final technical report
TRANSCRIPT
University of Houston Mechanical Engineering Department
N207 Engineering Building 1
Houston, TX 77204
May 3, 2015
Dr. Richard BannerotUniversity of Houston4800 Calhoun RdHouston, TX 77004
Dear Dr. Bannerot:
I am submitting the attached final technical report entitled Microchannel Heat Exchanger Flow Loop.
This report contains the results of the construction and validation of a microchannel heat exchanger flow loop suitable for use in the thermal fluids lab at the University of Houston. Computing power is increasing at an exponential rate and modern cooling methods are becoming increasingly insufficient as the number of transistors per unit area substantially increases the heat produced by the system. In order to mitigate this problem, water is becoming a necessary replacement for air as a convection agent due to its higher convection coefficient and therefore increased ability to remove heat. The flow loop described within this report will serve as an experimental gateway for students in the thermal fluids lab to learn applications of their fluids and heat transfer coursework related to a new, lucrative technology. The flow loop was constructed and manuals for two labs, as well as a manual for general operation were completed so that students can conduct the enclosed experiments and analyze the system with the amount of heat removed from the heat exchanger.
Hopefully, this report will educate students and generate future studies into effective methods of heat dissipation so that the next era of technological advancement may come to fruition.
Sincerely,
Jeremy Evans, Olivia Pacheco, Michael Russo, John-Roland EspinosaTeam 16 MECE 4341
Attached: Microchannel Heat Exchanger Flow Loop
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Microchannel Heat Exchanger Flow Loop
FINAL TECHNICAL REPORT
FOR
Department of Mechanical Engineering
Project Advisor: Dr. Dong Liu
Team Number 16
Team Members:John-Roland Espinosa
Jeremy EvansOlivia PachecoMichael Russo
MECE 4341: ME Capstone IIDepartment of Mechanical Engineering
University of Houston
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Abstract
This document is a technical report for the design and construction of a
microchannel heat exchanger flow loop for the cooling of electronic components. It was
prepared by Team 16 for the senior capstone project for MECE 4341. The design,
construction and testing of the finished project was completed on time and the results
have been validated and approved by Dr. Dong Liu. Based on testing of the flow loop,
the microchannel heat exchanger is an effective tool for removing heat from sources that
produce high heat fluxes such as those from modern integrated circuits. Experiments
were run to compare the measured Nusselt number and friction factor data to their
theoretical values and it was shown that the measured values were close to those
predicted. After construction and testing of the flow loop, Dr. Liu reviewed the flow loop
and experimental data and gave his approval of the results, thus validating the project.
Microchannel heat sinks are proving to be a viable option for the cooling of modern
electronic components. The project will be used by future mechanical engineering
students in MECE 4371 to learn micro scale applications of their fluids and thermal
knowledge from MECE undergraduate coursework. This project will demonstrate an
emerging field in research in thermal management of modern integrated circuits while
reiterating fundamental concepts of heat transfer and fluid mechanics.
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ContentsIntroduction.................................................................................................................................................................... 4
Statement of Work....................................................................................................................................................... 6
Accomplishments.........................................................................................................................................................6
Methodology.................................................................................................................................................................. 7
Process.............................................................................................................................................................................. 9
Constraints....................................................................................................................................................................13
Standards.......................................................................................................................................................................15
Results........................................................................................................................................................................... 16
Conclusion....................................................................................................................................................................22
Recommendations..................................................................................................................................................... 23
Appendix A – Friction Factor Experiment........................................................................................................24
Appendix B – Nusselt Number Experiment.....................................................................................................30
Appendix C – Flow Loop Operational Manual...............................................................................................36
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IntroductionThis document is a technical report for the design and construction of a
microchannel heat exchanger flow loop for the cooling of electronic components. It has
been prepared by Team 16 for the senior capstone project for MECE 4341.The sponsor
for this project is the mechanical engineering department and the advisor is Dr. Dong Liu.
The results of this project will be used by future mechanical engineering students at the
University of Houston for the MECE 4371 lab. This project addresses the problems
associated with the large amounts of heat generated by integrated circuits (ICs) in modern
computers. A microchannel heat exchanger was used to remove heat from ICs and keep
the temperature of the electronics at an acceptable level. A coolant flow loop was used to
circulate coolant through the microchannels of the heat exchanger and cause convection
to occur within the channels to remove heat. Parameters such as flow rate and heat flux
were determined to simulate the heat flux produced by a modern IC. Parts were designed
and fabricated to complete the coolant flow loop and to cover the micro-channels,
ensuring that there is no fluid communication between the channels. Planning for this
project began in August 2014 and construction and testing of the flow loop was
completed in April 2015.
According to Moore's law, the number of transistors on a two square centimeter
IC doubles approximately every two years as shown in Figure 1. As transistors in ICs get
smaller, the heat produced per unit area increases on the silicon chip in which the IC is
etched. When the temperature of the circuit components increases past a certain limit the
performance of the IC suffers. Proper thermal management is needed to keep the
components of ICs at an acceptable temperature to maintain optimal processor and
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computer performance. Thermal flux, q’’, is related to the surface temperature, Ts, and
the external temperature, T∞, as shown in Equation 1. Microchannel heat exchangers
allow engineers to obtain large values of the heat transfer coefficient, h, without losing
surface area due to the large number of microchannels that are etched into the surface of
the heat exchanger. Micro-channel heat exchangers can be fabricated using micro-
electrical-mechanical-systems (MEMS) machining techniques and are a viable option for
thermal management in future ICs.
