AKALYTICAL AND KF'ERItlENTAL INVEST1~ATIO:I

3F SATZLLITE PASSIVE THEXJAL CO!JTROL USING

PHASE-CFANGE MATERIALS (PCM I)

S o l 2. F i x l a r

S p e c i a l i s t Thermodynamics Engineer Space and Nissile Systems Divis ion

ile?ublic Aviat ion Corporation, Farmingdale, New York

I ., INTRODUCTIOK AND SUiU1IWY

A. IKTRODUCTIOW B. SUfRARY

Experience has shown t h a t the r e l i a b i l i t y of a s a t e l l i t e or s p a c e c r a f t i s g r e a t l y enhanced if its dependence upon powei and upon t h e endurance ?f noving p a r t s is elirilinated o r a t l e a s t reduced t o a minimum. Even i f proper design, s u f f i c i e n t redundancy, and a r igorous t e s t program permi t one t o p o s t u l a t e a h igh system r e l i a b i l i t y , t h e r e s t i l l can be unknown space hazards t h a t have n o t been designed f o r . rr.eans of p a s s i v e thermal c o n t r o l systems is, there- f o r e , a t t r a c t i v e f o r l o n g- l i f e satel l i tes and spacecra f t . A pass ive thermal c o n t r o l system i s here defined as one tha t does no t i n c o r p o r a t e mov- ing p a r t s o r rroving f l u i d s , a rd does no t require any power f o r i t s opera t ion .

The e x p l o r a t i o n of var ious n o s s i b l e

&en t h e i n c i d e n t o r b i t a l h e a t fluxes on a s a t e l l i t e vary over a wide range, o r the on-board equipment h e a t d i s s i p a t i o n i s widely f l u c t u a t i n g , p a s s i v e thermal c o n t r o l by t h e a p p l i c a t i o n of phase-change materials (!XK) p r e s e n t s an a t t r a c t i v e approach. Bas ica l ly , t h e PCM thermal c o n t r o l system c o n s i s t s of a c o r e f i l l e d w i t h a substance capable of undergoing a phase change ( s o l i d t o l i q u i d and vice v e r s a ) a t a predetermined temper- a t u r e . The c o r e i s sandwiched between the equip- ment t o be c o n t r o l l e d and t h e o u t e r s u r f a c e of t h e sa te l l i te which s e r v e s as a space r a d i a t o r . scheme i s shown schemat ica l ly i n F igure 1.

t o r ) i s exposed t o e x t e r n a l r a d i a t i o n o r t o in- t e r n a l equipment h e a t d i s s i p a t i o n , the phase- change m a t e r i a l w i l l absorb t h e excess heat and nelt a t a c o n s t a n t temperature ( t h e material mel t ing t m p e r a t u r e ) . When t h e s a t e l l i t e o u t e r s w f a c e f a c e s space, away from t h e sun, and h u t i s being l o s t , o r i f t h e h e a t - genera t ing equipment is s h u t o f f , t h e FCM w i l l s o l i d i f y and g i v e off t h e h e a t it absorbed during melting. The equipment w i l l thus b e i n a s t a b i l i z e d tamp- e r a t u r e environment r e g a r d l e s s of t h e f l u c t u a- t i o n s i n t h e i n c i d e n t e x t e r n a l or i n t e r n a l h e a t f luxes .

t o a n a l y t i c a l l y and experimental ly determine t h e f e a s i b i l i t y of s a t e l l i t e thermal c o n t r o l by the use of phase-change materials. t i v e was t o perform tests on d i f f e r e n t phase- change r a t e r i a l s and on d i f f e r e n t h e a t- t r a n s f e r c o r e conf igura t ions , and t o compare t h e a n a l y t i c a l arid eqeriri e n t a l r e s u l t s .

