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CAPILLARY PUMP LOOP (CPL)

HEAT PIPE

DEVELOPMENT STATUS REPORT

August 1982

Prepared for

National Aeronautics and Space Administrat ionGoddard Space F l i g h t CenterG re en be lt , M ~ r y lnd 20771

I n Response t oCo nt ra ct NAS5-26660, Task 005

Prepared byOAO Co rp or at io n

7500 Greenway CenterGreenbelt , Maryland 20770

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CONTENTS

Sec t i on .ase

1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . -12. CPL HEAT PIPE OPERATING PRINCIPLE . . . . . . . . . . . . . . . -1

3. 9ROTOTYPE DESIGK . . . . . . . . . . . . . . . . . . . . . . . . -1

3.1 De sig n Sumnary . . . . . . . . . . . . . . . . . . . . . . -3

3.2 Eva pora tor De sign. . . . . . . . . . . . . . . . . . . . . -7. . . . . . . . . . . . . . . . . . . . ..3 Condenser De sig n 3-7. . . . . . . . . . . . . . . ..4 L iq u id Inv en to ry Management 3-7. . . . . . . . . . . . . . . . . . . ..5 Temperature Co nt ro l 3-11

. . . . . . . . . . . . . . . ..6 Pressure Pr im ing . . . . -123.7 Non -Cond ensible Gas Tra p . . . . . . . . . . . . . . . . . -13

4 . PROOF-OF-CONCEPT TESTS. . . . . . . . . . . . . . . . . . . . . -1

4.1 T es t Set.Up . . . . . . . . . . . . . . . . . . . . . . . . -1

4.2 Heat Tra ns po rt Performance . . . . . . . . . . . . . . . . -3

4.3 Heat Tr an sf er Performance . . . . . . . . . . . . . . . . . -6. . . . . . . . . . . . . . . . . . . . ..4 Pressu re Pr im ing 4-7. . . . . . . . . . . . . . . . . . . ..5 Temperature C o c tr c l 4-10. . . . . . . . . . . . . . . . . . . . ..6 Heat Load Sh aring 4-10

5 SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . -1

REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . EF-1

ILLUSTRATIGNS

F igure Page. . . . . . . . . . . . . . . . .-1 C a p i l l a r y Pump Loop Schematic 2-2. . . . . . . . . . . . . . . . . . . .-1 CPL Heat Pi pe P ro to ty pe 3-2

. . . . . . . . . . . . . . . . . . .-2 C a p i l la ry Pump Evap orator 3-8. . . . . . . . . . . . . . . . .-3 L i q u i d I n v e n t o ry D i s t r i b u t i o n 3-10

. . . . . . . . . . . . . . . . . . .-4 Wn-Condensi b le Gas Tr ap 3-15

. . . . . . . . . . . . . . . . . . .-1 CPL He at P ip e Tes t Set.Up 4-2

. . . . . . . . . . . . . . . . . .-2 Ty pi ca l Recc very Under Load 4-8

. . . . . . . . . . . . . . . . . . .-3 Temperature Cycle P ro f i l e 4-11

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CONTENTS ( cont .ILLUSTRATIONS (cont )

F i gure Page

. . . . . . . . . . . . . . . .-4 Heat Load Sharing Test Profile 4-12

TABLES

Table- Page

. . . . . . . . . . . .-1 CPL Heat Pipe Prototype Design Surmary 3-4

. . . . . . .-2 CPL Heat Pipe Prototype Theoretical Performance. 3-6

. . . . . . . . .-1 CPL Heat Pipe Prototype Performance Siqmary. 4-4

. . . . . . . .-1 CPL Heat P'pe Development Status-February 1982 5-2

iii

2,-w-,

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SECTION 1. INTRODUCTION

A s ign i f i c an t ~d van ce n hea t p ipe technology has rece n t ly been rea l i zed by

the re- int roduct ion of the Capi la ry ?ump Loop ( C P L ) concept as a po ten t ial

candidate for th e management of large heat loads. The CPL h e a t p i p e , f i r s t

developed by NASA/Lewis i n the mic 1960's (Reference I ) , i s a two-phase heat

t r a n s f e r d ev ic e ca pa bl e of t r a n s f e r r i n g h ea t e f f i c i e n t l y , w ith l i t t l e

temperature d if fe re n ti a1 and no ext ern al power (pump) requirements. I t

o f f e r s 1arge heat load car rying c apa ci t i es together wi th subs tant i a1

n icking he igh t in g rav i ty . I t is curren t ly be ing evalua ted fo r po ten t i a l

app l ica t ion i n the thermal management of large space structures (Reference

2 1Experimental l m p s te s te d by NASA/Lewis in th e mid 196 0's dem onstrated th e

fe as ib i l i t y , th e po ten t i a l h igh hea t load capaci ty and low grav i ty sens i -

t i v i t y of t he CPL heat pipe concept. Rece ntly, a CPL heat pipe prototype

was developed and te st e d f o r NASA/GSFC to demof i s t ra te the p rac t i ca l i ty o f

such a design inc lud ing the fe a s i b i l i ty o f mul t ip le para l l e l evapora to r/

condenser zones within a sin gl e loop, heat load sharin g between

evapora to rs , 1 quid inventory/temperature control fe at ur e, priming under

load and entrapment of non-condensi b le g ases. The desi gn a1 so i nco rpo r-a tes very l z n g yapor and l iq ui d re tu rn headers (approximately 32 f e e t )

designed t o demonstrate the abi 1 ty of t ra ns fe rr ing heat loads over a large

d i s tance .

The primary ob jec tive s of development and t e s t e ff o rt s conducted t o date

were t o e s t ab l i s h f e as i b i l i t y o f t h e CPL hea t pipe des ign. The scope of

these ef fo r t s were 1imited and the CPL heat pipe prototype was designed and

fa br ic at ed using comnerci a l l y av ai la ble hardware and materi a ls . The scope

of the t e s t e f f o r t s wers a l so l imi ted to p roof-of -concept t es t ing . Despite

t h ese 1 n i t a t i o ns , sign if i can t pro gres s has been nade i n the development

of the CPL heat pipe. The cu rr eo t development s t a tu s of th e CPL heat pipe

is presented i n the sec t ion tha t fo l lows .

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SECTION 2. CPL HEAT P I P E OPERATING PR I NC I P L E

The ba sic desig n of th e CPL h ea t pip e, i l l u s t r a t e d s c h e m a t ic a l ly i n F i g u r e

2-1, cons i s t s o f an evaporator , which inc lude s a wick st ru ct ur e, and a

cont inuous loop which i s devo id o f any w ick ma ter ia l . The loop, wh ich can

be made of smooth w al l tubing, p ro vi des a f l ow passage f o r t h e vapor, hea t

t r an s fe r a rea i n t he condenser and l i q u i d r e t u r n t o t h e evapora to r .

