Fundamentals of Heat Pipes

1
Fundamentals of Heat Pipes

• With Applications to Electronics Cooling
• -- Widah Saied

2
Introduction

• Things to be discussed
• Basic components
• Advantages
• Ideal thermodynamic cycle
• Applications
• Types
• Heat transfer limitations
• Resistance network
• Wick design
• Choosing the working fluid
• Container design
• Heat pipes in electronics cooling
• Current research in electronics cooling

3
Basic Components
Adiabatic section
evaporator
wick
condenser
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4
Advantages of Heat Pipes

• Very high thermal conductivity. Less temperature
  difference needed to transport heat than
  traditional materials (thermal conductivity up to
  90 times greater than copper for the same size)
  (Faghiri, 1995) resulting, in low thermal
  resistance. (Peterson,1994)
• Power flattening. A constant condenser heat flux
  can be maintained while the evaporator
  experiences variable heat fluxes. (Faghiri, 1995)
  
• Efficient transport of concentrated heat.
  (Faghiri, 1995)

5
Advantages of Heat Pipes

• Temperature Control. The evaporator and condenser
  temperature can remain nearly constant (at Tsat)
  while heat flux into the evaporator may vary
  (Faghiri, 1995) .
• Geometry control. The condenser and evaporator
  can have different areas to fit variable area
  spaces (Faghiri, 1995) . High heat flux inputs
  can be dissipated with low heat flux outputs only
  using natural or forced convection(Peterson,1994)
  .

6
Thermodynamic Cycle

• 1-2 Heat applied to evaporator through external
  sources vaporizes working fluid to a
  saturated(2) or superheated (2) vapor.
• 2-3 Vapor pressure drives vapor through adiabatic
  section to condenser.
• 3-4 Vapor condenses, releasing heat to a heat
  sink.
• 4-1 Capillary pressure created by menisci in wick
  pumps condensed fluid into evaporator section.
• Process starts over.
• (Faghiri, 1995)

7
Ideal Thermodynamic Cycle
(Faghiri, 1995)
8
Heat Pipe Applications

• Electronics cooling- small high performance
  components cause high heat fluxes and high heat
  dissipation demands. Used to cool transistors and
  high density semiconductors.
• Aerospace- cool satellite solar array, as well as
  shuttle leading edge during reentry.
• Heat exchangers- power industries use heat pipe
  heat exchangers as air heaters on boilers.
• Other applications- production tools, medicine
  and human body temperature control, engines and
  automotive industry.
• (Faghiri, 1995)

9
Types of Heat Pipes

• Thermosyphon- gravity assisted wickless heat
  pipe. Gravity is used to force the condensate
  back into the evaporator. Therefore, condenser
  must be above the evaporator in a gravity field.
• Leading edge- placed in the leading edge of
  hypersonic vehicles to cool high heat fluxes near
  the wing leading edge. (Faghiri, 1995)
• Rotating and revolving- condensate returned to
  the evaporator through centrifugal force. No
  capillary wicks required. Used to cool turbine
  components and armatures for electric motors.
• Cryogenic- low temperature heat pipe. Used to
  cool optical instruments in space. (Peterson,
  1994)

10
Types of Heat Pipes

• Flat Plate- much like traditional cylindrical
  heat pipes but are rectangular. Used to cool and
  flatten temperatures of semiconductor or
  transistor packages assembled in arrays on the
  top of the heat pipe.

(Faghiri,1995)
11
Types of Heat Pipes

• Micro heat pipes- small heat pipes that are
  noncircular and use angled corners as liquid
  arteries. Characterized by the equation rc /rh?1
  where rc is the capillary radius, and rh is
• the hydraulic radius of the flow
• channel. Employed in cooling
• semiconductors (improve
• thermal control), laser diodes,
• photovoltaic cells, medical
• devices.
• (Peterson,1994)

12
Types of Heat Pipes

• Variable conductance- allows variable heat fluxes
  into the evaporator while evaporator temperature
  remains constant by pushing a non- condensable
  gas into the condenser when heat fluxes are low
  and moving the gas out of the condenser when heat
  fluxes are high, thereby, increasing condenser
  surface area. They come in various forms like
  excess-liquid or gas-loaded form. The gas-loaded
  form is shown below. Used in electronics cooling.
  (Faghiri,1995)

