Page 215 - Mechanical Engineers' Handbook (Volume 4)

204 Heat-Transfer Fundamentals

rejection region, and the adiabatic or isothermal region. Heat added to the evaporator region
of the container causes the working fluid in the evaporator wicking structure to be vaporized.
The high temperature and corresponding high pressure in this region result in flow of the
vapor to the other, cooler end of the container where the vapor condenses, giving up its
latent heat of vaporization. The capillary forces existing in the wicking structure then pump
the liquid back to the evaporator section. Other similar devices, referred to as two-phase
thermosyphons have no wick, and utilize gravitational forces to provide the liquid return.
Thus, the heat pipe functions as a nearly isothermal device, adjusting the evaporation rate
to accommodate a wide range of power inputs, while maintaining a relatively constant source
temperature.

Transport Limitations
The transport capacity of a heat pipe is limited by several important mechanisms. Among
these are the capillary wicking limit, viscous limit, sonic limit, entrainment, and boiling
limits. The capillary wicking limit and viscous limits deal with the pressure drops occurring
in the liquid and vapor phases, respectively. The sonic limit results from the occurrence of
choked flow in the vapor passage, while the entrainment limit is due to the high liquid vapor
shear forces developed when the vapor passes in counterflow over the liquid saturated wick.
The boiling limit is reached when the heat flux applied in the evaporator portion is high
enough that nucleate boiling occurs in the evaporator wick, creating vapor bubbles that
partially block the return of fluid.
To function properly, the net capillary pressure difference between the condenser and
the evaporator in a heat pipe must be greater than the pressure losses throughout the liquid
and vapor flow paths. This relationship can be expressed as
P P P P P v
l

c

where P net capillary pressure difference
c
P normal hydrostatic pressure drop

P axial hydrostatic pressure drop

P viscous pressure drop occurring in the liquid phase
l
P viscous pressure drop occurring in the vapor phase.
v
If these conditions are not met, the heat pipe is said to have reached the capillary limitation.
Expressions for each of these terms have been developed for steady-state operation, and
are summarized below.
P
2
Capillary pressure c,m
r c,e
Values for the effective capillary radius, r , can be found theoretically for simple geometries
c
or experimentally for pores or structures of more complex geometry. Table 26 gives values
for some common wicking structures.
Normal and axial hydrostatic pressure drop P gd cos
v

l
P gL sin

l
In a gravitational environment, the axial hydrostatic pressure term may either assist or hinder
the capillary pumping process, depending on whether the tilt of the heat pipe promotes or
hinders the flow of liquid back to the evaporator (i.e., the evaporator lies either below or
above the condenser). In a zero-g environment, both this term and the normal hydrostatic
pressure drop term can be neglected because of the absence of body forces.