Proceedings of ICMM2005
3rd International Conference on Microchannels and Minichannels
June 13-15, 2005, Toronto, Ontario, Canada
Paper No. ICMM2005-75070
DEVELOPMENT OF AN EXPERIMENTAL FACILITY FOR INVESTIGATING SINGLE-PHASE
LIQUID FLOW IN MICROCHANNELS
Mark E. Steinke
Thermal Analysis and Microfluidics Laboratory
Mechanical Engineering Department
Kate Gleason College of Engineering
Rochester Institute of Technology, Rochester, NY USA
J. H. Magerlein
T.J. Watson Research Center
IBM Corporation
Yorktown Heights, NY USA
Satish G. Kandlikar
Thermal Analysis and Microfluidics Laboratory
Mechanical Engineering Department
Kate Gleason College of Engineering
Rochester Institute of Technology, Rochester, NY USA
Evan Colgan
T.J. Watson Research Center
IBM Corporation
Yorktown Heights, NY USA
ABSTRACT
An experimental facility is developed to investigate singlephase liquid heat transfer and pressure drop in a variety of
microchannel geometries. The facility is capable of accurately
measuring the fluid temperatures, heater surface temperatures,
heat transfer rates, and the differential pressure in a test section.
A microchannel test section with a silicon substrate is used
to demonstrate the capability of the experimental facility. A
copper resistor is fabricated on the backside of the silicon to
provide heat input. Several other small copper resistors are
used with a four point measurement technique to acquire the
heater temperature and calculate surface temperatures. A
transparent Pyrex cover is bonded to the chip to form the
microchannel flow passages. The details of the experimental
facility are presented.
The experimental facility is intended to support the
collection of fundamental data in microchannel flows. It has
the capability of optical visualization using a traditional
microscope to see dyes and particles. It also has the capability
to perform micro-particle image velocimetry in the
microchannels to detect the flow field occurring in the
microchannel geometries. The experimental uncertainties have
been carefully evaluated in selecting the equipment used in the
experimental facility.
The thermohydraulic performance of microchannels will
be studied as a function of channel geometry, heat flux and
liquid flow rate. Some preliminary results for a test section,
with a channel width of 100 micrometers, a depth of 200
micrometers, and a fin thickness of 40 micrometers are
presented.
Alan D. Raisanen
IT Collaboratory,
Director of Technology
Rochester Institute of Technology
Rochester, NY USA
INTRODUCTION
Cooling of high heat flux microprocessor chips using
single-phase liquid flow presents an attractive option as the heat
fluxes exceed the present limit of air cooling (approximately
800 kW/m2). The majority of available literature and ongoing
research programs are focusing on two-phase flow systems to
perform the high heat flux removal. However, the single-phase
cooling option offers considerable advantages over a two-phase
flow system.
Kandlikar and Grande (2003, 2004) describe the
fabrication techniques and possible trends in microchannels.
They remark that the fluid flow in microchannels is further
reaching than just efficient heat transfer. It could open up
completely new fields not possible only a few years ago.
The upper limit of single-phase liquid-cooled systems is
not clearly established. It is therefore necessary to conduct
experiments in an effort to arrive at an acceptable cooling
system performance and the ability to predict the performance.
Establishing this boundary will also help to find ways to extend
this limit with other advanced techniques, such as incorporating
enhanced structures within microchannel flow passages.
The single-phase cooling system will have less system
complexity, less variability resulting from flow boiling, and
smaller pressure drops compared to a two-phase flow system.
In addition, the heat rejection side of the system will also have
less complexity because the need for a condensation process is
absent.
Steinke and Kandlikar (2004a) identified single-phase heat
transfer enhancement techniques for use in microchannels and
minichannels. They speculate that this increase in heat transfer
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Copyright © 2005 by ASME
performance from these techniques could place a single-phase
liquid system in competition with a two-phase system. Thus,
simplify the over all complexity and reliability. However, they
point out that the added pressure drop resulting from the
techniques should be carefully evaluated.
An experimental test facility that allows for the
determination of important parameters influencing heat transfer
and fluid flow in microchannels, as well as determining the
associated experimental uncertainties, is desired. The facility
should also have the ability to incorporate a variety of
visualization techniques that are helpful in microfluidics.
LITERATURE REVIEW
There are over 150 papers that address the single-phase
flow of liquids in microchannels. Bailey et al. (1995) provided
a review of heat transfer and pressure drop in microgeometries.
Their main focus was the pressure drop occurring in the
microgeometry. Palm (2001) presented a review of singlephase and two-phase flow in microchannels. Some of the
important parameters that govern the behavior were identified.
Palm concluded that more work is needed to advance the
understanding of fluid flow and heat transfer in microchannels.
Sobhan and Garimella (2001) performed a comparative analysis
of some of the existing works on microchannels. They cite the
need for further study on all fronts to allow for the explanation
of the discrepancies reported in literature. Recently, Morini
(2004) performed a literature review and focused on the
pressure drop and friction factor correlations available. They
also point to the need for more experimental work to develop a
greater fundamental understanding.
Table 1 shows selected works that have acquired data for
heat transfer and pressure drop in microchannels. The most
well known work is by Tuckerman and Pease (1981). This is
often considered to be the pioneering work in microchannels.
They used a pressure vessel to drive the flow through the
microchannels. There are several parameters for each work
reported in the table. The parameters include the hydraulic
diameter, Reynolds number, mass flux, and heat flux. There
are many methods for driving the flow through a microchannel.