The

A s the o u t e r sa te l l i te s u r f a c e (space r a d i a-

The primary o b j e c t i v e of t h e i n v e s t i g a t i o n w a s

A secondary objec-

A n a n a l y t i c a l and experimental i n v e s t i g a t i o n has been conducted on t h e f e a s i b i l i t y of s a t e l l i t e thermal c o n t r o l by phase-change materials. F i n i t e d i f f e r e n c e techniques and e lectr ical analogs w e r e used t o s o l v e the governing h e a t- t r a n s f e r equation on t h e IBI 709b d i g i t a l ccarputsr. o b j e c t i v e of the experimen-cal program was t o de te r - p i n e t h e f e a s i b i l i t y of PCM thermal c o n t r o l

The primary

A copparison u i t h o t h e r (semi a c t i v e ) thermal c c n t r o l systems, on an a n a l y t i c a l b a s i s , shows that t h e PCM thermal c o n t r o l scheme has - i n addi- t i o n t o being completely pass ive (absence of moving p a r t s and non-dependence on e x t e r n a l power) - a d e f i n i t e weight advantage over louvered o r f o r c e d c i r c u l a t i o n systems. W i t h p r e s e n t l y a v a i l a b l e phase-change materials ( h e a t of f u s i o n = 130 BTU/lb), it is p o s s i b l e to a t t a i n a weight index (weight of phase-change material needed p e r u n i t of absorbed hea t ) of 0.00385 lbs/BTU. The analytical approach presented, could b e used e f f e c t i v e l y (with some modif ica t ions) t o p r e d i c t the experimental p e r f o r- mance of PQ? Thermal Control Systans.

Two d i f f e r e n t phase-change m a t e r i a l s , Technical Eicosane (C H ) and Polyethylene Glycol (carbowax 600), and two

( f inned and f i n l e s s conf igura t ions) w e r e t e s t e d i n t h e Republic thermal-vacuum chamber. A t o t a l of 11 t e s t runs, each s imulat ing d i f f e r e n t external and i n t e r n a l sa te l l i te h e a t flux condi t ions , were made. During each test r u n t h e heat f l u x var ied between zero and a maximum, to simulate alternate sa te l l i te sun and space exposures. A t o t a l of 25 o r b i t a l cyc les were run. A simulated space thermal environm t ranging between 8 x 10-3 TORR and 2.2 x 10% TOFR, and -250% was obtained. Simulated e x t e r n a l h e a t l o a d s v a r i e d from zero t o 2% Bnr/hr-ft2 (absorbed n o t i n c i d e n t ) and i n t e r n a l d i s s i p a t e d equipment h e a t l o a d s v a r i e d from zero t o 2h watts p e r square f o o t of r a d i a t o r area.

20 it2 ifferent PCM core des igns

The tes t r e s u l t s show that sa te l l i te temper- ature c o n t r o l by phase-change materials i s f e a s i b l e ; that it i s p o s s i b l e wi th a PCM thermal c o n t r o l system t o maintain s a t e l l i t e equipment and struct- ural components w i t h i n narrow temperature 1in;its under widely f l u c t u a t i n g satel l i te e x t e r n a l h e a t loads. Most of the tes t da ta i n d i c a t e d t h a t t h e PCM thermal p r o t e c t i o n system reduced on-board equipment temperature f l u c t u a t i o n s by a t least 75% (compared t o an unprotected s a t e l l i t e ) . Much b e t t e r r e s u l t s could be expected with ohase-change

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materials especia l ly formulated f o r PCM.thennal cont ro l systens.

The experimental da ta showed that t h e finned PCM core configuration is more ef fec t ive , on a weight basis , than the finless core i n damping out s a t e l l i t e equipment temperature f luctuations. The r e s u l t s also showed that Technical Eicosane has a better PCM thermal protec t ion performance under the conditions tes ted than Polyethylene Glycol.

11. ANALYTICAL STUDY

A. MATHRUTICAL MODEL

When a l i qu id is undergoing a change of phase from l iqu id t o so l id , thermal energy i n t h e form of t h e l a t e n t heat of fus ion is released at a moving solid- liquid in ter face . e f f e c t takes place when a so l id undergoies a change of phase from sol id t o l iquid . cases the locat ion and time h i s to ry of t h e moving solid- liquid in t e r f ace is unhm. The problem is t o determine t h e temperature d i s t r ibu t ion i n the melted and unmelted port ions of t he phase- change mater ia l and t h e way i n which the melting is propogated.

