i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 4 3 9 e1 4 5 4
Available online at www.sciencedirect.com
w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
Two-phase flow patterns in U-bends and their contiguous
straight tubes for different orientations, tube and bend
diameters
Ricardo J. Da Silva Lima a,b,*, John R. Thome a
a
Laboratoire de Transfert de Chaleur et de Masse (LTCM), Institut de Génie Mécanique (IGM), Sciences et Techniques de l’Ingénieur (STI),
École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
b
École d’Ingénieurs et d’Architectes de Fribourg (EIA-FR), Bd de Pérolles 80, CP 32, CH-1705 Fribourg, Switzerland
article info
abstract
Article history:
In two-phase systems there are many singularities in piping systems, one of which in
Received 3 August 2011
particular are U-bends (also called 180 return bends or U-tubes) found very often in air
Received in revised form
conditioning coils, refrigerant systems, etc. Contrary to the literature concerning two-
10 January 2012
phase flow patterns in straight tubes, where many studies are available, only a limited
Accepted 7 February 2012
number of studies concerning bent tubes can be found. In this study, observations of two-
Available online 3 April 2012
phase flows in U-bends and their contiguous straight tubes were made with R134a at 5 C
for three different orientations (horizontal flow, vertical upflow and vertical downflow).
Keywords:
Five different glass test sections with three different internal diameters (13, 11 and 8 mm)
Refrigerant
and five different bend diameters (67, 38, 55, 32, 25 mm) were used. The experiments were
Two-phase flow
made for three mass fluxes (150, 300 and 500 kg s1 m2) and covered vapor qualities
Experimentation
ranging from 0.05 to 0.95. A total of 342 flow pattern observations were made during this
Imaging
experimental campaign. Analysis of the videos showed that, for the present range of
U-tube
experimental conditions, the centrifugal force was dominant over that of the gravity. In the
Horizontal tube
U-bend, the flow patterns are qualitatively similar to those observed in the straight tube
Vertical tube
sections before the bends. Upstream of the U-bend, in the straight tubes the flow pattern
are not modified. Downstream of the U-bend the flow patterns are generally reestablished
shortly, except for vertical upflow orientations.
ª 2012 Elsevier Ltd and IIR.
Configurations de l’écoulement diphasique dans les coudes
un U et les tubes droits contigus pour différentes orientations,
et pour plusieurs diamètres de tuyaux et de coudes
Mots clés : Frigorigène ; Écoulement diphasique ; Expérimentation ; Imagerie ; Tube en U ; Tube horizontal ; Tube vertical
* Corresponding author. École d’Ingénieurs et d’Architectes de Fribourg (EIA-FR), Bd de Pérolles 80, CP 32, CH-1705 Fribourg, Switzerland.
Tel.: þ 41 26 429 66 54; fax: þ 41 26 429 66 00.
E-mail addresses: rs_lima@hotmail.com (R.J. Da Silva Lima), john.thome@epfl.ch (J.R. Thome).
0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR.
doi:10.1016/j.ijrefrig.2012.02.002
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Nomenclature
Latin
D
d
G
h
L
_
m
P
Q_
R
T
x
Bend diameter, m
Internal diameter, m
Mass flux, kg s1 m2
Enthalpy, J kg1
Length, m
Mass flow rate, kg s1
Pressure, Pa
Heat power, W
Bend radius, m
Temperature, K or C
Vapor quality, e
Subscripts
in
Inlet
L
Liquid
out
Outlet
1.
Introduction
In two-phase systems, besides the straight tubes, there are
also many singularities, in particular U-bends (also called
180 return bends or U-tubes). However, contrary to the
literature concerning two-phase flow patterns in straight
tubes, where many studies are available, only a limited
number of studies concerning bent tubes can be found. The
number of studies is even smaller if one considers refrigerant
two-phase flows.
The first study on pressure drop in bent tubes with singlephase flows was made in the 18th century by Du Buat (1786),
while the complexity of the flows inside bent tubes was only
discovered at the beginning of the 20th century, experimentally by Eustice (1910, 1911) and then analytically by Dean
(1927, 1928a, 1928b). The complexity of flows inside bent
tubes is due to their intrinsic characteristic, an additional
force caused by the curvature: the centrifugal force. In singlephase flow, this force is directed from the instantaneous
centre of curvature to the outer wall of the bend, and
combined with the presence of a boundary layer at the wall,
produces a secondary flow ideally organized into two
symmetrical vortices. The fluid in the core moves outwards
and in the region near the wall inwards. The secondary flow is
superimposed on the main stream along the tube axis,
resulting in a helical shape to the streamlines. This
complexity is further increased with two-phase flows.
Because of the density difference between the two phases, the
heavier phase tends to go towards the outer wall, modifying
the two-phase flow pattern in the bend. Thus, the flow may be
different, not only in the bend, but also in the straight tube
sections up- and downstream of the bend. Another level of
complexity can be added if one considers different orientations, in which case the gravitational, viscous and centrifugal
forces may be combined in many different manners depending on their orientation.
