International Journal of Heat and Mass Transfer 64 (2013) 1216–1225
Contents lists available at SciVerse ScienceDirect
International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt
Pool boiling heat transfer enhancement over cylindrical tubes with
water at atmospheric pressure, Part II: Experimental results and bubble
dynamics for circumferential V-groove and axial rectangular
open microchannels
Jeet S. Mehta, Satish G. Kandlikar ⇑
Department of Mechanical Engineering, Kate Gleason College of Engineering, Rochester Institute of Technology, 76 Lomb Memorial Drive, Rochester, NY 14623, USA
a r t i c l e
i n f o
Article history:
Available online 10 May 2013
Keywords:
Pool boiling
Heat transfer enhancement
Rectangular microchannels
V-groove microchannels
Tube orientation
Bubble dynamics
a b s t r a c t
A two-part experimental study is conducted on pool boiling heat transfer over enhanced cylindrical
microchannel test surfaces with water at atmospheric pressure. In this Part II of the study, the effects
of circumferential V-groove microchannels and axial rectangular microchannels are reported. These
experiments were performed in the horizontal as well as vertical orientations. The heat transfer performances of the modified surfaces are compared with that of a plain surface. At a heat flux of 1070 kW/m2 a
maximum heat transfer coefficient of 96 kW/m2 K was achieved with an axial rectangular microchannel
test section in the vertical orientation. Videos captured using a high speed camera were analyzed and the
boiling heat transfer mechanisms seen at low and medium heat flux conditions were discussed. The
enhancement in the heat transfer performance, and the improvement of the critical heat flux observed
with these modified test sections have been attributed to the liquid rewetting phenomenon of the heated
surfaces through the microchannels.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Pool boiling is a highly advantageous mechanism for efficiently
dissipating large quantities of heat from a surface. The increased
performance is due to the latent heat of evaporation required to
change the phase of a fluid from liquid to gas. Further enhancements in the overall heat transfer performance are achieved by
modifying the heat transfer surface. Numerous techniques of
enhancing the surface performance have been developed over
the past three decades. An extensive literature review conducted
for pool boiling of a liquid over enhanced cylindrical surfaces
was presented in the part I paper [1]. Some of the widely used
techniques for surface modification are re-entrant cavities, porous
surfaces, and microchannels/integral fins.
The microchannel enhancement technique employed by Cooke
and Kandlikar [2,3] has shown superior performance in the overall
heat transfer as compared to boiling over flat surfaces. In order to
expand their findings this two part study was conducted to analyze
Abbreviations: CHF, critical heat flux; CRM, circumferential rectangular microchannels; CVM, circumferential V-groove microchannels; ARM, axial rectangular
microchannels.
⇑ Corresponding author. Tel.: +1 585 475 6728.
E-mail address: [email protected] (S.G. Kandlikar).
0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.04.004
the performance enhancements and effects of microchannels over
cylindrical surfaces. The interactions of bubbles nucleating and
growing, in and over the microchannel surfaces, have been observed to be partially responsible for the performance enhancement. The rewetting phenomenon and the pinning of the bubble
on to the fins of the microchannels, as explained by Cooke and Kandlikar [2,3], aided the dissipation of heat from the surface and reduced the wall superheat. They attained much lower surface
temperatures with microchannel surfaces as compared to a plain
surface.
The bubble dynamics over the heat transfer surfaces has been
analyzed by a few researchers in the recent past with the development of digital high speed cameras and other low cost imaging
techniques. In 1998, Chien and Webb [4] conducted experiments
to visualize nucleate boiling using high speed cameras. Enhanced
tubes consisting of sub-surface tunnels and surface pores were
used in their study. Experiments performed at lower heat fluxes
for saturated boiling revealed vapor filled tunnels except for the
channel corners which had liquid menisci. Kulenovic et al. [5] performed experiments on re-entrant cavity tubes, and visualized the
bubble dynamics using high-speed video systems. Using Fourier
analyses and other correlation techniques, they determined the
bubble generation frequency and quantified the upward bubble
flow velocity. Hsieh and Yang [6] used porous layer coated tubes
J.S. Mehta, S.G. Kandlikar / International Journal of Heat and Mass Transfer 64 (2013) 1216–1225
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Nomenclature
q00r
DT
h
resultant radial heat flux at the outer diameter, W/m2
wall superheat, K
heat transfer coefficient, W/m2 K
with wires spirally wrapped around them, and conducted experiments with saturated R134a and R600a. They performed experiments at low and medium heat fluxes, and their visualization
revealed a linear increase in the bubbling frequency with heat flux.
A slight decrease in the bubble diameter was also observed with an
increase in the heat flux condition.