The two most common methods used are a pressurized
vessel and a pump. The pressure vessels are typically
containers that contain the working fluid and can be pressurized
using an inert gas, usually nitrogen. The other method is to use
a pump. The style of pump used in the test loops varies.
However, the most desirable feature for all of them is a steady
flow rate. The most common pump used for steady flow is a
gear pump. The gear pump is a positive displacement pump
that delivers a constant volume of fluid at a steady rate, despite
changes in upstream pressure. By contrast, a centrifugal
pump’s flow rate depends upon the upstream pressure and
therefore is less desirable.
A wide range of Reynolds numbers and heat fluxes are
encountered in single-phase microchannel flow. A test facility
that produces a wide range of flow rates would be desirable. In
addition, an appropriate power supply for delivering high
current would be required to generate large heat fluxes.
OBJECTIVES OF PRESENT WORK
The main objective of the present work is to develop an
experimental facility capable of providing accurate data for
single-phase liquid heat transfer and pressure drop in
microchannels. The experimental system should provide
accurate measurements to generate microchannel heat transfer
and pressure drop data. Additionally, the test section should
provide the ability to perform microchannel flow visualization.
The development of the test section for a variety of fluid flow,
heat transfer, and geometry arrangements will be performed.
EXPERIMENTAL TEST FACILITY
The experimental test facility contains all of the supporting
equipment that supplies a metered working fluid to the test
section and the measurement of heat transfer and pressure drop.
Figure 1 shows the design schematic for the experimental
system.
General Description
The flow loop can be operated in an open-system or a
closed-system mode. The type of experiment conducted
determines which is better. The system represented in Fig. 1 is
an open flow loop. The flow loop begins with a polycarbonate
supply tank.
A positive displacement, micro-gear pump drives the flow
through the loop. The gear pump is a Micropump brand pump
that delivers 1 to 300 mL/min of flow at a maximum pressure
of 4.8 bars. This style of pump is chosen to give a constant
flow rate at the test section regardless of the pressure drop
occurring in it. This is because the pressure drop can change
significantly due to different microchannel geometries being
tested. The micropump is chosen because of its small footprint,
accurate flow rates, and ability to dispense small volumes.
Next, a membrane filter follows the pump. The filter is
housed in a stainless steel casing that can maintain high
pressures and temperatures. The membrane used in the filter
housing a standard 47 mm diameter replaceable nylon filter.
There are a variety of pore sizes readily available, ranging from
0.2 µm to 25 µm in diameter. Typically, a 1.0 µm pore size is
used in the filter.
The next item in the flow loop is a flow meter bank that
contains flow meters in a parallel arrangement with different
flow rate ranges. Each flow meter has a different range to
improve the accuracy of the flow measurement. The advertised
accuracy of the rotameter is 2% of full scale. However, the
measured accuracy is actually 1.25% for the bottom range and
less than 0.5% for the mid an upper flow ranges. The flow
meters are a rotameter type. For the flow range of the pump,
the rotameter gives reasonable accuracy for a moderate cost.
For even lower flow rates, a coriolis flow meter is utilized.
The flow meters have an overall flow range of 0.01 mL/min to
4,000 mL/min. The individual flow meter ranges usually
increase by an order of magnitude. In other words, one flow
meter would have a 3 to 75 mL/min flow range and the next
available flow meter would have a 30 to 300 mL/min flow
range.
A miniature shell and tube heat exchanger and a
recirculation bath are used to control the fluid temperature
entering the test section. The tube diameters are 3 mm and the
entire heat exchanger is constructed from stainless steel. The
inlet heater is designed to heat the working fluid up to 90 °C.
A typical load for the heat exchanger is 100 W.
The experimental system is instrumented to measure
temperatures, pressures and flow rates. Integrated resistors on
the test device are used to measure local temperatures.
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Copyright © 2005 by ASME
Table 1: Selected Works for Single-Phase, Liquid Flow in Microchannel Passages.
Year
Fluid
Shape
Dh ( µm )
Re
G ( kg / m2s )
q” ( W / cm2 )
Method for Driving Flow
Tuckerman & Pease
Missaggia et al.
Riddle et al.
1981
1989
1991
water
water
water
rectangular
rectangular
rectangular
92 - 96
160
86 - 96
291 - 638
2350
96 - 982
2615 - 5678
12463
950 - 9545
187 - 790
100
100 - 2500
pressure
pressure
pressure
Gui & Scaringe
Peng et al.
1995
1995
water
methanol
trapezoid
rectangular
338 - 388
311 - 646
834 - 9955
1530 - 13455
2110 - 21937
174 - 1179
12 - 112
0.2 - 10
pressure
pump
Peng & Peterson
Vidmar & Barker
1996
1998
water
water
rectangular
circular
133 - 200
131
136 - 794
2452 - 7194
579 - 4448
16005 - 46954
5 - 45
506 - 2737
pump
pump
Ravigururajan & Drost
Lee et al.
Qu & Mudawar
1999
2002
2002
R124
water
water
rectangular
rectangular
rectangular
425
85
349
135 - 1279
119 - 989
137 - 1670
3.2 - 30.6
1196 - 9944
335 - 4093
0.8 - 13
35
100 - 200
pump
pump
pump
Author
Figure 1: Experimental Flow Circuit. Open system mode shown.