Flow structure, pressure drop and heat transfer in U-bends
as well as before and after U-bends are an old unresolved
sat
V
Saturation
Vapor
Abbreviations
ANR
Annular flow pattern
BBY
Bubbly flow pattern
DYT
Dryout flow pattern
EIA-FR École d’Ingénieurs et d’Architectes de Fribourg
EPFL
École Polytechnique Fédérale de Lausanne
IGM
Institut de Génie Mécanique
IMT
Intermittent flow pattern
LTCM
Laboratoire de Transfert de Chaleur et de Masse
MST
Mist flow pattern
SLG
Slug flow pattern
STD
Stratified flow pattern
STI
Sciences et Techniques de l’Ingénieur
SST
Slug-stratified flow pattern
SWT
Slug-stratified-wavy flow pattern
TS
Test section
design problem in the heat transfer industry, especially in
coils which have a multitude of U-bends. These are thus
crucial aspects for optimizing heat exchanger design.
The objective of the present study is to investigate the
effects of U-bends on two-phase flows before, in and after the
U-bends, and in particular the influence of the orientation of
the U-bend and the flow directions. This is part of a much
larger study in which the two-phase pressure drops were
measured for five different U-bends for R134a and R410A in
highly instrumented copper U-bends, results which will be
presented in the near future.
2.
Experimental program
2.1.
Test facility
The experimental test facility used in this project is the LTCM
two-phase intube refrigerant test facility in a similar configuration as used by Silva Lima et al. (2009). This test facility was
previously used for various studies of refrigerants flowing
inside straight tubes concerning flow pattern observations
(Kattan et al., 1998a; Zürcher et al., 2002); dynamic void fractions measurements (Ursenbacher et al., 2004; Wojtan et al.,
2004, 2005c); heat transfer coefficient measurements in
circular plain tubes (Kattan et al., 1998b; Zürcher et al., 1998b,
1999; Wojtan et al., 2005b; Silva Lima et al., 2009), in circular
microfin tubes (Zürcher et al., 1998a) and flat channels
(Moreno Quibén et al., 2009b); pressure drops measurements
in circular plain (Moreno Quibén and Thome, 2007a, 2007b)
and flat channels (Moreno Quibén et al., 2009a). Recently, this
test section was used by Silva Lima and Thome (2010) to
measure peripheral and axial pressure distributions as well as
pressure drops in U-bend test sections and contiguous
straight tubes.
The test facility consists of three circuits: refrigerant, hot
water and a cold water-glycol mixture. For the sake of
simplicity, only the refrigerant circuit is described here. From
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1441
heavily insulated to avoid any heat exchange with the
surroundings.
The counter-current evaporator, depicted in Fig. 2, is a tube
in tube heat exchanger with the refrigerant flowing in the
center tube and the hot water flowing in the annulus of the
double pipe system. More details can be found in Silva Lima
and Thome (2010); Silva Lima (2011).
2.2.
Fig. 1 e Simplified schematic of the refrigerant circuit.
the last configuration (Silva Lima et al., 2009), this test facility
was modified to allow the connection of different U-bend test
sections. Moreover, a new frame was built (with a special
tilting mechanism) to support the U-bend test section in
several orientations and respective instrumentation. The test
facility and test sections are described in detail in Silva Lima
and Thome (2010) and Silva Lima (2011).
A simplified schematic of the refrigerant circuit is depicted
in Fig. 1. A magnetically driven gear pump (oil free) is used to
circulate the refrigerant fluid in the test loop. After the pump,
the fluid passes through a Coriolis mass flow meter and then
goes into a series of three horizontal electrical pre-heaters.
Then, the refrigerant enters the counter-current heat
exchanger (with hot water heating) where it is partially
evaporated and goes to the U-bend test section before going
back to the condenser.
The electrical pre-heaters and counter-current heat
exchanger are used to set the inlet vapor quality of the
refrigerant. The operating pressure (saturation temperature)
is controlled by the amount of refrigerant in the facility for
which the circuit is equipped with a vaporeliquid reservoir.
All the tube-to-tube connections have a smooth interface
in order not to disturb the flow patterns. All the pipes are
Test section
Fig. 3 depicts a schematic diagram of the U-bend test section.
From the flexible tube (that connects the test facility to the
U-bend test section) the refrigerant enters the U-bend test
section through a flow stabilizer and goes through the U-bend
measurement section. From the U-bend measurement
section the refrigerant enters the visualization test section
and flows to the outlet U-bend section were it exits the
U-bend test section and goes to the flexible tube, and then
back to the test facility (condenser). The visualization test
section was mounted at the outlet of the measurement
section in order to prevent measurement perturbations. For
security reasons (in case of rupture of the glass visualization
test section) the saturation temperature of the R134a was
limited to 5 C and the visualization test section was enclosed
in an aluminum framed chamber (with MAKROLON transparent panels) equipped with an exhaust fan. Details of the
measurement test section, instrumentation and data reduction can be found in Silva Lima and Thome (2010) and Silva
Lima (2011).
The visualization test sections, depicted in Fig. 4, are
a glass tubes manually bent with the help of a die (to ensure
geometrical homogeneity) into a U-tube. Table 1 gives the
dimensions of the five visualization sections, where d is the
internal diameter, D is the bend diameter (centerline to
centerline) and L the straight lengths before and after the
U-bend.
