Angirasa Devarakonda
Sest, Inc., Middleburg Heights, Ohio
William G. Anderson
Advanced Cooling Technologies, Lancaster, Pennsylvania
Thermo-Physical Properties of Intermediate
Temperature Heat Pipe Fluids
NASA/CR—2005-213582
March 2005
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Angirasa Devarakonda
Sest, Inc., Middleburg Heights, Ohio
William G. Anderson
Advanced Cooling Technologies, Lancaster, Pennsylvania
Thermo-Physical Properties of Intermediate
Temperature Heat Pipe Fluids
NASA/CR—2005-213582
March 2005
National Aeronautics and
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Glenn Research Center
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Thermo-Physical Properties of Intermediate Temperature
Heat Pipe Fluids
Abstract. Heat pipes are among the most promising technologies for space radiator systems. The paper reports further
evaluation of potential heat pipe fluids in the intermediate temperature range of 400 to 700 K in continuation of two
recent reports. More thermo-physical property data are examined. Organic, inorganic and elemental substances are
considered. The evaluation of surface tension and other fluid properties are examined. Halides are evaluated as
potential heat pipe fluids. Reliable data are not available for all fluids and further database development is necessary.
Many of the fluids considered are promising candidates as heat pipe fluids. Water is promising as a heat pipe fluid up
to 500-550 K. Life test data for thermo-chemical compatibility are almost non-existent.
INTRODUCTION
The National Aeronautics and Space Administration (NASA) is developing advanced space power conversion systems
for deep space science missions. As a part of this effort, the necessary radiator technologies for rejecting large amount
of waste heat into space are also being developed. In order to realize the twin objectives of higher power conversion
efficiencies and smaller radiator size, the temperature range of 400 – 700 K for heat rejection is considered as an
envelope.
Heat pipes are among the most promising heat transport and heat spreading devices for incorporating into space
radiator systems. A heat pipe is a passive two-phase sealed device that transports large amount of heat with minimal
temperature drop. Heat pipe technologies differ considerably depending on the operational temperature range. The
thermo-physical properties of the heat pipe fluid and the thermo-chemical compatibility of the envelope and wick
material with the fluid are the main scientific issues. The temperature range of 400-700 K is defined as Intermediate
(Devarakonda and Olminsky, 2004). While heat pipe technologies have matured in other temperature ranges over the
years, in the intermediate range they are less developed (Anderson et al., 2004). NASA Glenn Research Center is
investigating intermediate temperature heat pipe technologies. As an aspect of this technology development drive, an
evaluation of as many potential heat pipe fluids in this temperature range as possible was undertaken. This paper is a
continuation of this evaluation effort following two other recent reports (Anderson et al., 2004; Devarakonda and
Olminsky, 2004). More accurate thermodynamic and thermo-physical property data for some of the fluids discussed in
the earlier reports have since been obtained and data for some other fluids have been gathered and analyzed for this
report.
INTERMEDIATE TEMPERATURE HEAT PIPE FLUIDS
Devarakonda and Olminsky (2004) list the requisite properties of potential heat pipe fluids for intermediate temperature
heat pipes. These may be summarized as
Angirasa Devarakonda
Sest, Inc.
Middleburg Heights, Ohio 44130
William G. Anderson
Advanced Cooling Technologies
Lancaster, Pennsylvania 17601
NASA/CR—2005-213582 1
• wet a metallic solid surface
• high latent heat of evaporation
• melting point below ~400 K
• critical point above ~800 K
• chemically stable in the above temperature range, i.e., no ionization and dissociation
• low liquid viscosity
• high surface tension
• non-toxic, non-carcinogenic
• non-volatile/low volatility
These requisites are only indicative for the entire intermediate temperature range, i.e. 400 to 700 K. This range
appears somewhat arbitrary. The rationale behind it is that below 400 K, water heat pipe technology is well-
established, and above 700 K, alkali metal heat pipes work very well. Even if a fluid is potentially suitable for only a
segment of this range, it is still considered for evaluation.