Figure 1: Moore's law demonstrating transistor density as a function of time.
http://home.fnal.gov/~carrigan/pillars/web_Moores_law.htm
q ' '=h(T s−T ∞) Equation 1
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Statement of WorkTeam 16 designed, fabricated and tested a coolant flow loop containing a micro-
channel heat exchanger for use in MECE 4371 at UH as specified by Dr. Liu. The flow
loop and recorded data were then reviewed by Dr. Liu for validation of the project.
AccomplishmentsTeam 16 successfully completed all milestones set at the beginning of the
academic year. The first important accomplishment was component fabrication and
procurement which was completed over the course of a couple of months. The first major
shipment arrived in early March and included the stainless steel cart, reservoir,
thermocouples, pressure transducers, piping, G10 material and heat cartridges. All
necessary components that could not be found at a local hardware store were procured
before April. Sealants, braces, bolts, nuts and washers were purchased as needed. Not all
components were initially known and thus Team 16 required multiple trips to local
hardware stores.
After receiving all of the aforementioned components, Team 16 completed the
flow loop on April 18th. The flow loop and the data that it produced were inspected and
approved by the advisor, Dr. Liu, on April 28th. The advisor made several
recommendations for future work on the project that will be addressed by Team 16.
The final written deliverables were completed on time and accepted by the
advisor, including two lab manuals and one operational safety manual. The first lab
verifies that the friction factor varies linearly with the Reynolds number for developing
laminar flow. The second lab shows that Nusselt number varies linearly with the
Reynolds number under the same conditions. The operational manual provides
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information about the flow loop lay out and components, as well as operational
instructions required to run the flow loop safely while minimizing experimental error
without risking damage to the components.
MethodologyAfter determining the requirements and nature of the project, Team 16 began with
thorough research on the topic of microchannel heat exchangers by consulting the
internet, prior MECE coursework and Dr. Dong Liu’s thesis. A review of internal flow
convection problems in heat exchangers was completed. This included the Nusselt
Number and friction factor relations compared to the Reynolds number. Dr. Liu’s
doctorate thesis was closely analyzed in order to further inform the direction of the
project.
At the completion of the initial project research, the main components to complete
the flow loop were determined to be as follows: a stainless steel reservoir, a variable
drive gear pump, a 7 micron filter, a rotameter, the microchannel heat exchanger and
assembly as provided by Dr. Liu, a liquid to liquid heat exchanger, thermocouples,
pressure transducers, a data acquisition system and ¼ inch stainless steel piping with
ferrule tube fittings. The necessary specifications were calculated for the pump and heat
cartridges to remove an approximate 100 watts per square centimeter from the
microchannel heat exchanger to simulate a modern IC. The components were tested as
they were received to ensure proper function. With all components received and in
working order, the flow loop was then constructed in a methodical and careful manner.
The layout was determined such that the bending in the piping and the height variation
were minimized while all flow loop components were contained on the top tier of a
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stainless steel cart and all the electronics and electrical components were contained on the
bottom tier. Following precise measurements, the steel piping was bent and cut, the G10
insulation base was cut to size and holes were drilled into it for the microchannel heat
exchanger assembly bolts to pass through. Fixtures for the pump, rotameter,
microchannel heat exchanger assembly and the liquid to liquid heat exchanger were built
and installed. DataLogger software was installed and configured on a local computer
which was compatible with the data acquisition system containing a multiplexer with
pressure transducers and thermocouples routed into it.
Once the flow loop construction was completed, it was tested by plotting both the
Nusselt number and friction factor versus Reynolds number for the system against the
theoretical predictions. The discrepancies were accounted for and deviations were
deemed acceptable for the project. The results were approved by Dr. Liu to complete the
project.
ProcessThe analysis of the components and the determination of their specifications
required many careful calculations. All calculations were completed in order for the
components to successfully work together to produce a flow loop capable of removing
the heat and providing accurate and useful data. The microchannel heat exchanger copper
block was provided by Dr. Liu at the project’s start without any machine drawings due to
the extensive amount of time that had passed since its manufacture. Team 16 used the
MECE department’s shadow graph in order to measure the channel cross section and
determine the number of microchannels. Furthermore, a caliper was used to determine the
length of the channels, as well as the diameter and length of the heat cartridge holes.
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These measurements provided the critical dimensions for the channels and the geometric
constraints for the heat cartridges as shown in Table 1 below.
Table 1: Critical dimensions for the microchannel heat exchanger
Critical Dimensions
Hydraulic Diameter: Dh 214µm
Length: L 2.54cm
Channel Width: W c 192µm
Channel Height: H c 242µm
Number of Channels: N 50
The next task for Team 16 was to determine the specifications of the heat
cartridges. In order to provide the amount of heat flux needed to simulate the heat from a
CPU, 100 W/cm^2, eight heat cartridges were ordered, each capable of supplying 50 W
of power as shown in the calculations below.
Cross sectional area of top part of mic rochannelheat exchanger=3.884 cm2
Total power ¿be supplied=( 100Wc m2 ) (3.884 c m2 )=388.4 W
Power ¿be supplied byeach of 8cartridges=388.48
≈ 50 W
It was determined that the flow rate required to avoid boiling with a heat flux of
100 W/cm^2 was approximately 170 mL/min, thus creating a pressure drop across the
microchannels of approximately 0.8 psi as shown in the calculations below.
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m=qs ' ' PL
C p (Tm, o−T m , i)=169.5 mL /min
Where m is the mass flow rate in mL/min and qs' ' is the heat flux at 100 W
cm2 , P is
perimeter, L is channel length, C pis the specific heat and T m,o and T m,i are the mean
outlet and inlet temperatures respectively.