The CPL hea t p i pe i s a cont inuous l oop i n which bo th t ne vapor and l i q u i d

a lways f l ow i n t h e same d i r e c t i o n . As heat i s app l i e d t a t he evapora to r ,

l i q u i d evaporates from the sa tu ra ted w ick and fl ows t h rough t he l oop t o t he

condenser zone where heat i s removed. Flow i n th e condenser zone i i ti 1 l y

cons i s t s o f h i gh -ve l oc i y vapor p l us 1 q u id w a l l f i l m which subsequent ly

t u r n s i n t o l i q u i d " s l u g f l o w " . The l i q u i d i s r e tu rn e d t o t h e e va po ra to r

v i a a subcooled 1 q u i d r e tu rn header w9 ich co l l apses any r sn a i n ing vapor

bubbles. The uniquenes s of t h e CPL h ea t p i p e i s t h a t a w ic k s t r u c t u r e i s

requ i red i th e evap orator zone only . Capi11ary a c t i on i s 1 m i e d t o t h a t

zone, where it p r ov id e s t h e n ec es sa ry p re s su re d i i f o r e n t i a1 t o i n i t i a t e

vapor f low. i n the remainder o f the loop, pressure gxer ted by th e f l o w o f

vapor on the 1i ui d c o l unn ahead o f i t d r i v e s t h e 1i u i 4 back t o t h eevaporator by a posi t i ve "p i s ton" a c t i on.

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SECTION 3. PROTOTYPE DESIGV

The design of the CPL heat pipe , i l l us t ra ted in Figure 3-1, d i f f e r s

s ig n if ic a n tl y from the ea r ly NASAILewis ca p il la ry pumped loop. The major

differences include:

a. Para l 1el Ev apo rato rs and Condensers. Paral 1el ci rcui t s a re

es se nt i a1 to mi nimi ze pr es su re drops; t o accomnodate mu1 t i pl e

heat sources and heat sinks; and to f a c i l i t a t e the design and

con stru ction of cold pla tes . Pressu re drop consi de rat i ons a re

especi a1 l y c r i t i cal because of the 1 mi ted pumpi ng head that can

be developed by capi 11 ary a c t i on .

b. Reservoir. A re se rv oi r has been included as an int eg ra l p art of

t h e CPL heat pipe design. I t s primary funct ion i s l iq uid

inventor y co nt ro l. Addi t i onal fe at ur es derived from the use of

th e res ervo ir i ncl ude tcvperature control ; pressure primi ng under

load and/or against gravi ty; and reduced s e ~ s i t i v i t y o f l u i d

1 aks .c. Non-Condensi b le Gas ( N C G ) Trap. A we1 1 e st ab li sh ed weakness of

high composite capillary systems (such as the CPL heat p ipe) , i s

the i r sens i t i v i ty t o t he p resence of non-condensi bl e gas es . The

one-way a ct io n of th e c a p il la ry punped loop and th e p hysica l

se pa ra tion of the vapor and 1 qu id flow c han nels a1 lows th e

introduction of a tr a p to prevent th e displacement of non-

condensi bl e gases i to the evaporator zones.

Other features incorporatedi

to theCPL

heat pipe design i ncl ude s ep ar at es iz in g of the vapor header and 1 quid re tu rn channel s t o optimize pressure

drops; and th e use of axi a ll y grooved tubi ng in th e condenser t o enhance

heat t ransfer .

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3.1 DESIGN SUMMARY

A CPL h e at p i p e p ro t o t y pe ( F i g u re 3-1) c o n s i s t i n g o f two p a r a l e l c a p i l l a r y

pump evap orato rs and two pa ra l e l a x ia l 1y grooved condensers was developed

fo r p roo f-o f-co nce pt t es t i ng . The evap orato r zone and condenser zone arein te r c o n n e c te d by a 1/2 i n . d iamete r X 0.035 in . w a ll vapor header and a

1/4 in . d iameter X 0.035 i n . w a l l l i q u i d r e t u r n . Both the vapor header and

the l i q u i d re t u r n a re made of smooth wa l led a luminum tu b in g ben t i n t o

seve ra l passes t o p rov ide an ad i aba t ic t r ans por t leng th o f 9.8 me te rs (32

fee t ) . The CPL h ea t p i p e p ro to t y p e d e sign a 1 so inc ludes a 100 c c c a p a c i t y

non-condensible gas t r ap loc a ted a t the o u t l e t c f th e condenser tone and a

1000 cc capa c i ty l i q u i d res e r vo i r (accumu la to r ) connec ted t o the evapora-

to rs as shgwn i n F i gu re 3-1. A l l tu b i ng connect ions were made w i th s ta in -

1ess s tee l compression f i t t i n g s .

D es ig n c h a r a c te r i s t i c s of t h e CPL heat p ipe pro t o ty pe are summar ized i n

Tab1 e 3-1. Theoret ica l per formance, based on heat p i pe th eo ry (Ref erencs

3 ) , i s shown i n Tab le 3-2. Wick pro pe r t i es were der ive d f rom open a i r

s t a t i c w ick i ng he ig h t and f low measurement t es ts pe r fo rmed w i t h a lcoho l on

the as- f ab r ica ted ca p i l la ry pump evapora to rs p r i o r t o loop assemb ly . The

analys is per formed cons idered both laminar and turbu lent vapor f l ow.

L i q u i d f l o w a n a l y s is i n t h e ev a po ra to r c o ns id e re d r a d i a l f l o w c o n d i t i o n s

f rom a sma l le r w ick inne r d iamete r t o a la rg e w ick o u t te r d iamete r, as we l l

2s t i l e e f f e c t o f t h e p e r f o r a t e d l i q u i d r e t u r n tu be .

The CPL heat pipe was assembled on an 8 ft. X 2 f t . test frame made of

a luminum ang les. A t tachments were made v i a phen o l ic and ny lon i so la to rs .

A luminum hea ter b loc ks equ ipped w i th ca r t r i dg e heaters and water f lo w chan-

ne ls were c lamped on opp osi te s ides o f each evap orato rs. 'dater f l o w

channels were p rov id ed f o r hea t loa d sha r in g exper imen t. Heat s ink inp i n

the condenser reg ion c ons is ted o f a g rooved b rass p la te t o which the

condenser tu b i n g was clamped down w i t h t h e r m a l j o i n t C O ~ ~ O I J ~ ~etween the

su r faces. The b rass p l a t e i s equipped w i th a copper co o l i ng c o i l which i s

connected t o GSFC' s Thermophysics L ab or at or y T i nney cool: ng 1oop.

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Table 3-1. CPL Heat Pi pe Pr ot ot yp e De sign Sumnary

Evaporators-r Number o f Ev ap or a t ~ r s n Para1 e l 2

e To ta l Leng th 0.38 m (15 in . )

r Ac t iv e Length 0.31 m (12 in.)

Condensers

r Number o f Condensers i n Pa r a l l e l 2

r Maximum A c t i v e Len gth Per 1.8 m (72 i n . )Condenser

r Subcooled Se cti on Length 0.61 m (27 in.)

Wick-r Ma te r i a l A lu m in a/Si 1 c a F e l t

Mean Fiber Diameter 2.5 Micron

r Pack ing Dens i ty 40%

r Effec t ive Pumping Radius 9.7 Micron(Measured )

r Permeabi 1i y (Measured) 2.6 X 10- 13 ,2

r Geometry See F ig u re 3-2

Evaporator Tube

e M a t e r i a l

e Permeabi 1i y

e Flow Area

e Geometry

Vapor Header

r M a t e r i a l

r Inner Diameter

e l e n g th

~ l u m .A x i a l l y Gror ved Tub ing8 24.57 X 10' m

-5 26 . 9 X 1 0 mTag 54-5 (See Figure 3-2)

Alum. Smooth Wa lled Tub in g

1.1 X m (0.43 in.)

9.8 m (386 i n . )

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Table 3-1. CPL Heat P ip e Pro tot yp e Design Sumnary ( co nt .)

