university of waterloo faculty of engineering
TRANSCRIPT
UNIVERSITY OF WATERLOO
Faculty of Engineering
Nanotechnology Engineering
Nanotechnology Engineering Fourth Year Design Project Interim Report
NE_2010_05
Nanofluid-based Microfluidic Heat Exchanger for CPU Cooling
This report is submitted as the interim report requirement for the NE.408 course. It has been written solely by us
and has not been submitted for academic credit before (other than appendices A-G) at this or any other academic
institution.
Contributing Members
Yusuf Bismilla
Robert Black
Geoffrey Lee
Graeme Williams
Jason Wu
Project Supervisor
Professor Richard Culham
Mechanical and Mechatronics Engineering
Submitted on
December 4, 2009
1
Abstract
This proposal is for the development of a microfluidic heat exchanger (MHE) using a nanofluid as the
heat dissipation medium. This device has direct applications in cooling the CPU component of both
laptop and desktop PCs. The finalized device will consist of an array of microfluidic channels with widths
on the order of 200m. The channels will be milled from a piece of copper using very fine milling tools.
An aqueous, functionalized carbon nanotube (CNT) solution will flow through the microfluidic channels
in order to act as the heat exchange medium. A colloidal CNT solution was chosen due to its significantly
enhanced thermal conductivity compared to traditional heat exchanger fluids. The CNT solution, driven
by a micropump, will cycle throughout the channels and out to a radiator in order to dissipate the heat
into the environment. The device dimensions allow for a small vertical profile that can be incorporated
into a computer by mounting it directly onto the CPU. It is expected that this design will match and
potentially exceed the performance of current heat sink technologies.
Acknowledgements
We would like to thank the following individuals for their guidance in the work completed on this fourth
year design project:
Dr. Richard Culham – for helpful conversations, supplying of lab space and loaning test
equipment
Elmer Galvis – for helpful conversations, training and assistance with lab equipment
Dr. Aiping Yu – for helpful conversations, loaning of carbon nanotubes and use of equipment
Dr. Linda Nazar – for use of lab space and equipment, and use of various consumables
2
Glossary of Terms and Acronyms
CPU - Central Processing Unit
CNT - Carbon Nanotube
ID - Inner Diameter
MHE – Microfluidic Heat Exchanger
MHTL – Microelectronics Heat Transfer Laboratory
Microfluidic Chip – a thin copper block with microchannels milled into it
Nanofluid – a colloidal system of nanoparticles
Nanosolution – see nanofluid
OD - Outer Diameter
PC - Personal Computer
RBF - Round Bottom Flask
SEM - Scanning Electron Microscopy/Microscope
3
Table of Contents
Abstract ......................................................................................................................................................... 1
Acknowledgements ....................................................................................................................................... 1
Glossary of Terms and Acronyms ................................................................................................................. 2
List of Figures ................................................................................................................................................ 5
List of Tables ................................................................................................................................................. 6
1. Introduction to the Problem and Design Scope .................................................................................... 7
2. Overview of the Complete Project Plan ................................................................................................ 9
2.1. Time Allocation and the Associated Design Process ..................................................................... 9
2.2. Project Inventory and Budget ..................................................................................................... 12
2.3. Allocation of Tasks to Group Members ...................................................................................... 13
3. Summary of MHE Functional Specifications and Associated Verification and Test Procedures ........ 15
3.1. Explanation of the Fabrication and Testing Process ................................................................... 15
3.2. Heat Transfer Specifications, Verification Steps and Test Plans ................................................. 15
3.3. Microfluidic Chip and MHE Integration Specifications and Tests ............................................... 17
3.4. Safety and Noise Specifications and Tests .................................................................................. 19
4. Design Specifications for the MHE and Notes on COMSOL Simulations ............................................ 20
4.1. High Level Overview of the MHE ................................................................................................ 20
4.2. Major MHE Design Considerations and Microfluidic Chip Modelling ......................................... 20
5. Completion of the MHE Design to Date and Revised Timelines ......................................................... 23
5.1. Overview of the Colloidal System ............................................................................................... 23
4
5.1.1. Determination of the Colloidal System for Use in the MHE ................................................... 23
5.1.2. Synthesis and Functionalization of CNTs ................................................................................ 24
5.1.3. Characterization and Verification of the CNT Solution ........................................................... 27
5.2. Overview of the Test Setup and Determined Test Parameters .................................................. 28
5.2.1. Description of the Original Test Setup .................................................................................... 28
5.2.2. Initial Determination of Test Parameters ............................................................................... 29
5.2.3. Initial Heat Measurement Setup ............................................................................................. 30
5.2.4. Verification of Fluid Flow using the Test Setup....................................................................... 31
5.3. New Timeline for the Completion of the MHE ........................................................................... 32
6. Conclusions and Further Areas Required for Project Completion ...................................................... 34
Works Cited ................................................................................................................................................. 36
Appendix A – Customer Requirements ..................................................................................................... A-1
Appendix B – Project Plan and Milestones ................................................................................................ B-1
Appendix C – Design Flow .......................................................................................................................... C-1
Appendix D – Functional Specifications .................................................................................................... D-1
Appendix E - Verification Plan for the Paper Design Document ................................................................ E-1
Appendix F - Test Plan for the Constructed Prototype Design Document ................................................ F-1
Appendix G – Design Specifications .......................................................................................................... G-1
Appendix H – Verification Data ................................................................................................................. H-1
5
List of Figures
Figure 1 – Apparatus used for functionalization of SWNTs 25
Figure 2 - Block Diagram of Original Test Setup 28
Figure 3 – Mock Microfluidic Channel 29
Figure A-1 - Illustration of a Typical Heat Sink Mounted to a CPU A-1
Figure A-2 - Graphical Representation of Moore’s Law for Increasing Transistor Count A-1
Figure D-1 - High Level Design of the Microfluidic Heat Exchanger D-1
Figure D-2 - Schematic of the Microfluidic Chip – Maximum Size Constraints D-3
Figure E-1 - Test Bed Verification Block Diagram E-4
Figure F-1 - MHE and Associated Components Sitting in a CPU Tower F-1
Figure G-1 - Detailed MHE Setup with Measurement Positioning G-1
Figure G-2 - Cross-Section of the Microfluidic Chip with Device Dimensions G-2
Figure G-3 – Side View of the Microfluidic Chip Sitting on a CPU G-3
Figure G-4 - Side View of Radiator System G-4
Figure G-5 - Front View of Bent Copper Tube in the Radiator System G-5
Figure G-6 - MHE System Integration G-5
Figure H-1 Total Velocity Profile in MHE Inlet H-2
Figure H-2 x-Velocity in MHE Inlet H-2
Figure H-3 Channel Inlet Velocities H-3
Figure H-4 Velocity Distribution at Channel Inlet H-4
Figure H-5 Channel Simulation Drawing H-5
Figure H-6 Channel Velocity Profile H-5
Figure H-7 MHE Far Edge Temperature Distribution H-7
6
List of Tables
Table 1 - Updated Timeline for Project Completion 33
Table A-1 - Summary of Customer Design Specification Requirements A-4
Table B-1 - Project Milestones B-1
Table B-2 - Detailed Project Breakdown B-5
Table B-3 - Equipment Requirements B-7
Table B-4 - Software Requirements B-8
Table B-5 - Required Supplies B-8
Table B-6 - Anticipated Budget for the Design Project B-9
Table D-1 - Data Sheet: Minimum Functional Requirements and Supplementary Requirements D-1
Table G-1 - Summary of System Volumes G-6
7
1. Introduction to the Problem and Design Scope
As electronic devices continue to scale to smaller dimensions, a larger number of devices are able to fit
on a single wafer. In the case of a central processing unit (CPU), this translates into higher and higher
transistor densities, with each transistor dissipating a certain amount of heat due to its regular
operation. The downscaling of devices also results in the dissipation of a greater amount of heat from a
single transistor due to high ‘off’ currents in the devices. The end result of these issues is a significant
increase in the regular operating temperatures of CPUs. The problem of increased heat dissipation is of
critical importance as it affects the performance and durability of the CPU. This issue is especially
problematic in large-scale server farms and laptop computers, where a large amount of heat is
generated in a closed and isolated environment. For these particular applications, it is essential to
quickly and effectively dissipate the heat to maintain standard operating temperatures.
The purpose of this design project is to examine a microfluidic heat exchanger (MHE) using a colloidal
system of nanoparticles (nanofluid) for its applications in removing heat from a CPU. The MHE device
comprises of four main components:
- a colloidal system of suspended nanoparticles in water
- a copper heat sink with milled microchannels
- a microfluidic pump
- a radiator
The premise of this design is to use the microfluidic pump to flow the suspended nanoparticles through
the microchannels in the heat sink, where they will pick up heat from the CPU. The heated
nanoparticles will then travel outside of the microchannels and through a metal radiator, where they
will transfer the heat into the surrounding environment. As will be detailed in the body of this report,
the combination of the microfluidic channels and nanofluid is intended to increase the effective rate of
heat transfer from the CPU to the fluid, where it can be transferred away from the computer.
The scope of the design of the MHE is restricted only to desktop personal computers (PCs) for consumer
applications, with slimmer designs present only as secondary or tertiary considerations. This eliminates
the restriction of producing an extremely low profile device and eliminates the need for complex
integration for the case of a laptop computer. In terms of the MHE device, the scope of the design
8
focuses on the design of the microchannels and the colloidal system of nanoparticles, with little
emphasis on the optimization of the radiator design. Furthermore, the design of the micropump is not
within the scope of this design – only the operational parameters of the micropump will be considered.
As detailed in Customer Requirements, shown in Appendix A, current CPUs generate heat near 70-
100W, requiring heat sinks that have thermal resistance values near 0.5oC/W. As such, this value of
0.5oC/W remains the minimum goal for the thermal resistance values determined with the MHE
completed in this project. Since this project goes to great lengths to make use of a nanofluid to enhance
thermal transfer, it is also desired to see an improvement of the nanofluid-based MHE over a water-
based MHE (with at least a 1.1-times improvement). Furthermore, in order to integrate the microfluidic
device onto the CPU, its main body, denoted as the microfluidic chip, must not exceed the dimensions of
typical copper, fin-based heat sinks. These requirements are primary requirements that must be met.
Appendix A further details the baseline requirements for MHE noise generation and CNT weight loading
in the fluid of the MHE. The primary contributor to noise for the MHE is the micropump, and given that
the micropump is primarily a purchasing step, it is desired to purchase a pump with noise less than
65dB. The nanoparticle loading is an environmental and safety concern (deemed to be a priority
requirement), and the solids weight loading of the nanofluid should not exceed 3%. The basic
specifications outlined above are further summarized in Table A-1 in Appendix A.
This document serves to examine the first pass design process involved with the MHE. This document
first details the initial project plan, allocation of tasks and the basic design flow. This aspect of the
report makes use of Appendices B and C. The document then elaborates on the functional
specifications of the MHE and the methods chosen to verify that both the individual components of the
MHE and the fully-integrated system are suitable. This aspect of the report makes use of Appendices D
through F. The following section of the report then discusses the finalized design specifications, and
uses these specifications to model the behaviour of the fluid within the microfluidic chip. These aspects
of the report make use of Appendices G and H. In the final aspect of this report, the completion of the
MHE design, as accomplished to date, will be discussed in detail. Any alterations to the initial design
stages and specifications will be noted in this final discussion.
9
2. Overview of the Complete Project Plan
2.1. Time Allocation and the Associated Design Process
Of the four major components in the fabrication of the MHE established in Section 1 and Section 2.1
above, two have been determined to be of critical importance in the completion of this project. In
particular, the synthesis of the nanofluid and the fabrication of the microfluidic channels serve as the
most important design features. These two aspects most directly determine the efficacy of heat
removal of the system and, consequently, determine the success of meeting the primary customer
requirements established in Section 1 and Appendix A.
The importance of the nanofluid and microchannel in the success of this project has been emphasized in
Table B-1 in Appendix B, which outlines a rough breakdown of project milestones. The first ten weeks
of the project, which have already passed at the time of this document submission, have been dedicated
to the understanding and optimization of these systems. In the detailed work breakdown in Table B-2 in
Appendix B, six weeks were dedicated to the synthesis, chemical modification and suspension of
nanoparticles. In the completion of this step, various nanofluids have been researched for both their
long-term stability and their efficacy in heat removal, as detailed in Section 5.1.1. After numerous
considerations regarding both thermal conductivity enhancements and stability, carbon nanotubes were
determined to be the ideal nanofluid for use in the MHE.
The importance of the stability of the nanofluid is further emphasized by the numerous anticipated
design challenges, detailed in Section B.2 of Appendix B. The continuous suspension of the CNTs in
solution and the possibility for channel clogging have both been identified as potential design issues in
this project. From preliminary data, as discussed below in Section 5.1.2, the continued suspension of
the CNTs does not appear to be a significant issue. Furthermore, the design of the microfluidic chip, as
detailed in Appendix H, has been revised to include wider microchannel widths that should decrease the
probability of channel clogging.
A large portion of the design project time has been allocated to the testing of the nanofluid outside of
the channels, as detailed by the verification plan in Appendix E. Of particular importance is the
10
suspension of the CNTs within solution after time has been given for them to settle, as this action may
lead to clogging of the microchannels and reduce the efficacy of the channels for heat transfer. Time
has been allocated for various tests (shown in Table B-2 in Appendix B), including the ‘validation of
particle suspension’ and ‘validation of performance within a mock microchannel’ tests, early on in the
MHE project completion. The solution is to be rigorously tested for stability and heat transfer
characteristics when it is fully-integrated into the system and flowing through the microfluidic chip, as
detailed in the test plan in Appendix F. Additional time for tests relating to long term stability and
particle dispersion within channels has been allocated in the final testing stages of the MHE.
With regard to the design, a significant portion of the project completion time was allocated to the
modelling of the microfluidic chip, as shown in Table B-2 in Appendix B. This was originally considered
to be one of the most critical tasks, as there was significant doubt regarding the pressures and flow
velocities that would be required to remove heat from the system. These channel parameters were also
deemed to be extremely useful for the purchase of a microfluidic pump. However, as detailed in
Sections 5.2 and 5.3, this task has been revised due to the temporary acquisition of a mock
microchannel and a test micropump from the project supervisor. With these items, empirical data,
which are more effective for determining flow velocities required for heat removal, have been obtained.
Empirical data is also useful for the determination of relevant parameters for pump purchasing.
It is important to note that the modelling of the microfluidic channels has still been performed and is
detailed at depth in Section H. While many of the system parameters were determined experimentally,
theoretical modelling has served to provide important data regarding the general behaviour of the fluid
within the microchannels. In order to solve this system, it was necessary to decouple the state variables
involved in the simulation, as suggested in the expected design challenges in Section B.2 of Appendix B.
The creation of the microchannels themselves has been granted only a few weeks for completion and
subsequent analysis by scanning electron microscopy (SEM). This was done purposely, as many of the
challenges associated with the fabrication of the microfluidic chip, detailed in Section B.2 of Appendix B,
are alleviated due to the assistance provided by Elmer Galvis. Elmer’s expertise in milling microfluidic
channels from copper should prove invaluable in these project design steps and allow for the creation of
11
a microfluidic chip with minimal cross-channel leakage and an excellent seal. While the milling of the
microfluidic chip was originally scheduled to begin in November, the loaning of the mock microfluidic
chip has allowed for this step to be pushed back to a later time. This will be detailed further and shown
in the revised timeline in Section 5.3.