∆ P=fρum
2 Dh=0.8 psi
Where ∆ P is pressure drop in psi, f is the friction factor from the Moody diagram,
ρ is the density, um is the mean velocity calculated from the flow rate, and Dh is the
hydraulic diameter.
These flow rate and pressure drop requirements were used to determine which
pump and pressure transducers were needed. It was decided that the operating
temperature would vary from room temperature to 100⁰ C at the absolute maximum in
order to maintain single phase flow. Therefore, type T thermocouples were chosen due to
their relatively low temperature range and higher resolution. In order to ensure no leakage
occurred between components and piping, Team 16 followed recommendations from Dr.
Liu to go with ferrule type tube fittings to allow for easy connection/disconnection while
sealing properly. The data acquisition requirements were determined so that
thermocouple and pressure transducer voltage data could be converted to temperature and
pressure values and recorded as the flow rates varied. To meet those requirements while
remaining cost effective, an HP Agilent 34970A data acquisition and a corresponding
multiplexer module were purchased.
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In order for Team 16 to determine which correlations were needed for the
experimental and theoretical data, the fluid and thermal entrance lengths first needed to
be calculated to determine if the flow was fully developed or developing. The fluid entry
length varied depending on the Reynolds number and thus a relationship was used that
accounted for flow that was developing or fully developed with regards to the friction
factor experiment. For the entire channel length, the flow was thermally developing,
therefore it was determined that correlations and formulas relating to developing flow
could be used with minimal error in reference to the heat transfer experiment. The entry
length calculations and the decided upon correlations will be discussed at length in the
Results section of this document.
The flow loop construction required specialized tools, sealants and thermal
compounds. A pipe cutter and bender were utilized for forming the ¼ inch stainless steel
piping sections while maintaining precision and keeping the pipe from being crushed. For
the machining of the G10 insulation and the L brackets for fixturing, the table saw, drill
press and related safety equipment from Dr. Song’s lab were employed. A hand drill was
used to create bolt holes in the cart for fixturing and hand taps were used in order to re-
thread the ports in the polymer block. The thermocouple welder in the thermal fluids lab
was used to connect the ends of the thermocouples. Several different sealants, such as
Loctite, RTV, exhaust and silicone sealants, were utilized to prevent leakage from around
the thermocouples and from threaded connections. After it was observed that flow was
being drawn into the multiplexer by the capillary effect, the thermocouple sheaths were
sealed to prevent damage to the multiplexer. A method of pump priming was determined
in order to minimize the amount of air in the lines of the pump and to minimize the risk
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of the pump running dry. It was found after initial flow loop testing that that a sizeable
amount of heat was being lost to the surroundings. As a solution to this problem, exhaust
header wrap was tightly wound around the microchannel heat exchanger in layers to
prevent excessive heat loss to the air.
ConstraintsThe flow loop design was mainly constrained by the space and portability desired
by the advisor. In order to facilitate portability, Team 16 purchased a two-tiered stainless
steel cart with each tier 19.5 inches wide by 35 inches long and 4 inches depth. The top
tier of the cart held the entire flow loop in order to prevent pressure drops from height
differentials. Therefore, all of the loop's components had to fit within the available space
of the top level. The bottom level held the data acquisition hardware. In order to prevent
accidental spillage of water onto the lower level, a cart with a high wall on the top level
was purchased.
The design specifications for this project stated that the water must remain
laminar and single phase as it passes through the microchannels. This narrowed the
available pumps since it was necessary to calculate the mass flow rate of the water
compared to the hydraulic diameter of the piping between components. The Reynolds
number was then calculated to determine whether this would result in laminar or
turbulent flow within the microchannels. Furthermore, it was necessary to eliminate back
flow and head loss as much as possible between components with varying entry and exit
diameters. Proper fittings were necessary to facilitate these diametric conversions while
maintaining steady flow.
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All software and computer systems attached to the data acquisition hardware had
to be compatible. Team 16 ensured that the hardware and software could acquire inputs
from both pressure transducers and thermocouples and that the software was readily
available. Type T thermocouples were chosen due to their relatively low temperature
range and higher resolution and due to the operational limitations requiring temperatures
below 100°C to maintain single phase operation. Furthermore, the experiment required
several feet of thermocouple wire to accommodate the seven input sites and the length
required to reach the data acquisition system. The heat cartridges and pressure
transducers necessary for this experiment were fitted for the lengths and diameters of
their corresponding cavities.
Time and cost restraints were stringent therefore it was necessary to conserve both
as much as possible. Initially, the budget was estimated to be approximately $7,000
which was within the capabilities of the MECE department. Through price comparisons
and thorough market research, Team 16 was able to reduce this cost to about $5,000. The
two semesters given to the capstone course provided the main time constraint. The
involved nature of this project necessitated that Team 16 spend the majority of the first
semester learning about this application of heat transfer while planning out the
construction of the flow loop. Therefore, the components were not fully known until the
end of the fall semester. Many of the more specialized components required a month to
ship from the vendor, thus reducing the time allowed for the construction of the loop.
After all components arrived, Team 16 had slightly less than a month to construct the
flow loop.
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Material considerations played a large role in component selection. Stainless steel
was chosen for the majority of the flow loop due to its low corrosivity. The primary piece
of the loop is the copper microchannel heat exchanger. Copper is used due to its high
thermal conductivity. All other components were selected around the design of this
copper microchannel heat exchanger. Dr. Liu’s original thesis called for deionized water
to be pumped through the flow loop. After some research, it was discovered that
deionized water causes corrosion in copper components over extended periods of time.