13
Types of Heat Pipes

• Capillary pumped loop heat pipe- for systems
  where the heat fluxes are very high or where the
  heat from the heat source needs to be moved far
  away. In the loop heat pipe, the vapor travels
  around in a loop where it condenses and returns
  to the evaporator. Used in electronics cooling.
• (Faghiri, 1995)

14
Main Heat Transfer Limitations

• Capillary limit- occurs when the capillary
  pressure is too low to provide enough liquid to
  the evaporator from the condenser. Leads to
  dryout in the evaporator. Dryout prevents the
  thermodynamic cycle from continuing and the heat
  pipe no longer functions properly.
• Boiling Limit- occurs when the radial heat flux
  into the heat pipe causes the liquid in the wick
  to boil and evaporate causing dryout.
• (Faghiri, 1995)

15
Heat Transfer Limitations

• Entrainment Limit- at high vapor velocities,
  droplets of liquid in the wick are torn from the
  wick and sent into the vapor. Results in dryout.
• Sonic limit- occurs when the vapor velocity
  reaches sonic speed at the evaporator and any
  increase in pressure difference will not speed up
  the flow like choked flow in converging-diverging
  nozzle. Usually occurs during startup of heat
  pipe.
• Viscous Limit- at low temperatures the vapor
  pressure difference between the condenser and the
  evaporator may not be enough to overcome viscous
  forces. The vapor from the evaporator doesnt
  move to the condenser and the thermodynamic cycle
  doesnt occur.
• (Faghiri, 1995)

16
Heat Transfer Limitations

• Each limit has its own particular range in which
  it is important. However, in practical
  operation, the capillary and boiling limits are
  the most important. The figure below is an
  example of these ranges.

(Peterson,1994)
17
Heat Transfer Limitations

• Actual performance curves, capillary limit and
  boiling limit, are the limiting factors.

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ticles/SEP96/sep96_02.htm
18
Capillary Limit

• For a heat pipe to function properly, the
  capillary pressure must be greater or equal to
  the sum of the pressure drops due to inertial,
  viscous, and hydrostatic forces, as well as,
  pressure gradients.
• If it is not, then the working fluid is not
  supplied rapidly enough to the evaporator to
  compensate for the liquid loss through
  vaporization. If this occurs, there is dryout in
  the evaporator.

(Peterson, 1994)
19
Capillary Limit

• Equation for minimum capillary pressure

(Peterson, 1994)
20
Boiling Limit

• The Boiling limit is due to excessive radial heat
  flux all the other limits are due to axial heat
  flux.
• The maximum heat flux beyond which bubble growth
  will occur resulting in dryout is given by

(Peterson, 1994)
21
Boiling Limit

• Keff given by the table below

22
Resistance Network
(Peterson, 1994)
23
Heat Pipe Resistance

• In certain applications the temperature
  difference between the evaporator and the
  condenser needs to be known, such as in
  electronics cooling. This may be done using a
  thermal circuit.
• The main resistances within the heat pipe are
• Resistance Order of Magnitude
• Rw,a? liquid-wick resistance in the adiabatic
  section 104
• Rp,a? axial resistance of the pipe wall 102
• Rw,e? liquid-wick resistance in the
  evaporator 101
• Rw,c? liquid-wick resistance in the
  condenser 101
• Rp,e? radial resistance of the pipe wall at the
  evaporator 10-1
• Rp,c? radial resistance of the pipe wall at the
  condenser 10-1
• Other resistances exist but most are small
  relative to the above resistances.
• The external resistances the resistances
  transferring the heat to and from the heat pipe
  are also important in some cases.

(Peterson, 1994)
24
Heat Pipe Resistance

• The liquid-wick combination for the three heat
  pipe sections are given by
• Keff given on a previous slide
• The radial and axial resistances can be
  determined from traditional resistance equations
  for cylindrical shapes and flat plates depending
  on the shape of the heat pipe.