The flow pattern images were obtained with a fast acquisition camera (PHOTRON FASTCAM ). The camera was fixed on
the top of the tube while a uniform lighting, provided by a LED
light (20 20 cm2) was placed behind the glass tube. For the
vertical orientations, refer to Fig. 5, the liquid to vapor interface was perpendicular to the LED light and thus, the video
Fig. 2 e Counter-current heat exchanger before the U-bend test section.
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Fig. 3 e U-bend test section schematic.
Fig. 4 e Flow visualization sections.
camera could directly capture the images. For the horizontal
orientation, the liquid to vapor interface was parallel to the
field of view of the camera, and thus no image could be
captured. Therefore, for the horizontal orientation an
arrangement of mirrors was made to allow the visualization of
the liquid to vapor interface.
The mirror system is set up differently depending on the
pffiffiffi
value of the curvature ratio D/d. If D=d > 1 þ 2 2, then the
space between the two straight tubes was large enough to
allow for two mirrors (each one reflecting one straight tube
image) to be placed between the inlet and outlet straight
pffiffiffi
tubes. In the other case, where D=d < 1 þ 2 2, only one mirror
is set for the outlet straight tube.
2.3.
Instrumentation & data acquisition system
Details on the instrumentation and data acquisition system
can be found in Silva Lima and Thome (2010) and Silva Lima
(2011).
Table 1 e Flow visualization section dimensions.
TS (e)
1
2
3
4
5
d (mm)
2R (mm)
2R d1 (e)
L
L d1
13
67
38
55
32
25
5.15
2.92
5
2.91
3.13
610
655
550
550
400
47
50
50
50
50
11
8
2.4.
Experimental conditions
The experimental conditions as well as their respective
uncertainties are summarized in Table 2. More details on the
calculation of the uncertainties can be found in Silva Lima and
Thome (2010) and Silva Lima (2011).
2.5.
Data reduction
The main elements to be considered here are the physical
properties and the vapor quality at the inlet of the U-bend
visualization test section.
Anywhere from the inlet of the flow boiling heat exchanger
down to the exit of the U-bend measurement test section and
inlet of the U-bend visualization test section, the local saturation temperatures and any other physical properties of the
refrigerant are always based on an absolute pressure
measurement. The physical properties of the fluid are tabulated in small pressure intervals (equivalent to 0.5 C). This
small interval justifies a first order interpolation between two
data (two pressures) rather than an nth order polynomial
interpolation over a wide range of saturation pressures, which
would produce a much higher interpolation error. Thus, the
saturation temperature at location j is expressed by:
Tsat;j ¼ aj þ bj Pj
(1)
where a and b are the first order polynomial coefficients. The
same modus operandi is used for all the required physical
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1443
Fig. 5 e Visualization system with camera for vertical and horizontal orientations.
properties, substituting Tsat by the required physical property.
The physical properties of the liquid water follow the same
method, but all are based on temperature measurements
instead of pressure measurements.
The vapor quality calculation is based on an energy
balance taking into consideration the local conditions (so that
the flashing due to the pressure drop is also accounted for). In
the general case for a considered element, this energy balance
can be expressed as:
_ in hV;in þ Q_ ¼ mð1
_ xout ÞhL;out þ mx
_ out hV;out
_ xin ÞhL;in þ mx
mð1
3.
Flow pattern observations
This section focuses on the flow pattern observations made
during the experimental campaign. Because flow visualizations were made for all the experimental conditions reported
in this study (342 videos), it is not possible to show them all in
the scope of this article. Instead, only selected images are
shown to illustrate the physical phenomena occurring in the
(2)
which re-arranged, leads to:
xout ¼ xin
Q_
hLV;in Dout
hL
in
þ
_ LV;out
hLV;out hLV;out mh
(3)
where hL and hLV are the liquid and latent heat enthalpies, Q_
_ the refrigerant mass flow rate. This
the transferred heat and m
is applied sequentially from the outlet of the condenser down
to the inlet of the visualization test section.
A summary of the experimental conditions as well as their
respective uncertainties can be found in Table 2.
Table 2 e Range of experimental conditions and
respective mean uncertainties.
Parameter
Fluid
Tsat [ C]
G [kg s1 m2]
x [e]
d [mm]
D [mm]
Values
Mean uncertainty
R134a
5
150, 300 and 500
0.05 to 0.95
8, 11 and 13
25, 32, 55, 38, 67
e
0.1
3
0.01
0.05
0.5
Fig. 6 e Flow pattern map of Wojtan et al. (2005a) for R134a
at 5 C flowing at 310 kg sL1 mL2 inside the visualization
section with d [ 13 mm (and D [ 55 mm). The dryout and
mist boundaries are generated in the counter-flow heat
exchanger. The flow patterns names were coded as: STW:
stratified-wavy, SSW: slug-stratified-wavy, MST: mist flow,
DYT: dryout, SLG: slug, IMT: intermittent and ANR: annular.
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U-bends and in the inlet and outlet straight tubes near the Ubend. Generally, the images were chosen at three instants:
when a wave approaches the U-bend, when it leaves the Ubend and when it leaves the field of view of the test section.