The opposite

I n both

I n the PCM thermal cont ro l scheme a s l a b of phase-change mater ia l is exposed to two d i f f e ren t heat inputs f r o m two opposite faces. ur ternal face the heat f l u x cons i s t s of t h e ne t interchange between the incoming so la r f l u x (var iable) and t h e outgoing radia t ion (variable) . On the internal f a c e there is the heat input due t o equipment heat d iss ipa t ion which may o r may not be constant. Figure 2.

On the

The conditions are presented in

The heat conduction equation f o r ont-dimen- s ional heat flow with constant thermal propert ies i n the sol id and l i qu id regions can be wr i t ten respect ive ly as*

(1)

If the fus ion temperature is Tf and the s o l i b l i qu id in t e r f ace is a t x i l ( t ) , one boundary condi- t i o n t o be s a t i s f i e d is

( 3 )

a t x - xi1 . A second boundary condition concerns t h e

absorption o r l i b e r a t i o n of l a t e n t heat of fusion a t the solid- liquid in ter face . region x 7 x i l ( t ) contains l iquid a t temperature TL (x , t ) and t h a t t h e region x < x i l ( t ) contains so l id a t temperature TS (x, t) . solid- liquid in t e r f ace moves a dis tance d x i l , a quanti ty of heat Qf e dx+l per u n i t area is l ibe r-

Suppose that the

Then, when the

ated and must be removed by conduction. This may be expressed mathematically by

equation b holds f o r f reez ing as well as f o r melt- ing. dimensions as t h e fus ion f r o n t progresses, it will be assumcd that

I n order t o avoid changes i n the physical

c=e,=e (The mater ia ls tes ted during t h i s program ac tua l ly exhibited t h i s property, which is des i rable f o r an operational FCM T h e m 1 Control Systan). similar analysis will hold fo r the solid- liquid in ter face on the left s i d e of Figure 2 a t xi2(t).

Two addit ional boundary conditions t o be satisf ied a re

A

and

( 7 )

An exact ana ly t i ca l solution t o this problem is not available1 and the problem must be attacked by numerical methods. General approach type equat ons alone, as those presented i n the l i te ra ture$ , do not provide a complete solution.

B. FINITE DIFFERENCE FETHOD

Calculated temperature d i s t r ibu t ions through the PCM test model were determined using f i n i t e difference techniques with the Lockheed Missi le Systan Co. Thermal Network Analyzer Program 3 on the IBM 7094 computer. The model w a s essent ia l ly considered t o be separated i n t o i ts component pa r t s w i t h the physical parameters lumped i n t o d i s t i n c t nodes and represented as an e l e c t r i c a l network w i t h beat f lux, temperature, conductivity, and heat capacity being represented by current , voltage, resistance, and capacitance. Figure 3 shows t h e PCM model i n its canponent pa r t s indicating t h e number of nodes used i n the calculat ions. Since t h e model and t h e heat inputs a r e symmetrical, only one quarter of t h e model was used f o r t he nodular breakdown.

9 The temperature a t t h e nodes is obtaine the end of each f i n i t e time s tep Ae , from

9 See Appendix f o r l i s t of symbols

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When C i i s zero t h e expression is

( 9 )

The thickness of phase-change mater ia l and separator w a s divided i n t o twenty nodes i n order t o obtain an accurate temperature d i s t r ibu t ion through the model. faces w a s represented by s ingle nodes s ince the temperature va r i a t ion through the aluminum would be r e l a t i v e l y s m a l l . The phase-change material , aluminum, and separator were considered t o be i n int imate contact. t he phase-change material i n l i qu id and sol id- state was assumed to be pure conduction.

Each run consisted of a heating period axld a cooling period (no external load) t o siniulate o r b i t a l conditions of s a t e l l i t e s o l a r and space exposures. f o r the case where the heating period i s equal t o the cooling period. used f o r t h e analys is w a s Technical Eicosane (C2oH42). proper t ies of Eicosane a re l i s t e d i n Table I.