(Peterson, 1994)
25
The Wick and its Design

• Main Purpose- provides structure and force that
  transports the condensate liquid back to the
  evaporator. Also, ensures working fluid is evenly
  distributed over evaporator surface.

(Peterson, 1994)
26
Capillary Pressure

• The driving force that transports the condensed
  working liquid through the wick to the evaporator
  is provided by capillary pressure. Working fluids
  that are employed in heat pipes have concave
  facing menisci (wetting liquids) as opposed to
  convex facing menisci (non wetting liquids).
• Contact angle is defined as the angle between
  the solid and vapor regions. Wetting fluids have
  angles between 0 and 90 degrees. Non wetting
  fluids have angles between 90 and 180 degrees.
• (Faghiri, 1995)

27
Capillary Pressure

• Wetting angle

Water Wetting liquid
Mercury Non wetting liquid
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28
Capillary Pressure

• The shape of a fluids meniscus is dependent on
  the fluids surface tension and the solid-fluid
  adhesion force. If the adhesion force is greater
  than the surface tension, the liquid near the
  solid will be forced up and the surface tension
  of the liquid will keep the surface intact
  causing the entire liquid to move up.
• http//hyperphysics.phy-astr.gsu.edu/hbase/hframe
  .html
• When the liquid in the evaporator vaporizes, the
  radius of curvature of the menisci in the wick
  decreases. As the vapor condenses in the
  condenser, the radius of curvature of the menisci
  in the wick increases. The difference in the
  radius of curvature results in capillary pressure
  (Peterson,1994) . Capillary pressure is also due
  to body forces and phase-change interactions
  (Faghiri, 1995).

29
Capillary Pressure

• The capillary pressure created by two menisci of
  different radii of curvature is given by
• Where RI and RII are radii of curvature and s is
  the surface tension.
• Called the Young-Laplace Equation
• (Peterson,1994)

30
Capillary Pressure

• To maximize capillary pressure, the minimum radii
  is needed. For a circular capillary the minimum
  radii is
• Substituting these values into the formula for
  capillary pressure
• For max capillary pressure theta must be zero

(Peterson,1994)
31
Capillary Pressure

• Wetting fluids have a cos? value that will be
  positive. This results in a positive capillary
  pressure that creates a pushing force on the
  liquid in the wick near the condenser this
  forces the liquid to move to the evaporator.
• Non-wetting fluids will have cos? values that are
  negative, resulting in a negative capillary
  pressure that creates a suction force on the
  liquid in the wick. The liquid is prevented from
  moving to the evaporator.
• For this reason, the working liquid in heat pipes
  must be a wetting liquid.

(Peterson,1994)
32
Wick Design

• Two main types of wicks homogeneous and
  composite.
• Homogeneous- made from one type of material or
  machining technique. Tend to have either high
  capillary pressure and low permeability or the
  other way around. Simple to design, manufacture,
  and install (Faghiri, 1995) .
• Composite- made of a combination of several types
  or porosities of materials and/or configurations.
  Capillary pumping and axial fluid transport are
  handled independently (Peterson,1994) . Tend to
  have a higher capillary limit than homogeneous
  wicks but cost more (Faghiri, 1995).

33
Wick Design

• Three properties effect wick design
• 1. High pumping pressure- a small capillary pore
  radius (channels through which the liquid travels
  in the wick) results in a large pumping
  (capillary) pressure.
• 2. Permeability - large pore radius results in
  low liquid pressure drops and low flow
  resistance.
• Design choice should be made that balances large
  capillary pressure with low liquid pressure drop.
  Composite wicks tend to find a compromise between
  the two.
• 3.Thermal conductivity - a large value will
  result in a small temperature difference for high
  heat fluxes.

(Peterson,1994)
34
Wick Design
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ticles/SEP96/sep96_02.htm
(Peterson,1994).
35
Choosing the Working Fluid

• Heat pipes work on a cycle of vaporization and
  condensation of the working fluid, which results
  in the heat pipes high thermal conductivity.
  When choosing a working fluid for a heat pipe,
  the fluid must be able to operate within the heat
  pipes operating temperature range. For instance,
  if the operating temperatures are too high, the
  fluid may not be able to condense. However, if
  the operating temperatures are too low the fluid
  will not be able to evaporate. Watch the
  saturation temperature for your desired fluid at
  the desired heat pipe internal pressure.
• In addition, the working fluid must be compatible
  with the wick and container material.