Also, the indicated saturation temperatures (5 C) correspond
to the saturation temperature at the inlet of the U-bend test
section.
3.1.
Generalities
Fig. 6 depicts the flow pattern map of Wojtan et al. (2005a) for
the experimental conditions obtained with R134a at 5 C
flowing at 310 kg s1 m2 inside the 13 mm internal diameter
tube (bend diameter of 66 mm and horizontal configuration).
All the flow patterns are represented, including the dryout and
mist regimes. Note that the test section is adiabatic but the
dryout and mist regimes, for which a heat flux is required, are
generated in the heat exchanger test section. At low vapor
quality the flow pattern is slug (SLG). As the vapor quality
increases the process path enters the intermittent (IMT )
regime. At a vapor quality of about 0.3, the process path crosses
the intermittent to annular (ANR) boundary. The vapor quality
is going to increase until the dryout inception is reached at the
dryout (DYT ) boundary. In this regime the liquid film intermittently wets the perimeter of the tube. As the vapor quality
increases the liquid film reaches the dryout completion and
the process path enters the mist (MST ) regime.
Fig. 7 depicts the expected number of occurrences of each
flow pattern in the straight tubes as predicted by the flow
pattern map of Wojtan et al. (2005a, 2005b) for all the experiments. The dryout and mist regimes are generated in the heat
exchanger test section. For the lower mass flux of
150 kg s1 m2, Fig. 7(a), the following flow patterns were
predicted to occur: stratified-wavy, slug-stratified-wavy,
annular and dryout. For the medium mass flux of
300 kg s1 m2, Fig. 7(b), the following flow patterns were
predicted to occur: slug, intermittent, annular, dryout and
mist. For the higher mass flux of 500 kg s1 m2, Fig. 7(c), the
a
b
c
d
Fig. 7 e Flow pattern distribution at the inlet of the U-bend visualization test section covering all the experimental database
for: (a) G [ 150 kg sL1 mL2, (b) G [ 300 kg sL1 mL2, (c) G [ 500 kg sL1 mL2 and (d) for all mass velocities. The flow pattern
regimes were predicted by the flow pattern map of Wojtan et al. (2005a). The dryout and mist boundaries are generated in
the counter-flow heat exchanger. The flow patterns names were coded as: STW: stratified-wavy, SSW: slug-stratified-wavy,
MST: mist flow, DYT: dryout, SLG: slug, IMT: intermittent and ANR: annular.
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following flow patterns were predicted to be present: slug,
intermittent and annular. It can be seen that for this mass flux
the expected number of occurrences of intermittent flow is
higher than of annular flow, which is due to the capabilities of
the facility. For these conditions, due to the higher pressure
drop, the experimental range of vapor quality was limited to
a maximum value of 0.5 by the pump. Qualitatively, the predicted flow patterns were found to match very well with the
experimental observations before the U-bend.
3.2.
Horizontal flows
Regarding the horizontal U-bend flow observations, the flow
patterns observed were: stratified-wavy, slug-stratified-wavy,
intermittent, annular and dryout and mist. They will be discussed below at the three mass fluxes tested.
3.2.1.
Low mass flux, 150 kg s1 m2
Fig. 8 depicts the flow with a wave approaching the U-bend,
leaving the U-bend and leaving the visualization test section.
At this low mass flux and low vapor qualities the flow regime
is stratified-wavy. With no wave at the U-bend it can be seen
that the liquid tends to be pushed against the outer wall of the
U-bend, forming a liquid layer that is oriented perpendicular
to the stratified layer at the inlet. When the wave passes
through the U-bend the liquid layer thickness on the outer
wall of the U-bend increases. At the outlet of the U-bend the
liquid stratifies again on the bottom of the tube. Note that at
the U-bend, depending of the height of the wave, a sort of
annular flow is generated with the liquid moving around the
periphery of the tube, up to the top of the tube. This particular
flow pattern differs from the usual straight pipe annular flow
because in this case the liquid is moving with a helical
trajectory around the periphery, as opposed to straight tube
annular flow, where the liquid moves parallel to the axis of the
straight tube. This particular flow persists until the outlet of
the U-bend. At the outlet of the U-bend, the liquid film falls
towards the bottom of the perimeter, until the flow is
stratified-wavy again, which occurs approximately at 2d
(internal diameters) downstream from the U-bend. The wave
then tends to oscillate between the inner and outer side of the
straight tube. With large waves, this distance can increase up
to the limits of the view in the frame.
As the vapor quality increases (not shown here), the liquid
height decreases and the wetted fraction of the perimeter
increases. The perturbation caused by the U-bend seems to
propagate further along the downstream straight tube. For
vapor qualities of 0.29 (not shown here) it was observed that
the top of the tube is wetted downstream to 4d, while for 0.36
(not shown here) that distance is about 5d.
At a vapor quality of 0.48 the flow becomes annular as
illustrated in Fig. 9. On the second half of the U-bend, liquid
droplets detach from the interfacial waves which then hit
either the outer side of the U-bend or the outer side of the
outlet straight tube. At even higher vapor qualities, for
example at 0.62 (not shown here), the shape of the interfacial
waves is modified at the inlet of the U-bend. These
phenomena, occurring in the annular flow subsist for higher
vapor qualities, until dryout occurs.