L iquid Return

0 M a t e r i a l

a Inner Diameter

0 Length

Condenser

a Mater i a l

a Vapor Diameter

Alum. Smooth Wal led Tu bing

4.6 X m (.I8 i n . )

9.8 m (386 i n . )

Alum. Axially Grooved Tubing

4.' X rn (0.185 in.)

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Tab1 e 3-2. CPL Heat Pipe Prot oty pe The or et ic al Performance

V o rk i n g F lu 1 1 Freon-11

Op er at in g Temperature 20°c

A c t i v e 1 ength Per Condenser* 0.5 m (20 i n . )

Capi 11 a ry Pumping Head 3940 n/m2

Maximum Heat Transport (Zero Elevat ion; 400 Watts

Maximum Sta t ic Wick ing Height 0.27 m (10.5 in.)

P re s s u re D ro p D i s t r i b u t i o n

r Wick

r Vapor - Evaporator

e Vapor - Header

Vapor - Condenser

r L i q u i d - Condenser

e L i q u i d - Retu rn L ine

r L i q u i d - Evaporator

* Based on ob se rv ~d emperatu re p r o f i 1e w i t h t h e c o l d p l a t e t e mpe rature a t

approx ima te ly - 3 ' ~

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EVAPORATOR UESI N

Co ns t ruc t i on o f the cap i 11 ar y pump evapo rators, i 1u s t r a t e d i n F i g u r e 3-2,

cons i s t ed o f 1.15 i n . Tag 54-5 a x i a l l y g ro ov ed a l m i n u n t u b i n g p acke d w i t h

a lu n in um /s i l i c a f i b e r w ick . The w ick was made by c u t t i n g washers f rom 1i n . t h i c k f e l t t h a t was i n se r t ed ar,d packed i n t o t he Tag 54-5 t u b i n g t o a n

approximate densi ty of 40 percent. Approximate1 y 140 washers were

i n s e r t e d i n t o each o f t h e c a p i l l a r y punp evapo ra t o rs . Loca ted i n t he

c e nt e r o f t h e w ic k i s a 3/8 i n . d ia me te r p e r f or a te d l i q u i d r e t u r n w i t h a

1/4 i n . d ia me te r r e s e r v o ir i n j e c t o r tu b e l o c a t e d i n s ~ d eh e 3 /8 i n . l i q u i d

r e t u r n . The compacted wick ma te r ia l i s reta ine d by an a lun inum re ta in er

f i t t i n g a t t he vapo r ou t l e t end snd a p ress f i t t e d a1 umin ls l washer a t t he

1i ui d re tu rn end. 'c lel ded a1 un i nun end f i t t i n g s complete che ass enbly o f

t h e c a p i 11ary pump evaporator.

CONDENSER DESIGY

Ax i a1 l y grooved tub in g was used i n the design o f the condensers t o erlhance

heat t r a ns fe r chz ra c te r i s t i c s . The condenser zone co ns i s t s o f two 1.8

metsrs ( 6 f t . ) p a ra l l e l pa ths and a 0.6 meters ( 2 f t . ) subcooled zone.

3.4 LIQUID INVENTORY MANAGEMEaT

St ab l e ope r a t ion o f t he CPL heat p ipe req u i re s comple te condensati on o f t he

vapo r phase be f o r e t he wo rk ing f l u i d e n t m t he evapo ra t o r zone. I n

add i t i on , l i q u i d en te r i ng the evapora to r must Se a t a tempera tu re l ower

than sa tu ra t i on . Th i s i s because a f i n i t e amount of heat , conducted

t hr ough t he l i q u i d r e t u r n end f i t t i n g , wou ld cause vapo r bubb les t o be

generated i n a sat ura ted l i q u id . S ince these vapor bubbles cannot pass

t hr ough t he w ick s t r uc t u r e , t hey wcul d accumu la ted w i t h i n t he l i q u i d

r e t u r n l i n e un t i l t he r e su l t i n g b lockage would deprim e t he w ick. To avo i dt he above cond i t i ons , a po r t i o n o f t he c~nd ens e rmust be a1 lo c a te d as a

subcooled regio n. Th is can be accomplished by cha rging th e CPL heat p ipe

w i t h s u f f i c i e n t ! i q u i d t o e ns ure a p a r t i a l l y blo ck ed c ondenser a t a l l

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times. Predetermined charg ing of th e CPL heat pipe, however, presents

sev era l problems which includ e the f o l l owing:

a. Precise f luid inventory calculations and accurate charging

procedures t h a t would be req uir ed s in ce condenser zone volume i ssmall compared to to t a l volume. That is , even small va rian ces i n

to ta l inventory wil l s ig nif ica nt l y af f ec t extent of condenser

blockage.

b. Liquid entrapment in the vapor header can significantly affect

condenser zone block age. The amount of entrapment, i f any,

cannot be predetermined.

c .

If a non-condensi b le gas( K G !

tr;p is used in the systen design,liq uid in vento ry wi ll be displac ed as NCG's are co lle ct ed .

Additional blockage of the condenser by the displaced 1iq u i j wi 11

cause a performance degradation.

d . Fluid densit ies vary as a function of temperature. A CPL heat

pipe charged to have a partially blocked condenser at low

ope ratin g temperatures could be com pletely blocked at high

ope ratin g temp erature s. The ope ratin g temperature of a CPL heat

pipe with a fixed charge would be limited depending on ther e la t iv e volume of th e condenser zone as compared to th e

remainder of the system.

To avoid the above problems, a reservoir (accumulator) has been I ncorpor-

ated as part of the CPL heat pipe design. As i l lu st ra te d schematically i n

Figure 3-3, dis t r i but i on of the : quid between the reservoir and the loop

i s mai ct ai ned by a pr essu re bal ance between t he sa tu ra ti on pre ssur e in th e

re se rv oi r and the sa tu ra ti on pre ssu re in th e loop. Any change in loo?

opera ting temperature due to vari a ti ons in condenser Sl ockage caused by

the above fa c to rs or va ria ti on s in heat load and/or sink tempe rature wi 11

cause a pressure imbalance which w ill re su lt i n a d~splacementof l iquid

int o or out of the res er vo ir. Condenser blockage is thus increased or

decreased until equilibrium i s restored.

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In ef fec t , tbe reservoir sa tura t ion condi t ions wi l l contro l the loop

saturat ion condit ions as long as:

a. The res erv oir i s of su ff i ci en t volune to accomodate the range of

l iquid displacement required t o ~nai tain varia t ions in operatingcondi t i ons .

b. The res er vo ir i s only pa rti a1 l y f i l led under a1 1 condit ions.

c. A p a rt i a1 l y blocked condenser i s maintained a t a1 1 times.

To sa t i s f y the l a s t cond i tion , the r e se rvo ir mu s t be maintained at a

temperature which i s equal to or g re ate r than th e temperature at which

l iqu id displacement from th e subcooled regic n w ill oc cur. Any lower

re se rv oi r temperature wil l dra in an excessive mount of ii qu id from the

loop to a point where subcooling con dition s a re no longer s a t i s f i e d and th e

CPL heat pipe wi ll deprime. A1though th e re se rv oi r can be operated a t any

range of te mp eratu re, a si n gl e s e t p oin t designed t o accom ,odate maximum

heat 1oad/maximun si nk condi t i on will s a ti s fy a1 1 other ope rati ng

condit ions. Thus, a simple co ntro ller wil l sa t i s c y CPL heat pipe reservoir

rr qu i rement s .3.5 TEWE,RATIIRE CONTROL

The need to maintain temperature control of the reser voir i s p red icated by

the need to control l iquid inventory in the CPL heat pipe for reasons

previously discussed. A1 though a temperature co ntrol led re se rv oi r auds

ccnrtqlexity to the C?L heat p ipe design , the benef i t s der ived fa r o u t ~ e ih t

the disadlldntages. In addition to the abil i t y to compensate fo r any nunber

of va! .:~ b l e s n th e ccjntrol of liq ui d inventory, a temperature con tro lled

re . .rvoir provides the f 01 1owing addi t i onal be ne fit s:

a. Operating temp erature of the CPL heat pipe i s co ntrol led by th e

s e t poin t of th e re se rv oi r. Temperatures of components mounted

on the CPL heat pipe are automati cal l y mai nta i ned over th e en ti re

,-ange of ope ratin g c ond itions .