The remaining design features of this project are not entirely within the scope of the project
requirements, and are therefore not allocated as much time for completion. From Table B-2 in
Appendix B, the radiator design is relatively simple and should not require more than two weeks to
fabricate. Furthermore, while the micropump is critically important for the operation of the integrated
system, it is a highly complex device and a fourth year design project in its own right. Hence, the
micropump will be purchased based off of the information attained from the test micropump loaned to
the group from the project supervisor. While the fabrication of the radiator and the purchasing of the
pump were originally planned to occur much earlier in the project timeline, the loaned radiator and test
micropump have also allowed these step to be pushed back to a later date. This will be detailed further
and shown in the revised timeline in Section 5.3.
The project plan has been updated to accommodate for the acquisition of the loaned items from the
project supervisor and is shown in Appendix C. This flowchart may be used to better understand the
relative level of completion of the project. While much of this information will be provided in much
greater detail in Section 5 and Appendix H, a brief summary of the completion of the flowchart is
detailed below:
- the initial research on water soluble nanoparticle synthesis methods has been completed
- the aqueous CNT colloidal system has been successfully synthesized, and has adequately passed
both solubility tests qualitatively, but quantitative measurements need to be made
- the test bed has been constructed with 1/8th inch ID tubing
- water has successfully flowed through the mock microchannels
- system parameters have been acquired (including total volume, pressure drops and flow rates)
- fluid flow and heat models have been completed for the microfluidic channels using COMSOL
- theoretically, the system has been proven to be capable of dissipating the heat generated by
current commercial CPUs
- basic fluid flow tests of the nanofluid in the mock microchannels have been completed
12
As such, most of the steps on the first page of the flowchart have been completed, with the exception of
the additional nanofluid validation steps in the mock microchannel, milling of the microfluidic channels
and pump purchasing. By the measures set in Table B-2 in Section B of this report, the project is slightly
behind schedule. However, a revised project schedule detailed in Section 5.3 of this report serves to
correct this problem. In light of this revised schedule, the remaining aspects of the project are discussed
in detail in Section 6 of this report.
2.2. Project Inventory and Budget
The materials, equipment, lab space and associated budget for this project are detailed at depth in
Section B.3 of Appendix B. As discussed in Section B.3.1 of Appendix B, the major lab space
requirement is the Microelectronics Heat Transfer Laboratory (MHTL). This lab contains most of the
required equipment detailed in Table B-3 of Appendix B, and it is used for most of the basic device tests.
This lab space has been provided by the project supervisor. Additional lab space requirements are
primarily to make use of machining tools, SEM, electronic testing equipment, and synthesis of the
nanofluid. This includes the engineering student machine shop, the nanotechnology characterization
labs and the undergraduate engineering electronics lab.
Explicit materials and their associated costs are given in Tables B-5 and B-6 in Appendix B. Consumables
are primarily composed of the chemicals needed for the fabrication of the nanofluid. Additional
supplies include the tubing and copper necessary to construct the MHE and the computer required to
test the final device. Software requirements are detailed in Table B-4 in Appendix B, and largely include
university-licensed programs for modelling or data analysis of the MHE. Of particular note is the
program Super Pi, which will be used to stress the CPU for MHE testing.
This materials listing and the associated budget have been updated to accommodate for the purchase of
CNTs that were originally hoped to be provided by a faculty member. The money for this purchase was
reallocated from money that was originally intended for chemicals, which were supplied in kind by Dr.
Linda Nazar’s lab. Furthermore, group member Robert Black performed all CNT functionalization
experiments in Dr. Nazar’s lab, so the use of C2-168 and C2-275 were not required for the completion of
this project.
13
Only small reductions to the budget have been made at this point in the project completion. This is due
to the fact that neither the micropump nor the milling tools have been purchased, for reasons detailed
in Section 2.1 above and Section 5 below. At the time of this document completion, purchases have
only been made on carbon nanotubes and tubing connectors. The remaining budget should be
sufficient for completion of this project, although this will be much clearer once the more expensive
items, such as the microfluidic pump, have been purchased.
2.3. Allocation of Tasks to Group Members
The primary allocation of tasks to group members is discussed at depth in Section B.1.2 in Appendix B.
These descriptions represent the basic duties of group members as determined at the beginning of the
project; however, variations of these responsibilities and further specializations in roles have naturally
occurred throughout project completion. In this manner, each group member has adopted two tasks.
Each group member is directly responsible for the proper completion of both tasks assigned to him.
These tasks are detailed for each member below:
- Yusuf Bismilla:
o Yusuf is the primary contributor to the modelling of the microfluidic channels.
o Yusuf is one of the principle members responsible for the experimental testing of the
MHE. In this role, he has developed the test bed for the MHE, and run tests with both
water and the nanofluid in the mock microfluidic chip.
- Robert Black:
o Robert is the primary contributor to the nanofluid synthesis, including the
functionalization of the carbon nanotubes. In this role, he has developed a method to
produce CNTs that remain reasonably well-dispersed in aqueous solution.
o Robert is also a strong contributor to the documentation involved with this project.
- Geoffrey Lee:
o Geoffrey is one of the principle members responsible for the experimental testing of the
MHE. This entails responsibilities similar to those discussed above for Yusuf.
o Geoffrey is also a strong contributor to the documentation involved with this project.
- Graeme Williams:
14
o Graeme is the primary contributor to the documentation involved with this project. In
this role, he has organized the data involved with the project and has served as the
principle editor of all submitted documents.
o Graeme is also involved with the experimental testing of the MHE.
- Jason Wu
o Jason is one of the principle members responsible for the experimental testing of the
MHE. This entails responsibilities similar to those discussed above for Yusuf.
o Jason is also a contributor to the documentation involved with this project.
15
3. Summary of MHE Functional Specifications and Associated
Verification and Test Procedures
3.1. Explanation of the Fabrication and Testing Process
As discussed in Section 2, this project is composed of four primary components, of which two have been
identified as critically important. In order to guarantee the highest chance of success for this design
project, these components are first examined on an individual basis both theoretically and
experimentally. These theoretical and experimental tests are referred to as verification steps and are
detailed in Appendix E. These steps ensure that the individual components will not lead to failure to
meet the functional specifications, outlined in Appendix D. Once the MHE passes its verification steps,
it will be integrated into the final system with a fully functioning desktop PC, as shown in Figure F1 of
Appendix F. At this point, the system will undergo various test procedures, outlined in Appendix F, to
ensure that the MHE fully meets all of the functional specifications outlined in Appendix D.
3.2. Heat Transfer Specifications, Verification Steps and Test Plans
From the data sheet in Table D-1 in Appendix D, the heat transfer requirements have been identified
and are summarized for convenience below. The complete justification and detailed explanations for
each of these requirements can be found in Section D.2.1 of Appendix D.
- The maximum thermal resistance of the MHE device must be 0.5oC/W.
- The maximum temperature of the CPU must not exceed 50 oC.
- The CNT-based MHE should show an improvement over the water-based MHE by at least a
factor of 1.1-times.
In order to verify that the MHE has the potential to meet these requirements prior to the integration of
the MHE design components, a number of verification steps have been considered. In order to reach
any measure of stable heat transfer, it is first necessary that the nanofluid is stable and the CNTs within
the fluid remain disperse during operation. If the CNTs crash out of solution, any gains to the thermal
conductivity of the solution will be lost. Therefore, verification and testing of these particular
specifications involves both the nanofluid stability tests and the actual heat transfer tests.
16
The verification steps for the nanofluid stability involve studying the short-term and long-term stability
of the dispersed CNTs, as described at depth in Section E.1 of Appendix E. In brief, these tests involve
allowing time for the colloidal CNT solution to settle, providing a light agitation to the solution and
examining the final dispersion of the CNTs. The capacity for the CNTs to re-suspend with light agitation
over both the short term (24 hours) and the long term (> 1 week) has been verified qualitatively, as
described in Section 5.1.3. These tests remain to be verified quantitatively with UV-Vis; however, given
the CNT solution’s success in flowing through the microfluidic channels in a mock microfluidic chip, it is
believed that the solution will pass this verification test easily.
In order to gauge the general behaviour of the nanofluid within the microfluidic channel, theoretical
modelling of the system was also taken as a verification step. Through the use of COMSOL, the relative
success of the project may be estimated by applying realistic boundary conditions and physics to the
model of the MHE. This step has been completed and is detailed in Appendix H. In general, it has been
found that a water-based cooling system with the current microfluidic chip design, as discussed in
Section 4 below, should allow for the attainment of the maximum thermal resistance value of 0.5oC/W.
The temperature of the system also remained under 50oC, meeting the second basic requirement of this
particular design aspect. Improvements to thermal conductivity by replacing water with the nanofluid
should allow for further improvements to these values, as discussed in Appendix H.
As a final verification step to ensure that the MHE is able to meet the basic thermal requirements, the
heat transfer characteristics were studied using the mock microchannel and the test micropump. The
procedure for this test is detailed further in Section E.2 of Appendix E. This step has been completed for
water, but has yet to be verified for the nanofluid, as detailed in Section 5.2. In particular, it has been
verified that water solutions flowing through the mock microchannel can adequately cool a resistive
heating element at the operating pressures provided by the test micropump. It has also been verified
that a simple radiator (comprised of bent copper tubing mounted onto a fan) is capable of adequately
cooling the liquid before it re-enters the microfluidic chip. While the nanofluid has been tested in the
mock microchannel, it has not yet been tested with the heating element.
17
In order to test the heat transfer characteristics of the MHE once it has been fully integrated into the PC,
a number of tests have been created, as described in Sections F.3.1 and F.6.2 of Appendix F. These
tests include measurements of the performance of the MHE itself, tests on the flow rate of the
micropump and tests on the heat output of the radiator. While the design of the micropump and
radiator are not a primary concern in this project, it is important that they are not limiting factors in the
removal of heat from the CPU. The tests procedures for the micropump and radiator are summarized in
Sections F.4 and F.5 of Appendix F respectively. Since this project is not yet completed to the point
where these tests can be fully completed, the basic measurement parameters and experiments are
summarized below for convenience:
- measurement of thermal resistance of the MHE at initial steady state temperatures
- temperature stability of the MHE over longer periods of time, remaining below 50oC
- nanofluid suspension tests with the MHE in both the on and off state over long periods of time
- thermal resistance improvement of the nanofluid-based MHE over the water-based MHE
- measurement of micropump flow rates and pressures during both short and long term
operation
- measurement of radiator efficiency during MHE operation
These tests will be conducted with the use of thermocouples, flow meters and pressure meters placed
at various regions in the flow lines of the MHE.
3.3. Microfluidic Chip and MHE Integration Specifications and Tests
From the data sheet in Table D-1 in Appendix D, the requirements specific to the microfluidic chip and
the MHE integration have been identified and are summarized for convenience below. The complete
justification and detailed explanations for each of these requirements can be found in Section D.2.2 of
Appendix D.
- The absolute maximum dimensions of the microfluidic chip are 80mmX100mm (width/length)
X60mm (height).
- Given the above microfluidic chip dimensions, the maximum inlet and outlet height dimensions
are 30mm, as illustrated in Figure D-2 in Appendix D.
18
- The maximum tolerable inner tube diameter is 1/4th inch. This is necessary in order to decrease
the total volume of the system and to effectively decrease the cost induced by the use of a
nanofluid.
- In order to better integrate the MHE with the system, it is desired that the micropump uses a
voltage below 12V, such that it can be connected to the PC’s internal power supply.
Further optional requirements that have not been summarized above have also been discussed in
Section D.2.2. These requirements, however, address only secondary and tertiary requirements noted
in Appendix A, and are not necessary for project completion and success.
In general, the above specifications act primarily as design rules. During the fabrication and assembly of
the MHE, it is necessary to adhere to these rules by purchasing the proper tubing, machining the correct
dimensions and purchasing the ideal micropump. SEM will be used to confirm inner channel dimensions
of the microfluidic chip, which are described further in Appendix G and Section 4. Callipers will be used
to verify the dimensions of all of the external parameters. From work already completed with the mock
microfluidic chip, it has been found that the external dimensions of the microfluidic chip can be
substantially decreased in size from the maximum possible dimensions. In particular, the vertical
dimensions of the finalized model will be significantly reduced in order to promote better heat transfer
from the CPU to the nanofluid.
In addition to the above functional requirements, it is also necessary that the microfluidic chip provide
stable and reasonable flow rates and pressure drops when used with the micropump, as detailed in
Section E.2.1 in Appendix E. These parameters are also useful for pump purchasing, as they ensure that
the purchased micropump functions suitably with the MHE. Pressure drops and associated flow rates
have already been determined with the mock microfluidic chip and the test micropump, as detailed in
Section 5.2. Reasonable flow rates and fluid behaviour within the channels has also been verified
through modelling with COMSOL in Appendix H.
Once the MHE system has been fully integrated, long term stability tests and stress tests will be
performed on the microfluidic chip in order to detect critical errors that may alter the flow parameters.
As discussed in Section F.3.2 in Appendix F, issues such as cross-channel leakage, channel clogging, joint
19
leakage and material deformation are of significant concern. These critical errors should be detectable
by analyzing the system for any significant variations to flow rate, pressure drops or heat removal
efficiency.
3.4. Safety and Noise Specifications and Tests
From the data sheet in Table D-1 of Appendix D, the safety and noise requirements have been identified
and are summarized for convenience below. The complete justification and detailed explanations for
each of these requirements can be found in Section D.2.3 of Appendix D.
- CNTs must be studied extensively for their safety when suspended in an aqueous medium, and
the CNT weight loading in this project must not exceed 3%.
- Seals must be ensured to be completely leak-free, and wires must be properly electrically
insulated.
- The noise level of the MHE must not exceed 50dB
As detailed in Appendices D and E, the verification steps for ensuring proper sealing and electrical
insulation with the MHE are mainly qualitative. The safety documentation regarding CNTs is also a
qualitative requirement. Furthermore, the weight loading of CNTs is determined experimentally during
the functionalization and suspension of the CNTs in solution. The noise level of the system will be
verified once it is fully integrated with the use of a decibel meter, as detailed in Section F.7 of Appendix
F. As detailed in Appendix D, the primary culprit for noise in this system is the micropump. Since the
test micropump loaned by the project supervisor has a noise output well below 50dB, this particular
requirement is not believed to be of significant concern.
20
4. Design Specifications for the MHE and Notes on COMSOL Simulations
4.1. High Level Overview of the MHE
A simplistic high-level overview of the MHE is provided in Figure G-1 in Appendix G. This diagram
outlines the flow of the nanofluid through the tubing and to the various components of the device. As
discussed in Section G.1 of Appendix G, the nanofluid is kept in a reservoir, where it is connected to a
micropump. The micropump forces the nanofluid through the tubing, allowing for heating of the
nanofluid to occur as it passes through the microfluidic chip and cooling as it passes through the
radiator. Figure G-1 also makes note of important temperature sensors, which are used for the
verification and system tests that were described at depth throughout Section 3.
4.2. Major MHE Design Considerations and Microfluidic Chip Modelling
As detailed in Section G.3, a water-based nanofluid is desired for this particular application, as it is
minimally reactive with the copper microfluidic chip. Furthermore, the continued suspension of the
CNTs is of critical importance in the synthesis and use of the colloidal CNT solution. Both of these
considerations have already been taken into consideration in the creation of the functionalized CNTs for
the MHE, as detailed below in Section 5.1.2. Through various functionalization experiments, it was
determined that simple refluxing with nitric acid over a set period of time produced CNTs that were
reasonably soluble and viable for use in the MHE. In the preliminary tests of this experiment, detailed in
Section 5.2, it has been shown that these functionalized CNTs should remain disperse within the MHE.