For this reason, reverse osmosis water was chosen in order to preserve the copper block
and do to its wider accessibility. This decision allowed for reduced cost because copper
became permissible in place of stainless steel for the liquid to liquid heat exchanger.
Finally, it was necessary that the housing of the copper heat exchanger be completely
insulated in order to prevent heat loss to the surroundings and obtain an accurate
representation of inlet and outlet flow temperatures. To this end, G10 material was
chosen and the entire housing was covered in insulating exhaust wrap. In order to view
the flow and the inlet and outlet thermocouples, the housing was covered by a
transparent, polycarbonate top.
StandardsThe pump, condenser, pressure transducers, thermocouples, reservoir, flow meter
and the instrumentation follow ANSI, ISO, ASTM, ASME, EPA, GLP, and GMP
standards, respectively. Standards are accreditations that give assurance to the consumer
of the quality of the product and calibration of the instruments that are being used.
While ordering parts we encountered many of these standards. Although there
were many standards that were not readily available to the public, acknowledging their
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existence is important. A private company, Innocal, applied testing standards to the pump
which pertained to verifying flow rates, power, pump head and pump curves. The
standards used were ISO, EPA, GLP and GMP standards. The heat exchanger was
manufactured by Lytron and was certified under ISO, 9001:2008 and AS9100:2009 under
managerial standards as well as codes of practice in reference to heat exchangers. Codes
of practice are common within the industry and have been widely used for components
such as the reservoir that is considered an unfired pressure vessel in ASME standards.
ASME has several standards that it produced especially for stainless steel fittings that
will be used for piping within the flow loop. The fittings and piping also follow ASTM
and ANSI standards for material properties that have been tested and documented online
for reference. ASTM is also responsible for standards such as calibration that were
utilized in the manufacturing process of the pressure transducers, data acquisition system,
and thermocouples.
ResultsThe final results of construction are shown below in Figures 2 and 3. The top tier
of the mobile cart is presented in Figure 2. The contents and detailed reasoning behind
these component choices were presented in the methodology section of this document. As
shown, all components were arranged in a way that worked well geometrically and was
aesthetically pleasing. Every component could be easily broken down from the others to
facilitate maintenance and cleaning. The bottom tier, which contained all of the electrical
equipment, is shown in Figure 3. This equipment powered the pump, pressure
transducers, heat cartridges and the data acquisition system. The high lip of the top tier
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was designed to protect this equipment in case of accidental leakage during maintenance
or operation.
Figure 2: Flow loop assembly on top tier of cart.
Figure 3: Electronic components on bottom tier of cart.
The heart of the system is shown below in Figure 4. Upon close inspection it is
possible to see what appears to be a rough surface on top of the copper block. Those are
the microchannels of this heat exchanger where fluid flows during normal operation. The
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heat exchanger is visible in Figure 2 through the transparent top of the housing. There
was a sufficient seal provided by the polycarbonate top so that there would be no
communication between the individual microchannels of the copper block. There were
thermocouples attached to the five inlet sites shown in the T-shape of the heat exchanger,
as seen in Figure 4. Each thermocouple gave realistic readings that corresponded to the
appropriate temperature distribution resulting from the heat flux with and without fluid
flow. Furthermore, the inlet and outlet of the channels each had a thermocouple for these
flow temperatures. The water was able to sufficiently remove heat from the copper block
in order to simulate the heat that would be removed by fluid in a modern integrated
circuit.
Figure 4: Copper microchannel heat exchanger.
The received data followed the same trends as the theoretical predictions as
shown below in Figures 5 and 6. Any offsets can be explained by minor head losses and
imperfect insulation of the entire system. The trends are very close to predicted values
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and show a strong correlation when the factors mentioned previously are included. In
order to find the measured friction factor, f, Equation 2 was used. The pressure drop, ΔP,
was measured using the pressure transducers at the inlet and outlet of the fluid flow. The
pressure drop was divided by the density of water, ρ, in order to find the major head loss,
hl. The characteristic length, L, and hydraulic diameter, Dh, were previously measured
using a shadowgraph. The velocity, V, was found through the rotameter by dividing the
flow rate by the cross sectional area of all 50 microchannels combined. Then, it was
necessary to find the Reynolds number, Re, through Equation 3. Here, µ is the dynamic
viscosity which can be found from tables and all other variables were described
previously. The theoretical value for the friction factor for fully developed flow in a
channel with an aspect ratio, α, was determined using Equation 4. It was determined that
the developing hydrodynamic entrance length in the microchannels was significant for all
Reynolds numbers tested according to Equation 5. Therefore, Equation 6 was used to
account for the mixture of developing and fully developed flow. This equation solved for
the non-dimensional flow distance, x+, which is dependent on variables described above.
The non-dimensional flow distance was then used to find the apparent friction factor, fapp,
through Equation 7. Finally, Equation 7 was used to create the theoretical curve shown in
Figure 5.
hl=f LV 2
2 Dh=∆ P
ρEquation 2
ℜ=ρV D h/µ Equation 3
fRe fd=96¿ Equation 4
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xe,l
Dh=0.05ℜ Equation 5
x+¿=L/(Dh ∙ ℜ)¿ Equation 6
f app ℜ=¿¿ Equation 7
Figure 5: Friction factor vs. Reynolds number.
