Development of an Instrumented Glass Microchannel Device
For Critical Heat Flux Visualization and Studies
Abhishek Jain1, Theodorian Borca-Tasciuc1,
Anand P. Roday1, Michael K. Jensen1, Satish G. Kandlikar2,
1
Department of Mechanical, Aerospace and Nuclear Engineering,
Rensselaer Polytechnic Institute, Troy, NY
2
Department of Mechanical Engineering,
Rochester Institute of Technology, Rochester, NY
Email: [email protected]
Abstract
Boiling in microchannels is an important candidate for
cooling of electronic chips. A crucially important factor in the
design of microchannel boiling heat transfer is the critical heat
flux (CHF). This work presents the development of a glass
microchannel device for quantitative investigations of CHF
and visualization of boiling phenomena at microscale. The
device is instrumented with heaters and temperature sensors to
map the temperature distribution in the axial direction. The
use of low thermal conductivity glass reduces the heat
conduction losses, and improves the accuracy of CHF values
extracted from experimental results. This work presents the
fabrication and packaging of a single microchannel device.
Keywords
Micro-fabrication technique, temperature sensors
1. Introduction
Small, fast and densely populated computer chips have
resulted in very high heat dissipation rates that must be
accommodated with advanced cooling techniques. The
aerospace, air separation/cryogenics, and automotive
industries have investigated ultra-compact heat exchangers to
save weight and volume and to increase heat transfer rates.
The common feature of these and other thermal control
(cooling) and thermal processing (heat exchangers)
applications is the use of boiling heat transfer in single and
multiple parallel channels that have diameters in the range of
50 to 1000 µm. The CHF puts an upper thermal limit on the
phase-change heat transfer. Some of the earliest studies
related to the measurements of CHF were conducted by
Bergles, and Bergles and Rohsenow [1], [4]. Recent surveys
of the field are presented in Palm [5], Mehendale et al. [6],
and Kandlikar [7]-[8]. The CHF condition varies with the type
of the channel [9]-[12], type of fluid [13]-[16], and operating
conditions (e.g., flow rate, pressure level, inlet temperature).
Boiling phenomenon in micro tubes and microchannels with
hydraulic diameters ranging from 25 µm to 1 mm has been
studied in [17]-[21]. Microchannels fabricated in silicon [22][24] and stainless steel tubes [25]-[29] have been employed to
study single phase and CHF conditions; both single and
multiple parallel channels have been used. However, heat
losses have not been taken into account accurately so the
values of CHF from the experimental results may not be
accurate. In addition, with multiple channels, parallel channel
two phase flow instabilities have an effect on the results, but
the magnitude of the effect is unknown.
0-7803-8985-9/05/$20.00 ©2005 IEEE
Therefore, this paper describes an instrumented
microchannel device in glass for CHF studies and
visualization purposes. The use of low thermal conductivity
glass reduces heat conduction losses, improves the accuracy
of CHF values from experimental results, and helps in
visualization of the boiling phenomena. Moreover, the results
of CHF for the present described microfabricated single
microchannel device is unaffected by parallel channel effects.
Microfabrication and micropackaging technology of the
device are discussed.
Hence the objectives of the present work are to fabricate
and to demonstrate proof-of-concept of the microchannel
device in glass and to perform preliminary testing of the flow
loop. The work also addresses the packaging issues involved.
The packaged microchannel MEMS device will be tested for a
range of operating conditions.
Figure 1: Schematic of the cross-section of the device
Figure 2: Top view of the first glass substrate
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to be made with the electrode pads on the bottom glass
substrate. The heater in the bottom substrate is in direct
contact with the microchannel. This configuration maximizes
the boiling heat transfer and together with the low thermal
conductivity substrate ensures that the most of the power
dissipated by electrical heating goes for boiling purpose. Also
the configuration is unaffected by the parallel channel effect
and heat losses by conduction through the substrate.