As detailed in Section G.4, the main restriction on the radiator design is its size. Since this project makes
use of a somewhat costly nanofluid, it is desired to minimize the total volume of the system to reduce
the total amount of colloidal CNT nanofluid that needs to be prepared and functionalized. As such, it
was determined that the radiator should use 1/8th inch inner diameter (ID) copper tubing for the
fabrication of the radiator. In the actual fabrication of the radiator, 95cm of this thin tubing will be bent
after 10cm lengths to produce a radiator with reasonably high surface area, which can be attached to
copper fins and a fan for enhanced radiation to the environment. This design is illustrated in Figures G-4
and G-5 in Appendix G. This portion of the design project has yet to be completed.
21
In order to further minimize the total volume of the system, it was determined that 1/8th ID tubing
would be used to connect all major aspects of the design. This was determined through experimental
work, as detailed in Section 5.2, where the use of 1/4th ID tubing resulted in very large system volumes.
Switching to smaller diameter tubing results in a total theoretical minimum volume of the system as
18.6mL. Such a small amount of nanofluid is ideal, as it also reduces the safety risks associated with
using CNTs as the coolant within the MHE.
As detailed in Section G.5, it was determined that the primary micropump parameters would be
determined experimentally since the group has access to a mock microfluidic chip and a test
micropump. These values have largely been determined, and are discussed further in Section 5.2 of this
report. These values have also been used in the modelling of the flow within the microfluidic chip to
attain reasonably meaningful data regarding the behaviour of the fluid within the microchannels, as
detailed Appendix H. In particular, the flow rates that were experimentally determined were used in
the initial conditions of the inlet of the microfluidic chip in the COMSOL model.
The design parameters for the microfluidic chip are detailed at depth in Section G.2 of Appendix G. In
this regard, the lengths of the microfluidic channels were taken at 4cm, slightly longer than the
encapsulation used in modern CPUs. Furthermore, the importance of drilling the inlet and outlet wells
slightly lower than the channels and the inclusion of an o-ring to prevent leakage are noted. The deeper
inlet and outlet wells may increase the possibility for turbulence within the MHE, which should
accomplish two important tasks:
- The turbulence will mix the nanofluid, further dispersing the CNTs prior to the entrance into the
microchannels.
- The turbulence will ensure reasonably even distribution of the nanofluid throughout the
channels. This has been taken into consideration in the modelling of the microfluidic chip, as
detailed in Appendix H.
With regard to the microfluidic chip, it was also determined that minimum channel widths and pitches
of 200m were necessary in order to ease the fabrication of the microfluidic chip. An example cross-
section of the microfluidic chip is shown in Figure G-2 of Appendix G. In the channel simulations in
22
Appendix H, channel widths and pitches of 250m are used instead of this minimum value. Using a
larger channel width serves to decrease the contact surface area between the nanofluid and the walls of
the microfluidic chip, resulting in decreased heat transfer. However, the larger channel widths should
reduce the likelihood of clogging. Furthermore, the use of a high thermal conductivity nanofluid should
offset these losses to heat transfer.
Taking all of the above design specifications into consideration, a finalized model containing both the
fluid flow and heat transfer mechanics was produced, as detailed in Appendix H. This model showed
that despite the inclusion of lowered inlet and outlet wells, some degree of variation in the channel
velocities will always be present. The variation in velocities, however, did not significantly affect the
finalized heat profiles, as shown in Figure H-7 in Appendix H, where variations in temperature across the
channels did not exceed 1oC.
Most importantly, the model discussed in Appendix H verified the baseline efficacy of the MHE design
with water as the choice coolant fluid. In particular, it was seen that when the MHE is used, a CPU
generating 75W of energy resulted in only a 20oC increase in the operating temperature of the device.
This corresponds to a thermal resistance of only 0.27oC/W, which is well below the primary thermal
requirements established earlier in Sections 1 and 3. Furthermore, the system remained below the
maximum allowed temperature of 50oC. This theoretical validation of the most important primary
requirements of this project is excellent news for the overall outlook of the project.
23
5. Completion of the MHE Design to Date and Revised Timelines
5.1. Overview of the Colloidal System
For this design to be successful, it is required that stable suspension of the desired nanomaterial is
achieved and overcomes the challenges in detailed in Appendix B. To obtain the desired nanofluid, the
fluid must meet the specifications outlined in Appendix D. This section outlines the approach taken to
achieve the desired nanofluid, and the current status of this project component.
5.1.1. Determination of the Colloidal System for Use in the MHE
From literature searches, it was found that spherical insulating nanoparticles, such as SiO2, CuO and
Al2O3 nanoparticles, may be viable choices for the nanofluid in the MHE due to their high stability and
resistance to chemical degradation. The governing factor for the thermal conductivity of a nanofluid has
been determined to be due to the Brownian motion of the nanoparticles within the fluid. Consequently,
the thermal conductivity of the solution may be improved simply by decreasing the size of the
nanoparticles [1]. Furthermore, given this behaviour, the insulating nature of the bulk material that
comprises the nanoparticles may be disregarded if the nanoparticles can be synthesized at very small
sizes and remain stable at high temperatures. Previous studies have shown aqueous colloidal solutions
of Al2O3 and CuO at 3% volume fractions to have improvements in thermal conductivity over water by
6% and 10% respectively [2].
From literature searches, carbon nanotubes were also determined as a possible choice for the nanofluid.
CNTs suspend reasonably well in aqueous media when they are properly functionalized, and they are
very stable and resistant to chemical degradation. Their relatively simple acquisition and subsequent
functionalization, as will be discussed in greater detail in Section 5.1.2 below, also makes them an ideal
material for this system. Dispersed carbon nanotubes have been shown to improve the thermal
conductivity of their base fluid significantly, with literature values showing improvements over water by
20% at 1.14% weight fractions. At even higher loadings, approaching the limits of the functional
specifications noted in Appendix D, this improvement may feasibly be further increased.
24
Given the above considerations, an aqueous colloidal system of CNTs was determined to be the ideal
nanofluid for use in the MHE. As such, it was first considered for its applicability in this project, as
detailed below. If the system exhibits any critical failures in further verification or tests, as detailed by
the system verification and test plans in Appendices E and F, the alternative colloidal solutions
described above will be examined in greater detail. However, given the promising data described in
Sections 5.1.2 and 5.2 below, it is unlikely that such failures should occur.
5.1.2. Synthesis and Functionalization of CNTs
It has been determined that single walled carbon nanotubes (SWNT) suspended in water will serve as
the nanofluid for this project. As detailed directly above and in Appendices A and G, dispersed SWNTs
have the potential to provide the required heat conductivity to meet the customer requirements.
However, SWNTs are hydrophobic materials and do not disperse well in water. In order to overcome
this problem, the SWNTs were functionalized in order to create oxide-groups on their surface, allowing
them to become hydrophilic and dispersible in water with minimal aggregation. This was achieved
through an acid bath treatment, with the current optimized procedure performed as follows:
1. Fill a 500 mL single-neck round bottom flask (RBF) with 56.25 mL of 16M nitric acid (HNO3) and 93.75
mL deionized water to give an acid concentration of 16M
2. Add 1 g of AP-SWNTs to the RBF solution
3. Assemble the RBF into an apparatus as shown below in Figure 1 – Note: the addition of a reflux
condenser keeps the volume of the solution constant as the reaction occurs
4. Place the RBF into a sand bath and water turned on to flow through the condenser
5. Stir the solution lightly (warning – heavy stirring will deposit the SWNTs along the RBF walls), and heat
the vessel to 100 °C
6. The reaction has started once the solution begins to reflux
7. Let reaction proceed for 6 hours. One will observe a dull red gas form and cover the inner contents of
the reaction apparatus. This is the decomposition of the HNO3. Ensure that liquid drips from the
condenser back into the RBF
8. After 6 hours of refluxing, quench the reaction with 200 mL of deionized H2O. This stops the reaction
and prevents the formation of amorphous carbon
25
9. Filter the SWNTs from the solution using a 0.08 µm PTFE filter, and perform suction filtration with
deionized water. Overall, the total amount of water used to filter and wash the SWNTs should be near
1.5 L, after which time the filtrate should read a pH of 6-6.5 (the pH of the water used). Only when the
pH is close to neutral are the SWNTs rid of any acid residue
10. Cover the SWNT “cake” left on the PTFE filter paper and dry it in a 60 °C oven over night. The final
yield from ~ 1g of SWNTs was ~550 mg in the original run through this experiment.
Post-functionalization steps: The above yield was lightly ground via pestle and motor and placed in a
measured amount of water to give a desired concentration. Sonication was used for approximately 10
minutes to disperse the SWNTs.
Figure 1 – Apparatus used for functionalization of SWNTs
The above synthesis procedure is the optimized route thus far to obtain a sizeable portion of dispersed
SWNTs in solution. The losses in yield are attributed to the fact that the SWNTs used had ~30% catalysts
by weight as part of their composition, which were removed during the synthesis procedure through the
26
acid bath. The remaining weight loss can be due to sample lost that was stuck to the RBF vessel, and lost
through filtration.
Previous attempts at functionalizing the SWNTs involved similar processes, but to which certain reaction
parameters proved to create an undesired product. It was discovered that having the reaction continue
for extended periods of time (~2 days) results in the SWNTs becoming over-oxidized, resulting in the
SWNTs being transformed into amorphous carbon. This amorphous carbon tends to aggregate much
more than the SWNTs, creating large particle sizes which are not dispersible in water to any degree. The
same effect happens if the acid concentration used for the synthesis is too high (~10M).
Furthermore, due to the small size of the SWNTs, other attempts at isolating the functionalized SWNTs
from solution were performed, largely producing negative results. At first, centrifugation was performed
to isolate and wash the SWNTs. However, this resulted in a large amount of product being lost while
decanting the wash, with the pH of the solution being very difficult to bring to the desired neutral range.
Filters other than PTFE filters caused the SWNTs to become entangled in the filter, becoming
irremovable. Simply adding sodium hydroxide (NaOH) to the solution as well to make it neutral, hence
removing all filtration steps,, did not work. Adding NaOH resulted in the creation of large amounts of
NaCl, and caused the SWNTs to attract together, forming large, non-dispersible aggregates.
Future attempts in terms of dispersing the SWNTs will depend on their performance in terms of both
heat conductivity and their continuous flow through the microchannels. Initial studies have shown that
they are able to travel through the microchannels without any clogging. Additionally, anionic surfactant
such as sodium dodecyl sulphate (SDS) can be used to increase the dispersion of the SWNTs. However,
this may come at a cost of reduced thermal conductivity due to the addition of non-conductive organic
molecules to the solution. Future attempts at further optimizing the synthesis will only be performed if
the SWNTs fail the verification tests described in Section 3 above.
27
5.1.3. Characterization and Verification of the CNT Solution
The success of this preparation was investigated using the verification plan outlined in Appendix E and
discussed in Section 3. It was found that upon conducting the short-term particle stability test,
approximately 25-40% of the SWNT would fall out of solution and settle. However, mild agitation of the
solution would re-suspend the SWNTs without any difficulty. Similar results occurred for long term
stability verification. There was no observable change in colour of the actual fluid itself. For the time
being, the solution will be kept in its current state for testing. Diluting the solution to potentially
increase the amount of SWNTs that stay in the water is an option, but is not desired due to the decrease
in the conductivity that would inevitably occur. Acid treating the sample again to re-functionalize the
surface of the insoluble SWNTs is not an option since literature shows that the SWNTs will begin to
degrade and become amorphous carbon after extended acid treatments.
While the SWNTs did not stay fully suspended in solution, the slight agitation needed to re-suspend the
SWNTs may overcome the problem. In this regard, the turbulence in the cooling radiator and at the
inlet of the microfluidic chip may be enough to re-suspend the SWNTs. In order to verify if this was a
possibility, the solution was run through the mock microfluidic chip at about 0.5% w/w (due to the
amount of volume required of the system and the amount of SWNTs available). It was found that the
fluid was able to pass through the microchannels unimpeded for approximately ten minutes. Upon
inspection of the microchannels afterwards, it was found that some very minimal sediment was present,
but this did not prevent flow along any of the microchannels. Therefore, it was determined that short-
term stability issues can be overcome due to turbulence associated with the fluid flow system.
The next stages of the colloidal suspension are to determine its ability to dissipate heat by running the
fluid through a fully functioning apparatus. If the colloidal solution does not meet the thermal resistivity
requirements, the current concentration of 0.5% w/w needs to be increased. It is currently unknown
whether this concentration will be able to pass through the microchannels. Failure to pass through
channels will lead us to the addition of surfactant to increase solubility. This has potential to decrease
the heat conductivity of the fluid by a small amount, but may be necessary to avoid catastrophic failure
caused by channel clogging.
28
5.2. Overview of the Test Setup and Determined Test Parameters
For this design to be successful, it is required that the microfluidic channels overcome the challenges
outlined in Appendix B. In order to obtain the desired functionality of the system as outlined in
Appendix D, a test bed was created to obtain the appropriate test parameters. This section outlines the
approach taken to build a test set up that is able to obtain specific data from the system, and outlines
the results of preliminary testing.
5.2.1. Description of the Original Test Setup
A test set up was provided from Elmer Galvis, a supervising graduate student for this project. The
original test set up was created with 1/4 inch tubing, and a block diagram of the setup is shown in Figure
2.
Micropump
MHE
Radiator
Filter Flow Meter
T
T
Thermocouple
Thermocouple
Differential
Pressure
Meter
Figure 2 - Block Diagram of Original Test Setup
An initial test flow was conducted using this test setup. A mock channel was provided by Elmer and can
be seen in Figure 3. The mock channel consisted of 200μm wide channels with dimensions that fit withal
of the functional and design specifications noted in Sections 3 and 4.
29
Figure 3 – Mock Microfluidic Channel
5.2.2. Initial Determination of Test Parameters
The main goals of this portion of the project were to become familiar with the system and to ensure that
there were no detectable leaks in the system. Past problems associated with the setup involved the
accumulation of dust in the microfluidic channel, as suggested by Elmer. The dust would fall into the
open water reservoir, and would accumulate in the channels. Using a filter helped to alleviate this
problem.
From initial testing with water, it was determined that too much fluid was in the system for our
particular application. Without the inclusion of the radiator, two methods were used to determine the
total volume of fluid in the system. The first and most accurate way of determining the volume of the
system was to fill the entire system with fluid. The reservoir was detached from the system and the fluid
was drained into a beaker. This was done twice. A volume of 100 mL ± 10 mL was recorded. The second
method of determining the volume of the system was to heavily agitate the system while the system
was running. This involves extreme shaking of the reservoir until bubbles appeared. Time was tracked
while the bubbles flowed throughout the entire system (28 seconds). The time was multiplied by the
flow rate of the system, tracked by the flow meter in units of mL/min (221 mL/min). Multiplying these
two values result in a volume of 103 mL.
From functional and design considerations discussed above, it was determined that the volume of the
system needs to be minimized. This is due to the relatively high cost of the nanofluids, especially with
an initial estimated concentration of the CNT solution (1% w/v). It was determined that the 1/4 inch ID
30
tubing was too wide, which resulted in large volume of the system. The tubing was minimized to 1/8
inch ID tubing. Theoretically, reducing the diameter in half will reduce the volume by 1/4. There was
also an excessive amount of tubing used in the original setup which was removed. The sensor that
monitors pressure difference is also extremely far from the source. Pressure differences caused by the
tubing could have a significant effect on the output readings.