For the smaller bend diameter the phenomena described
above are intensified. For example, the liquid thickness on the
outer side of the bend is increased, even with no occurrence of
waves.
3.2.2.
Fig. 8 e Flow pattern images for the horizontal
configuration with R134a at 5 C flowing at 150 kg sL1 mL2
and x [ 0.12 inside the visualization section with
d [ 11 mm, D [ 55 mm. Time intervals are D21 t [ 130 ms
and D32 t [ 80 ms. The U-bend shows the top view while the
mirrors inside show the two side views.
1445
Medium mass flux, 300 kg s1 m2
At a medium mass flux the phenomena observed are similar
to what was described for the lower mass flux, except that the
phenomena are intensified. At this mass flux, before the
annular regime is reached, the perimeter is completely wetted
but large swarms of droplets travel together with the liquid
film. When these swarms of droplets reach the U-bend, Fig. 10,
they are driven against the outer wall of the U-bend. This
tends to pack these droplets together and form a consistent
liquid layer.
As the vapor quality increases, the flow becomes annular
before reaching the dryout. For annular flow the same
phenomena as for the lower mass flux were observed. For
example, the modification of the shape of the interfacial
waves at the approach of the U-bend can be seen in Fig. 11. At
dryout inception (generated in the counter-current heat
exchanger) the liquid intermittently wets the wall of the tube,
until it dries out. Fig. 12 shows the path described by this
wetting liquid film. The thin liquid film starts by wetting the
bottom of the straight tube. When it arrives close to the
U-bend, the liquid film is pushed against the inner wall of
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Fig. 9 e Flow pattern images for the horizontal configuration with R134a at 5 C flowing at 150 kg sL1 mL2 and x [ 0.48
t [ 2 ms.
inside the visualization section with d [ 11 mm, D [ 55 mm. Time intervals are DiD1
i
the U-bend. As the liquid film thickness increases, the wetted
angle increases and the liquid film starts to reach the top of
the tube. Again, close to the U-bend the thin liquid film is
pushed against the inner wall of the U-bend.
3.2.3.
High mass flux, 500 kg s1 m2
At high mass flux the flow observations are very similar to
what was described for the medium mass flux. For this reason
no additional descriptions are given here.
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1447
not affect the flow pattern, except at the outlet of the U-bend
and respective outlet straight tube (refer to Figs. 13 and 14). At
these locations, the liquid film on the top of the tube persists
further in the outlet straight tube (i.e. about 2e3d ).
A further increase of the vapor quality (not show here)
leads to an increase of the wetted perimeter on both inlet and
outlet straight tubes as well as on the U-bend. Furthermore,
the wetted length on top of the outlet tube is also increased.
The flow regime tends to become annular. No significant
change in the flow pattern occurs at the U-bend. At the outlet
of the U-bend and along the straight outlet tube, a thicker
liquid film persists on the top of the tube. The thickness of this
liquid film is similar to that of the liquid stratified at the
bottom of the tube. The difference in thickness fades out along
the outlet straight tube.
In the annular flow regime and at high vapor quality, where
the high vapor velocity increases shear on the liquid film, Fig. 15
shows some liquid droplets detaching from the liquid film and
splashing against the outer wall of the U-bend. Also, the shape
of the interfacial waves at the inlet of the U-bend is modified.
These phenomena were already observed for horizontal flow.
3.3.2.
Fig. 10 e Flow pattern images for the horizontal
configuration with R134a at 5 C flowing at 300 kg sL1 mL2
and x [ 0.25 inside the visualization section with
d [ 11 mm, D [ 55 mm. Time intervals are D21 t [ 50 ms
and D32 t [ 40 ms.
3.3.
Vertical downflows
As for horizontal flows, for vertical downflows the flow
patterns observed were: stratified-wavy, slug-stratified-wavy,
intermittent, annular, dryout and mist.
3.3.1.
Low mass flux, 150 kg s1 m2
At low mass flux and low vapor quality the flow pattern is
stratified-wavy as shown in Figs. 13 and 14. From the inlet
straight tube the refrigerant enters the U-bend with no
significant change in flow pattern. The liquid layer stratified at
the bottom of the tube is splashed against the outer wall of the
outlet sector of the U-bend. The remaining liquid falls down
much in a manner of a freely falling liquid. With the occurrence of small intermittent waves, the outer side of the inlet
sector of the U-bend is intermittently wetted. At the outlet of
the U-bend, a thin liquid layer remains at the top of the tube
while a thicker layer stratifies at the bottom. On the straight
outlet tube, the thin liquid layer persists over a very short
length (i.e. about one internal diameter d) while it falls down
to the bottom from the top. Small waves are generated on the
bottom liquid layer. This layer seems to spread peripherally,
increasing the fraction of wetted perimeter (in comparison to
the inlet straight tube).
