for the galileo mission

£ J 330^9 .,,P3

SELENIDE ISOTOPE GENERATOR for the

GALILEO MISSION

AXIALLY-GROOVED HEAT PIPE;

ACCELERATED LIFE TEST RESULTS

TES-33009-49

AUGUST 1979

Prepared for the U.S. Department of Energy under Contract DE-AC01-78ET33009

e U "J S a e G ^

1 ed

S ales G V -i r

^^^TELEDYNE ENERGY SYSTEMS 110 W. TIMONIUM RD., TIMONIUM, MD. 21093 PHONE: 301-252-8220 TELEX 8-7780 CABLE: TELISES

\y> tJSTRIBUTieN 8f THIS OOEUV.ENT iS U?iLi!«!Tiftf

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

TES-33009-49 ii

NOTICE

"This report was prepared as an account of work sponsored by the United

States government. Neither the United States not the United States Department of

Energy, nor any of their employees, nor any of their contractors, subcontractors,

or their employees, make any warranty, expressed or implied, or assumes any

legal liability or responsibility for the accuracy, completeness, or usefulness of

any information, apparatus, product, or process disclosed, or represents that its

use would not infringe privately-owned r ights ."

TES-33009-49 ill.

TABLE OF CONTENTS

Notice Table of Contents List of Figures

I. Introduction

n . Description of Heat Pipe life Test Fixtures

A.

B.

c. D.

Introduction

Instrumentation

Temperature Controls

Data Handling

m . Test Program

A.

B.

c.

Non-Condensible Gas Test

Heat Transfer Test

Heat Transport Test

IV. Test Results

A.

B.

C.

Non-Condensible Gas

1. Introduction 2. The Arrhenius Rate Model 3. Data Analysis 4. Assessment

Heat Transfer

i . Introduction 2. Data Analysis

Heat Transport

1. Introduction 2. Data Analysis

Page

11 iii iv

I- l

n -1

n - 1

n-4

n-4

n-5

m - 1

n i - i

ni-2

ni-2

IV-l

IV-l

IV-l IV-l IV-2 IV-8

IV-15

IV-15 IV-16

IV-21

IV-21 IV-21

TES-33009-49 iv.

TABLE OF CONTENTS (Cont.)

Page

V. Summary and Conclusions V-1

1. Introduction V-1 2 . Non-Condensible Gas V-1 3 . Reliability V-2 4 . Heat Trans fe r /Transpor t V-2

VI. References VI-1

APPENDIX A A-1

TES-33009-49 V.

LIST OF FIGURES

Figure Title Page

1 LCP 10023 1-2

2 LCP 10024 n - 2

3 LCP 10024 n - 3

4 Axially-Grooved Heat Pipe, Fixture No. 1 IV-3





9 Non-Condensible Gas Generation Rate of Axially- IV-9 Grooved Heat Pipes v s . Temp. (Fixture Averages)

10 Locations, Test Hours and Status:of Heat Pipes IV-11

11 Non-Condensible Gas Generation Rate of Axially- IV-12 Grooved Heat Pipes v s . Probabili ty at Heat Pipe Temp. = 125°C

12 Fixture Position No. 1-3 S/N: LT-12 IV-16

13 Fixture Position No. 5-1 S/N: LT-29 IV-17

14 Fixture Position No. 1-1 S/N: LT-79 IV- l8

15 Fixture Posit ion No. 2-6 S/N: LT-37 IV-19

16 Fixture 2-7 LT-83 Gross Watts v s . Delta Temp. (E-V) IV-21

17 Fixture 2-12 LT-85 Gross Watts v s . Delta Temp. (E-V) IV-22

A-1 Plate 54 A-1

TES-33009-49 I-l .

I. INTRODUCTION

This report presents the results through SIG/Galileo contract close-out of accel

erated life testing performed on axially-grooved, copper/water heat pipes (Fig. 1) fabri

cated for Teledyne Energy Systems (TES) by B&K Egnineering of Towson, MD. The test

was begim in June, 1978 and results presented herein cover the period from the start of

the test through mid-June, 1979. The primary objective of the test was to determine the

expected lifetime of axially-grooved copper/water heat pipes (hereinafter called heat

pipes or simply pipes). This requires that the heat pipe failure rate (due to either a leak

or a build-up of non-condensible gas) be determined. The seconday objective of the test

was to determine the effects of time and temperature on the thermal performance para

meters relevant to long-term (>50,000 hr) operation on a space power generator.

The vast majority of the data in this report pertains to 60 component develop

ment heat pipes fabricated for the Selenide Isotope Generator/Galileo Mission (SIG/GM)

program in accordance with LCP 10023 (Figure 1). Some data on three predevelopment pipes

fabricated and tested by B & K Engineering is also included. Heat pipes with sintered

wicks, fabricated by the Hughes Aircraft Co. , were originally included in the life test,

but they are not discussed in this report because they are no longer under consideration

for use on space power generators. All axially-grooved heat pipes on test have a 3. 5 inch

long evaporator, a 2 8.5 inch lor^ condenser, and a 90° bend between the two. They

have internal grooves coated with cupric oxide (CuO) to improve the wetting angle,

and are charged with water. Fifty of the 60 pipes (-009 configuration) were made

from 0.313 inch OD copper tubing, while the other 10 pipes (-019 configuration)

were made from 0.440 inch OD copper tubing. Internal dimensions were the same

for both configurations, but the -019 pipes had thicker walls to withstand the higher

I TES-33009-49 1-2

FIGURE 1

TES-33009-49 1-3.

internal pressure caused by operating at 225°C. Thirty-four of the -009 pipes

and 8 of the -019 pipes were clamped into four test fixtures. Twelve -009 pipes

were soldered to a panel and then placed in a fifth test fixture. The rest of the

pipes were kept as spares.

