Track 2 TOC
Proceedings of 2003 ASME SHTC
Proceedings
HT2003
ASME Summer Heat
TransferofConference
ASME July
Summer
Heat
Transfer
21-23,
Las
Vegas,Conference
Nevada, USA
July 21-23, 2003, Las Vegas, Nevada, USA
HT2003-47175
HIGH SPEED PHOTOGRAPIC OBSERVATION OF FLOW PATTERNS DURING FLOW BOILING
IN SINGLE RECTANGULAR MINICHANNEL
Prabhu Balasubramanian
Satish G. Kandlikar
[email protected]
Mechanical Engineering Department
Rochester Institute of Technology
Rochester, NY 14623
ABSTRACT
The present study extends the range of flow patterns
observed during boiling of water in a single minichannel of
height 197 µm and width 1054 µm. The calculated hydraulic
diameter of the channel is 333 µm. High-speed photography is
used to obtain video images of the flow field. The top covers of
the channels are made of Lexan to permit visual observation,
while the minichannel is machined in a copper block heated
with cartridge heaters . The flow patterns are studied as a
function of mass flux, heat flux, and location along the flow
channel length. The results provide important information
regarding the boiling behavior in a single channel.
INTRODUCTION
Complex interactions are seen between the liquid-vapor
phase and the channel walls during flow boiling in small
diameter channels. These interactions have remarkable effects
in altering the flow patterns. [Kandlikar et al. 2001a]. The
effect of surface tension forces is expected to be important.
[Triplett et al. (1999)]
The high pressure drops encountered in micro and
minichannels limit the flow rate, and the mass fluxes
encountered in these channels are generally smaller than
employed in the conventional diameter tubes. The low mass
flux combined with the small channel diameter yield all liquid
Reynolds numbers that are in the laminar region. In many
cases, the Reynolds number is on the order of 100 or lower.
The complex heat transfer and fluid flow interactions are
difficult to model, and currently we have to resort to
experimental methods to obtain more information about the
flows in these geometries.
In the present work, we will use the channel classification
proposed by Kandlikar and Grande (2002a) as follows:
Conventional channels Dh > 3mm
Minichannels –
3mm ≥ Dh > 200 µm
Microchannels –
200 µm ≥ Dh > 10 µm
Transitional Channels10 µm ≥ Dh > 0.1 µm
Transitional Microchannels -10 µm ≥ Dh > 1 µm
Transitional Nanochannels - 1 µm ≥ Dh > 0.1 µm
Molecular Nanochannels 0.1 µm ≥ Dh
Under this definition, the present study falls under the
minichannel classification.
NOMENCLATURE
d,w
Channel depth and width ( m )
A
Area ( m2 )
G
Mass flux ( kg/m2 s )
h
Heat transfer coefficient ( W/m2 K )
l
Length ( m )
m&
Mass flow rate ( kg/s )
P
Pressure ( kPa )
Q
Heat transfer ( W )
q"
Heat flux ( W/ m2 )
Re
Reynolds number ( = G*Dh / µ )
Surface temperature ( °C )
Ts
Difference in temperature between test
∆Ts-amb
section surface and ambient
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Copyright © 2003 by ASME
LITERATURE REVIEW
Two-phase flow in micro and mini channels has been a
subject of investigation of a few recent studies. Kandlikar et al.
(2001a), Kandlikar (2001b, 2001c, 2002c) report the flow
patterns in a 1mm square stainless steel channel heated with the
flow of water. They noted significant flow oscillations due to
localized boiling with parallel minichannels. The expanding
bubbles at any location experience very large growth rates and
cause the liquid fronts outward on both sides of the bubble.
The resulting flow reversal introduces significant pressure
fluctuations in the channel. The reversed flow in the channels
causes the vapor to flow back into the inlet manifold leading to
flow maldistribution in the inlet header. This behavior leads to
premature CHF condition.