In the end, an optimized system was created in order to use less fluid, monitor pressure drop more
effectively and to use less space in general. The test bed was heavily modified such that the pressure
sensor was located at the inlet and outlet of the microfluidic chip. This allows for measurement of direct
pressure difference. An updated test bed setup block diagram can be seen in Figure G-1 in Appendix G.
The fluid flow was measured using a flow meter. Using a 12 V bias on the micropump, a flow of 200
mL/min ± 10 mL/min was observed. The influence of pump warm up and fluctuating voltage supply
could have provided the drift in flow rate. The resultant volume of the set up, excluding the radiator is
approximately 35 mL. Similar tests to those stated earlier in this section were performed to obtain the
volume. The pressure drop across the channel was observed to be approximately 2 PSI.
5.2.3. Initial Heat Measurement Setup
In order to satisfy the design specifications a maximum of 0.5:C/W must be observed in the system. This
means that an output of 60 W will increase the temperature of the system 30:C above the ambient.
Considering 20:C as ambient, the maximum operating temperature must be in the regime of 50:C
maximum. The output wattage can be determined by the applied voltage, the resistance of the heater,
and the formula P=V/R2. The resistance was measured to be 4.4 Ω. Hence, to achieve 60 W, a voltage of
16.24 V must be applied.
In order to measure the temperature of the heater, a thermocouple must be present. At first, a
thermocouple was mounted onto the heater using Kapton tape. However, due to the size of the heater,
Kapton tape did not provide enough adhesive strength to combine the two components securely. In
order to overcome this problem, Elmer suggested the idea of soldering an additional aluminum layer on
top of the existing heater and use epoxy at the interface of machined aluminum blocks to secure the
thermocouple into place.
31
The microfluidic chip was placed on the heater and clamped down using approximately 3 pound/in2 of
force. A voltage supply of 9 V was applied to the heater, resulting in a temperature increase to 70:C.
Boosting the applied voltage to 10 V resulted in a further temperature increase to 80:C. Although the
CNT solution was not used, the results were troubling, as with just the basic fluid, the device could not
operate at 50:C at a power output of 22 W. Upon further inspection, it was determined that the
contact was not very good. A small addition to the force used to compress the heater and the
microfluidic chip was applied, resulting in a drop of 3 degrees over 30 seconds. From this data, it was
concluded that a better thermal interface was needed. Elmer suggested that a graphite interface layer
should be placed between the heater and the microfluidic chip. This suggestion will be carried out in the
following week of the design schedule.
5.2.4. Verification of Fluid Flow using the Test Setup
As explained in Section 5.1.3, the nanofluid was used to determine the stability of the particles. This
section explains the alterations made on the test setup to accommodate the nanoparticles. The 1/8 inch
inner diameter tubing is already used in order to minimize the volume of the system. The effective
volume approximately decreased by a factor of 3. While the flow of the nanoparticles through the
microchannel resulted in no permanent clogs, the filter used in the test system caught many of the
particles. The physical appearance of the white filter after testing showed areas of black particles. In
order to remove this problem, two solutions were considered. Firstly, a new filter can be obtained to
increase the pore size, allowing a larger amount of particles to pass through. However, ordering a new
filter would result in a delay in experimentation.
A second consideration was made. The filter could be removed from the system. The original use of the
filter was to eliminate the dust particles that accumulate in the open water reservoir. If a new and
enclosed reservoir was made, no dust particles would enter the system. In this regard, a 125 mL
Nalgene bottle was purchased. Two holes were drilled into the bottle, one at the bottom, and one near
the top. Epoxy was used to secure tube connectors into the drilled holes. The bottle was uncapped and
nanofluids were added to the system. The lid was reapplied to seal off the system. A test was run
performed on the system. No observable clogs were seen in the channels.
32
5.3. New Timeline for the Completion of the MHE
As outlined throughout Section 2 of this report numerous alterations to the timeline for completion of
the MHE have taken place. Table 1 on the following page shows the alterations to the original timeline.
It should be noted that completed tasks have not been updated to show true completion times. Rather,
tasks which have yet to be completed have been updated to show new projected completion dates.
Tasks highlighted in green are completed to date. Tasks highlighted in red were originally scheduled for
completion by submission of this report, but have yet to be completed. The task in pink indicates a task
partially completed at the time of submission. Finally tasks in orange have been delayed from the
original plan; however these tasks were not initially planned to be completed by submission of this
document.
One key alteration to the design timeline is related to pump purchasing and validation. The availability
of a micropump for use in Professor Culham's laboratory significantly decreased the original high priority
for acquisition of a pump. Validation of this pump was completed as outlined in Appendix H, along with
simulation that indicates that this pump should meet primary design requirements. This acquisition has
allowed for continued work on the project without delay caused by pump purchase and delivery,
previously acting as a bottle neck to the completion of any experimental tasks.
Secondly, since the validation of the nanofluid in a mock microfluidic chip has not yet been fully
completed, final microchannel design cannot be completed and thus milling has been delayed.
However, the assistance and expertise of Elmer Galvis will significantly reduce the time for milling of the
channel. This has subsequently pushed back SEM characterization and stress testing of the
microchannels.
The end result of these time line alterations has reduced the time allocated for final testing and
benchmarking from 4 weeks to 2 weeks. It is believed that this should be sufficient for completion of
these specific tasks outlined in Appendix F. Project completion is anticipated within the design cycle
given these alterations to the project timeline.
33
Table 1 - Updated Timeline for Project Completion
Sept
Oct
Nov
Dec
Jan
Feb
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Tasks 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Synthesis, Modificatiion and Suspension of Particles
x
X X X
X
Heat sink channel modeling of flow of liquid
x
X X X
X
Test: Validation of Particle Morphology and Suspension
x
X X X
X
Test: Validation of Particle Morphology and Suspension
x
X X X
X
Test: Validation of Nanofluid Performance within a Mock Microchannel
x
X X X
X
Diffusion Modeling and Simulations
x
X X X
X
Heat Dissapation Modeling and Simulation
x
X X X
X
Pump Purchase
x
X X X
X Test: Validation of Pump Parameters
x
X X X
X
Test: Use of Pump with Mock Channel and Nanofluid*
x
X X X
X
Machining of Microchannels for the Heat Sink
x
X X X
X Machining of Radiator
x
X X X
X
Test: Flow of liquid through channels with pump*
x
X X X
X
Test: Uniform particle dispersion
x
X X X
X
Test: SEM characterization of channels
x
X X X
X Test: Micro-channel stress tests
x
X X X
X
Test: Long term stability
x
X X X
X
Assemble Prototype
x
X X X
X Final testing and benchmarking
x
X X X
X
TASKS LEGEND
DATES LEGEND
Completed from Plan
Completed Partially Completed from Plan
Original
Incompleted from Plan
Revised Delayed from Plan
34
6. Conclusions and Further Areas Required for Project Completion
In Section 1 of this document, the initial problem of CPU heat generation was presented, and a broad
overview of one potential solution involving the use of microfluidics combined with a high thermal
conductive nanofluid was suggested. The device that would be constructed throughout this project was
termed the MHE. Section 2 served to elaborate the major aspects of the MHE design process, and
described a timeline to effectively produce this high efficiency CPU heat sink. Section 3 provided the
functional details of the MHE and suggested methods for testing both the individual components of the
MHE and the fully integrated system. Section 4 provided the specific design details for the four major
components to the design of the MHE with specific notes on modelling of the system. The remainder of
the report has been dedicated to providing some of the preliminary experimental and verification data
achieved throughout the first half of the project completion timeline.
Work on the nanoparticle suspension was completed and initial results suggest that the nanofluid
appears stable. Some flocculation of the particles is seen if the solution is left to settle; however,
particles are easily re-suspended with slight agitation. It is believed that turbulence in the radiator as
well as inlet of the MHE should provide sufficient agitation to prevent sedimentation. Early validation of
the nanofluid in the mock microfluidic chip confirms this suspicion. Suspension of the CNTs in the
weight percent ranges required for this project was possible without the addition of the originally
planned surfactant.
Simulation and modelling of the system was completed, albeit with several simplifications, and is
outlined in Appendix H. Initial project planning indicated the use of simulations as a key data piece for
completion of the design; however, difficulty in correctly modelling the full system places modelling
results as a source of design reference as opposed to a driving force for the design. The acquisition of a
test micropump and a mock microchannel from the project supervisor has also decreased the necessity
of modelling for determination of MHE system parameters. Current simulations, however, do show that
a water-based MHE should provide sufficient cooling. Therefore, it is believed the nanofluid shall
provide sufficient improvement to outweigh the non-idealities of the simulation.
35
In-lab experimentation showed that proper thermal contact between the MHE and the heat source is a
primary immediate concern for the project. Contact specifications were identified as per Intel
documentation and driven into design specifications as outlined in Appendix G. This shall prove a key
issue in ensuring adequate performance of the MHE.
Work done to date indicates that customer requirements as per Appendix A and functional
specifications as per Appendix D will be met. The project plan appears to be on budget as outlined in
Appendix B showing sources of chemicals and lab space. Group members have also adapted to their
specializations and tasks in order to meet the needs of the current design project requirements, as
detailed in Section 2.
The main priority in moving the project forward is completion of the validation of the nanofluid
performance. Following this, the purchase of a micropump must be completed. Concurrent to
nanofluid validation, channel design must be finalized and machining of microchannels must take place.
A major design trade-off or consideration will be made in maximizing surface area for contact (minimum
channel width) while ensuring no clogging of the microchannels with the nanofluid (increasing channel
width). Extreme caution in prototype assembly must be taken to ensure thermal contact is maximized.
While the project remains behind the original outlined schedule as seen in Appendix B, the design flow
in Appendix C is being followed reasonably closely. Alterations to the schedule to ensure successful and
on-time completion of the project have been summarized in Section Error! Reference source not found..
The design team has full confidence that they will complete the design by week 9 of the 4B term.
36
Works Cited
N.B. Some references may be repeated throughout the main document and the following appendices, as each appendix is built to act as a standalone document.
[1] Jang, S.P. and S.U.S. Choi, "Cooling performance of a microchannel heat sink with nanofluids", Applied Thermal Engineering, vol. 26, p. 2457-2463, 2006.
[2] Lee, S., S.U.S. Choi, S. Li, and J.A. Eastman, "Measuring thermal conductivity of fluids containing oxide nanoparticles", Journal of Heat Transfer-Transactions of the Asme, vol. 121, p. 280-289, 1999.
[A-1] Gwinn, J.P. and R.L. Webb, "Performance and testing of thermal interface materials", 34, vol. 215-222, 2003.
[A-2] Pastukhov, V.G. and Y.F. Maydanik, "Active coolers based on copper-water LHPs for desktop PC", Applied Thermal Engineering, vol. 29, p. 3140-3143, 2009.
[A-3] Rutter, D. "CPU Coolers Compared," Dan's Data April 2008. [Online] Available: http://www.dansdata.com/coolercomp.htm [Accessed: September 20, 2009].
[A-4] "Super Pi," Xtreme Systems February 2006. [Online] Available: http://www.xtremesystems.com/superpi/ [Accessed: September 20, 2009].
[A-5] Choi, S.U.S., Z.G. Zhang, W. Yu, F.E. Lockwood, and E.A. Grulke, "Anomalous thermal conductivity enhancement in nanotube suspensions", Applied Physics Letters, vol. 79, p. 2252-2254, 2001.
[A-6] Choi, T.Y., M.H. Maneshian, B. Kang, W.S. Chang, C.S. Han, and D. Poulikakos, "Measurement of the thermal conductivity of a water-based single-wall carbon nanotube colloidal suspension with a modified 3-omega method", Nanotechnology, vol. 20, p. 6, 2009.
[A-7] "FrostyTech Heatsink Review," Frosty Tech September 2009. [Online] Available: http://www.frostytech.com/articlesearch.cfm?Category=198&CategorySearch=Get+Listing [Accessed: September 20, 2009].
[G-1] Khan, A.W., R.J. Culham, and M.M. Yovanovich, "Optimization of Microchannel Heat Sinks Using Entropy Generation Minimization Method", IEEE Transaction on Components and Packaging Technologies, vol. 32, p. 243-251, 2009.
[G-2] Choi, S.U.S., Z.G. Zhang, W. Yu, F.E. Lockwood, and E.A. Grulke, "Anomalous thermal conductivity enhancement in nanotube suspensions", Applied Physics Letters, vol. 79, p. 2252-2254, 2001.
[G-3] Choi, T.Y., M.H. Maneshian, B. Kang, W.S. Chang, C.S. Han, and D. Poulikakos, "Measurement of the thermal conductivity of a water-based single-wall carbon nanotube colloidal suspension with a modified 3-omega method", Nanotechnology, vol. 20, p. 6, 2009.
[G-4] "Intel Core 2 Duo Processor E8000 and E7000 Series and Intel Pentium Dual-Core Processor E6000 and E5000 Series, and Intel Celeron Processor E3x00 Series Thermal and Mechanical Design Guidelines," Intel, Document Number: 318734-001, August 2009.
A-1
Appendix A – Customer Requirements
A.1 Preamble
The goal of this design project is to develop a heat sink technology for both consumer-based and next
generation, high efficiency central processing units (CPUs). In particular, a microfluidic liquid cooling
approach with the use of a high thermal conductivity nanofluid is to be investigated and applied to
current CPU technology in order to more effectively remove heat from computer chips. This project
serves as an initial investigation into the feasibility of nanofluid-based microfluidics for heat removal
from CPUs. It is therefore suggested that the most basic requirements for the project adhere to
standards set by commercial, copper fin, fan-based cooling systems. Furthermore, this microfluidic heat
exchanger should be easily interchangeable with simple heat sinks, by mounting and clamping directly
on the CPU, as illustrated in Figure A-1 below.
Figure A-1 – Illustration of a Typical Heat Sink Mounted to a CPU
This design project is to be completed in direct response to ongoing miniaturization of CPU transistors
and their associated gains in heat output. The empirical relation known as Moore’s Law has allowed for
the doubling of transistors incorporated into CPUs every year since 1971, as shown in Figure A-2 below.
Figure A-2 – Graphical Representation of Moore’s Law for Increasing Transistor Count
As transistors approach nanoscale dimensions, device oddities and inefficiencies can lead to higher
leakage currents. When coupled with higher transistor densities, these next-generation CPUs result in
significant increases in heat generation. The progression from Intel Pentium II processor chips to Intel
A-2
Pentium IV 170-nm processor chips resulted in a decrease in die size from 25.4mm to 12.5mm, and an
associated gain in heat output from 33W to 80W [A-1]. Furthermore, more recent 45nm and 32nm
technologies are believed to approach heat outputs of 130W to 180W, with traditional fin-based, copper
heat sinks reaching their limits at 70-100W [A-2]. It is clear that alternative, advanced solutions will be
required to accommodate for future technologies.
A.2 Heat Removal Requirements
The heat removal requirements of the microfluidic heat exchanger detailed in this subsection are
Priority One requirements, as they outline the basic function of the design project. The average thermal
resistivity of current CPU heat sink technology averages at approximately 0.5 °C/W [A-3]. It is therefore
required that the microfluidic heat exchanger performs with a maximum average heat resistance to CPU
die of 0.5 °C/W. This deliverable can be confirmed by controlled temperature measurements with
thermocouples attached to CPUs running both with and without the microfluidic heat exchanger. The
CPUs should be stressed with a calculation-intensive program, such as Super Pi [A-4], during
temperature measurements. Specifications of the heat energy output of the processor of interest can
be determined with product specifications in order to perform the thermal resistance calculations.