On November 6, 1978, one week after pipe LT-23 was found to have failed, all

unsoldered pipes were removed and inspected. During this inspection, two other pipes,

LT- i l and LT-i9, were found to have holes in their evaporator walls similar to those

found in LT-23. A detailed and lengthly investigation was conducted to determine the

reasons for these failures. While this investigation was underway, only the twelve

soldered pipes remained on test. Although some questions remained unanswered by the

investigation, the perforations in the evaporator walls were attributed to crystalline insula

tion beads becoming trapped in the thermal grease between the evaporator and the saddle

then penetrating the evaporator wall when the pipe was clamped in place.

When the life test was resumed in Feb. 1979, the insulation around the pipes

was wrapped with foil and the thermal grease was eliminated to prevent any more

penetrations of the evaporator walls. The total number of pipes on test was reduced

from 54 to 45 due to the removal of the three failed pipes, the removal of eight other

pipes for destructive analysis, the addition of three spares, and the removal of one

pipe now in the possession of Quality.

TES-33009-49 I l - i .

II. DESCRIPTION OF HEAT PIPE LIFE TEST FIXTURES

A. INTRODUCTION

All heat pipes undergoing life tes ts a re mounted on five separate test fixtures

designed by B&K Engineering and located in the Materials Engineering & Test Laboratory

at TES. Fixtures 1 thru 4 (see LCP 10024) shown in Figures 2 and 3, a re nearly iden

tical to one another and a re equipped with heat pipe saddles into which heat pipes a re

individually clamped in place. The saddles in fixture 4 are la rger than the saddles in the

other fixtures to accept heat pipes with thicker walls (and larger diameters) required for

high tempera ture (225°C) testing. Fixture 5 (LCP 10025) is basically s imi la r to the other

fixtures except that it contains heat pipes which a re soldered to a panel having integrally

machined saddles . Thus, in comparison to clamped pipes at the same test t empera ture ,

the heat pipes on fixture 5 should theoretically show the effects, if any, of being heated

to ~ 230°C for ~ 30 minutes during the soldering process .

The nominal tes t t empera tu re , original number of pipes , and current number

of pipes on each tes t fixture a r e given in Table 1.

TABLE 1

Nominal Test Tempera tu res and Number of Heat Pipes on Test Fixtures

Number of Pipes 6/78-11/78 2/79 - 6/79

12 8

12 7

10 10

8 8

12 12_

54 45

Fix ture No.

1

2

3

4

5*

Totals

Nominal Tes t Tempera tu re

(°C)

125

165

185

225

125

* heat pipes a r e solder-bonded to a panel

TES-33009-49 II-2

FIGURE 2

TES-33009-49 n-3

FIGURE 3

TES-33009-49 II-4.

B. INSTRUMENTATION

The heat pipe position i of each fixture is instrumented with 10 Chromel/Alumel

thermocouples (T/C ' s ) , while all other pipes on the fixture have only 4 T / C ' s . The

pipe in position 1 has three T / C ' s on the evaporator, six on the condenser, and one half

way between the two to approximate the vapor tempera ture . Pipes with 4 T /C ' s have one

that reads evaporator t empera tu re , two that read condenser t empera tures , and one that

reads vapor t empera ture . All heat pipe T / C ' s a re attached to that par t of the heat pipe

wall diametrical ly opposite the heat pipe-saddle interface area . Exact T/C locations

a r e specified in Ref. 1.

C. TEMPERATURE CONTROLS

Under normal life tes t conditions, about 125 watts is input to the evaporator heater

block of each heat pipe. Since this is not enough heat to maintain any of the pipes at their

specified tes t t empera tu res , (even with insulation around them) a high temperature fluid

loop is used to ra i se the condenser t empera tu re to the desired level. The high t empera

tu re fluid loop, which uses MOBILTHERM No. 603 in all fixtures as the working fluid,

i s a l so used to maintain heat pipe t empera tu res during heat t ranspor t t e s t s .

Each tes t fixture is also equipped with a low tempera ture coolant loop which is only

used during a non-condensible gas tes t to cool the last four inches of the condenser. The

low tempera ture coolant loop uses water at 5-20°C as the worldng fluid.

Since a pipe could rupture due to internal p re s su re if it is operated at an excessively

high t empera tu re , a l imits ta t (thermostat plus relay) will shut off the power to a pipe's

heater block if the pipe's t empera ture exceeds a specified maximum value.

TES-33009-49

D. DATA HANDLING

n-5.

Temperature and heat input data are acquired from the test fixtures by the 143

channel Doric Digitrent 240 Process Monitor. Although the evaporator temperatures

of all pipes on test can be monitored simultaneously by the Digitrend, it is more

conomonly used to collect data from only one fixture at a time. It provides a paper

tape record of the T/C and power transducer output which is then read by a teletype

machine, transmitted to a data file and stored on magnetic disc at the ITEL computer

in Dallas, Texas to await further processing.

