ICMF2001, by T. Saito & R.H. Mudde
Performance of 4-tip Optical Fiber Probe and Bubble Characterizations
by the Probe in Turbulent Bubbly Flows
Takayuki Saito1 and Robert F. Mudde 2
1
Department of Mechanical Engineering, Faculty of Engineering, Shizuoka University
3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan
Phone & Fax: +81-53-478-1601
E-mail: ttsaito@ipc.shizuoka.ac.jp
2
Kramers Laboratorium voor Fysische Technologie
Delft University of Technology
Prins Bernhardlaan 6
2628 BW Delft, Netherlands
Phone: +31-15-278-2834
Abstract––Probe methods are widely used for measuring void fraction and bubble
characterizations in turbulent gas-liquid two-phase flows. However, knowledge of
interfacial motion of a bubble being in contact with a probe as well as its motion of the
center of gravity is insufficient. In the present paper, both motions of a bubble contacting
with a 4-tip optical fiber probe are discussed on the basis of visualization of them using
two sets of high-speed video camera and microscope. The optical fiber is a plastic one of
250µm in inner diameter. The tip of each probe is cut under an angle of 30° with the fiber
axis. As a result, both motions of the bubble are not influenced by the presence of the
probe. After a discussing accuracy of this probe method, we describe bubble
characterizations in a turbulent bubbly flow in a large-diameter loop (149mm in diameter
and 10m in height) using the 4-tip optical fiber probe. Radial profiles of void fraction and
bubble velocity, and distribution of bubble diameter are discussed.
Key Words: Optical Fiber Probe, Bubble, Interfacial Motion, Motion of the Center of
Gravity, Turbulent Bubbly Flow
1. INTRODUCTION
Turbulent bubbly flows are very often encountered in various fields such as mechanical,
chemical and bio industries, and so on. Since the global warming, due to the increase of
atmospheric CO2 concentration, must be solved for sustainable growth of our mankind,
importance of the flows is increasing also in the fields of environmental science and
technology. Investigations on the turbulent bubbly flows are necessary: to understand the
mechanism of mass and heat transfer between wavy sea surface; to develop direct injection of
gaseous CO2 into shallow seawater (Haugen et al. 1992); to realize deep sea sequestration of
CO2 utilizing a gas lift pump (Saito et al. 1999, 2000, 2001). For these investigations, in
particular, the large-scale or global-scale structure of the flows in a large diameter pipe has to
be understood [Lance et al. 1991; Mudde et al. 1997(a), Mudde & Saito 2001] deeply and
quantitatively by means of experiments. Hence, reliable methods are necessitated to measure
characteristics of the flows.
A Laser Doppler Anemometer (LDA) has been widely employed to measure motion of the
liquid phase of the flows [Adorian and Yao 1987, Mudde et al. 1997(b)]. On the other hands,
some pares of electric conductivity probes [Iguchi et al. 1994] or optical fiber probes (OFP)
[Hatano et al. 1986] have been usually used to characterize the bubbles of the flows. One can
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ICMF2001, by T. Saito & R.H. Mudde
calculate diameters and velocities of the bubbles as ell as void fraction of the gas phase from
the time for a probe piercing each bubble. Many literatures have been discussing the problems
of applying the probe methods to the turbulent bubbly flows [Abuaf et al. 1978; Clark et al.
1988]. They are summarized as follows:
1) Response of the measuring system.
It refers to compatibility with the bubble sizes, the interface velocities and physical
dimensions of the probe tips.
2) Chord length pierced by the probes.
Pierced time depends on a pierced chord length; an angle between velocity vector of
bubble and the probe; velocity of the bubble. Imagine characterization of an ellipsoidal
bubble using a probe. The probe piercing an edge of the bubble decreases pierced time; a
probe piercing the center of the bubble increases the time.
3) Relationship between bubble size and velocity distributions calculated from the probe
methods and those of the original two-phase flow.
4) The influences of the presence of the probes on the liquid phase flow, and both motions
of the center of gravity and of interface of a bubble.