The relationships necessary to find the Nusselt number, Nu, are found through
Equations 8-14 presented below. Begin by finding the total heat transfer, q, from
Equation 8. The fluid properties are the density, ρ, and the specific heat, Cp, which were
found in the fluid tables. The volumetric flow rate, Q, was found using the rotameter,
while the inlet and outlet temperatures, Tm,o and Tm,i were obtained using the
thermocouples that were placed directly into the flow. The effective heat flux, q”eff, was
found according to Equation 9 where N is the number of microchannels on top of the heat
exchanger and A is the wetted area. Then, Equation 10 can be used to find the heat
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transfer coefficient, h. Tw can be attained by reading the temperatures of the
thermocouples at the top of the microchannel heat exchanger and using Equation 11
where TAVG is the average of the temperatures of the thermocouples along the length of
the channel for a given flow rate. The variable s is the distance between the
thermocouples and the channels, and kcu is the thermal conductivity of the copper. Tm is
an average of the inlet and outlet flow temperatures. Tm is valid due to the fact that Tm
varies linearly across the channel length due to the constant heat flux. Afterwards, the
Nusselt number is realized through Equation 12 where kf is the conduction coefficient of
the fluid. It was determined that the flow in the microchannels was thermally developing
for the entire length of the channel for all Reynolds numbers tested using Equation 13.
Here, xt,l represents the thermal entry length, the Prandtl number, Pr, is a thermal fluid
property of the water and the other variables were described previously. Also, the
Reynolds number, Re, was found the same way as described for the friction factor
experiment. The final theoretical relation is shown in Equation 14 and plotted in Figure 6.
Here, the Reynolds number is found the same way as was described above for the friction
factor experiment. The dynamic viscosity of the water at its mean temperature and the
dynamic viscosity of the water at the wall temperature are represented by µf and µw,
respectively.
q=ρ C pQ(T m, o−T m,i) Equation 8
qeff} = {q} over {NA¿ Equation 9
h=qeff } over {( {T} rsub {w} - {T} rsub {m} ) ¿Equation 10
T w=T AVG−sqeff} / {k} rsub {cu ¿ Equation 11
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Nu=h Dh
k fEquation 12
xt ,l
D h=0.05 RePr Equation 13
Nu=1.86 ( RePr Dh /L )1 /3( μ f
μw)
0.14
Equation 14
Figure 6: Nusselt number vs. Reynolds number.
ConclusionThermal management in computers is becoming more of a priority as technology
advances and the number of transistors on a standard size integrated circuit continues to increase.
Microchannel heat exchanger liquid coolant systems for ICs are a viable option due to the high
thermal conductivity and specific heat of the liquids used. Several methods have been used to
construct and test the flow loop. Methods include taking measurements and calculations into
consideration when ordering components, working with tools to construct the flow loop and
creating laboratory documents after administering several tests. The measured data from the flow
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loop closely followed predictions made by theoretical correlations. Dr. Liu reviewed the
construction of the flow loop and the measured data and validated the project. The project has
been completed within the desired time and the team has successfully submitted all required
deliverables to the MECE department.
RecommendationsThere are several recommendations that are a part of the self-evaluation process
of the capstone design project. Moving through the flow loop, starting with the
microchannel heat exchanger, suggestions were made to improve the overall performance
of the flow loop. There was a small gap in between the copper block and the ported white
polycarbonate top. This gap exists due to uncertainty in machining the top and can be
fixed by adding a small layer of sealant between the two components. A silver based
thermal compound was placed between the heat cartridges and the copper block in order
to reduce thermal resistance. Team 16 recommends that a silicone based thermal
compound be used for future work due to the unavoidable fact that the compound can get
too close to the leads and if electrically conductive, it can cause temporary arcing until
the compound dissipates. Laminar flow was the only flow regime that could be analyzed
due to limitations imposed by the pressure transducers which were rated only for 0 – 15
psi, which does not allow for a high enough inlet pressure to produce turbulent flow. It’s
recommended that pressure transducers of 0 – 50 psi range are ordered for future use. If
more precision is required, a rotameter with smaller increments than the current 100ccm
could be implemented.
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Appendix A – Friction Factor Experiment
Experiment #1 – Darcy Friction Factor Verification
*Note: cartridge heaters not necessary for this first experiment*
Objective: Show that experimental results for friction factor (f*Re) correlate closely to the theoretical predictions for laminar flow through micro-channels.
Background:
This experiment examines the pressure drop that occurs as fluid flows through
channels that have hydraulic diameters that are on the scale of micrometers. It also
examines the relationship between the Reynolds number and friction factor for
developing, laminar, internal flow. The results are used to compare fluid flow
relations on the micro and macro scale. Pressure drop is a phenomenon that occurs
as a result of a fluid experiencing internal flow. For this experiment the internal flow
will be through a channel.
For internal flow, the observed pressure drop is caused by two factors, major and
minor head loss. The major head loss accounts for the pressure drop caused by
friction effects in the channel. The major head loss will be used to calculate the
friction factor in this experiment. The equation used to find the friction factor for
fully developed flow is as follows:
hl=f LV 2
2 Dh=∆ p
ρ Equation 1
Where hl is the major head loss, f is the friction factor, L is the length of the channel,
V is the average velocity of the fluid, p is the measured pressure, is the fluid ρdensity and Dh is the hydraulic diameter. The average velocity can be found by
dividing the volumetric flow rate of the fluid by the cross-sectional area of the
channel. For laminar flow, the friction factor is proportional to the Reynolds
number.
