International Group on Research Reactors Conference 2014
November 17-21, 2014, Bariloche, Argentina
Experimental study on downward two-phase flow
in vertical narrow rectangular channel
T.H. Kim, B.J. Yun and J.H. Jeong*
School of Mechanical Engineering
Pusan National University
November 17, 2014
Contents
1. Introduction
2. Experimental apparatus and method
3. Flow regime
4. Conclusion
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1. Introduction (1)
 Research background
 The Ki-Jang research reactor (KJRR)
 Reactor type : open pool
 Power : 15 MW
 Coolant : water
 Fuel: U-MO plate type
 Active fuel length : 600 mm
 Fuel box size : 76.2 x 76.2 mm
 Width of fuel meat : 62.0 mm
 Normal operation
 Inlet pressure : 0.18 Mpa
 Inlet temperature : 35 ℃
 KJRR employ a plate type fuel and bottommount CRDM and allow downward flow of
water through fuel channels
 Thermal-hydraulic characteristics for
downward flow in rectangular channel are
required for safety analyses (e.x. PSAR)
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1. Introduction (2)
 Two-phase flow in vertical channels
 Most of studies performed with upward flow in circular tubes
 Several works for upward flow in rectangular channels
 Study on vertical downward flow in rectangular channels is relatively scarce.
Flow direction
Channel type
Circular
Upward
Rectangular
Downward
Circular
Rectangular
etc.
Author and year
Remarks
Abundant previous works available in the open literature
- Gap : 0.3 ~ 17 mm
Wilmarth and Ishii (1994, 1997) ,
Flow pattern transition criteria suggested.
Xu (1999), Hibiki and Mishima
- Gap : ~ 0.3 mm
(2001), etc.
A new theory needs to be developed.
Spedding and Nguyen (1980),
Barnea et al. (1982), Usui and
Sato (1989),Goda et al. (2003),
Swanand et al. (2012), Julia et
al. (2013) , etc.
The upward and downward two phase flows
were observed to be different in terms of
interaction of the buoyancy, gravity and liquid
inertia forces.
Little works in the open literature
Paranjape et al. (2011); bundle
Mishima and Ishii (1984), Sun
et al. (2004), Julia and Hibiki
(2011) ; annulus
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Rod bundle and annulus upward flow.
Cap-bubbly flow motions were found to be
relatively steady in narrow gap region.
1. Introduction (3)
 Flow regime maps
Circular upward flow
Taitel et al.(1980)
Rectangular upward flow
Wilmarth and Ishii(1994)
Circular downward flow
Kim et al.(2004)
Rectangular downward flow
?
Flow regime map for downward two phase flow in vertical rectangular channels
with the gap of 2.35 mm is to be suggested in this study
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2. Experimental apparatus and method (1)
 Experimental apparatus
 Air-water two phase flow behavior were observed using a rectangular channel
A schematic of experimental apparatus and test section
Photograph of experimental apparatus
The measurement uncertainties of flow rate, temperature, and pressure were 0.05% of reading, 0.3 ℃,
and 0.05% of span, respectively.
