Marc A. Gibson and Donald A. Jaworske
Glenn Research Center, Cleveland, Ohio
Jim Sanzi
SEST Inc., Middleburg Heights, Ohio
Damir Ljubanovic
Gilcrest Electric, Elyria, Ohio
Thermosyphon Flooding in Reduced Gravity
Environments Test Results
NASA/TM—2013-217905
July 2013
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Marc A. Gibson and Donald A. Jaworske
Glenn Research Center, Cleveland, Ohio
Jim Sanzi
SEST Inc., Middleburg Heights, Ohio
Damir Ljubanovic
Gilcrest Electric, Elyria, Ohio
Thermosyphon Flooding in Reduced Gravity
Environments Test Results
NASA/TM—2013-217905
July 2013
National Aeronautics and
Space Administration
Glenn Research Center
Cleveland, Ohio 44135
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NASA/TM—2013-217905 1
Thermosyphon Flooding in Reduced Gravity
Environments Test Results
Marc A. Gibson and Donald A. Jaworske
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio 44135
Jim Sanzi
SEST Inc.
Middleburg Heights, Ohio 44130
Damir Ljubanovic
Gilcrest Electric
Elyria, Ohio 44035
Abstract
The condenser flooding phenomenon associated with gravity aided two-phase thermosyphons was
studied using parabolic flights to obtain the desired reduced gravity environment (RGE). The experiment
was designed and built to test a total of twelve titanium water thermosyphons in multiple gravity
environments with the goal of developing a model that would accurately explain the correlation between
gravitational forces and the maximum axial heat transfer limit associated with condenser flooding. Results
from laboratory testing and parabolic flights are included in this report as part I of a two part series. The
data analysis and correlations are included in a follow on paper.
Nomenclature
Density of the liquid
Density of the vapor
D Thermosyphon inner diameter
g Acceleration of gravity
Surface tension
Bond number
Vapor area
Heat of vaporization
RGE Reduced Gravity Environment
1.0 Introduction
Fission power systems have long been recognized as potential multikilowatt power solutions for
lunar, Martian, and extended planetary surface missions. These power sources are especially attractive in
places where solar intensity is limited by providing uninterrupted power, day or night, for extensive
periods of time that can span one to two decades. Typically, 30 to 40 percent of the reactor heat gets
converted to electricity and the remaining 60 to 70 percent gets rejected to space through large surface
area heat rejection radiators. Current heat rejection technology for fission surface power systems has
focused on titanium water thermosyphons embedded in carbon composite radiator panels with the
working fluid temperature range of 300 to 450 K (Ref. 1). Figure 1 provides a graphical representation of
NASA/TM—2013-217905 2
Figure 1.—Notional 40 kWe fission surface power system.
a potential 40-kWe Moon-based fission surface power system that has a total heat rejection surface area
of 185 m (Ref. 2) with over 300 thermosyphons. The thermosyphons, or wickless heat pipes, are used as a
redundant and efficient way to spread the waste heat from the power conversion unit(s) over large radiator
surface areas where it can be rejected to space. It is well known that thermosyphon performance is reliant
on gravitational forces to keep the evaporator wetted with the working fluid. One of the performance
limits that can be encountered, if not understood, is the phenomenon of condenser flooding. This occurs
when the gravity forces acting on the condensed fluid cannot overcome the shear forces created by the
vapor escaping the evaporator throat. When this occurs, the heat transfer process is stalled and may not
re-stabilize to effective levels without corrective control actions. The flooding limit in Earth’s gravity
environment is well understood as experimentation is readily accessible, but when the environment and
gravitational forces change relative to other planetary bodies, experimentation becomes difficult. An
innovative experiment was designed and flown on a parabolic flight campaign to achieve the reduced-
gravity environments (RGE) needed to obtain empirical data for analysis, as none was found during
literature searches. The 1-g laboratory and parabolic reduced-gravity results will be compared to existing
models to determine if they can accurately predict the flooding limit. If not, new correlations will be
established to model the behavior and give new insight into the flooding limit in actual RGEs.