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b. Complete shut-down of t he CPL heat p ipe can be achieved.

Component temperature w i l l be ma in ta ined as lun g as su f f i c ie n t

power i s a v a i l a b l e t o c om pensate f o r p a r a s i t i c h e at l ea k s.

c. Automatic shut-down of the condenser under condit ions where heat

lo ad sh ar in g between components i s req u i red.

The t e mpe ratu re c o n t ro l f e a t u re i s d e r i v e d f r o m a p re s s u re b alan c e t h a t

mus t b e ma in ta i ne d b etue en t h e s a tu r a t i o n p re s s u re i n h e r e s e r v o i r and t h e

s a t u r a t i o n pr es su r e i n t h e l oo p. As s t a t e d e a r l i e r , a ny change i n l c o p

temp e ra tu re due t o v a r i a t i o n s i n h e a t l o a d a nd /o r s i n k t e i p e ra t u r e w i l l

cause a pressure imbalance which w i 1 r e s u l t i n a d is pl ac em e nt of 1 q u i d

i n t o , o r ou t of th e re se rvo i r . Condenser b lockage i s thus inc reased c r

dec reased un t i l equ i1 i r i m i s r es to re d. C o nt ro l o f th e r e s e r v o i r

t e mp e ra ture a s t m a t c a l l c o n t r o l s t h e v ap or te mp eratu re i t h e CPL hea t

pipe. The CPL hea t p ipe thus ope ra tes 1i e a va r i ab le conductance heat

p ip e w i t h t h e e x c e p t i o n t h a t l i q u i d b lo ck ag e, i n s te a d o f n on -c on de ns i b l e

gas b lo ck ag e, i s vsed t o v a r y t h e a c t i v e 1ength o f the condenser .

The f a c t th a t sa tu ra t ion p ressu re b a l ance must be rnai n t a i ned a1 so pr ov id es

the a b i l i t y fo r complete shut-down o f th e loop and hea t load sha r in g

between components. If ower f rom components i s n ot s u f fi i e n t t o mai n t a i n

s a t u r a t i o n c o nd i t io n s , c om p le te f l o o d i n g o f t h e l o o p o r t o t a l d e p l e t i o n o f

t h e r e s e r v o i r i n v e n t o r y ( de pe nd in g on r e l a t i v e v o l m e s ) w i l l r e s u l t . I n

ei th er case, shut-Gown of the CPL heat p ip e i s ach ieved. S im i l a r ly , i fheat sha r ing i s req u i r ed between components , p a r t i a l o r comple te shut-down

of the condenser w i l l o cc ur i n o rd er t o m a i n t a i n s a t u r a t i o n pr,P ~ ~ ~ba la nce . The l i m i t i n g f a c t o r i n l o a d s h a r in g i s t h e need t o m a i n t a i n

s ub co oled i n l e t c o n d i t i o n s i n to t h e a c t i v e e v a po ra to r s.

3.6 PRESSURE PR IM I NG

A h e a te d r e s e rv o i r p ro v id e s t h e a b i l i y t o p r ime th e CPL heat p ipe under

loa d and/or aga in s t gr av i t y . Pr im ing can be ach ieved by r a i s i n g t h e

re s e rv o i r t e mp era tu re t o d e v e lo p s u f f i c i e n t p res s u re t o a1 l ow d i s? l aczment

o f 1i u i d i to the evapora to rs .

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In a typical dry-out of the CPL heat pipe, ei th er thermal or mechanical,

depletion of the liquid inventory in the evaporators causes a total

blockage of the condenser zone which extends into the vapor header. By

maintaining the vapor header temperature at higher than evaporztor dry-out

temperature, and by cycl ing th e re se rv oi r t o a temperature above th e dry-

ou t tem per atur e, pr es su re priming of th e eva po rato rs can be achiev ed. Two

factors make this priming mechanism possible: 1) the high saturat ion

pressure established in a heated vapor header which allows liquid to be

injected into the evaporators as if the evaporators were subcooled and 2 )

complete shut-down of th e loop by vi rt u e of the blocked condenser zone

which allows priming under an a r t i f i c i a l "no-load" co rd iti on . Priming

under load i s t h u s po ssible as long as the ra te of evaporator temperature

r i se under the dry-out condi tion i s s uf f ic ien t ly slow to a llow the tmpera -

tu re cyc ling of the re ser vo ir before th e dry-out temperature exceeds vaporheader temperature and/or before physical damage of the loop and the

components mounted to the loop. P.lso, heat m u s t be applied to the vapor

header t o main tain an el sv ate d temp erature in th a t zone. The power level

required can be minimal as long as the vapor header is well insulated.

During normal operation, power applied to the vapor header helps prevent

li q u id entrapment. The need to ma intain power on the vapor header can be

eliminated for pressure priming purposes with a rese rvoi r f lu id inventory

capable of f looding the ent i re loop.

3.7 YON-CONDENSIBLE GAS TRAP

Any of the non-condensible gases in the CPL heat pipe will be swept along

in the flow stream of t he working f lu id . Sin ce they cannot be condensed,

they begin forming small bubbles in th e condenser. Even tually th e bubbles

w i l l migrate into the evaporators where surface tension forces at the wick

inte rface wi l l prevent fur the r migra tion. I f suff ic ie nt qua nt i t i e s of

non-condensible gases are present i n th e system, accumulation of bubbles

in the evaporators will continue until th e CPL heat pipe deprimes.

To ensure reliable CPL heat pipe o pe rat ion , non-condensibl e gases must be

reduced to su ff ic ie n tl y low q ua n ti ti es or they must be trapped in an area

where they wi 11 not detrim enta l1 y a f f e c t performance. Pa st exp erien ce

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with conventional hea t pipes has demonstrated the i m pr ac tic ali ty of

producing gas f r e e hea t pipes. The problem i s compounded in th e CPL heat

p i p e due t o extremely long tubing lengths and ti g h tl y packed wick str uc tu re

which makesi t

impractic le to proper ly evaluate the systea pr ior t ocharging. In add ition, reflu x bo ile r methods of gas separ atic n used in

conventional heat pipe processing cannot easily be applied to the CPL heat

pipe due to i t s complex geometry and because gases tend t o c o ll e c t in th e

evaporator ( in ste ad of the condenser) where they cannot e as il y be removed.

Far these reasons a nun-condensible gas trag was introduced in to the d e ~ i g n

of the CPL heat pip e. The gas tr ap i s loca ted a t the exit of the condenser

and is designed (Figure 3-4) to separate gas bubbles from the l iquid by

means of buoyancy forces exerted on the gas bubbles. T h i s design was

se lec ted f o r i t s si m pl ici ty and ease of implementation.

