Fundamentals of Heat Pipes
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01-10-2010, 12:01 PM

Fundamentals of Heat Pipes

With Applications to Electronics Cooling
-- Widah Saied

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

Attached Files
.ppt   FundamentalsofHeatPipesII.ppt (Size: 1.66 MB / Downloads: 128)
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15-11-2012, 04:01 PM

Fundamentals of Heat Pipes

.ppt   Fundamentals of Heat.ppt (Size: 1.66 MB / Downloads: 51)

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)
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) .

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.

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.

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.

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.

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.

Choosing the Working Fluid

Heat pipes work on a cycle of vaporization and condensation of the working fluid, which results in the heat pipe’s high thermal conductivity. When choosing a working fluid for a heat pipe, the fluid must be able to operate within the heat pipe’s 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.

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 fluid’s interactions with the heat pipe container and wick.

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.

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.

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