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Shah & London provide a theoretical correlation for f*Re with respect to the aspect
ratio for fully developed laminar flow as shown below in Equation 2 and another
correlation to modify for developing flow as shown in Equation 3.
Equation 2
Where is the aspect ratio for the microchannels. Since the hydraulic diameter and αlength of the channels are small, we can assume that the flow is not fully developed. The relation used to calculate the apparent friction factor for developing laminar flow can be seen in Equation 3.
f app ℜ=¿¿ Equation 3
where entrance length is defined as
x+¿=L/(Dh ∙ ℜ)¿ Equation 4
Pre-Lab Questions:
1. Familiarize yourself with the apparatus and the flow loop operations manual.2. Determine expressions for the uncertainty of ℜ ,∆ P ,∧f .3. Calculate the entrance length for a microchannel with critical dimensions as
described and determine if flow is transitional or fully developed.
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fRe fd=96¿
Apparatus:
Figure 2: Piping and Instrument Diagram (P&ID)
26
Figure 3: Microchannel Heat Exchanger
Figure 4: Profile of Microchannels
27
Critical Dimensions
Hydraulic Diameter: Dh 214µm
Length: L 2.54cm
Multiplexer Channel Measurement and Location
1 TC – flow inlet
2 TC – Copper block – top right (near outlet)
3 TC – Copper block – top middle
4 TC – Copper block – top left (near inlet)
9 PT – flow inlet
10 PT – flow outlet
18 TC – Copper block – second from bottom
19 TC – Copper block – bottom
20 TC – flow outlet
Procedure:
1. Have the TA check the data acquisition hardware and software to make sure it is set up properly.
2. Provide water flow to heat exchanger by turning on the sink.3. Turn on the pump and vary flow rate and therefore pressure drop along the
microchannels. Then obtain the measured data of the flow rate and the corresponding voltage produced by the pressure transducers between the inlet and outlet of the test sections.
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In-Lab Analysis:
1. Calculate average velocity and Reynolds Number to confirm laminar flow:
ℜ=ρU Dh
μ Equation 5
Where ρ is the density of the reverse osmosis water, U is the average velocity, Dh is the hydraulic diameter, and μ is the dynamic viscosity.
Calculate the pressure drop across the microchannels using the calibration data for the pressure transducers.
2. Calculate friction factor, f, for each data point using Equation 1.
3. Plot Reynolds number vs. Friction factor and show that for laminar flow, the slope is approximately constant.
4. Calculate theoretical friction constant f*Re for laminar fully developed flow and compare using the theoretical equations and assumptions in Dr. Liu’s thesis
Deliverables:
Worksheet 1: Measured voltage data
Worksheet 2: Measured pressure drop, Reynolds number, friction factor, theoretical
friction factor and uncertainties for each.
Worksheet 3: A plot of the measured and theoretical friction factor. Explain any
discrepancies between the measured data and the theoretical curve.
Post-Lab Analysis:
1. Compare the experimental data for f vs. Re with the correlation in Equation 2. Explain the possible causes behind the discrepancies between the two sets of data.
2. Derive an expression for major head loss from the following energy balance equation:
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Q−W s−W shear−W other=∂∂t ∫CV
❑
eρdV +∫CS
❑
(u+ ρv+ V 2
2¿+gz )ρ V ∙dA ¿
Equation 6
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Appendix B – Nusselt Number Experiment
Experiment #2 –Nusselt Number vs. Reynolds Number Verification
Objective: Show that experimental results for Nu as a function of Re correlate closely to the theoretical predictions for developing laminar flow through microchannels.
Background:
This experiment examines the variation of Nusselts number with increasing Reynolds number for developing laminar fluid flow through microchannels. The Nusselts number is then used to find heat transfer coefficients for the flow. The results are used to compare heat transfer relations on the micro and macro scale.
The Nusselts number for laminar developing flow in a channel is a function of both Reynolds number and Prandtl number. There are several empirically determined relations for the average Nusselts number for developing flow. The theoretical Sieder-Tate relation that will be used in this experiment is shown in Equation 1.
Nu=1.86(RePr Dh
L)1 /3
(μ f
μw)
0.14
Equation 1
Where Re is Reynolds number, Pr is Prandtl number, Dh is the hydraulic diameter, L is the length of the channels, µf is the dynamic viscosity of the fluid at the mean fluid temperature and µw is the dynamic viscosity of the fluid at the wall temperature. The average heat transfer coefficient for the flow can then be determined by using the relation in Equation 2.
Nu=h Dh
k f Equation 2
Where kf is the thermal conductivity of the fluid and h is the average heat transfer coefficient. Because the hydraulic diameter and length of the channels are small we can assume that the flow is not fully developed for the entire length of the channel for low Reynolds numbers.
Pre-Lab Questions:
1. Familiarize yourself with the apparatus and the flow loop manual.
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2. Determine expressions for the uncertainty for Re and Nu
Apparatus:
Figure 5: Piping and Instrument Diagram (P&ID)
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Figure 6: Microchannel Heat Exchanger
Figure 7: Profile Microchannel
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Critical Dimensions
Hydraulic Diameter: Dh
214µm
Length: L 2.54cm
Channel Width: W c 192µm
Channel Height: H c 242µm
Number of Channels: N
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Multiplexer Channel Measurement and Location
1 TC – flow inlet
2 TC – Copper block – top right (near outlet)
3 TC – Copper block – top middle
4 TC – Copper block – top left (near inlet)
9 PT – flow inlet
10 PT – flow outlet
18 TC – Copper block – second from bottom
19 TC – Copper block – bottom
20 TC – flow outlet
Procedure:
1. Review the voltage to pressure conversion chart for the pressure transducers and the method used to read a flow meter.