The thickness of t h e aluminum

The heat t r ans fe r mode through

I W machine runs were made f o r three cases.

The heat inputs t o these runs were

The phase-change material

The physical, chemical, and thermal

The analytical results, f o r one of the three cases studied, are shown i n Figure 4, giving the Eicosane temperature d is t r ibut ion , the outer and inner aluminum p l a t e temperature h is tor ies , and the in t e r f ace propagation h i s to r i e s during heating and cooling. (during heating) there is only one solid- liquid in t e r f ace and i t s propagation i s indicated by the upward-pointing arrow. appearance of t h e second solid- liquid in ter face , whose propagation i s indicated by t h e downward- pointing arrow, i s noted. The existance of two solid- liquid in t e r f aces i s due t o the Eicosane exposure to two heat f luxes from opposite s ides (external heat .f lux and equipnent heat f lux). The time delay i n the second solid- liquid in t e r f ace is due t o t h e r e l a t i v e l y small equipment heat flux i n comparison t o the l a rge external heat flux.

It should be observed that i n i t i a l l y

A t t - 0.53 tou r s t he

C. OVER-ALL PERFORMANCE OF PCM THERMAL CONTROL SYSTEMS

I n order t o evaluate t h e merita of t h e FOd thermal cont ro l pr inc ip le , an analys is was per- formed on a s a t e l l i t e i n a synchronous orbi t . An energy balance on the s u n l i t s ide of a plana PCM panel w i l l give ( see Figure 5)

On the dark side a similar balance can be wr i t ten as

I n t h e above energy balance, t he Earth thermal emission and the ref lec ted s o l a r r ad ia t on terms

synchronous a i t i t u d e s these terms are negligible. From the last two equations one can ge t expressiors for P and 6 as follows:

nere not included s ince it can be shown t t h a t a t

For ts = t d , equaticns (12) and (13) reduce t o - - / . - QL’St w=

&Qf (14)

The l a s t two equations are interdependent and their interdependence is shown i n Figure 6 f o r various phase-change mater ia ls i n a f l a t panel under synchronous o r b i t conditions. The in terpre ta t ion of Figure 6 is as follows: for a given s o l a r absorpt iv i ty and phase-change material, t he PCM weight and the required emissivity t o d i s s ipa t e a given amount of i n t e rna l power can be determined f o r the conditions s t a t ed i n t h e notes on the f igure. systems f o r other than synchronous o rb i t s is dis- cussed below.

The performance of PCM thermal protectior.

D. COXPARISON WITH OTHER SYSTDIS

It is of i n t e r e s t t o compare the FQ4 thermal protection system with other systems. ing s t r i c t l y passive thermal cont ro l weight penalty associated with such systems must be analyzed. Figure 7 gives t h e required weight of phase-change mater ia l per square f o o t of surface t o be protected, a s a function of the surface solar-exposure time per orbit , and the type of phase-change material. Solar-exposure time of a s a t e l l i t e surface i s equal t o half the s a t e l l i t e o r b i t a l period due t o self-shadowing ef fec ts on a non-spinning s a t e l l i t e i n a noon-midnight o r b i t (an o r b i t t h a t contains the sun-earth vector).

I n evaluat- systems, the

I n Figure 7 the half-period times fo r a number of s a t e l l i t e s of i n t e r e s t a r e superimposed on the graph. The graph is based on a solar absorpt iv i ty of 0.1 and an average value of so lar f l u x t h a t a f l a t satel l i te surface would be ex- posed t o during half an o r b i t ( the average value of a sinusoid is 0.637 x peak value). t i v i t y of 0.1 is obtainable w i t h Corning fused s i l i c a s . containment of the phase-change material . The weight obtained from Figure 7 is, therefore, the t o t a l system weight.

An absorp-

A 100% weight allowance was made f o r the

For t h e Numbus orbi t , f o r example, the t o t a l PCM system weight would be 0.1876 pounds per squan foot using Trans i t He e t 869 as the phase-change

* f groprietary fornula t ion m a d e by Cryo-Therm n .

