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Proceedings of the ASME 2013 Summer Heat Transfer Conference
HT2013
July 14-19, 2013, Minneapolis, MN, USA
HT2013-17441
Experimental Investigation of Flow Boiling Performance of
Open Microchannels with Uniform and Tapered Manifolds
(OMM)
Ankit Kalani, Satish G. Kandlikar*
Department of Mechanical Engineering
Rochester Institute of Technology
Rochester, NY, USA
*sgkeme@rit.edu
ABSTRACT
Boiling can provide orders of magnitude higher cooling
performance than a traditional air cooled system especially
related to electronics cooling application. It can dissipate large
quantities of heat while maintaining a low surface temperature
difference. Flow boiling with microchannels has shown a lot of
potential due to its high surface area to volume ratio and latent
heat removal. Flow instabilities and early critical heat flux have
however prevented its successful implementation. A novel flow
boiling design is experimentally investigated to overcome the
above mentioned disadvantages while presenting a very low
pressure drop. The design uses open microchannels with a
tapered manifold (OMM) to provide stable and efficient
operation. Distilled, degassed water at atmospheric pressure
was used as the fluid medium. Effect of tapered block with
varied dimension is investigated. Pressure drop data for
uniform and tapered manifold for plain and microchannel chip
are presented. A maximum heat flux of 281 W/cm2 at 10 °C
wall superheat was recorded with microchannel and tapered
manifold without reaching CHF. The maximum pressure drop
obtained for the above mentioned configuration was only 3.3
kPa.
1. INTRODUCTION
Single phase cooling has been the dominant mode of heat
transfer for the last several decades due to its low cost and
reliable operation. Miniaturization of electronics and increase
in chip power densities has generated a need for high heat flux
dissipation. Boiling has the ability to dissipate large quantities
of heat due to its latent effects. Cooling with microchannels has
shown a lot of potential since the pioneering work of
Tuckerman and Pease [1]. Colgan et al. [2] used enhanced
microchannels in a single phase study to dissipate heat fluxes of
over 1 kW/cm2. They were limited by the high chip temperature
and large pumping power requirement. The state of research of
flow boiling in microchannels has been well reviewed by many
researchers [3–5]. The current literature review was focused on
the heat transfer and pressure drop performance of flow boiling
in microchannels.
For two-phase flow, Kandlikar [6] had pointed out that
flow instability, low heat transfer coefficient and flow
maldistribution were some of the key issues for the poor
performance of the flow boiling system. Various authors have
used different techniques to prevent high pressure drop
fluctuation in the system. Kandlikar et al. [7] used artificial
nucleation sites and inlet restrictors to provide stable flow
boiling system. Wang et al. [8] studied flow boiling instability
using three different inlet/outlet configurations. Low
fluctuations were obtained for the configuration which had inlet
restrictions, while no restrictions were placed on the exit side.
Wu and Cheng [9] and Lee et al. [10] used trapezoidal cross
section, parallel microchannels in their flow boiling study. Lu
and Pan [11] achieved stability in their microchannel system
using diverging, parallel microchannel as proposed by
Mukherjee and Kandlikar [12] with artificial nucleation sites.
Design with evenly distributed cavities along the channel
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Copyright © 2013 by ASME
showed the best performance. Hetsroni et al. [13] used parallel
triangular microchannels with varied experimental parameters.
Zhang et al. [14] extensively studied the Ledinegg instability in
microchannels. They concluded that presence of inlet
restrictors, increase in the system pressure and the channel
diameter, reduction in the number of channels and the channel
length lead to a more stable flow in the microchannels.
Balasubramanian et al. [15] used straight and expanding
microchannel in their flow boiling study. The authors observed
lower pressure drop and wall temperature fluctuation with
expanding microchannel geometry. Cho et al. [16] and
Megahed [17] used cross-linked microchannel in their
experimental work. Recently, Sitar et al. [18] used square
parallel microchannel of 25 × 25 µm and 50 × 50 µm cross
section with FC-72 and water. The authors used a combination
of inlet/outlet restrictors, inlet/outlet manifolds and fabricated
cavities to limit the instabilities. The authors observed
reduction in the onset of nucleate boiling temperature and an
even flow distribution.