The early studies in literature focused on gas -water flows;
Fukano et al. (1989) using horizontal capillary tube with inner
diameter of 1.0mm to 4.9 mm and Wambsganss et al. (1991)
using horizontal round tube of inside diameter 2.90 mm provide
a good survey of literature on this topic. Kasza et al. (1997)
using rectangular channel of size 2.5mm by 6.0mm compared
the flow pattern transitions with the available flow pattern maps
and confirmed the presence of bubbly, plug, slug, and annular
flow patterns, with stratified region limited to very small flow
rates. Triplett et al. (1999) using circular minichannel with
inner diameter of 1.1mm and 1.45mm explains that due to the
dominance of surface tension, stratified flow is essentially
absent, slug (plug) and churn flow patterns occur over
extensive ranges of parameters, and the slip velocity under
these patterns is
The flow pattern observation was done in a systematic
fashion by Cornwell and Kew (1992) with R-113 in 0.9 mm x
1.2 mm rectangular minichannels. They identified a flow
pattern referred to as the confined bubble region. Here the
bubbles nucleate and grow to occupy the entire channel. Large
vapor bubbles fill the channel with liquid slugs between the
consecutive elongated bubbles. A good summary of flow
pattern visualization in minichannels is given by Kandlikar
(2001).
Recent study by Hetsroni et al. (2000) was conducted on
microchannels of 103-129 µm hydraulic diameter with all water
Reynolds number in the range 20-70. They visualized the flow
to be annular, with intermittent dryout and rewetting of the
channels. The nucleation was not observed during the annular
flow. Vapor was observed in the inlet plenum, which was
similar to the observations made by Kandlikar et al. (2000) for
the 1-mm parallel channels. The instability resulting from the
boiling in parallel channels was confirmed in microchannels as
well.
There is a need to obtain more information on the twophase flow behavior in minichannels. This information will be
helpful in understanding the heat transfer mechanism during
flow boiling in these channels.
OBJECTIVES OF THE PRESENT WORK
The present work is aimed at visualizing the two-phase flow
behavior of water boiling in minichannels. The interaction of
nucleating bubbles and the liquid flow, and other flow
characteristics, will be investigated as a function of flow rate
and heat flux. In addition, the two-phase flow near the exit
manifold will also be studied.
EXPERIMENTAL SETUP
The experimental setup for the horizontal minichannel
flow loop is designed to deliver a constant mass flow rate to a
minichannel heat exchanger. In addition, the water delivery
system must deliver de-ionized water with different air
concentrations. Figure 1 shows the schematic of the water
delivery system. The system includes a pressure chamber, a
flat plate heat exchanger, a throttling needle valve, a flow
meter, a test section, and a condensate collection container.
The test section details are shown in Fig. 2. The test
section is a combination of three layers. The top layer is made
of Lexan, an optically clear polycarbonate material. The water
inlet and outlet plenums are machined into the polycarbonate
layer. This is done to eliminate the heat transfer in the inlet and
outlet manifolds. The second layer is a copper block that
contains the minichannels. The copper is an Electrolytic Tough
Pitch alloy number C11000. It is comprised of 99.9% copper
and 0.04% oxygen (by weight). The thermal conductivity is
388 W/m-K at 20°C. The third piece is a Phenolic. It is a
laminate of epoxy and paper. It is a laminate of epoxy and
paper with a thermal conductivity of 0.2 W/m-°K and acts an
insulator on the lower surface of the copper plate. It is used to
secure the test section with the help of ten mounting screws. A
cartridge heater supplied with a constant DC input power is
used for heating the test section.
High-Speed
Camera
Microchannel
Test Section
Constant pressure
water supply
Condenser
Flow
meter
Flat plate HX
Figure 1: Fluid Delivery System. Pressure Chamber, Flat
Plate Heat Exchanger, Throttling Needle Valve, Flow Meter,
Test Section, and Condensate Collection.