TES-33009-49 m-i.

m . TEST PROGRAM

As described in Ref. 2, the following five tests were scheduled to be performed

afteiT heat pipe life tests were resumed in Feb. , 1979: heat transport, heat transfer,

non-condensible gas, radiographic examination, and destructive analysis. However,

due to the termination of the SIG Program, the radiographic examinations and destruc

tive analyses were not performed, and the heat transport test was performed once on

three of the fixtures. Those tests which actually were performed are described below.

A. NON-CONDENSIBLE GAS TEST

This is the most important test that was conducted during the life test since gas

generation and the resultant growth of a gas slug at the cold end of the condenser is

judged to be one of the time/temperature induced degradation mechanisms which could

ultimately severely reduce the performance of the pipe. The gas measurement test is

performed with the pipe at a positive elevation (~ 0.5 inch), a heat load of about 25W on

the pipe, and the low temperature coolant loop in operation. The heat pipe vapor tempera

ture and the average coolant temperature are measured and then used to calculate the

quantity of non-condensible gas in the pipe according to the procedure described in Ref. 3.

Since the coolant loop cools only the last 4 inches of the condenser, and 0.5 to 1. 0

inch of that length is needed to remove the 25W input, about 3.0 to 3.5 inches are occupied

by non-condensible gas during a gas test. A high vapor temperature indicates that a high

vapor pressure is required to compress the non-condensible gas into the last 3 to 3.5

inches of the condenser; hence the higher the vapor temperature the larger the quantity of

gas that has accumulated in the pipe.

TES-33009-49 in-2.

B. HEAT TRANSFER TEST

This test measures the effective thermal conductance, Q/AT, where Q is the net

heatload carried by the pipe and AT is the temperature difference between the evapora

tor wall and the condenser wall. It is performed with the pipe operating at its nominal

test temperature, with a positive elevation of 0.75 inch, and a gross heat input of 125W.

The evaporator, vapor, and condenser temperatures are measured and then used to

calculate AT (evaporator to vapor AT), and AT (evaporator to condenser AT). A

degradation in the pipe's heat transfer capability would appear as an increase in AT

and AT with time. Although the degradation rate of a heat pipe's heat transfer cap

ability is an important performance parameter, it is not required to determine the

expected lifetime of the pipe. Therefore, heat transfer measurements were considered

to be of secondary important during the life test.

C. HEAT TRANSPORT TEST

This test measures the maximum amount of heat a pipe can transport without

burnout^ ^ (Q ) at three negative elevations, usually -0 .25 , -0.50, and -0.75 inch.

It is performed by measuring AT^ as a function of heat load, Q. A plot of AT, vs. Q

shows a slow rise in AT, for Q <Q and then a sharp increase in AT, for Q > Q 1 max ' 1 max

The heat transport test is considered to be of secondary importance for the same reasons

given for the heat transfer test, i. e . , it is not required to determine the expected life

time of a heat pipe.

See Ref. 1 for the definition of burnout.

I

TES-33009-49 IV-l.

IV. TEST RESULTS

A. NON-CONDENSIBLE GAS

1. *" Introduction

As mentioned in the previous section, the bvdld-up of non-condensible gas as a

function of time is the most important of the three heat pipe parameters that were

actually measured since it is one of the most probable failure modes for axially-grooved

heat pipes (the other, according to Ref. 4, being corrosion or erosion of the heat pipe

wall). If all heat pipe materials (copper tubing, CuO coating, and water) are assumed

to be 100% pure, there is no known chemical reaction that could generate any non-

condensible gases inside a sealed heat pipe. Therefore, it is theorized that any gas

which is generated must come from: (1) an outgassing process, and/or (2) a self-

limiting chemical reaction involving minute amounts of impurities in the heat pipe

materials. If this hypothesis is correct, the rate of gas generation should decay

exponentially to zero as a function of time. Also, the initial gas generation rate should

be higher at higher temperatures. Of course, theoretical predictions should not be used

as a justification for choosing one particular fitting fimction (e. g. decaying exponential)

over another (e. g. linear) when it comes to analyzing the gas generation data.

2. The Arrhenius Rate Model

Since the Arrhenius acceleration rate model has previously been successfully

applied to non-condensible gas generation in heat pipes (Refs. 4 and 5), it is believed to

be the best choice for this analysis as well. In its simplest form, the Arrhenius model

assumes a time independent degradation rate, where degradation, in this case, is taken

to mean gas generation. Although a constant gas generation rate contradicts the theoretical

predictions presented earUer, it cannot be ruled out by the measured data. Furthermore,

TES-33009-49 IV-2.

conservatism favors the choice of a constant gas generation rate over an exponentially

decaying one, since it would predict larger quantities of gas at end of mission. The

cons'tant gas generation rate hypothesis could be abandoned in the future if a statistical

analysis of additional test data shows that the gas generation rate acutally does decrease

with time.

As discussed in Ref. 5, the Arrhenius model allows one to use the measured degra

dation rate at an elevated temperature (e.g. 165°C, 185"C, or 225"C) to predict the

degradation rate at some reference temperature (e.g. 125°C), where t ^ hours at the

elevated temperature is the equivalent of t ( > t ) hours at the reference temperature.

The ratio tg/t , is called the acceleration factor, J.