(Peterson, 1994).
36
Choosing the Working Fluid

• Operating temperature ranges for various working
  fluids

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37
Choosing the Working Fluid

• Generally, as the operating temperature range of
  the working fluid increases, the heat transport
  capability increases.
• Choice of working fluid should also incorporate
  the fluids interactions with the heat pipe
  container and wick.

(Peterson, 1994).
38
Choosing the Working Fluid

• Chi(1976) developed a parameter of gauging the
  effectiveness of a working fluid called the
  liquid transport factor
• Where?? is the latent heat of vaporization and ?
  is the surface tension. Subscript ? refers to
  the liquid
• For electronics cooling applications, occurring
  in low to moderate temperatures, water is the
  liquid with the highest liquid transport factor.
  Another common fluid is ammonia.

(Peterson, 1994).
39
Container Design

• Things that should be considered for container
  design
• Operating temperature range of the heat pipe.
• Internal operating pressure and container
  structural integrity.
• Evaporator and condenser size and shape.
• Possibility of external corrosion.
• Prevent leaks.
• Compatibility with wick and working fluid.
• (peterson,1994)

40
Container Design

• Stresses
• Since the heat pipe is like a pressure vessel it
  must satisfy ASME pressure vessel codes.
• Typically the maximum allowable stress at any
  given temperature can only be one-fourth of the
  materials maximum tensile strength.
• (peterson, 1994)

41
Container Design

• Typical materials
• Aluminum
• Stainless steel
• Copper
• Composite materials
• High temperature heat pipes may use refractory
  materials or linings to prevent corrosion.
• (Peterson, 1994)

42
Heat pipe Compatibility

• When designing a heat pipe, the working fluid,
  wick, and container must function properly when
  operating together. For example, the working
  fluid may not be wettable with the wick or the
  fluid and container may undergo a chemical
  reaction with each other.

(Peterson, 1994)
43
Heat pipe Compatibility

• Working fluid/
• material
• compatibility.

(Faghiri, 1995)
44
Heat Sink/Source Interface

• The contact resistance between the evaporator and
  the heat source and between the condenser and the
  heat sink is relatively large and should be
  minimized.
• Methods used to join the parts include use of
  thermally conductive adhesives, as well as,
  brazed, or soldered techniques.

• (Peterson, 1994)

45
Heat Pipes in Electronics Cooling

• Cooling of electronics has one primary goal
  maintain a components temperatures at or below
  the manufacturers maximum allowable temperature.
  As the temperature of an electronic part
  increases the rate of failure increases.
• (Peterson, 1994)

46
Heat Pipes in Electronics Cooling

• Heat pipes are excellent candidates for
  electronics cooling because of their high thermal
  conductivity, high heat transfer characteristics,
  they provide constant evaporator temperatures
  with variable heat fluxes, and variable
  evaporator and condenser sizes.
• Therefore, they are good alternatives to large
  heat sinks, especially in laptops where space is
  limited.
• They are good alternative to air cooling because
  of their better heat transport capabilities. Air
  cooling may still be used to remove heat from the
  condenser.

• (Peterson, 1994).

47
Heat Pipes in Electronics Cooling

• Common heat pipes used in electronics cooling
• Micro heat pipes
• Capillary looped heat pipes
• Flat plate heat pipes
• Variable conductance heat pipes

48
Heat Pipes in Electronics Cooling

• In single component cooling, the heat pipes
  evaporator may be attached to an individual heat
  source (power transistor, thyristor, or chip).
• The condenser is attached to a heat sink to
  dissipate the heat through free or forced
  convection.

• (Peterson, 1994)

49
Heat Pipes in Electronics Cooling

• Cooling can also occur with multiple arrays of
  devices or entire printed wiring boards.

