Proceedings of NHTC’01
35th National Heat Transfer Conference
June 10-12, 2001, Anaheim, California
NHTC01-11262
HIGH-SPEED PHOTOGRAPHIC OBSERVATION OF FLOW BOILING OF WATER IN
PARALLEL MINI-CHANNELS
Satish G. Kandlikar
[email protected]
Mark E. Steinke
Shurong Tian
Levi A. Campbell
Mechanical Engineering Department
Rochester Institute of Technology
Rochester, NY 14623
ABSTRACT
The use of smaller passage dimensions is becoming more
prevalent in flow boiling applications. Passages with hydraulic
diameters on the order of 1mm provide higher heat transfer
coefficients resulting in more compact heat exchanger
arrangements. Passages with 1mm hydraulic diameters do not
pose severe clogging or blocking constraints as opposed to
micro-channels and have a less pronounced pressure drop
penalty. The present paper explores the flow regimes during
flow boiling of water in mini-channels. High-speed photography
is used to obtain visual pictures of the flow phenomena from
subcooled flow boiling all the way to critical heat flux
conditions. The tests include one single-channel and a set of six
parallel channels with electric heating from three sides of the
channel. The top cover is made of Lexan to permit visual
observations. The results are used to identify specific features
of flow boiling in smaller diameter mini-channels.
NOMENCLATURE
Ac: Cross-Section area
G: Mass flux of water through test section
(m
Ac )
m : Mass flow rate of water through test section (kg/s)
Q”: Heat flux through channel walls to water (kW/m2)
TA: bulk temperature at the test section inlet (°C)
TB: bulk temperature at the test section outlet (°C)
TS: Average surface temperature of the test section (°C)
vapor m fluid
x: vapor mass fraction at outlet m
(
)
INTRODUCTION
The need to increase heat transfer coefficients on the
evaporating liquid side in an evaporator is becoming
increasingly important in many applications including
automotive air conditioning, heat pipes, direct refrigeration
cooling of electronic devices, and fuel cells. Although
evaporation in small diameter channels received considerable
attention in the 1960’s (for example, Bergles, 1964), its use in a
compact evaporator configuration with multiple channels is
currently receiving wide attention. The complexities associated
with evaporation in multiple channel passages are not clearly
understood. The present study focuses on providing an insight
into the two-phase flow characteristics during evaporation of
water in 1-mm hydraulic diameter, multiple channel, electrically
heated evaporator section.
LITERATURE REVIEW
There are very few publications available in literature
addressing the flow patterns in multi-channel evaporators with
small diameter channels. Conventional compact evaporators
are plate-fin type, with the evaporating liquid flowing between
two parallel plates that have uniformly spaced bumps for
brazing the plates together. These obstructions provide a heat
transfer enhancement on the evaporation side. However, the
evaporating liquid is free to flow across the plate width.
The use of small diameter channels, each of 1-mm or less
hydraulic diameter presents quite a different scenario. Here the
pressure drop between the two manifolds is quite high and the
evaporating liquid cannot flow across into another flow channel
as in the case of a plate-fin type evaporator.
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Copyright © 2001 by ASME
Evaporation in 1-mm diameter tubes was studied by Lin et
al. (1999). They obtained the heat transfer data for evaporating
R-141b over the entire range of quality. The heat transfer
coefficient showed a similar trend as in the larger diameter
tubes (say, id>6 mm). However they observed pressure and
temperature fluctuations in the wall temperature and pressure
drop. The pressure drop fluctuations were analyzed by Kew
and Cornwell (1996), and they presented a theoretical model
that showed how an evaporating slug can lead to pressure drop
fluctuations in a single channel. The large pressure drop in
narrow channels leads to fluctuations in local saturation
pressure leading to fluctuating flow conditions.