Kim et al. [7] studied the boiling behavior over plain and microporous wires. They concluded that the microporous coatings augmented the heat transfer performance through the increase in
the latent heat transfer due to higher nucleation site density. Kang
[8] performed experimental work on the effects of tube inclinations on the heat transfer enhancement during pool boiling. He
used photographs to compare the bubble formation and departure
from the tubes at various inclinations and heat flux conditions. He
observed an increase in the bubble size and strong bubble movement with an increase in the heat flux. As the inclination angle
was increased the generated bubbles were seen to coalesce on
the underside of the tube, and slide off as a large bubble slug.
The available literature does not clearly focus on the activities
of the bubble nucleating and growing inside the microchannel
grooves. In order to provide some insight and reveal the fundamental heat transfer mechanisms and bubble dynamics responsible for the performance enhancements, the current work was
performed and is presented in this paper.
In the first part of this study, the design and development of the
experimental setup was described in detail. Also, the experimental
results obtained for circumferentially grooved rectangular crosssection microchannels were presented. The effects of the geometric
parameters were also discussed in the previous part. In this part of
the study, the experimental results for circumferential V-groove
microchannels and axial rectangular microchannels are presented
along with figures showing bubble activity, which were recorded
using high speed cameras.
The experiments were conducted at saturation conditions after
allowing the pool of water to boil for at least 30 min. The main heater was inserted into the test section assembly. The system was designed so as to provide heat to the test surfaces in the radially
outward direction. To reduce the heat losses in the axial direction,
ceramic insulation material was inserted on either side of the test
section. Additional details of the experimental setup and the test
section assembly are provided in the part I paper of this study.
The test sections used in this study were fabricated from copper
alloy 101 which has a thermal conductivity of 391 W/m K. Each
test section tube was 20 mm long and had inner and outer diameters of 9.53 mm and 15 mm, respectively. Parametric effects of the
rectangular and V-groove microchannel dimensions on the boiling
heat transfer performance are evaluated and analyzed. In the part I
paper [1], the parametric effects of the geometric dimensions in
the Circumferential Rectangular Microchannel (CRM) test sections
were evaluated. In this part the design and experimental results for
Circumferential V-groove Microchannel (CVM) test sections and
Axial Rectangular Microchannel (ARM) test sections are described
and discussed. The details of the design of these modified test sections are provided in the following sub-sections.
2.1. Circumferential V-groove microchannel (CVM) test sections
Six circumferentially grooved test sections were designed and
fabricated by varying their geometrical parameters. These micro-
2. Approach
A pool boiling experimental setup was designed and fabricated
to study the heat transfer performance over cylindrical surfaces.
Water was used as the working fluid and the experiments were
conducted at atmospheric pressure. The microchannel enhancement technique was employed to study the heat transfer performance enhancement during pool boiling over the cylindrical test
surfaces. Experiments were performed in the horizontal as well
as vertical orientation for all the modified test section surfaces.
In part I paper [1] of this study, the key components of the experimental setup were described and explained in detail. The data
acquisition systems that were used to read and record the generated data and the data reduction methodology and equations employed while taking into account the uncertainties in the
measurements were also explained in detail in the previous part.
Fig. 1 shows a picture of the assembled experimental setup and
some of the key components seen through the large visualization
window. As detailed in the figure, the test section assembly and
the auxiliary heater are horizontally orientated and submerged in
the pool of water. The thermocouple used for measuring the bulk
fluid temperature was placed in the central region between the
test section assembly and the auxiliary heater as shown in figure.
Fig. 1. Assembled experimental setup with the test section horizontally oriented.
Fig. 2. Key geometric parameters required to define the CVM test sections.
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Table 1
Dimensional details of geometric parameters for CVM and ARM test sections.
Test section
Depth (mm)
Pitch (mm)/angular pitch (°)
Included angle (°)
Channel width (mm)
Area enhancement factor
CVM1
CVM2
CVM3
CVM4
CVM5
CVM6
ARM1
ARM2
ARM3
ARM4
ARM5
ARM6
0.24
0.31
0.37
0.43
0.32
0.46
0.22
0.39
0.22
0.22
0.37
0.51
0.39
0.55
0.54
0.70
0.40
0.55
6
6
6
8
8
8
60
60
60
60
45
45
–
–
–
–
–
–
–
–
–
–
–
–
0.40
0.40
0.52
0.54
0.53
0.53
1.68
1.60
1.74
1.65
2.03
2.03
1.57
1.95
1.56
1.42
1.71
1.95
channel test sections were defined and differentiated based on
their depth, pitch and included angle as shown in Fig. 2. The
dimensional details of all the CVM test sections are provided in
Table 1. The modifications on the surface of the test sections results in an increase in the total wetted surface area. This increase
was defined as the area enhancement factor and is also given in
the table. Precision machining was employed to fabricate the
microchannels on the outer surface of the test sections and to
achieve small tolerances of ±15 lm. Dimensional measurements
performed using a confocal laser scanning microscope showed
dimensional errors of less than 10 lm. From these analyses it
was also concluded that the designed sharp V-groove channels
at the bottom were not successfully reproduced. This was because of the limited strength at the tips of the miniature cutting
tools at their tip. Hence a small fillet at the bottom of the Vgroove microchannel was produced as seen in the 3D surface
profile generated for a CVM test section by the microscope as
shown in Fig. 3.
