1. A Presentation on Heat Pipes
Submitted to: Submitted By:
Dr.-Ing. Jyotirmay Mathur Subhash Patel
Associate Professor (2011PME5264)
MNIT, Jaipur
2. Heat Pipe Background
• 1800s – A. M. Perkins and J. Perkins developed
Perkins tube
• 1944 – R. S. Gaugler introduced the use of a wicking
structure
• 1964 – G. M. Grover published research and coined
the “Heat Pipe” name
3. Transfer of Heat
Heat Added Heat Released
Heat Sink
Heat Pipe
Heat
Processor
*Drawing is not to scale.
4. Heat Transfer within
a Heat Pipe
Heat Absorbed Container Heat Released
Wick Structure
Evaporation Condensation
Wick Structure
Heat Absorbed Container Heat Released
*Drawing is not to scale.
5. Component of heat pipe
Container
• Metal Tubing, usually
copper or aluminum.
• Provides a medium with
high thermal
conductivity.
• Shape of tubing can be
bent or flattened.
6. Working Fluid
• Pure liquids such as helium, water and liquid silver
• Impure solutions cause deposits on the interior of the
heat pipe reducing its overall performance.
• The type of liquid depends on the temperature range
of the application.
7. Examples of Working Fluid
BOILING PT. AT
USEFUL
MELTING ATM. PRESSURE
MEDIUM RANGE
PT. (° C ) (° C)
(° C)
Helium - 271 - 261 -271 to -269
Ammonia - 78 - 33 -60 to 100
Water 0 100 30 to 200
Silver 960 2212 1800 to 2300
8. Choosing the Working Fluid
Chi(1976) developed a parameter of gauging
the effectiveness of a working fluid called
the liquid transport factor:
l
Nl
l
l
Where is the latent heat of vaporization and
is the surface tension. Subscript refers to
the liquid
(Peterson, 1994).
9. The wicking structure
Axial Groove Wick
Created by carving out grooves on the interior core
of the Heat Pipe.
12. Purpose of the Wick
• Transports working fluid from the Condenser to the
Evaporator.
• Provides liquid flow even against gravity.
13. How the Wick Works
• Liquid flows in a wick due to capillary action.
• Intermolecular forces between the wick and the fluid
are stronger than the forces within the fluid.
• A resultant increase in surface tension occurs.
14. 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.
16. 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.
17. 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)
18. 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)
19. 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.
20. 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)
21. 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)
22. 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 dry
out in the evaporator. Dry out 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 dry out.
23. 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 dry out.
• 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 doesn’t move
to the condenser and the thermodynamic cycle
doesn’t occur.
24. The stages in the design :
( i) Select wick and Wall materials
(ii) Select working fluids
Criteria - limitations
- pressure
-priming
-handling
-purity etc .
(iii) Examine wick types :
Homogeneous rejected
Arterial selected
25. The stages in the design :
(iv) Determine artery sizes
(v) Examine radial resistance to heat flow
(vi) Examine overall pressure balance of proposed
design
(vii) Select final configuration on basis of (vi) and
such features as manufacturing difficulties etc.
26. 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.
• Stresses:
– Since the heat pipe is like a pressure vessel it must
satisfy ASME pressure vessel codes.
27. Container Design
• Typical materials:
– Aluminum
– Stainless steel
– Copper
– Composite materials
– High temperature heat pipes may use refractory
materials or linings to prevent corrosion.
28. Sample Design
A heat pipe is required which will be capable of
transferring a minimum of 15 W at vapour
temperatures between 0 and 80 0C over a distance of
1 m in zero gravity (a satellite application) .
Restraints on t he design are such that the evaporator
and condenser sections are each 8 cm long , located
at each end of t he heat pipe , and the maximum
permissible temperature drop between the outside wall
of the evaporator and the outside Hall of the
condenser is 6 0 C. Because of Height and volume
limitation, the cross - sectional area of the vapour
space should not exceed 0 . 197 cm 2 • The heat pipe
must also with -stand bonding temperatures.
29. Selection of Material
• The selection of wick and wall material is based in
various criteria.
• In this problem mass being an important parameter
• So Aluminium alloy (HT30) is chosen for the wall,
and Stainless Steel for the Wick.
30. Selection of Working Fluid
Working fluid compatible with the wall and wick
materials, based on available data, includes:
• Freon 11
• Freon 113
• Acetone
• Ammonia
The limitations on heat transport must now be
examined for each working fluid.