In order for the nanofluid to be effective within the microfluidic heat exchanger, it must also meet basic
requirements. It is expected that the colloidal suspension of nanoparticles will achieve thermal
conductivities greater than water. Previous reports have shown a 2.5X increase in thermal conductivity
over the oil-based solvent with a 1% volume loading of CNTs [A-5]. For aqueous systems, an
improvement of thermal conductivity of 20% is more realistic [A-6]. In this design project, the nanofluid
is integrated into a much larger microfluidic system, and its specific thermal conductivity is not of great
concern for the success of the project. A more suitable test would be to compare the efficacy of a
water-based microfluidic heat exchanger to a nanofluid-based microfluidic heat exchanger. For this
design project, the microfluidic heat exchanger with a nanofluid running through it should see at least a
1.1-times decrease in thermal resistivity over a simple water solution.
A.3 Physical Dimension Design Constraints
It is required that the microfluidic heat exchanger itself has a smaller size profile than current heat sink
technology, which uses bulky copper fins attached to a fan. Top of the line CPU heat sinks have
A-3
dimensions on the order of approximately 150 mm x 120 mm x 100 mm [A-6]. The proposed design for
the microfluidic heat exchanger must have a slimmer design profile, with maximum dimensions of
60mm x 100mm x 80mm. This is a Priority One requirement. The required tubing and micropump to
operate the microfluidic flow must also be limited in physical dimensions in order to allow easy
accommodation around the remaining interior computer hardware. Dimensions for the heat sink and
required peripherals can be confirmed by the customer by direct measurements, and by the ease of
incorporation of the heat exchanger into a desktop personal computer. Further slimming to allow the
microfluidic heat exchanger to fit into a laptop case remains as a Tertiary/Non-Essential requirement.
A.4 Ease of Installation and Integration into Personal Computers
The microfluidic heat exchanger technology is anticipated to be used commercially in high performance
personal computers. The use of microfluidic cooling for CPUs is inherently more complicated than
traditional fan-based cooling systems; however, it is still crucial that no additional training or knowledge
of microfluidics is required for simple installation. Since computer hardware experts typically assemble
computers, the ease of installation and integration into current technology is a Priority Two
requirement. The microfluidic heat exchanger should be easily mounted onto and should make direct
contact with the computer chip. Furthermore, the micropump required for the microfluidic heat
exchanger must be powered by the power supply of the desktop computer, removing the need for an
external power source. As a final point of interest, it is desired that the micropump flow rate is
controllable by pump interfacing software, allowing for optimal heat removal. In order to confirm that
this requirement is met, a computer technician should report minimal difficulty in the attachment and
operation of the microfluidic heat exchanger on a personal computer.
A.5 Noise Level Requirements
The heat sink is required to provide lower noise than current heat sink technology. Typical fan-based
CPU heat sinks tend to operate in the noise range of 25 dB – 65 dB [A-6]. The microfluidic heat
exchanger should stay within the low end of this noise range, with a maximum noise of 50 dB. This is a
Priority Two design requirement. Since the majority of the noise in this design will come from the
micropump driving the fluid flow, it is necessary to select a pump that offers the required pressures,
while maintaining noise below 65 dB. This design requirement will be confirmed with the use of a sound
A-4
meter or through simple audible hearing tests. In general, it is necessary to ensure that the noise
generated by the microfluidic heat exchanger is not a distraction.
A.6 Safety and Green Requirements
The issue of safety has become a primary concern with new technological advances, especially with
regard to nanotechnology and the use of nanomaterials in commercial products. It is therefore required
that use of the microfluidic heat exchanger has absolutely no ill effects on one’s health or well-being.
This is a Priority One requirement. In order to ensure this goal is met, the materials used to create the
heat sink device must be present in low enough weight percentages to have little to no effect on human
health. For the purposes of this proof of concept design project, a maximum mass weight per volume of
3% should be used, although this value may have to be tweaked in later iterations of the model if it is
used in a commercial setting. Furthermore, the nanomaterials used in the microfluidic heat exchanger
must be properly sealed, in order to avoid device leakage. This requirement may be ensured by visual
inspection of the device itself, and with data accumulated regarding the materials used, their weight
percentages in the device, and their potential ill effects on human health.
In addition to maintaining a safe product, the materials used in the microfluidic heat exchanger should
have minimal impact to the environment. Since this is still the preliminary stage of this work, this green
requirement is a Tertiary/Non-Essential requirement. In order to validate this requirement,
documentation regarding the materials used in the device, their methods of fabrication and their
potential ill effects on the environment should be provided.
A.7 Summary of Specific Customer Requirements
A summary of the quantitative customer requirements is provided in Table A-1 below.
Table A-1 - Summary of Customer Design Specification Requirements
Design Specification Requirement
Maximum Dimensions 60mmx100mm (w/l) x 80mm (h)
Maximum Heat Resistance 0.5oC/W
Efficiency of CNT-based System w.r.t Water-based System
1.1x improvement in heat resistance
Maximum Noise < 65dB
Maximum Nanofluid Weight Percent (m/v) 3%
B-1
Appendix B – Project Plan and Milestones
B.1 Project Milestones and Tasks, and Assigned Group Member Duties
Appendix B serves to provide information regarding the timeline for project completion, with specific
milestones and tasks outlined and assigned to group members. This appendix also details the project
budget, including the required equipment, lab space, software, and supplies. The key project milestones
for the project are outlined in Table B-1. A more detailed project timeline, showing extensive details for
each milestone, is presented in Table B-2. The timeline and milestones have been created on a worst
foreseeable case basis. This assumes that several technical difficulties, as detailed in Section B.2, will be
encountered in the completion of the project. As such, a time scaling factor between two and five has
been applied to each milestone depending on the complexity of the task.
In Table B-1, the stated weeks correspond to weeks throughout the 4A and 4B fall and winter terms,
starting with week 1 as the week of September 13-19, 2009. This notion is further elaborated in Table B-
2, where the project week heading is shown below its corresponding monthly week heading. Based on
the assumptions generated by the time scaling factor, and the inclusion of breaks for midterm and final
examinations throughout the term, the project is expected to be completed in 24 weeks.
Table B-1 – Project Milestones
# Milestones Weeks Examined
1 Establishing Colloidal System and Verification of Suspension and Uniform Dispersion
1-9
2 Modeling and Simulations of Microfluidic Channels Nanofluid Dispersion and Heat Transfer
2-10
3 Machining of Micro-channel Heat Sink and Radiator 9-10
4 Testing of Micro-Channel Heat Sinks 11-18
5 Prototype Assembly 19
6 Final testing and Benchmarking 20-24
B.1.1 Project Milestones and Assigned Group Member Duties
The following assignments have been made with regard to the project milestones:
1. Robert Black, Graeme Williams, Geoffrey Lee
2. Yusuf Bismilla, Jason Wu, Graeme Williams
3. Yusuf Bismilla, Jason Wu
4. Yusuf Bismilla, Robert Black, Geoffrey Lee, Graeme Williams, Jason Wu
B-2
5. Geoffrey Lee, Jason Wu
6. Yusuf Bismilla, Robert Black, Geoffrey Lee, Graeme Williams, Jason Wu
B.1.2 Detailed Task Allocation for Group Members
Yusuf Bismilla Yusuf Bismilla is responsible for the modeling and simulation of the flow of both water and nanofluid
solutions through microfluidic channels. He will also provide support for group members Graeme
Williams and Jason Wu in nanofluid dispersion and heat dissipation modeling and simulation. These
simulations will be done in COMSOL Multiphysics, and are intended to assist in the determination and
verification of pump parameters.
Both Yusuf and Jason Wu are jointly responsible in the preparation of the CAD file for micromachining
the microchannels. Furthermore, both Yusuf and Jason will design and machine a simple radiator for the
finalized device.
With regard to the testing of the microchannel system, Yusuf is responsible for testing the mock channel
with the nanofluid solution, validation of the purchased pump parameters, and verification of proper
flow through channels with the purchased pump.
Robert Black Robert Black is primarily responsible for the synthesis, chemical modification, and suspension of
nanoparticles to achieve a stable colloidal solution with high thermal conductivity. He will then provide
support for the subsequent validation of particle morphology and proper nanoparticle suspension in
aqueous solution. Nanoparticle morphology will be confirmed with scanning electron microscopy or
atomic force microscopy.
With regard to the testing of the microchannel system, Robert will be responsible for the scanning
electron microscopy characterization of microchannels, and the determination of the long term stability
of the system.
Geoffrey Lee Geoffrey Lee will assist Robert Black in the synthesis, chemical modification, and suspension of
nanoparticles in the development of the nanofluid.
B-3
With regard to the testing of the microchannel system, Geoffrey will confirm the proper flow of liquid
through channels on the machined device and test for uniform particle dispersion. He is also
responsible for testing the purchased pump with the nanofluid in mock channels.
Both Geoffrey and Jason Wu are responsible for the assembly of the prototype system. This task
includes proper mounting of the system within a PC. At this stage, members must also ensure basic
device and computer functionality within the prototype system.
Graeme Williams Graeme Williams will also provide assistance to Robert Black in the synthesis, chemical modification,
and suspension of nanoparticles in the development of the nanofluid.
Graeme will be responsible for the modeling and simulation of heat dissipation in the microfluidic heat
exchanger. He will also provide support to Yusuf Bismilla in the modeling of fluid flow, and to Jason Wu
in the simulation of nanofluid dispersion in the channels. These simulations will be done in COMSOL
Multiphysics and are intended to assist in the determination and verification of pump parameters.
With regard to the testing of the microchannel system, Graeme will be responsible for testing for
uniform particle dispersion. He will also use scanning electron microscopy for the characterization of
the machined microchannels, and will be involved in subsequent stress tests to the microfluidic heat
exchanger.
Jason Wu Jason Wu is responsible for the modeling of the nanofluid dispersion in the microchannels. He will also
provide support to group members Yusuf Bismilla and Graeme Williams in the modeling and simulation
of fluid flow and heat dissipation. These simulations will be done in COMSOL Multiphysics and are
intended to assist in the determination and verification of pump parameters.
Both Jason and Yusuf Bismilla are jointly responsible in the preparation of the CAD file for
micromachining the microchannels. Furthermore, both Jason and Yusuf will design and machine a
simple radiator for the finalized device.
With regard to the testing of the microchannel system, Jason is responsible for the validation of the
pump parameters. In this role, he must ensure that the ordered pump can perform to its quoted values
prior to its integration with the system. Jason will also contribute toward microfluidic heat exchanger
stress tests and long term stability tests.
B-4
Both Jason and Geoffrey Lee are responsible for the assembly of the prototype system. This task
includes proper mounting of the system within a PC. At this stage, members must also ensure basic
device and computer functionality within the prototype system.
Final Benchmarking and Testing All team members will be responsible for the final benchmarking and testing of the system. This time
will be used to elucidate the characteristic behaviour of the microfluidic heat exchanger, allowing each
member to gain an understanding of all device parameters. Data regarding the device performance will
also be ascertained. This is intended to allow for all members to acquire an intimate knowledge of the
working device in order to be well-prepared to field questions regarding any aspects of the design,
including the previously delegated tasks.
B-5
Table B-2 – Detailed Project Breakdown
Sept Oct Nov Dec Jan Feb Mar Apr
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Tasks 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Synthesis, Chemical Modification and Suspension of Nanoparticles
X
X X X
X
Heat Sink Channel Modeling of the Flow of Liquids
X
X X X
X
Test: Validation of Particle Morphology and Suspension
X
X X X
X
Test: Validation of Nanofluid Performance within a Mock Microchannel
X
X X X
X
Determination of Pump Parameters
X
X X X
X
Diffusion Modeling and Simulation
X
X X X
X
Heat Dissipation Modeling and Simulation
X
X X X
X
Pump Purchase
X
X X X
X Test: Validation of Pump
Parameters
X
X X X
X Test: Use of Pump with Mock
Channel and Nanofluid
X
X X X
X Machining of Microchannels for
the Heat Sink
X
X X X
X Machining of Radiator
X
X X X
X
Test: SEM of Microchannels
X
X X X
X Test: Flow of Liquid Through
Channels with the Pump
X
X X X
X Test: Uniform Particle Dispersion
X
X X X
X
Test: Micro-channel Stress Tests
X
X X X
X Test: Long Term Stability
X
X X X
X
Assemble Prototype
X
X X X
X Final Testing and Benchmarking
X
X X X
X
B-6
B.2 Expected Design Challenges
As detailed in Section B.1, the timeline used in this design project assumes a worst-case scenario,
allowing for sufficient time to overcome design challenges. Some of the most notable design challenges,
with suggestions of methods to overcome the challenges, include:
Functionalization of nanotubes to ensure suspension over a long period of time
o Carbon nanotubes tend to settle out of solution, especially if they have not been
sufficiently treated with acids or oxidizing agents
o A portion of the budget has been allocated to the purchase of P2/P3 functionalized
carbon nanotubes, which suspend well in polar solvents
o Carbon nanotubes may also be treated with surfactant to increase their stability
Simulation of the fully-coupled system
o Highly-coupled systems, such as the microfluidic heat exchanger that couples variables
in the flow, mass diffusion and heat transfer profiles, are very difficult to solve through
traditional numerical methods with COMSOL
o It may be necessary to decouple the system and make simplifying assumptions in order
to attain reasonably accurate data regarding the operation of the device
Flow issues with the microfluidic channels
o Joint leakage can be minimized by precision machining and proper sealing
o Proper flow through all of the microfluidic channels can be addressed through modeling
of various input designs and precision milling
o Cross-channel leakage can be minimized by precision milling and planarization
Channel or micropump clogging
o The use of a nanofluid within fine tubing, 100-200m channels and the specialized
micropump may result in clogging and critical failure of the device
o Blank water solutions will therefore be run through the system at high pump speeds in
between nanofluid tests in order to remove nanoparticle residue
o Filters will also be used to remove agglomerated nanoparticles from the system
Heating of the micropump during its operation
o Micropump heating under continuous operation can lead to its failure
o A computer fan may be used to cool the micropump during operation
B-7
B.3 Project Equipment, Supplies, Space and Budgetary Considerations
B.3.1 Lab Space Requirements
In order to complete this project, lab space will be required to fabricate, characterize and test various
design components, including the nanofluid, microfluidic channel and radiator. These activities will be
conducted in the following lab space:
Microelectronics Heat Transfer Laboratory (MHTL) (E3-2133)
o Provided by consultant Professor Richard Culham
o For the fabrication of the microchannel and testing of the device performance
Dr. Linda Nazar’s Lab (ESC 131)
o For the functionalization of CNTs
Engineering Student Machine Shop (E3-2101)
o For the fabrication of the test bed
Undergraduate nanotechnology characterization labs (DC-3702)
o For the characterization of the microchannel array and the nanofluid
Undergraduate nanotechnology engineering electronics lab (E2-3346)
o For the testing and assembly of electronic components of the design
B.3.2 Equipment Requirements
The equipment requirements for this design project can be split into three major components:
fabrication, characterization, and testing. Table B-3 below summarizes the equipment required to
complete the project, with a brief description of each piece of equipment provided for clarity.
Table B-3 - Equipment Requirements
Equipment Purpose
Fabrication Milling Machine Fabrication of the microchannel Machine tools Fabrication of the microchannel Assorted machining equipment Fabrication of mounting device, case
modifications Characterization SEM Characterization of microchannel Optical Microscope Testing and characterization of
microchannel Thermocouples Temperature monitoring Testing Power Supply Powering the microfluidic pump Microfluidic pump Generating nanofluid flow
B-8
B.3.3 Software Requirements
The software required for the completion of the design project are all readily available through either
current University of Waterloo licenses, or are available as free software. The software requirements
are summarized in Table B-4 below.