In zero-g op er ati on , a mechanism oth er than buoyancy w i l be required t o

sepa rate non-condensible gases from the liq uid . One approach will be to

use a gas trap design which incorporates a wick struc tur e de si ~n ed o

prevent further migration o f gas bubbles by sur face tens ian forces a t the

wick interface.

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GZAV ITY

S I R E YSH SC?EE!tCO V E R I N G

Figure 2-4. Non-Condensi ble Gas T rap .

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SECTION 4. PROOF-OF-CONCEPT TEST

CPL he at p ip e proo f -of -conce pt te s ts have been conducted on se ve ral

occas ions du r in g th e pas t 10 months. Te s t i n g was 1 m i ed i n scope and

d es ig n p r i m a r i l y t o e s t a b l i s h f e a s i b i l i t y o f d esign, t o o b t a i n an und er-

s t and i ng o f o pe r a t i ng behav i o r and t o e s t ab l i s h bas i c pe rf orm ance

ch a r ac t e r i s t i c s . Res u l t s o f t e s t s conduct ed on t h r e e sepa r at e occas ions

a re sum nar ized be1 ow.

4.1 TEST SET-UP

P r i o r t o t e s t i n g , t h e CPL h e a t p i p e was b r i e f l y ev ac ua te d w i t h a m e ch a ni ca l

pump and charged w i t h comnerc i a l l y a v a i l ab le Freon-11. No speci a1 at tempt

was made t o m in im iz e non-condens i b l e gases.

The CPL t e s t set -up (F ig ur e 5 - 1 ) i n c l u d e d i n d i v i d u a l c o n t r o l o f e v a po r at o r

power t o e s t a b l i s h t h e a b i l i t y o f o p e r at in g p a r a l l e l c i r c u i t s a t d i f f e r e n t

power l ev e l s . P r o v i s i on s were a l so made t o coo l t he evapo r a to r s t o

e s t a b l i s h t h e f e a s i b i l i t y o f h e a t lo a d s h ar in g. The s e t- up a l s o in c l ud e d a

vapo r header hea t e r des i gned t o p r even t l i q u i d en tr apm en t and t o p r ov i de a

back p r essu r e du r i ng p r i m i ng , and a r e se r v o i r hea t e r f o r t em pera t u re

co n t r o l and p r essu r e p r i m i ng cyc l es . A t t h e condenser end, t h e p a ra l l e l

condenser tubes were c lamped t o a co ld p l a t e connected t o t he GSFC Themo-

phys i cs Labo r a t o r y T i nney coo l an t l oop . The en t i r e I oop was i n su la te d w i t h

f i b e r g l a s s i n s u l a t i o n b l a n k e ts w i t h t h e e x c ep t io n o f t h e c o l d p l a t e w hic h

was l e f t u n -i n su l a te d t o o b t a i n v i s u a l o b s e rv a t io n o f t h e l i q u i d / v a p o r

i n t e r f a c e 1oca t i on i n t h e condenser . V i su a l observa t i ons were made

p o s s i b l e b y m a i n t a i n in g t h e c o l d p l a t e a t a l ow enough t m p e r a t u r e t o f o m

f r o s t wh ich was subsequent l y thawed as the vapor f r o n t advanced i n t o th e

condenser zone. F in a l l y , i ns t ru me nta t i o n o f t he CPL heat p i pe p ro to ty pecon s i s te d o f copper -cons tan tan thermocouples ( see F ig ure 5 -1 fo r

1oc a t i ons) .

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4.2 HEAT TRANSPORT PERFORMANCE

The CPL heat p ip e was f i r s t op erated a t th e end o f August 1981. A f t e r an

i n i t i a l su ccess , s t a r t - u p o f t h e CPL p ro ve d t o be d i f f i c u l t . An excess i ve

amount of non-con densible gases was determine d to be pre se nt i n the system.

A f t e r s e v e r a l b l e e d in g o p e ra t io n s , t h e CPL h ea t ~ i p e as success fu l l y

opera ted a t power l ev e l s i n excess o f 200 wat t s and a t e leva t i on s o f 2.0

i c hes. Performance tests were repeated on several occasions

s u b s t a n ti a t i ng t he CPL hea t p i pe ' s capab i l i t y of t r a ns f e r r i n g l a r g e heat

l o ad s o ve r lo n g d i st a n ce s and a t s u b s t a n t i a l e l e v a t io n s . The i n i t i a l t e s t s

a l so i n d i c a t e d t h a t a p a r a l l e l c i r c u i t d esig n i s f e a s i b l e and t h a t

unba lanced heat l oads can be app l i e d to the evapora to rs . I n add i t i on , t he

fa c t t h a t t h e CPL heat p ip e was su cce ss fu l l y opera ted w i t h m in ima l a t tempts

t o co n t r o l non-condens ib le gases i n d i c a te s t h a t t he p roblem o f gas bubb les

may be c i rcumvented wi th th is design.

Per formance tests were repeated at the end of September lbeginning of

October 1981 and i n February 1982. Res ul ts , in c lu d in g evaporator and

condenser conductance, are sumnar iz ed i n Ta ble 4-1. The SeptemberIOctober

t e s t r e s u l t s e s s e n t i a l l y d u p li c at e d i n i t i a l t e s t r e s u l ts . C ap ac it ie s i n

th e range of 200 wa tts a t an e le v a ti o n of 2.0 inch es were obtaned. On th e

bas i s o f t he da ta ob ta ined , ex t rap o la ted zero e lev a t i on per fo rmance o f 250

wat ts was achieved as compared t o th eo re t i c a l p re di c t io ns of 400 wat ts . An

a tte mp t, a t th e end o f t h i s t e s t e f f o r t , t o ab ta in performance data a t 4.0

inches of e le va t i on was unsuccessful .

Dur ing the September IOctober per formance test ing , t became apparent that

s i gn i f i ca n t q ua n t i t i e s o f non -condensi bl e gases were s t i 11 p r e se n t i n t h e

system. This determ inat ion was made on the basis o f the r e la t iv e l y h ig h

loop opera t ing temperature as compared t o the r as er vo i r s et p o in t tempera-

t u r e . A f t e r s ev e r al r e s e r v o i r b l ee d i ng o p e ra t io n s , a s i g n i f i c a n t

r ed uc t i on i n 1oop opera t i ng tempera tu re w i th respec t t o reservo i r t empera-

tu re was noted. Fu r the r at tempts t o remove non-condensi b l e gases f rom the

system were d i s c o n t i r g e d b ecause o f t h e i n a b i l i t y t o c l e a r l y e s t a b l i s h a

gas i n t e r f a ce i n t he r ese r v o i r . Th is was due t o t he f a c t t ha t t he

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r es e r vo i r was hea t ed a l ong i t s e n t i r e l en g t h which masked any grad ien ts

developed by non-condensib le gas/vapor sep arat io n,

A f t e r a four-month i n ac t i v e per iod , t h e CPL heat p ipe was reac t ' va ted i

February 1982. The pr im ary ob je c t i ve was to determi ne i f any res idual non-

conde nsib le gases were pres ent i n t h . system, t o removs th e gases i f

pos s i b l e and t o de te r m ine any e f f ec t qas r educ t i on ~ o u l d have on

performance. A new heater, con f i ned t o t he bo tt om o f t he r ese r vo i r , was

i n s t a l l e d and a p o s i t i v e i d i c a t i on of non-condensi b l e gases was o b ta i ned.