Under these conditions of low mass flux and low vapor
quality, decreasing the bend diameter D from 55 to 32 mm did
Medium mass flux, 300 kg s1 m2
For a medium mass flux and a low vapor quality the flow
pattern is slug flow in Figs. 16 and 17. The top of the tube is
intermittently wetted by slugs (high amplitude waves in this
case) and by droplets that detach from the crests of these
waves. At the U-bend most of the liquid is splashed against
the outer wall of the U-bend. At the outlet of the U-bend and
along the outlet straight tube a liquid film persists on the top
of the tube over out several internal diameters. The comparison between the two bend diameters (refer to Figs. 16 and 17)
does not allow one to comment about the increase of length of
the liquid film on the top of the tube due to the reduction of
the bend diameter. Indeed, for the lower bend diameter,
depicted in Fig. 17 the image is not clear due to some
condensation on the external surface of the glass tube. Taking
into consideration what was observed for the lower mass flux,
it can be inferred that for this condition the smaller internal
diameter could have the same effect, that is, increasing the
downstream length of the liquid film on the top of the tube.
At this mass flux, the liquid detachment in the U-bend with
droplets moving from the inner wall to the outer wall of the
U-bend is intensified. Also at this mass flux and for waves or
slugs entering the U-bend, a small hydraulic jump can be
observed at the outlet of the U-bend. This hydraulic jump
generates waves with crests from which droplets are detached.
A further increase of vapor quality makes the flow become
intermittent and then annular (not shown here). As for the
lower mass flux, no significant change occurs. In annular flow
and at the approach of the U-bend the interfacial waves start
bending, with the top of the wave bending backwards. At
a given point, the liquid at the bottom of the wave seems to
detach and splash against the outer wall of the U-bend, while
the interfacial wave seems to fade out.
3.3.3.
High mass flux 500 kg s1 m2
At a high mass flux, the observations are very similar to those
made for the medium mass flux. Thus, no additional
descriptions are given here.
1448
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 4 3 9 e1 4 5 4
Fig. 11 e Flow pattern images for the horizontal configuration with R134a at 5 C flowing at 300 kg sL1 mL2 and x [ 0.72
t [ 2 ms.
inside the visualization section with d [ 11 mm, D [ 55 mm. Time intervals are DiD1
i
3.4.
Vertical upflows
As for the previous U-bend orientations, the flow patterns
observed were: stratified-wavy, slug-stratified-wavy, intermittent, annular, dryout and mist.
3.4.1.
Low mass flux, 150 kg s1 m2
At a low mass flux and a low vapor quality the flow pattern is
stratified-wavy flow as shown in Fig. 18 before the U-bend.
From the inlet straight tube the refrigerant enters the U-bend
with no significant change in flow pattern. The liquid thickness of the stratified layer remains quasi-constant until
almost half the way up the U-bend. From there, the liquid
detaches and a sort of annular flow is generated. This sort of
annular flow persists along the outlet straight tube up to 5d. At
the bottom of the outlet straight tube the height of the liquid
increases with the distance from the U-bend. If a large wave
enters the U-bend, the stratified liquid at the bottom persists
all the way up to the outlet of the U-bend so that at the outlet
of the U-bend most of the liquid is at the top of the tube. Then,
the liquid falls from the top down to the bottom, generating
a wave. For this case, the top of the tube will remain wetted for
more than 10d downstream of the U-bend.
With the increase of the vapor quality (not shown here),
the height of the liquid at the bottom decreases, but the
wetted perimeter increases. A sort of annular flow is
generated almost at the inlet of the U-bend. This sort of
annular flow persists until about 7d downstream of the Ubend. At the outlet of the U-bend and all along the outlet
straight tube, while the liquid film is moving downwards to
the bottom of the tube, droplets detach either from the
falling liquid film or from the crests of the waves generated
by the impact of the liquid film at the bottom of the tube.
These droplets are entrained in the vapor over a certain
distance until they eventually impact into the liquid stratified at the bottom.
At a vapor quality of 0.53 in Fig. 19, the flow pattern is
annular. As for horizontal and downflow regimes, close to the
bend the shape of the interfacial waves is modified. In the
second half of the U-bend, droplets detach from the inner side
of the U-bend and splash against the outer side, that is,
against the top of the outlet of the U-bend. This effect persists
for higher vapor qualities.
For the smaller bend diameter the physical phenomena are
similar (not shown here). At low vapor quality the distance
over which the liquid persists as stratified at the outer side of
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 4 3 9 e1 4 5 4
Fig. 12 e Flow pattern images for the horizontal
configuration with R134a at 5 C flowing at 300 kg sL1 mL2
and x [ 0.93 inside the visualization section with
d [ 11 mm, D [ 55 mm. Time intervals are D21 t [ 40 ms,
D32 t [ 100 ms, D43 t [ 160 ms and D54 t [ 260 ms.
1449
Fig. 13 e Flow pattern images for the vertical downflow
configuration with R134a at 5 C flowing at 150 kg sL1 mL2
and x [ 0.15 inside the visualization section with
d [ 11 mm, D [ 55 mm. Time intervals are D21 t [ 163 ms
and D32 t [ 84 ms.
U-bend is increased, compared to the larger internal diameter.
In this case it reaches half way up the U-bend. At high vapor
qualities, the detachment of liquid droplets from the inner
side of the U-bend is also increased.
3.4.2.