Table B-4 - Software Requirements
Software Purpose
COMSOL Multiphysics Design and simulation of device AUTOCAD Creation of drawings for fabrication MATLAB Data analysis C-Compiler Programming of pump if required Super Pi Stressing the CPU for device testing
B.3.4 Requirements on Supplies
The supplies for this project can be divided in the same manner as the equipment described above, and
are summarized in Table B-5 below.
Table B-5 – Required Supplies
Supply Purpose
Nanofluid Ethylene Glycol Suspension liquid Sodium Lauryl Sulphate Suspension stabilizer Sulphuric Acid CNT functionalization Nitric Acid CNT functionalization Carbon Nanotubes Heat conductivity enhancer Device Copper Microchannel material Plastic Tubing Connection between MHE, pump, etc. Electrical Wire Connectors Connection to power source Radiator Radiation of heat Testing Thermal Paste Improving device-CPU contact Computer Test bed
B.3.5 Detailed Budget Outline
The expected budget for the design project, along with the supplier for the equipment, software and
materials, is provided below in Table B-6.
B-9
Table B-6 - Anticipated Budget for the Design Project
Item Supplier Cost
Nanofluid
Carbon Nanotubes CNT Solutions $238.10
Ethylene Glycol (1L) Nazar Lab --
Sulfuric Acid (500 mL) Nazar Lab --
Nitric Acid (500 mL) Nazar Lab --
SDS (10g) Nazar Lab --
Microfluidic Heat Exchanger
Copper Professor Culham $50.00
Machine Tools Professor Culham $250.00
CAM Design Professor Culham $250.00
Fluid Tubes (25 ft) Cole Parmer $35.47
Fluid Tube Fitters (2) Cole Parmer $70.00
Thermal Paste NCIX $7.99
Micro-Pump $1,300.00
External Labour
Machining Costs Professor Culham - -
Radiators Professor Culham - -
Simulation
COMSOL NE - -
MATLAB NE - -
Testing
Test Bed
-Dual Core Processor
-2 GB Ram
-80 gig Hard drive
-Motherboard & Power Supply
NCIX $337.10
Computer Heat Sink (Later Disassembled to be used as a clamp for MHE)
NCIX $50.95
Thermocouples and Temp. Profiler Previous Employers - -
Device Fabrication Equipment
- Drills
-Dremmels
-Milling Machine
Professor Culham
Thermal Testing Laboratory and Equipment Professor Culham - -
Material Characterization
-SEM
- Optical Microscope
NE Labs - -
Extra
- Super Pi
- Temperature Monitor
Internet Freeware - -
TOTAL $ 2,589.61
Shipping and Taxes (30%) $ 3,366.49
C-1
Appendix C – Design Flow
C-2
D-1
Appendix D – Functional Specifications
D.1 Functional Overview of the Microfluidic Heat Exchanger
The microfluidic heat exchanger (MHE) uses a colloidal suspension of nanoparticles (nanofluid) that
flows through microchannels in order to remove heat from a central processing unit (CPU). Pressure is
maintained within the microchannels with the use of a micropump, and the heat generated by the CPU
is pulled away and emitted into the environment with a simple radiator. The high level design for this
system is shown below in Figure D-1.
Figure D-1 - High Level Design of the Microfluidic Heat Exchanger (Arrows Indicate the Flow of Heat)
The high level design summarized above satisfies the MHE’s primary capacity to remove heat from CPUs
in an effective manner. Further functional requirements are summarized in Table D-1 below.
Table D-1 – Data Sheet: Minimum Functional Requirements (White) and Supplementary Requirements (Blue)
Function or Property Performance Specification Priority
Active Temperature Measurement Required 1
Operating Temperature < 50oC 1
Thermal Resistance of the MHE < 0.5oC/W 1
Thermal Resistance of Nanofluid w.r.t. Water 1.2 X therm-NF < therm-H2O 1
Dimensions of the Microfluidic Chip Maximum: 60mmX100mmX80mm 1
Nanomaterial Concentration < 3% by weight 1
Nanomaterial Data and Safety Info Available, included 1
Tubing Inlet to Microfluidic Chip Side – for slimmer vertical profile 3
Dimensions of the Microfluidic Chip for Laptop Maximum: 30mmX80mmX60mm 3
Voltage Source Required for MHE <= 12V 2
MHE Snap-On Clip Attachment to CPU Required 2
Noise Level of the MHE < 50dB 2
Supplementary Encasement for the MHE Required 3
Flow Modulation with Temperature Variation Enabled 3
D-2
D.2 Detailed Functional Specifications
For clarity, primary requirements are elaborated in detail below. Secondary and tertiary requirements
are noted as “optional” throughout the following sections. Only functional requirements have been
considered throughout this document. Design details such as the specific test CPU heat output,
required flow rates, pressure of the microfluidic channels and microfluidic pump choice are examined in
detail in Appendix G: Design Specifications.
D.2.1 Heat Transfer Considerations
The efficiency of the MHE is measured in terms of thermal resistance, which refers to the difference in
temperature across the MHE for every watt of energy across it. In order to maintain and verify low
thermal resistance, the temperature of the system must be actively monitored and the heat output of
the CPU in watts must be known. The overall temperature of the MHE and CPU must always be
maintained below a critical CPU failure temperature. It follows that the performance of the MHE with a
nanofluid must be superior to the performance of the MHE with a simple water solution. The following
functional requirements and notes are then observed:
- Active temperature monitoring of the CPU with and without the MHE is required
- Temperature monitoring of the fluid before and after flowing through the microfluidic chip is
required
o Thermal resistance may be roughly estimated as:
𝑅𝑀𝐻𝐸 =∆𝑇
𝑄𝐶𝑃𝑈=
𝑇𝑎𝑓𝑡𝑒𝑟 − 𝑇𝑏𝑒𝑓𝑜𝑟𝑒
𝑄𝐶𝑃𝑈 (by spec's)
- Maximum monitored temperature: 50oC
o CPUs typically slow or fail at T>60oC; a 10oC buffer is given
o Otherwise, depending on the specific test CPU choice, CPUs should operate at
manufacturer’s recommended temperatures
- Maximum thermal resistance of 0.5oC/W
o This value meets current heat sink standards
o A processor with heat output of 60W (near current standards) yields a 30oC increase in
temperature over ambient temperatures; at 20oC ambient, the MHE temperature is
50oC = maximum allowable temperature
- Thermal resistance of water-based MHE > 1.1 X thermal resistance of nanofluid-based MHE
D-3
o Aqueous CNT nanofluids offer 1.2x increases in thermal conductivity over water; a 1.1x
factor is taken as an estimate for improvement over pure water, based on potential
design issues and considerations that are discussed in Appendix G: Design
Specifications
D.2.2 Microfluidic Chip Size and MHE Integration Considerations
The microfluidic chip must easily integrate into standard personal computers (PCs). PCs currently make
use of radiator-inclusive heat sinks with fans that blow air across bulky aluminum or copper fins. This
style of direct-radiator heat sink has dimensions on the order of 150x120x100mm. The microfluidic chip
must surpass these design requirements to allow room for inlet and outlet tubing, which will enter the
chip from the top-side of the chip. The length and width of the microfluidic chip may also be reduced, as
less area is needed because the radiator is placed away from the chip. A schematic of the microfluidic
chip design is shown below in Figure D-2.
Figure D-2 – Schematic of the Microfluidic Chip – Maximum Size Constraints
As detailed above, the following functional size constraints are suggested:
- Microfluidic chip width X length: 80mmX100mm
- Microfluidic chip height: 60mm
- Inlet/outlet tube radius: ¼ inch = 6.35mm
- Inlet/outlet maximum height: 30mm
Optional: In order to meet laptop design considerations, the following constraints may be met:
- Inlet and outlet connections made at the side of the microfluidic chip
D-4
o this allows for a much smaller vertical profile, but may be more difficult to accomplish
from a design standpoint, as detailed in Appendix G: Design Specifications
- Microfluidic chip width X length: 60mmX80mm
- Microfluidic chip height: 30mm
- Inlet/outlet tube radius: ¼ inch = 6.35mm
- Placement of the radiator and micropump outside of the laptop casing
Optional: In order to integrate the MHE with the CPU in the simplest manner, the following constraints
may be made:
- Micropump operating voltage <= 12V
- Micropump powered through the internal computer power supply
o this constraint is limited by the micropump choice and purchase, which is primarily a
design consideration
o an external power supply that may be mounted inside the computer case must be used
if the micropump requires a voltage greater than 12V
- Simple clip-on attachment of the microfluidic chip to the CPU
o thermal paste will likely be required to ensure good contact between the CPU and the
microfluidic chip
Optional: Micropump flow may be controlled based on the temperature measurements in order to
better integrate the MHE with the CPU and to ensure a constant operating temperature. The following
functional requirements are made in order to achieve this feature:
- this constraint assumes a flow-limited heat removal regime, where heat is transferred to the
solution faster than the solution can be moved onto the radiator
o as the temperature increases, the flow must be increased to improve heat removal from
the microfluidic chip
- the pump may be programmed to modulate its applied pressure as a function of active
temperature measurements, thereby maintaining a constant CPU temperature
o this factor is strongly dependent on the pump choice, which remains a design
consideration
D-5
D.2.3 Safety and Noise Considerations
Many nanomaterials have been identified as potentially toxic substances to humans due to their
capacity to diffuse through cell membranes. It is paramount that the project is not harmful to the user
or the members involved in the fabrication of the MHE. The following constraints and notes are made:
- all nanomaterials are thoroughly researched and investigated for both short term and long term
toxic effects
- nanomaterial concentration in solution: < 3% by weight
o typical nanosolutions are made at concentrations < 2% by weight , with most much
lower (< 0.5% by weight)
o this limitation should not affect device performance
- all seals must be ensured to be leak-proof prior to operation of the MHE
- All electrical connections need to be well insulated to prevent shock to the user or damage to
the computer system.
- Optional: Add an additional plastic encasement around the MHE and CPU to prevent spreading
of the nanofluid if a leak does occur during operation
Optional: Since the MHE does not use a fan, it is anticipated that it will be quieter than current heat sink
technology. However, further functional requirements are made in order to ensure the MHE is not
distracting:
- MHE noise level < 50dB
o this noise level is typical of current heat sink technology
E-1
Appendix E - Verification Plan for the Paper Design Document
This appendix is intended to outline the verification steps planned to ensure the validity of the design of
the microfluidic heat exchanger (MHE). In order to prevent any critical design flaws, the various
components of the system are verified both prior to and after integration. The high level overview of
the verification steps has been previously summarized briefly in Appendix C: Design Flow. This
document will elaborate these verification steps with specifics on their completion, including their
implementation for this specific project and the subsequent attainment of meaningful data.
E.1 Nanoparticle Verification
E.1.1 Short-Term Particle Solubility
In order to ensure proper suspension of the nanoparticles within solution, a short-term solubility test
will be completed. Following particle synthesis, the nanoparticle solution will be left in a scintillation vial
over a period of 24 hours. The vial will be periodically observed for any sedimentation or "crashing out"
of the nanoparticles. Observations will be made both qualitatively and quantitatively through UV-Vis
measurements.
For UV-Vis measurements, absorption curves over the near-UV and visible spectrum (~300nm to
~800nm) will be made immediately after fabrication. For carbon-based nanosolution, such as a colloidal
CNT solution, the absorption curve will show a broad absorption band over the entire visible spectrum.
For colloidal semiconducting nanoparticles, strong absorption bands will be witnessed at energies
corresponding to the semiconductor band gaps, near the blue end of the visible spectrum or the near-
UV regime. Over the 24 hour test period, subsequent UV-Vis measurements will be made and
compared to the initial absorption curve. If the nanoparticles grow in size, a shift in the absorption
bands will be witnessed. However, if the particles begin to agglomerate, the absorption bands will begin
to weaken in intensity and then disappear entirely.
While some sedimentation is anticipated, significant crashing out implies error within the particle
synthesis or insufficient solubility of the chosen nanoparticles. Allowing for some degree of crashing
out, the particles should be able to re-suspend with a small amount of agitation (solutions will be
agitated slightly prior to UV-Vis measurements). This agitation simulates the fluid flow that the
E-2
nanosolution would experience when it is in the microfluidic channels and the micropump is operating.
Over this relatively brief period of 24 hours, a decrease at the absorption band of interest greater than
20% of the original intensity indicates a test failure, as the nanoparticles have clearly agglomerated and
are unable to re-suspend in solution.
More than five failures of this verification step indicate either a design oversight or incorrect
assumptions with particle selection and synthesis. In the latter case, the nanosolution will lose its
enhanced thermal conductivity when the nanoparticles fall out of solution. As such, the nanosolution
will provide no gains over water and the design project will be unable to meet one of its primary
requirements. In addition to this critical failure, the nanoparticles that fall out of solution have the
potential to clog microfluidic channels and the micropump itself. This could cause additional critical
design failures later in the project. In the case of failure for this verification step, alternative synthesis
approaches and/or particles will be investigated in order to meet customer requirements.
E.1.2 Long-Term Particle Stability
If proper short-term stability of the nanoparticle solution is achieved, long-term stability tests will be
performed. These tests will comprise of week-long observations for sedimentation or "crashing out" of
the nanoparticles from the solution. The same qualitative observations and quantitative UV-Vis
experiments, as described above in Section E.1.1, will be performed to verify the stability of the
nanosolution. The overall allowed decrease in absorption intensity will be taken as 30%. If the particles
remain in solution for this duration, the nanosolution will be deemed satisfactory and able to meet
project stability requirements. As such, the particle choice and synthesis methods will be adopted for all
future project tests. Long-term stability testing of this nanosolution will continue until failure or end of
the design cycle in order to quantify the final stability of the nanoparticle solution.
As with the short term stability tests, complete failure for this test occurs if a given particle choice and
synthesis method have more than five observed failures. In this case, there has been a design oversight
or incorrect assumption in particle selection or synthesis method. In this case, alternative synthesis
approaches and/or particles will be investigated in order to meet customer requirements.
E-3
E.1.3 Nanosolution Thermal Conductivity
The original intent of the design project was to test the absolute thermal conductivity of the
nanosolution. However, the project supervisor has suggested that solution-based thermal conductivity
tests (such as the hot wire test) are poorly-defined, and comparisons to literature are nearly impossible
due to variations in test conditions and irreproducibility of results among different machines. As such, it
has been suggested that the performance of the nanofluid can be measured directly using a mock
microchannel and a resistive heater, as described below in Section E.2.1. The thermal dissipation
capabilities of the nanofluid will be tested and compared to water. Once a proper flow rate has been
determined both water and the nanosolution will be run through the mock microfluidic channel at a
constant flow rate, as described in Section E.3.2.
E.2 Test Bed Verification
E.2.1 Mock Microchannel Flow
In parallel with the nanoparticle verification, the microchannel flow concept will be confirmed on a test-
bed with a mock microchannel of similar dimensions to the chosen design and pump. This mock
microchannel and test micropump have been loaned to the group by the project supervisor for the
completion of this project. The mock microchannel will be placed in the test bed with a resistive heater
(of known heat output in watts) mounted below the channels in order to simulate the heat output of a
CPU.