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Copyright © 2005 by ASME
They are formed by depositing copper in a very thin line,
more details given in a later section. The remaining system
temperatures are read using thermocouples; do to the
inexpensive yet accurate measurement capabilities. E type
thermocouples are chosen for their accuracy and their
temperature range.
Pressure is measured using a differential pressure
transducer from Omega. The pressure transducer range, from
0.02 to 6.8 bars, is selected to give the highest level of
accuracy.
Flow Visualization
Studying the fluid flow in microchannels can be achieved
in party by visualizing the flow field. There are two main
visualization techniques that can be employed with the
experimental test facility, . The experimental facility is capable
of performing both flow visualization techniques by utilizing an
optically clear microchannel wall.
A standard optical microscope has been included in the
experimental facility. Long distance objectives have been
incorporated to increase the working distance. A high-speed
CCD camera is attached to the microscope to capture images of
the flow.
A micro-particle image velocimetry (µPIV) system is
utilized with a visualization test section to investigate the
velocity field in the microchannel. A high frame rate camera is
utilized with this system to visualize higher Reynolds number
flows. The velocity vectors in the microchannel can be
determined using this technique. The flow distribution in the
microchannels, the flow in the header region, and the entrance
length in the microchannel are some of the aspects that can be
studied with this experimental facility.
TEST SECTION DESIGN
The design of the test section used to conduct the
experiments in the present work is discussed in this section.
The test section is located in the test flow circuit and provides
support for the test device. The test fixture is part of the test
section and provides a platform to contain the electrical and
fluidic connections.
Test Device
The test device contains the microchannels, the heater and
the temperature sensors. The geometries of the microchannels
that have been fabricated are presented in Table 2. Figure 2
shows the location of the geometry variables used in Table 2.
The parameters are the microchannel width is a, fin thickness
separating the microchannels is s, depth of the microchannel is
b, the total flow length of the microchannel is L, and the
number of microchannels is n.
There are twelve different microchannel configurations
selected for the study. The channels are 10 mm in total length
and are 8 mm wide. The number of channels varies to
completely fill the 8 mm width. The resulting number of
channels varies between 18 and 100. The test section is
fabricated using two different layers bonded using an adhesive.
The first layer is a silicon substrate that contains the
microchannels, the heater, and the temperature sensors. The
second layer is made of Pyrex 7740 and contains the fluidic
connections.
s
a
b
Figure
2:
Geometric
Microchannels
Parameters
for
the
Table 2: Channel Geometries of the Test Device.
a
s
pitch
Number
µm
µm
µm
G1-001
G1-002
G1-003
G1-004
G1-005
G1-006
G1-007
G1-008
G1-009
G1-010
G1-011
G1-012
40
40
70
70
100
100
200
200
200
250
250
250
40
100
40
100
40
100
40
100
200
40
100
200
80
140
110
170
140
200
240
300
400
290
350
450
n
L
µm
100
57
73
47
57
40
33
27
20
28
23
18
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
The microchannels are fabricated using deep reactive ion
etching (DRIE). A very high aspect ratio with straight
sidewalls can be achieved using this method. Figure 3(a)
shows an example of the microchannels etched into the silicon.
There are also inlet and outlet headers formed in the silicon.
The back side of the header is rounded to help direct the flow
and minimize stagnation regions. Finally, the entrance to each
channel is rounded to reduce the pressure drop due to the
entrance. The radius is one half the fin thickness, s.
An insulating layer of silicon nitride is deposited on the
backside of the device. Then, a layer of copper is deposited
and patterned on the backside to form the heater and sense
resistors. Finally, a nitride cap is deposited to help minimize
corrosion of the copper. The copper is patterned into a long
serpentine resistor for the heater. The total length is 82 mm
with a resistance of 9.7 ohms. The current is controlled to
allow for the introduction of a targeted heat flux of 200 W/cm2.
The current supplied to the heater can be as large as 5 amps.
The six temperature sensor resistors are kept very small to
increase the spatial resolution. A constant current is supplied to
each sense resistor. The resistance of the sensor is a function of
temperature allowing for the temperature to be determined.
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Copyright © 2005 by ASME
Each sense resistor requires four connections, two for supply
current and two for potential reading. The sensor will return a
line averaged temperature along the length of the resistor.
Figure 3(b) shows the backside of the device with the heater,
sensors, and the bonding pads.
(a)
(b)
acetal resin from DuPont. It is a lightweight plastic that is very
stable and does not experience any significant creep.
The test fixture is comprised of the following major parts;
the pogo probe block, the main block, the bottom retaining
plate, the device retaining plate, and the mounting plate. The
pogo probes are inserted into the pogo probe block. The pogo
probes are a high current version that can handle up to 10 amps.
Figure 4 shows the pogo probes inserted into their respective
block. Their pattern there matches the contact pad pattern in
Fig. 3(b). The pogo probe block is inserted into the main block.
Figure 5 shows the test section assembly.
Pogo
Probes
Pogo Probe
Block
Power
Resistor
Sensor
Resistor
Figure 3: Microchannels in Silicon Substrate. (a)
microchannels etched into front side of silicon, (b)
electrical layout of copper back side.
To discuss the location of features on the test device, let
the origin be located at the center of the device. There are six
temperature sensors located on the backside of the device. The
locations of the sensors are at -4.750 mm, -3.375 mm, -1.875
mm, 0.375 mm, 3.375 mm, and 4.750 mm. The first and last
sensor is placed beneath the inlet and outlet plenums. The
remaining four sensors are interlaced within the serpentine of
the heater.
