A prototypic heat pipe system for an out-of-core thermionic reactor was built and tested. The emitter of the concentric thermionic converter consists of the condenser of a tungsten heat pipe utilizing a lithium working fluid. The evaporator section of the emitter heat pipe is radiation heated to simulate the thermal input from the nuclear reactor. The emitter heat pipe thermal transport is matched to the thermionic converter input requirement. The collector heat pipe of niobium, 1% zirconium alloy uses potassium as the working fluid. The thermionic collector is coupled to the heat pipe by a tapered conical joint designed to minimize the temperature drop. The collector heat flux matches the design requirements of the thermionic converter

Kroeger, E. W.

Miskolczy, G.

English

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https://ntrs.nasa.gov/search.jsp?R=19780018447 2020-03-22T03:11:40+00:00Z
NASA TECHNICAL	 NASA TM-78946
MEMORANDUM
(NASA-TM-78940)
	 LITHIUM AND POTASSIUM HEAL
FIFES FOR THERMIONIC COFYEFIERi (NASA)
	
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1478-26390
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G3/34 23315
LITHIUM AND POTASSIUM HEAT PIPES
FOR THERM IONIC CONVERTERS
by Gabor Miskolczy
Thermo Electron Corporation
Waltham, Massachusetts
and
Erich Kroeger
Lewis Research Center
Cleveland, Ohio
TECHNICAL PAPER to be presented at the
Thirteenth Intersociety Energy Conversion Engineering; Conference
sponsored by the SAE, ACS, AIAA, ASME, IEEE, AIChE, and ANS
San Diego, California, August 20-25, 1978
.,r
1
LITHIUM AND POTASSIUM HEAT PIPES
FOR THERMIONIC CONVERTERS
Gabor Miskolczy *
Thernio Electron Corporation
Waltham, Massachusetts 02154
Erich Krueger
NASA Lewis Research Center
Cleveland, Ohio 44135
ORIGN. AL Pp,GE lb
OF P WR (AIJ a.L11T
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W
ABSTRACT
A prototypic heat pipe system for an out-of-
core thermionic reactor has been built and tested.
The emitter of the concentric thermionic conver-
ter consists of the condenser of a tungsten heat
pipe utilizing a lithium working fluid. The evap-
orator section of the emitter heat pipe is radia-
tion heated to simulate the thermal input from the
nuclear reactor. The emitter heat pipe thermal
transport is matched to the thermionic converter
input requirement. The collector heat pipe of
niobium, 1 °J -zircor.iun; alloy uses potassium as
the working fluid. The thermionic collector is
coupled to the heat pipe by a tapered conical joint
designed to minimize the temperature drop. The
area ratio of the evaporator to condenser is 16:1,
which increases the radiation area. The com-
posite wick structure c„nsists of seven arteries
and cylindrical wraps. The collector heat flu-:
matches the design requirements of the therm-
ionic converter.
CYLINDRICAL THERMIONIC CONVERTERS with
heat pipe-heated, heat pipe-cooled electrodes are
the best representation of state-of-the-arttech-
nology in space power systems. Such converters
can be used for a wide range of space power ap-
plications, including nuclear electric propulsion,
space solar thermionics, and space isotopic
power systems. Heat pipe thermionics results
in a light-weight, mechanically simple power
system: there are no moving parts; relatively
small radiators are required because of high heat
rejection temperatures; and, because thermionic
power systems are modular in construction,
single-point failures are automatically eliminated
Furthermore, development costs are reduced
because power systems can easily be tested by
building scaled models. Such a scaled, three-
converter module has been designed, faoricated,
and tested. Each heat pipe converter in the
module consists of three major components: the
emitter emitter heat pipe assembly, the collector
* Work supported by NASA Contract No. NAS
3-20270.
assembly, and the collector heat pipe. Figure I
shows a cutaway view of the completed cylindrical
converter.
A
COLlfc,e+ 
Nf., n ►[
FNO.^ io
^COLLEC,OR
COL
”
 FEIL	
- rr '•EA
ll'
.14wL.,nr7 t
	
