Page 347 - Mechanical Engineers' Handbook (Volume 4)
P. 347
336 Heat Pipes
Heat input Heat output
wick
Vapor flow
Evaporator Adiabatic section Condenser
Figure 1 Schematic of a heat pipe.
the pumping pressure produced by the surface tension cannot overcome the summation of
the total pressures, the heat transport occurring in the heat pipe reaches a limit known as
the capillary limit. There are several other limitations disconnecting the return of the working
fluid from the evaporator to the condenser or from the condenser to the evaporator. Among
these are the boiling limit, sonic limit, entrainment limit, and viscous limit. When the heat
flux added to the evaporator is sufficiently high, nucleate boiling occurs. The bubble formed
in the wick significantly increases the thermal resistance, causing the heat-transfer perform-
ance to be significantly reduced. More importantly, when the heat flux is so high, the bubbles
block the return of the working fluid and lead to a dryout of the evaporator. The boiling
limit plays a key role in a high heat flux heat pipe. When the vapor velocity is high and the
cross-sectional area variation of the vapor space in a heat pipe cannot meet the flow con-
dition, chocked flow occurs and the vapor flow rate will not respond with the amount of
heat added in the evaporator. This will lead to a sonic limit. The entrainment limit is due to
the frictional shear stresses caused by the vapor flow at the vapor–liquid interface. The
viscous limit occurs in a low heat flux heat pipe, where the vapor pressure difference in the
vapor phase cannot overcome the vapor pressure drop in the vapor phase.
From a thermodynamics point of view, the thermal energy added to the evaporator in a
functional heat pipe produces the mechanical work to pump the working fluid. No external
power is needed for a typical heat pipe. The phase-change heat transfer occurs almost in the
quasi-equilibrium state. The heat pipe has a very high efficiency to transfer the thermal
energy from a higher-temperature heat source to a lower-temperature heat source. An oper-
ational heat pipe can provide an extra-high effective thermal conductivity and reach a higher
level of temperature uniformity. The working fluid medium in a heat pipe can be selected
from a variety of fluids, depending on the operating temperature and compatibility with the
shell material. The heat pipe can be operated from a temperature lower than4Ktoa high
temperature up to 3000 K. Because the evaporator and condenser of a heat pipe function
independently, the heat pipe can be made into any shape, depending on the design require-
ment. Due to these unique features, the heat pipe has been widely used in a wide range of
applications.