NASA/TM—2013-217905 3
2.0 Experiment Design
Specific design constraints and requirements were developed using input from the parabolic flight
provider, heat pipe design codes, literature review, and experienced personnel. Initially, these inputs
helped determine the overall power levels of the thermosyphons, which was largely based on aircraft
power availability. A baseline of 2 kW at 115 V was used as the maximum electrical constraint for the
experiment design while in flight. The next consideration was to determine the number and size of
thermosyphons to meet the electrical constraints while still being able to reach the desired flooding limit.
Knowing that the flooding limit was going to be extremely difficult to obtain in parabolic flight, it was
determined that 12 thermosyphons, if possible, would be a good balance between getting multiple chances
at capturing the flooding event while keeping in mind the electrical constraints. Using 12 thermosyphons,
each heater would have a total of 165 W of supply power while in reduced gravity.
The length and diameter of the thermosyphon was mainly based on the flooding limits of the
thermosyphons as predicted by analytical and empirical heat pipe models, but also took into account the
geometric constraints of the intended flight rack. A significant effort was taken to study different
correlations from several sources with the intensions of verifying the predictive models or proposing new
correlations based on the test results. Initial research led to two models developed by Faghri et al. (Ref. 2)
and Tien and Chung (Ref. 3), which took into account the bond number, Equation (3). The bond number
is increasingly important as the thermosyphon diameter decreases below 0.5 in. (12.7 mm), and takes into
account the densities of the liquid and vapor as well as the surface tension, acceleration of gravity, and the
diameter of the thermosyphon. Other sources were investigated (Refs. 4 to 15) but many had limited or no
correlations with water, had limited or no test results using small diameter thermosyphons, or had
equation variables that could only be determined after testing. Equations (1) to (3) describe Faghri’s semi-
empirical correlation built from multiple test sources and different working fluids, which has been shown
to agree well with water, but has had limited comparisons to diameters less than 0.5 in. Note, none of
these flooding models have been proven in RGE.
(1)
.
(2)
(3)
Equations (4) and (5) describe Tien and Chung’s correlation, which was later modified by Faghri,
with slight differences between C
K
in Equation (5) and K in Equation (2). Equation (1) was used for early
predictions as it represented the more conservative approach to staying within the aircraft electrical
budget. The diameter of the thermosyphon would be estimated by graphically evaluating the flooding
limit of 165 W and a lunar gravity value of 1.7 m/s
2
. Both lunar and Martian gravity environments were
analyzed but the lesser lunar gravity was chosen as the desired target because it would allow a wider
range of data for the intended correlation as well as require less heater power. The decision to test mostly
in lunar gravity, as opposed to half lunar and half Martian, was due to the fact that during the parabolic
flights only a limited number of parabolas are dedicated to reduced gravity and the experiment needed as
much time as possible to pass through a flooding event.
(4)
√
3.2tanh0.5
(5)
NASA/TM—2013-217905 4
Figure 2.—Predictive models of thermosyphon flooding by
Faghri et al. (Ref. 1) and Tien and Chung (Ref. 2) for both
Earth and lunar gravity.
After careful examination, the final decision was to
design the experiment with a total of 12 thermosyphons
made from 0.25 by 0.035 in. (6.35 by 0.889 mm) wall
titanium tube using water as the working fluid. The total
thermosyphon length of 24 in. (60 cm) was built with a
2.5 in. (6.35 cm) evaporator, a 2.5 in. (6.35 cm) adiabatic
section, and a 19 in. (45.7 cm) condenser, providing a
length to diameter (L/D) ratio of 130, similar to the
thermosyphons designed for the fission surface power
system in Figure 1. Two wraps of 100-mesh titanium
screen were used in the evaporator section to increase
fluid flow during nucleation and prevent dryout. The
condenser was designed to be air cooled, using a finned
aluminum tube that would enhance heat transfer and
allow the internal fluid temperature to be altered via a
variable speed fan. Plots of the flooding limit from
Equations (1) and (4) are provided in Figure 2 using the
above thermosyphon geometry and water properties. The
equations show significant differences but provided a
good baseline model to compare with test results.
Electrically, the experiment also needed extensive
preparation for the data acquisition, heater control, power
conditioning, and safety interlocks. A National
Instruments PXI chassis and real-time controller was
employed using customized LabVIEW (National Instruments) programming to provide the system logic
and user interface functions. The end product used a laptop computer that was linked to the PXI controller
inside the flight rack and let the operator view and collect data signals, control the heater power, view
alarms, and record notes. Several revisions of the data acquisition and control (DAC) system were
implemented to establish a streamline process that allowed quick communication and control needed
during parabolic maneuvers. Pictures of the experiment hardware can be seen in Figure 3.