Two quantitative factors used in evaluating the suitability of a given fluid for heat pipe application are its vapor
pressure and a merit number (also variously called the liquid transport factor or the figure of merit), M, defined as
L
L
M
µ
λ
⋅
σ
⋅
ρ
= (1)
The merit number is formed by grouping the desirable properties of the fluid in the numerator and the less desirable in
the denominator. Hence, the larger the value of M, the more suitable is the fluid for heat pipes. The importance of each
of these properties individually and in combination in the evaluation of heat pipe fluids will be discussed. First a list of
potential heat pipe fluids is chosen based on their melting and critical points. Stull (1947) presented vapor pressure data
for an exhaustive list of elemental, organic and inorganic fluids.
The initial screening and evaluation of a heat pipe fluid is done with the vapor pressure and merit number data. If a
fluid shows promise on this basis, then thermo-chemical compatibility studies with potential envelope and wick metals
will be undertaken.
SOME CANDIDATE FLUIDS AND PROPERTIES
Available test data support water heat pipe’s operational temperature up to 400 K (Devarakonda and Olminisky, 2004).
The rule of thumb for maximum operational temperature for a fluid is about 100 K below its critical temperature.
Hence water heat pipes are candidates for operational temperatures ~550 K and somewhat above. Test data are needed
for water heat pipes in the range 400 to above 550 K. One important issue that needs to be addressed with water at
these temperatures is its high vapor pressure. The vapor pressure of a heat pipe fluid at the operational temperature is an
important factor. High vapor pressures require thick envelope walls as well as stronger braze/welds. Too low vapor
pressure will result in large temperature gradients along the length of the heat pipe.
The list of potential heat pipe fluids for the desired operating temperature range is shown in Tables 1 and 2. Data are
collected from various sources including CRC Handbook (1976), Janz (1988), Meyer et al. (1993), Ohse (1985), Perry
and Green (1984), Reid, Prausnitz and Poling (1987), Stull (1947) and Smithells (1983). Water data are given as
benchmark and as a potential heat pipe fluid up to 550 K. Because of their wide ranging applications in commercial
electronic cooling, water heat pipe technologies have been refined over the years. However, most of the life test data
are at lower temperatures, which are not suitable for intermediate temperature radiators. Anderson and Stern (2005)
report on some on-going life test data, which indicate that titanium and Monel K-500 are potential envelope and wick
materials. Life test data at the intermediate temperatures for copper-water heat pipes are not available.
Halides and Elemental Pure Substances
A halide is a compound of the type MX, where M may be another element or organic compound, and X may be
fluorine, chlorine, bromine, iodine, or astatine. Some of the halides are thought to have the desirable properties and
NASA/CR—2005-213582 2
characteristics of an intermediate temperature heat pipe fluid. In addition, elements like sulfur and iodine are also
surveyed.
Unfortunately, the physical property data for many of the halides are incomplete. As shown in Table 2, many of the
fluids do not have data for at least one of liquid density, liquid viscosity, or surface tension data. The known
properties of one halide in a family may be used to estimate the properties for related halides, i.e., use the AlCl
3
data
to estimate AlBr
3
and AlI
3
properties. The approach taken below is to use the fluid property estimation methods
discussed in Reid, Prausnitz, and Poling (1987). If a method is found that can calculate known properties in a
family, then that method is used to estimate liquid viscosities in other families, based on the known data. In some
cases, there are no data for any fluids in a particular halide family. These halides were not considered further, since
there were no data (typically surface tension and/or liquid viscosity) for any member of the family:
• BCl
3
, BBr
3
, BI
3
• SiCl
4
, SiBr
4
, SiI
4
Possible halide fluids are then the halides of Al, Bi, Ga, Sb, Sn, and Ti.
