Information about power electronic cooling, and carbon nanotubes.

1. General Information: Power
Electronics Cooling and Carbon
Nanotubes.
By: Salman Saleh

2. Power Electronics Cooling
• Heat Transfer Intro.
• Power Electronics Failures and Causes
• Methods for Power electronic cooling
• Microchannel Single and Two Phase
• Passive Two Phase Cooling System - Heat
Pipes
• Jet Impingement Two Phase and Single Phase

3. Heat Transfer Intro.
Thermal conductivity (k): is the ability of a material to
transfer heat, through a unit thickness for a unit area per a
temperature gradient (W/mC).
Heat transfer coefficient (h or HTC): “The convection heat
transfer coefficient h is not a property of the fluid. It is an
experimentally determined parameter whose value
depends on all the variables influencing convection such as
the surface geometry, the nature of fluid motion, the
properties of the fluid, and the bulk fluid velocity.” (W/m2C)
Emissivity (𝜺): “The property emissivity, whose value is in
the range 0 ≤ 𝜀 ≤ 1, is a measure of how closely a surface
approximates a blackbody (a perfect emitter) for which 𝜀 =
1"
𝑄 = 𝜀𝜎𝐴(𝑇𝑠
4 − 𝑇𝑠𝑢𝑟𝑟
4 )
Heat transfers as a result of temperature difference
between one medium and another, by three mechanisms
Conduction
Occurs when heat transfers from particles with
high kinetic energy to particles with low kinetic
energy through direct contact.
The process factors are:
∆T: temperature gradient
A: cross section area.
dx: Distance traveled
k: Thermal conductivity
Convection
Occurs when a fluid (gas or liquid) is heated by
passing over a hot surface.
The process factors are:
𝑇𝑠𝑢𝑟𝑓𝑎𝑐𝑒 − 𝑇𝑙𝑖𝑞𝑢𝑖𝑑
ℎ: ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑐𝑜𝑒𝑓𝑒𝑐𝑖𝑒𝑛𝑡
𝐴: 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑐𝑜𝑛𝑡𝑎𝑐𝑡
Radiation
Occurs due electromagnetic waves, generated
by the random movement of atoms .
The process factors are:
𝑇𝑠𝑢𝑟𝑓𝑎𝑐𝑒
𝐴: 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑛𝑔 𝑠𝑢𝑟𝑓𝑎𝑐𝑒
𝑇𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔
𝜀: 𝑒𝑚𝑖𝑠𝑠𝑖𝑣𝑖𝑡𝑦
𝜎 = 5 × 10−8 𝑊/(𝑚2 𝐾4)

4. Power Electronic Failures and Causes
CAUSES OF POWER ELECTRONICS FAILURE
Alternative substrates to Silicon has been developed
such as SiC, and GaN to improve the performance,
and meet higher power density demands. However
these new substrates is limited by thermal
challenges due to variations of the coefficient of
thermal expansion (CTE) of its components.
1. Packaging 2. Junction & Medium Temperature
The power cycling lifetime, which refers to
turning the device on and off, decreases as the
difference between the junction and the
medium temperature increases.
CTE
Variation
Electric
performance
improvement
Examples of Power
Electronic Failure
Degradation in the power
electronic performance,
due to thermal stresses will
lower the vehicle driving
range
Substrate in power
electronics fracture, due to
thermal fatigue
Voiding: Due to cyclic loads
voids will start develop in
the solder

5. Methods for Power Electronics Cooling
The low cost and availability of air give it an advantage over the other
cooling methods. “However, due to its very low thermal conductivity (0.0257
W/(mK) at 20 C and atmospheric pressure) and low heat capacity (specific
heat capacity of 1005 J/(kgK)) it is a poor heat transfer medium.” Therefore,
with increase in demand for better cooling methods and high heat thermal
coefficient, liquid cooling was an alternative for the air cooling
Methods of
convection cooling
Liquid Cooling
Two-Phase: Involves
phase change
Passive (Capillary
Pumping)
Active
Single Phase: No phase
change occur during
fluid flow
Air Cooling
Notes on Air Cooling
Ranges of heat thermal coefficient for varies cooling methods Ratings of cooling methods
Two Phase
cooling
• The highest in complexity: Heat
pipe, and spray cooling.
• The highest in weight: Jet and
spray cooling.
• The highest in cost: Heat pipe.
• The highest in cooling capacity: Jet
Impingement.

