LOS ALAMOS SCI ENTIFIC LABORATORY of the

University of California LOS ALAMOS NEW MEXICO

The Effect of Vibration on

Heat Pipe Performance

UNITED STATES ATOMIC ENERGY COMMISSION

CONTRACT W-7405-ENG. 36

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LA-3798 UC-38, ENGINEERING AND EQUIPMENT

LOS ALAMOS SCIENTIFIC LABORATORY -7 % of the !'ilt

University of California LOS ALAMOS NEW MEXICO

Report writtcn: October 4. 1901 Report distributed: November 22, 1967

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THE EFFECT OF VIBRATION ON HEAT PIPE PERFORMANCE

by J. E* mrall

ABSTRACT

A water heat pipe was operated while being subjected t o typical sinwoidal and ran- vibrations encountered during a missile lauuch t o determine the effect of vibration on heat pipe performance. The results of the experiment indicate that vibration tend4 t o imprave heat pipe performance as it prawtes better wetting of the wick structure by the fluid.

I. Introduction 1 The heat pipe i s a heat transfer device which

has considerably less weight and orders of magni-

tude greater thermal conductance than any type of solid heat conductor. It i s a sealed container in

which a fluid i s continuously evaporating and con-

densing, transferring heat by mass flow and uti l iz-

ing the latent beat of vaporizt%tion.* A complete flow cycle is formed by return of condensed liquid

t o the evaporator through a capillary wick struc-

ture. By proper choice of fluids, heat pipes can

be constructed for operating temperatures from be-

law o0 to 20000~.3

Because of the heat pipe's high thermal con-

ductance, l ight weight, and abi l i ty t o transfer heat with essentially no temperature drop, it has

great potential for the solution of heat t;ransfer

problems in space applications. The thermal prob-

l em encountered i n satel l i tes range from low tem-

perature thenual controls4 where water heat pipes

could be used, t o high temperature heat rejection

systems and thennionic power devices in the liquid- metal heat pipe range.

To establish the fact that heat pipes w i l l

function praperly under space conditions, an exper-

iment was conducted i n which a water heat pipe was

operated in an earth orbit. The results of tha t

experiment indicated that the absence of gravifa-

tional forces was not detrimental t o heat pipe per-

formance. In many space applications, however, it

may also be necessary for a heat pipe system t o

operate during the launch period which would subject

it t o rather severe vibrational forces. The object of th i s experiment was t o investigate the effects of

sinu8oida.l and random vibrations, similar t o missile launch conditions, on the operation of a water heat

pipe. 11. Heat Pipe Assembly

The heat pipe used for the tes t was a 12 in.

by 3/4-in.-o.d. stainless s teel tube lined with

three layers of 100 mesh stainless s teel screen t o provide a wick structure. Closure of the pipe was made by welding en end cap into each end. A 1/8-in.-

o.d. annealed nickel capillary tube, welded into one

end cap, provided a means for sealing the fluid

under vacuum after loading. Heat was supplied t o the heat pipe by a copper-

clad electric heater which was wound around the pipe

and soldered in place with pure t i n for a good me-

chanical and thermal bond.

After the heat pipe was assembled and helium

leak-tested, it was loaded with the proper amount

of dis t i l led water. The pipe was f i r s t evacuated

for several hours unt i l the pressure was 10'~ mm a. The vacuu!n l ine was then sealed off, and a valve t o

a pipette containing dis t i l led water was opened. When the desired amount of water had been metered

into the heat pipe, #e pipette valve was closed.

The pipe was sealed by cutting the snnealed nickel

capillary tube with a pinch-off tool. The aammt of

@-.y; - Fig. 1. Heat Pipe Assembly. ,. -

~7;La- . - ~ G L ' " t ~ 8 A' . I L 7 ;id %

water requirk to saturate t h ~ ~ c k , with no excess, equalization-analyser console for random vibration

was 7.5 B*

The heat pipe waa supported between two brack- ets which were attached to an aluminum baseplate,

testing. The exciter had a frequency range of

5-2000 cps,and the rate of frequency change could

be programed for autaarrtic control. A remote

with the bracket at the heater end 1/8 in. higher frequency meter, connected to the control room, was

than that at the condenser end. This waa done so placed beside the exciter sa thrrt the olswvex

the heat pipe would have a slight pitch when the

Psseplate was mounted horizontaUy, to ensure that

liquid return was by capillary action. The heater

leads were supported by clamping them to the bracket

at the heater end. The complete assembly is sham

in Fig. I.

111. Instrumentation

Temperatures along the length of the heat pipe

were measured by chraoel-ahrmel thermocouples spot-

welded to the surface of the pipe. The thermo-

couples were spaced along the letigth, CLI1 s n m in

Fig. 2, w i t h six placed along the top surface sdd

aix underneath. A 12-point recorder, with a sweep-

rate of one temperature reading per second, was used

to record the temperatures. A variable tmnsiomer

provided control of the heater input power to obtain

the desired temperatures.

could note the frequency on %he temperature record-

ing sheet as a function of time.

A small bench-model vibrator was also used to

test the heat pipe performance when operating at

different angles. This was a 60-cycle, constant

frequency model with a variable force inprf. , I

V. Test Procedure I

For the variable frequency and random vibra-

tion tests, the heat pipe assembly was bolted to

the Ling exciter in a horizontal position. The

m0ul1Lbg arraugeure1iL Is a l ~ w n in Fig. 3 rLOh the

temperature recorder and variable power aupply in

the background. Sinusoidal and random vibration

tests were made st two different heat pipe equili- brium tenpe-twes, 60' and gO°C. The follawiag

specifications were used for the tests.