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Copyright © 2003 by ASME
Copper
m icrochannel
(Top View)
Lexan
Copper
Microchannel
Cartridge
Heater
Phenolic
Retaining
Plate
Figure 2: Test Section Cross Section.
There are six parallel minichannels machined into the
copper substrate. Using a microscopic vision system, the
channel depth and width are measured at six locations along the
channel. These measurements are used to determine the
average dimensions of the channels. The average channel
dimensions are 1054 µm wide by 197 µm deep and 57.15 mm
long. All of the measured values fall within ± 5% of the
average values. The header is fabricated to allow water flow
though only one channel, while the remaining five channels
remain dry without any flow through them. The interactions
among parallel channels are thus eliminated in these runs, and
we are able to see the behavior of an isolated single channel. A
total of six channels are machined on the copper block so that
the same test section can be used for single as well as for
parallel channel testing with the use of suitably designed
headers.
The pressure drop for the minichannel was measured
between the inlet and exit plenum locations. The pressure drop
was used to calculate the friction factor for the minichannels,
after accounting for the entrance region, area changes, bends,
and exit losses.
Images of flow pattern are recorded using a high-speed,
microscopic image acquisition system. A high-speed, digital
CCD camera, which can run up to maximum of 8,000 framers
per second was used to gather the images. The majority of the
images are recorded at 250 and 500 fps due to limiting capacity
of the light source. The lens used is a long distance microscope
lens. It has a field of view of 0.5 mm at a focal length of 15
cm.
EXPERIMENTAL PROCEDURE
The experimental procedures for preparing degassed water
and the experimental data collection are outlined in this section.
8 MΩ de-ionized water was used in the experiment. The
details of the degassing procedure are given by Kandlikar et al.
(2002b).
For preparing the degassed water, the same 8 MΩ deionized water is used. The water is added to a commercially
available pressure cooker equipped with deadweights for
pressure of 15 psi. Once the pressure is attained as the cooker is
heated, the deadweight is removed and the chamber is allowed
to blow down and return to atmospheric pressure. Thus, a very
vigorous boiling occurs within the chamber. The air dissolved
in the water is forced out of the chamber along with the steam
as the chamber depressurizes. The chamber is once again
pressurized following the heating.
The water is degassed to a saturation temperature of about
121 °C. The remaining dissolved air will not precipitate from
the water as long as it stays below the respective degassing
temperatures. However, steam continually escapes from the
chamber, which maintains a constant pressure when the
deadweight is again applied. Thus by using the pressure cooker
the water is degassed and supplied to the test section while
maintaining a constant pressure. Care is taken to refill the
pressure chamber if the water level falls below a certain limit
and causes fluctuations in the flow line.
Once the test section is assembled and well insulated, heat
loss experiments are conducted. The heat loss calibration chart
is plotted between the supply power to the test section in watts
and the difference in temperature between test section surface
and ambient. For example, ∆Ts-amb of 50°C and 90°C had
corresponding heat losses of about 3.26 W and 5.86 W
respectively. The heat loss data is used in calculating the actual
heat carried away by water flowing in the test section.
The experiment was performed with degassed water. The
water is drawn from the bottom of the pressure vessel and
passes through a flat plate heat exchanger (not shown in Fig. 1)
to provide the desired water inlet temperature to the test
section. A flow meter is used to measure the flow rate. The
accuracy of the flow meter is 3% of the full scale, or 0.25
cc/min. LabVIEW is used to monitor the thermocouples
measuring temperature.
The accuracy of temperature
measurement is ± 0.1 °C. Flow is started in the channel and the
cartridge heater is powered. The mass flux is held constant
while the input power is varied through the desired range. The
resulting thermal performance was recorded in terms of water
flow rate, inlet and outlet water temperatures, six temperatures
inside the copper block along the flow length, and the inlet and
exit pressures.
The images are acquired after the system has reached
steady state to detect nucleation in the flow channel. A
microscopic lens is used to get a close view of the channel and
to detect the locate nucleation sites. Once the entire channel is
imaged, the microscopic lens is used to gather detailed images
of specific features and events
The uncertainty of the experimental data was determined.