The microchannel cooling concept will first be verified with water as the liquid coolant by determining
the flow rate required to keep temperatures below those specified in Appendix D: Functional
Specifications. This will be done by altering the flow rate of the micropump while monitoring the
thermocouple readout temperatures. This step is also critical in determining the pump parameters for
future microfluidic pump purchasing. This process is shown in Figure E-1 below.
E-4
Micropump Radiator
Mock Microchannel
Resistive Heater
Thermocouple Thermocouple
Fluid Flow
Heat Generation
Monitor
Differential
Pressure SensorFlow Rate Sensor
Figure E-1 – Test Bed Verification Block Diagram
Fluid temperature will be measured directly with k-type thermocouples and a digital multimeter. Data
will be recorded by hand at various intervals in order to determine the time to reach a steady state
temperature.
The micropump flow is controlled with a DC-voltage source operating between 0 and 24V. The flow rate
will be monitored through a flow rate sensor. The flow rate will be probed over the full range of applied
voltage values of the micropump, and a flow rate vs. voltage curve will be generated in order to properly
interpret future test data.
In order to ensure that steady state flow is reached, differential pressure sensors will compare the
pressure before and after entering the mock microchannel. When the differential pressure is seen to be
constant over 5 minutes, steady state flow can be assumed within the channel. Datasheet information
on the pressure sensor will convert a measured voltage, as determined by piezoelectric elements within
the pressure sensor, to a pressure drop in psi.
The resistive heater will be turned on in order to generate heat corresponding to the specifications
outlined in Appendix D: Functional Specifications. The flow rate will be adjusted until thermocouple
readings maintain a temperature at or below 50°C over a period of 1 hour. This flow rate will be
E-5
recorded in addition to the pressure drop across the microchannel required to maintain the constant
flow.
Completion of this verification step will allow for the determination of critical micropump specifications
that are necessary for purchasing. Furthermore, this step shows the functionality of the test bed prior
to integration with the nanosolution. This ensures that monitoring of various parameters can be
completed effectively in the simplest, baseline system. As such, any errors that are encountered while
testing with the nanoparticle solution infer incompatibility with the nanosolution and are not necessarily
a test bed design error. The data and test setup from this verification step will also be applied to
confirm the nanosolution's capacity to cool the CPU more effectively than water, as detailed in Section
E.3.2 below.
E.2.2 Pump Testing
The tests performed in Section 0 will be completed in order to verify that the purchased pump meets
the necessary requirements for this project. As such, the total pressure across the system and the
desired flow rates will be determined and confirmed prior to pump purchasing. Failure of the pump to
meet the outlined specifications will require consultation with the pump manufacturer in order to
isolate and correct pressure mismatches.
E.3 Microchannel Verification
E.3.1 Nanofluid Heat Dissipation Modelling
Nanofluid heat dissipation will be performed in a COMSOL model by coupling the Navier-Stokes
incompressible fluid flow module and the conductive and diffusion heat module. The basic microfluidic
chip will be replicated in COMSOL and the flow rate parameters determined in Section E.2.1 will be
applied to the model. Since the absolute thermal conductivity values of the nanofluid cannot be
measured directly, values will be taken from literature and applied to the nanofluid. In order to simplify
calculations, the nanofluid will initially be assumed to be a homogenous solution. This is a reasonable
estimate given that flow throughout the channels sufficiently agitates the solution and the nanosolution
passes the solubility verification steps listed in Section E.1.
E-6
The model will then introduce a heat source to the microfluidic chip, and the efficacy of the MHE and
the nanosolution will be confirmed from a theoretical standpoint. The primary goal of this step is to
verify the potential for the nanofluid as a coolant. In later steps, it will also serve to attain a rough
estimate of the nanosolution’s absolute thermal conductivity, as the final temperature values of the
experimental system can be used to elucidate the required thermal conductivities. Failure in this step
will warrant further research into the nanoparticle selection, synthesis and its weight loading in the
solution.
E.3.2 Diffusion of the Nanoparticles in the Mock Microchannel
As briefly described in Section E.1.3 and Section E.2.1, the nanofluid will also be run through the mock
microchannel in order to gain a rough estimate of its efficacy at heat removal when compared to water.
This testing will be performed in a similar manner to the method described in Section E.2.1. In addition
to the simple proof of concept tests, the temperature of both the nanofluid and water will be measured
at pre- and post-mock microchannel, and rough thermal resistance values will be calculated. These
values will indicate the relative performances of the two solutions, and following Appendix D:
Functional Specifications, the nanosolution must show at least a 1.2X improvement in thermal
resistance.
This verification step will also be extended over a longer period of time, so as to ensure that the device
is capable of operating for an extended length of time while maintaining relatively uniform flow rates.
This will verify that the nanoparticles do not clog the microchannels or the micropump, and confirm that
the stability testing completed earlier in Section E.1 is valid. This is the final verification step in ensuring
the correct nanofluid choice. Failure in this verification step warrants an alternative particle selection or
an alternative synthesis method. Failure in this verification step may also imply that the mock
microchannel and borrowed micropump are unsuitable for this project, and further alterations to the
project microfluidic chip design and proposed micropump purchase may be necessary.
E.3.3 Microchannel Fabrication
Verification of the automated machining process must be completed following microchannel machining
of the designed channel. This will comprise of inspection by optical microscopy and scanning electron
E-7
microscopy (SEM) in order to ensure dimensions are within tolerance specifications. Furthermore,
proper seals between the channel, the cover, and inlet and outlets will be checked for by flooding the
system with water and observing for leaks. Failure in this verification step will warrant a re-fabrication
of the microfluidic chip or modification to the microfluidic chip design depending on mode of failure.
E.4 Test Computer Verification
E.4.1 Power Supply
The computer power supply will be verified to be able to supply a constant voltage to the micropump. If
current or voltage requirements are not met, an external 12/24V power supply will be purchased to
ensure reliable operation of the MHE.
E.5 Radiator Verification
E.5.1 Heat Dissipation
Prior to construction of the prototype, the designed radiator will be verified in the test bed using water
as outlined in Section 0, with the designed microchannel, and purchased pump. Failure in this section
will be established if the radiator is unable to dissipate sufficient heat to meet the customer
requirements. This will ultimately lead to re-design or re-fabrication of the radiator. .
F-1
Appendix F - Test Plan for the Constructed Prototype Design Document
This appendix serves as a reference test plan to verify that the prototype of the microfluidic heat
exchanger (MHE) meets all the performance standards. These requirements, as detailed in Appendix A
– Customer Requirements and Appendix D – Functional Specifications, must be met by the final
prototype in order to ensure a successful design. This document also outlines the performance testing
methods that will be used on the final prototype in order to evaluate and validate final performance
specifications. This test plan is currently set to be performed beginning in week 20 of design.
PLEASE NOTE: These are all post-construction tests that are completed in order to confirm earlier
verification and validation of the individual design components (as detailed in Appendix E –
Verification Plan). These tests are concerned with the functionality of the components when they are
integrated into the finalized system, and hence, are contained within this particular appendix.
F.1 Prototype Testing Apparatus
The prototype of the MHE will be tested while integrated into a fully functioning desktop personal
computer (PC). The MHE will be secured to the CPU, and the external components will be placed
accordingly either on the interior or exterior of the desktop case. Figure F1 below shows a schematic of
the final testing apparatus within the PC assemblage. Once completed, each specific component of the
system will be monitored using the appropriate techniques and tools detailed in the subsequent
sections of this appendix. This will allow for the testing and verification of all the associated parameters
in real time and under normal operating conditions.
Figure F-4 - MHE and Associated Components Sitting in a CPU Tower
F-2
F.2 Compact Design and Installation
The overall MHE dimensions and accompanying extremities are required to have a low dimensional
profile that will allow ease of installation into a desktop tower, as outlined in Appendix D: Functional
Specifications. In the construction of the test apparatus, the compact design of the MHE and its user-
friendly installation will be ensured based upon is the ease of MHE encasement and attachment to the
CPU. Furthermore, the accompanying components of the MHE, such as the tubing, micropump and
radiator, will be visually tested to determine if they meet the requirements be concealed within the
computer case while maintaining the required performance specifications.
This section is considered a pass as long as the components are able to fit inside a standard ATX case
without interfering with any electrical components.
F.3 Microfluidic Heat Exchanger (MHE)
F.3.1 Heat Transfer
In order to monitor and verify the heat transfer from the CPU to the MHE, the PC will be run with a
stress-testing program (Super-Pi) that will cause the CPU to perform a large number of strenuous
calculations increasing both power consumption and heat output. The temperature change across the
MHE via thermocouples as well as the absolute temperature of the CPU will be monitored. The thermal
resistance value of the MHE device will be calculated and compared with the desired thermal resistance
value of 0.5°C/W.
The following steps will be taken in order to determine the thermal resistance of the MHE device:
1. Two thermocouples will be placed at the inlets and outlets of the MHE device.
2. The system will be allowed to run for 10 minutes while running SuperPi to allow the system to
reach steady state.
3. Temperature readings will be taken from both thermocouples.
4. Once both temperature readings are taken, the thermal resistance of the MHE device can be
calculated.
The temperature of the CPU with the MHE attachment, while maintaining constant heat output from
the CPU, will be noted at specific time intervals over 72 hours. Measurements will be made more
F-3
frequently at the beginning of the test, and will be taken over longer time intervals (~4 hours) as the test
nears its end.
The thermal resistance of the MHE, given the temperature change measurements above, will be
calculated using equation (f1) below.
𝑅𝑀𝐻𝐸 =∆𝑇
𝑄𝐶𝑃𝑈=
𝑇𝑎𝑓𝑡𝑒𝑟 −𝑇𝑏𝑒𝑓𝑜𝑟𝑒
𝑄𝐶𝑃𝑈 (f1)
, where Tbefore is the temperature is the temperature reading going into the MHE device, and Tafter is the
temperature reading exiting the MHE device. This is a reasonable estimate for the thermal resistance of
the device. QCPU is the amount of power produced by the CPU, which is obtained from the CPU’s
manufacturer’s specification document.
Additionally, it is required that the CPU does not exceed an operating temperature of 50°C. This
requirement is confirmed by monitoring the temperature of the CPU using the onboard CPU
temperature monitor during the 72 hour test time.
F.3.2 Sustainability and Durability
Potential durability issues that may occur with the MHE include joint leakages, cross-channel leakage,
clogging, and material deformation due to the heating of the MHE from the CPU. Each of these issues
may lead to catastrophic failure of the MHE. As such, the final prototype will be tested for these issues
through long-term operation within the desktop tower. The final prototype will be run through a
minimum of one week of operational stress testing (CPU under load) in a humid and warm environment
to simulate effects of material degradation. Thermocouples will be attached to the entrance and exit of
the MHE in order to observe the MHE performance during this time and to test for any performance
degradation. Degradation in heat transfer of the MHE can be isolated to any of the operation issues
mentioned above through physical inspection of the MHE after testing is performed.
Finally, the strength of the hold of the MHE device on the CPU will be tested by simply applying a force
of roughly 40 N in the four lateral directions. The MHE should remain reasonably stationary and stay
stable in its attachment with the application of this force.
F-4
F.3.4 Physical Dimensions
In accordance with having a low physical profile, the physical dimensions of the MHE will be measured
with callipers. The final device must be within 5% tolerance for all dimensions.
F.4 Micropump
F.4.1 Flow Rate
The efficiency of the MHE to properly transfer heat relies on an optimal fluid flow rate to maximize heat
transfer from the MHE into the fluid, as well as heat dissipation from the fluid through a radiator. The
chosen flow rate will be confirmed acceptable via monitoring of the temperature of the nanofluid and
CPU. Instability of these temperatures may indicate an improper flow rate in the final prototype. A flow
meter may be used in the final prototype design to ensure a consistent and adequate flow rate for heat
removal.
F.4.2 Operating Voltage
The micro-pump is to be operated through the power supply of the PC or if necessary an external power
supply. Therefore, it must have a minimum operating voltage and minimal noise. Verification of this
operating voltage can be tested through using the PC power supply as the power source, and ensuring
that an adequate and consistent flow rate is maintained using the methods outlined previously.
F.4.3 Durability and Sustainability
As with the MHE, the pump is supposed to be durable and functional for an extended period of time. To
test this, the pump parameters will be monitored for 72 hours with the MHE system in full operation.
Flow rates and input voltage will be monitored for uniformity during the time. Averages and standard
deviations of both the flow rate and input voltage will be calculated. To ensure uniformity in flow rates
and voltage, the standard deviations of both parameters should not exceed 25% of the average values.
The pump performance at the end of this period will be tested to ensure the pump maintains the
desired flow rate and hence meets the requirements of the design.
F-5
F.5 Radiator
F.5.1 Heat Dissipation
The radiator is required to dissipate sufficient heat from the nanofluid before it goes back into the MHE.
Failure to dissipate enough heat will result in the inability of the device to maintain a low CPU
temperature over a large period of time. In order to test the radiator performance, two thermocouples
at the entrance and exit of the radiator will be used within the PC set-up to monitor the effective heat
exchange of the nanofluid to the outside environment.
F.6 Nanofluid
F.6.1 Stability
Nanofluid performance is inherently limited by the suspension of nanoparticles. In this manner, the
nanoparticles must not crash out of solution and effectively inhibit the flow through the microchannels
of the MHE. In order to test particle stability over the long term, the system will be left to rest for
approximately one week. After this time, the device will be run again using the same specifications used
the week prior. While the nanofluid may not remain completely stable at rest, the flow of the fluid
should cause sufficient turbulence to re-suspend the nanoparticles. The initial start-up motion of the
fluid will indicate if there is a large agglomeration of particles within the system causing impeded flow.
Furthermore, poor device performance compared to the original performance over a week-long resting
period could indicate an agglomeration of the nanoparticles, resulting in a decrease in the thermal
conductivity of the nanosolution.
F.6.2 Heat Dissipation
Heat dissipation efficacy of the nanofluid will be tested by comparing the thermal resistance values
between the MHE device running only with water and the MHE device running with the nanofluid.
First, temperature readings will be obtained and thermal resistances calculated with the nanofluid in the
system. The nanofluid will then be pumped into a storage container, and the MHE system flushed with
water for 15 minutes. The device will be run with water and temperature readings obtained and
thermal resistances calculated. Finally, the device will be flushed once again, and run once again with
F-6
the nanofluid to ensure the methodology of the test. Performance should match that prior to the water
test.
Temperature readings are taken at both the inlet and the outlet of the MHE device. These temperature
readings correspond to the temperatures across the MHE device—Tbefore and Tafter, respectively. Thermal
resistance values will be calculated using equation (f1), as detailed earlier in Section F.3.1.
F.7 Noise
The amount of noise produced by the system will be measured using a decibel meter. The MHE device
should not exceed 50dB of noise. To test this noise requirement, an initial reading of the computer
system, while under load, will be taken without the MHE device installed. After the initial measurement
is taken, the MHE device will be installed and a measurement will be taken while the CPU is under load.
The difference in decibels of noise produced by the MHE should not be greater than +50dB.
G-1
Appendix G – Design Specifications
G.1 High Level Overview
The microfluidic heat exchanger (MHE) combines the enhanced thermal conductivity of a nanosolution
with the efficient heat transfer in microfluidic channels to remove heat from next-generation
processors. In its simplest incarnation, the MHE is composed of a microfluidic chip, a micropump and a
radiator. A reservoir of the nanosolution is fed into the microfluidic pump, and thin tubing is used to
connect the main aspects of the MHE. Various measurement tools are attached throughout the tubing,
including temperature, pressure and flow meters. The basic setup is shown below in Figure G-1.