Figure 3: Top view of the second glass substrate
2. Instrumented Glass Microchannel
A cross-section of the microchannel test section is shown
in Fig. 1, and the top views of the two glass substrates are
shown in Figs. 2 and 3, respectively.
The device consists of two layers: 1) the bottom layer
contains the fluid entry/exit ports, the heater, and the
temperature sensors; and 2) the top layer contains the
microchannel. The heaters and temperature sensors are made
of platinum film. The temperature sensor array monitors the
heater temperature along 2 mm sections. Temperature of each
section of the heater is inferred from the change in resistance.
Note that the heater and the temperature sensors are separated
by a small gap from the platinum layer used as an electrode
for anodic bonding. The two glass substrates are anodically
bonded; first they are heated to 4000C for 2 hours, and then a
voltage of 450 V is applied for 1 hour.
3. Micro-fabrication Technique
The micro-fabrication steps used for the preparation of the
microchannel device are shown schematically in Fig. 5. For
the corresponding heater width and glass thickness the
technique allows the fabrication of microchannels of
hydraulic diameters ranging from 50 µm to 300 µm. Steps (a)
to (k) show the procedure for fabricating the fluid ports and
the electrode in the bottom glass substrate. The channel in the
top substrate is etched in a similar manner following the
procedure through step (g) and using the appropriate mask.
Step (a) shows the patterning of the photoresist (PR) layer
defining the fluid inlet/outlet ports in the bottom layer. After
developing the PR as shown in step (b) a layer of chromium
(200 Å) and gold (3000 Å) is deposited (step (c)) as a mask to
protect the desired regions on the glass substrate during HF
etching. Thereafter the mask is removed in Au and Cr
etchants (steps (f) and (g)). A second photolithography step
(h-j) is employed to pattern the photoresist defining the
heater/temperature sensor array. The platinum layer of
thickness 1000 Å is deposited using e-beam (k) and patterned
by lift-off (l). The channel in the top glass is fabricated using
the same glass wet-etching technology, except the etching is
timed to obtain the required channel depth. The two substrates
are bonded together using anodic bonding between the
platinum bonding electrode and the top glass layer.
(a) Photoresist Exposure
(b) Developing Photoresist
Figure 4: Schematic of the microchannel device after
bonding
Figure 4 shows the top view of the device after the two
layers are bonded. The ports for the fluid entry and exit in the
top layer interfaces with that in the bottom layer. The top
layer is smaller in width which allows electrical connections
Jain, et al, Development of an Instrumental Glass …
(c) Deposition of Au and Cr
(Figure 5: Continued on next page)
(d) Lift-off
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(e) Etching with Hydrofluoric acid
(f) Au etchant
(g) Cr etchant
(h) Spin coat Photoresist
the corresponding heater width and glass thickness the
technique allows the fabrication of microchannels of
hydraulic diameters ranging from 50 µm to 300 µm. Steps (a)
to (k) show the procedure for fabricating the fluid ports and
the electrode in the bottom glass substrate. The channel in the
top substrate is etched in a similar manner following the
procedure through step (g) and using the appropriate mask.
Step (a) shows the patterning of the photoresist (PR) layer
defining the fluid inlet/outlet ports in the bottom layer. After
developing the PR as shown in step (b) a layer of chromium
(200 Å) and gold (3000 Å) is deposited (step (c)) as a mask to
protect the desired regions on the glass substrate during HF
etching. Thereafter the mask is removed in Au and Cr
etchants (steps (f) and (g)). A second photolithography step
(h-j) is employed to pattern the photoresist defining the
heater/temperature sensor array. The platinum layer of
thickness 1000 Å is deposited using e-beam (k) and patterned
by lift-off (l). The channel in the top glass is fabricated using
the same glass wet-etching technology, except the etching is
timed to obtain the required channel depth. The two substrates
are bonded together using anodic bonding between the
platinum bonding electrode and the top glass layer.
(i) Patterning electrode
(j) Developing Photoresist
Figure 6: Microchannel device
Figure 6 shows a photograph of a microfabricated
microchannel device. The microchannel is 35 µm deep.