10
60
110
160
210
260
310
360
410
460
0
50
100
150
200
250
300
Thermosyphon Flooding Limit (W)
Adiaba c
Temperature
(C)
Faghri
1g
Chung
1g
Faghri
Lunar
Chung
Lunar
Constant
Tem
p
erature
Constant
Powe
r
Figure 3.—Experiment flight rack with data acquisition
system, 12 thermosyphons, power conditioning,
safety interlocks, and variable fans.
NASA/TM—2013-217905 5
3.0 Test Results
3.1 Earth Gravity
Testing was conducted in the laboratory well before any flight testing took place to address the
functionality of the experiment and determine how the sensors would be used to detect the flooding limit.
The thermosyphons were taken to their flooding limit using several procedures, which were eventually
down selected into the most appropriate for parabolic flight. During the constant temperature procedure,
the heaters were taken up in temperature using a voltage ramping function that would vary the heater
voltage at 1 V/min. Using the variable speed fans, the operator adjusted the airflow across the finned
condenser to control the adiabatic temperature of the thermosyphon. Using this strategy, the power
increases as the adiabatic temperature stays constant and eventually the thermosyphon passes through the
flooding limit. This can be seen graphically by the “Constant Temperature” arrow in Figure 2. Another
method that was incorporated into the test procedures was to keep the power constant and increase the
airflow, thus cooling the adiabatic temperature and ultimately passing through the flood limit from a
different angle (see “Constant Power” arrow, Fig. 2). Both methods produced similar results and would be
used for parabolic flight.
When the condenser floods, heat transfer is stalled and the heater temperature increases while the
condenser temperature decreases. This can be seen in the thermocouple data as a change in slope and is
easily visible during testing. An example of a 1-g flooding event can be seen in Figure 4 and depicts the
change in slope of both the heater and condenser temperatures. During the laboratory and parabolic
testing, 12 thermosyphons would be monitored visually to detect if a flooding event had occurred and
would initiate shutdown of the individual heaters. Using this philosophy, all thermosyphons were set and
ramped at exactly the same settings, providing a total of 12 flooding data points during an ideal test run.
This procedure was used to gather multiple flooding points over a large temperature range providing the
needed data to compare to Equations (1) and (2) as shown in Figure 5.
In comparing the 1 g test results with the predictions from Equations (1) and (4) it was evident that
neither correlation model fit the data very well. Figuring out what caused the differences was of great
interest. Both of these equations came from semi-empirical methods and incorporated the smaller
diameter, working fluid properties, and gravitational constants, which should provide a good estimate, but
the results are in disagreement.
Figure 4.—Typical 1-g flooding event while cooling the condenser.
NASA/TM—2013-217905 6
Figure 5.—The 1 g test results using 2 grams of working
fluid versus predictions from Equations (1) and (4).
Figure 6.—The 1-g test results of initial thermosyphons 6,
8, and 12 to investigate the affect of fluid charge on test
results.
One idea that had surfaced throughout initial 1 g testing, as well as the first parabolic flight campaign,
was the exact amount of working fluid in each thermosyphon and how it affected the test results.
Throughout the testing, it was noticed that data scatter between individual thermosyphons was more than
desirable, so a further investigation was initiated. Of the initial 12 thermosyphons, 3 were tested
individually to their flooding limits and are reported in Figure 6. It was thought that these differences
were due to the amount of working fluid, but to determine this, the thermosyphons would have to be cut
open. After the September 2011 flight week, the thermosyphons were all weighed and cut open so that the
water could be baked out of the tubes. The assemblies were reweighed and the difference was known to
be the fill charge. The thermosyphons had fluid charges ranging from 0.76 grams to 2.5 grams and could
be related to the performance of the individual units with number 6 having 0.76 grams of fluid, 8 with
2.5 grams, and 12 with 2.1 grams. The fluid charge differences in these initial 12 units were attested to
filling procedures that worked well with larger diameter thermosyphons but would prove difficult using
the 0.125 in. (3.2 mm) fill tube associated with the 0.25 in. (6.35 mm) thermosyphons.