Surface Tension Estimation
Reid, Prausnitz, and Poling (1987) suggest that surface tension can be estimated using a corresponding states
equation, when the critical properties and normal boiling point are known. A parameter Q is first calculated as
()
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
−
+=
Critical
Boil
Critical
Critical
Boil
T
T
P
T
T
Q
1
ln
11196.0
(2)
where P
Critical
has units of bar and T
critical
, Kelvin. The surface tension is then calculated with
m
N
10
T
K
1
K
T
Bar
P
Q
3
9
11
Critical
3
1
Critical
3
2
Critical
−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
⎟
⎠
⎞
⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛
=σ (3)
This equation was used to estimate the iodine surface tension. Better agreement with known values of halide can be
obtained by multiplying the above equation by 2/3 as
σ=σ
3
2
Halide
(4)
Experimental and estimated values of surface tension are shown in Fig. 1. The agreement between the two is
excellent for the three fluids where both surface tension and critical properties are known.
Vapor Pressure and Merit Number
Anderson et al. (2004) discussed the evaluation of the organic fluids, aniline, naphthalene, toluene, hydrazine, and
phenol. In general, the vapor pressures as well as the merit numbers of the organic fluids are lower than those of
water at a given temperature. Organic molecules are heavy. Hence, an important issue that needs experimental
verification is the chemical stability (i.e., whether or not they decompose) of the fluids at elevated temperatures
subjected to radiation, and the continuous cycling of evaporation and condensation. For heat pipes suitable for space
radiators, cesium is a good working fluid at temperatures above ~ 700 K (Anderson, 2005). It has been used in both
heat pipes and loop heat pipes (Anderson et al., 1995). Below 700 K the vapor pressure is low, vapor velocities are
high, and the viscous and sonic limits control the heat pipe. Vapor pressure data for a number of fluids are plotted in
Fig. 2 with water as the high end limit and Cesium as low end. The vapor pressures of all the substances considered
NASA/CR—2005-213582 3
fall within this range. Judging purely from the point of view of vapor pressure, any or all of these fluids are
candidates for intermediate temperature heat pipes.
Tem
p
erature
(
K
)
250 300 350 400 450 500 550 600 650
Surface Tension (N/m)
0.00
0.01
0.02
0.03
0.04
AlCl
3
Experimental
AlCl
3
Estimated
SnCl
4
Experimental
SnCl
4
Estimated
TiCl
4
Experimental
TiCl
4
Estimated
AlCl
3
SnCl
4
TiCl
4
FIGURE 1. Surface Tension Calculated with the Corresponding State Equation Shows Good Agreement with the Experimental
Value.
Temperature (K)
400 500 600 700 800
Vapor Pressure (Pa)
1e+2
1e+3
1e+4
1e+5
1e+6
1e+7
Water
SnCl
4
TiCl
4
Iodine
Cesium
TiI
4
TiBr
4
BiI
3
GaCl
3
SbBr
3
BiCl
3
AlBr
3
FIGURE 2. Vapor Pressure Data for Intermediate Temperature Heat Pipe Fluids.
Merit number data are presented in Fig. 3. Water has the highest merit number by a large margin even at 600 K,
although, steeply declining, and its vapor pressure, sharply increasing with temperature. Provided the high vapor
pressure issue is addressed as discussed above, water appears to be an attractive option as a heat pipe fluid in the
temperature range up to about 550 K. The merit numbers for BiCl
3
, GaCl
3
, and GaBr
3
use a constant latent heat at
the normal boiling point, since their critical temperatures are not known.
The viscosity of sulfur is too high in this temperature range to make it a candidate fluid. However, the addition of
small amount of iodine (90%S-10%I) will offset this adverse effect (Polasec and Stulc, 1976; Rosenfeld and
Lindemuth, 1992). Pure Iodine is also a candidate fluid, and has been used in a heat pipe (Rosenfeld and Lindemuth,
1992). Iodine’s two disadvantages are (1) low thermal conductivity (sintered metal wicks must be used in the heat
pipe) and (2) corrosiveness, since it is a very reactive fluid. Lindemuth and Rosenfeld (1993) fabricated and tested
an iodine heat pipe with a 304 SS wall. The wick was two wraps of 400-mesh 304-stainless- steel screen. The pipe
was operated successfully with a relatively high temperature drop, but was not life tested.