The silicon substrate contains the microchannels with three
walls. A Pyrex cover piece is placed on top of the silicon
substrate to form the complete microchannel test device. The
cover piece has the same overall dimensions of the silicon
substrate. The thickness of the cover plate is 750 µm. The
cover piece is laser drilled with two holes to match with the
location of the inlet and outlet headers. The diameter of the
plenum hole is 1.5 mm.
Adhesive Bonding
Adhesive bonding is the method chosen to attach the Pyrex
cover to the silicon substrate. This method provides wide
latitude with the surface preparation as well as surface
roughness. However, both parameters are still important in the
success of the bond interface. The adhesive used is M-Bond
610. It is a two-component, solvent thinned, epoxy-phenolic
adhesive. The operational temperature range is from -269 ºC to
370 ºC for a short period and the long term temperature range is
-269 ºC to 260 ºC. The approximate composition is 60%
Tetrahydrofuran, 30% Bisphenol F epoxy resin, 10% Methyl
Ethyl Ketone.
Test Fixture
The test fixture is the part of the test section that provides
the support structure for the microchannel test section. The
fixture houses the electrical and the fluidic connections. The
material used for the test fixture is black delrin. Delrin is an
Figure 4: Pogo Probe Block with Probes Inserted.
Test
Device
Figure 5: Test Fixture Assembly.
Inlet
Pressure
Injection
Port
Outlet
Thermocouple
Figure 6: Device Retaining Plate.
A critical part in the test fixture assembly is the device
retaining plate shown in Figure 6. This part provides a
clamping force on the microchannel test section and the fluidic
connections.
The piece is made of an optically clear
polycarbonate material. The fluid enters the piece and moves
into an inlet plenum. The fluid then enters the test device and
leaves through the exit plenum. The device is sealed using two
double sealing O-rings.
Two miniature thermocouples are fabricated and placed in
the plenums. The thermocouple bead is 0.5 mm in order to
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Copyright © 2005 by ASME
minimize the effect on the flow. They are also coated with an
epoxy to seal them and prevent corrosion.
Pressure ports are located in each of the plenums to get the
pressure drop across the test device. The plenums themselves
are carefully designed to allow for sufficient volume to reduce
the fluid flow velocity. This ensures that the measured pressure
drop is indeed measuring the change in static pressure and has
no dynamic pressure component included with the
measurement. Experiments are conducted to ensure that true
static pressure is measured.
EXPERIMENTAL UNCERTIANTY
The experimental uncertainties can become quite large for
a microchannel heat exchanger because the magnitudes of some
of the measurements are very small.
In addition, the
propagation of errors in the system can become troublesome.
The physical size of the system being measured presents a
challenge. It is very difficult to fabricate a thermocouple small
enough to have any significant size resolution and attach it to
the chip at a desired location. A better approach is to use a
resistance element that has a known temperature dependence.
The magnitudes of the measurements represent a problem
as well. The heat transfer in microchannels is very efficient.
Therefore, the changes in temperature or the temperature
difference can be very small. The ∆T can be on the order of
only a few degrees. A typical experimental uncertainty value
for temperature is ± 0.5 °C on a two point calibration. An
experimental uncertainty of ± 0.04 °C can be achieved by using
the data collection procedure presented later.
Fortunately, several of the standards for experimental
uncertainties still apply.
The two best standards for
determining experimental uncertainties are ASME PTC 19.1
(1998) and NIST Technical Note 1297 (1994). There are many
similarities between these standards and others. The primary
standard discussed here will be the ASME standard. In general,
the total uncertainty is comprised of two parts; bias error and
precision error given by:
U =2
B
2
2
+
2
σ
(1)
N
where U is the total uncertainty, B is the bias error, σ is the
standard deviation, N is the number of samples. The bias error
is a measure of the systematic error and the precision error is a
measure of the random errors in the system.
There are several rules to follow when propagating errors
from measured variable to calculated values. In general, Eq.
(2) gives the uncertainty of a calculated parameter.
Up =
n
i =1
∂p
u σi
∂σ i
2
(2)
where p is the calculated parameter. The uncertainty in any
parameter is the sum of the uncertainties of the components
used to calculate that parameter.
A measurement technique is utilized to reduce the
experimental uncertainties.
In a data burst DAQ (data
acquisition) mode, several hundred samples of the same
quantity, like temperature, are measured at a high frequency.
The mean and standard deviation of the sample set is
determined and used to report the parameter being measured.
The systematic errors are ones that result from biases in the
experimental setup and can be minimized by using calibrations.
These errors are typically repeatable and can be eliminated.
The uncertainties for the major parameters are presented in
Table 3 as well as the parameter mean values.
Table 3. Uncertainties for Mean of Experimental Data.
Parameter
Mean
Uncertainty (%)
Q
G
Re
q”
T
θ
∆p
f
( mL/min )
( kg/m2s )
40
741
212
29
40
0.4
2.9
0.4
( W/cm2 )
( °C )
( °C cm2 / W )
( kPa )
0.5
5.1
6.1
3.3
0.2
6.0
3.0
6.5
Steinke and Kandlikar (2005a, 2005b) present a more
detailed discussion of the experimental uncertainties occurring
in microchannel pressure drop and heat transfer.
The
uncertainty for calculated parameters is derived in terms of the
measured parameters. This allows for the contribution from
each measured parameter to be determined.