3. Data Analysis

Figures 4 thru 7 present the amount of non-condensible gas in p. g-moles versus

heated time for the individually clamped heat pipes operating at temperatures of 125 "C

(fixture 1), 165°C (fi?cture 2), 185"C (fixture 3), and 225''C (fixture 4), respectively.

Figure 8 presents similar data for the heat pipes soldered to a panel and operating at

125°C (fixture 5). The procedure for calculating the quantity of gas is discussed in detail

in Ref. 3. Each point shown on Figures 4 thru 8 represents a single gas measurement

on one pipe, and the solid and dashed lines indicate the linear regression mean and 95%

probability band (two standard deviations), respectively, for all pipes on the fixture.

The only data points which were not included in these figures or in the statistical analysis

of the data were four unusually high gas measurements taken on two failed pipes (LT-11

and LT-23) prior to the discovery that they had lost their charge. In removing these data

points, it was assumed that some abnormal process occurred prior to the detection of

the failures, such as a severe degradation in the pipes' heat transfer capabilities, which

G

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45

48

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25

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15

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8

FIGURE A-AXIALLY-GROOUED HEAT PIPE

HOH-CONDENSIBLE GAS FIXTURE NO. 1

W I CO CO o o 1

^

TERPERATURE = 125 C 12 PIPES CLAHPED TO FIXTURE HEAH AND 95'/. PROBABILITY INTERMAL EQUATIOH: H«7.58+1.72E-e41

r

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TIHE <HOURS> <t>

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45

48

35

38

25

28

15

10

5 I

0

1

0

FIGURE 5 AXIALLY-GROOUED HEAT PIPE

HOH-CONDEHSIBLE GAS FIXTURE HO. 2

TEMPERATURE » 165 C 12 PIPES CLAMPED TO FIXTURE ME AH AHD 95'/. PROBABILITY IHTERUAL EQUATIOH: M=12.95+4.55E-84t

za I CO CO o o CD I > ^ to

1888 2888 3888 4888 5888

TIME (HOURS) <t)

6888 7888 8888

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TEMPERATURE = 185 C 18 PIPES CLAHPED TO FIXTURE MEAN AND 95^. PROBABILITY INTERUAL EQUATION: H=8.68+2.38E-e3t

H W CO I CO CO o o «5 I

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8 1888 2880 3888 4888 5888

TIME <HOURS) <t)

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40

35

30

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H W w 1 CO CO o o CD 1

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TEHPERATURE = 225 C 8 PIPES CLAHPED TO FIXTURE MEAN AND 955i PROBABILITY INTERUAL EQUATIOH: M=ll.94*6.82E-03t

/

8 1888

^

G t M 0 L E S

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H0H-C0HDEHSI6LE GAS FIXTURE HO. 5

H

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TEMPERATURE « 125 C 12 PIPES SOLDERED TO PAHEL MEAH AHD 95' PROBABILITY IHTERUAL EQUATION: M=9.56+4.81E-84t

r •—" "w^ ^ •'^ ^"^ "* ^ ^ •

••—*-r- J j_ . i

1

0 1888 2880 3888 4888 5888

TIME (HOURS) (t)

6888 7888 8868

I

TES-33009-49 IV-8.

would justify the removal of these points from the data set. This assumption is a

reasonable one considering the corrosion that was detected inside these two pipes

after their leaks were detected. Since the third failed pipe (LT-19) had no internal

corrosion, and its pre-removal gas measurements were close to the average for

that fixture, it was not deleted from the population.

Although linear fits to the data in Figs. 4 thru 8 were performed, a close

inspection of the data points on Figs. 4, 5, 6, and 8 show an initial build-up of gas

during the first 2000 ± 1000 hours, followed by an apparent leveling off. This trend

lends some credence to the outgassing/self-limiting chemical reaction theory p re

sented earlier. However, in view of the relatively wide distribution in the data, more

data points at longer t imes, especially at 185°C and 225 °C, would be necessary to

verify this conclusion in a statistically significant manner.

In addition to showing an increase in the quantity of gas as a function of t ime.

Figs. 4 thru 8 also show a higher rate of gas generation ( i . e . , steeper slopes) at

higher temperature. This trend, which is in good agreement with the theory behind

the Arrhenius rate model, is probably genuine, even though there are large statistical

uncertainties in the slopes of the higher temperature lines. The quantitative relation

ship between gas generation rates and temperature are discussed in greater detail in

Appendix A.

4. Assessment

As discussed earl ier in this report, the assessment of the data is based on the

Arrhenius acceleration rate model which indicates a linear relationship between the

log of the gas generation rate versus the reciprocal of absolute test temperature.

Figure 9 presents a semi-log plot of the slopes of the lines shown in Figs. 4 thru 8

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TES-33009-49 IV-10.

versus 1000/T, where T is the test temperature in degrees K. The solid and dashed

lines on Fig. 9 show the linear regression mean and 95% probability range, respec

tively. The coefficient of correlation (COC) for the linear fit is 0. 92, which indicates

that the data correlates very well to a straight Une, and that decisions may be made on

the basis of that fit. Thus, the assumption that the Arrhenium model is applicable to

this data has been si^jported.