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material. the t o t a l P C M system weight would be 0.172 lb / f t2 of covered surface. lou ered temperature son t ro l system designed f o r OGO weighs 0.6 l b / f t and t h e exp cted weight f o r t h e Nimbus system i s 1.5 l b / f t . For a forced c i r cu la t ion loop the radia tor alone, integrated with the vehicle s t ruc ture , would weigh approximately 0.5 lb/f t2. t i o n t o the advantages of being a completely passive system, the FCN thermal control s y s t m has a d e f i n i t e weight advantage over the louvered and the forced c i r cu la t ion systems. pointed out that the louvered and forced c i rcula- t i o n systems have a considerable weight advan- tage aver other ac t ive and semi ac t ive thermal cont ro l systems.

Similarly, f o r the OGO (POGO) o r b i t

A semi-active

5

Thus, i n addi-

It i s t o be

In terms of volume, t he PCM thermal cont ro l scheme shows d e f i n i t e promise. Fleet 86 as the phase-change mater ia l f o r t he Nimbus and POGO o r b i t s the PCM thermal protec- t ion system would occupy l e s s than 0.002 cubic f e e t per square f o o t of protected area. considerably l e s s than the volume required f o r t h e shu t t e r s plus the actuating sub-system i n a louvered system o r t h e volume requirements f o r a forced c i r cu la t ion system.

Using Trans i t

This is

111. EXPERIMENTAL PROGRAM

A . TEST OBJECTIVES, APPROACH, AND SCOPE

The primary objec t ive of t h e experimental program w a s t o determine the f e a s i b i l i t y of s a t e l l i t e thermal contro l by the use of phase- change materials . w a s t o perform tests on various phase-change materials and on d i f f e r e n t PCM core configuratiorq and t o compare the ana ly t i ca l and experimental resul t s .

A secondary program objective

The bas ic t e s t approach f o r t h e PCM thermal control program was t o cons t ruc t two similar t e s t panels that would sirrmlate a typica l equipment bay i n a satellite. One panel, the PCM t e s t panel, had a layer of phase-change mater ia l interposed between t h e equipment and the space radia tor ( s a t e l l i t e skin). ed t e s t panel, was iden t i ca l to the PM panel except t h a t it did not have the layer of phase- change material . Both panels were subjected t o iden t i ca l external and i n t e r n a l heat conditions i n t h e Republic space chamber and t h e r e su l t i ng temperatures recorded and compared. This test approach provides a f a i r l y direct comparison. The e f f e c t of t h e FCM pr inc ip l e could thus be iso la ted and objec t ive ly evaluated. of the instrumented panels equipped Kith four pyrohel imeters

The other panel, the unprotectr

A photograph

is shown i n Figure 8.

Two d i f f e ren t PCM panel core configurations were tested. One configuration incorporated f i n s t o assist i n the heat t r ans fe r through t h e phase- change material . The other configuration was f in - less . See sub-section I11 B f o r a description of the test m o d e l s . Tu0 phase-change materials were t e s t ed f o r comparative purposes vie., Technical Eicosane and Polyethylene Glycol (Carbowax 6m). The physical chemical, and thermal proper t ies of these two phase-change mater ia ls are given i n Table I.

Three PCM theml-vacuum tests, each w i t h a d i f f e r e n t test objective ( i n each test a d i f f e ren t PCM-core combination was subjected t o d i f f e ren t external and in t e rna l heat loads), were Considered t o be adequate t o demonstrate f e a s i b i l i t y of t h e PCM thermal control scheme and t o point out Bny areas t h a t requi re fu r the r study and experimental ver i f ica t ion . I n the f i r s t thermal-vacuum t e s t , a ( r e l a t ive ly ) high-melting-temperature material was tes ted i n t h e finned pcE4 core configuration. I n t h e second t e s t , a ( r e l a t ive ly ) low-melting- temperature mater ia l w a s tes ted i n the same core. I n the t h i r d t e s t , a r e l a t ive ly high-melting- temperature material was tes ted i n the f in l e s s PCM core configuration.