32. Conclusion on Selection of
Working Fluid
• After the various examination like Sonic Limit,
Entrainment Limit, Wicking Limit, Priming of the
Wick Acetone is Selected.
• Properties of Acetone are shown
39. Circumferential liquid
distribution
• The circumferential wick thickness is limited by
the fact that the temperature drop between the
vapour space and the outside surface of the heat
pipe and vice versa should be 3 0 c maximum
Assuming that the temperature drop through the
aluminum wall is negligible , the thermal
conductivity of the wick may be determined and used
in steady state condition.
43. Computational Study of Improving the
Efficiency of Photovoltaic Panels in the UAE
The efficiency of photovoltaic cells decreases as
temperature increases, therefore cooling is essential at
elevated illumination situations for instance concentrating
systems, or hot and humid conditions.
•With the average temperature in the UAE reaching up to
42 C in the summer the cell temperature could reach up
to 80 C which decreases the output power by up to
0.65%/K, fill factor to 0.2%/K and conversion efficiency
to 0.08%/K of the PV module, above the operating
temperature .
44. •a reduction by 20 C will give an increase in
efficiency between 0.6 and 1%.
•The overall reduction in the highest possible
output power (Pmax) of a solar cell decreases as
the cell temperature increase, shown in Fig.
45.
46. Heat Produced by Photovoltaic
Cells
•When PV modules are exposed to sunlight it converts
only 10% to 15% of the light to electricity the rest is
converted to heat.
•PV panels are rated at 25 C and isolation of 1 kW/m².
The power output of PV cells can be estimated from the
expected Nominal Operating Cell Temperature (NOCT),
defined as the open circuit temperature of the module at
800 W/m² irradiance (on cell surface), air temperature of
20 C, 1 m/s wind velocity and mounted with an open
back.
47. Ross, R.G. (1980) approximation can be
used to calculate the cell temperature (T
cell)
48. Heat Pipe Selection
•The two main assessments for the heat pipe design is
the selection of the heat pipe’s working fluid and
envelope (wick) materials for compatibility with the
heat pipe.
•The second main decision is the designing of the wick
to cool the PV panel reliably, under any orientation and
environmental conditions.
49. Heat Pipe Materials
•Falling under the temperature range of -20 to 1000 C,
two potential heat pipe wall and wick materials are
aluminum and copper.
•In this study copper is selected for its higher thermal
conductivity as compared to aluminum.
•Compatible working fluids for copper according to
surveys by Dunn and Reay, Brennan and Kroliczek and
Anderson are:
Compatible with copper: Water, Methanol, Ethanol
Incompatible/Unsuitable with copper: Ammonia ,
Acetone
50. Heat Pipe Fluid Selection Selection
Typical results of the compatibility of working fluid
and wall material are being shown in Fig. and it is
shown that the power output of copper/water heat pipe
is six times greater than the other fluids.
51. Description of the Proposed Finned
Heat Pipe
• After the choice of heat pipe and working fluid, the
next step was the selection of fin arrangements. In this
case, the fins were arranged according to the
constrained of the need to fit between the rear side of
the PV panel and the result of cooling the panel.
• The 3D profile of the proposed arrangement used is
shown in Fig.
52.
53.
54.
55. •the glass provides protection for the solar cells and in
some cases anti-reflection coatings are applied for
reduction in light scattering. T
•The PV panel is attached to an aluminum frame to be
is beneficial for the proposed finned heat pipe
arrangement due to the high conductivity that
aluminum can achieve.
•The proposed finned heat pipe arrangement consist a
copper heat pipe with attached aluminum fins and an
aluminum saddle acting as a heat sink for the finned
heat pipe.
60. Discussions
•the use of fins on heat pipe is more efficient as compared
to heat pipes alone.
•the cooling of PV panels to its maximum operating
efficiency by maintaining the solar cell operating
temperature under the UAE’s climatic conditions can be
obtained with the help of the proposed finned heat pipe.
•This study confirms the advantages of a finned heat pipe
for practical use, especially in the high-temperature
region.
•The proposed finned heat pipe can be used to passively
remove the heat, accepting high heat flux by natural
convection, at a much lower heat flux.
61. REFERENCES
• Computational Study of Improving the Efficiency of
Photovoltaic Panels in the UAE: Ben Richard
Hughes, Ng Ping Sze Cherisa, and Osman Beg,
World Academy of Science, Engineering and
Technology 73 2011.
• Fundamentals of Heat Pipes by Widah Saied
• Heat Pipes by P.D. Dunn and D.A. Reay
• Nptel