Fig. 3. 3D surface profile of CVM5 generated using the confocal laser scanning
microscope.
2.2. Axial rectangular microchannel (ARM) test sections
Test sections consisting of rectangular cross-section geometries
were also designed and fabricated with axially oriented microchannels. A representative CAD model of an ARM test section
showing the axial orientation of the microchannels on the cylindrical tube surface is given in Fig. 4(a). The test sections ARM1–ARM6
were defined by the depth and width of the rectangular microchannels as shown in the Fig. 4(b). The angular pitch was also required to define the total number of channels on the test surface
and evaluate the total wetted surface of the test section. The
dimensions of the various geometric parameters for the ARM test
sections and their corresponding area enhancement factors are given in Table 1.
Fig. 4. (a) ARM test section and (b) key geometric parameters required to define an
ARM test section.
3. Results and discussion
3.1. Experimental results for CVM test sections
In this section, the experimental results for circumferential Vgroove microchannels and axial rectangular microchannels are
described and discussed in detail. The experimental data sets
generated for these test sections were reduced to represent their
respective heat transfer performances as boiling curves. The
dimensional effects of the microchannel geometric parameters
on the heat transfer performance were analyzed using their
respective boiling curves. The effects of tube orientation are also
discussed in this section. To better understand the enhancements due to microchannels, the results were re-calculated to
generate area normalized heat fluxes, thereby eliminating the
performance enhancements due to the increased total wetted
surface area.
The boiling curves generated from the results of the plain test
section (P0) and test sections CVM1–CVM6 in the horizontal and
vertical orientations are shown in Fig. 5 and Fig. 6, respectively.
The results obtained show good enhancements in the overall performance for the microchanneled surfaces. The plain test section
P0, was only tested up to a heat flux of 670 kW/m2 at which it
yielded a heat transfer coefficient of 38 kW/m2 K at a wall superheat of 17.8 K in the horizontal orientation, whereas in the vertical
orientation a heat transfer coefficient of 36 kW/m2 K was obtained
at a wall superheat of 18.8 K. At an approximate heat flux of
700 kW/m2 test section P0 reached the Critical Heat Flux (CHF)
condition and the nucleate boiling mechanism transitioned into
the film boiling regime. Hence for safe operation of the experimen-
J.S. Mehta, S.G. Kandlikar / International Journal of Heat and Mass Transfer 64 (2013) 1216–1225
1219
the horizontal orientation. It is also highly likely that due to the
narrow channel features, the liquid circulation in the channels
was affected, leading to poorer performance compared to the performance of the other CVM test sections. The detailed results for
the CVM test sections in both orientations at their respective maximum heat flux conditions are given in Table 2. The overall
enhancement factor was defined as the ratio of the heat transfer
coefficient of the modified surface to that of the plain surface at
their respective maximum heat flux conditions. Overall heat transfer enhancement factors of 1.8–2.4 were achieved in the horizontal
orientation and enhancements of 2.0–2.3 were obtained in the vertical orientation for the CVM test sections. From the obtained results it was difficult to factor out the effects of the geometric
parameters on the heat transfer performance.
3.2. Experimental results for ARM test sections
Fig. 6. Boiling curves for the CVM test sections in the vertical orientation.
In this section, performance of six test sections having axially
oriented microchannels is reported. The experimental results for
the ARM test sections show some good enhancements in the overall heat transfer performance. All the ARM test sections were successfully tested up to a heat flux of 1070 kW/m2 and the CHF limit
was not reached for any of these surfaces. The performance curves
for the ARM test sections evaluated from the generated experimental data in the horizontal and vertical orientations are given in
Fig. 7 and Fig. 8, respectively. As seen from the figure, ARM2 produced the best performance in the vertical orientation, yielding a
maximum heat transfer coefficient of 96 kW/m2 K at a wall superheat of 11.1 K. The heat transfer coefficients for other ARM surfaces
in the vertical orientation were in the range of 75–90 kW/m2 K
with the wall superheats in the range of 11.9–14.2 K. In the horizontal orientation the heat transfer coefficients evaluated were in
the range of 70–88 kW/m2 K with the wall superheat in the range
of 12.1–15.3 K. The detailed results for the ARM test sections are
provided in Table 2. The overall heat transfer enhancement factors
of 2.1–2.7 were achieved in the vertical orientation for the different
ARM test sections. In the horizontal orientation the enhancement
factors achieved were in the range of 1.9–2.3. A steady increase
in the heat transfer coefficients was observed with increasing heat
flux conditions as seen in Fig. 9. The figure also shows the observed
enhancement in the heat flux limit from 670 kW/m2 for the plain
surface to 1070 kW/m2 for the enhanced surfaces without reaching
CHF.