The accuracy of the instruments is reported as: T = ± 0.1 °C,
DP = ± 0.01 psi, Volts = ± 0.05 V, I = 0.005 Amp. The bias
and precision errors were estimated, and the resulting
uncertainty in the heat transfer coefficient is 8.6 % and in the
friction factor is 7.2 %.
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Copyright © 2003 by ASME
RESULTS
FLOW VISUALIZATION IN A 1054 µM WIDE BY 197µM
DEEP MINICHANNEL
The single channel tests are performed using high speed
camera and microscopic lens. The channel is 1054 µm wide
and 197 µm deep. The inlet and outlet headers were designed
to provide flow through the single channel.
In the following figures, image sequences from high speed
camera are shown. The visible light and dark regions surround
the flow channel. Changes in image capture rate, ambient light,
and exposure time may have some affect on the appearance of
the channel and its surroundings. The bulk flow in the channel
is from left to right.
Figure 4: Successive frames (a) through (j) at 2ms interval
showing rapid sequence of nucleation, bubble and slug
growth in the middle of the channel. G=62 Kg/m2 s,
q"=47.25 KW/m2 , Ts=102°C
Figure 3: Successive frames (a) through (j) at 12ms interval
of single channel showing local nucleation, bubble growth
and slug formation near the channel inlet. G=62 Kg/m2 s,
q"=47.25 KW/m2 , Ts=102°C
Figure 3 shows a local nucleate site, where a bubble starts
to grow and subsequently develops into a slug. Note in Fig.
3(a), a bubble is nucleating in the middle of the channel. Fig.
3(b), 12ms later, the bubble has grown in size. Fig 3(c) to Fig.
3(g) show the gradual increase in the size of the bubble until it
blocks the entire width of the channel. Fig. 3(h) to Fig. 3(k)
shows the bubble further expanding both sides and developing
in to a slug. Fig. 3(k), 12ms after Fig. 3(j) shows the movement
of the slug in the direction of bulk flow, with change in contact
angles with the channel walls. This process of bubble growth,
slug formation and slug movement is seen to occur periodically
Figure 4 illustrates a rapid sequence of bubble nucleation,
growth, slug formation and departure. This pattern was
observed in the middle of the channel. Fig. 4(a), at 0ms shows a
nucleation site along the bottom wall of the channel, where a
bubble nucleates and starts growing. Fig. 4(b) and 4(c), which
are at 2ms and 4ms later, shows the increase in size of the
bubble. Fig 4(d) to Fig. 4(h) shows the bubble expanding to it’s
right but still sticking to the walls of the channel. Fig. 4(i) and
Fig. 4(j), which are at 16ms and 18ms later, show the bubble’s
liquid vapor interface getting collapsed and the subsequent
departure of the bubble.
Fig. 4(k), 20 ms later, shows three more nucleation sites on
the upper and lower walls of the channel. Out of these
nucleation sites, there is another bubble growth seen in the
upper right side wall of the channel. This bubble follows the
same pattern of growth and departure as explained before. The
differences between this when compared to Fig. 3 is that the
nucleation site is seen along the channel wall and the slug
formation is partial and it does not block the entire channel.
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Copyright © 2003 by ASME
Figure 5: Successive frames (a) through (n) at 5ms interval
of single channel showing dryout and rewetting.
G=81Kg/m2 s, q"=54.69KW/m2 , Ts=101°C
Figure 6: Successive frames (a) through (l) at 2ms interval
of single channel showing flow reversal. G=106 Kg/m2 s,
q"= 85.40 KW/m2 , Ts=101.43°C
Dryout is represented in Fig. 5. For the flow pattern shown,
the liquid film is reduced until the channel wall is dry and only
vapor flows in the channel. Fig. 5(a) to Fig. 5(n) are at equal
time intervals of 5ms. This flow pattern was not seen as a stable
one and occurs only intermittently. Figs 5(a) to 5(c) show the
channel filled with liquid. The vapor flow is seen in Fig 5(d)
and it’s more enhanced as seen from the subsequent frames. A
close look in to the figures from 5(d) to 5(m) shows that the
vapor flow does not fill up the entire width of the channel and
there is a thin film of liquid trapped at the edges of the channel.
