Page 369 - Mechanical Engineers' Handbook (Volume 4)

358 Heat Pipes

the input power is low, the vapor pressure inside the heat pipe is low. The volume of
noncondensable gas expands and reduces the condensing area. When the input power is high,
the vapor pressure increases, resulting in the contraction of vapor volume and directly in-
creasing the condensing area. As a result, the temperature drop from the evaporator to the
condenser can be maintained fairly constant. As shown in Fig. 15, the noncondensable gas
is used to moderate the conductance as the input power is varied, giving the device the name:
gas-loaded variable heat pipe. Based on the same principle, several other VCHPs have been
developed to moderate the conductance change in the heat pipe, such as vapor flow-
modulated heat pipes, excess-liquid heat pipes and liquid-flow-modulated heat pipes (thermal
diodes). 6

5.6 Rotating Heat Pipes
The rotating heat pipe consists of a sealed hollow shaft, which contains a fixed amount of
working fluid. The rotating heat pipe can be divided into two types: those with an internal
taper and those without an internal taper. Comparing with the conventional heat pipe, the
rotating heat pipe has the following features: (1) the condensate in the rotating heat pipe is
returned to the evaporator by the centrifugal force; (2) the rotational speed plays the most
important role for its heat-transfer performance; (3) the heat-transfer performance is enhanced
for a rotating heat pipe with an internal taper because the removal of the condensate from
the cooled liquid surface by the centrifugal action helps the condensate back toward the
evaporator; and (4) the sonic, entrainment, boiling, and condensing limits are the primary
factors limiting the heat-transfer capacity in the rotating heat pipe. For the detailed infor-
6
mation related to the rotating heat pipes, please see the books written by Peterson and
Faghri. 9

5.7 High-Temperature Heat Pipes (Metal Heat Pipes)
Because most working fluids in high-temperature heat pipes are metal, the high-temperature
heat pipe is also called the metal heat pipe. Due to the higher surface tension, higher latent
heat of vaporization, and higher thermal conductivity, the high-temperature heat pipes can
transport large heat load and reach a very high level of temperature uniformity, and have
been employed for the advanced energy system such as advanced thermophotovoltaic, ad-
vanced gas turbine engines, and nuclear reactors. Table 4 lists some common used working
fluids and their temperature ranges. However, comparing with those heat pipes operating at
other temperature ranges, within the high-temperature heat pipe exists some obstructions
such as corrosion/reliability concern, high chemical reactivity, start-up control, and severe
work conditions due to too high an operating temperature.

5.8 Cryogenic Heat Pipes
Cryogenic fluids are used in the cryogenic heat pipe. These working fluids are either a
chemically pure material such as helium, argon, krypton, nitrogen, or oxygen or a chemical
compound such as methane, ethane, or Freon. The operating temperature ranges for these
fluids are listed in Table 4. For cryogenic fluids at a low temperature, the surface tension,
thermal conductivity, and latent heat of vaporization are relatively low and the liquid vis-
cosity is much higher. As a result, the heat pipe optimized for zero-g operation would not
properly prime in a one-g environment due to the low surface tension. The very high vapor
pressure in the heat pipe during storage and the low operating temperature, where the cooling
methods to remove heat from the condenser are limited, are also of concern for cryogenic