Calculating-the acceleration factor J for some elevated temperature is simply a

matter of substituting the reference temperature, the elevated temperature, and the

slope of the Arrhenius equation into the equation for J given in Ref. 5. AU axially-

grooved heat pipes ever tested on fixtures 1 thru 5 plus a few vendor tested pipes are

listed in Fig, 10 to show their heated hours, acceleration factors, accelerated hours,

and current test status. The acceleration factors given in Fig. 10 are significantly

larger than those given in Ref. 2 because they are quite sensitive to the slope of the

line in Fig. 9, which, in turn, is very sensitive to the location of any one of the five

points on it. Thus, there is a great deal of vuacertainty in the acceleration factors in

Fig. 10 which can be attributed directly to the imcertainties of the gas generation rates

at elevated temperatures, particularly 225 °C. This can best be corrected by obtaining

more high temperature test data.

Using the mean and 95% probability limits on the gas generation rate at 125 °C

from Fig. 9, the probability of exceeding a specified gas generation rate is plotted in

Fig. 11.. For the case of a 50,000 hour lifetime at 125°C (tjqjical SIG/Galileo numbers),

the mean amoimt of gas generated would be 12 \i g-moles, and 99% of the pipes would

have a build-up of less than 80 p,g-moles. Converting these results to inches of block

age^ ' (Ref. 6) yields a mean increase in blockage 0. 24 inch with 99% of aU pipes having

^ 'The conversion given in Ref. 6 is peculiar to the SIG/Galileo generator since it depends on external factors that affect the temperature distribution in the blocked region.

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TES-33009-49 IV

an increase under 1.6 inches. Note that the average initial amount of gas in a pipe (the

average of the intercepts of the lines in Figs. 4 thru 8) is 10 |i g-moles. Thus, the

average pipe would start with a 0.2 inch blockage at t = 0 and have a total blockage of

only 0.5 (0.44) inch after 50,000 hours. Compared to the initial active condenser length

of SIG/Galileo heat pipes (~25 inches), this represents a decline of about 1% in the

active condenser length after 50,000 hours.

To determine the maximum heat pipe failure rate with the Thomdyke chart^ ' ,

one need only know the test time accrues, the desired confidence level, and the number

of failures that occurred during the test. Figure 10 accrued test times and it also lists

three pipe failures. However, Ref. 2 states that the penetrations in the evaporator walls

of the failed pipes were almost certainly caused by crystalline insulation beads being

dislodged from the insulation in the fijctures, adhering to the thermal grease in the

evaporator saddle, and then being forced deep into the evaporator wall when the pipes

were clamped to the saddles. Thus, the three pipes that leaked can be neglected in

computing the expected reliability of a heat pipe on a space generator since it would not

be susceptible to the same failure mechanism. Heat pipe failure rates are presented in

Table 2 for a 50% confidence level and zero catastrophic failures.

The Thomdyke Chart is a graphical representation of the Poisson Exponential Binomial Limit.

TES-33009-49 lV-14.

TABLE 2

HEAT PIPE MAXIMUM FAILURE RATES

Total Test Time (hr.)

158,664

54,113

47,812

33,222*

930,145**

,429,125***

Operating Temperature

(°C)

125

165

185

220-225

125

125

Maximum Heat Pipe Fai lure Rate (%/1000 hr)

0.44

1.28

1.45

2.09

0.075

0.049

Heat Pipe Demonstrated Reliability @ 50% Confidence Level for 50,000 h r . Mission

80

53

48

35

96

98

•Includes pipes tes ted by B&K Engineering. **Includes acce lera ted hours on pipes tes ted by TES only.

***Includes acce lera ted hours on pipes tes ted by TES and B&K.

B . HEAT TRANSFER

Introduction

In Ref. 7, severa l problems associa ted with the measurement of the heat t r a n s

fer capability (Q/AT) of the axiaUy-grooved heat pipes on life tes t we re discussed. One

example of these problems i s that measu red heat pipe AT's for a constant heat load, Q,

exhibit a ve ry high degree of var iabi l i ty . This is caused p r imar i ly by instrumentation

and fixture design p rob lems , but it is exacerbated by the fact that heat pipe AT's a r e

obtained by subtract ing one la rge number from another. For these reasons , no quan

titative t r ea tment of AT measurements was attempted. The next section contains a

qualitative analysis of AT t rends based on data taken through Nov. 6, 1978 (Ref. 8) as

weU as recent ly measured data from May and June, 1979.

TES-33009-49 IV-15

2. Data Analysis

Plots of AT^ (evaporator to vapor AT) and AT (evaporator to condenser AT)

were presented in Ref. 8 for all 54 axially-grooved heat pipes originally placed on life

test. These plots were recently updated for 28 of the 45 pipes now on test. Data on the

other pipes was either unavailable or unusable. Four typical AT vs. time plots are p r e

sented in Figs. 12 thru 15.

Figures 12 and 13 illustrate the large cariances in the measured data which would

render any statistical treatment of the data virtually meaningless. Furthermore, since

the raw AT data was edited to eliminate any suspiciously high or low ( < 0) values, the

variance in the unedited data would be even larger than it would appear to be from look

ing at the plotted (edited) data. A significant reduction in the AT variances can only be

accomplished by an improvement in the quality, not the quantity, of the measurements.

A plot resembling Fig. 15, with 7 data points, is statistically more significant than one

such as Fig. 12, with 13 data points.