2. Ensure that the data acquisition system is set up properly for the experiment by asking the TA.
3. Supply cooling water to the second heat exchanger by turning on the valve for the sink.
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4. Turn on the transformer and set the voltage to 60VAC to power the heater cartridges. Once the maximum measured temperature reaches 70⁰C turn on the pump so that the flow rate is 200ml/min.
5. Vary the flow rate over four points from 200ml/min to 500ml/min and record the corresponding pressure drop for each flow rate. Allow for temperatures to reach steady state (+/- 0.2⁰C) for each flow rate before moving to the next flow rate.
6. Calculate average velocity in the channels using the measured volumetric flow rate.
In-Lab Analysis:
1. Calculate Reynold’s Number and confirm laminar flow:
ℜ=ρU Dh
μ Equation 1
Where ρ is the density, U is the average velocity, Dh is the hydraulic diameter, and μ is the dynamic viscosity of the reverse osmosis water.
2. Calculate the steady state heat gain of the coolant for each flow rate using the following formula:
q=ρ C pQ(T m, o−T m ,i) Equation 2
Where C p is the specific heat, Q is the flow rate, and the temperatures are the mean fluid temperatures at the outlet and inlet, respectively. Note: all fluid properties should be calculated for water at the mean fluid temperature.
3. Calculate the effective heat flux in the micro-channels:
qeff} = {q} over {NA¿ Equation 3
Where N is the total number of channels and A is the area available for convection per channel which would equate to:
A=L(W c+2 H c) Equation 4
Where L is the length of the channel, Wc is the width of the channel and Hc is the height of the channel
4. Determine the average heat transfer coefficient
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h=qeff } over {( {T} rsub {w} - {T} rsub {m} ) ¿ Equation 5
Where Tw is the temperature of the wall of the channel (extrapolated from the nearest thermocouple) and the mean fluid temperature.
5. Calculate the average Nusselt’s number
Nu=h Dh
kf Equation 6
Where kf is the conductivity of the fluid.
6. Plot Reynold’s number vs. Nusselt’s Number and show that at Nu increases approximately linearly with Re at low flow rate (laminar flow) with a different constant slope at high flow rates.
7. Obtain the theoretically predicted curves for Re vs. Nu by using Equation 8 and compare the experimental results with the theoretical predictions and discuss.
Nu=1.86( RePrDL )
13 (
μf
μw)
0.14
Equation 7
Deliverables:
Worksheet 1: Measured Temperature Data
Worksheet 2: Average temperature for each channel at each flow rate at steady state, q, qeff, Tm, and Tw, and uncertainty calculations for each.
Worksheet 3: Measured h and Nu, theoretical Nu, Reynolds number and uncertainty calculations for each.
Worksheet 4: A graph comparing the measured Nu vs Re and the theoretical Nu vs Re and an explanation of any deviation of your measured values from theory.
Post-Lab Analysis:
1. Compare the experimental data for Nu vs. Re with the correlation in Equation 7. Explain the possible causes behind the discrepancies between the two sets of data.
2. Using Equation 8 prove that the mean temperature, Tm, varies linesrly with position along the microchannels, thus validating our use of an average for Tm in this experiment.
T m (x )=Tm, i+qs ' 'm c p
x withqs ' '
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Appendix C – Flow Loop Operational ManualMicrochannel Heat Exchanger Flow Loop for MECE 4371
Purpose:
This document provides operational instructions for the microchannel heat
exchanger flow loop developed by Team 16 as a part of the MECE 4341 Capstone
Design course at the University of Houston. The loop will be used in the thermal fluids
lab of MECE 4371.
Layout
Visual representations of the flow loop are provided below in Figures 1 and 2.
Flow travels through each component through ¼” stainless steel piping. Each connection
is provided by Swagelok ferrule fittings that provide a tight seal to prevent leakage. The
flow originates from the reservoir where clean, reverse osmosis water is stored. It is
important that the reservoir remains sealed, unless it is necessary to add or remove water
since impurities can easily clog the microchannels. Water is then drawn from this
reservoir into the pump and then filtered before going into the rotameter where the flow
rate is measured. The flow then proceeds into the housing which contains a
microchannel heat exchanger which is a copper block with small channels that were
etched into the top through photolithography.
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Figure 8: Flow loop layout on top tier of steel cart.
Figure 9: Piping and instrument diagram (P&ID) for flow loop.
The housing consists of a translucent polycarbonate top, a ported polymer block, a
G10 insulating base, exhaust wrap insulation, the copper microchannel heat exchanger
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and instrumentation for measuring pressure drop and temperature as shown in Figure 3
below. The polycarbonate top separates the individual microchannels and prevents fluid
communication between them. The exhaust wrap insulation ensures that heat lost to the
surroundings is minimized. The white ported polymer block separates the copper from
the polycarbonate thus providing the inlet and outlet to the microchannels as well as ports
for the pressure transducers and thermocouples. The ends of the polymer block were
threaded in order to accept the fittings necessary for inlet and outlet piping.
This block was manufactured specifically to accept the top part of the copper
block with a tight fit. An o-ring made from Buna-N type material is placed in a small
groove etched at the top of the white polymer block. This o-ring creates a seal between
this block and the polycarbonate top that prevents water from leaking out of the sides.
Another o-ring of the same type was placed at the bottom of the polymer block in order
to prevent leakage in this area.