Medium mass flux, 300 kg s1 m2
At low vapor quality the flow regime is slug-stratified-wavy as
shown in Fig. 20 before reaching the U-bend. When the flow is
stratified or wavy the phenomena are similar to those seen for
the lower mass flux. Only the distance over which the top of
the outlet straight tube is wetted is longer, reaching about 7d.
when large waves occur at the inlet of the U-bend, they will
flow around the U-bend and continue their way through the
outlet straight tube. In this case the top of the tube remains
wetted for distances longer than the frame of view of the
camera as shown in Fig. 20.
As for the horizontal configuration, at higher vapor qualities in Fig. 21 before the flow regime reaches annular flow,
a large swarm of droplets enter the U-bend, they impact
together on the outer wall of the U-bend to form a liquid layer.
When this liquid layer reaches the outlet of the U-bend it
starts falling down and droplets are released again.
As the vapor quality increases, the flow regime reaches
annular flow (not shown here), and what was observed for this
flow pattern of the lower mass flux remains valid here.
Fig. 14 e Flow pattern images for the vertical downflow
configuration with R134a at 5 C flowing at 150 kg sL1 mL2
and x [ 0.15 inside the visualization section with
d [ 11 mm, D [ 32 mm. Time intervals are D21 t [ 137 ms
and D21 t [ 78 ms.
1450
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 4 3 9 e1 4 5 4
Fig. 15 e Flow pattern images for the vertical downflow
configuration with R134a at 5 C flowing at 150 kg sL1 mL2
inside the visualization section with d [ 11 mm.
3.4.3.
High mass flux, 500 kg s1 m2
At the high mass flux, the observations are very similar to
those made for the medium mass flux. Thus, no additional
descriptions are given here.
Fig. 16 e Flow pattern images for R134a for the vertical
downflow configuration with 5 C flowing at
300 kg sL1 mL2 and x [ 0.11 inside the visualization
section with d [ 11 mm, D [ 55 mm. Time intervals are
D21 t [ 56 ms and D32 t [ 28 ms.
Fig. 17 e Flow pattern images for the vertical downflow
configuration with R134a at 5 C flowing at 300 kg sL1 mL2
and x [ 0.11 inside the visualization section with
d [ 11 mm, D [ 32 mm. Time intervals are D21 [ 89 ms and
D21 [ 37 ms.
Fig. 18 e Flow pattern images for the vertical upflow
configuration with R134a at 5 C flowing at 150 kg sL1 mL2
and x [ 0.18 inside the visualization section with
d [ 11 mm, D [ 32 mm. Time intervals are D21 t [ 130 ms
and D21 t [ 154 ms.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 4 3 9 e1 4 5 4
1451
Fig. 19 e Flow pattern images for the vertical upflow configuration with R134a at 5 C flowing at 150 kg sL1 mL2 and x [ 0.53
t [ 2 ms.
inside the visualization section with d [ 11 mm, D [ 32 mm. Time intervals are DiD1
i
4.
Discussion
4.1.
Observations
The flow pattern observations provide a view of the perturbations generated in the U-bend, which appear to be driven by
the centrifugal force. The centrifugal force overcomes the
gravity force, independent of the orientation of the U-bend.
For this reason all the phenomena that are observed for
horizontal flow are also observed for vertical downflow and
vertical upflow. The intensity of the phenomena varies, but
the phenomena observed are qualitatively the same.
For flow regimes of the stratified type, the liquid is
systematically pushed against the outer side of the U-bend by
the centrifugal force. This behavior is also visible for large
swarms of droplets, which are pushed by the centrifugal force
against the outer side of the U-bend, patching together to form
a liquid layer. Nevertheless, at the outlet of the U-bend the
gravity force takes control again and forces the liquid to
stratify at the bottom of the tube.
For the annular flow regime, the shape of the interfacial
waves is modified at the proximity of the U-bend. For
example, for vertical flows the tip of the interfacial wave on
the inner side bends towards the U-bend. In the U-bend,
driven by the centrifugal force, droplets detach from the
interfacial wave and are pushed against the outer wall of the
U-bend. The interfacial wave fades out as the droplets
detach.
At dryout inception vapor qualities, however, the liquid
thin film is pushed not against the outer side, but against the
inner side of the U-bend. Indeed, the flow is mainly constituted of vapor and it behaves as a pseudo single-phase flow.
As for single-phase flows in bent tubes, two counter rotating
vortices are generated in the U-bend. These vortices turn from
the outer to inner side. For example, for horizontal flows the
bottom vortice pushes the bottom liquid film towards the
inner side of the U-bend.
Exception for several particular cases referred to earlier,
the flow patterns observed in the U-bends are qualitatively
similar to those that can be observed in the straight tubes, i.e.
annular in the straight tube remains annular in the U-bend.
Nevertheless, minor variations occur, which are generated by
variations of the velocity profile. These will have an impact on
the void fraction in and after the U-bend and naturally, on the
pressure drop and heat transfer.
1452
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 4 3 9 e1 4 5 4
Fig. 20 e Flow pattern images for the vertical upflow
configuration with R134a at 5 C flowing at 300 kg sL1 mL2
and x [ 0.10 inside the visualization section with
d [ 11 mm, D [ 55 mm. Time intervals are D21 t [ 60 ms
and D21 t [ 31 ms.
4.2.