A f t e r re pe ate d r e s e r v o i r b l eedi ng op er at i on, i became apparent that the

l i q u i d i nve n t o r y was be i ng dep l e ted and t he CPL heat pi pe was charged w i th

ad d i t i o na l F reon -11. A f t e r t he r es e r v o i r b l eed i ng ope r a t i on was

c m p l eted, pr ev io us ly achieved performance cou ld not be du pl ic ate d. TheCPL heat pi pe was then b le d a t th e non-condensi b l e gal. t r a p valve, t o

ensure tha t i t was no t t o t a l l y b locked , and a t t he va l ve l oca t ed a t t he end

o f t he l i q u i d r e t u r n l i n e ( se e F i g ur e 3 -1 f o r v a lv e l o c a t i o n ) . A p o s i t i v e

disp lacement of non-condensi b l e gas was obta ined at both lo ca t io ns . The

CPL hea t pi pe wzs the n p erf ormanco te st ed a t an e le v a t i o n of 1.5 i che; and

a heat t r a n s p o rt ca pa ci ty of 340-350 wat ts was achieved. Thi: per formance

le ve l was repeated dur ing severa l t e s t runs. The measured per formance

ex t rap o l a tes t o approx imate l y 400 wat t s a t zero -e iev a t i on wh i ch ii d en t i c a l t o t h eo r e t i c a l y p r ed i c t ed perfo rm ance. The sma l l e l evi? ti n

d i f f e rence , cornpared t o p rev ious te s t i n q , c a l l r i ~ t xp l a i n t he pe r f g rmance

d i f f e r en ce . Possi b l e exp l ana t i ons i cl ude b l zedi ng o f non-condensi b l e

gases, addi tional 1i uid charge (prev ious charge i adequate due t 9

r epea ted r e s d v o i r b l eed i nq ope r a t i ons ) and/ or F reon- 11 i s be t t e r a01 e t o

b r i d g e ( fo r m a s l u g r a t h e r th a n a p ud d le ) t h e i n n e r d i a me te r o f t h e l i q u i d

r e i u rn a t 1ower e l ev a t i on.

The most 1i e ly exp la na t i o n i s t ha t remova l o f non-condensab le gases t o a

1ow enough le v e l a l l awed th e CPL heat p ipe to be opera ted w i thou t p remature

de prim i ng. D is p l acemeot of non-condensi b l e f roni the : ui d re tu rn may be

espec ia ! l y t roub lesome because o f t h e very l ong l i q u i d re t u rn and th e

p o ss i b i l i y o f non-condensi b l e gas bubble ent rapnent due ';J t h e mu1 t i p l e

pass a rrangement used i n t h i s se t-up . S ince a i t e r na te ' leng ths of t h e

l i q u i d r e t u r n a r e t i 1 e d downward, t h e l i q u i d r e t u r n m ust a c t a g a i ns t

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buoyancy fo rce s t o disp lac e any gas bubbles. In ad di tio n, Freon-11 wicking

height properties are very low, making i t d i f f i c u l t f o r t h e f l u i d t o g a p

the inner diameter of the lCquid ret ur n by ca pi ll ar y actio n. I t is thus

poss ib l e fo r the liquid return to have a tendency to by-pass non-conden-

s ib l e gas inclusions, making the displacement of gas bubbles, by entrap-

ment, a difficul t and slow process. During a significant number of tests

where burnout occurred at relat7ve7y ?3w power inputs, the amount of watt-

hours transported prior to burnout were most often i n excesr of that

requi red to d i splace to ta l loop liquid inventory at least orlce and often

twice. This would tecd t o suppo rt the poss ibi 1 t y th at non-condensible

gases trapped i n the liq ui d retu rn were slow!y mig rating and hindered the

abi l i ty to achieve fu l l capac i ty in ea r ly tes t s ; and tha t the d isplacement

of such gases from the liquid return is a very slow process die to ent rap-ment in the multipl,; pass set-up. Future tes t s wi l l cons ider th i s poss i -

bility including means of effectively removing residual non-condensible

gases from the liquid return and means of determining the effectiveness of

th e non-condensi bl e gas t)-ap .

4.3 HEAT TRANSFER PERFORMANCE

Evaporator and condenser f i lm c oe ff ic ie nt s derived from performance t e s t s

conductsd t i date are sumnarized in Table 4-1. A range of v alus s i s

indicated t o account for uncertainties in temperature measurements,

loca tion of thennocouples, e stab lishn ent of a referenc e sa tur atio n vapor

temp erature, e f f e c t s of non-unif orm heat inp ut/ ou t?u t and dete rm inat ion of

act ive zone in the condenser. Establishment of a re fe rence sa tura t ion

vapor temperature i s made d i f f i c u l t sin ce a heated vapor header i s used in

th is set-up. Determination of the active condenser tone was also difficult

s i n c e i t s n o t passibie to establish the lengtb of annular flow and slug

flow zones since both are subcooled and cannot be clearly detected w i t h

thermocoup 1es .Accurate determi nation of heat t ra ns fe r ch ar ac te ri st ic s wi 1 1 requi re

control led te st condit ions including in struven tat ion ~nore su i tab le f or

th is purpose. Nevertheless, a high heat tra ns fe r eff icie nc y of the CPL

heat pipe i s cle ar ly ind ic3f sd from the data obtained t o date. Evaporator

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fi lm c oe ff ic ie nt s are two to four t imes those of conventional axial groove

hea t pip e de signs. This perf crmance improvement i s consi s ta n t with

observations made by Saaski for inverted m i niscus evaporator designs

(Reference 4 ) . Very ef f i c i en t condenser performance i s a1 so apparen t from

the observat ion that only short , fu l l y act ive zones were required t o re jec t

1arge q u an ti ti e s of hea t. Minimum 1eng ths asso ci ated with subcool ed

regions ( i .e ., annular, s lug and a ll l iq uid phase zones) have yet to be

established; however, since they represent 1ow heat d i s s ipa t ion areas ,

they should not significantly affect an overall system design w i t h

regards t o radi ato r surf ace area a1 1ocation.

4.4 PRESSURE PRIMING

An

important design fea tu re of theCPL

h ea t p ip e i s t h e ab i l i t y t oesta bl ish condi t ions favorable fo r pressure priming and to in i t ia te a

priming cycle on command. The CPL heat p ige was sub jec ted to a ~ e r i e s f

priming cycl es during perf omance te st in g to determine prining c h a r a c t~ r -

is t ics and to es tabl ish condi t ions necessary to achieve rel izble dn d

consi s ta n t priming.

The ab i l i t y of the CPL heat pipe to prime under load and/or against gravity

i s i l l u s t r a t ed i n F i g u re 4-2. After an in i t i a l s teady s ta te was

established at an elevation of 2.0 inches and evaporator power i n p u t of 1C3

watts, power was then increased to 180 watts. A t this power level,

evaporator dry-out was indicated. Power to the etl orators was reduced to

20 watts, to avoid rapid and exce ssive temperature ri s e of the ev aporators

due ta th e ir smail thema : mass, and, simultaneou sly, the re se rv oir was

cycled to a temperature above th a t of th e evaporators. The re se rv oir was

than allowed to cool down t o i t s in i t i a l s e t point . Once recovery was

indi ca te d, noted by a reduction i n evaDorator temperature, evaporator

power was then increased to 100 watts to confirm p r i m i n q . Subsequently,

power was stepped up t o 180 watts a t which pgin t

the test was terminated.The priming cycle illustrated in Figure 4-2 was repeated o n a t l ea s t s i x

different occasions during the September/October test effort ,

demonstrating the pote nti a1 o f CPL heat pip e priming mechani sm.