A qualitative assessment of the change in AT's with respect to time reveals that

in the majority of cases ( -64%), there is some increase AT with time (e.g. , Fig. 15).

About 30% of the pipes show either an ambiguous (e.g. , Figs. 12 and 14) or horizontal

trend, and the remainder show a decrease in AT with time. These findings are similar

to previously observed trends in AT (Ref. 8). Although no definitive explanation has

been found for the observed increase in AT with time, it is probably caused by a

degradation of the CuO coating and/or the groove geometry (Ref. 8).

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TES-33009-49 IV-20,

C. HEAT TRANSPORT

1. Introduction

The problems encountered in measuring the heat transport capability of the heat

pipes were somewhat less severe than those that affected the AT measurements (Ref. 7).

This is because a relative trend in AT as a function of heat load (Q) is all that is required

to determine the burnout point of a heat pipe, whereas an absolute measurement of AT at

a known Q is needed to determine a pipe's heat transfer capability. The former is easier

to obtain than the latter. Nevertheless, a quantitative treatment of the heat transport

data was not attempted for reasons discussed in Ref. 7. A brief discussion of some of

the most recent heat transport data appears below.

2. Data Analysis

Plots of AT versus Q are shown in Figs. 16 and 17 for pipes LT-83 and LT-85.

These pipes were selected for analysis because their prior burnout test data had already

been well documented (Ref. 2). Although the two most recent burnout tests on both pipes

show a decline in AT between 80 and 110 watts input, it is not clear whether or not this

effect is due to a masking of the burnout point. If it i s , it could conceivably be due to the

location of the T/C's and/or a partial burnout^ ' (Ref. 7). On the other hand, it could be

an effect unrelated to burnout. In either case, there is no significant degradation of

either pipe's heat transport capability. In fact, if the drop in AT is ignored, there

appears to be an increase in the burnout point of both pipes with time. Thus, using the

data that is currently available, it can be qualitatively concluded that there is not signifi

cant reduction in a heat pipe's heat transport capability with time.

A partial burnout occurs when some of the grooves are dried out and others are not.

TES-33009-49 IV-21.

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TES-33009-49 V - 1 .

V. SUMMARY'AND CONCLUSIONS

1. Introduction

The p r imary failure modes for an axially-grooved heat pipe a r e : (1) loss of

charge ( i . e . , a leak) and (2) generation of non-condensible g a s . Data relevant to these two

failure modes have been acquired from a premature ly concluded life tes t conducted at TES

between Jvme 1978 and June 1979. Data on heat t r ans fe r / t r anspor t t rends with respect to

t ime were acquired, but were considered to be of little s ta t is t ical value.

2. Non-Condensible Gas

The gas generation ra te appears to be constant with t ime after an initial sharp r i se

although the re a r e indications that it d rops to approximately ze ro beyond «-2000 hours .

More data points would be needed to prove such a trend ex i s t s . The relationship between

the gas generat ion ra te and t empera tu re obeys the Arrhenius equation:

M = exp ( A - | )

where :

M = gas generation ra te ( |j .g-moles/hr)

T = absolute t empera tu re ("K)

A , B = constants

A = 7 . 2 8 2

B =6213.5'"K

Fo r the SIG/GM radia tor design, the mean length of heat pipe blockage is expected to grow

from 0.2 inch at 0 hours to ~ 0 . 5 in at 50, 000 hours .

The gas generation can be explained by desorption mechanisms and gives no evidence

of ar i s ing through corros ion products .

TES-33009-49 V-2.

3. Reliability

During the life test, the following pipe-hours were accumulated: 159,000 at

125"'C, 54,000 at 165''C, 48,000 at ISS^C, and 8,500 at 225''C. Heated hours per

pipe ranged from 1000 to 7500 with an average of 4720. Applying calculated acceleration

factors yields the equivalent of 930,000 pipe-hours at 125°C. Including the accelerated

hours on vendor tested pipes raises this number to 1,430,000 pipe-hours at 125°C.

If the three heat pipe failures are attributed to crystalline insulation beads

penetrating into the evaporator walls (Ref. 2), then the maximum failure rate can be

computed by using the Thomdyke chart with zero failures. Thus, for a heat pipe tempera

ture of 125°C and a mission time of 50, 000 hours, the demonstrated heat pipe reliability

is between 80% (based on 159,000 actvial pipe-hours at 125°C) and 98% (based on 1,430,000

accelerated pipe-hours at 125° C).

4. Heat Transfer/Transport

Measurements indicate some degradation of heat transfer with time, but no detect

able degradation of heat transport.

TES-33009-49 VI-1.

VI. REFERENCES

1. Kroliczek, E. and Collins, J. P . , "SIG Heat Pipe Test Procedure," LCP 10069, Oct. 21, 197J.

2. "Heat Pipe Failure Evaluation Report-Response to Action Item No. 13 from SIG/GM Design Review Meeting of Nov. 2 8-30,1978" Teledyne Energy Systems Topical Report.

3. "Gas Front Computer Analysis and Utilization Report", BK038-1013, Oct. 1977.

4. Anderson, W. T. , "Hydrogen Evolution in Nickel-Water Heat Pipes", AIAA paper No. 73-726, July, 1973.

5. Budesheim, G., "Arrhenius Model Technique Applied to Axially-Grooved Heat Pipe Life Test Data for EOM Reliability Assessment", SIG-GWB-1576, October 20,1978.