J.
fMl„[r
T”,
[rn t[r^ I
Fig. 1 - Cutaway view of cylindrical converter
The emitter/emitter heat pipe dss,^mbly was
made from a single tube of either arc-cast or CVC
(chemical vapor deposition) tungsten. The emitter
heat pipe had an outside diameter of 19 mm and
was 222 mm long. The actual emitting surface,
the condenser end of the heat pipe, had an area of
35 cm 2 . A tungsten wire mesh was used for a
wick, and the working fluid was lithium. The col-
lector was constructed from a niobium, 1
zirconium alloy.lloy. The active collector surface
consisted of a vapor-deposited layer of tungsten
oxide. The collector heat pipe was also made
from niobium, 1 47-zirconium. This heat pipe had
an outside diameter of 70 mm and was 515 mm
long. A multiple screen wick artery systen: was
used for the liquid return using potassium as the
working fluid. The collector heat pipe was bonded
onto the collector r.ssembly with a tapered conical
joint.
4-
ik
Three heat pipe converters were then assem-
bled into a completed module. This paper gives
the details of the design, construction, and testing
of the emitter and .ollector heat pipes. The con-
struction and testing of the converter and module
have been described previously. (1)*
EMITTER HEAT PIPE
FABRICATION-The emitter of the thermionic
converter is heated by radiation using a heat pipe
that is integral with the emitter. The heat pipe
allows the heat source to be remote from the
emitter surface. The design requirements spe-
cified a tungsten emitter surface; thus, the whole
emitter-heat pipe assembly was fabricated from
a single tungsten tube. The crystal structure of
the tungsten was closely controlled, thus allowing
the welding of the end caps. The desirable elon-
gated crystal structure can be obtained by extrud-
ing an arc -vacuum -cast tungsten billet, and mak-
ing the tube from the extrusion by electrical dis-
charge machining. Alternatively, the tube can be
formed by CVD on a mandrel that is later dis-
solved. Heat pipes were fabricated by both
methods.
Two end caps are attached to the tungsten tube
by electron beam welding. One end cap has a pro-
jection used to center the emitter in the collector
assembly; the other end cap is equipped with a
thin-walled tungsten tube, as shown in Figure Z.
4	 - _--
57.7111T
(EMITTER)
 *__I9304-
(00)
II
	