Miller and Mitchie (1969) described the principle of an OFP method; they showed that
Snell’s law could be applied to measuring bubbles. Abuaf (1978) investigated response of an
OFP with controlled tip geometry and established a basis of the OFP method to determine
local void fraction. Clark (1988) and Liu (1995) developed a statistical way to transform
chord- lengths calculated from results of a probe method to both originally local bubble-sizes
and its distribution. For a large bubble such as spherical cap and slug, the influence of a probe
on bubble motions is negligible: because the bubble length pierced by a probe is enough
larger than degree of a bubble deformation due to contacting with the probe; the dynamic
pressure (~ ρlUB2 ) is much larger than the surface tension pressure (~ σ/DE). For a small
bubble (DE<0.1mm), the surface tension pressure is much larger than the dynamic pressure.
The probe cannot detect such a small bubble. For a bubble of 1mm < DE < 5mm, the surface
tension is the same order as the dynamic pressure. The degree of bubble deformation due to
contacting with a probe might be almost the same order as a chord length pierced by the probe.
The influence of a probe on kinetics of the bubble has been pointed out, however their
knowledge obtained is not satisfactory. Hence, quantitative knowledge of bubble deformation
with being in contact with a probe is required to improve the quality of the probe methods.
In the present paper, a kinetic process of a bubble of about 3.5 mm in equivalent diameter
contacting with a 4-Tip optical fiber probe (Mudde and Saito 2001) is quantitatively
investigated using high-speed digital video cameras and microscopes. After comparing both
motions of interface and the center of gravity of the bubble with those of a free bubble, we
will discuss influence of the presence of the probe on the motions. Probe calibration is
clarified on the basis of high-speed video images and signals from the four photo multipliers
simultaneously sampled. Finally, we characterize bubbles of a turbulent bubbly flow in a
large-diameter pipe (149mm in inner diameter, 10m in height) using the 4- Tip OFP to
demonstrate its performance.
2. EXPERIMENTS
2.1 Structure of a Single Optical Fiber Probe
We used a plastic optical fiber (Polymethyl Meta Acrylate) of 250 µm in outer diameter;
200 µm in core diameter; 25 µm in clad thickness, for a single optical fiber probe (S-OFP).
The structure of the S-OFP is shown in Fig. 1. An optical fiber (1) for detecting a bubble is
2
(1)
250 µm
250 µm
ICMF2001, by T. Saito & R.H. Mudde
E-3
(4)
(2)
E-1
30°
(3)
E-4
E-2
(5)
500 – 2000 mm
500 – 2000 mm
(1) Optical fiber for detecting a bubble, (2) Optical fiber for a light source, (3) Optical fiber
for a photo multiplier, (4) Halogen lamp, (5) Photo multiplier
Figure 1: Structure of a single optical fiber probe.
φ 3.1mm
(4)
(6)
(3)
(2)
1 – 3 mm
(5)
(5)
(6)
(3)
(2)
(1)
(1)
(1) Center probe, (2) (3) (4) Outer probe, (5) Stainless tube, (6) Stainless capitally
Figure 2: Structure of 4-Tip optical fiber probe.
jointed with the bonded ends of optical fibers (2) and (3) for a halogen lamp (4) and for a
photo- multiplier (5). The beam from the halogen lamp (4) of an output of 100W passes
through the optical fiber (2) and the optical fiber (1). During the end E-1 being in a liquid
phase, most of the beam enters the liquid phase obeying Snell’s law. On the other hand, during
the end being in a gas phase, most of the beam is reflected at the end back into the fiber (1)
and introduced into a photo multiplier (5) (type R212, Hamamatsu Photonics Co., Ltd.)
housed in an electromagnetic shield. The end E-1 is sharply cut into an angle of 30° with the
fiber axis, and the ends E-2, E-3 and E-4 are cut cylindrically.