The following equation is recommended for use in
microchannel fluid flow. The uncertainty in the fRe product is
shown in Eq. (3), Steinke and Kandlikar (2005a).
2⋅
U f Re
2
Uρ
+
ρ
UL
= +
f Re
L
Uµ
µ
2
U
+5⋅ b
b
+ 3⋅
2
UQ
Q
2
U ∆p
+
2
1/ 2
2
∆p
U
+ 5⋅ a
a
Ua
+ 2⋅
a+b
2
2
Ub
+ 2⋅
a+b
(3)
2
where Q is the volumetric flow rate, ρ is the density, µ is the
viscosity, ∆p is the pressure drop, a is the microchannel width,
b is the microchannel depth, and L is the microchannel length.
It can be seen that the most dominant terms in the fRe
uncertainty are the measurements of microchannel width and
height. Therefore, even with the most careful pressure drop
measurements, there is still going to be a large uncertainty due
to the measurement errors of the microchannel dimensions and
the flow rate.
SYSTEM CALIBRATION AND EXPERIMENTAL
PROCEDURE
The approach used for the system calibration and the
experimental procedure are described in this section. The three
main topics include the system calibration, the experimental
procedure, and the preparation of the working fluid.
Experimental Calibration
The experimental system requires calibration and some
specific data acquisition techniques to generate the most
accurate data possible for a given system configuration. The
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Copyright © 2005 by ASME
thermocouples, temperature sensors, pressure transducers, flow
meters, and the pump all require calibration and will be
discussed in detail.
The thermocouples are calibrated using a heated block and
an ice bath. The ice bath gives a reference temperature of 0 °C.
A calibrated hot block from Omega is used to give the other
known temperature points. The range of operation for the
thermocouple calibrator is 40 °C to 480 °C. The operating
range for the experimental system is from 0 °C to 200 °C. The
thermocouples are calibrated using twenty points chosen within
the operational range of the system. A linear curve fit is
assumed and applied to the collected points to determine the
calibration equation. Finally, the calibration process is repeated
using the equation that was determined. The temperature
measurements are verified in this manner and excellent
agreement is found using this method. All of the temperatures
are measured to within ± 0.03 °C.
The temperature sensors on the test device are calibrated
by applying a known temperature to the device. Recall that
these sensors are actually copper resistors and the resistance is
a function of temperature. The efficient heat transfer that
occurs in the microchannels will be used to our advantage.
Temperature controlled water is passed through the test device
at a moderate flow rate. A sufficient amount of time is allowed
to let the test device reach the fluid temperature, within ± 0.05
°C. Several external thermocouples are used to ensure that an
accurate temperature and steady state temperature is achieved.
Finally, the potential is measured for that applied temperature.
A linear curve fit is assumed and applied to the data. Once
again, the calibration is repeated with the calibration equation
to verify the accuracy of the fit. The test devices are calibrated
prior to experimentation and calibrated just after
experimentation to ensure good linearity.
The pressure transducers are calibrated using known
pressures and the measured response of the transducer. A
pressure calibrator from Omega is used to apply a known value
of pressure. The range of the pressure calibrator is -100 kPa to
200 kPa. The high side pressure port on the differential
pressure transducer is exposed to the known values of pressure.
Over twenty points are taken with in the range of the specific
pressure transducer. A linear curve fit is assumed and used to
generate the calibration equation.
The flow meters and pump are calibrated at the same time.
The pump is set for a specific flow rate. The fluid is collected
in a flask and timed. Then, the mass of the fluid is measured on
a calibrated balance for that period of time. Several points are
taken over the range of operation of the pump and for each of
the flow meter ranges. A polynomial curve fit is applied to get
the calibration equation for the pump and flow meters.
Experimental Procedure
A data acquisition (DAQ) system monitors several
thermocouples, pressure transducers, and sensing resistors. In
addition, the data acquisition system controls the pump used to
drive flow in the system. The DAQ is based upon the signal
conditioning SCXI system from National Instruments. This
system conditions all incoming signals and gives the ability to
have high channel counts.
The DAQ system used in the experimental system is
capable of sampling at a rate of 100,000 samples per second.
The present work is focused on steady state performance.
However, the fast sampling will be used to increase the
accuracy. The procedure for data collection begins with the
sampling rate. The channels are sampled at a rate of 1.0 kHz.
The data collected is in the form of a waveform. The channel
information contains both amplitude and frequency
information. A thousand samples for each channel are
collected. The data are analyzed to determine the mean and the
standard deviation, σ, of the data set. Then, the mean and twice
the standard deviation, 2σ, are recorded. The calibration for
each measurement is applied offline of the data collection.
A test device is loaded in the test fixture. First, the top
retaining plate is used to create the fluidic connections. Then,
the electrical connections are made via the pogo probes. The
flow loop is brought to the desired inlet temperature by using a
test section bypass. Next, the flow enters the test device and
the input power is applied. After all of the data has been
collected at the specified heat flux, the input power is changed
while the flow rate remains constant. Therefore, the data is
collected by varying the heat flux for a fixed mass flow rate.
The system reaches steady state before the data collection
begins. The measured values are considered to be at steady
state if the values do not fluctuate more than the uncertainty
values. Typically, the data collection begins 5 minutes after the
system reaches steady state. The data is a time averaged mean
of over 5,000 points.