tal setup, the testing with the plain test section was limited to a
maximum heat flux of 670 kW/m2. Most of the CVM test sections
were successfully tested up to a heat flux of approximately
1070 kW/m2. Test section CVM2 consisting of a cross-section
geometry that yielded the least area enhancement, reached the
CHF condition at a heat flux of slightly over 900 kW/m2 in the horizontal orientation. However in the vertical orientation it was successfully tested up to a heat flux of approximately 970 kW/m2.
The results for four of the six CVM test sections at high heat flux
conditions showed very similar heat transfer performances. In the
horizontal orientation the heat transfer coefficients obtained for
these test sections were in the range of 85–90 kW/m2 K, with the
wall superheat in the range of 12.1–12.6 K. In the vertical orientation the heat transfer coefficients were in the range of 74–82 kW/
m2 K, with the wall superheat in the range of 13.1–14.5 K. The
CVM5 and CVM6 test sections consisted of narrower V-grooves
with included angles of 45°. Furthermore the microchannel geometry of the CVM5 test section consisted of comparatively shallower
grooves and narrower channels. A coupled effect of such microchannel dimensions was responsible for the comparatively poor
heat transfer performance achieved by the CVM5 test section in
3.2.1. Effects of microchannel dimensions
The results obtained for the different geometric configurations
of the ARM test sections were distinct and comparable to each
other. The effects of the channel depth on the heat transfer performance results in the vertical orientation are shown in Fig. 10. The
performance curves for ARM1 and ARM2 having microchannel
depths of 0.22 mm and 0.39 mm, respectively are shown in this figure. The other dimensions of the microchannel geometry for this
pair of test sections were the same. The performance observed
for ARM2 having deeper microchannel grooves was relatively better than that observed for ARM1 which had shallower grooves.
Similar comparison between ARM5 and ARM6 having channel
depths of 0.37 mm and 0.51 mm, respectively was made and their
boiling curves are also shown in the figure. At high heat flux conditions ARM6 showed better performance than ARM5. From these
results it was concluded that axial rectangular cross-section microchannels with greater depths perform slightly better than shallower grooves.
The effects of channel width on the heat transfer performance
were more prominent than the channel depth effects for the
ARM test sections. The results for ARM1 and ARM3 having channel
widths of 0.40 mm and 0.52 mm, respectively were compared and
Fig. 5. Boiling curves for the CVM test sections in the horizontal orientation.
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Table 2
Results for all the CVM and ARM test sections in the horizontal and vertical orientation at their highest heat flux condition.
Test section
P0
CVM1
CVM2
CVM3
CVM4
CVM5
CVM6
ARM1
ARM2
ARM3
ARM4
ARM5
ARM6
Horizontal orientation
Vertical orientation
q00r
kW/m2
DT
K
h
kW/m2 K
hcrm/hpo
-
q00r
kW/m2
DT
K
h
kW/m2 K
hcrm/hpo
-
667
1065
848
1087
1083
1066
1067
1066
1069
1068
1066
1064
1066
17.8
12.5
10.3
12.1
12.3
15.6
12.6
13.2
12.1
15.1
14.3
14.7
15.3
38
88
83
90
88
69
85
81
88
71
74
72
70
–
2.3
2.2
2.4
2.3
1.8
2.3
2.2
2.3
1.9
2.0
1.9
1.9
667
1066
966
1086
1082
1066
1066
1069
1069
1068
1066
1064
1069
18.8
14.5
11.8
14.3
14.0
14.9
13.1
11.9
11.1
14.2
13.8
12.5
12.1
36
74
82
76
77
70
80
90
96
75
78
85
88
–
2.1
2.3
2.1
2.2
2.0
2.3
2.5
2.7
2.1
2.2
2.4
2.5
Fig. 7. Boiling curves for the ARM test sections in the horizontal orientation.
Fig. 9. Heat transfer coefficient comparison between the ARM test sections in the
vertical orientation.
Fig. 8. Boiling curves for the ARM test sections in the vertical orientation.