6. Collins, J . P . , "Calculation of the Non-Condensible Gas Slug Length for the Heat Pipe Life Test", SIG-PC-1534, Sept. 26, 1978.

7. Collins, J. P . , "Problems Associated with the Axially-Grooved Heat Pipe Acceptance, Verification, and Life Test Data", SIG-JPC-1565, Oct. 5, 1978.

8. Budesheim, G., "Analysis of Axially-Grooved Heat Pipe Life Thermal Transfer Test Data", SIG-GWB-1649, Dec. 12, 1978.

J. Slfcitcr, J . , Packer Engineering Associates Report 3-345.101, Jan. 26, 1979, and #3.345.103, Dec. 31, 1978.

10. Elliott, R . P . , "Constitution of Binary Alloys - First Supplement", McGrew-Hill Book Co., N. Y. 1965.

11. Jeans, J . H . , "Dynamical Theory of Gases", Cambridge University Press , Cambridge, UK, 1925.

12. Miller, A .R . , "The Adsorption of Gases on Solids", Cambridge University Press , Cambridge, UK., 1949.

13. Walker, R. D. J r . , 6th Semi-Annual Report, NASA Research Grant NGR 10-005-022, Feb. 25, 1969, Univ. of Florida, Gainesville.

TES-33009-49 A - 1 .

APPENDIX A

ANALYSIS OF PROBABLE GAS GENERATION MECHANISM

TES-33009-49 A-2.

APPENDIX A

The data from the long term heat pipe tests presented in the body of this report,

was examined to determine the probable gas buildup, mechanism. Packer Engineering

Associates performed gas analyses on several freshly charged heat pipes and several

heat pipes which had been life tested at various temperatures (Ref. 9). The average

total gas contents of these pipes are listed in the last column of Table A-1 . An Arrhenius

plot of this data yields an apparent activation energy of 12.3 kcal/mole. This is the kind

of value one could expect for weak (molecular) chemisorption, especially on a nearly fully

occupied surface.

We use this data plus a few other facts to make a roi^h estimate of the amoimt of

non-condensible gas chemisorbed and physisorbed on the heat pipe walls and the contribution

from gases disolved in the fluid charge. These numbers are then compared to the experi

mental life test results and preliminar}' co: lacl, ^t, drawn concerning corrosion versus

outgassing in the system. 2

The nominal designed internal area of the heat pipes is 500 cm . The needle-like

CuO surface is assumed to have an effective area of about 100 times this. Examination of

photomicrographs of the as prepared coating shows this may be a reasonable assumption

(Fig. A-1). Thus the effective gas adsorption area on the heat pipe surface would be of the

4 2 order of 5 x 10 cm .

CuO has a monoclinic structure of space group A 2/a with four formula units per e s o

unit cell (Ref. 10). Its crystallographic constants are a = 4.684 A, b = 3.425 A, c = 5.129A,

and P = 99°28'. The unit areas of the 010, 100, and 001 faces are thus 24.258 x lo""*" cm^,

—16 2 —16 2 17. 567 X 10 cm , and 19.649 x 10 cm each with one copper atom per face. If we

TABLE A-1

Gas Buildup in Life Test Heat Pipes

Temp. (°C)

25 125

165

Hours

0 2338

2560 2680 2680

Calculated ( moles /HP) in Operating Heat Pipes Linear Regress ion (Time Only)

"Initial"

—

7.58

12.95 12.95 12.95

Buildup

—

0.40

1.16 1.22 1.22

Total

—

7.98

14.11 14.17 14.17

Multinle "Initial"

8.87

10.62 10.62 10.62

Regress ion (Time Buildup

—

0.72

2.76 2.89 2.89

and Temp.) Total

—

9.59

13.38 13.51 13.51

Experimental (Packer Data)

0 .08-1 .4 (4 pipes) 0.58

1.65 0.60 0.34

>-

vx

O

o CO 1

(O

185 1422 8.68 3.38 12.06 11.50 2.09 13.59 1.04

I 09

TES-33009-49 A-4

FIGURE A-1

TES-33009-49 A-5.

assume that the actual crystal faces exposed are random distribution of the various

members of the (100) form, then each copper atom would be in an average area of

—16 2 14 2

20.49 x 10 cm . This would lead to an effective exposure of 4.88 x 10 cu atoms/cm .

Since each of these copper atoms is bound to an oxygen it would be very reasonable

to assume that any gaseous species would be adsorbed on the surface in molecular rather

than atomic form. This is quite in keeping with the activation energy determined from the

life tes ts .

The distance of closest approach of copper atoms in each of the above crystal planes

is (001): 2.663A, or 3.144A, (010): 3.473A, and (100): 3.084A. Since the effective molecular e

diameter of the oxygen molecule is 3.64A (Ref. 11) it is not possible to completely cover the

surface on a 1:1 basis. It can be shown on a minimum energy basis that the most effective

packing scheme is one covered surface atom with the four nearest neighbors vacant (Ref. 12).

This leads to 9= 0.5 for chemisorption, with a foirly strorf "an der V/aals' layer above

the vacancies, also with 9 = 0 .5 .