Working Fluid Preparation
The working fluid for the present work is distilled, deionized and degassed water. The present work is focused on
generating fundamental single-phase flow data for a variety of
microchannel geometries. It is therefore important to eliminate
all of the interfering variables. The amount of dissolved gas in
the water will affect the heat transfer performance at very large
∆Ts. The non-condensables can outgas during the experiments
and cause heat transfer enhancement.
The amount of dissolved gas in the water needs to be
precisely controlled to eliminate the heat transfer changes
resulting from out-gassing of dissolved gases. Steinke and
Kandlikar (2004b) conducted an experimental investigation
concerning the control of dissolved gases. They demonstrated
that the effects due to the out-gassing of dissolved gases can be
eliminated if the water is treated to reduce the dissolved oxygen
content to 5.4 parts per million (ppm) at 25 °C.
To be
conservative, the dissolved oxygen level used in these
experiments is maintained at 3.2 ppm. The water is checked
randomly throughout the course of the experiments.
A brief outline of the degassing procedure is given. A
pressure vessel is filled with de-ionized, distilled water. The
vessel is heated to generate steam.
A deadweight
corresponding to 1 atm of pressure is applied. When the vessel
reaches the desired pressure, the deadweight is removed. A
vigorous boil results from the sudden change in pressure. As a
result, the dissolved gases are released from the water.
EXPERIMENTAL RESULTS
Some experimental results obtained from the Single-Phase
Experimental Facility will be presented to demonstrate the
parameters that can be investigated with the system. The
results shown are for microchannel with a width of 250 µm, a
7
Copyright © 2005 by ASME
depth of 200 µm, and a fin thickness of 200 µm. The heat
transfer performance will be characterized in terms of the heat
flux, temperature difference and unit thermal resistance of the
microchannel test section. The pressure drop performance will
be characterized in terms of the total pressure drop and
apparent friction factor. Table 4 presents the range of
parameters investigated for this microchannel geometry.
θ=
Table 4. Range of Experimental Data
Parameter
Min
2
( kW/m )
( °C )
( °C m2 / W )
( kPa {psi} )
68
1262
364
548.9
54.6
16900
4.8 {0.70}
0.8
°C
j
− Ti )
(4)
q"
200
G = 219
180
G = 342
G = 513
G = 726
140
G = 977
120
G = 1262
100
80
60
20
0
0.00
0.10
0.20
0.30
0.40
0.50
0.60
2
Heat Flux, q" MW/m
Figure 8: Unit Thermal Resistance vs. Heat Flux.
G01-012; a = 250 µm, b = 200 µm; G = 219, 342, 513,
2
726, 977, 1262 kg/m s; Re = 61, 91, 132, 187, 250, 324.
0.6
G = 219
G = 342
25
∆ Tavg,(Tj-Tin)
(T
40
30
20
15
q"
=
160
Heat Flux, q" MW/m
The following results do not reflect the maximum
performance for a silicon microchannel. The smaller pitches
and microchannel widths will provide a much higher heat
transfer performance. There are many papers that present
better performance.
For example, Colgan et al. (2005)
investigated the performance of silicon microchannels with
some enhancement features and dissipated more than 2,750
kW/m2. The test section used has the coarsest pitch and the
widest microchannels. The intention is to demonstrate some of
the parameters that can be determined from the experimental
facility and the associated uncertainty.
Figure 7 shows the difference between the average
temperature of the resistive sensor on the back side of the test
section and the inlet water temperature versus heat flux. The
data are represented by points and the uncertainty is shown as
error bars. As expected, the temperature difference increases
linearly with heat flux and decreases with increasing mass flux.
∆T j
2
12
219
61
34.9
24.8
4100
0.7 {0.10}
0.11
θ ° C m / MW
( mL/min )
( kg/m2s )
Max
2
Q
G
Re
q”
T
θ
∆p
f
The slope of the line in Fig. 7 is the unit thermal resistance
of the test section. Figure 8 shows the unit thermal resistance
for the different mass fluxes. The unit thermal resistance is a
common parameter in the electronics cooling industry. It is
meant to give a direct comparison of performance for different
cooling techniques and a lower value is desired. The unit
thermal resistance can be calculated using Eq. (4).
G = 219
G = 513
0.5
G = 726
G = 977
0.4
G = 1262
0.3
0.2
0.1
G = 342
G = 513
10
G = 726
0.0
G = 977
5
0
G = 1262
0
0.0
0.2
0.4
0.6
0.8
2
Heat Flux, q" MW/m
Figure 7: Temperature Difference vs. Heat Flux. G01012; a = 250 µm, b = 200 µm; G = 219, 342, 513, 726,
2
977, 1262 kg/m s; Re = 61, 91, 132, 187, 250, 324.
100
200
300
Reynolds Number, Re
400
Figure 9: Heat Flux vs. Reynolds Number. G01-012; a
= 250 µm, b = 200 µm; G = 219, 342, 513, 726, 977,
2
1262 kg/m s; Re = 61, 91, 132, 187, 250, 324.
For a mass flux of 1262 kg/m2s and a ∆Tavg of 23.5 °C, the
unit thermal resistance is 0.40 °C cm2/W and the thermal
resistance would be 0.15 °C/W. A typical air-cooled heat sink
8
Copyright © 2005 by ASME
where fapp is the apparent friction factor, Re is the Reynolds
number, µ is the viscosity, V is the mean velocity, L is the
microchannel length, and Dh is the hydraulic diameter. The
theoretical value for fRe can be used to normalize the friction
factor data to give the non-dimensional fRe ratio, C*. The nondimensional fRe ratio versus Reynolds number is shown in
Figure 11. This is a more appropriate method to compare
theoretical prediction of the apparent friction factor, Steinke
and Kandlikar (2005a). A C* value of 1.0 would be a perfect
match between experimental data and theoretical value.