ff	 a	 o(ADIABATIC)
222 25—
858 TT
(HEATED)
Fig. 2 - Emitter heat pipe
`Numbers in parentheses designate References
at the end of the paper.
A tungsten wick and wick retainer are inserted
prior to welding the second end cap. The moly-
rhenium emitter sleeve assembly is welded on at
this time. A photograph of a completed emitter
heat pipe is shown in Figure 3.
Fig. 3 - Photograph of completed emitter heat pipe
The distillation of the lithium is performed
in a vacuum system. The lithium distillation con-
tainer is terminated in a thin, hollow tungsten
needle, which is inserted into the tungsten tube of
the heat pipe. The slip fit of these two tubes per-
mits the heat pipe to be outgassed prior to the
distillation of the lithium. The lithium can con-
tains a U trap so that only the lithium can enter
the heat pipe, and the impurities (mainly oxides)
remain in the can. Heaters are provided on the
lithium can, the heat pipe, the heat pipe filling
neck, and the tungsten fill tubes. The tempera-
tures in the system are suitably varied so that
the lithium can be distilled through the can into
the heat pipe, or alternately from the heat pipe
back into the can. At any point during the distilla-
tion process, the heat pipe can be :. pecked to
determine if it has been charged with the proper
amount of lithium. This is accomplished by oper-
ating the heat pipe in the isual mode and checking
for uniform temperature along its length.
The final closure of the heat pipe is , ,:com-
plished once proper heat pipe operation has been
observed. A current (x700 A) is passed through
the concentric fill tubes causing them to melt and
form a leaktight tungsten bead at the end of the
tube. The completed heat pipe, under test, is
shown in Figure 4.
TESTING-The emitter heat pipes are heated
by a radiation source enclosed in a MULTI-FOIL
insulated furnace, 152.4 mm in diameter and
152.4 mm high, consisting of 30 layers of tungsten
foil and 30 layers of molybdenum foil. The foil
spacing is achieved by zirconia particles.
SONIC
LIMIT
ENTRAINMENT
LIMIT
21 MESH
/	 -ENTRAINMENT/	 LIMIT,
/	 50 MESH
/AOIATION AND ELECTRONCOOLING GT IO Aicm2
CONDUCTION
COOLED
AOIATION
COOLED
1200	 1300	 1400	 1500	 1600	 1700
TEMPERATURE (K)
100,000
10,000
x
2
J
x
e 1000
100 L.
1100
ORIGINAL PAGE IF
OF ")k QrlALITY
S
Fig. 4 - Photograph of emitter heat pipe during
test
The characteristics of the furnace were mea-
sured with a water-cooled calorimeter that was
installed in the furnace. The furnace temperature
was determined by an optical pyrometer sighting
on a blackbody hole in a molybdenum target at the
bottom of the furnace. The power output of the
furnace to the water-cooled load was determined
by measuring the water flow rate using a gradu-
ated cylinder and a stopwatch. The water inlet
and outlet temperatures were determined by test
thermometers graduated at 0.1 K intervals. The
furnace losses, mainly conduction losses through
the tungsten filaments, were determined by sub-
tracting the output from the input. The heat input
to the heat pipe was determined, and the axial heat
flux calculated, as shown in Figure 5. The heat
loss from the heat pipe, which is mainly by radia-
tion, A-as calculated from the measured surface
temperature of the condenser of the heat pipe.
For a lithiurr. heat pipe that was conduction cooled,
a higher heat flux was reached. (2)
The heat flux expected to be imposed during
thermionic converter operation is also shown in
Figure 5. The principal heat losses in a therm-
ionic converter are radiation and electron cool-
ing. (3) Thus, if the electron cooling (measured
at 10 A/cm Z ) is added to the observed radiation
loss, an estimate of the total thermal load on the
heat pipe during converter operation is obtained.
It is seen that the required performance is far
below the sonic and entrainment limits.
Fig, 5 - Heat flux in emitter heat pipe
The theoretical heat flux limits for a lithium
heat pipe are also shown in Figure 5. The sonic
axial heat flux limit, Q/A, for the lithium heat
pipe was calculated from the following expres-
sions: (4, 5)
Q8 = p LV8	 V	 d
`' S = ( YR 0T/M) 1/Z	(2)
where p is the vapor density, L is the latent heat
of vaporization, V s is the sonic velocity, Y is the
ratio of specific heats, R o is the gas constant, T
is the temperature, and hi is the molecular weight.
It is seen that, at a low operating temperature and
at startup, the sonic limitation is the most severe
restriction in the operation of the heat pipe.
The entrainment axial heat flux limit for the
heat pipe was also calculated. This limit occurs
when the vapor exiting from the evaporator pre-
vents the return of the liquid from the condenser
by removing the droplets from the pores of the
screen. This limit, Q E /A, is given as follows:
QE/A = L (Pvy/Z) 1/Z	 (3)
where Z is the characteristic distance of the pores
for a s creen wick, gene rally taken as the distance
between the wires. The entrainment limit for the
(1)
94
lithium heat pipe also is shown for two screens,
200 mesh and ^0 mesh. It is seen from Figure 5
that the axial heat flux is not limited by either of
these parameters of the desired heat pipe oper-
ating range.
COLLECTOR HEAT PIPE
FABRICATION-The collector and the collec-
tor heat pipe are made from niobium, I" -
zirconium alloy. They are joined by a tapered
seat. The working fluid is potassium. Due to the
unavailability of proper size tubing, the heat pipes
were fabricated by rolling and electron beam
welding sheets into the desired tubes. After the
first rolling operation, the edges of the sheet
were trimmed so that, upon welding and rerolling,
the desired tube size was obtained without further
machining. A drawing of the collector heat pipe
is shown in Figure 6. Microgrooves were
machined on the heat pipe surface to increase its
apparent emissivity. The tube was then cleaned
and annealed. The collector heat pipe transfers
heat from the collector of the thermionic conver-
ter, a concentric cylinder about 70 mm high, to
the outside cylinder, the radiator of which is
515 mm high. The liquid from the radiator is
returned to the collector through a series of wick
arteries that bridge the angular gap between the
collector and the radiator at the bottom portion of
the heat pipe, as shown in Figure 7. The wicks
and arteries were inserted, the end caps were
welded on, and the charging and evacuation tubes
were attached. The heat pipe was outgassed using
electrical resistance heaters. Then the potassium
capsule was cracked and the potassium distilled
into the heat pipe. Electron beam pinch-offs on
the niobium, 1 °J -zirconium tubes were made. As
an additional precaution, pinch-off protector caps
were electron beam welded over the pinch-offs.
r
Fig. 7 - Photograph of collector heat pipe arteries
in Figure 9. If it is assumed that all the heat,
Q R , radiated to the copper-cooler' shield was
transferred by radiation, then the effective emis-
sivity, E, can be calculated from the known radi-
ator temperature, T R , and sink temperature, TS,
from the fallowing equation:
WATER COOLED
COPPER SHIELD
THERMOCOUPLE
LOCATIONS
AT PIPE
BOMBARDMENT
Fig. 8 - Collector heat pipe test arrangement
20
e
unr^
Fig. 6 - Collector heat pipe
TESTING-Upon fabrication, each heat pipe was
instrumented with thermocouples and mounted in
a water-cooled copper heat receiver. An electron
bombardment filament was inserted in the collec-
tor cavity of the heat pipe to simulate the collector
heat load, as shown in Figure 8. Each heat pipe
was tested up to an 850 K radiator temperature.
The heat radiated to the water-cooled shield was
measured by noting the temperature rise and flow
rate of the cooling water. The results are shown
O HEAT FLUX
&EMISSivitr
r
^ W
2
o e
0 6
0
0
o
600	 650	 700
	 750	 BOO	 $50	 900
SURFACE TEMPERATURE (10
Fig. 9 - Heat pipe radiator heat flux measurements
E IS
- 10
i
i
a
a os
°a
0
PA(1r 1 1P
5
8
6
Q R
 = 4,; ( T R } - :' S 4 )	 (4)
where is the Stefan-Boltzmann constant. Also
shown in this figure is the apparent emissivity of
the surface, based on measured radiator surface
temperature. The temperature profiles along the
heat pipe for several operating conditions are
shown in Figure 10. The locations of the thermo-
couples are shown in Figure 8.
1000
AXIAL
MEAT
FLUX
( W/C,2)
153
132
107
90
66
36
600.
	