0
50
100
150
200
250
300
350
400
0 50 100 150 200 250 300 350
Thermosyphon
Flooding
Limit
(W)
Adiaba c
Temperature
(C)
Laboratory
1g
Results
Faghri
1g
Tien
and
Chung
1g
0
50
100
150
200
250
300
350
400
0
50
100
150
200
250
300
Thermosyphon
Flooding
Limit
(W)
Adiaba c
Temperature
(C)
Faghri
1g
Tien
and
Chung
1g
Thermosyphon
#6
Thermosyphon
#8
Thermosyphon
#12
NASA/TM—2013-217905 7
Figure 7.—Dryout and flooding events of thermosyphon number
12 using 2.0, 1.0, and 0.3 grams of water as the fluid charge.
Before the thermosyphons were hermetically sealed, a test was completed to determine, for this
specific thermosyphon geometry, what the best fluid charge is, and how the charge affects the flooding
limit. Figure 7 reports the results as the charge was changed from 0.3 grams up to 5.0 grams. The results
are clear that fluid charge has a significant impact on the heat transfer limit of the thermosyphon. When
using too little fluid, the evaporator dries out before ever getting to the flooding limit and when too much
fluid was used, the heat pipe would not work at all. Although not reported in the figure, the maximum
amount of fluid that could be used was around 3.0 grams. At 3.0 grams and above, the thermosyphons
could not be started. Also worth mentioning is the fact that 2.0 grams of fluid took up 5 in. (12.7 cm) of
the 24 in. (60 cm) total length. This volume of fluid was needed to achieve the maximum flooding limit,
but may not be practical in some design applications. It was determined that 2.0 grams of fluid would be
used as the new fluid charge. The 1-g data in Figure 5 was compiled using the newly filled
thermosyphons with 2.0 grams of working fluid.
Notice in Figure 6 that thermosyphon number 6 agrees well with Equation (1) and numbers 8 and 12
do not. The data suggests that the differences in Equations (1), (4), and the test data may very well be
related to fluid charge and whether or not the flooding limit is actually reached, or if the evaporator is
drying out before flooding occurs. It appears that the differences between past and current test results
might be explained by fluid charge. Typically, after a flooding event starts, it is quickly followed by
evaporator dryout, but knowing that the fluid charge determines which event happens first makes the
analysis much more difficult. Through careful temperature measurement, both below, inside, and above
the evaporator, it was possible with this experiment to determine whether dryout had occurred before or
after flooding. With the use of a wicked evaporator, the dryout limit could be detected in the data when
the lower evaporator temperature changed slope and started increasing before condenser temperatures
started falling. The lower evaporator can be best described as a 1-in. adiabatic section just below the
heater, which served as a fluid reservoir. As the fluid left the reservoir to increase the mass flow needed to
transfer the increased heat output, the thermocouple in that section would show an increase in
temperature. This signified the start of dryout and depending on the fluid charge, may or may not be close
to the flooding limit. After some time, the evaporator section directly under the heater block would also
dry out and start the familiar slope increase of the heater block temperature, which would ultimately limit
the heat transfer. Conversely, when flooding occurred as shown in Figure 4, the lower evaporator
temperatures would initially not show signs of dryout, but the stalled heat transfer due to flooding would
suddenly increase the heater block temperature. The timing of these events can be used to help determine
whether or not flooding is actually occurring or if the thermosyphon is running out of working fluid as in
NASA/TM—2013-217905 8
the dryout case. Understanding this difference is key to finding the maximum heat transfer limit of the
thermosyphons in all gravity fields.
The proposed theory that correlations between different sources might be explained by fluid charge
and test methods, will be hard to prove without a large test program covering numerous thermosyphon
geometries and working fluids, which is not under the scope of this project. As with many heat transfer
and fluids experiments, it is important to update existing models to improve the understanding of the
engineering and physics associated with the process and hardware. Correlation of the test data resulting
from this experiment will be covered in a follow on paper.