6. Microchannel Single and Two Phase
Forcing fluid through channels with
hydraulic diameters that ranges
between 50–1000 μm, to transfer
heat either through one or two
phase mechanism.
Principle of Operation Parameters
Hydraulic Diameter
HTCPressuredrop
Heattransferred/pumping
power
Heat exchanger length
Pressuredrop
PressuredropFrictionfactor
Microchannel Hydraulic Diameter
• Hydraulic Diameter= 4*Area/Parameter
• At lower hydraulic diameter we have high
HTC, which means better heat transfer.
However, at small hydraulic diameters we
have high pressure drop.
Microchannel Cross Section
• The cross section of the
microchannel affect its HTC and the
pressure drop, thus affecting its
performance.
• Data shows circle cross section
results in higher HTC and a low
pressure drop.
Channel Shape
• The shape of the channel
influence the ability of the
channel to remove heat.
• Curved and Zic-Zac channels
have higher HTC than the
straight channels, However,
they have a higher pressure
drop.
Two phase Vs Single Phase Cooling
Single Phase Limitations
• Small Channels => high
pressure drop=>High
pumping power.
• Low performance for
refrigerants
• Temperature gradient along the
cooled Chip
Two Phase Limitations:
• Heat transfer coefficient
depends on vapor quality
• Backflow and instabilities
• Critical heat flux (hot spots).
Limitations
Thermal conductivity ratio
Hydraulic Diameter
Single Phase
Cooling
Two Phase Cooling
Heat Flux Capability (W/cm^2) 10-20 300 – 1000
Isothermality across the device Low High
Pumping Power High Low
Flow Rates High Low
Technology Maturity High Emerging
System Complexity Less More

7. Passive Two Phase Cooling System - Heat Pipes
Principle of Operation
• The heat pipe is divided into 3 parts: the
evaporator section, adiabatic (transport)
section and condenser section.
• Liquid evaporates due heat applied at the
evaporator and flows to the condenser due
to vapor pressure.
• The condensed liquid from the condenser is
pumped back to evaporator through the wick
structure.
• This capillary pressure gradient circulates the
fluid against the liquid and vapor pressure
losses, and adverse body forces such as
gravity or acceleration.
Limitations
Capillary Limit: Occurs when the capillary pumping does not vapor and
pressure drop, and therefore the condensed liquid does not flow back to
the evaporator. In addition it occurs when the heat pipe is at frozen state
and exposed to high heat flux.
Sonic Limit: Occurs when the vapor pressure speed at the end of the
evaporator section reaches the local speed of sound. Cooling at this rate
will result a high temperature gradient.
Boiling limit: Occurs when the bubbles due boiling liquid, fill the wick
structure holes and thus preventing the liquid from reaching the
evaporator.
Entrainment Limit: Due to the opposite flow of the liquid and the vapor, a
shear force might torn liquid droplets and thus carried by the bubbles to
the condenser=>dry-out at high rates occurrence of the entrainment.
Frozen Startup: Due heat pipe design, and under frozen state the heat
pipes vapor generated may freeze at the condenser.
Advantages
Freedom of Design: Heat pipes geometries range from simple cylindrical
or flat heat pipes, to curved plate heat pipes.
Maintenance-free: Due to the unneeded electrical and mechanical
devices Heat pipes can operate for long periods without maintenance
Types of Heat Pipes
Vapor Chambers
Loop Heat Pipes
Wicking parameter
Vapor Chambers
Thermosyphon
Overview of Capillary Pumping:
Is the liquid flow inside small tubes due to
intermolecular forces (cohesion and surface
tension) between it and the surface.
Parameter:
The capillary pressure should be greater than
the vapor pressure drop, liquid pressure drop,
and the gravitational force.
∆P𝐶> ∆P𝑣 + ∆P𝑙 + ∆P𝑔
∆𝑃𝑐 = 𝜎𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑡𝑒𝑛𝑠𝑖𝑜𝑛 ×
1
𝑟1 (𝑐𝑢𝑟𝑣𝑎𝑡𝑢𝑟𝑒)
+
1
𝑟2
Three parameters should be considered while
designing the wick are:
Minimum Capillary radius to enhance capillary
pressure.
High permeability of the axial liquid flow, to
lower the liquid pressure drop.
High thermal conductivity to prevent large
temperature gradient through the wick.