2.2 Structure of 4-Tip-Optical Fiber Probe
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ICMF2001, by T. Saito & R.H. Mudde
In- house optical glass fiber probes have been utilized for measuring profile of void
fraction as well as profiles of bubble size and velocity [Frijlink, Cartellier]. The 4-Tip optical
fiber probe (4-Tip OFP) developed in the present investigation is of a bundle of four S-OFPs
as shown in Figure 2. Each S-OFP is put in a stainless capillary of 0.54 mm in outer diameter
(6). Three tips of the outer probes (2) – (4) are of the same length and form an equivalent
triangle. Center probe (1) is positioned through the center of gravity of the triangle. The radial
distance from the center probe to each of the others is 0.54 mm. Five kinds of values, namely
1, 1.5, 2, 2.5 and 3 mm, were given to the distance between the tip of the center probe and the
other three tips to consider influences of the distance on measured results.
Each optical signal of the probes is detected using the photo multiplier (one for each).
Amplified DC signals of the photo multipliers are sampled at a sampling frequency of 40
kHz.
(1)
(2)
(11)
(7)
(6)
(8)
(9)
(10)
(3)
(12)
(12)
(4)
(5)
(a) Overall diagram of experimental setup.
(10)
(1)
Front view
(8)
50
Bubble
(9)
(2)
(10)
50
Side view
(9)
(8)
(b) Location of visualizing system in a ground plane.
(1) Acrylic cubic- vessel, (2) 4- Tip optical fiber probe, (3) Capillary, (4) Syringe, (5) Stepping
motor, (6) Controller, (7) Pure-air cylinder, (8) High-speed video camera, (9) Microscope,
(10) Stroboscope, (11) Tracing paper, (12) Precision optical stages
Figure 3: Experimental setup used for calibration of the 4-Tip optical fiber probe.
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ICMF2001, by T. Saito & R.H. Mudde
6
6
4
4
2
2
0
0
1
2
3
-1
QG (10 cm3/s)
4
5
0
VB (10-8 m3 ), AI (10-5 m2 )
DS , DL , DE (10-3 m)
2.3 Experimental Setup Used for Calibration of 4-Tip Optical Fiber
To grasp bubble motions, including the interfacial motion, in the contacting process with
the 4-Tip probe, visualization of the bubble motions was made using an experimental setup as
shown in Figure 3 (a). The bubble train is generated at a capillary (3) of 0.78 mm in inner
diameter that is placed at the bottom of an acrylic cubic-vessel (1) (30cm*30cm*30cm) filled
with tap water (temperature: 291 – 293 K). Pure air (CO<1ppm, CO2 <1ppm and CH4 <1ppm) is
carefully pushed into the capillary at a constant rate using a syringe (4) driven by a stepping
motor (5). The center tip of the 4-Tip OFP is placed about 20 mm above the capillary. The
bubble almost vertically rises below this position. Hence, velocity vector of the bubble is
almost parallel to the axis of the probe.
The average size and shape of the bubbles generated by this method is shown in Figure 4.
Average equivalent diameter of the bubbles was calculated from directly measured volume
and the number of the bubbles trapped in a small glass bottle. The others were obtained from
carefully processing images of the bubbles simultaneously filmed in the high-speed video
cameras. As shown in the figure, bubbles can be generated well uniformly and controllably by
means of this method. The average equivalent diameter D E of the bubbles is 3.64 mm, the
average short diameter D S 1.92 mm, and the average long diameter D L 4.63 mm.
□ : Average equivalent diameter D E , ○ : Average short diameter D S , △ : Average long
diameter D L , ▲: Average volume V B , ■: Average interfacial area AI . The average values
were calculated from about 500 bubbles and their images simultaneously taken from two
directions perpendicularly intersecting each other.
Figure 4: Characteristics of the bubbles generated at the capillary.