100	 200
	
400
	 500	 600
AXIAL DISTANCE FROM HEATER (rnm)
Fig. 10 - Temperature profiles in collector heat
pipe
It is seen from Figure 11 that, at the highest
temperature tested (cs 850 K), the heat pipe flux is
limited by the ability of the radiator to reject heat
to the environment, not by the thermal perfor-
mance of the heat pipe. When the heat pipe is
SONIC LIMIT
POTASSIUM
ENTRAINMEN'
LIMIT
200 MESH
SODIUM
^/ J
POTASSIUM
RADIATION
COOLED
600	 700	 Soo	 900	 1000	 1100
T EMPERATURE (K)
Fig. 11 - Axial heat flux for collector heat pipe
operated at the lowest temperature tested ( ,600 K)
the operation is near the sonic limit. It is seen
that a heat pipe operating with sodium as the work-
ing fluid could not operate below ;700 K. In order
to extend the range of operation of the thermtonic
device to lower temperatures, potassium was
. hosen rather than sodium.
CONCLUSIONS
The heat pipes for operating radiation-heated
and radiation-cooled thermionic converters were
shown to be within the theoretically calculated
performance limits for such heat pipes. It was
shown that experimentally observed performance
of these heat pipes fulfilled the design require-
ments imposed by the thermionic converters.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the advice
of Mr. David Lieb during this study, and the
cooperation of Mr. Harry Hardister and Mr.
Alan 7_erigian in the construction and testing of
the hardware.
REFERENCES
1. Gabor Miskolczy, David P. Lieb, Douglass
B. Goodale, Erich Kroeger and James Morris,
"Performance of a Thermionic Converter Module
Utilizing Emitter and Collector Heat Pipes. 11
Presented at 1978 IEEE International Conference
on Plasma Science, Monterey, California, May
1978.
2. D.M. Ernst, E.K. Levy and P.K.
Shefsiek, "Heat Pipe Studies at Thermo Electron
Corporation, 11 Proceedings of the 1968 Therm-
ionic Conversion Specialists Conference,
Framingham, Massachusetts, October 1968,
p. 254.
3. G. N. Hatsopoulos and E. P. Gyftopoulos,
"Thermionic Energy Conversion, Volume 1,
Processes and Devices, " Cambridge, Massa-
chusetts, The MIT Press, 1973.
4. P. Dunn and D.A. Reay, "Heat Pipes,
New York, Pergamon Press, 1976.
5. "Heat Pipe Design Handbook, 11 Dynatherm
Corporation, prepared for NASA under Contract
No. NAS9-11927, August 1972.
i
-, 900
Y
W
i 900
Cr
W
~ 700
10.000
1000
E
s
x
V
a
100
10


Lithium and potassium heat pipes for thermionic converters

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