Fig. 10. Boiling curve comparison to analyze the effects of the channel depth.
analyzed. From the boiling curves shown in Fig. 11 for their vertical
orientation results it was concluded that the surfaces with narrower microchannels yielded relatively better performance. Simi-
lar conclusion was reached by comparing the performance curves
of ARM2 and ARM5 having channel widths of 0.40 mm and
0.53 mm, respectively. At high heat flux conditions ARM2 consist-
J.S. Mehta, S.G. Kandlikar / International Journal of Heat and Mass Transfer 64 (2013) 1216–1225
Fig. 11. Boiling curve comparison to analyze the effects of the channel width.
ing of smaller channel widths yielded significantly better heat
transfer rates than ARM5. Hence, for axially oriented rectangular
base microchannels over a cylindrical surface, it was concluded
that deeper and narrower channels aided in enhancing the overall
heat transfer performance.
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cussed in Section 3.3, any modifications on the test section surface
resulted in an increase in the total wetted surface area. In order to
eliminate the influence of the surface area enhancement on the
heat transfer performance results and isolate the enhancements
due to other factors, the normalized heat fluxes were re-calculated
for all the CVM and ARM test sections using their respective area
enhancement factors. Area normalized performance curves for
the CVM test sections in the horizontal orientation and for the
ARM test sections in the vertical orientation are shown in Fig. 12
and Fig. 13, respectively.
The area normalized results for the modified surfaces still show
considerable performance enhancements over the plain surface.
This leads to the conclusion that the factors other than area
enhancement were also responsible in augmenting the overall
boiling heat transfer performance. Comparing ARM1 with ARM3,
the channel depth, pitch and area enhancement factor are same
but the channel widths are 0.40 mm and 0.52 mm, respectively.
The performance of ARM1 is better than ARM3, indicating that narrower channels, or wider fins, are desirable. In case of ARM2 and
ARM5, although the channel width of ARM2 is smaller than
ARM5, the pitch is different resulting in nearly equal fin widths.
These effects are seen to balance out, with nearly equal performance for the two channels on the area normalized basis shown
in Fig. 13. Comparing ARM1 and ARM2 with different depths, it
is seen that their performance is very similar on the area normalized basis as seen in Fig. 13. This indicates that the depth did not
have a major effect on the heat transfer mechanism for these cases.
3.3. Effects of tube orientation
4. Bubble dynamics
The results obtained for the CVM test sections in the horizontal
and vertical orientation are shown in Fig. 5 and Fig. 6 respectively.
After analyzing these results it was concluded that the heat transfer performance for the circumferentially grooved microchannels
was comparatively superior in the horizontal orientation. Exactly
the same conclusion was drawn for the CRM surfaces, the results
of which were discussed in the part I paper [1] of this study. The
CRM test sections also showed superior heat transfer performance
in the horizontal orientation. The results for the ARM test sections
in the horizontal and vertical orientation are shown in Fig. 7 and
Fig. 8, respectively. For these test sections relatively superior performance was observed in the vertical orientation compared to
that in the horizontal orientation. These experimental results
clearly show a significant influence of the tube orientation and
the microchannel groove orientation on the overall heat transfer
performance. For the circumferentially grooved surfaces better
performance was observed in the horizontal orientation whereas
for axially grooved surfaces better performance was observed in
the vertical orientation. It can be concluded that the bubble interactions on the microchannel surfaces may be responsible for this
enhancement relationship. For the CRM and CVM surfaces in the
horizontal orientation, the bubbles generated on the bottom surface of the tube were observed to slide on the side wall when they
reached their critical departure diameters. Similarly for the ARM
surfaces in the vertical orientation, the bubbles generated on the
lower part of the test sections were observed to slide on the surface
while pinned to the microchannel surface. Hence from these results it was concluded that the microchannel groove orientation
preferentially enhanced the overall heat transfer performance
depending on the tube orientation.
In order to understand the heat transfer mechanism and visualize the bubble dynamics over the microchannels an experimental
setup with visual access to the heated surface was developed. As
described in the approach section earlier, a high speed microscope
camera was used to capture the bubble nucleation and growth on
the modified cylindrical surfaces. KeyenceÒ VW5000 motion analyzing microscope coupled with a VH-Z00R lens was used to capture the videos at a magnification of 50 and a frame rate of
1000 or 4000 fps as required. Light from external sources was provided to improve the brightness and video clarity for higher frame
rate videos. The videos were recorded using the circumferentially
grooved test sections on their top and bottom surfaces so as to
clearly visualize and study the bubble activity inside and over
3.4. Area normalized results
The results described above were evaluated based on the projected surface area at the outer diameter of the test section. As dis-
Fig. 12. Area normalized boiling curves for the CVM test sections.