2.4 Experimental Setup of Vertical Loop
A vertical loop with an inner diameter of 149 mm and a height of 10.0 m as shown in
Figure 5 was used to verify the performance of the 4- Tip OFP in application to a real turbulent
bubbly flow. Pure air is injected into a gas buffer (1) after its flow rate measured and
controlled by a mass flow controller (13) (MASFLO, Oval Co. Ltd.). Bubbles of almost
uniform size are generated through a bubble disperser (2), and smoothly rise into an inlet pipe
through a mixing vessel (3) with a bell mouth. Tap water pumped from a water vessel (9) is
introduced into a vertical acrylic pipe (6) through liquid inlet screens (5) installed on the pipe
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ICMF2001, by T. Saito & R.H. Mudde
(7)
(6)
2,400
(8)
(11)
(9)
(10)
10,800
10,000
(19)
(18)
(17)
(16)
(5)
(4)
(3)
(2)
(1)
(15) (14) (13)
(12)
(1) Gas buffer, (2) Bubble disperser, (3) Mixing vessel, (4) Inlet pipe, (5) Liquid inlet screen,
(6) Vertical acrylic pipe, (7) Knockout tank, (8) Screen, (9) Water vessel, (10) Centrifugal
pump, (11) Electromagnetic flowmeter, (12) Gas cylinder, (13) Mass flow controller, (14)
Thermo couple, (15) Pressure transducer, (16) Probe stage, (17) Probe traverse, (18) 4-tip
probe, (19) Lift
Figure 5: Outline of a vertical test loop used for bubble characterization in a real
turbulent bubbly flow by means of 4-Tip OFP.
wall. The water flow rate is measured by electromagnetic flow meter (11 ) (ADMAG-AE-50A,
Yokogawa Electric Corporation). The 4-Tip OFP (18) fitted into a capillary that is bent at the
end at an angle of 90° is mounted on a probe traverse (17) to be positioned at a desired radius
in the vertical pipe. Four Axes of the probe is parallel to the direction of the main stream of a
two-phase flow.
2.5 Visualization of Bubble and Digital Image Processing
The interfacial motion of the bubble as well as the motion of the center of gravity was
visualized by two sets of a high-speed digital video camera of 500 frames/sec (NAC,
Memorycam-C3 ) and a stroboscope synchronizing with the video as shown in Figures 3 (a)
and (b). Each camera (8) with a microscope (9) (SZ11, Olympus Co. Ltd.) is mounted on a
precision optical stage with 6 degrees of freedom. Shadow images of the bubbles are filmed
6
ICMF2001, by T. Saito & R.H. Mudde
from two directions perpendicularly intersecting each other as shown in Figure 3 (b). The
most suitable lens opening and lightning were decided to take clear and precise shadow
images of bubbles before performing experiments for the calibration.
After recording a scale and background images, bubble trains are filmed. The original
images are processed using median filter. Absolute difference between the processed
background image and that of the bubble is calculated. Hence, only bubble image is extracted.
Processing the bubble images taken from the two directions, long and short diameters of the
bubble at every moment are calculated by digital image processing.
(a)
(b)
(c)
(a) Sample of bubble image, (b) Background image, (c) Extracted bubble image: Calculating
absolute difference between the images of (a) and (b), image (c) is obtained.
Figure 6: Procedure of digital image processing to extract bubbles.
3. RESULTS
3.1 Motions of a Free Bubble
Sample of time series images of a free bubble rising in a rest water column is shown in
Figure 7. The bubble is categorized into a type of oblate ellipsoidal. The position of the
bubble at 0ms corresponds to that of bubble hitting the probe. The bubble vertically rises
without a wobbling motion in this period, however one can find that a periodically interfacial
motion of the bubble.
3.2 Motions of a Bubble hitting the 4-Tip OFP
Both motions of the interface and the center of gravity of a bubble hitting the 4-Tip OFP
are sampled in Figure 8. In this case, the distance between the tip of the center probe and the
other three tips is 3 mm. At 14 ms, the bubble hits the center tip. It is found that even after
hitting, the bubble continues almost the same motion as those before hitting. Moreover,
comparing bubble motions in Figure 8 with those in Figure 7, both the motions of bubble
contacting with the probe is almost the same as those of the free bubble.