Additional pressure priming evaluation t2sts were conducted in February

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F i g u r e 4-2. Ty pi ca l Recovery Under Load

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1982. Priming was judged to be successful in two o u t of fo ur a ttem pts made

on th e same day. Review of t he data af t e r te st; ng in di ca ted some

differences between the successful an d the unsucces. f ul cy cle s. This

inc luded the fac t tka t the reservoir temperature was not alloked

su ffi cie nt time to cycle down prio r to power ap pl icat io n to t he re ser vo ir

during th e unsuccessful cyc les. Another observation i s tha t a sig nif: :aht

amount of watt-hours were transpor ted ( a t le a st tx ic e tha t required t o

c i rcu la te to ta l loop inven tory) p r io r to burnout ind ica t ions af t e r the tho

unsucc essful priming cy cl es , which may in di ca te the exi sta nc e of some

re;idual non-condensible gase s i n the l iqu id re tu rn l in e and/or t ha t the

non-condensible gas t rap i s ine ffe ct iv e.

Evaluation of pressure priming to date has been made difficult by the f a c t

that Cstz was obtained manually. Sin ce 2 number of factors can icfluence

priming anq sin ce time con stan ts may be si gn if ic an t, ad diti on al te st in g

and evaluation w i t h improved ar~d quic ker da ta ga the rin g methods w i 7 1 be

required before the re l i a b i l i t y of pressure priming can be es tabl ished .

Although p res sur e priming has ye t t o be achieved on a re pe at ab le and

co ns ist an t ba sis, succ essfu l priming has been achieved on a si g n if ic a nt

number of occasions indica t ing tha t once the cantr ol l in g fa cto rs are

es ta bl ished and u nderstood, a r e i i abl e procedu re may be p oss ible . Obser-va tion s made t o d at e which tend t o su?por t th i s p r i l iv inary conclus ion

i nc lu d e t h e f a c t t h a t a s i g n i f i can t r i s e i n reservo i r l in e t empera ture (T /C

$31, Figure 4-1) and shut-down of t h e condenser have co n s is ta n tl y been

noted prior to any noticeable rise in evaporator temperature. I n addi t ion,

a r i s e i n re se rv oi r temperature was notsd ind ic ati ng tha t vapor and/or hot

l i q u i d are d i sp laced in to the reservo i r d u r i n g dry-out. The a b i l i t y to

de tec t dry-out before si gnif i cant ev aporatcr temperature r i s e shoul d prove

valuable i n allowing priming cycles t o be i n it ia te d w i t h minimum reduction

in appl i ed system power. Condenser shut-down and di sp l acement of

vaporj l iquid i n to the reser voir i ndi ca te tha t condi t ions favorable to

pres sure priming are being es ta bl ish ed during dry-out. Back-flow i n t o th e

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r es e r v o i r a i so i nd i ca t es t h a t d i q 1 cement of non -condens i b le gas bubbles

may be poss i b l e .

4.5 1 tWERATIIRE CONTROL

T em pera ture c o n t r o l c a p a b i l i t i e s o f t h e CPL h e a t p ip e i s i l l u s t r a t e d i n

F i g u r e 4-3. k ' i t h 120 wa t t s app l i ed t o t he evapo ra t or , a r es e r vo i r se t

po in t t empera tu re o f 2 3 '~ and a s ink tempera tu re o f -s°C, th e condenser i s

p a r t i a l l y opened m ai n t a in in g an evap orator temperature of approx imate ly

25'~. As t9 e bink i s cyc led to -30°c, aq e a r l y t o t a l shu t-down o i t h t

t onde :~se r i s ach ieved. A t a s il lk t m ~ e r a t u r e f +15% t he copdenser i s

f u l l y open. No no t i ce ab le change i n evaporh to r t eqperacure can t~ b e ~ e c t e d

t hr o ug h o ut t h e e n t i e c y c le . S i m i l a r t e m p e r a t u r e ~ u n t r o l h a r z c t e ri s t i c s

were abserved w i th resp ec t t o power va r i a t i on s . As expec ted , a s l g h t r i s eo r d ro p i n e v a po r at or te n p e ra t u re t h a t i s p r o p o r t io n a l t o e v a p o ra t or

conductance and power in pu t was noted du r in g power v a r ia t i o n cy cle s. These

t e s t s v e r i f i e d t h e a b i l i t y u s in g t h e r e s e r v o i r t o c o n t r o l t h e te mp er atu re

o f t h e CPL heat p ipe.

4.6 HEAT LOAD SHARING

The a b i l i t y t o share heat between components represents a s ig n i f i c an t

advantage fo r power savings i n space syscens app l ica t ion s. Re su l ts of a

t e s t c ond ucte d t o e s t a b l i s h f e a s i b i l i t y o f h ea t l oad sha r i ng i n t he CPL

h e a t p i p e i s shown i n F i g u r e 4-4. I n i t i a l l y , power was a p p li e d i n

increments t o each evaporator u n t i l a t o t a l 300-wat t loa d was achieved.

Th is s tepwise i nc rease i n power deqons tra tes th e a b i l i t y o f t he CPL heat

p i p e t o c o n t r o l t em p er at ur e as a f u n t i o n o f power i n p u t . A f t e r t h e i n i t i a l

r i s e to opera t i ng tempera tu re , on1y m a 11 temperature i crements res ul te d

w it h la rg e power increase s. The power increment f rom 100 wa tts per

evapo r at o r t o 150 wa t ts pe r evapo ra to r r esu l t ed i n on l y 1 .5 '~ r i s e i n

e va po ra to r t em p era tu re w hic h i s c o n s is t a n t ~ i t hhe measured range ofevaporator f i l m coe f f i c i en t s . A t t he 300-watt i npu t , a f u l l y open

c ond en se r c o n d i t i o n ( w i t h r e s p e c t t o t h e r e l a t i v e l o c a t i o n o f T/C # 17 ) i s

ind ica ted . As can be seen, th e condenser tenp eratu re i s on ly lo t below

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NAPaRAia7- ESERVOIR

NAPORATW ?OK?: iD YATTS

ELEVATION 2 IN.

REERVOIR?!lYR L . 4 Z A r n

Figure 4-3. Temperature Cycle Prof l e

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Figure 4-4. Heat Load Sha r i n g Te s t Prof i le

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th at of the rese rvo ir s atu rat ion se t point temperature indicat ing a very

high fi lm coefficient in the ful ly act ive sect ion of the condenser.

As the power to one of the evapora tors was sh ut -o ff , i t s temperature

reduced to a point sl ig ht iy above th at of the res ervo ir set-p oint . Atemperature drop in the second (a c ti ve) evaporator operat: ~g temperature

is also apparent. Although th is i s yet to b? fu l ly explained , a t l e as t

p a r t o f this temperature droo can be accaunted for by the reduction of

vapor pressure drop between the evaporator and condenser resclting from

the smaller total applied heat load. A reduction in condenser temperaturs

i s als o noted indic atin g the condenser i s no longer fu ll y activ e to the

point at which the condener thermocouple is located.

