Establishing low-power operating limits for liquid metal heat pipes Page: 4 of 10
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heat pipes. Data from the LTE w"! he used
in the development of the computer code
used to model mass migration phenomena.
Conclusions derived from that data will be
used when designing the high temperature
experiments (HTE).
LTE Working Fluid Selection
Prior to conducting the LTE experiments, it
was necessary to select a working fluid and
to validate its usefulness through prelim-
inary heat pipe performance tests. Selection
of the LTE working fluid was made with the
objective of achieving operation in a similar
manner to a liquid metal in a heat pipe.
Criteria for selecting a low temperature
heat pipe working fluid are listed below:
- Useful operating temperature below
470 K in order to facilitate bench
testing. Instrumentation require-
ments and insulation requirements
are greatly simplified as the operating
temperature approaches room
temperature. In addition handling and
data collection are simplified, requir-
ing no specialized equipment.
Thermophysical properties affecting
useful operating temperature include
vapor pressure, liquid and vapor
kinematic viscosities, latent heat of
vaporization, surface tension and
liquid density.
* Free molecular flow behavior near
the triflepLin.Qa The working fluid
vapor pressure and density must be
very low near the triple point (i.e.,
on the order of 107 atm.).
* Triple point above the dew point.
This prevents mass accumulation on
the outside of the heat pipe while
under operation as a result of conden-
sation of water from the surroundings.
* Relatively low saturation vapor
pressure at the operating temqr-
atILLL This helps to minimize the
mass required for the containment
structure. A saturation pressureslightly below atmospheric is prefer-
able because this mitigates the hazard
to the experimenters in the unlikely
event of a rupture or leak in the
containment vessel.
- Compatible with materials of
construction. A useful working fluid
must be nonreactive with the selected
materials of construction.
- Low toxicity. hazard potential and
nuisance factor.
Hundreds of candidate fluids wero screened
based on the criteria established above.
Those with greatest merit are listed in
Table 1 along with some of their notable
properties and characteristics.
Of the candidates listed in Table 1
nonadecane, octadecane and polyethylene
glycol E600 were considered most favor-
able. Because more property data were
available in the literature for octadecane
than for " 3 other two, octadecane was
selected or use in the low temperature heat
pipe experiments. Octadecane appears to
satisfy all of the design criteria.
Preliminary Heat Pipe Tests
The intent of the preliminary heat pipe tests
was to determine whether the selected
Working fluid would perform s itisfactorily
in a heat pipe, in particular, to determine
whether it would transport sufficient heat
and exhibit sufficient wicking capability.
For these experiments, a simple
transparent heat pipe was constructed. It
comprised a quartz envelope and a
homogeneous, 250 mesh staless steel
wick. The heat pipe was operated at steady
state for successively greater static lift
heights. The temperature profile at each
orientation was recorded. A heat pipe
performance limit was indicated when the
evaporator temperature rose sharply. Such
a condition is a result of insufficient fluid
circulation. This condition is often termed a
capillary limit.3
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Secary, J. (Phillips Lab., Kirtland AFB, NM (United States)); Merrigan, M.A. & Keddy, M.D. (Los Alamos National Lab., NM (United States)). Establishing low-power operating limits for liquid metal heat pipes, article, January 1, 1992; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1052626/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.