High heat flux testing has also been undertaken by various
researchers. Qu and Mudawar [19] tested a microchannel heat
sink with 21 parallel channels and obtained a maximum heat
flux of 130 W/cm2. Kuo and Peles [20] used 200 µm × 253 µm
parallel microchannel with structured reentrant cavities. Mass
flux was varied from 83 kg/m2s to 303 kg/m2s. The authors
concluded that lower boiling incipience and increased CHF
were observed with structured reentrant cavities. Heat fluxes of
up to 643 W/cm2 at 80 °C wall superheat were recorded with
303 kg/m2s mass flux. Liu and Garimella [21] experimentally
investigated flow boiling in microchannels with inlet water
temperatures of 67 – 95 °C, and mass fluxes of 221 – 1283
kg/m2s. The authors obtained a heat flux of 129 W/cm2 and at
exit quality of 0.2. Recently, Kandlikar et al. [22] and Kandlikar
[23] used the open microchannel with tapered manifold
configuration (OMM) to simultaneously increase the CHF and
the heat transfer coefficient. They and obtained a heat flux of
506 W/cm2 at a wall superheat of 26.2°C without reaching
CHF. Preliminary work with tapered manifold showed low
pressure drop and high heat transfer performance.
In the current work, tapered microchannel configuration is
further investigated and its effect on pressure drop is studied.
Plain and microchannel chips are used with distilled and
degassed water as the working fluid at atmospheric pressure.
Three tapered manifolds having the same inlet manifold height
(0.127 mm) and gradually increasing exit manifold height (200,
400 and 600 µm) are used and their effect on the pressure
fluctuations are studied. Results are compared with the uniform
manifold for both plain and microchannel chips.
2. NOMENCLATURE
kCu
q”
x
Tsat
Tc
thermal conductivity of copper, W/m K
heat flux, W/m2
distance, m
saturation temperature, K
chip temperature, K
Twall
ΔTsat
G
ṁ
Ac
h
wall temperature, K
wall superheat, K
mass flux, kg/m2s
mass flow rate, kg/s
cross-sectional area, m2
heat transfer coefficient, W/m2 K
3. EXPERIMENTAL SETUP
Figure 1 shows the flow boiling test setup used in the
current study. It was similar to that used in an earlier study by
Kandlikar et al. [22]. The test setup consisted of a heating
block, base plate, intermediate plate and a manifold block. The
heater consisted of a copper block with eight 200 W cartridge
heaters. The top half of the heater had three equally spaced
holes for thermocouple probes and the tip was 10 mm × 10 mm
square section contacting the test chips. The manifold block
consisted of inlet and outlet openings for the fluid flow. The
base plate used was used to support the intermediate plate and
the manifold block. The test section was placed in the base
plate and a silicone gasket helped in sealing the system. A
gasket thickness of 0.127 mm was used for all the test runs.
This presents a fixed manifold height in the uniform manifold
case, and the inlet manifold height in the case of tapered
manifolds.
A Micropump© pump was used to provide the required
flow rate for the system. A flow rate of 80 mL/min was used for
the current work. A supply tank with distilled water was
degassed initially before supplying the water to the system. The
supply tubing from the tank to the system was wrapped in
fiberglass insulation to minimize the heat losses. A subcooling
of 10°C was employed at the water inlet into the test section.
Figure 1. Schematic of the flow boiling test setup [22].
The intermediate plate consisted of tapered and uniform
manifolds. The tapered manifold allowed a tapered gap as seen
in Fig. 2 above the microchannel while the uniform manifold
had no recess in the intermediate plate. The tapered manifold
was designed to allow a larger flow cross-sectional area on the
exit side of the fluid path. Three different tapered manifolds
were used having the same inlet height (0.127 mm) and
gradually increasing manifold heights to 200 µm, 400 µm and
600 µm on the exit side. Polysulfone was used for the manifold
block and the intermediate plate.
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Copyright © 2013 by ASME
Figure.3 Schematic of the microchannel copper chip.
x
Twall = Tc − q"(k 1 )
Figure 2. Schematic of the tapered manifold [22].
Heat flux was calculated using the three thermocouples
inserted in the copper block using the one-dimensional heat
conduction equation.
dT
q" = −k Cu
(1)
dx
Three-point backward Taylor’s series approximation was used
to calculate the temperature gradient dT/dx.
dT
dx
=
3T1 −4T2 +T3
2∆x
(2)
A differential Omega® pressure sensor was used for the
pressure drop reading. An NI cDaq-9172 data acquisition
system with NI-9213 temperature module and NI-9205 pressure
module was used to record the temperature and pressure
respectively. A LabVIEW® virtual instrument (VI) was used to
display and record temperature, pressure and heat flux.