The phase-change mater ia ls considered fo r an operational PCM thermal cont ro l system should have t h e following properties:

1. High heat of fusion 2. Reversible solid- to- liquid t r ans i t i on 3 . t.Ielting temperature i n ~e range of 50’ t o

100% (can be a l te red t o s u i t speci f ic require- ments)

&. Low coeff ic ient of volumetric expansion i n both t h e l iquid and so l id phases

5. High dens i ty 6. Low change i n dens i ty during change of phase 7. Nontoxic and noncorrosive 8. High thermal conductivity i n both phases 9. Low vapor pressure i n t h e v i c i n i t y of the

melting point 10. High spec i f i c heat i n both phases 11. Reasonable commercial a v a i l a b i l i t y

B. TEST MODELS, INSTRIMENTATION, AND SPACE SIMULATION

1. Test Models - Various configurations were examined f r m t h e point of view of contain- ment of t h e phase-change material, low contact res is tance between the in t e rna l heat generating equipment and t h e space radiator, and good thermal conductivity within the core. following design approaches were considered:

For t h e lat ter the

a. Fins b.

c. Honeycanb core s t ruc tu re d. Corrugated core s t ruc tu re e. S tee l wool or f ibrous alumirnun core

Metall ic suspensions in t h e phase- change mater ia l

One finned and one f i n l e s s t e s t core configur- at ions were designed and bu i l t . finned PCM core consisted of an all-aluminum, f l a t container 6 3/blI by 6 3/411 and 3/btt thick. outer p l a t e of t h e core served as a space radia tor and on the inner p l a t e were mounted f i v e 50 watt, 300 o h (1%), r e s i s t o r s connected i n p a n l l e d t o simulate hea t generating equipment. A th ick cavi ty between t h e two p la t e s contained t h e phase- change material , A photograph of t h e instrumented PCM t e s t core configuration showing the r e s i s to r s , points of thermocouple attachments and the panel c m e r is presented i n Figure 9.

The aluminum f ins (p ins) , 1/8” i n diameter,

Basically, the

The

were pressed i n t o the two p l a t e s and formed a one- inch center-to-center f i n pattern. arranganent w a s such that when t h e two pla tes were

The f i n

assembled, a 3 inch center-to-center f i n pa t tern

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TABLE I. PROPERTIES OF PHASE-CHANGE MATERIALS DESIGNATED FOR TESTING

MATERIALS

Chemical Fornula

Molecular Weight

Density, lb / f t3

Ivielting Point, OF

Viscosity, CKS

Specific Heat, BTU/lb%

Heat of Fusion, BTU/lb

Solubi l i ty Fn water, %

Vapor Pressure, mn Hg

Flash Point, %

Surface Tension, dynes/cm

Boiling Point

Volumetric coeff ic ient of expansion

Thermal Conductivity BTU/hr-f t-%

POLYETHYLENE GLYCOL ( CARBOMX 600)

570-630

70 @ 2OoC

68-77

10.5

0.54

63

100 mr 20%

5 . 2 ~ 1 0 ~ @ l 0 0 O C

475

44.5 @ 25OC

0.0075/°C @ 5SoC 0.0924 @ 1 2 2 % 0.C922 @ 194OF

TECHNICAL EICOSANE

C20Hh2

2@2.5&

46.0 @ 176%

94.64

Solid: 0.528 @ 100% Liquid: 0.481 @ 100%

71.21

loo @ 2O0C

205OC

O.ooos/% i n l iquid phase

0.133

resulted. The e n t i r e core w a s of a welded con- struction. were used t o reduce the thermal contact resistance between the res i s to r s and the a l u m i m plate.

The finless test core configuration was

Indium f o i l gaskets (0.01s" th ick)

similar t o t h e finned configuration, except f o r t h e absence of the f ins , larger cavity t o accommodate the phase-change material, and differences i n construction. th ick instead of a quarter inch i n the finned con- figuration. The differences i n construction were t h a t the f i n l e s s core was bonded instead of welded, and tha 9 insulator (spacer) between the two aluminum pla tes was made of phenolic res in f ibe r- glass ixlstead of aluminum.