From these considerations for the chemisorbed layer with N = number of surface sites and

9 = fractional occupancy: 9N = 0.5 x 4. 88 X 10^^ = 2.44 x 10^^ 0„ molecules/cm^ s i

®^s 2.44 X 10^^ . f,r, .<,-10 1 ^ / 2 = R^ = 4.05 X 10 moles On/cm . N 6.023x10** ^

o There would be the same amoimt of gas condensed in the associated van der Waals' layer.

4 2 Taking the effective area of 5 x 10 cm as previously described:

-10 2 4 2 -5

4. 05 X 10 moles On/cm x 5 x 10 cm = 2.02 x 10 moles O^.

Thus with total coverage we could expect 20 fxmoles of chemisorbed 0„ (or other non-

condensible species) per heat pipe. Again, 20 (Jimoles would also be present in the closely

bound van der Waals' layer.

TES-33009-49 A-6. •

Data for the solubility of gases in pure water (Ref. 13) was extrapolated to the

heat pipe as charged pressures using Henry's Law. This indicated a solubility level of about

0.1 \i.To.ole per heat pipe of each species at room temperature. These levels are "background"

in all of the measurements made here. This mechanism should be of negligible importance,

especially since the charge water is freeze outgassed to much lower levels before being

introduced to the heat pipe. However, the widely varying levels of COg found by the Packer

analyses indicate that some residual gases may still be present in solution in some of the pipes.

The measured amounts of gas in several heat pipes opened at Packer Engineering is

compared with the analytical predictions from the life test data in Table A-1 . Both of the

regression fits indicate a rapid gas buildup labelled 'Initial", although the first readings

were taken after approximately 300 hours of operation. This would represent any dissolved

gases and all of the loosely bound van der Waals' layer plus some of the chemisorbed layer.

The gas indicated ir> the bui.Mup columns would then represent the slow evolution of chemi

sorbed species.

Since for the low temperature heat pipes the initial rapid loss is 8 |imoles or less ,

we could assume either that the van der Waals layer was partly depleted during charging

operations or that the estimated area is high by a factor of 2 or more. If the latter is the

case then the total amount of gas remaining in the chemisorbed layer would only be about

5 to 10 |Jimoles, and after this is desorbed, no further gas buildup would be obseived. On

the other hand if the original area estimate were correct and the surface was full of chemi

sorbed species, approximately 15 jimoles coxild be released eventually. However, at the

125°C rate indicated only about 10 jimoles additional would be released in 60,000 hours.

Examination of the Packer data for opened heat pipes shows very little difference

between as charged'heat pipes and life tested ones, even though the life tested heat pipes

TES-33009-49 A-7.

have apparently increased their gas content by over an order of magnitude while operating.

This observation fits very well with a desorption mechanism. Thus if a heat pipe is

removed from testing and allowed to stand for an appreciable time, a large fraction of the

desorbed gases would again adsorb on the walls (at a slower rate however, since they must

work through the water film in this case).

This same mechanism could also explain the apparent drop in observed gas volume

when several of the pipes were removed from test and reinstrumented after cooling bath

problems occurred. The generally proposed explanation has been that the new thermocouples

had a better thermal interface with the heat pipe and therefore indicated less of a tempera

ture drop, which in the test method used would indicate less gas volume. However, if the

gas desorbed from the walls and swept to the condenser end of the heat pipe during normal

operation is allowed to rediffuse back through the length of the pipe under isothermal con

ditions, it could gradually redissolve in the working fluid and, to some extent, readsorb on

the pipe walls. Thus when the test is restarted the non-condensible gas volume would really

be lower than when the pipe was taken off test , and changes in the thermocouple attachment

efficiency wouldn't have to be proposed to accoimt for the observations.

The gas content of the heat pipes opened at Packer Engineering Associates showed

no obvious correlation to test temperature or time. The quantities of nitrogen and oxygen

generally vary roughly in the same manner from pipe to pipe. However they are not in

the ratio expected from dissolved a i r . There is excess oxygen. The N2:02 ratio is 2 or 3:1

rather than 4:1. This is reasonable from the adsorption mechanism, oxygen would be more

tightly bovmd than nitrogen and some of the latter could be removed in the purging operations.

The amoxmt of hydrogen observed also exhibited no correlation with heat pipe operating

history, or with the amount of oxygen or the oxygen/nitrogen ratio. Thus it appears to be

TES-33009-49 A-8.

the result of outgassing rather than chemical reaction.

Argon and methane are present at about the 1 ji mole level in the heat pipes

regardless of the amounts of the other gases. They were probably dissolved in the

charge water at this level. Similarly COn is present probably as a result of incomplete

purging of the charge water.

From these considerations there is no evidence of any corrosion reactions generating

gas in these heat pipes. All of the gas buildup observed thus far can readily be explained

in terms of adsorption/desorption mechanisms.

In reaching these conclusions several assumptions have been made:

a) the area of the CuO needles is'*'100 times the area of the heat pipe copper surface.

b) the exposed surfaces of the CuO belong to the (100) form.

c) 9 = 0.5 for chemisorption, and all sites are occupied.

d) zero order kinetics apply.

e) the gas solubilities in the charge water obey Henry's Law.

f) the method of measuring gas plug size on the life test heat pipes is insensitive

to saddle and interface conductances.

All of these can be checked experimentally, although the effects of a, b, and c may

have to be combined in one measurement by obtaining accommodation coefficient and volume

absorption data. However sufficient information can be obtained to check the correctness of the

hypotheses.

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