To discuss more quantitatively, average velocity components of the center of gravity and
average interfacial velocities of bubbles are calculated. These velocities are plotted in Figures
9 (a) and (b). For this short period, velocity components UBY and UBX are almost constant.
Therefore, hitting and contacting with the probe do not influence the motions of the center of
gravity of bubbles. In Figure 9 (b), ULI and UUI represent interfacial velocities at the upper
side and lower side contacting points with the probe, respectively. Considering the
periodically interfacial motion of the free bubble as shown in Figure 7, the interfacial
velocities is not influenced by the contacts. As stated above, both motions of interface and the
center of gravity are not influenced by the presence of the 4-Tip OFP.
7
ICMF2001, by T. Saito & R.H. Mudde
0 ms
Time interval: 2 ms
12 ms
0mm
3mm
Figure 7: Sample images of free-bubble motions.
t = 0 ms
time interval = 2 ms
t = 26 ms
0mm
3mm
The bubble just hits the center tip at 14 ms and the other three tips at 24 m/s.
Figure 8: Sample images of bubble motions contacting with the 4-Tip OFP.
0.4
0.5
0.2
U UI , U LI (m/s)
U BX , U BY (m/s)
U BY
Bubble contacts with
the 4-Tip OFP.
U BX
0
Bubble hits
the 4-Tip OFP.
-0.2
0
20
0.3
0.2
Bubble hits
the 4-Tip OFP.
0.1
Bubble leaves
the 4-Tip OFP.
10
0.4
0
30
0
10
t (ms)
(a) Velocities of the center of gravity.
Bubble leaves
the 4-Tip OFP.
t (ms)
20
(b) Velocities of bubble interface at
contacting points
Figure 9: Time series velocities of bubbles.
8
30
ICMF2001, by T. Saito & R.H. Mudde
3.3 Calibration of Probe Signal
Figure 10 is a typical sample of a signal outputted from the photo multiplier when the
single OFP completely pierces a bubble; the tip completely enters the bubble. Before t1 , the
tip is in water. At t1 , the bubble is beginning to hit the tip; in the strict sense the upper surface
of the bubble just touches the lower edge of the fiber core as shown in Figure 10. At t4 , the
upper surface touches the upper edge of the fiber core; at t5 the lower surface the lower edge;
at t8 the lower surface the upper edge. Between t1 and t4 the bubble gradually covers the core,
hence the output increases with the covered area. One can find that the output increases
almost lineally between (A) and (B). If the tip is entirely in the bubble between t4 and t5 , the
output continues almost a certain value. Between (C) and (D), the output decreases much
faster, and reaches the output value of the tip in water; it will be related with the surface
tension of the bubble and the surface characteristic of the fiber.
High and low thresholds are employed to analyze the signal as follows: First the points of
(A) and (B) are obtained using the high threshold; Second the points of (B) and (D) are done
using the low one. Starting time t2 of touching the tip for analyzing the OFP signal is
calculated from an intersection of the strait line (6) connecting the points of (A) and (B), and
the strait line (9) of the output value of the tip in water. The output of the tip in water is
constant as indicated in the figure, however, that of the tip in a bubble is not constant. Hence,
several values of high threshold are used to determine t3 . Average value of the difference
between t1 and t2 is 4.5% of calculated total-piercing time (namely t7 – t2 ), and that between t3
and t4 is 3.9% of the total time. On the other hand, the difference between t5 and t6 as well as
between t7 and t8 is negligible. On the basis of these considerations, starting time of touching
the probe is shifted from t2 to upstream by 4.5% of the calculated total-piercing time; time of
2
Bubble position
(6)
(7)
(1)
(A)
(2)
(8)
Optical fiber
t8
t5
VP [volt]
(4)
(C)
1
(D)
(B)
(5)
(3)
0
(9)
t4
t1
bubble
-1
t 1 t2
3.3
t3 t 4
t 5 t6 t7 t 8
3.35
t [10 sec]
3.4
-5
(1) Probe signal, (2) High threshold, (3) Low threshold, (4) Rectangular wave by high
threshold, (5) Rectangular wave by low threshold, (6) Strait line connecting points (A) and
(B), (7) Strait line connecting points (C) and (D), (8) Output value of the tip in bubble, (9)
Output value of the tip in water
Figure 10: Typical sample of signal of detecting a bubble.