The PCM cavity was a half an inch

As s ta ted i n sub section I11 A, an unprotec- ted panel was tes ted i n the space chamber alongside the PCM panel so t h a t a direct comparison between the resu l t s could be made. The unprotected panel core was an exact repl ica of t h e PCM panel except that it had no FCM cavity and hence was made of one sol id aluminum plate.

I n order t o duplicate the thermal mass of the PCM panel core the unprotected core thickness was increased so t h a t both panels had approximately the same thermal mass. This was done fo r the f i r s t thermal vacuum test (see sub section I11 A). For

t h e f i r s t two t e s t s the mass of the unprotected panel w a s approximately one half the mass of the PCM panel.

Both the F C M core and the unprotected core, were covered with an insulated cover t o simulate a s a t e l l i t e equipment compartment interacting w i t h the space environment through the space radia tor ( s a t e l l i t e skin). The cover, seen i n Figure 9, was made of aluminum, and covered with 40 wrinkled sheets of aluminized mylar super insulation during the f i rs t and third thermal vacuum tes ts , and with 2 0 sheets during t h e second thermal-vacuum tes t .

The outside surfaces of the cores were coated w i t h a Carol1 Black 1019 epoxy-based coat- ing having a 0.92 infrared emissivity. The coated surface of the core served a s the space radia tor and simulated t h e s a t e l l i t e outer skin. An assm- bled panel, coated, insulated and instrumented is shown i n Figure 10.

2. Instrumentation - The two t e s t panels were instrumented with 29 thermocouples far the first two thermal vacuum t e s t s and with 31 thenno- couples f o r the th i rd tes t . In both instances the unprotected (conventional) test panel had 13 thennocouples. w i t h 16 thermocouples for t h e f i rs t two tes t s and w i t h 18 during the l a s t test.

The POI panel was instrumented

The thermocouple

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posi t ions f o r measuring r e s i s t o r , core surface, and POI temperature d i s t r ibu t ions a r e shown i n Figure 11 f o r the PCM t e s t core.

I n the f i r s t t w o t e s t s t he re were 3 i n t e r n a l thermocouples i n the posit ions shown i n Figure 11 (Nos. 12, 13 and 1b). same f o r all 3 therniocouples ( l / 8 of an inch). For t h e 3rd test where the co re thickness was 3 an inch, there were 5 i n t e rna l thermocouples, with varying immersion depths t o record t h e tempep a tu re d i s t r ibu t ion scross t h e P a ? . Nos. 1 2 , 13, and 1 b were located 1/8", $It, and 3 / P respect ive ly from the f r o n t p la te . couples 15 and 16 were immersed 1/8" and 3/811 respect ive ly but a t a d i f ferent l a t e r a l location.

Figure 1 2 shows schematicallq the instrumen-

The immersion depth w a s t he

Thermocouples

Thermo-

t a t i o n layout f o r t he unprotected test panel. instrumentation of t h i s panel was t h e same f o r a l l 3 PM thermal vacuum te s t s . The thermocouple instrumentation of t h e panel covers can be seen on t h e photograph i n Figure 9. instrumentation and equipment used during the PCM thermal-vacuum t e s t s a r e shown i n t h e block diagram i n Figure 13.

3 .

The

The addi t ional

Space Simulation - The three PCM thermal- vacuum t e s t s were conducted i n the Republic 131x18' space chamber that ha capab i l i t y of less than h1O2 TORR when empty and outgassed. assembly of cryogenic walls arranged t o enclose the two test panels. cooled with l i q u i d nitrogen and were maintained a t -2509. The POI experiments were suspended in- s ide t h e cryogenic enclosure as shown i n Figure a. Radiant heat was supplied t o the t e s t panr ls from heat lamps which were mounted on the in s ide sur- faces of t he cryogenic panels. These were G.E. #250OT3 c l e a r quartz lamps, which w e r e vacuum metallized with aluminum on one s ide t o r e f l e c t t he heat away from the cryopanels and toward the test panels.