4. TEST SECTION
A copper chip with an overall dimension of 20 mm × 20
mm × 3 mm was used as the test section. Only the central 10
mm × 10 mm area of the chip was exposed to the fluid through
an opening in the silicone gasket. A square 2 mm × 2 mm
groove, as shown in Fig. 3, on the underside of the chip was
provided to reduce the heat spreading effect. A thermocouple
hole was provided on the side of the test section, to measure the
actual chip temperature. A microchannel chip of 450 µm depth
and with 181 µm wide channels and 195 µm wide fins was also
tested. CNC machines were used to make the microchannels on
the copper chip.
The wall temperature at the top of the chip surface was
calculated using the heat flux obtained from the measured chip
temperature Tc and the temperature drop in the copper substrate
over the distance x1, which is the distance between the
thermocouple location and the top surface.
(3)
Cu
The maximum heat flux tested was limited to 300 W/cm2
due to the heater design. CHF was not reached in any of the
testing.
5. RESULTS
Both, uniform and tapered manifolds were tested with the
plain and microchannel chips and the results for heat transfer
and pressure drop are presented in this section. Heat flux was
calculated using Eq. 1 for a heater area of 100 mm2. The
pressure data was obtained using the differential pressure
sensor. For all the test runs, the flow rate was kept constant at
80 mL/min, and a gasket of thickness 0.127 mm was used to
provide a fixed height at the inlet manifold. The effect of a
uniform height manifold and three tapered manifold is also
discussed.
Table 1. Manifold configuration and mass fluxes at inlet
and outlet.
Manifold
Taper
height
(exit)
Inlet
height
(µm)
Exit
height
(µm)
Ginlet
(kg/m2s)
Goutlet
(kg/m2s)
Uniform
0
127
127
372
372
Taper A
200
127
327
372
238
Taper B
400
127
527
372
175
Taper C
600
127
727
372
138
The configurations for the uniform and the tapered
manifolds used in the current system are shown in Table. 1. The
inlet height remains constant for all test runs, while the exit
height changed depending on the type of manifold used. Both
the inlet and exit heights were referenced from the top plane of
the microchannel to the manifold cover. The mass flux for inlet
and outlet are calculated using the following equation.
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Copyright © 2013 by ASME
ṁ
(4)
𝐴𝑐
where the cross-sectional area Ac, is the actual flow area at
a given section calculated as the sum of the microchannel and
gap height provided by the gasket at that section. The
maximum exit quality observed in the current testing is below
0.1.
𝐺=
5.1 Uniform manifold testing:
The uniform manifold with plain and microchannel chips
were tested first so as to establish the baseline results. The
uniform manifold consisted only the constant height provided
by the gasket over the chip. Both the inlet and exit manifolds
had a height of 0.127 mm.
Figure 4 shows the boiling performance of the two chips
with uniform manifold. The plain chip showed a slight boiling
overshoot at a wall superheat of 17 °C. Linear increase in heat
flux was observed with wall superheat. A maximum heat of 227
W/cm2 at 22 °C wall superheat was recorded for plain chip.
Microchannel chip showed similar overshoot but performed
significantly better than plain chip. A maximum heat flux of
283 W/cm2 at a wall superheat of 13 °C was obtained. Testing
was not continued at higher heat fluxes.
Figure 5. Pressure drop versus heat flux for plain and
microchannel chip with uniform manifold.
5.2 Tapered manifold testing:
Tapered manifold was designed to provide an additional
flow area in the system along the flow direction to
accommodate the vapor flow and reduce the pressure drop.
Figure 6 compares the boiling performance for the uniform
and tapered manifold C for a microchannel and a plain chip.
Expectedly, the microchannel chip performed better than the
plain chip. The highest heat flux tested for the microchannel
chip was 281 W/cm2 at 10 °C wall superheat. For the plain
chip, a maximum heat flux of 208 W/cm2 at a wall superheat of
16 °C was recorded. Boiling overshoot was not observed for
both plain and microchannel chips.
Figure 4. Boiling performance showing heat flux versus
wall superheat for plain and microchannel chip with
uniform manifold.
Figure 5 shows the heat flux and its corresponding pressure
drop for the same. The plain chip shows pressure fluctuations
from 100 kPa at low heat fluxes to 160 kPa at high heat fluxes.
The introduction of microchannel does show a reduction in
pressure drop as the extra area provided by the microchannels
reduces the flow resistance. At low heat fluxes, a pressure drop
of 40 kPa is observed. For high fluxes, a maximum pressure
drop of 60 kPa is seen near a heat flux of 250 W/cm2. The
overall pressure fluctuation for the microchannel chip increased
with the increase in heat flux.
