Page 359 - Mechanical Engineers' Handbook (Volume 4)

348 Heat Pipes

T T v,e (38)
v
1 (RT /h )ln(p /p )
v,e
e
ƒg
v,e
where T is the vapor temperature in the evaporator and p is the vapor pressure in the
v,e v,e
evaporator. Vapor flows through the adiabatic section to the condenser region. At the con-
densing section, vapor condenses due to the phase-change driving force. The condensed fluid
then is drawn into the wicking structure by a capillary force. Assuming that the heat pipe is
charged with the proper amount of working fluid, the wick surface can be assumed to directly
contact the saturated vapor. Therefore, the film thickness on the wicking structure is neglected
while the temperature drop across the liquid–vapor interface in the condenser is equal to
zero.
4 HEAT-PIPE FABRICATION PROCESSES

4.1 Wicks
The wicks in a heat pipe are used to pump the condensate from the condenser to the evap-
orator. Figure 7 lists several of wick structures currently being used in heat pipes. Among
the wick structures shown in Fig. 7, the grooved, sintered metal, and wrapped screens are
the most common wicks. The grooved wicks have been widely used in the laptop computers
for a relatively low heat load. The groove dimensions significantly affect the capillary lim-
itation and effective thermal conductivity. When the groove configuration is different, as
shown in Fig. 8, the heat-transfer performance changes. For a given application, there exists
an optimized groove configuration for the best heat transfer performance.
Another wick having higher effective thermal conductivity is the sintered metal wick.
The sintered wick can be produced at a temperature 50–200 C below the melting point of
the sintering material. The porosity of the sintered wick depends on the surrounding pressure,
sintering temperature, and time. Also, the environmental gas can significantly help the sin-
tering process. As hydrogen is used as the environmental gas, for example, it helps sinter
the copper material. The heat-transfer performance in a heat pipe with sintered wicks largely
depends on the wick thickness, particle size, and porosity. For a given heat flux, there exists
an optimum design for the evaporating heat transfer in a heat pipe with sintered wicks. As
shown in Fig. 9, when the particle radius decreases, the optimum thickness increases. And
14
it is concluded that, if it is possible to decrease the particle radius while maintaining a
constant porosity, the particle radius should be as small as possible. The impact of these
results is that thicker sintered wicks, which are more readily manufactured and assembled
into heat pipes, can provide heat removal capabilities equivalent to the thinner wicks.
The screen wick has been widely used in the conventional heat pipe. Stainless steel,
copper, nickel, and aluminum meshes are commercially available for the screen wicks. Al-
though the smallest pore size for the copper meshes is about 100 pores per inch, the smallest
pore size for the stainless steel meshes can reach less than 5 m in the hydraulic diameter.

4.2 Working Fluid Selections
Because a heat pipe cannot function below the freezing point or above the critical temper-
ature of its working fluid, the selected working fluid must be within this range. Table 4 lists
some working fluids that can be used in the heat pipe for a given operating temperature. In
addition, the vapor pressure, surface tension, contact angle, and viscosity in the heat pipe