When water cooling was i n i t i a l l y applied ( a t approximately 40 wat ts) to the

inactive evaporatcr, no significant change in either evaporator tempera-

tu re s could be detected. The in ac tiv e evaporator continued to ope rate

isotherm ally and a fu rt he r reduction in condenser temperature, indic ating

ad di tio na l shut-down, was app aren t. I t was al so noted, as might be

expected, that sbocooli ng temperature entering the active evaporator

increased as cooling of the inactive evaporator was initiated.

As cooiing t o the in ac tive evapo rator was incre ased to 70 wat ts (near ly 50

percent of th e applied heat to the act ive evaporator), a sig ni fic an t

decrea se in condenser temp erature was noti ced combined with a s l i g h t

decrease in ac tive evaporator temperature. The ina ctiv e evaporator

remained isotherm al with no apparen t change in i t s oper atin g temp erature.

After 10 minutes of operation at the increased cooling load, inlet tempera-

ture to the active evaporature increased to saturation temperatures which

was followed by a depriming and burnout of the active evaporator.

The above behavior can be explained by the fact that the impedance to vaporflew between evapo rators i s much lower than the impedance to t he ca nde nse r.

Vapor, the re fo re , wi 11 f 1ow pref eren t! a1 ly betxeen evapo rators and, s in ce

pressure equilibrium with respect t o the reservoir set point must be

maintained, a condenser shut-down results, as coolirlg in the inactive

evaporator is increased . In a l l b u t one respect, preferential shutdown o f

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th e condenser i s an ide al cond ition f o r heat load sh arin g. However, the

problem th at develops i s tha t , s ince there i s l i t t l e i f any l iquid blockage

of the inact ive evaporator, the l iquid re turned t o the loop is a t

sa tu rat io n temperature . As discussed ea rl ie r , some subcooling is requiredfor proper operation of CPL hea t pipe. Heat load sh ar ing becomes 1imited by

the condit ion af the l iquid re turning to the act iv e evaporator.

Additional tests and evaluations will be required not only t o confirm the

above observations, b u t to establ ish more c learly the l imits that might be

imposed on he at load sh ar in g by subcool i ng considerations. Also,

evaporator d esigns with reduced s e n si ti v it y of subcool ing and th e abi 1i t y

t o provide some means of subcooli ng between evaporators should be investi-

gated before further conclusions on heat load sharing can be formulated.

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SECTION 5. SUMMARY AND CONCLUSIONS

D e s p i te t h e l i m i t e d l e v e l o f d ev elo pm en t and t e s t e f f o r t s c on du cte d t o

dat e, s i g n i f i c a n t p r og r ess has been made i n es t a b l i s h i n g t h e CPL hea t p i pe

a s p o t e n t i a1 c a n d id a t e f o r s yste ms a p p l i c a t i o n r e q u i r i n g 1arge heat l oad

ca r r y i ng ca pa c i t i es ove r l ong d i s t ances . Cu r r en t CPL hea t p i p e deve lop -

m e ~ ~ tt a t u s i s ~ u m n a r i z e d n T a ble 5 . 1 . A l l aspec t s o f t he CPL hea t p i pe

d e s ig n ha ve been de mo ns tr ate d a t l e a s t i n p r i n c i p l e . H ea t lo a d c a r r y i n g

c a p a c i t i e s d u r i n g p r o o f - o f - co nc ep t t e s t s a r e co n s i s t a n t w i t h t h e o r e t i c a l

p r ed i c t i on s . Mu1 t i - k i l o w a t t ca pa c i t i e s w i t h F reon- 11 and an o r de r o f

magnitude, b e t t e r per formance w i t h Ammonia can be reason ably ex t ra po la te d

f o r l a rg er system des igns . The CPL heat p ipe has a l s o p roved t o be an

e f f i c i e n t hea t t r ans f e r dev i ce w i t h measured evapo r a t o r and condenser f i l m

c o e f f i c i e n t s t w i c e t o fo u r t im e s t h a t of c o n v e n ti o n a l a x i a l g ro o ve h e a t

p i p e desi gns . O t he r f ea t u r es t h a t have been c l e a r l y dem onst ra ted i nc l ud e

t h e f e a s i b i l i t y o f m u l t i p l e p a r a l l e l c i r c u i t s design and th e a b i l i t y t o

c o n t r o l o p e r a t i n g t em p er at ur e by c o n t r o l l i n g t h e t em p er at ur e o f t h e l i q u i d

i n v e n t o r y r e s e r v o i r .

A re as r e q u i r i n g f u r t h e r t e s t s a nd e v a l u a t i o n t o r e ac h f i r m c o n c lu s i o ns

i n c l u d e :

a. Press ure pr im ing .

b. Head l o a d s h a r i n g c h a r a c t e r i s t i c s and l i m i t a t i o n s .

c. S e n s i t i v i t y t o gas bu bb le s e ntra pm en t i n t h e l i q u i d r e t u r n l i n e

and f e a s i b i l i t y o f d i s p l a c i n g g as b ub bl es i n t o t h e r e s e r v o i r .

d. S e n s i t i v i t y t o l i q u i d r e t u r n dia me te r i n a 1 -g o p er a ti o n.

A d d i t i o n a l t e s t s c u r r e n t l y p l an ne d f o r t h e n ea r f u t u r e s h o ul d r e s o l v e m ost

o f t hese que s t i on ab le a reas. On the bas i s o f t es t s and eva lua t i o ns

conduc ted t o da te , t he CPL heat p ip e has i n most aspec ts unqu es t i on3b ly

p ro v e n i t s a b i l i t y t o p e r fo r m as p r e d i c t e d . Even i n q u e s t io n a b le a re as o f

p erfo rm an ce , a s u f f i c i e n t l y 1arge degree of success has been achieved

i n d i c a t i n g a good t o e x c e l l e n t p o t e n t i a l f o r f a v o ra b l e r e s o l u t i o n s o f

t hese a reas . The po t e n t i a1 l ev e l s o f pe r fo r mance ach i evab l e w i t h i n

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curre nt s ta te- of - th e-a rt make i t po ss ib le to co nsi de r capi 11 ary pumped

two-phase heat transfer designs for 1arge space s t r uc tur e app l i ca t ion s

with ca pa ci t i es in the t e n ' s of ki; owatt range. Furthermore, 'mprovements

and additional development i n the wick material properties beyond that of

cu rr en tl y av ai l ab le comnercial prod ucts, may make i t poss ib le i;o acll ieve

another order of magnitude improvement i n ki lowat t capaci ty , thus making

i t poss ib le t o c onsider designs wi th mu1 t i -nundred ki lowat t capaci t ies .

8/8/2019 19850009014_1985009014


REFERENCES

1.. S t e ~ g e r , F.J. "Expsrimenta l F sa s i b i ' l i t y S tudy o f Water F i l l e d Cap i l - .

1ary-Pumped Heat T ra n rf er Loops ," NA5A Te ihn i ca l N a~ or en du ~~ lASA-TH-

X- 1310, Ntvernber 1966.

2. "Developnent o f Heat P ip e Des i gn C - i - ep t s f o r ii e i gh Capac i t y

I n s t r m e n t Module Heat Tra ns po rt System," BK079-1001, B&K f ng i neer i fi g

I c . Towssn , H a ry l and 21204.

3. "Heat Pipe Design Handbook," B&;( Eng ineer ing I;Ic.,olume I, June

1379 NTIS N81-70H2.

4. Saaski , E.W., " I n i f e s t i ga t i o n of an In ve rt ed Ni ni sc us Heat Pipe Ai:%

Concept," NASA CR-137, 724, August 1375.

Documents

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