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Unformatted text preview: he tube by extruding axial
grooves along its inner surface, but this approach presents manufacturing
The performance of a wick depends on its structure. The characteristics
of a wick can be changed by changing the size and the number of the pores
per unit volume and the continuity of the passageway. Liquid motion in the
wick depends on the dynamic balance between two opposing effects: the
capillary pressure, which creates the suction effect to draw the liquid, and
the internal resistance to flow as a result of friction between the mesh surfaces and the liquid. A small pore size increases the capillary action, since
the capillary pressure is inversely proportional to the effective capillary radius of the mesh. But decreasing the pore size and thus the capillary radius
also increases the friction force opposing the motion. Therefore, the core
size of the mesh should be reduced so long as the increase in capillary force
is greater than the increase in the friction force.
Note that the optimum pore size will be different for different fluids and
different orientations of the heat pipe. An improperly designed wick will
result in an inadequate liquid supply and eventual failure of the heat pipe.
Capillary action permits the heat pipe to operate in any orientation in a
gravity field. However, the performance of a heat pipe will be best when
the capillary and gravity forces act in the same direction (evaporator end
down) and will be worst when these two forces act in opposite directions
(evaporator end up). Gravity does not affect the capillary force when the
heat pipe is in the horizontal position. The heat removal capacity of a horizontal heat pipe can be doubled by installing it vertically with evaporator
end down so that gravity helps the capillary action. In the opposite case,
vertical orientation with evaporator end up, the performance declines considerably relative the horizontal case since the capillary force in this case
must work against the gravity force.
Most heat pipes are cylindrical in shape. However, they can be manufactured in a variety of shapes involving 90° bends, S-tur...
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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