The actual flow patterns existing in such parallel multichannel configurations are also not well studied. Table 1
presents a summary of a few important papers that investigate
flow patterns in small diameter channels. A number of studies,
such as those by Wambsganss et al. (1990), Barajast and Panton
(1993), Lin et al. (1998), and Triplett et al. (1999) studied the
flow patterns in air-water systems. The flow patterns observed
were similar to those for large diameter tubes except for a new
flow pattern, called rivulet flow, in which the liquid flows in
streams along the tube walls. However, the evaporating liquid
flows studied by Kasza et al. (1997) for water, and Kuznetsov
and Shamirzaev (1999) with R318C provide more useful
information regarding the existence of bubbly flow even in
small diameter tubes. Although Kuznetsov and Shamirzaev
observed suppression of nucleate boiling, Kasza et al. (1997)
report an increase in nucleation activity in the thin film adhering
to the flow channel walls.
Cornwell and Kew (1996) observed flow pattern during
evaporation of R-113 in two parallel multichannel geometries:
75 channels, 1.2 mm wide x 0.9 mm deep, and 36 channels 3.25
mm wide and 1.1 mm deep. They observed that there were
considerable fluctuations in heat transfer as well as flow
behavior. They identified three flow patterns: isolated bubble,
confined bubble, and annular-slug. Isolated bubbles were small
bubbles which move in the liquid, while confined bubbles
completely filled the flow cross section. For each region, they
developed a heat transfer correlation scheme.
Lin et al. (1998) present pressure drop characteristics
associated with different flow patterns observed during airwater flow in 2.1 mm diameter glass tubes. They observed
significant pressure drop fluctuations for annular flow, with
pressure drop value fluctuating between 0 and 6000 Pa for a gas
velocity of 4.309 m/s and a liquid velocity of 0.145 m/s. They
did not observe annular-slug flow pattern over the range of
parameters tested in their experiments. They also presented a
comprehensive table showing flow pattern investigations prior
to 1996 in a single small diameter channel.
Hestroni et al. (2000) studied the evaporation of water in
multi-channel evaporators consisting of 21-26 parallel flow
passages. They observed periodic behavior of the flow patterns
in these channels. The flow changed from single-phase flow to
annular flow with dryout in some cases. The dry-out, however,
did not result in sharp increase in the wall temperature. This
clearly indicates that there is still some evaporating liquid film
on the channel walls that could not be observed from the video
images. They also reported the presence of vapor phase in the
inlet plenum. The channel dimensions studied by Hestroni et al.
are around 0.103-0.129 mm in hydraulic diameter.
OBJECTIVES OF THE PRESENT WORK
As seen from the literature survey, there are very few
studies available on flow patterns with evaporating liquids in
small diameter multi-channels with hydraulic diameters around
1 mm. The present study is directed toward obtaining an
understanding of the flow pattern and two-phase flow behavior
in multi-channel evaporators with six parallel channels, each
with 1-mm hydraulic diameters. One of the main objectives is
to establish the flow characteristics and specific reasons for
severe pressure drop fluctuations observed in small diameter
multi-channel evaporators
EXPERIMENTAL SETUP
Figure 1 Experiment System: Reservoir (1), Pump (2),
Valve (3), Flow Meter (4), Valve (5), Test Section (6),
Condenser (7), Coolant Inlet (8), Flow Meter (9), and
Thermocouples A,B,C,D,E
The experiment setup (Fig. 1) consists of (1) a 5 gallon
reservoir for distilled water, (2) an Oberdorfer gear pump, (3) a
bypass valve for water flow rate control, (4) an Omega water
flow meter, (5) a control valve, (6) the test section, (7) a
condenser/calorimeter, (8) coolant flow for the condenser, and
(9) an Omega water flow meter. The flow meters (4) and (9)
each are accurate to ±3% of the measured flow rate. K-type
thermocouples (±0.5°C) measure the bulk water temperature at
locations indicated by the letters A-E in Fig. 1. Distilled water
flows from the reservoir through the pump to a water flow
meter. The bypass valve and control valve allow precise flow
rate control. Water then flows through the test section and the
condenser.
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Copyright © 2001 by ASME
Table 1 Summary of Two-phase Flow Patterns in Small Diameter Channels
Author
Year
Exptl.