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2. Experimental apparatus and method (2)
 Experimental apparatus
 Bubble distribution
 Air-water inlet mixer
0.4
0
30
60
500mm
0.3
0.2
Water
0.1
0.0
0.4
270mm
0.3
0.2
Porous media
Void fraction
0.1
Porous media
0.0
0.4
120mm
0.3
Height
dependency
0.2
0.1
0.0
0.4
80mm
0.3
Air
Air
0.2
0.1
0.0
0.4
30mm
0.3
0.2
0.1
0.0
0
30
60
Width [mm]
0.8
0.7
2.35 mm
Void fraction
Test section
0.6
0.5
0.3
jL=0.2 jG=0.3
jL=0.2 jG=0.5
jL=0.2 jG=0.75
jL=0.2 jG=1.25
0.2
0.1
0.0
0
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Velocity
dependency
at 500 mm
0.4
10
20
30
40
Width [mm]
50
60
2. Experimental apparatus and method (3)
 Experimental procedure
1. Gas flow is set at minimum level to prevent water from entering air supply line
2. Liquid flow rate is set at pre-determined level
3. Gas flow rate is increased in a stepwise manner
4. Visually observe and wait until flow reaches a quasi-steady state
5. To capture images using a high speed camera
6. Go to step 3 and repeat to increase gas flow rate
7. Go to step 2 and repeat to increase liquid flow rate
 Local velocity measurements
1. PIV system to measure liquid velocity around bubbles
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2. Experimental apparatus and method (4)
 Image Processing
 PIV system with LASER
 Image capture system with Xe-light
• Shadow image capture
Air + Water
Air + Water
High speed
camera
Cylindrical Lens
High speed
camera
Mirror Arm
Continuous
Xe lighting
Laser sheet
Light sheet
Test section
Pulsed
Nd:YLF laser
Synchronizer
PC
Test section
Laser
Oscillator
PC
High speed camera : 5,000 frames per second
1024 × 512 pixels
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2. Experimental apparatus and method (5)
 Image Processing
Test procedure
Image processing procedure
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3. Flow Pattern (1)
 Flow pattern identification for downward flow in a rectangular channel
 Two-phase flow patterns are usually classified into four or five different patterns
 Two-phase flow in rectangular channel would be influenced by two-phase
interfacial interaction, narrow-side walls, wide-side walls, and reverse gravity
(buoyancy)
☞ Six different flow patterns are identified in downward rectangular channel based
on the average void fraction and visual observation as follows:
Annular
Churn-turbulent
Slug
Cap-bubbly
Bubbly
Large-bubbly
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3. Flow Pattern (2)
 Bubbly flow (B)
1.0
 Continuous liquid flows with small dispersed
bubbles
Equation
Void Fraction
 Near spherical shape due to surface tension
Void Fraction
Intercept
Void Fraction
Slope
Standard Error
0.08727
0.00121
0
--
Bubbly
Average
0.6
0.4
0.2
 Average void fraction is less than 0.25 (same for
upward flow)
0.0
0.0
0.1
0.2
0.3
0.4
Time (s)
 Bubble behavior is similar to upward flow in
circular tube except direction
jL(m/s)
Test condition
1
0,1
0,01
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0
Value
 Most bubbles smaller than the gap size
jL = 2 m/s, jG = 0.12 m/s
y = a + b*x
Adj. R-Square
0.8
0,1
jG (m/s) 1
10
3. Flow Pattern (3)
 Downward Cap-bubbly flow (CB)
 Large flat slug bubbles appear but do not fill whole
channel width
1.0
0.8
Void Fraction
 As the gas flow rate increases, bubbles coalesce and
the confinement by the channel cause the growing
bubbles to be flattened and distorted to appear as
small overturned caps
0.6
0.4
Equation
0.2
y = a + b*x
Adj. R-Square
0
Value
Void Fraction
Intercept
Void Fraction
Slope
Cap-Bubbly
Average
Standard Error
0.52216
0.00474
0
--
0.0
 Transition from cap-bubbly flow to slug flow occurs
at averaged void fraction of approximately 0.7.
0.0
0.1
0.2
0.3
0.4
Time (s)
jL(m/s)
Test condition
1
0,1
0,01
jL = 1 m/s, jG = 0.52 m/s
0,1
jG (m/s)
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1
10
3. Flow Pattern (4)
 Slug flow (S)
 Coalescence of cap-bubbles is enhanced due to
wake entrainment to become large slug bubbles:
diameter approaches the channel width (Large
Taylor bubble)
1.0
Void Fraction
0.8
 Large Taylor bubbles are separated by liquid
bridges containing small bubble.
0.6
0.4
0.2
Equation
Value
 Average void fraction fluctuates due to alternation
of large Taylor bubbles and liquid bridges
Intercept
Void Fraction
Slope
Slug
Average
Standard Error
0.75209
0.01035
0
--
0.1
0.2
0.3
0.4
Time (s)
jL(m/s)
Test condition
1
0,1
0,01
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0
Void Fraction
0.0
0.0
jL = 1 m/s, jG = 1.96 m/s
y = a + b*x
Adj. R-Square
0,1
jG (m/s) 1
10
3. Flow Pattern (5)
 Churn-turbulent flow (CT)
 Further increase in the gas flow rate, large Taylor
bubbles break into irregular bubbles
1.0
0.8
Void Fraction
 Liquid bridge is also distorted and broken
 Liquid slugs are frothy and chaotically disordered
0.6
0.4
C-Turb.