9
ICMF2001, by T. Saito & R.H. Mudde
V P [volt]
2
1
P-C
P-3
0
P-2
P-1
-1
1
1.1
1.2
t [10-5 sec]
1.3
1.4
P-C: Signal of center tip, P-1, P-2 and P-3: Signals of the other three tips.
Figure 11: Typical signals of 4-Tip OFP of detecting a bubble.
bubble upper surface touching the upper edge of the core is sifted from t3 to downstream by
3.9% of the calculated total-piercing time.
3.4 Signal Analysis (4-Tip OFP)
The local gas fraction α is obtained from the output sc (t j) of the center probe:
bc (t j ) = 1 if sc (t j ) > thα
α=
1
N
∑
N
j =1
(1)
b c (t j )⋅ ∆t s
(2)
N ∆t s
where bc represents the binary data, thα the threshold, t j the time, N the number of bubbles
detected, t s the sampling rate.
Rising velocity and diameter of the bubbles are obtained from output of center tip and those si
(i = 1, 2, 3) of the other three tips as follows:
if sc ( t s ) > thv bubble hits the center tip at time t s,
(3)
if si ( t k ) > thv bubble hits the other tip i at time t k,
(4)
τ ic = t k − t s
(5)
Average time difference τ is obtained
1 3
∑ τ ic .
3 i =1
These time differences are evaluated using a given tolerance:
τ=
if
τ ic − τ
< δ t ∀i , then valid bubble detection,
τ
10
(6)
(7)
ICMF2001, by T. Saito & R.H. Mudde
else reject and reset.
Bubble rising velocity is given by
∆s
.
τ
The vertical bubble size db is calculated:
if sc(t e) < thv the center tip leaves bubble
T = te - tc
db = v b * T
vb =
(8)
(9)
(10)
3.5 Accuracy
The inaccuracy of the bubble velocity and size depends on various factors. The most
important ones are: the poison at which the bubble hits the probe and the angle that the bubble
velocity vector makes with the probe. To estimate the accuracy of 4-Tip OFP method, the
Monte Carlo simulations are performed under given size distributions of bubble and the ones
of velocity vectors. The accuracy is estimated at about 10% for size and about 11% for
velocity.
4. DEMONSTRATION OF 4-Tip OFP IN A REAL BUBBLY FLOW
The 4- Tip OFP is employed to measure profiles of void fraction, rising velocity of bubbles
in a real turbulent bubbly flow in a pipe of 149mm in inner diameter. The experiment was
performed at a superficial water velocity of 0.175m/s (Reynolds number of 2.6·104 based on
the pipe diameter and the superficial velocity) and a superficial gas velocity of 0.0183m/s.
The radial profile of void fraction is plotted in Figure 12 (a), and the one of bubble rising
velocity in Figure 12 (b).
8
0.4
0.2
4
0
v (m/s)
α (%)
6
2
0
-1
-0.2
-0.5
0
r/R (-)
0.5
(a) Radial profile of void fraction
-1
1
-0.8
-0.6 -0.4
r/R (-)
-0.2
0
(b) Radial profile of bubble velocity
Figure 12: Experimental results on void fraction and bubble velocity in a real bubbly
flow.
5. CONCLUSION
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ICMF2001, by T. Saito & R.H. Mudde
The 4- Tip OFP as well as the measuring method using the OFP developed in the present
investigation is useful for measuring void fraction and characterizing bubbles in turbulent
bubbly flows. To improve the accuracy of this method, more detailed investigations are
required. Next step of our future investigation, it will be performed.
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