3.2 Lunar Gravity
The parabolic flights took place in September of 2011 and May of 2012. Typically, each flight week
consists of 4 days of flights with 40 parabolas per day. These 40 parabolas can consist of several different
gravity targets and are usually agreed upon between the experiment principal investigators and the flight
engineer at the beginning of the flight week. During the September 2011 campaign, each of four flights
consisted of 12 lunar, 3 Martian, and 25 zero gravity parabolas. During the May 2012 flight week, each
flight consisted of 15 lunar and 25 zero gravity parabolas. The thermosyphons need some gravitational
force to send the fluid from the condenser back to the evaporator and therefore will not function in zero
gravity environments. In 2011, the experiment was shut down during the zero gravity parabolas. During
the May 2012 campaign, four thermosyphons were replaced with four fully wicked heat pipes that would
be operated during the zero gravity parabolas to gather additional research data, but results will not be
included in this report.
Determining the flooding limit during parabolic flight was more difficult than expected. The reduced
gravity portion of the parabola only lasts about 20 to 30 sec depending on the gravity target, leaving little
time to decipher the flooding status of 12 thermosyphons. Immediately following the reduced gravity
portion of the parabola is hyper-gravity, which occurs during the bottom of the parabola as the aircraft
pulls out of the nose down position and begins to climb altitude for the next parabola. Gravity levels are
usually between 1.5 and 2 g’s during this portion of the parabola and force the fluid back down to the
evaporator. The positive side of the sinusoidal occurrence of gravity levels is that it allows the
thermosyphons to hydraulically “reset” if they had flooded in the previous parabola, and allows the
operator to change power levels or cooling rates to try and “recover” for the remaining parabolas. The
down side to the alternating gravity levels is that it requires the thermosyphons to redistribute the working
fluid after hyper-gravity and continue functioning near their maximum capacity. Figure 8 shows a data
plot from the September 2011 flight campaign of thermosyphon number 8 going through parabolic
maneuvers. The graph shows power and temperature on the left axis, gravity levels on the right axis, and
Figure 8.—Flooding event data of thermosyphon number 8 taken
during Martian and lunar gravity parabolas in September 2011.
NASA/TM—2013-217905 9
Figure 9.—Lunar gravity flooding data from September 2011 and
May 2012 flight campaigns compared to Equation (6)
correlation.
time on the horizontal axis. The gravity levels can be seen alternating between reduced gravity and hyper-
gravity with the first three parabolas at Martian gravity and the remainder targeted at lunar gravity. At
lunar parabola 3, a flooding event can be seen taking place. Notice the change in slope of the heater block
temperature as well as the change in the lower and upper evaporator temperatures during reduced and
hyper-gravity. This is a good example of what flooding looks like from the captured reduced gravity data.
Thermosyphon flooding Data from both the September 2011 and May 2012 flight weeks have been
compiled into Figure 9. This represents all the flooding events that occurred after the first parabola. In
many occurrences, one or more of the thermosyphons would flood during the first parabola which meant
that the power level and temperature targeted was above the flooding limit and would not allow the heat
to transfer axially down the thermosyphon. Each full parabola only lasted about 1 min, meaning that for
the 12 or 15 lunar parabolas there was only 12 to 15 min to pass through the flooding limit. This required
the initial setting to be as close as possible to the predicted flooding point which ultimately led to many
first parabola floods. In total, there were only 4 chances to capture the intended flooding data (once per
flight), which was amplified by using 12 thermosyphons for the experiment. Out of the total 96 chances
from the 8 flights of September 2011 and May 2012, only 34 data points were captured that would
explain the maximum heat transfer limit. The perseverance in building the experiment and gathering the
lunar gravity flooding data in parabolic flight will provide the needed information to formulate the
required correlations for future thermosyphon designs for reduced gravity environments.
4.0 Conclusions
Thermosyphons can be used in fission surface power applications for the Moon, Mars, or other
planetary surfaces as a redundant and efficient way to spread waste heat from the power conversion
system. This research effort set out to verify leading semi-empirical models related to the flooding limits
of simple cylindrical thermosyphons in reduced gravity environments. Testing was completed in the 1 g
laboratory environment as well as in reduced gravity environments using parabolic flights. Equations
from Faghri et al. (Ref. 1) and Tien and Chung (Ref. 2) were used as a baseline to study the flooding
phenomenon. Early laboratory testing and parabolic flights showed that the fill volume of the
thermosyphons could possibly explain some of the differences between the predictive models knowing
that evaporator dryout and flooding limits are hard to distinguish. After finding the optimum fill volume,
the flooding limit of the current thermosyphons were shown to differ from existing models. Determining
new correlating models to explain the test results will be the next step in the research.