Condition
Wambsgans
s,
Jendrzejczy
k, and
France
1990
Mass flux:
50~2000kg/
m2s
Barajast
Panton
and
Lin, Kew, and
Cornwell
Kasza,
Diascalou, and
Wambsganss
1993
Gas
velocity:
0.1~150m/s
1997
Mass flux;
kg/m2s
q: 110kW/m2
21
1998
Mass
1~10000kg/m2s
Kuznetsov and
Shamirzaev
flux:
1999
Mass flux:
200~900 kg/ m2s
q: 2~110 kW/m2
Channel
Geometry
Rectangular
Circular
Rectangular
Circular
Annulus
Channel
Size
19.05×3.18m
m
Aspect ratio:
6~1/6
5.47mm
i.d: 1.6mm,
300mm long
2.5×6.0×500mm
i.d: 2.1mm,
470mm long
0.9mm-annulus gap
400mm long
1.6mm
3.53mm
2.1mm
Air
and
water
Horizontal
Air and water
Water
Air and water
Horizontal
Horizontal
Isothermal
Isothermal
Two-phase
Not studied
Two-phase
Not reported
Dh
Test Fluid
Orientatio
n
Heating
Method
Phase
Pressure
Drop
Heat
Transfer
Flow
Pattern
Remarks
Triplett,
Ghiaasiaan,
Abdel-Khalik,
and
Sadowski
1999
Gas
velocity:
0.02~80m/s
Liquid velocity:
0.02~8m/s
Circular,
semitriangular
Hetsroni,
Segal,
and Mosyak
2000
Re: 20~70
q: 80~360 kW/m2
Triangular
(Parallel
Multi channel n=
21,26)
1.1, 1.45mm for 15mm long, θ =55°
circular,
200mm
long
0.129mm, 0.103mm
R318C
Circular:1.1,
1.45mm, SemiTri:1.09, 1.49mm
Air and water
Water
Vertical
Horizontal
Horizontal
Horizontal
Electrical
Isothermal
Electrical
Isothermal
Two-phase
Not reported
Electrical (uniform
and nonuniform)
Two-phase
Not clear
--
--
--
Not reported
Two-phase
Two-phase
Fluctuation range:
Not studied
Slug:-4255~8511Pa/m
Chum:-4255~6383Pa/m
Annular:-4255~14894Pa/m
-hmeasured: 1~20kW/m2 K
Plug flow,
bubble flow,
slug
flow,
wave flow,
annular flow
The
small
channel
dimensions
did
not
suppress
bubble flow.
Plug flow, slug
flow, rivulet flow,
annular
flow,
bubble
flow,
dispersed flow
Bubble flow,
slug flow
Slug flow, churn flow,
annular flow
A
new
flow
pattern:
rivulet
flow was found.
θ >90°, the contact
angle
had
a
significant effect
on
transition
boundaries.
Thin liquid-wall
films that are
formed cause both
bubble size and
generation
frequency
to
increase at wall
nucleation sites.
Existing
flow
maps
available
for
flow
regimes in small tube
except the boundaries
Two-phase
Not reported
--
Confined bubble flow, Cell Bubble flow,
churn flow, Slug
flow,
flow,
Annular flow
slug-annular flow,
annular flow
The capillary forces define Five flow patterns
the
flow
pattern. were
Decreasing of the film distinguished in
thickness
leads
to small channel.
suppression of boiling heat
transfer and a transition
from boiling heat transfer
to forced convection.
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Copyright © 2001 by ASME
Not clear
Periodic
flow
annular
Periodic annular flow
observed
in
microchannels. There
is
a
significant
enhancement of heat
transfer during flow
boiling
in
microchannels.
of the channel surroundings should be ignored. The dark band
in the center of each frame is the channel, in which the direction
of bulk flow is from left to right.
Figure 2 Test Section Detail
As shown in Fig. 2, the test section includes a chemically
etched stainless steel plate that features six parallel 1mm square
(nominally) channels 60mm long. The etching process creates
channels that have a substantial radius at each corner. The
etched plate is instrumented with five K-type thermocouples.