Sinusoidal Sweep

IV. Vibr%tion - Equipent 5 - 45 cps @ 0.125 inches D.A. displacement

The facility used for this experiment was a 45 - 165 cps % 12.0 G1s peak

Ling Electronics vibration exciter uith a 90-kW out- l 6 5 - 2000 cps @ 9.0 G1s peak

put power amplifier and an 80-filter automatic

Fig. 2. Thennocouple Spacing.

Fig. 3. eat Pipe Mounted on Vibration Exciter.

R a n d m Vibration.

zo - 65 q3s 63 9.64. o2 per cps 65 - 3.25 cps @ 9.0 db per octave

2 225.- 700 cps 63 0.3 0 paC

700 - 915 cps b -18.9 Bb per octmue 913 - 2000 cps 0 0.06 o2 per cps

The sinusoidal sweep was started a t 5 cps and

increased to 2000 cps a t a sweep rate of 3 minutes

psr octave. 'Ilse ~8ndcm Yibmtion tegt w run 4 minutes a t each temperature level.

A t the s tar t of Qe test, the power supply was

tucned on and adjusted until the heat pipe reached an equilibrium temperature of 6 0 ' ~ . The sinusoidal

ribsation sweep was then started,and the frequency,

aa indicated by the r w t e frequency Ipeter, was recorded a t various intervals on the temperature

-ecording chart. When this phase had been can-

pleted, the heat pipe was then subjected to the ran- d m vibration. Dlaring b&h tellts a continuoua mon- itor of the taperatares was nude by the t-a- ture recorder. The heafi pipe temperahre was men raised to 90°c and the two tests repeated.

For the -cycle vibration test , the heat pipe

was =$ieU &% 70°0 w i t h the heater end elevated

a t various W e s '* 0' to 40'. The heat ppipe

was bolted to the vibrator plate, and the m e , waa

varied by elmt* onc: a d of #e vibration uni t as shown in Fig. 4. k t each aagle, the hsst pipe

was e;Uawed to attain t;Eanperature eqpilibrim and

then 'the vibrahr wm turned on for I minute. In an effort to olMeme the effect o t vibration on

the aietribution of a e W in a heat pipe, a

PlexQzhr model with na. &eater waa a o tested* In

this madel the wick structure did not e r t d the

f u l l length of the tube so that a transparent

Fig. 4. Vibration Test a t Various Angles.

section a t one end permitted observation of vibra- except a t point No. 12 which was 3/4% low. This

t ion effects on a liquid pool (see Fig. 5). was due t o a slight amount of excess liquid col-

lected a t th i s point. During the vibration sweep

VI. Reaul6s %here was no change i n the temperature distribu-

A t the beginning of the sinusoidal sweep test , tion except between 500 and 1000 cps. In th i s

the heat pipe wse essentially isothermaJ. a t 60°c range, temperature reading NO. 12 increased t o 60°c

Fig. 5. Transwent Plexiglas Model.

and a l l temperature^ plotted as a s t ra ight l ine.

A t t h i s temperature there was no change i n the tem-

perature distribution during the random vibration

t e s t .

When the temperature was increased t o 90°c,

point No. 12 was approximately 2°C low because the

quantity of excess l iquid was larger owing t o

thermal expansion of the liquid. During both the

sinusoidal and random vibration t e s t s t h i s tempera-

ture .difference was only 1°C.

In the 60-cycle t e s t , the mount of excess

l iquid a t the end opposite the heater increased

with the angle of inclination of the heat pipe,

owing t o draining of the wick structure. This led

t o a lower temperature a t the end of the pipe. A t

40 " elevation, the wicking-1Mt height, thermo-

couples No. 6 and No. 12 indicated temperatures of

7" and 12-1/2'~, respectively, below the other pipe

temperatures. A radiograph at t h i s angle showed

that the -l iquid pool was large enough t o contact

both of these temperature points. However, when

the vibrator was turned on a t each of the angles

tested,the temperature differences disappeared and

the heat pipe operated isothermally within 1°C.

The Plexiglas model was tested in an effor t t o

tletermine what caused the heat pipe t o operate iso-

thermally under vibration. It was observed that a

pool of water, formed by inclining the.pipe, was

broken up into small droplets and thrown back up

into the wick structure. It was also noted tha t

when the wick was almost completely drained, it

would re-wet much fas te r and more completely under

vibration.

VII. Conclusions

The resul ts of ' these t e s t s indicate tha t

sinusoidal. and random vibrations, within the

spectrum tested, are not detrimental t o heat pipe

performance. It was considered possible tha t

vibration would remove liquid from the wick

structure; however, these t e s t s show that vibra-

t ion aids in the wetting of the wick, forcing

l iquid in to all parts of the wick structure, and

improves heat pipe performance. In fac t , it ap-

pears tha t a f t e r heat pipes have been loaded with

fluid, the wettin6j-in process could be greatly

accelerated by applying vibration.

References

1. Grover, G. M., Cotter, T. P., and Erickson, G. F., "Structures of Very High Thermal Con- ductance," J. Appl. Phys. s, No. 6, 1990-1991 ( ~ u n e 1964).

2. Cotter, T. P., "Theory of Heat Pipes," Los . Alamos Scientific Laboratory' Report, ~ -3246-= ,

February 1965.

3. Deverall, J. E., and Kemme, J. E . , "High Thermal Conductance Devices Utilizing the Boiling of Lithium or Silver," Los Alamos Scientific Laboratory Report, LA-3211, October 1964.

4. Deverall, J. E., and Kemme, J. E., "Sate l l i te Heat Pipe," Los Alamos Scientific Laboratory Report, LA-3278-~~, January 1965 .

5. Deverall, J. E., Salmi, E. W., and Knapp, R . J., "Orbital Heat Pipe Experiment," Los Almos Scient i f ic Laboratory Report, LA-3714, June ,

1967.