Capp* = 1.0
1.20
G = 219
1.15
G = 342
G = 513
1.10
G = 726
G = 977
1.05
Capp*
thermal resistance alone would be approximately 1 °C/W. This
does not take into account the thermal resistances of the silicon,
thermal interface material, contact resistances, and the heat
spreader. Therefore, this technique is already an improvement
over air cooled heat sinks. A typical two-phase system would
have a thermal resistance would be approximately 0.10 °C/W.
Once again, the present work does not attempt to get maximum
performance from the microchannels. However, the thermal
resistances of the single-phase and the two-phase system can be
on the same order of magnitude.
The heat flux versus Reynolds number is shown in Figure
9. The junction temperatures were limited to 55 °C to
minimize corrosion of the copper resistors. The heat flux was
adjusted for each mass flux to obtain the data as long as all of
the junction temperatures remained less than 50 °C. As a
result, the number of data points collected for each mass flux is
not the same. As expected, the higher mass fluxes can support
higher heat fluxes. The largest heat flux dissipated is 54.9
W/cm2. An increase in the maximum heat flux is demonstrated
with an increase in Reynolds number.
The pressure drop in the test section is measured in the
inlet and outlet plenums. The entrance and exit losses are
determined and subtracted from the measurement to determine
only the pressure drop occurring in the microchannel. Figure
10 shows the resulting pressure drop in the microchannels. As
expected, the pressure drop increases with the Reynolds
number. The small variation seen in the Reynolds number is
due to the water properties being calculated using the mean
water temperature. The mean temperature will increase as the
outlet temperature rises with an increase in heat flux.
G = 1262
1.00
0.95
0.90
0.85
0.80
0
100
200
300
Reynolds Number, Re
400
*
Figure 11: Capp vs. Reynolds Number. G01-012; a =
250 µm, b = 200 µm; G = 219, 342, 513, 726, 977, 1262
2
kg/m s; Re = 61, 91, 132, 187, 250, 324.
There seems to be good general agreement with the friction
factor data. The Capp* ratio takes into account the developing
flows in the microchannels. The experimental uncertainties are
shown as error bars in the figure. There are a few cases that
still have a 5% difference that the theoretical value and is an
encouraging result. A more detailed discussion on the friction
factors and the procedure for correcting for the entrance and
exit losses and the developing flow region is presented by
Steinke and Kandlikar (2005a).
Figure 10: Pressure Drop vs. Reynolds Number. G01012; a = 250 µm, b = 200 µm; G = 219, 342, 513, 726,
2
977, 1262 kg/m s; Re = 61, 91, 132, 187, 250, 324.
The apparent friction factor can now be calculated from the
pressure drop. The inlet and exit losses are removed from the
measured pressure drop. The conventional expansion and
constriction area correlations were used. The apparent friction
factor is calculated using Eq. (5), Kakaç et al. (1987).
∆p =
2( f app Re )µV L
Dh
2
(5)
CONCLUDING REMARKS
A single-phase experimental test facility has been designed
and fabricated. The system is capable of delivering a working
fluid to a test section with a variety of inlet temperatures and
flow rates. A wide range of mass fluxes and heat fluxes can be
applied. The equipment has been carefully selected to give the
maximum amount of accuracy in the flow ranges.
The experimental facility also has the ability to perform
flow visualization using a traditional optical microscope and a
micro-particle image velocity system.
The system can deliver fluid from a flow range of 1
mL/min to 4,000 mL/min. It can provide an input power to the
test section of 1.5 kW.
The experimental uncertainties have been carefully
controlled to produce experimental data with very low
uncertainty. Steinke and Kandlikar (2005a, 2005b) present a
9
Copyright © 2005 by ASME
more detailed discussion of the experimental uncertainties
occurring in microchannel pressure drop and heat transfer.
The experimental results shown represent some of the
parameters that can be determined using the single-phase test
facility. The facility can now be used to determine the
performance of single-phase microchannels.
The unit thermal resistance shown in Fig. 7 seems to be
dependent upon the Reynolds number for low mass fluxes and
independent of the heat flux at the higher Reynolds numbers.
The diabatic friction factors are in agreement with conventional
laminar Poiseuille flow theory after accounting for the entrance
and exit losses and developing region effects.
NOMENCLATURE
a channel width, m
b channel height, m
B systematic error
Dh hydraulic diameter, m
f
Fanning friction factor
G mass flux, kg m-2 s-1
L
n
N
p
P
q
q”
Q
R
Re
s
T
channel length, m
number of channels
number of measurements in sample
pressure, Pa
power, W
heat transfer rate, W
heat flux, W m-2
volumetric flow rate, L s-1
thermal resistance, °C W-1
Reynolds number
fin thickness, m
temperature, °C
V mean velocity, m s-1
ui
U
uncertainty of parameter i
uncertainty
Greek
∆ difference
η fin efficiency
µ viscosity, N s m-2
ρ density, kg m-3
σ standard deviation
θ unit thermal resistance °C m2 W-1
Subscripts
app apparent
avg average
f fluid
h heater
i
inlet
o outlet
s surface
AKNOWLEDGEMENTS
The fabrication of the test devices was performed at IBM
T.J. Watson Research Center. The device bonding and
experimental investigation was performed at Rochester Institute
of Technology in the Thermal Analysis and Microfluidics
Laboratory. In addition, the support from the IT Collaboratory
for device bonding is appreciated.