NASA/CR—2005-213582 4
All the halides evaluated are potentially good candidates as intermediate temperature heat pipe fluids because of
their moderate vapor pressures and reasonably good values of merit numbers. It should be noted that the halides
shown are the only halides for which sufficient data exist to estimate merit number; other halides might actually be
better heat pipe fluids. Data of thermo-chemical compatibility with envelope and wick metals are scarce and only a
small amount of life test data exist (Saaski and Tower, 1977, Anderson et al, 2004). Further tests are in progress in
NASA Glenn Research Center.
From Fig. 3, two interesting halides are antimony tribromide (Sb Br
3
) and bismuth trichloride (Bi Cl
3
). Anderson
(2005) evaluates heat pipes with these two working fluids. Bismuth trichloride is similar in behavior to sulfur/10%
iodine. Its vapor pressure is not high in the intermediate temperature range and its merit number is lower only to
that of water at elevated temperatures. The merit number increases with temperature in the range where water’s is
steeply falling. As shown in Fig.4, it has a very high liquid viscosity. This is partially offset by their high surface
tension, as shown in Fig. 5.
Tem
p
erature
(
K
)
200 300 400 500 600 700 800
Figure of Merit (W/m
2
)
1e+9
1e+10
1e+11
1e+12
Sulfur/Iodine
Iodine
TiCl
4
SnCl
4
BiCl
3
AlBr
3
GaCl
3
GaBr
3
Water
SbBr
3
FIGURE 3. Merit Number for Various Intermediate Temperature Fluids.
Temperature (K)
300 400 500 600 700
Liquid Viscosity (cP)
0
5
10
15
20
25
30
35
Water
Sulfur/Iodine
BiCl
3
FIGURE 4. Experimental Liquid Viscosities for water, Sulfur/10% iodine, and BiCl
3
.
NASA/CR—2005-213582 5
Temperature (K)
300 400 500 600 700
Surface Tension (N/m)
0.00
0.02
0.04
0.06
0.08
Water
AlCl
3
BiCl
3
BiBr
3
Sulfur/Iodine
Water
AlCl
3
Sulfur/10% Iodine
BiCl
3
BiBr
3
FIGURE 5. Experimental Values of Surface Tension for Water, Sulfur/10% Iodine, and the Aluminum and Bismuth Halides.
TABLE 1. Intermediate Temperature Heat Pipe Fluids – Organic and Elemental.
Fluid Composition
Melting
Point, K
Boiling
Temp., K
Critical
Temp., K
Critical Pressure,
MPa
Water H
2
O 273 373 647 22.12
Dowtherm A Diphenyl/Diphenyl Oxide 285 530 770 3.135
Sulfur S 386 718 1314 20.7
Sulfur/10% Iodine S/10%I 390 — — —
Bromine Br 266 332 588 10.3
Iodine I
2
387 458 785 11.6
Naphthalene C
10
H
8
354 491 748 4.05
Phenol C
6
H
6
O 314 455 694 6.13
Toluene C
6
H
5
CH
3
178 384 592 4.1
Hydrazine N
2
H
4
275 387 653 14.7
Aniline C
6
H
7
N 267 458 699 5.3
NASA/CR—2005-213582 6
TABLE 2. Candidate Fluids and Their Properties, Including Halides with Incomplete Data.
Fluid
Melting
Point, K
Boiling
Point, K
Critical
Temp., K
Critical
Press.
Atm.
