach Int
004

5 Dep
Vol-





5 -


0 50 100
Time t (ms)


150 200


Figure 5: Evolution of the bubble radius R and the bub-
ble foot radius rf.


1 .Horizontal duct Vertical duct 02
2 0 z A z o 0 4
08 y0 yo 016
06 012
1 04 008 3 03
15 0 03

E 0 10 20 30 40
S1 -t(ms) 02



0o- ,/ v'' 01


0 -10


t (ms)


Figure 6: Bubble translational motion.


-90 A
o
x 60 n;
o 80
40
0 H o 7.0 5,

0 -
0 .


I "--.". '
t-'- '^ ,^ "---


upstream contact angle a downstream contact angle /3
x* G + /OG
SG (horizontal) G (horizontal)
SG (vertical) G (vertical)
n I i .


0 50 100
Time t (ms)


150 200


Figure 7: Contact angles



is seen in the behavior at the bubble departure. Fitting
curves to the data shown in the figure will be used later













Force


Added mass force

Drag force

Lift force

Capillary force


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



Table 2: Different forces acting on a bubble.
Explicit expression
F pd [r1 + T (w W) i] + -. ', 2 (RVb) (2)

F, C7R2 I U' U' (3)

F, pC VbU' x V (Wz) (4)


F, -arrf Jr sin ndpy


r ff f, cos p cos ndzp


Contact pressure force F, r 7r aHy


Buoyancy force


F, -pVbg


in the force calculation.
Figure 7 shows the time evolution of the contact an-
gles (a, 3) in the pG experiment, with tendency curves
shown in solid and broken lines. At the beginning
(t < 80 ms), the two angles a and 3 are approximatively
equal to each other. The bubble has a symmetrical shape.
This suggests that hydrodynamical effects of the flow are
not sufficient to overcome the dominancy of capillary ef-
fect because of the small size of the bubble. The inter-
val for the observation of this symmetrical bubble cor-
responds to the bubble size smaller than R ~ 0.15 mm,
according to fig. 5. After the bubble attains the latter
size, the flow is important and can break the symmetry.
The upstream contact angle a increases, while the down-
stream one 3 rests constant (t > 120 ms). In the inset of
the figure, measured contact angles are shown for the G
experiments. It is seen that, in both duct configurations,
the angles are approximatively equal to each other, sug-
gesting the symmetrical shape of these bubbles. This is
in agreement with the fact that the bubble size is always
smaller than 0.15 mm in the present G experiments.

Discussions

In this section, we calculate the different forces acting on
the bubbles in the experiments presented in the previous
section and examine the validity of the mechanistic ap-
proach to predict the vapor bubble behavior. The forces
mentioned in the introduction are summarized in table 2,
with explicit mathematical expressions, that we will use
for modeling the bubble behavior. The bubble is con-
sidered to be a sphere tangent to the wall surface except
when the capillary and contact pressure forces are con-
cerned. In the modeling, we have essentially followed
the work of Montout (2008), where he found favorable
agreements of the model with experimental vapor bub-
ble behavior on a wall in upward vertical flow. In the


table, z and y are the unit vectors along the flow and
perpendicular to the wall, respectively.
For the added mass force F,, the result for a spher-
ical bubble of volume Vb above a solid wall calculated
by Duhar et al. (2009) is chosen. The distance of the
bubble from the wall has been set to equal the bubble
radius. The bubble velocity, vy + wz, in eq. (2) and
its acceleration are calculated from the center position
(ye, z,) by v = dy/ldt, w = dz/ldt, dv/dt = '., /dt2
and dw/dt = d2z6/dt2, using the experimentally deter-
mined fits shown in fig. 6. The flow velocity U = W
and the shear rate OW/Oy are calculated at the bubble
top y = 2R with the turbulent velocity profile eq. (1).
U' is the relative velocity between the liquid and the
bubble. It is approximately equal to U. For estimat-
ing the drag coefficient C, and the lift coefficient C, in
eqs. (3) and (4), the results of a numerical study by Leg-
endre et al. (2003) are employed (For very small val-
ues of the bubble Reynolds number Reb 2WR/v,
the drag coefficient is set 16/Reb for avoiding nega-
tive values). In eq. (5) for the capillary force Fc, the
angle p is the azimuthal angle around the y axis mea-
sured from the z direction. The bubble surface contacts
the wall with the angle 7 at the azimuthal position p
(7 = a at p = 7 and y = f at p = 0). The polyno-
mial interpolation proposed by Klausner et al. (1993) is
used to calculate 7 at an arbitrary p (-7r < p < 7):
-y + (a )[3(|p|/)2 2(|p|/7)3]. To calculate
the contact pressure F,, the curvature H of the bubble
surface at the foot is required. Klausner et al. (1993)
proposed 1/5R for the curvature and used this estima-
tion for explaining their experimental data on the bubble
departure diameter. In the present experiment in pG and
G, the bubbles were found almost spherical. It seems
reasonable to take H 1/R in the present study. The
gravitational acceleration vector g is set 0 for pG ex-
periments and gy and gi for G experiments with the












horizontal and vertical duct configurations, respectively.