• (Peterson, 1994)

50
Heat Pipes in Electronics Cooling

• An arrayed heat pipe cooling system

(Peterson, 1994)
51
Heat Pipes in Electronics Cooling

• Heat pipe cooling a component set up in an array

• (Peterson, 1994)

52
Heat Pipes in Electronics Cooling

• Since many semiconductors are small, micro heat
  pipes may be used for cooling individual
  semiconductors or an array. Good for applications
  
• where space is limited
• like laptops.
• (Peterson, 1994)

53
Heat Pipes in Electronics Cooling

• When the electrical power is high and the heat
  rejection requirements large and nucleate pool
  boiling occurs, another method of cooling a heat
  source may be employed.
• Nucleate pool boiling causes a large temperature
  drop. To reduce the drop, you can make the device
  a part of the wick structure to ensure that fresh
  liquid is always in contact with the heat source.
  Further providing cooling to the transistor.
• In the image to the right the heat source (a
  transistor chip) is in contact with the working
  liquid and the working liquid is being evaporated
  away, cooling the transistor.

• (Peterson, 1994)

54
Heat Pipes in Electronics Cooling

• Summary
• Heat pipes enable devices with higher density
  heat dissipation requirements and greater
  reliability.
• Low cost
• Proven alternative to conventional methods of
  electronics cooling.
• (Peterson, 1994)

55
Current Research in Electronics Cooling

• Laptops today perform well and are small
  therefore, they have high heat dissipation
  demands.
• Excess heat may slow down the processors speed
  or shut the laptop off. (Junnarkar, 2003)
• First time a heat pipe used in a laptop was in
  1994. Current heat pipes move the heat from the
  CPU to a small heat sink. (Ali et al., 1999)

56
Current Research in Electronics Cooling

• Because micro heat pipes are small they are very
  useful in cooling of laptops where space is
  highly restricted.
• Wang and Peterson (2003) have come up with two
  different micro heat pipe setups for laptop
  cooling
• Micro heat pipes configured into flat plate
  shapes were employed to cool a CPU. The condenser
  was attached to a heat sink. The heat sink was
  smaller in size than one not attached to a heat
  pipe because the base of the heat sink attached
  to a heat pipe experiences more uniform
  temperatures and therefore, an increased
  efficiency.

57
Current Research in Electronics Cooling

• Two different configurations were developed
• Both were 152.4 mm long and 25.4 mm wide
• Layers of copper screen mesh, with parallel wires
  and two copper sheets were formed, in the shape
  of a flat heat pipe, to form an enclosed space.
• No capillary wick structure needed because of the
  micro heat pipes sharp corners.
• The fan is strategically placed to provide forced
  convection to the heat sink.

58
Current Research in Electronics Cooling
59
Current Research in Electronics Cooling

• Main Results
• In configuration 1, tilt angle effected the
  amount of heat dissipated
• In configuration 2, tilt angle had no effect on
  amount dissipated.
• Important because laptops experience operation in
  many orientations.

60
Current Research in Electronics Cooling
Mesh number is defined as the number of openings
per linear inch. (About,2006)
61
Current Research in Electronics Cooling

• Things that increased heat transport capacity
• Increasing mesh number
• Increasing wire diameter

62
Current Research in Electronics Cooling

• The thermal resistance from the heat sink to the
  device junction, due to the cooling of the heat
  pipe with forced convection, is greater for case
  1 than case 2 at all air velocities. The values
  were determined from the relation
• Where Qc is the heat dissipated through the heat
  sink

63
Current Research in Electronics Cooling
64
Current Research in Electronics Cooling

• Other discoveries
• Within the CPUs operating temperature limit, the
  heat capacity of a micro heat pipe is restricted
  by the heat sinks ability to transfer heat
  through convection
• Heat transfer not restricted by the capillary
  limit.

65
Current Research in Electronics Cooling
The maximum heat transfer limit provided by the
heat pipe, for the most part, is not reached due
to deficiencies in the heat sinks ability to
transfer heat through convection.
66
Current Research in Electronics Cooling

• Case 2 provided a lower thermal resistance and a
  greater heat transport capacity than Case 1.
• Case 2 transported 52W at 85?C and .85 ?C /W
  resistance.
• Case 1 transported 24W at 85?C and 1.55 ?C /W
  resistance.

67
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