Latent
Heat
(Boiling
Point)
kJ/kg
Liquid Density
kg/m
3
Surface Tension,
N/m Viscosity, cPoise
Water 273 373 647 218.3 2257 832 (500 K) 0.0315 (500 K) 0.118 (500 K)
Cs 302 941 2045 114.7 531.7 1667 (600 K) 0.0557 (600 K) 0.250 (600 K)
S/I 390 718 (S) 1314 (S) 204.3 (S) 309.3 1717 (600 K) 0.0490 (600 K) 21.0 (600 K)
I
2
387 458 785 116 171.4
3740 (453 K) 0.031 (550 K) e 1.27 (475 K)
AlCl
3
451 456 sub 620 26.0 — 1202 (500 K) 0.0072 (500 K) 0.244 (500 K)
AlBr
3
370 528 763 28.5 85.5 2331 (500 K) 0.016 (500 K) e 0.809 (500 K)
AlI
3
464 658 983 — 158.2 3133 (500 K) — 2.10 (500 K)
BCl
3
165.9 285.8 455 38.2 203.15 1340 (285 K) — —
BBr
3
227 364 581 — — 2643 (291 K) — —
BI
3
323.1 483 773 — — 3350 (323 K) — —
BiCl
3
503 714 — — 229.6 3693 (600 K) 0.0586 (600 K) 19.4 (600 K)
BiBr
3
491 734 — — 168 4477 (600 K) 0.0586 (600 K) —
BiI
3
681 ~773 — — — 4866 (600 K) — —
GaCl
3
351 474 — — 356.7 1743 (500 K) 0.0124 (500 K) 0.449 (500 K)
GaBr
3
395 552 — — 189.4 2770 (500 K) 0.0195 (500 K) 1.027 (500 K)
GaI
3
485 618 sub — — — 3590 (500 K) — —
SbCl
3
346 556 — — 190.7 2329 (500 K) — 0.337 (500 K)
SbBr
3
370 553 1178 55 163.2 3193 (500 K) 0.026 (550 K) e 0.957 (500 K)
SbI
3
443 674 — — 136.7 3776 (600 K) — —
SiCl
4
204.3 330.8 508.1 35.4 170.1 1259 (400 K) — —
SiBr
4
278.6 427 663 — — 2852 (298 K) — —
SiI
4
393.7 560.5 944 — 93.7 3589 (500 K) — —
SnCl
4
240 388 592 37.0 130.1 1944 (400 K) 0.0155 (400 K) 0.360 (400 K)
SnBr
4
303 478 — — 93.5 — — —
SnI
4
418 638 — — 80.15 3834 (400 K) — —
TiCl
4
243 409.6 638 46.0 190.9 1543 (400 K) 0.0211 (400 K) 0.391 (400 K)
TiBr
4
312 506 795.7 — 120.8 2758 (400 K) — —
TiI
4
427 650 1040 — 101 3258 (500 K) — —
Sub= Sublime
e=estimate
CONCLUSIONS
Further evaluations are reported for potential intermediate temperature fluids in the range, 400 to 700 K. Only
limited amount of data could be obtained for halides, many of which may be suitable as heat pipe fluids. Some
properties are estimated based on standard procedures. Limited data are available for chemical compatibility tests
with potential envelope and wick materials. Life test data are nearly non-existent. More detailed compatibility
studies are currently undertaken at NASA Glenn Research Center.
NASA/CR—2005-213582 7
NOMENCLATURE
M =Merit number (W/m
2
)
ρ
L
=Liquid density (kg/m
3
)
σ =Surface tension (N/m)
λ =Latent heat (J/kg)
µ
L
=Liquid viscosity (kg/m s)
P
Boil
=Boiling Temperature, 1 atm (K)
T
Critical
=Critical Temperature (K)
P
Critical
=Critical Pressure (Bar)
REFERENCES
Anderson, W. G., and Stern, T., “Heat Pipe Radiator Trade Study for the 300-550 K Temperature Range,” STAIF 2005,
Albuquerque, NM, February 13-17, 2005.
Anderson, W. G., “Evaluation of Heat Pipes in the Temperature Range of 450 to 700 K,” STAIF 2005, Albuquerque, NM,
February 13-17, 2005.
Anderson, W. G., Rosenfeld, J. H., Angirasa, D., and Me, Y., “Evaluation of Heat Pipe Working Fluids in the Temperature
Range 450 to 700 K,” STAIF-2004, Albuquerque, NM, 2004.
Anderson, W. G., et al., “High Temperature Loop Heat Pipes,” Proceedings of the 30th Annual IECEC, Vol. 1, pp. 543-548,
Orlando, FL, 1995.
CRC Handbook of Chemistry and Physics, 57
th
Edition, CRC Press, Cleveland, OH, 1976.
Devarakonda, A., and Olminsky, J. K., “An Evaluation of Halides and Other Substances as Potential Heat Pipe Fluids,” 2
nd
International Energy Conversion Engineering Conference, Providence, Rhode Island, 2004.