The etched plate is clamped to a Lexan window and a heater
cartridge. A thin Grafoil gasket is placed between the Lexan
window and the etched plate. The heater cartridge consists of a
240W maximum electrical resistance heater, mineral-fiber type
insulation, and a rigid stainless steel plate.
Temperatures at various locations are recorded using
Labview software. In the case of subcooled boiling, heat
transfer rates are obtained by performing a heat balance on the
water side of the test section (thermocouple locations A and B).
High-speed digital pictures are taken of the mini-channels
through the Lexan window using a MotionScope 8000S PCI
camera.
EXPERIMENTAL PROCEDURE
The experiments are conducted by first initiating the flow
of distilled water through the test section and tap water through
the condenser coolant loop. The flow rate is adjusted to a
desired level using the bypass and control valve. The Labview
system is activated. Then the power supply to the electrical
resistance heater is activated and set to deliver 157W to the
heater cartridge. Floodlights are used to illuminate the test
section. The high-speed digital camera is placed directly above
the test section. Once the Labview system indicates that the test
section outlet and surface temperatures are steady, digital
photograph sequences are taken to capture the boiling
phenomena visually. The Labview system is then used to record
temperature data to accompany the digital images.
RESULTS
The following flow patterns were observed in the 1mm
square channels: nucleate boiling, bubbly flow, slug flow,
annular/slug flow, annular/slug flow with nucleate boiling, and
dryout.
In the following figures, visible light and dark regions
surround the flow channel. Changes in image capture rate,
ambient light, and flow regime require adjustments to exposure
time and lighting that affect the appearance of the channel
surroundings. In some of the following figures the channel
surroundings are painted black to reduce glare. The appearance
Figure 3 Successive Frames (a) through (g) at 2ms
Intervals of one Channel in Nucleate Boiling- G=155
2
2
kg/m s, Ts=92.6°°C, TA=35.2°°C, TB=71.8°°C,q=133kW/m
Fig. 3 shows an example of nucleate boiling in one of the
channels. The vapor bubbles shown here are approximately
0.14 mm in diameter. Smaller bubbles were apparent during the
observation but are not visible in the photograph.
Notice in Fig. 3(a) that there is a nucleation site on the
channel wall (upper-left-hand area). In Fig. 3(b), 1ms later, the
bubble has grown. Fig. 3(c) shows the detachment of the vapor
bubble 2ms after the first frame. In Fig. 3(d), 3ms after frame
(a), the bubble collides with another and the newly formed
bubble travels along with the liquid stream in Figures 3(e), (f),
and (g) (4, 5, and 6ms after the first frame).
Fig. 4 illustrates bubbly flow. Vapor bubbles here range
from 0.16mm to 0.24mm in diameter. Although there is active
boiling occurring in the channel, the bulk outlet temperature of
the water is below the saturation temperature (100°C),
indicating subcooled boiling.
Fig. 4(a) is a picture of bubbly flow in a single channel;
several vapor bubbles are visible here as they travel along with
the fluid flow. The frames (a)-(h) shown in Fig. 4 are at 1ms
successive time interval.
Slug flow is represented in Fig. 5. Vapor bubbles in this
regime grow nearly to the channel width (1mm) then expand in
both directions along the channel before traveling to the outlet.
Only a thin film remains on the channel walls.
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Copyright © 2001 by ASME
In Fig. 5(a), there are three visible bubbles in the left
portion of the channel and a fully formed slug on the right. The
time interval between each successive frame shown in the figure
is 4ms. In Fig. 5(b), the three bubbles on the left side travel to
the right yet the fluid/vapor interface at the left side of the slug
moves very little. This would indicate that the vapor slug is
expanding in the direction opposite the fluid flow. In Fig. 5(c),
the slug has begun to move with the fluid flow. Figures 5(d)
through (h), the vapor bubbles begin to expand into slugs as
they move through the channel.
Fig. 6 shows an example of annular/slug flow. In this
regime, a fluid film travels along the channel walls and vapor
travels in the center. In the figure, an average film thickness of
0.23mm is observed (this is a very approximate value). Annular
flow is maintained throughout the entire pictured section in each
frame- these pictures are indexed by 4ms. In Fig. 6(a), there is a
visible fluctuation in the annular fluid/vapor interface near the
center of the frame. In Fig. 6(b) and (c), this fluctuation moves
along the channel to the right in the direction of flow. In Fig.