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4.1. CVM test section at low heat flux condition
Fig. 13. Area normalized boiling curves for the ARM test sections.
Fig. 14 shows the sequential images of nucleation, growth and
departure of bubbles from the top surface of the CVM1 test section
in the horizontal orientation at a low heat flux of approximately
27 kW/m2. The video was recorded at a high frame rate of
4000 fps. In frame (a), a large bubble is seen departing, while a
new bubble nucleates inside the microchannel groove. As this bubble grows within the groove, it occupies the entire microchannel
region filling the V-groove. It then grows in the groove as shown
in frames (b)–(d). As the bubble reaches its departure diameter,
it is expelled from the groove. In frame (e) the coalesced bubble
pins to the tip of the fins and slowly grows. At 2.25 milliseconds
another bubble nucleated from the active cavity and is seen to coalesce with the pinned bubble as shown in frame (f). It is also seen
that for this brief moment of time, the microchannel groove area is
completely blocked for rewetting of the microchannel. The presence of vapor in the groove adversely affects the heat transfer
performance.
Under similar heat flux conditions, a significantly lower bubble
generation frequency was observed on the underside of the same
test section surface compared to the top surface. The bubbles
nucleating inside the grooves on the bottom surface were initially
seen to be ejected downwards out of the microchannel. Immediately after the ejection of the bubbles, the buoyancy forces took
over and were responsible for the pinning of these bubbles on to
the fins. While pinned to the surface, these bubbles did not intrude
or fill the groove area. The grooves were only filled during the
nucleation and growth of the bubbles inside the channels. The pinned bubbles after reaching an unstable diameter were seen to slide
around the test section and finally detach from the test section surface. After complete departure of the previous bubble, a new bubble was seen to nucleate and a continuous cycle of bubble
nucleation, growth and departure was observed. Even while the
bubbles were growing while pinned to the bottom fins, the grooves
were flooded with the bulk liquid thereby rewetting the heated
surface. At low heat flux conditions the departure diameter of
the bubbles on the top surface was relatively much smaller compared to the bottom surface. Also the dwelling period of the bubbles pinned to the surface was comparatively much lower on the
top surface, due to the buoyancy forces.
4.2. CVM test section at medium heat flux condition
Fig. 14. Bubble dynamics on the top surface of CVM1 test section at a low heat flux
of 27 kW/m2.
the microchannel groove area. Hence for this purpose the videos of
rectangular and the V-groove cross-section geometries were studied and compared. Three different heat flux levels were studied:
27 kW/m2, 75–80 kW/m2 and 150 kW/m2. The image sequences
generated from the various videos are presented and discussed in
this section.
An image sequence of the bubble activity observed on the bottom surface of a horizontally orientated CVM1 test section is
shown in Fig. 15. The video for this image sequence was recorded
at a frame rate of 1000 fps and a heat flux of 75 kW/m2. The frame
(a) in the figure shows a growing bubble attached to the fins on the
underside of the tube. In less than a millisecond the bubble detaches from the surface and a new bubble nucleates as seen in
frame (b). The active nucleation site is located on the side wall of
the V-groove. At 2 milliseconds, the second bubble grows inside
the microchannel while almost touching all the walls of the groove.
At this point the previously nucleated bubble has completely detached from the surface and initiated its interaction with the larger
bubble on the underside of the tube as seen in frame (c). The second bubble is ejected from the microchannel groove and starts coalescing with the first bubble as seen in frame (d). The frame also
shows the nucleation of a third bubble immediately after the ejection of the second bubble. Frames (e) and (f) show the coalescence
of the first and the second bubble and their eventual merger into
the larger bubble at the end of 5 milliseconds. The third bubble is
seen to grown inside the microchannel groove and eventually pins
to the fins. An important observation of micro bubbles feeding into
a larger bubble was made using the visualization data recorded at
J.S. Mehta, S.G. Kandlikar / International Journal of Heat and Mass Transfer 64 (2013) 1216–1225
Fig. 15. Bubble dynamics on the bottom surface of CVM1 test section at a heat flux
of 75 kW/m2.
1223
similar and higher heat flux conditions. As the heat flux was increased the bubble generation frequency increased and simultaneous growth of numerous larger bubbles by the micro bubbles
feeding mechanism was observed.