NASA/TM—2013-217905 10
References
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Power. Cleveland: NASA/TM, 2008. TM 2008-215166.
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Thermosyphons. Faghri, A., Chen, M.M. and Morgan, M. Houston: ASME, 1998. 1988 National Heat
Transfer Conference. pp. 291-303.
3. Entrainment Limits in Heat Pipes. Tien, C.L. and Chung, K.S. 1979, AIAA, pp. 643-646.
4. Wallis, G.B. One-Dimensional Two-Phase Flow. New York: McGraw-Hill, 1969.
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6. The Hanging Film Phenomenon in Vertical Annular Two-Phase Flow. Wallis, G.B. and
Makkenchery, S. 1974, J. Fluids Eng., pp. 297-298.
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pp. 1181-1188.
8. Dunn, P.D. and Reay, D.A. Heat Pipes. New York: Pergamon Press Inc., 1982.
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Mass Transfer, pp. 169-186.
10. Faghri, Amir. Heat Pipe Science and Technololgy. New York, NY 10001: Taylor and Frances, 1995.
11. Vapor Flow Considerations in Conventional and Gravity-Assisted Heat Pipes. Kemme, J.E. Bologna:
s.n., 1976. Proc. 2cd Intl. Heat Pipe Conf. pp. 11-22.
12. Theorecticl Investigation of Heat Pipes Operating at Low Vapor Pressures. Levy, E.K. 1968, ASME
J. Engineering Industry, Vol. 90, pp. 547-552.
13. A Visualization Study of Flooding and Entrainment in a closed Two-Phase Thermosyphon. Shatto,
D.P., Besly, J.A. and Peterson, G.P. New Orleans, LA: AIAA, 1996. 31st Thermophysics Conference.
14. Performance Limits of Gravity-Assisted HEat Pipes. Prenger, F.C. 1984, Proc. 5th Int. Heat Pipe
Conf., pp. 1-5.
15. An analytical study on the critical heat flux of countercurrent boiling in a vertical tube with a closed
bottom. Katto, Y. and Watanabe, K. 1992, Int. J. Heat Mass Transfer, pp. 3021-3028.
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Thermosyphon Flooding in Reduced Gravity Environments Test Results
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5f. WORK UNIT NUMBER
WBS 887359.01.04
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
John H. Glenn Research Center at Lewis Field
Cleveland, Ohio 44135-3191
8. PERFORMING ORGANIZATION
REPORT NUMBER
E-18221
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Washington, DC 20546-0001
10. SPONSORING/MONITOR'S
ACRONYM(S)
NASA
11. SPONSORING/MONITORING
REPORT NUMBER
NASA/TM-2013-217905
12. DISTRIBUTION/AVAILABILITY STATEMENT
Unclassified-Unlimited
Subject Category: 20
Available electronically at http://www.sti.nasa.gov
This publication is available from the NASA Center for AeroSpace Information, 443-757-5802
13. SUPPLEMENTARY NOTES
14. ABSTRACT
The condenser flooding phenomenon associated with gravity aided two-phase thermosyphons was studied using parabolic flights to obtain
the desired reduced gravity environment (RGE). The experiment was designed and built to test a total of twelve titanium water
thermosyphons in multiple gravity environments with the goal of developing a model that would accurately explain the correlation between
gravitational forces and the maximum axial heat transfer limit associated with condenser flooding. Results from laboratory testing and
p
arabolic fli
g
hts are included in this re
p
ort as
p
art I of a two
p
art series. The data anal
y
sis and correlations are included in a follow on
p
a
p
er.
15. SUBJECT TERMS
Heat pipe; Two-phase heat transfer; Fission surface power; Mars; Lunar; Microgravity research; Parabolic testing
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF
ABSTRACT
UU
18. NUMBER
OF
PAGES
18
19a. NAME OF RESPONSIBLE PERSON
STI Help Desk (email:[email protected])
a. REPORT
U
b. ABSTRACT
U
c. THIS
PAGE
U
19b. TELEPHONE NUMBER (include area code)
443-757-5802
Standard Form 298 (Rev. 8-98)
Prescribed b
y
ANSI Std. Z39-18
