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Unformatted text preview: ium. The liquid then returns to the evaporator end of the heat pipe through the wick as
a result of capillary action in the wick, completing the cycle. As a result,
heat is absorbed at one end of the heat pipe and is rejected at the other end,
with the fluid inside serving as a transport medium for heat.
The boiling and condensation processes are associated with extremely
high heat transfer coefficients, and thus it is natural to expect the heat pipe
to be a very effective heat transfer device, since its operation is based on alternative boiling and condensation of the working fluid. Indeed, heat pipes
have effective conductivities several hundred times that of copper or silver.
That is, replacing a copper bar between two mediums at different temperatures by a heat pipe of equal size can increase the rate of heat transfer between those two mediums by several hundred times. A simple heat pipe
with water as the working fluid has an effective thermal conductivity of the
order of 100,000 W/m · °C compared with about 400 W/m · °C for copper.
For a heat pipe, it is not unusual to have an effective conductivity
of 400,000 W/m · °C, which is 1000 times that of copper. A 15-cm-long,
0.6-cm-diameter horizontal cylindrical heat pipe with water inside, for
example, can transfer heat at a rate of 300 W. Therefore, heat pipes are
preferred in some critical applications, despite their high initial cost. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 549 549
CHAPTER 10 There is a small pressure difference between the evaporator and
condenser ends, and thus a small temperature difference between the two
ends of the heat pipe. This temperature difference is usually between 1°C
and 5°C. The Construction of a Heat Pipe
The wick of a heat pipe provides the means for the return of the liquid to
the evaporator. Therefore, the structure of the wick has a strong effect on
the performance of a heat pipe, and the design and construction of the wick
are the most critical aspects of the manufacturing process.
The wicks are often made of porous ceramic or woven stainless wire
mesh. They can also be made together with t...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
- Spring '10