Figure 6. Boiling performance of plain and
microchannel chips with the tapered manifold C.
Figure 7 compares the pressure drop for the tapered
manifold C for the microchannel and plain chip. The effect of
the tapered manifold is significant in terms of pressure drop
performance for both chips. The plain chip shows a maximum
pressure drop of 6 kPa at a heat flux of 208 W/cm2, while the
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Copyright © 2013 by ASME
microchannel chip showed a pressure drop of around 2 kPa for
a similar heat flux. The error bars shown are pressure
fluctuation for a given heat flux. The solid points shown on the
graph are the average values over the pressure range. At
intermediate heat fluxes, small negative values of pressure drop
are seen indicating a small backflow behavior. However, it is
quite infrequent and insignificant and is not affecting the heat
transfer performance adversely. The plain chip shows an
increasing trend of pressure drop with the heat flux, while the
microchannel chip shows only a slight increase with the
increasing heat flux.
Figure 8. Boiling performance of plain chip with
tapered and uniform manifold.
Figure 7. Pressure drop performance for plain and
microchannel chips with tapered manifold C.
5.3 Heat transfer and pressure drop performance with the
plain chip
The effect of the two manifolds on plain chip is discussed
in this section. Figure 8 shows the boiling performance of
tapered and uniform manifolds. The tapered manifold showed
an improved performance compared to the uniform manifold.
The uniform manifold showed a boiling overshoot and recorded
a heat flux of 227 W/cm2 at 22 °C wall superheat. In
comparison to the uniform manifold, the three tapered
manifolds showed similar performances to one another. Hence,
the effect of taper height itself was not significant as seen from
the figure. Boiling overshoot was not observed with any of the
tapered manifolds. The tapered manifold B recorded a heat flux
of 255 W/cm2 at a wall superheat of 17 °C.
The pressure drop performance of the plain chip with
uniform and the three different tapered manifolds is shown in
Fig. 9. The tapered manifolds show a significant pressure drop
reduction compared to the uniform manifold. The uniform
manifold showed the highest pressure drop values at both low
and high heat fluxes. For the tapered manifold, the values were
below 20 kPa over the entire range. The highest pressure drop
was observed with the tapered manifold B at a heat flux of 255
W/cm2 of 19 kPa. Tapered manifold C showed the lowest
pressure drop over the entire heat flux range. A maximum
pressure drop of 6.2 kPa at 208 W/cm2 heat flux was recorded
for the tapered manifold C.
The introduction of a tapered manifold drastically reduced
the pressure drop from 150 kPa (uniform) to 6 kPa (tapered
manifold C) for similar heat flux values.
Figure 9. Pressure drop performance of plain chip with
uniform and tapered manifolds.
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Copyright © 2013 by ASME
5. 4 Heat transfer and pressure drop performance with the
microchannel chip
Results of the microchannel chip with the uniform and the
tapered manifolds are discussed in this section. Figure 10
shows the heat flux versus wall superheat plot for the two
manifolds with the microchannels. Unlike the plain chip
performance, the effect of varying the taper was observed in the
heat transfer performance. The tapered manifold C recorded a
heat flux of 281 W/cm2 at a wall superheat of 10 °C. The
uniform manifold however showed a better performance than
the tapered manifold A, although the testing was not continued
to higher heat fluxes. CHF was not reached for any of the tests,
hence showing potential for greater heat dissipation. The
tapered manifold B dissipated a heat flux 225 W/cm2 at a wall
superheat of 9 °C. The slope of the tapered manifold A curve
suggests that at higher heat fluxes, it might perform better than
the uniform manifold. Further testing of tapered manifold A is
suggested and will be undertaken in future work.
Figure 11. Pressure drop performance of microchannel
chip with uniform and tapered manifolds.
DISSCUSSIONS
Figure 10. Boiling performance of microchannel chip
with tapered and uniform manifold.
Three tapered manifolds were tested with the microchannel
chip and their pressure drop performance is shown in Fig. 11.
The maximum pressure drop observed was 11 kPa for tapered
manifold A at a heat flux of approximately 170 W/cm2. A
maximum pressure fluctuation of around 6 kPa was observed
for all three tapered manifolds. The tapered manifold C showed
the lowest pressure drop of 3 kPa at a heat flux of 260 W/cm2 in
comparison with the other two tapered manifolds. A maximum
pressure drop of 10 kPa was observed with the tapered
manifold B, and it showed lower pressure fluctuations
compared to the tapered manifold A.