Janz, G. J., Thermodynamic and Transport Properties for Molten Salts: Correlation Equations for Critically Evaluated Density,
Surface Tension, Electrical Conductance, and Viscosity Data, Journal of Physical and Chemical Reference Data, Volume
17, Supplement No. 2, National Bureau of Standards, 1988.
Lindemuth, J. E. and Rosenfeld, J. H., “Heat Pipe Cooling of Faraday Shields”, Final Report Contract DE-F601-90-ER81058,
July 9, 1993.
Meyer, C. A., McClintock, R. B., Silvestri, G. J., and Spencer, R. C., ASME Steam Tables: Thermodynamic and Transport
Properties of Steam, 6th Ed., ASME Press, New York, New York, 1993.
Ohse, R. W., Editor, Handbook of Thermodynamic and Transport Properties of Alkali Metals, Blackwell Scientific Publications,
Boston, MA, 1985.
Perry, R. H., and Green, D., Perry’s Chemical Engineers Handbook, Sixth Edition, McGraw Hill, New York, New York, 1984.
Polasek, F., and Stulc, P., “Heat Pipe for the Temperature Range from 200 to 600°C,” Proc., Second International Heat Pipe
Conference, Bologna, Italy, 2, pg. 711, 1976.
Reid, R. C., Prausnitz, J. M., and Poling, B. E., The Properties of Gases and Liquids, Fourth Edition, McGraw Hill, New York,
New York, 1987
Rosenfeld, J. H., and Lindemuth, J. E., “Sulfur Heat Pipes for 600 K Space Heat Rejection Systems,” Final Report for NASA
LERC, Contract No. NAS3-26324, 1992.
Saaski, E., and Tower, L., “Two-phase Working Fluids for the Temperature Range 100-350°C,” American Institute of
Aeronautics and Astronautics, 12th Thermophysics Conference, Albuquerque, NM., June 27-29, 1977.
Smithells Metals Reference Book, 6th Edition, E. A. Brandes, Editor, Butterworths, Boston, MA, 1983.
Stull, D. R., “Vapor Pressure of Pure Substances”, Ind. Engrg. Chem., Vol. 39, 517-550, 1947.
NASA/CR—2005-213582 8
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E–15049
WBS–22–973–80–40
NAS3–03064
14
Thermo-Physical Properties of Intermediate Temperature Heat Pipe Fluids
Angirasa Devarakonda and William G. Anderson
Heat pipes
Unclassified -Unlimited
Subject Category: 20
Sest, Inc.
18000 Jefferson Park
Suite 104
Middleburg Heights, Ohio 44130
Prepared for the Space Technology and Applications International Forum (STAIF–2005) sponsored by the University of
New Mexico’s Institute for Space and Nuclear Power Studies (UNM-ISNPS), Albuquerque, New Mexico, February 13–17,
2005. Angirasa Devarakonda, Sest, Inc., 18000 Jefferson Park, Suite 104, Middleburg Heights, Ohio 44130, e-mail:
[email protected], 216–433–3914 and William G. Anderson, Advanced Cooling Technologies,
1046 New Holland Avenue, Lancaster, Pennsylvania 17601, e-mail: Bill.Anderson@1-ACT.com, 717–295–6059. Project
Manager, Duane E. Beach, Power and Electrical Propulsion Division, NASA Glenn Research Center, organization code
RPT, 216–433–6285.
Heat pipes are among the most promising technologies for space radiator systems. The paper reports further evaluation
of potential heat pipe fluids in the intermediate temperature range of 400 to 700 K in continuation of two recent reports.
More thermo-physical property data are examined. Organic, inorganic, and elemental substances are considered. The
evaluation of surface tension and other fluid properties are examined. Halides are evaluated as potential heat pipe fluids.
Reliable data are not available for all fluids and further database development is necessary. Many of the fluids consid-
ered are promising candidates as heat pipe fluids. Water is promising as a heat pipe fluid up to 500 to 550 K. Life test
data for thermo-chemical compatibility are almost non-existent.