6(d), the fluctuation is no longer visible. Figures 6(d) through
(h) show the time progression in the wall fluctuations in the
fluid/vapor interface. These fluctuations identify the fluid/vapor
interface and indicate annular flow.
Figure 4 Successive Frames (a) through (h) at 1ms
Intervals of One Channel in Bubbly Flow2
G=39.5kg/m s, Ts=120°°C, TA=24.2°°C, TB=99.3°°C, x>0
Figure 6 Successive Frames (a) through (h) at 4ms
Intervals of One Channel in Annular/Slug Flow2
G=28kg/m s, Ts=112°°C, TA=24.2°°C, TB=99.3°°C, x>0
Figure 5 Successive Frames (a) through (h) at 4ms
2
Intervals of One Channel in Slug Flow-G=28kg/m s,
Ts=112°°C, TA=24.2°°C, TB=99.3°°C, x>0
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Copyright © 2001 by ASME
nucleation sites yield bubbles too small to be seen with this
equipment.
Figure 7 Successive Frames (a) through (h) at 1ms
Intervals of One Channel in Annular/Slug Flow with
2
Nucleate Boiling- G=40kg/m s Ts=120°°C, TA=24.2°°C,
TB=99.3°°C, x>0
Fig. 7 illustrates annular/slug flow with nucleate boiling. In
this regime, the annular liquid film boils at nucleation sites
along the channel walls. In the figure, the film thickness is
approximately 0.23mm as seen from this angle, and the
approximate bubble diameters are 0.09-0.12mm. The frames
shown were taken at intervals of 1ms. The annular fluid/vapor
interface is present throughout the pictured section of the
channel in each frame. In Fig. 7(a), nucleation sites are visible
in the right-hand region of the frame. In Fig. 7(b), the bubbles
in the right-hand region have detached and entered the vapor
channel. In Figures 7(c) through (f), vapor bubbles are formed
at nucleation sites near the center region of each frame and
detach into the vapor channel. Figures (g) and (h) show new
nucleation in the thin fluid film along the channel walls.
Dryout is represented in Fig. 8. For the flow pattern shown,
the liquid film of the annular flow regime is reduced until the
channel wall is dry and only vapor flows in the channel. This
flow pattern was not seen as a stable one and occurs only
intermittently. Following the annular flow, a fluid interface is
visible near the center of the visible channel in early frames of
Fig. 8. For this figure, 1ms elapsed between frames. In Fig. 8(a)
nucleation in a thin annular film can be seen just to the right of
the interface region. In Figures 8(b) through (h), nucleation can
be seen in the thin film on the upper part of the right-hand side
of each frame. In Figures 8(h) through (j), an apparent dryout
condition exists on the lower-right-hand region of each frame. It
is possible, however, that a very thin film in which the
Figure 8 Successive Frames (a) through (j) at 1ms
2
Intervals of One Channel in Dryout- G=40kg/m s,
Ts=120°°C, TA=24.2°°C, TB=99.3°°C, x>0
Pressure drop across the 6 parallel channel configuration is
profoundly affected by boiling phenomena. Vapor generation in
a confined channel causes a fluid flow restriction as the vapor
bubble expands. When the bulk mass flow rate is held constant
across all the channels, the increased resistance to flow in a
vapor restricted channel causes faster flow in the remaining
channels. When all of the channels are experiencing annular
flow with intermittent dryout, a large differential pressure builds
at the test section inlet. When the differential pressure at the
inlet reaches a threshold value, a fluid slug overcomes the vapor
restriction and wets one or more channels while relieving the
pressure buildup. Fig. 9 shows a representative differential
pressure versus time plot for a similar test section undergoing
vigorous boiling.
Notice in Fig. 9 that negative pressure drops were
recorded. The visual study focuses on slug flow and boiling
phenomenon involved in pressure fluctuations that may result in
the inversion of the pressure gradient across the test section.
