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 

ߩ ௅Density of the liquid 

ߩ ௏Density of the vapor 

D Thermosyphon inner diameter 

g Acceleration of gravity

ߪ Surface tension 

ܤ ௢Bond number 

ܣ௩ 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 reducedgravity 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 semiempirical 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. 

ܳሶ

௠௔௫ ൌ ܭ݄௚௙ܣ௩ሾ݃ߪሺߩ ௅െ ߩ௏ሻሿ

ଵ

ସൗ ቂߩ௏

ିଵ

௅ߩ ൅ ସൗ

ିଵ

ସൗ ቃ

ିଶ

 (1) 

ܭ ൌ ቂ

ఘಽ

ఘೇ

ቃ

଴.ଵସ

ைܤଶ݄ܽ݊ݐ

ଵ

ସൗ ൌ ܴ	ݐ݄ܽ݊ଶܤை

ଵ

ସൗ (2) 

ሺഐಽషഐೇሻ೒ൣܦ ൌ௢ ܤ

഑ ൧

భ

మ (3) 

Equations (4) and (5) describe Tien and Chung’s correlation, which was later modified by Faghri, 

with slight differences between CK 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/s2

. 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. 

ܳሶ

௠௔௫ ൌ ܥ௄

ଶ݄௙௚ܣ௩ሾ݃ߪሺߩ ௅െ ߩ௏ሻሿ

ଵ

ସൗ ቂߩ௏

ିଵ

௅ߩ ൅ ସൗ

ିଵ

ସൗ ቃ

ିଶ

 (4) 

 

ܥ ௄ൌ √3.2tanh	ሺ0.5݋ܤଵ

ସൗ ሻ (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 

Temperature

Constant 

Power

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 hypergravity 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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Transfer Conference. pp. 291-303. 

3. Entrainment Limits in Heat Pipes. Tien, C.L. and Chung, K.S. 1979, AIAA, pp. 643-646. 

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Mass Transfer, pp. 169-186. 

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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. 

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-07-2013 

2. REPORT TYPE

Technical Memorandum 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Thermosyphon Flooding in Reduced Gravity Environments Test Results 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Gibson, Marc, A.; Jaworske, Donald, A.; Sanzi, Jim; Ljubanovic, Damir 

5d. PROJECT NUMBER

5e. TASK NUMBER

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 

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.

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:help@sti.nasa.gov) 

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)

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