Average
0.2
 Average void fraction fluctuates like that in slug
flow but low frequency component is reduced
Equation
y = a + b*x
Adj. R-Square
0
Value
Void Fraction
Intercept
Void Fraction
Slope
Standard Error
0.88216
0.00485
0
--
0.0
0.0
0.1
0.2
0.3
0.4
Time (s)
jL(m/s)
Test condition
1
0,1
0,01
jL = 1 m/s, jG = 5.57 m/s
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0,1jG (m/s) 1
10
3. Flow Pattern (6)
 Annular flow (A)
1.0
 The gas phase flows as a continuous phase in
the center of channel
Void Fraction
0.8
 The liquid phase flows filling the gap near the
wide-side walls of channel
0.6
0.4
Annular
Average
Equation
0.2
 Often appearance of thin water film on walls
y = a + b*x
Adj. R-Square
0
Value
Void Fraction
Intercept
Void Fraction
Slope
Standard Error
0.91273
0.00154
0
--
0.0
 Amplitude of average void fraction diminishes
0.0
0.1
0.2
0.3
0.4
Time (s)
jL(m/s)
Test condition
1
0,1
0,01
jL = 0.5 m/s, jG = 7.18 m/s
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0,1
jG (m/s) 1
10
3. Flow Pattern (7)
 Large Bubble (LB)
 Buoyance is influential at low liquid flow rate
1 jL = 0.5 m/s, jG = 0.03 m/s
jL(m/s)
 Counter flow in observer’s coordinate if
liquid flow is further reduced
1
1
2
0,1
0,01
0,1
jG (m/s) 1
10
2 jL = 0.5 m/s, jG = 0.19 m/s
1.0
Equation
y = a + b*x
Adj. R-Square
-9.64515E-4
Value
Void Fraction
0.8
Standard Error
Void Fraction
Intercept
0.22554
0.00609
Void Fraction
Slope
0.00204
0.01055
Large Bubble
Average
0.6
0.4
0.2
0.0
0.0
0.1
0.2
0.3
0.4
Time (s)
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3. Flow Pattern (8)
 Velocity profile around small bubbles
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jL = 1 m/s, jG = 0.1 m/s
3. Flow Pattern (9)
 Velocity profile around large bubbles(1)
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jL = 0.8 m/s, jG = 0.1 m/s
3. Flow Pattern (10)
 Velocity profile around large bubbles (2)
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jL = 0.5 m/s, jG = 0.1 m/s
3. Flow Pattern (11)
 Counter current or falling film flow
jL(m/s)
Test condition
2
jL = 0.4 m/s, jG = 0.07 m/s
3
jL = 0.4 m/s, jG = 0.1 m/s
4
jL = 0.4 m/s, jG = 0.18 m/s
1
2 3
4
1
0,1
0,01
0,1
jG (m/s)
1
1 jL = 0.2 m/s, jG = 0.03 m/s
10
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4. Flow regime map
Superficial Liquid Velocity
[m/s]
Bubble
Slug
Cap-Bubble
Churn-Turbulent
1
Large Bubble
Annular
Hibiki and Mishima (2001)
(upward rectangular)
B
CB
S
CT
LB
A
0,1
0,01
0,1
1
Superficial Gas Velocity [m/s]
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10
5. Conclusion
 Flow patterns of downward flow in vertical rectangular channel were
identified
 Six different flow patterns were identified: bubbly, downward cap-bubbly, slug,
churn-turbulent, annular and large bubble flows
 Downward cap-bubbly flow and large bubble flow are peculiar to downward flow
 In high liquid flow region, similar to upward flow in rectangular channel
 In low liquid flow rate region, buoyance becomes influential and transition to
counter flow may occur
 Flow regime map was constructed.
 Flow regime map in coordinates of liquid and gas superficial velocities
 Flow pattern transition lines are close to them for upward flow in vertical
rectangular channel in high liquid flow rate region (liquid momentum
dominating region)
 Transition criteria should be developed to consider buoyance effect in low liquid
& gas flow region
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Thank you
for your attention