REFERENCES
Bailey, D.K., Ameel, T.A., Warrington, R.O. and Savoie,
T.I. "Single phase forced convection heat transfer in
microgeometries-a review." Proceedings of the Intersociety
Energy Conversion Engineering Conference, v 2,
Environmental Impact (1995) 301-310.
Colgan, E.G., Furman, B., Gaynes, M., Graham, W.,
LaBianca, N., Magerlein, J.H., Polastre, R.J., Rothwell, M.B.,
Bezama, R.J., Toy, H., Wakil, J., Zitz, J. and Schmidt, R. “A
practical implementation of silicon microchannel coolers for
high power chips.” Semiconductor Thermal Measurement,
Modeling, and Management Symposium (2005).
Gui, F. and Scaringe, R.P. "Enhanced heat transfer in the
entrance region of microchannels." Proceedings of the
Intersociety Energy Conversion Engineering Conference
(1995) 289-294.
Kakaç, S., Shah, R.K. and Aung, W. Handbook of SinglePhase Convective Heat Transfer John Wiley & Sons: New
York (1987).
Kandlikar, S.G. and Grande, W.J. "Evolution of
Microchannel Flow Passages - Thermohydraulic Performance
and Fabrication Technology." Heat Transfer Engineering 24,
no. 1 (2003) 3-17.
Kandlikar, S.G. and Grande, W.J. " Evaluation of Single
Phase Flow in Microchannels for High Heat Flux Chip
Cooling—Thermohydraulic Performance Enhancement and
Fabrication Technology." Heat Transfer Engineering 24, no. 1
(2004) 5-16.
Lee, P.S., Ho, J.C., and Xue, H. "Experimental study on
laminar heat transfer in microchannel heat sink," The Eighth
Intersociety Conference on Thermal and Thermomechanical
Phenomena in Electronic Systems, 30 May-1 June 2002,
ITHERM 2002. IEEE Publications (2002) 379 -386.
Missaggia, L.J., Walpole, J.N., Liau, Z.L. and Phillips, R.J.
"Microchannel heat sinks for two-dimensional high-powerdensity diode laser arrays." IEEE Journal of Quantum
Electronics 25, no. 9 (1989) 1988-1992.
Morini, G.L. "Single-phase convective heat transfer in
microchannels: A review of experimental results."
International Journal of Thermal Sciences 43, no. 7 (2004)
631-651.
Palm, B. "Heat transfer in microchannels." Microscale
Thermophysical Engineering 5, no. 3 (2001) 155-175.
Peng, X.F., Wang, B.X., Peterson, G.P., and Ma, H.B.
"Experimental investigation of heat transfer in flat plates with
rectangular microchannels," International Journal of Heat and
Mass Transfer 38, No. 1 (1995) 127-137.
Qu, W. and Mudawar, I. "Experimental and numerical
study of pressure drop and heat transfer in a single-phase
micro-channel heat sink." International Journal of Heat and
Mass Transfer 45, no. 12 (2002) 2549-2565.
Ravigururajan, T.S. and Drost, M.K. "Single-phase flow
thermal performance characteristics of a parallel microchannel
heat exchanger." Journal of Enhanced Heat Transfer 6, no. 5
(1999) 383-393.
10
Copyright © 2005 by ASME
Riddle, R.A., Contolini, R.J. and Bernhardt, A.F. "Design
calculations for the microchannel heatsink." Proceedings of the
Technical Program - National Electronic Packaging and
Production Conference 1 (1991) 161-171.
Sobhan, C.B. and Garimella, S.V. "A comparative analysis
of studies on heat transfer and fluid flow in microchannels."
Microscale Thermophysical Engineering 5, no. 4 (2001) 293311.
Steinke, M.E. and Kandlikar, S.G. "Review of SinglePhase Heat Transfer Enhancement Techniques for Application
in Microchannels, Minichannels and Microdevices."
International Journal of Heat and Technology 22, no. 2 (2004a)
3-11.
Steinke, M.E. and Kandlikar, S.G. "Control and Effect of
Dissolved Air in Water During Flow Boiling in
Microchannels." International Journal of Heat and Mass
Transfer 47 (2004b) 1925–1935.
Steinke, M.E. and Kandlikar, S.G. “Single-Phase, Liquid
Friction Factors in Microchannels.” International Conference
on Microchannels and Minichannels, Paper # ICMM05-75112
ASME Publications (2005a).
Steinke, M.E. and Kandlikar, S.G. “Single-Phase Liquid
Heat Transfer in Microchannels.” International Conference on
Microchannels and Minichannels, Paper # ICMM05-75114
ASME Publications (2005b).
Tuckerman, D.B., and Pease, R.F.W. “High-Performance
Heat Sinking for VSLI,” IEEE Electron Device Letters EDL-2
(1981) 126-129.
Vidmar, R.J., and Barker, R.J. "Microchannel Cooling for
a High-Energy Particle Transmission Window, an RF
Transmission Window, and VLSI Heat Dissipation," IEEE
Transactions on Plasma Science 26, no. 3 (1998) 1031-1043.
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Copyright © 2005 by ASME