Another video of the bubble feeding mechanism on the top surface of the CVM5 test section was recorded and the important
events from the video are detailed in Fig. 16. This video was recorded at a frame rate of 4000 fps at a similar heat flux of
80 kW/m2. Frame (a) shows a nucleated bubble growing on the left
wall of the central microchannel groove. This growing bubble attaches to the microchannel fins and starts interacting with the previously departed bubble. At approximately 2.75 milliseconds after
nucleation, the two bubbles coalesce and the pinned bubble is
forced to detach from the surface due to the buoyancy forces acting
on the larger bubble as seen in frame (b). In the next 0.25 milliseconds this bubble completely detaches from the surface and immediately after that, at 3.25 milliseconds, a new bubble nucleates
from a different cavity on the right wall of the microchannel as
seen in frame (c). This newly nucleated bubble partially fills the
microchannel as it grows and pins to the fins at the surface as seen
in frame (d). This bubble was also seen to grow and similarly coalesce with the larger bubble and later depart from the surface.
Alternating bubble nucleation events from multiple active cavities
in a microchannel groove was observed in this video. Ejection of
the bubbles from multiple nucleation sites in close proximity of
each other were seen to feed and grow a larger bubble as shown
in Fig. 16 was similarly observed in other videos. The partial filling
of deeper V-grooves compared to the shallow channels which are
easy to completely fill as the bubble grows inside the round bottomed channel, was concluded be one of the contributing factors
responsible for enhanced heat transfer performance with deeper
V-grooved microchannels.
An important bubble growth mechanism was observed while
analyzing the recorded videos at relatively higher heat flux conditions. Continuous nucleation and ejection of micro bubbles from
the grooves to aid in the growth of larger bubbles pinned to the
test section surface describes the observed bubble feeding mechanism during nucleate boiling. It is also important to note the nucleating bubbles are instantaneous ejected from the microchannels
within 1–2 milliseconds, so as to make way for the bulk liquid to
rewet the heated surface. Simultaneous or alternating nucleation
from multiple active cavities in close proximity was also observed
at higher heat flux conditions.
4.3. CRM test section at high heat flux conditions
Fig. 16. Bubble dynamics on the top surface of CVM5 test section at heat flux of
80 kW/m2.
The best performing test section CRM3 reported in part I paper,
was used to capture videos of bubble dynamics with the rectangular groove cross-section geometry. The videos were captured at a
comparatively higher heat flux of 150 kW/m2 than that discussed
in the previous sub-sections. At higher heat fluxes it was difficult
to record clear and uninterrupted videos with these surfaces due
to extensive movement of the generated bubbles. Videos on the
top and the bottom surfaces of the test section were recorded at
a frame rate of 4000 fps. An image sequence of a recording on
the top surface of the CRM3 is presented in Fig. 17. In the frame
(a), a bubble nucleating from an active cavity on the microchannel
wall can be seen. In a time span of 2 milliseconds the bubble grows
inside the microchannel and pins to the fins as it is forced out of
the groove. At 3 milliseconds another micro bubble nucleates from
the cavity and feeds into the first bubble as shown in frame (c).
Two additional micro bubbles feed into the first bubble before its
initiates to detach and depart from the surface due to the increased
buoyancy forces. Frame (d) shows a new bubble ejected from the
active cavity, attaches to the opposite wall of the microchannel
as seen in frame (e) at 5.5 milliseconds. Instantaneously after the
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J.S. Mehta, S.G. Kandlikar / International Journal of Heat and Mass Transfer 64 (2013) 1216–1225
limit at 900 kW/m2 and the overall wall superheats recorded are
relatively higher for the CVM test sections. It can be concluded that
the corners in the rectangular channels cannot be filled by the
round bubbles while growing inside the microchannels giving
uninterrupted access to the working fluid for rewetting of the
heated surface. Whereas the filleted V-groove cross-sectional area
can be completely filled by the vapor bubble and obstruct the rewetting through the microchannels at high heat fluxes thereby
resulting in lower critical heat flux limits.
5. Conclusions
The results obtained in this two part study show that the microchannel groove orientation, cross-sectional geometry, microchannel geometric dimensions and the tube orientation greatly
influence the heat transfer performance over a cylindrical tube surface in a pool boiling system. A plain test section, eight CRM, six
CVM, and six ARM test sections were tested and their experimental
results were presented in these papers. These test sections fabricated with unique microchannel geometries were tested in both
horizontal and vertical orientation. The resultant radial heat flux,
the wall superheat, and the heat transfer coefficients were evaluated and are presented in the form of their boiling curves. The following conclusions were drawn after analyzing the experimental
results.
Fig. 17. Bubble dynamics on the top surface of CRM3 test section at a heat flux of
150 kW/m2.
ejection of this bubble, another bubble nucleates from the cavity as
seen in the frame. These bubbles grow and ultimately coalesce as
seen in the frames (f) and (g). This coalesced bubble nearly fills
the entire microchannel cross-section area. The rectangular base
in this channel is highly advantageous in rewetting the heated surface as the corner regions are always filled with liquid and continuously supply liquid to the active nucleation sites. As the bubble
grows while attached to the fins, it clears the microchannel area
providing access for the bulk fluid to completely rewet the surface.