In this section, the flow boiling performance of the uniform
and the three tapered manifolds for both plain and
microchannel chips is presented. The results of heat transfer
and pressure drop are further analyzed.
6.1 Comparison between the microchannel and the plain
chip with uniform and tapered manifold
Figure 12 shows the heat transfer performance for tapered
manifold C with the microchannel chip and the uniform
manifold with both chips. Expectedly, microchannel chip
showed significant performance improvement compared to
plain chip for both manifolds. The introduction of the tapered
manifold yields similar performance at the mid-range heat
fluxes, but the heat flux is seen to rise for a given wall
superheat at higher heat fluxes. The maximum heat flux
obtained with the tapered manifold is greater than uniform
manifold with the microchannel chip. Both plain and
microchannel chips show a small temperature overshoot with
the uniform manifold. No temperature overshoot is observed
with the tapered manifold.
Figure 12. Boiling performance comparison with
tapered manifold with microchannel and uniform manifold
with both chips.
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Copyright © 2013 by ASME
Figure 13 shows pressure drop versus the corresponding
heat flux with the uniform manifold and the tapered manifold
A. The highest pressure drop was observed with a uniform
manifold with a plain chip. At high heat fluxes (~225 W/cm2), a
pressure drop of 160 kPa was recorded with the plain chip. At a
similar heat flux, the microchannel chip with a uniform
manifold recorded a pressure drop of 50 kPa. The reduction in
the pressure drop was mainly due to the increase in the flow
cross-sectional area provided by the microchannels. The
tapered manifold C showed the lowest pressure drop of ~2 kPa
at a heat flux of 225W/cm2. The combination of tapered
manifold with microchannel clearly showed a significant
pressure drop reduction over the entire range of heat flux. The
expanding cross-sectional area along the flow direction was
able to accommodate the increased vapor flow and resulted in
an extremely low pressure drop. The overall increase in the
pressure fluctuation with increasing heat flux is also limited for
the tapered manifold, hence showing a more stable flow with
the tapered manifold in comparison to the uniform manifold,
while simultaneously offering a better heat transfer
performance in terms of higher heat flux at a given wall
superheat.
°C wall superheat with the microchannel chip, while a heat flux
of 250 W/cm2 at 18 °C wall superheat was obtained with the
plain chip.
Figure 14. Comparison of heat transfer performance
for plain and microchannel chip with tapered manifold (B
and C).
Figure 15 shows the heat transfer coefficient versus heat
flux for the microchannel and the plain chips with the tapered
manifolds B and C. Heat transfer coefficient is a very important
parameter in comparing the thermal performance of different
surfaces. Similar to the conclusion from Fig. 14, the
microchannel chip performed significantly better than the plain
chip for both tapered manifolds. At higher heat fluxes, a
maximum heat transfer coefficient 278 kW/m2K for both tapers
was recorded. A maximum heat transfer coefficient of 140
kW/m2K at 250 W/cm2 heat flux was observed for the plain
chip with taper B. For taper A, similar results were obtained.
Figure 13. Pressure drop performance comparison with
tapered manifold C with microchannel chip and uniform
manifold with both chips.
6.2 Comparison between the microchannel and the plain
chip with the tapered manifold
Figures 14 and 15 show the boiling performance
comparison between the two chips for taper manifolds, B and
C. The data for the tapered manifold A was not included in the
figures so as to avoid overcrowding of data points.
Higher heat dissipation at a given wall superheat is
observed with the microchannel chip for both tapered
manifolds as seen in Fig. 14. The plain chip shows no effect of
taper height on the boiling performance. For the tapered
manifold C, a heat dissipation of 282 W/cm2 was achieved at 10
Figure 15. Comparison of heat transfer coefficient for
plain and microchannel chip with tapered manifolds B and
C.
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Copyright © 2013 by ASME
Table 2 shows the maximum values for heat flux and heat
transfer coefficient and their corresponding wall superheat and
average pressure drop value for plain and microchannel chip.
Taper C with microchannel chip shows the best performance in
terms of pressure drop and heat transfer. For the plain chip,
similar heat transfer performance is obtained for all three
tapers. The tapered manifold show significant pressure drop
difference compared with uniform manifold for the plain chip.