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Copyright © 2001 by ASME
Unit pressure drop (Pa/m)
6000
4000
2000
0
-2000
0.00
50.00
100.00
150.00
200.00
Time (s)
Figure 9 Differential Pressure History for a 6 Channel
2
(1mm x 1mm) Parallel Configuration- G=48kg/m s,
∆Pmax/L = 4688Pa/m, ∆Pmin/L = -1793Pa/m, Ts = 125°C,
2
Q”=74.3kW/m , TB = 90.17°C.
In Fig. 11, the formation of a vapor slug and its subsequent
motion are shown. The lines through the frames in this figure
enclose the boundary of the developing slug in the first frame,
Fig. 11(a). Flow through the visible section progresses from left
to right and 2ms passes between frames. In Fig. 11(b), the slug
has grown in the direction of flow. In Fig 11(c), the slug has
grown in the direction of flow and in the direction counter to
flow as well. This phenomenon of the fluid/vapor interface
moving in a direction counter to the bulk flow continues in
Figures 11(d) through (h). In Fig. 11(i), the slug stops its
motion in the direction counter to the direction of bulk flow and
joins with a slug to the right. In Figures (j) through (l), the slug
is carried in the direction of bulk flow.
Fig. 10 illustrates a situation where two neighboring
channels differ in flow regime. The bottom channel is
experiencing a slug/dryout condition while the upper channel is
in the bubbly flow regime. In this figure, 2ms passes between
each frame.
Figure 10 Successive Frames (a) through (j) at 2ms
Intervals of Two Channels in Differing Flow Regimes2
G=40kg/m s, Ts=120°°C, TA=24.2°°C, TB=99.3°°C, x>0
Figure 11 Successive Frames (a) through (l) at 2ms
Intervals of One Channel Exhibiting Slug Formation
2
and Motion- G=40kg/m s, Ts=110.6°°C, TA=24.7°°C,
TB=99.3°°C, x>0
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Copyright © 2001 by ASME
Figure 12 illustrates another example of slug formation and
vapor/fluid interface motion counter to the bulk flow. The lines
across the frames in the figure enclose the boundary of the slug
of interest in Figure 12(a). The time elapsed between frames is
2ms. In Figures 12(b) and (c), the slug grows and moves in the
direction of bulk flow. The fluid/vapor interface on the left side
of the slug in Figures 12(d) through (g) is moving in the
direction counter to the bulk flow. In Figures (h) and (i), the
fluid/vapor interfaces of the slug are out of range of the viewing
window. With the fluid/vapor interfaces absent, Figures (h) and
(i) appear to be in the annular flow regime.
Fig. 13(c) although the outlet-side interface moves in the bulk
flow direction. In Fig. 13(d), however, the flow in channel (1)
moves along the direction of bulk flow but the inlet-side
fluid/vapor interface of the slug in channel (2) progresses in the
direction counter to the bulk flow. In Fig. 13(e), the inlet
fluid/vapor interfaces in both channels move in the direction
counter to bulk flow. In this situation, it appears that both of the
channels experience a vapor-clogging condition where
differential pressures across the channels increase due to vapor
generation and consequently reduce flow through the two
channels. The reaction to this condition in the other four
channels would be an increased flow rate. The pressure drop
across the entire system of six channels would increase since the
differential pressure across each of the parallel channels must
be equal.
In Fig. 13(f), the inlet-side fluid/vapor interfaces move past
the vertical reference line (x). In Figure 13(g), the slug in
channel (1) is shown displaced in the direction of bulk flow
while the slug in channel (2) continues in the direction counter
to bulk
Figure 12 Successive Frames (a) through (i) at 2ms
Intervals of One Channel Exhibiting Slug Formation
2
and Motion- G=40kg/m s, Ts=110.6°°C, TA=24.7°°C,
TB=99.3°°C, x>0
Fig. 13 illustrates a time progression of fluid/vapor
interactions in two adjacent channels. Both channels (1) and (2)
exhibit slug flow in the visible section. Vertical lines (y) and (z)
indicate the initial boundaries of a vapor slug in channel (2) in
Fig. 13(a). Vertical lines (x) and (w) are reference lines to aid
visualization of slug motion throughout the frames.