At these high heat fluxes, continuous generation of the micro bubbles from the active nucleation site was observed.
The rewetting phenomenon or the continuous liquid supply
through the corners of the microchannel groove under a growing
bubble helps in extending the critical heat flux limits and enhancing the boiling heat transfer performance significantly. The extension in the critical heat flux limit was seen to be more prominent in
the rectangular cross-sectional channels because of the continuous
liquid supply through the corners. Also one of the test sections consisting of V-grooved microchannels reached its critical heat flux
The CVM test sections showed good overall enhancements in
the heat transfer performance. Enhancement factors in the
range of 1.8–2.4 were achieved in the horizontal orientation,
and 2.0–2.3 were obtained in the vertical orientation. Similar
to the CRM test sections, the CVM test sections showed slightly
better performance in the horizontal orientation.
At around 900 kW/m2, CVM2 reached its critical heat flux limit
in the horizontal orientation due to its lower area enhancement
factor and filleted V-grooves. The parametric effects of the
microchannel dimensions for the six CVM test sections were
inconclusive and no trends on their effects were found.
The ARM test sections showed superior performance compared
to the CVM test sections. All the ARM test sections were tested
up to a heat flux of 1070 kW/m2 without reaching their critical
heat flux limits. Steady increase in the heat transfer performance was observed with increasing heat flux conditions.
Importantly the ARM test sections results showed 10–15% better heat transfer performance in the vertical orientation compared to the horizontal orientation.
The best performing test section, ARM2 having a highly favorable geometry for liquid rewetting, achieved a maximum heat
transfer coefficient of 96 kW/m2 K in the vertical orientation
at a heat flux of 1070 kW/m2 while maintaining the wall superheat of 11 K. The overall enhancement factors for the ARM test
sections were in the range of 1.9–2.3 in the horizontal orientation. Whereas in the vertical orientation the enhancement factors in the range of 2.1–2.7 were achieved.
The parametric study for the axially grooved rectangular crosssection microchannels showed that deeper and narrower
grooves aided in enhancing the surface performance. The area
normalized results clearly showed the influence of other factors,
such as the geometric parameters on the overall heat transfer
performance.
The circumferentially grooved CRM and CVM test sections
showed better heat transfer performance in the horizontal orientation whereas the axially grooved ARM test sections yielded
better performance in the vertical orientation. The groove orientation coupled with its preferential tube orientation aids in
enhancing the overall heat transfer performance up to 18%.
J.S. Mehta, S.G. Kandlikar / International Journal of Heat and Mass Transfer 64 (2013) 1216–1225
The rectangular cross-section geometry microchannels outperformed the V-groove microchannels. The circumferential rectangular microchannel test section CRM3, the experimental
results of which are presented in the part I paper [1] of this
study showed the greatest performance of all the test sections
designed and tested in this work.
The bubble interactions in and over the microchannels of a few
CRM and CVM test sections were recorded and analyzed to recognize the underlying boiling heat transfer mechanisms. Some of the
conclusion derived related to the bubble nucleation, growth and
departure are given below.
1225
for these test sections. Hence the overall heat transfer performance obtained with rectangular base test sections was comparatively superior to that obtained with V-groove test sections.
Acknowledgement
The experimental work was conducted in the Thermal Analysis,
Microfluidics, and Fuel Cell Laboratory at the Rochester Institute of
Technology in Rochester, NY. Part of this work was supported by
the National Science Foundation EPDT grant #0802100.
Appendix A. Supplementary data
At low heat flux conditions in the horizontal orientation, the
bubble generation frequency on the bottom surface is comparatively lower than that on the top surface, due to the attachment and growth of the bubbles on the bottom surface and
the buoyancy forces acting on the bubbles on the top surface.
The rapid detachment and departure of the generated bubbles
from the top surface leads to shorter dwelling times and smaller
departure diameters influencing the continuous re-nucleation
of active cavities.
At higher heat fluxes the bubble feeding mechanism was
observed where the bubbles nucleating from the active cavities
immediately merges with the larger bubbles in the vicinity and
aids in their growth. As the heat flux increases, multiple nucleating cavities in close proximity were observed to feed a single
large bubble. With further increase in the heat flux, simultaneous growth of such large bubbles was observed over the
entire microchannel surface.
The bubble nucleating inside the microchannels were instantaneously expelled out within a fraction of a millisecond, clearing
the groove area for rewetting of the heated surface. The rectangular cross-section groove was concluded to be highly advantageous for rewetting the heated surface since the corners of the
channels were always flooded with water, aiding in lowering
the wall superheat and extending the critical heat flux limits
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/
j.ijheatmasstransfer.2013.04.004.
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