(k) Deposition of platinum
(l) Microchannel device (bottom substrate)
Figure 5: Microfabrication steps for the bottom glass
substrate
4. Micro-fabrication Technique
The micro-fabrication steps used for the preparation of the
microchannel device are shown schematically in Fig. 5. For
Jain, et al, Development of an Instrumental Glass …
Figure 7: Details of the device holder block
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Figure 8: Picture of device-holder and the packaged
device mounted in the flow loop
5. Device Packaging
Before testing, the microfabricated microchannel device
must be packaged to allow for fluid delivery, pressure drop
measurements, and electrical connections for power and
temperature sensors. Figure 7 shows the schematic of the
device package. Ports have been made for the fluid entry and
exit. Pressure tappings, which connect with the entry/exit
ports, are used for measuring the inlet/exit pressures.
The major challenge encountered was fluidic packaging.
The interfaces between the fluidic ports on the device side and
package side were sealed using a thin layer of paraffin sheet.
Plastic covers are used to clamp the device near the inlet and
outlet ports. Figure 8 shows a photograph of the packaged
device mounted in a flow loop (without electrical
connections).
Degassed water is pumped using a micropump (Barnant
Com, 900-1012) through the test section after preheating.
Three flowmeters (Omega Engineering, Part No. 22376,
15514, 02616) are used to get the desired flow rate. Electrical
heating is done in the preheater section. The pressure at the
inlet and exit ports and the temperatures of different heater
sections are monitored using the differential pressure
transducer (Validyne Model CD 15 Carrier Demodulator) and
Data Acquisition and LabVIEW Software, respectively. The
water is sent to the heat exchanger where it is condensed and
then recirculated back to the loop.
For testing, the packaged device is inserted in the test
section region of the loop. Preliminary testing of the
microchannel test section in the flow loop was done for flow
rates ranging from 0.025 mL/min to 3.0 mL/min and for
pressure drops varying from 1.0 kPa to 40.0 kPa. Pressure
levels from 0.5 to 5 atm are possible. The packaged device
operated without any leaks which demonstrated that the
packaging scheme worked successfully.
7. Future research plan
An extensive program is planned to study boiling
characteristics, two-phase flow, and the CHF condition using
this newly developed test section. A wide range of pressures,
mass fluxes, and qualities will be covered. To determine the
effect of fluid properties, pressures from 0.5 to 5 atm will be
tested; sub atmospheric pressures are advantageous from the
point of view of reductions in the operating temperature. In
addition, two different fluids – water and R123 – will b used
because of their significantly different thermophysical and
thermodynamic properties. Data to be obtained are heat
fluxes, wall temperatures, mass flux, pressure drop, and
quality. Complementary modeling of the CHF condition will
be validated against the data obtained. Eventually, multiple
parallel channel tests will be microfabricated using the same
approach.
8. Conclusions
A first generation glass microchannel device was
fabricated using MEMS technology.
The device was
packaged in a specially designed holder for electrical and
fluidic access and tested for varying ranges of pressures and
flow rates. The device is currently tested in a closed loop with
degassed water for CHF measurements.
Figure 9: Schematic of the flow loop used for critical heat
flux characterization
6. Flow Loop
A schematic of the closed flow loop employed in the
critical heat flux measurements is shown in Fig. 9. Presence of
dissolved gas affects the onset of nucleate boiling, as
dissolved gas bubbles nucleate before the wall temperature
reaches the fluid saturation temperature. This, in turn, affects
the critical heat flux condition. Hence, for accurate and
reproducible data, the water must be degassed. The water is
boiled in the water tank, and degassing is done through
vacuum pump attached to the condenser. Degassing was
achieved to 2.5 ppm.
Jain, et al, Development of an Instrumental Glass …
Acknowledgments
The authors would like to thank the National Science
Foundation (under CTS grant 0245642) for providing
financial support for this research.
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