CONCLUSIONS
The current work involves an experimental investigation of
flow boiling performance with plain chip and chips with open
microchannels with the uniform and the tapered manifolds
(OMM). The testing was limited to heat fluxes below about 280
W/cm2 due to heater limitations Distilled water at atmospheric
pressure at a flow rate of 80 mL/min was used for all test runs.
Table 2. Summary of all test runs for plain and
microchannel chip including maximum heat flux, wall
superheat heat transfer coefficient and average pressure
drop.
Chip
Plain
µchannel
q”max
∆Tsat
h
∆Pavg
W/cm2
°C
kW/m2K
kPa
Uniform
Taper A
Taper B
227.1
228.6
255.1
22.1
15.8
17.7
102.4
144.4
144.3
158.4
12.6
19.6
Taper C
208.3
15.6
133.5
6.3
Uniform
Taper A
Taper B
283.2
263.8
239.1
12.9
14.1
8.6
217.9
186.7
277.6
62.1
7.5
6.2
Taper C
281.2
10.1
277.8
3.3
Manifold
1.
2.
3.
4.
The tapered manifolds with microchannel chips yield a
dramatic enhancement in heat transfer performance, while
providing an extremely low pressure drop value. This feature
makes it particularly suited to cooling the high performance IC
chips. The low pressure drop feature provides a very high
coefficient of performance (ratio heat removed to pumping
power) for 3D chip cooling architecture. For the tapered
manifold C with microchannel chip, a heat flux of 281.2
W/cm2 is dissipated at a wall superheat of 10.1 °C with a heat
transfer coefficient of 277.8 kW/m2°C. The corresponding
pressure drop is only 3.3 kPa. Further performance
enhancements are expected with optimizing the microchannel
geometry and the taper configuration.
The main mechanism responsible for reducing the pressure
drop with the tapered manifolds is the increase in the flow
cross-sectional area as the vapor is generated along the flow
direction. As seen from Table 2, the cross-sectional area
increases for tapered manifolds, and the pressure drop is
corresponding lower. The results for plain chips are affected
due to the presence of backflow under some heat flux
conditions. The liquid flows through the microchannels
promoting nucleation and is responsible for delaying the CHF.
Further work on establishing the CHF limits for these
configurations is suggested by redesigning the test section to
deliver higher heat fluxes. This work is continuing in the
authors’ lab.
5.
6.
7.
8.
8
Three tapered manifolds with a gradual increase in the
gap at the exit (200 µm, 400 µm and 600 µm) and a
uniform manifold of a 127 µm gap were tested with
the microchannel and the plain chips in the current
setup.
A heat flux of 227 W/cm2 at a wall superheat of 22 °C
was observed for the uniform manifold with a plain
chip, while a heat flux of 283 °C at 12 °C wall
superheat was recorded for the microchannel chip with
same manifold
The combination of microchannel chip and the tapered
manifold significantly reduced the pressure drop in the
system. Taper C with microchannel showed the best
performance with the lowest pressure drop of 3.3 kPa
compared to the 160 kPa pressure drop with the plain
chip and the uniform manifold.
A heat flux of 281 W/cm2 at 10 °C wall superheat with
taper C was recorded with the microchannel chip. The
microchannel chip with the tapered manifolds showed
significant performance improvement compared to the
plain chip with the uniform manifold.
Similar improved performance in heat transfer
coefficient for the microchannel chip with tapered
manifold was observed in comparison to the plain chip
with tapered manifold. A maximum heat transfer
coefficient of 278 kW/m2K was recorded with
microchannel chip and taper C.
The testing was not conducted to the CHF limit, which
was reported to be higher that 500 W/cm2 in an earlier
publication. The comparison presented here showed
that the microchannel chip with a taper C has the best
heat transfer performance among the chips and
manifolds tested.
The main mechanism responsible for the dramatic
reduction in pressure drop is due to the increased flow
cross-sectional area to accommodate the vapor
generated along the flow direction. This combines the
inherent benefits of microchannels in providing a
superior heat transfer performance.
The open microchannels with tapered manifold
(OMM) configuration is able to provide significant
heat transfer coefficient and CHF enhancements and
holds promise in overcoming the limitations posed by
the microchannels during flow boiling. Additional
testing to cover a higher exit quality and establish the
CHF limits of the OMM configuration is proposed.
Copyright © 2013 by ASME
ACKNOWLEDGEMENTS
The work was conducted in the Thermal Analysis,
Microfluidics and Fuel Cell Laboratory at the Rochester
Institute of Technology in Rochester, NY and supported by the
National Science Foundation under Award No. CBET-123602.
[14]
[15]
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