2. A heat pipe heat exchanger is a simple device which is
made use of to transfer heat from one location to another,
using an evaporation-condensation cycle.
Heat pipes are referred to as the "superconductors" of heat
due to their fast transfer capability with low heat loss.
What is a Heat Pipe?
3. Working Principle
• The heat input region of the heat pipe is called evaporator, the
cooling region is called condenser.
• In between the evaporator and condenser regions, there may be
an adiabatic region
5. 1.Container
The function of the container is to isolate the working fluid
from the outside environment.
Selection of the container material depends on many
factors. These are as follows:
Compatibility (both with working fluid and external
environment)
Strength to weight ratio
Thermal conductivity
Ease of fabrication, including welding, machineability and
ductility
Porosity
Wettability
6. Container materials
Of the many materials available for the container, three are by
far the most common in use—name copper, aluminum, and
stainless steel.
Copper is eminently satisfactory for heat pipes
operating between 0–200◦C in applications such as electronics
cooling.
While commercially pure copper tube is suitable, the oxygen-
free high conductivity type is preferable.
Like aluminum and stainless steel, the material is readily
available and can be obtained in a wide variety of diameters and
wall thicknesses in its tubular form.
7. The prime requirements are:
1.compatibility with wick and wall material
2.Good thermal stability
3.wettability of wick and wall materials
4.vapor pressure not too high or low over the operating
temperature range
5.high latent heat
6.high thermal conductivity
7.low liquid and vapor viscosities
8.high surface tension
9.acceptable freezing or pour point
8. Examples of Working Fluid
Medium
Melting Point
(°C)
Boiling Point at
Atm. Pressure
(°C)
Useful Range
(°C)
Helium -271 -261 -271 to -269
Nitrogen -210 -196 -203 to -160
Ammonia -78 -33 -60 to 100
Acetone -95 57 0 to 120
Methanol -98 64 10 to 130
Flutec PP2 -50 76 10 to 160
Ethanol -112 78 0 to 130
Water 0 100 30 to 200
Toluene -95 110 50 to 200
Mercury -39 361 250 to 650
Sodium 98 892 600 to 1200
Lithium 179 1340 1000 to 1800
Silver 960 2212 1800 to 2300
9. 1. It is a porous structure made of materials like
steel,alumunium, nickel or copper in various ranges of pore
sizes.
2. The prime purpose of the wick is to generate capillary
pressure to transport the working fluid from the condenser to
the evaporator.
3. It must also be able to distribute the liquid around the
evaporator section to any area where heat is likely to be
received by the heat pipe.
10. 4. Wicks are fabricated using metal foams, and more particularly
felts, the latter being more frequently used. By varying the
pressure on the felt during assembly, various pore sizes can be
produce.
5. The maximum capillary head generated by a wick increases with
decrease in pore size.
6. The wick permeability increases with increasing pore size.
7. Another feature of the wick, which must be optimized, is its
thickness. The heat transport capability of the heat pipe is raised
by increasing the wick thickness.
8. Other necessary properties of the wick are compatibility with the
working fluid and wettability.
11.
12. Wick Design
Two main types of wicks: homogeneous and composite.
1.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 .
2.Composite- made of a combination of several types or
porosities of materials and/or configurations. Capillary
pumping and axial fluid transport are handled
independently . Tend to have a higher capillary limit than
homogeneous wicks but cost more.
14. 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.
15. Fig: The actual test results of heat pipe with different wick structure at
horizontal and vertical (gravity assist) orientations.
17. Advantages Of Heat Pipes
May reduce or eliminate the need fir reheat,
Allow cost effective manner to accommodate new
ventilation standards,
Requires no mechanical or electrical input,
Are virtually maintenance free,
Provide lower operating costs,
Last a very long time,
Readily adaptable to new installations and retrofiting
existing A/C units and
Are environmentally safe.
19. Heat transport limitationHeat transport
limitation
description Cause Potential solution
Viscous Viscous forces prevent vapor flow
in the heat pipe
Heat pipe operating below
recommended operating
temperature
Increase heat pipe
operating temperature or
find alternative working
fluid
Sonic Vapor flow reaches sonic velocity
when exiting heat pipe evaporator
resulting in a constant heat
transport power and large
temperature gradients
Power/temperature
combination, too much power
at low Operating temperature
This is typically only a
problem at start up. The
heat pipe will carry a set
power and the large
temperature will self
correct as heat pipe warms
up
Entrainment(Floodi
ng)
High Velocity vapor flow prevents
condensate from returning to
evaporator
Heat pipe operating above
designed power input or at too
low an operating temperature
Increase vapor space
diameter or operating
temperature
Capillary Sum of gravitational, liquid and
vapor flow pressure drops exceed
the capillary pumping head of the
heat pipe wick structure
Heat pipe input power exceeds
the design heat transport
capacity of the heat pipe
Modify heat pipe wick
structure design or reduce
power input
Boiling Film boiling in heat pipe
evaporator typically initiates at 5-
10 W/cm2 for screen wicks and 20-
30 w/cm2 for powder metal wicks
High radial heat flux causes
film boiling resulting in heat
pipe dry out and large thermal
resistances
Use a wick with a higher
heat flux capacity or
spread out the heat load
20. 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.
Lift and Drag Drag-based wind turbine In drag-based wind turbines, the force of the wind pushes against a surface, like an open sail. In fact, the earliest wind turbines, dating back to ancient Persia, used this approach. The Savonius rotor is a simple drag-based windmill that you can make at home (Figure 1). It works because the drag of the open, or concave, face of the cylinder is greater than the drag on the closed or convex section. Lift-based Wind Turbines More energy can be extracted from wind using lift rather than drag, but this requires specially shaped airfoil surfaces, like those used on airplane wings (Figure 2). The airfoil shape is designed to create a differential pressure between the upper and lower surfaces, leading to a net force in the direction perpendicular to the wind direction. Rotors of this type must be carefully oriented (the orientation is referred to as the rotor pitch), to maintain their ability to harness the power of the wind as wind speed changes. Airflow over any surface creates two types of aerodynamic forces— drag forces, in the direction of the airflow, and lift forces, perpendicular to the airflow. Either or both of these can be used to generate the forces needed to rotate the blades of a wind turbine.