Figure G-1 - Detailed MHE Setup with Measurement Positioning Components include: A. Microfluidic Chip, B. Microfluidic Pump, C. Radiator, D. Nanosolution Reservoir
In the finalized model of the MHE, the microfluidic chip will be mounted onto a current-technology CPU.
The integration of the MHE into a personal computer (PC) has been discussed in great detail in Appendix
D: Functional Specifications, and is shown below in Section G.5. The remainder of this appendix will
serve to address the specific details of each component within the MHE.
G.2 Microfluidic Chip Considerations
The microfluidic chip is composed of an inlet, an outlet and numerous microchannels bridging the gap
between the inlet and the outlet. The nanosolution is forced to flow through these channels by an
applied pressure across the microfluidic chip. In order to meet the primary requirements of this
G-2
document, top-entrance inlets and outlets are used, as detailed in Appendix D: Functional
Specifications. A cross-section of the microfluidic chip with its given design parameters is shown in
Figure G-2. The details of these design parameters are discussed in great detail below.
Figure G-2 - Cross-Section of the Microfluidic Chip with Device Dimensions
The microfluidic channels will be milled from a copper block with ultra-fine, specialized drill bits. This
work is to be completed by a graduate student currently working for the project supervisor. The inlet
and outlet wells will be milled using regular drill bits. The inlet and outlet wells will be drilled slightly
deeper into the copper block than the microfluidic channels in order to encourage turbulent flow at the
inlet and outlet, which will allow for better nanosolution mixing and enhanced nanoparticle distribution
throughout the microchannels. Unfortunately, the presence of turbulent flow complicates the
modelling of the system, as turbulence typically results in divergent, erroneous solutions.
The microfluidic chip is comprised of two pieces: the bottom plate, with the microchannels and
inlet/outlet wells drilled into it, and the top plate, which is a planarized, thin sheet of copper that sits on
the bottom plate to enclose the top-side of the microchannels. A trench will be drilled and an O-ring will
be fitted into the bottom plate in order to ensure a good seal is made between the top and bottom
pieces when the microfluidic chip is assembled. It is necessary that both pieces of the microfluidic chip
are extremely flat in order to prevent cross-channel flow, which would potentially degrade the efficacy
of the MHE. In test conditions, this thin sheet of copper may be replaced with a glass slide, which can be
used to identify errors in flow, and to verify the distribution of the nanosolution throughout the
individual channels.
G-3
In order to ease the fabrication of the microfluidic chip, a minimum channel width of 200m will be
used. While larger channels decrease the efficacy of heat transfer from the walls of the microfluidic chip
to the nanosolution, the enhanced thermal conductivity of the nanosolution should account for any such
variations. Furthermore, the larger channel width should decrease the probability of microfluidic chip
clogging due to nanosolution agglomeration within the channels. It has been found that a pitch of half
the channel width is desired for optimal heat removal in microfluidic devices [G-1]. However, technical
considerations in the microchannel fabrication and further operational mechanical stability limit this
parameter. As such, a pitch of 200m between microchannels will also be used for the microfluidic chip.
The length of the microfluidic channels is taken at 4cm, which is slightly larger than the side-length size
of current commercial CPUs. The inlet and outlet wells occupy the remainder of the microfluidic chip
and are designed to hang over the side of the CPU. This is done in order to prevent rapid heat transfer
to the relatively stationary, turbulent flow in the inlet and outlet wells. A side profile of the microfluidic
chip is shown in Figure G-3 below. When the microfluidic chip is integrated into the full system, the top
plate will be in direct contact to the bottom plate, and the entire chip will be clamped to the CPU.
Figure G-3 – Side View of the Microfluidic Chip Sitting on a CPU
G.3 Nanofluid Considerations
In order to meet the heat removal requirements outlined in Appendix A: Customer Requirements and
Appendix D: Functional Specifications, a colloidal suspension of carbon nanotubes (CNTs) will be used
within the MHE. CNTs exhibit exceptional thermal conductivity, previously showing a 2.5-times increase
in thermal conductivity over their poly-(a-olephin) base fluid [G-2]. In a more practical test, the CNTs
exhibited 1.2X increases in thermal conductivity over their aqueous solvent [G-3]. A water-based
solvent is desired for this application, as it is minimally reactive with the copper microfluidic chip. As
such, CNTs dispersed in water are chosen as the nanofluid for the MHE.
G-4
As detailed in Appendix D: Functional Specifications, the most critical parameter for the nanofluid is
the capacity for the nanoparticles to remain suspended in solution during the operation of the MHE. In
order to aid in their suspension, raw CNTs will be treated with strong acid to functionalize them with
hydrophilic, oxide-based functional groups. These functional groups will serve to make the hydrophobic
CNTs more hydrophilic, and allow them to interact with the polar/aqueous solvent more readily. If the
CNTs still exhibit suspension problems, a small amount of surfactant, such as SDS, will be added to
solution.
G.4 Radiator Considerations
The main portion of the radiator is composed of an inlet, an outlet, and a series of 1/8th inch tubes. As
the heated CNT solution flows through the radiator, heat is transferred from the CNT solution in the
metal tubing into an array of metal fins. A fan, mounted on the opposite side of the fin grating, pulls air
from the heated fins and blows it into the surrounding environment. An overview of the full radiator
design is shown below in Figure G-4.
Figure G-4 - Side View of Radiator System
The radiator must be able to transfer heat to the environment with a relatively small solution volume
due to the limited amount of CNT solution. The production of the CNT solution is inherently limited by
cost considerations in the purchasing of raw CNTS and their subsequent functionalization and
suspension in water. In order to accommodate this constraint, small diameter tubing will be used. This
also allows for a larger surface area to volume ratio for more efficient heat transfer. In order to ensure
G-5
easy workability of the material, and taking into consideration the above points, 1/8th inch inner
diameter copper tubes will be used in the radiator design.
A 95 cm inch long copper pipe with an inner diameter of 1/8th inch will be used for the radiator
fabrication. After every 10 cm of straight tubing, a 1.5cm diameter, 180-degree bend will be made, such
that the copper pipe can loop back into the system. This is done in order to maximize the contact area
between the tubes and the metal fins that absorb and subsequently radiate the heat into the
environment. The radiator will comprise of eight turns of 10 cm long copper tubing, as shown in Figure
G-5. The summative dimensions of the looped copper tubing will be 13 cm x 11 cm. The effective
volume of this system will be approximately 8.0 mL.
Figure G-5 - Front View of Bent Copper Tube
A 12 cm x 12 cm micro-fin grating will be attached to the bent copper tubing via brazing or welding. This
attachment method allows for an effective contact between the two structures, increasing thermal
interfacing and conductivity. The use of fins helps to increase the surface area exposed to the
environment. The net effect is to remove heat off of the CNT solution in a highly efficient manner
through increased surface area contact among the components in the radiator design.
G.5 System Overview and Micropump Considerations
An overview of the MHE system is seen Figure G-6 (as shown previously in Appendix F: Prototype Test
Plan). The micropump, MHE, and radiator will be connected using plastic tubing with a 1/8th inch inner
diameter (ID) and 1/4 inch outer diameter (OD).
G-6
Figure G-6 - MHE System Integration
Approximately 4 feet of tubing is required to connect all components within the design. This comprises
of 1.5 feet from the micropump to the MHE, 1.5 feet from the MHE to the radiator, and 1 foot from the
radiator to the micropump. With this information, a total system volume is determined to be 18.6 mL,
as summarized in Table G-1.
Table G-1 - Summary of System Volumes
Component Volume (mL)
Tubing 9.5
Radiator 8.0
MHE 1.1
Total 18.6
Connection of the MHE to the CPU will be completed by altering a standard heat sink connector to
match the MHE, and to meet the standard connection points on the computer motherboard.
Approximately 18 - 70 pounds of force is recommended for the application of the MHE to the CPU,
which corresponds to 57-221 kPa [G-4]. The heat sink connectors are generally spring loaded clamps,
and both the CPU and motherboard are designed to sustain such forces.
The micropump flow rate must be determined experimentally to provide a pressure drop to sustain a
CPU temperature below 50°C with the nanofluid running through the MHE. This will be accomplished
with the aid of the loaned micropump from the project supervisor. A micropump will then be purchased
based on these experimental parameters. The heat removed from the CPU is known as the thermal
design power and, as outlined in reference [G-4], may be taken as 75W for this project. The micropump
will be mounted in an enclosure toward the front/top of the ATX case. This location is chosen as it is
away from other core components in the computer and therefore mounting should not pose an issue.
H-1
Appendix H – Verification Data
H.1. Simulation Work
This section outlines work completed in simulating both flow and heat dissipation in the MHE.
Simulation was split into two sections in order to ease computation and allow for simplifications when
necessary:
- Initially the inlet area was solved by itself without the inclusion of the microchannel geometry.
Since the inlet has an unknown flow profile, laminar assumptions cannot be made and thus this
aspect of the modelling is the most computationally intensive. Heat modelling was not done
here as it is assumed that the majority of heat transfer will take place in the main channel area
and not at the inlet or outlet.
- Secondly, the series of laminar flow channels were solved using a velocity distribution obtained
from the inlet area. Heat dissipation modelling was added and the two systems were then
solved independently of each other. That is, flow profiles were solved first and heat dissipation
solved second. This places the assumption that heating of the fluid does not significantly affect
its flow mechanics.
H.2. Inlet Simulation
The inlet geometry was designed based on previous work done in the supervising professor's laboratory.
The design consists of a 18𝑡ℎ Iinch ID inlet tube into a semicircular reservoir. The curved backend
serves to funnel flow toward the outer channels which are the greatest distance away from the inlet
tube. Channel inlets are located above the base of the channel such that if sedimentation occurs in the
channel inlet, it remains in the reservoir until turbulence re-suspends the particles, allowing them to
flow easily through the channels and to avoid clogging. This design also serves to more evenly distribute
the fluid throughout all of the channels. Channel widths were specific as 250 µm and heights of 500 µm.
Channels were separated by a 250 µm gap.
Inlet velocity was determined by lab measurements of the volumetric flow rate that the test micropump
is capable of providing through the mock microfluidic chip. This value was subsequently converted to a
H-2
flow velocity over inlet area. This was determined to be approximately 200 mL/min and implied an inlet
boundary condition velocity of approximately 0.4168 m/s.
All channel inlets were assumed to be located at 0 pressure relative to the inlet, and no-slip boundary
conditions were assumed along the sidewalls. Figure H-1 below shows the total velocity profile in the
inlet. As can be seen the greatest velocity is seen in the inlet tube at which point velocity drops
significantly over the larger volume of the reservoir.
Figure H-1 Total Velocity Profile in MHE Inlet
Figure H-2 shows the breakdown of this velocity into solely the x-component (towards the channel
inlets). Here the greatest velocity is seen immediately following the inlet as fluid rushes toward the low
pressure regions at channel inlets. This then begins to level out closer towards the channel; however,
there are still inequalities in the channel flow distribution as seen in Figure H-3. The effectiveness of the
semicircular reservoir is seen Figure H-2 by the existence of positive x-flow behind the inlet on either
end.
Figure H-2 x-Velocity in MHE Inlet
H-3
The simulation above confirms the results that were originally expected. There is an unequal flow
distribution entering the various channel inlets, with the greatest velocity seen where the channel inlet
is in line with the reservoir inlet. This is seen in Figure H-3 below. A semicircular reservoir serves to
funnel flow towards the outside channels thus reducing flow concentration to the center channels. It
should be noted that due to computational limitations the mesh size could not be as fine as desired.
Therefore, while these numbers will be used to carry out the second part of the simulation, this exact
distribution is likely not precise.
Figure H-3 Channel Inlet Velocities
The data carried onto the second part of the simulation is only the average channel velocity at each
channel. This was easily obtained by averaging the data over each individual channel to obtain a single
velocity as providing boundary conditions as a series of functions, or directly from this simulation proved
extremely difficult. The averages are seen in Figure H-4 and a rough fit of a 4th order polynomial is seen
to highlight the trend.
H-4
Figure H-4 Velocity Distribution at Channel Inlet
H.3. Channel Simulation
Channel simulations were done separately as explained above. Using the inlet flow distribution from
the inlet average velocity distributions, boundary conditions were created for the channels. Figure H-5
below shows the geometry as drawn. While unclear it consists of 75 250µm wide by 500 µm high
channels—each of 37.55mm length. No-slip boundaries were assumed at the walls and 0 pressure at
exits.
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0 10 20 30 40 50 60 70
Ave
rage
Ve
loci
ty (
m/s
)
Channel Number
Channel Inlet Average Velocity Distribution
H-5
Figure H-5 Channel Simulation Drawing
This was solved independently of any heat simulations and the resulting flow distribution can be seen in
Figure H-6 below. As predicted parabolic flow profiles are seen in each channel, with velocity
distributions matching that at the inlets.
Figure H-6 - Channel Velocity Profile
With velocity distribution determined, a heat distribution in the channel can be feasibly solved. Several
assumptions are made in order to allow for convergence of the model using readily available data.
0.00
0.05
0.10
0.15
0.20
0.25
0.00 0.01 0.01 0.02 0.02 0.03 0.03 0.04 0.04
Ve
loci
ty [
m/s
]
Distance from Edge [m]
Channel Velocity Distribution
H-6
Firstly, it is assumed that the solution properties do not change significantly as a function of
temperature. While this is untrue, a primary requirement of the system is that the temperature does
not exceed 50°C. At steady state, the variation in temperature across the channels should be minimal
(within a few degrees Celsius) and thus if the properties of water at this temperature are used, this
assumption should remain valid.
Secondly, heat dissipation from the exterior walls of the MHE is assumed to be minimal with respect to
the convective flux of fluid removing heat from the system. As such, the walls are set to thermal
insulation. While heat will indeed be lost to air, this assumption provides a 'worst case' scenario in that
it will serve to decrease the overall performance of the simulated device compared to the actual device.
As such, if device performance is met with this assumption, it can be safely assumed that performance
in the real device will be equivalent or greater.
Thirdly, it is assumed a perfect thermal contact between the chip and the MHE. That is, the assumption
is that all 75 W of dissipated power from the chip enter the MHE. This assumption is not realistic for a
real world device. Thermal resistance does occur between the chip and the MHE, and has been
highlighted as a major source of concern earlier in Section 5.2. As such, this assumption overestimates
the performance of the device. However, removal of this assumption significantly increases the
complexity of the simulation. Efforts were made to include the chip in the simulation and
approximations to thermal resistance were made, but were unsuccessful. It is hoped that the inclusion
of the nanofluid, combined with ensuring reasonably good thermal contact between the MHE and chip
will offset this overestimation of device performance.
Finally, it was found via iteration that a maximum inlet temperature of 29°C is required in order to
ensure the temperature across the MHE is below 50°C. This provides a baseline for radiator
performance required and can be used as verification in radiator selection. Maximum temperature will
be at the base of the MHE furthest away from the channel inlet. This distribution can be seen in Figure
H-7 below. Temperature can be seen to be lower where channel flow is greatest as expected further
justifying the need for uniform flow distribution. It is clear that with the assumptions stated above the
H-7
maximum temperature in the device will reach 49.5°C with an inlet flow rate of 200 mL/min and inlet
fluid temperature of 29°C.
Figure H-7 MHE Far Edge Temperature Distribution
321.9
322
322.1
322.2
322.3
322.4
322.5
322.6
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
Tem
pe
ratu
re (
K)
Distance from Edge [m]
MHE Far Edge Temperature Distribution