Comparison to existing studies
For the horizontal configuration, none of the flow pattern
observations made in this study corroborate the flow pattern
observations made in the horizontal configuration with
airewater mixtures of Chen et al. (2002), Wang et al. (2003,
2004). They identified several regions related to the variation of the flow motion which were not observed in this study.
For vertical upflow, similarly to what was observed in this
study for stratified and wavy type flows, Usui et al. (1980)
observed the liquid to be maintained at the outer face of the
tube in the U-bend and downstream of the U-bend, before
falling down to the bottom of the tube. Droplets entrained in
the vapor core were also observed to impinge the outer wall as
reported by Wang and Mayinger (1995). For the annular
regime, the flow was observed to be stable, similarly to what
was reported by Usui et al. (1980). On the other hand, Usui
et al. reported film inversion for high proportions of gas in
relation to liquid flow, which suggests liquid film break up of
the annular flow in the U-bend. None of these was observed,
except film inversion for the dryout regime, during the intermittent re-wetting of the tube. Also, no flow reversal or
flooding were observed for slug flow in the U-bend, as reported
by Usui et al. (1980) and Wang et al. (2005). Flow reversal and
flooding seem to occur only for very specific liquid to vapor
velocity ratios, which were not those of this study.
Fig. 21 e Flow pattern images for the vertical upflow
configuration with R134a at 5 C flowing at 300 kg sL1 mL2
and x [ 0.18 inside the visualization section with
d [ 11 mm, D [ 32 mm. Time intervals are D21 t [ 40 ms
and D21 t [ 25 ms.
For vertical downflow, many of the observations made by
other authors were also verified. For stratified type flows the
liquid was observed to hit the outer wall of the U-bend, similarly to what was reported by Traviss and Rohsenow (1973) and
Usui et al. (1981). Some of that liquid also flows along the sides
of the tube, creating a surge of liquid surface at the outlet of the
U-bend, as reported by Usui et al.. Of course, each of these
phenomena occur at different velocities, the first at medium to
high velocities, and the last one at low velocities. For slug or
wavy flows no flow reversal was observed, similarly to what
was reported by Usui et al.. For annular flow, Usui et al. reported
the same observations as for downflow, which was also verified in this study. Additionally, droplets entrained in the vapor
core were also observed to impinge the outer wall as reported
by Traviss and Rohsenow (1973). On the other hand, for annular
flow Travis and Rohsenow reported the migration of the liquid
film and vapor to the outer side and inner side of the U-bend,
respectively. This suggests liquid film break up, which was
never observed here, except film inversion for the dryout flow
pattern, during the intermittent re-wetting of the tube.
5.
Conclusions
The large visual database of flow patterns in U-bends has been
obtained and presented. The flow pattern database, with 342
visualizations, covers five test sections with three internal
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 4 3 9 e1 4 5 4
diameters (8, 11 and 13 mm), five bend diameters (25, 32, 38.1,
55 and 66 mm), three orientations (horizontal flow, vertical
upflow and vertical downflow), one fluid (R134a), one saturation temperature (5 C) and three mass fluxes (150, 300 and
500 kg s1 m2).
The flow pattern visualizations show the impact of the
U-bend on the flow pattern. Locally it can modify the inlet flow
pattern depending on the flow conditions and U-bend orientation. The recovery distance downstream of the U-bend
varies with experimental conditions, the U-bend’s orientation
and flow direction. Horizontal flows and vertical downflows
recover after a short distance downstream of the U-bend,
while vertical upflows recover over a larger distance. All the
observations show that the centrifugal force is dominant over
the gravity. Thus, the flow pattern modifications in the U-bend
are similar for all orientations.
In the U-bend, the flow patterns are qualitatively similar to
those observed in the straight tube sections before the bends.
Annular flows are not greatly modified by the U-bend, only
their interfacial waves are affected and the droplets entrained
in the vapor core are impacted against the outer wall of the
U-bend. Slug and intermittent flows keep their geometrical
characteristics, but the liquid phase is pushed by the centrifugal force to the outer side of the U-bend. Similar comments
can be made for stratified-wavy flows, although these may
suffer perturbations in the low vapor quality range, specially
for vertical orientations. At the outlet of the U-bend, the inlet
flow patterns are quickly recovered, except for stratified-wavy
flows in the vertical upflow configuration. From these observations, if one was to try to develop a flow pattern map for
U-bends, it would have to be not only a function of all traditional experimental parameters, but additionally, of the
angular position in the U-bend (i.e. at 74 the flow pattern has
different characteristics than at 38 ).
The flow pattern observations made in this study verify
many of those reported previously in the literature. The main
differences are due to the experimental conditions (velocity
ratios) and physical properties of the fluids.
An important step as been made with this study, which
clearly shows the effects of the U-bend on refrigerant twophase flow patterns over a wide range of experimental conditions, including different test sections and orientations.
Furthermore, the observations show that the straight tube flow
patterns are generally reestablished shortly after the U-bends,
such that straight tube flow patterns maps may also then be
applicable, an important point for flow pattern based models.
Acknowledgments
The authors wish to acknowledge the sponsorship of this work
by ASHRAE contract RP-1444 (contract from 2007 to 2011).
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