An important thing to note is that the flow patterns in
minichannel are not stable, they fluctuate periodically from all
liquid flow to slug flow, annular flow and dryout region. It is
not possible to assign a single flow pattern for a given set of
flow and heat flux conditions.
Reverse flow or flow reversal is seen in Fig. 6. The bulk
flow in the channel is from left to right. Fig. 6(a), 0ms shows
the channel filled with liquid. Fig 6(b), 2ms, shows the
appearance of an annular slug to the right of the channel.
Figures 6(c), (d) and (e), which are at 4ms, 6ms and 8ms,
shows the movement of the slug form right to left, which is
opposite to the direction of bulk flow in the channel.
In Fig 6(f), 10ms later the slug stops moving backward and
12ms later, as seen in Fig 6(g), the slug now changes its
direction and starts moving from left to right, which is along
the direction of bulk fluid flow. Figures 6(h) to (k) shows the
slug moving in the same direction and in Fig 6(l) the slug
passes away and the channel is filled with liquid.
By this observation, the occurrence of the reverse flow in a
single channel is also seen to occur, although it does not extend
all the way to the inlet manifold under the range of conditions
investigated here.
5
Copyright © 2003 by ASME
Figure 7: Successive frames (a) through (j) at 2ms time
interval of single channel showing two phase flow at the exit
manifold. G=106 Kg/m2 s, q"= 85.40 KW/m2 , Ts=101.43°C
Figure 7 illustrates the two-phase flow near the exit
manifold. The top view of the exit manifold is seen on the right
end of the each frame. Figures 7 (a) to (j) are at equal time
intervals of 2ms. Figure 7(a) shows the vapor filling up the
entire length of the channel leading to the exit manifold. Figure
7(b), 2ms later, shows a shift in the liquid vapor interface,
which straightens out later in Fig. 7(c) again filling the entire
channel. This change is the interface is observed to occur
periodically as seen from Figs. 7(d) to (j). In Figs. 7(g) and 7(h)
this shift in interface is more pronounced and the slug, which
fills the entire channel to the left, thins out in to a fine line as it
approaches the exit manifold. The flow rate and heat flux for
this figure are higher than those for the earlier observations.
This leads to a somewhat uniform flow pattern near the exit
manifold. For the lower mass flux and heat flux cases, flow
pattern changed periodically at the exit manifold, cyclically
changing from very short all liquid flow to almost all vapor
flow.
CONCLUSION
The following flow patterns were observed in a single
minichannel of height 197 µm and width 1054 µm
(Dh=333µm) rectangular minichannel: Nucleate boiling, slug
formation, Dryout, Reverse flow and Two phase flow at the exit
manifold.
•
The process of bubble nucleation and its subsequent
expansion into a slug is observed with changes in the
contact angle.
•
The rapid process of bubble nucleation, departure and
partial slug formation is also observed with disturbance in
the liquid vapor interface.
•
The occurrence of channel dryout and rewetting is
observed. This pattern was not seen as a stable one; it is
seen to occur intermittently with almost a fixed
periodicity to it.
•
The occurrence of the reversed flow in a single channel is
also seen, although it does not extend all the way to the
inlet manifold.
•
Two-phase flow at the exit manifold is observed with
continuous change in the liquid film thickness in response
to the upflow changes occurring in the channel.
ACKNOWLEDGMENTS
All of the work was conducted in the Thermal Analysis
Laboratory at RIT. The support provided by Rochester Institute
of Technology is gratefully acknowledged.
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Copyright © 2003 by ASME
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