In Fig. 13(b) channel (1) has flow in the direction of bulk
flow (left to right) and the vapor slug in channel (2) has
expanded in the direction of bulk flow yet the inlet-side
fluid/vapor interface has not moved. The fluid/vapor interface
on the inlet side of the slug in channel (2) is still stationary in
Figure 13 Successive Frames (a) through (e) at 2ms
2
Intervals of Two Channels Interacting- G=40kg/m s,
Ts=110.6°°C, TA=24.7°°C, TB=99.3°°C, x>0
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Copyright © 2001 by ASME
Figure 13 Successive Frames (f) through (j) at 2ms
2
Intervals of Two Channels Interacting- G=40kg/m s,
Ts=110.6°°C, TA=24.7°°C, TB=99.3°°C, x>0
Figure 13 Successive Frames (k) through (o) at 2ms
2
Intervals of Two Channels Interacting- G=40kg/m s,
Ts=110.6°°C, TA=24.7°°C, TB=99.3°°C, x>0
flow past the vertical line (w). It is evident that in the 2ms
between Figures 13 (g) and (h) that the inlet-side slug
fluid/vapor interfaces are moving in opposite directionsspecifically, in channel (1) the direction of motion is toward the
outlet and in channel (2) the direction of motion is toward the
inlet. Although the inlet-side fluid/vapor interface in channel
(2) in Fig. 13(i) is difficult to see, it is apparent that the slug in
channel (1) grows in the direction counter to bulk flow. In Fig.
13(j), the slug in channel (1) continues to grow in the direction
counter to bulk flow and the slug in channel (2) begins to move
in the direction of bulk flow.
In Fig 13(k), both channels are exhibiting slug growth in
the direction counter to bulk flow. When both channels
experience interface motion in the direction of the inlet, it is
reasonable to expect a sharp increase in differential pressure
across the system of channels. In Fig 13(l), channel (1)
continues to experience interface motion counter to the bulk
flow and channel (2) begins to move in the direction counter to
bulk flow. In Figures 13(m) and (n), both channels show slug
motion in the direction of bulk flow. When both channels
experience a change
in the direction of the motion of an interface from counter to
bulk flow to the direction of bulk flow, it is reasonable to expect
a sharp relief in differential pressure across the system of
channels. Fig. 13(o) shows the slug in channel (2) moving
toward the outlet and a new slug forming about the vertical line
(w) in channel (1).
CONCLUSIONS
• The following flow patterns are observed in a parallel
channel evaporator with six channels of 1 mm
hydraulic diameter: bubbly flow with nucleate boiling,
bubbly flow, slug flow, annular/slug flow, annular/slug
flow with nucleate boiling, and dryout.
• Large pressure fluctuations were evident. These
pressure fluctuations are attributed to boiling
phenomena, specifically the violent evolution of vapor
in the slug-flow, annular/slug flow, and annular/slug
flow with nucleate boiling regimes.
• Slug growth occurs in the direction counter to bulk
flow in parallel channel evaporators, forcing liquid and
vapor back into the inlet manifold - this is evidence of
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Copyright © 2001 by ASME
•
•
•
vapor clogging and a justification for severe pressure
fluctuations in parallel channels.
The reversed flow in the parallel channels and vapor
back flow into the inlet manifold have been clearly
observed for the first time in a small diameter multichannel evaporator. Although it is very difficult to
account for such flow reversals in heat transfer
modeling, future attempts should include this fact in
the analysis.
Annular flow in these observations only occurred as an
intermittent condition after the growth of a slug.
Dryout condition was not sustainable at the surface
heat fluxes employed in the present study.
Triplett, K. A., Ghiaasiaan, S. M., Abdel-Khalik, S. I., and
Sadowski, D. L., 1999, “Gas-liquid two-phase flow in
microchannels, Part: Two-phase Flow Patterns, International
Journal of Multiphase Flow, Vol. 25, pp. 377-394.
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