Unformatted text preview: temperature ranges for
some fluids used in heat pipes
Range, °C Helium
1600 cen58933_ch10.qxd 9/4/2002 12:38 PM Page 548 548
HEAT TRANSFER 1942. However, it did not receive much attention until 1962, when it was
suggested for use in space applications. Since then, heat pipes have found
a wide range of applications, including the cooling of electronic equipment. The Operation of a Heat Pipe
The operation of a heat pipe is based on the following physical principles:
• At a specified pressure, a liquid will vaporize or a vapor will condense
at a certain temperature, called the saturation temperature. Thus,
fixing the pressure inside a heat pipe fixes the temperature at which
phase change will occur.
• At a specified pressure or temperature, the amount of heat absorbed as
a unit mass of liquid vaporizes is equal to the amount of heat rejected
as that vapor condenses.
• The capillary pressure developed in a wick will move a liquid
in the wick even against the gravitational field as a result of the
• A fluid in a channel flows in the direction of decreasing pressure.
Initially, the wick of the heat pipe is saturated with liquid and the core
section is filled with vapor. When the evaporator end of the heat pipe is
brought into contact with a hot surface or is placed into a hot environment,
heat will transfer into the heat pipe. Being at a saturated state, the liquid in
the evaporator end of the heat pipe will vaporize as a result of this heat
transfer, causing the vapor pressure there to rise. This resulting pressure
difference drives the vapor through the core of the heat pipe from the evaporator toward the condenser section. The condenser end of the heat pipe is
in a cooler environment, and thus its surface is slightly cooler. The vapor
that comes into contact with this cooler surface condenses, releasing the
heat a vaporization, which is rejected to the surrounding med...
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