Calculated forces for the pG experiment are shown
in fig. 8. Only the bubble behavior until its departure
from the nucleation site is considered. In the perpen-
dicular direction to the wall, it is seen in (a) that the
dominant forces are the capillary and contact pressure
forces. Because the bubble is small, the other forces are
negligibly small. These dominant forces are in opposed
directions and balanced qualitatively until the bubble de-
parture. Even at the departure (t 190 ms), no indica-
tion of the violation of the force balance is not found:
extrapolating the behavior of these two forces beyond
t 190 ms, one can see these forces continue to be bal-
anced qualitatively. In the direction along the flow, the
drag and capillary forces are dominant compared with
the other forces as seen in the figure (b). The former that
tends to remove the bubble from the nucleation site in-
creases linearly in time until the bubble departure. Qual-
itatively, this force is balanced by the capillary force in
the opposite direction until t ~ 130 ms, except at the be-
ginning where measurements could include large errors
because of the small size of the bubble. After t 130 ms,
due to the increase in the upstream contact angle a (see
fig. 7), the capillary force decreases rapidly in such a
manner that the balance with the linearly increasing drag
force cannot be established. This qualitative break of
the force balance can be associated with the final depar-
ture at t 190 ms. In G experiments the behavior
of the force y components is found to be similar to that
in the pG experiment for both duct configurations. The
capillary and contact pressure forces are dominant and
qualitative balance between them is found. The z com-
ponents of the forces show different behavior as shown
in the In the insets of fig. 8. Since the bubble size be-
fore detachment are very small in normal gravity, the
determination of the upstream and downstream contact
angles is very inaccurate. It was not possible to distin-
guish a difference between these two angles and this is
the reason why the capillary force is close to zero in the
figure. Normally this force is the only one preventing
the bubble from sliding on the wall. Nevertheless, the
experimental data have been used to estimate the other
forces acting on the bubbles. For the horizontal duct, the
drag force is dominant. Due to the rapid growth of the
bubbles in the present G experiments, the added mass
force is also important in the same direction as the drag.
For the vertical duct, the buoyancy and the drag domi-
nate the dynamics and act in the same direction without
canceling each other.
As discussed in the previous paragraph, qualitative
breaks of the balance between the forces modeled by
eqs. (2-7) can be associated with the bubble departure
from the nucleation site. Due to the inaccuracy in the


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


measurements of the contact angles in normal grav-
ity it was not possible to validate this approach. But
this mechanistic approach applied to bubble growth in a
shear flow in microgravity seems very promising.
Indeed, if one sets the time of the bubble departure at
t 130 ms for the pG experiment from the calculated
forces in fig. 8 (b), it gives a prediction RD = 0.16 mm
according to the measured bubble radius in fig. 5 at this
time. This is smaller than the observed RD(= 0.18) only
by 11 percent. For the quantitative examination of the
mechanistic approach, predictions of the model on, e.g.,
RD should be compared with experiments. It is neces-
sary to refine the force modeling because the quantitative
aspects of the model depend on the details of the force
behavior, Further analyses on the experimental data ob-
tained in the present experimental setup with different
flow, gravitational and thermal conditions are being car-
ried out for this aim.


0 50 100
Time t (ms)


150 200


(a) Components perpendicular to the wall

005 0015

001 15
oo 0
5 -001 '" "' " -0 005 '
0 1 2 3 4 5 0 2 4 6 8
time t (ms) time (ms)

0



FD
1 ---. FL A
-1 .---- Fc \


0 50 100
Time t (ms)


150 200


(b) Components along the flow

Figure 8: Different force components before the depar-
ture from the nucleation site.



Conclusions

Dynamics of single vapor bubbles were investigated ex-
perimentally in pG environment. Detailed observation


----FA FC
FD ---- F











of the bubble behavior was used to calculate different
forces acting on the bubbles attached on the wall at the
nucleation site. Calculated forces show a break of the
force balance at the bubble departure and validate the
mechanistic approach, for the first time, to predict the
bubble behavior in the pG environment.


Acknowledgements

The authors gratefully thank Prof. C. van der Geld and
his laboratory stuff for fabrication of the bubble genera-
tors used in the present study. They also thank G. Ehses
for technical supports and pG experimentation. The fi-
nancial support from CNES (Centre National d'Etudes
Spatiales) and ESA (European Space Agency through
the MAP Project "Convective Boiling and Condensa-
tion") is acknowledged with appreciation.


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


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