Thermocouples were placed at the inlet and the outlet of the flow in order to
measure the temperature distribution across the copper block. The thermocouples were
held in place by an RTV silicone sealant that was used to fill the fittings and secure the
thermocouple wire in place. An additional five thermocouples were placed directly into
the copper block to obtain temperature distribution data throughout the block itself. A
constant heat flux is provided by eight heat cartridges which are placed in small cavities
at the bottom of the heat exchanger. A silver thermal compound was placed around the
cylindrical heat cartridges in order to minimize thermal resistance between the cartridges
and the copper block. In order to insulate the bottom of the heat exchanger, a flat plate of
G10 material was used. Bolts were used to create a tight seal within the housing that
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would prevent fluid communication between channels while avoiding deformation of the
channel walls. These bolts were driven through the polycarbonate top, the ported polymer
block and the G10 material. Eight holes were drilled into the G10 material so that the
heat cartridge wiring could be routed into the transformer discussed below. Finally,
pressure transducers were placed at the flow inlet and outlet to determine the pressure
drop across the microchannels. Loctite Thread Locker was used to seal the threads of the
pressure transducers and the inlet and outlet fittings. Flow then leaves the housing and
enters a liquid to liquid copper heat exchanger where tap water removes heat from the
RO water before the flow returns to the reservoir and the loop is complete.
Figure 10: Microchannel heat exchanger housing with polycarbonate top, ported polymer block, copper microchannel heat exchanger and G10 insulating base.
All of the components described above were placed on the top tier of a two-tier,
stainless steel cart as shown in Figure 1. The bottom tier contains all of the electronic
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components required for the system as seen in Figure 4. Output from the thermocouples
and pressure transducers is transferred to the data acquisition system on the bottom tier.
The data acquisition system then communicates with a local computer for data
processing. Data processing occurs through the Data Logger 3 program. The heat
cartridges required a transformer in order to convert the wall outlet voltage from 120
VAC to a variable AC voltage based on the heat required for each experiment. The
pressure transducers require an input between 9 and 30 volts. In order to power these, a
12 volt power supply is used that converts AC voltage to DC. A power strip was used to
provide outlets for the data acquisition system, the pressure transducers, the transformer
and the pump. Finally, clear plastic tubing was used for the inlet and outlet flows of the
tap water which passed through the liquid to liquid copper heat exchanger. The inlet was
fitted so that it could run from a generic garden hose. The fitting can be removed and the
tubing can be attached and hose clamped onto the barbed outlet of the laboratory sink if
necessary as well. The outlet is simple tubing that was made long enough to reach the
nearest drain.
Figure 11: Bottom tier of cart containing data acquisition and transformer.
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Operation Safety
1. Do not use pure deionized water or pure reverse osmosis water in this system!
The lack of ions in the water will cause it to strip copper ions from the
microchannel heat exchanger and the liquid to liquid copper heat exchanger
which will erode the components over time. The approved water for this
system is commercially available reverse osmosis water with some mineral
enhancement. The mineral parts per million should be fairly low, but it must
be present so that some ions will exist in the water. The system was initially
tested with Nestle ® Pure Life ® water purchased from CVS. This water is
clean enough to avoid clogging the microchannels and has enough ions to
avoid eroding the copper.
2. Always assure that the pump is sufficiently primed prior to operation. This
means that fluid must have contact with the pump’s moving components
before it can be safely activated. In order to prime the pump, undo the fitting
on the outlet of the reservoir. Rotate this piece of piping away from the pump
so that a tube can be fitted onto the end that was originally the reservoir outlet.
Elevate the tubing and use a funnel to direct water through the piping and use
gravity to drive the water into the pump. The water should fill the quarter inch
piping up to the highest bend that occurs right after the reservoir outlet. Then
reconnect the piping to the reservoir outlet. This will assure that the pipe is
mostly filled with water and only a small pocket of air will enter the pump
through the small piece of piping that goes into the reservoir. Excessively
running the pump while dry will destroy the unit! Furthermore, take care
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when running the pump past 1000 CCM according to the system’s rotameter.
Leakage began to occur at this flow rate and better sealing is necessary in
order to achieve a higher internal system pressure.
3. The heat cartridges are controlled through a variable voltage available through
the transformer. Do not push the transformer above 70 VAC without
approval from Dr. Liu! If fluid is not flowing over the microchannels,
then the heat exchanger can easily reach 100°C with less than 40VAC.
Excessive voltage or heating of the copper block without fluid flow will
create a fire hazard and possibly melt the white polymer housed on top of
the copper block. Any time that the heat cartridges are active, be sure to
watch and listen carefully. If any sounds come from the heat cartridges or
if a plume of smoke is seen, then turn off the transformer immediately to
prevent damage to the system.
4. When operating the data acquisition system (DAQ), assure that data is logical
for the experiment. Very inaccurate data may indicate that fluid has
entered the system. The thermocouples are currently sealed, but it is still
possible for the seal to break and for fluid to go through the outer sheath of the
thermocouple wiring and use the capillary effect to deposit water into the
thermocouple channels of the multiplexer. Be very careful and stop flow
operation immediately if data is wildly inaccurate. Furthermore, do not power
off the data acquisition system in the middle of a scan. The computer
software and the DAQ are in constant communication. If the DAQ hardware
is powered off mid scan, then the software will still interpret that a scan is
43
necessary. The software will reactivate the DAQ which will then proceed to
start a new scan instead of finishing the old one. This will result in the
retrieval of bad data, so always assure that scanning is complete before
powering off the DAQ.
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