8. Jet Impingement Two Phase and Single Phase
Principle of Operation
• “Impinging liquid or gas on the onto a surface
on a continuous basis”
• The three Configuration of jet impingement:
• Free-Surface Jet: Using liquid in a
less dense medium (air).
• Submerged Jet: Using fluid to
impinge in the same medium.
• Confined Jet
Parameters Single Phase
• Womac Single submerged and 4-jet
submerged resulted close HTC, even
though we have multiple jets.
Therefore, the spaces between jets
plays a role in the Jet performance.
• HTC gets higher as the when the jet is
closer to the target.
Notes on Two Phase
• “The jet diameter, jet orientation, number of jets, jet
configuration, and even jet velocity do not have much of an
impact on the heat transfer in nucleate boiling.”
• “During critical heat flux occurs, the temperature of the wall
shoots up because of dry-out conditions in which no liquid is in
contact with the surface to sustain boiling.”
Water Supremacy for Two Phase
Limitation
Parameter Values
Metal Copper
Liquid Filtered Water
Diameter 1.5 mm
Velocity 125 m/s
Erosion rate 0.0092 mm/hr
• At high velocities (>10m/s)
erosion of the heat transfer
surface material must be
considered.
• High velocities lead to
erosion of the surface and
splashing away without
properly wetting the
component resulting in poor
cooling.
• The HTC increases with the increase of velocity.
• The HTC increases with the increase number of
jets, for example at 12 m/s Womac 4 jet-free
surface is 70000 W/m2K, while for a single jet it
is 45000 W/m2K.
• The HTC is higher for submerged than for free-
surface
• Studies show that the water
scored the highest in the heat
transfer coefficient (HTC) and in
the heat flux, compared to
other fluids, for a two phase
microchannel experiments.

9. Polymer CNT composite
• Polymer CNT Composite
• CNT Introduction and Properties
• Types of CNT
• Parameters for the polymer/CNT composite
• CNT Sea Urchin Project Approach

10. CNT Polymer Composite- Sea Urchin CNT
What is CNT?
Carbon Nanotube Properties
E (TPa) 1
Tensile Strength (Gpa) 10-60
Strain at Break 10 %
Thermal Conductivity (W/m.K) >3000
Cylindrical carbon molecules with unique
properties that make them potentially useful
in a wide variety of applications.
Types of CNT
Based on number of tubes:
1. Single-walled CNT (SWNT).
2. Double-walled CNT (DWNT).
3. Multiple-walled CNT (MWNT).
Based on chiral (folding of the CNT):
1. Chiral
2. Armchair
3. Zic Zac
Notes About CNT Properties
Electrical properties of CNT:
• Geometric differences such as defects,
chirality, different diameters and the degree
of crystallinity of the tubular structure affect
electronic properties.
Mechanical properties of CNT:
• The results showed that CNTs are in fact very
soft in the radial direction. Thus, affecting
CNT nanocomposites mechanical properties.
Polymer/CNT Composite
Fibers Factors
CNT
structure
The structure of the CNT is dependent on the synthesis
process. Therefore this will affect the properties of the
composite material.
Polymer-CNT
microstructural
development
Interfacialforces between the CNT and the polymer should
be studied and reinforced.
Dispersion
During dispersion process the length of the CNT will get
shorter. If the CNT was below 500 nm it can not transfer its
stiffness or strength properties.
The
orientation of
the polymer
and the
Alignment of
the CNT.
CNT affects the orientation of the polymer, therefore its
mechanicalproperties. In addition the alignment of the CNT
also affects the composite properties.
Polymer/CNT Composite Fibers Factors
CNT dispersion

11. References
• Çengel, Yunus A. Heat Transfer: a Practical Approach. McGraw-Hill, 2003.
• Agbim, Kenechi A. “Single-Phase Liquid Cooling For Thermal Management Of Power Electronic Devices.” George W.
Woodruff School Of Mechanical Engineering, 2017
• McGlynn, D'Arcy. “Model Development for Micro-Channel Cooling Technology.” 2013
• SUBHEDAR, DATTATRAYA, and BHARAT RAMANI. “Recent Studies On Single Phase Fluid Flows And Heat Transfer In
Microchannel - A Review.” International Journal of Mechanical and Production Engineering Research and Development,
2 June 2013, pp. 169–180.
• “Advanced Thermal Management Technologies.” Advanced Cooling Technologies, 2018.
• Faghri, Ameer. “Heat Pipes: Review, Opportunities And Challenges.” Global Digital Central, 5 Jan. 2014.
• Skuriat, Robert. “Direct Jet Impingement Cooling of Power Electronics.” University of Nottingham, 2012.
• White, John. “LITERATURE REVIEW ON ADSORPTION COOLING SYSTEMS.” Latin American and Caribbean Journal of
Engineering Education, 2013.
• Song, Kenan, et al. “Structural Polymer-Based Carbon Nanotube Composite Fibers: Understanding the
Processing–Structure Performance Relationship.” Materials, vol. 6, no. 6, 2013, pp. 2543–2577.,
doi:10.3390/ma6062543.