an u l t imate vacuum

Erected in s ide t h e chamber was an

The cryogenic w a l l s were

To simulate the o r b i t a l heat flux f luc tua t io r s t h a t a s a t e l l i t e experiences during sun and space exposures, t he t e s t panels were exposed t o alter- nate heating and cooling periods by phased oper- a t ion of lamps and cryopanels. equipment heat load was kept constant during the heating and cooling periods. cooling period cons t i tu ted a complete cycle (simulated o rb i t ) . The cycles were repeated several times (ranging from 1 t o 4 times) t o demonstrate repeatabi l i ty . presented i n Table 11. loads l i s t e d i n t h e t a b l e are t h e mean values of the four pyroheliometers i n the pos i t ions shown in Figure 8.

The in t e rna l

Each heating and

A t e s t summary is The external r a d i a n t hea t

I V . TZST RESULTS, EVALUATION, AND COHPARISON

A. TEST RESULTS

Figure 15 shows t h e temperature h i s t o r i e s of the simulated s a t e l l i t e equipment ( r e s i s t o r s ) f o r Run II* during the first PCN thermal vacuum t e s t .

* Due t o l imi t a t ions on t h e s i z e of t h e paper, not a l l t e s t r e s u l t s w i l l be preserited. All test d a t a a r e included i n a Republic Aviation Report on t h e e n t i r e PCM thermal cont ro l program.

The equipment surface temperature of t he PCM t e s t panel, thermocouples 1, 2, 3, 4, and 5 are plotted along with t h e tu re s of t h e unprotected test panel, thermocouples 19, 20, 21, 22, and 23 (see Figurea 11 and 12) . The temperatures a t these locations were very c lose s o that they are represented by one point on the graph.

corresponding equipment tempera-

It should be noted that the equipment tmper- a tu re i n the unprotected t e s t panel f luctuated between &5OF and 153'F, while the equipment i n the PCbl t e s t panel stayed between 85% and 115%. represents a reduction from a 108% temperature excursion t o only 2 7 9 . Figure 16 shows the temp- era ture var ia t ion of the Eicosane during t h e r ad ian t heat cycling. A change i n slope, i n i t i - a t ing t h e typica l melting plateau, is observed i n the v i c i n i t y of the meltine point during t he heating and cooling periods.

Figure 1 7 shows the surface temperature

This

h i s to r i e s of t h e simulated s a t e l l i t e equipment f o r Run I of the second PCM thermal-vacuun t e s t . The equipment surface temperatures of the PCM t e s t panel a r e superimposed on the temperature p lo t s of t h e unprotected test panel. The damping e f f ec t of t he PCM panel i s apparent from the f igure. The equipment i n the unprotected t e s t panel experiencd a temperature excursion of 31°F while the equip- ment i n the PCM panel experienced a temperature excursion of only 7 9 . Similar r e s u l t s may be noted i n Figure 18 where the in s ide and outside core temperatures of both panels are compared. There w a s an undbskerniblb temperature gradient through the core. lene Glycol temperature h is tory f o r t h e same run. The absence of pronounced melting plateaus i n the Polyethylene Glycol temperature p ro f i l e i s mainly due t o t h e rapid heat influx caused by the shor t c l r cu i t i ng e f f e c t of the aluminum core spacers and f i n s .

Figure 19 shows the Polyethy-

The r e s u l t s of Run I auring the th i rd F'CM thermal-vacuum t e s t a r e shown i n Figure 20 where t h e equipment temperatures i n the PCM t e s t panel a r e compared with the equipment temperatures i n the unprotected t e s t panel. The temperature f luc- tuat ion damping e f f ec t of the pcpt panel is c l ea r ly shown in the f igure. The equipment i n the conven- t i o n a l panel f luctuated betueen 30' and %5?! while the PCM panel equipment stayed between 80 F and 110%.

I n F i w r e 2 1 a s shown the space r ad ia to r outside surface (core) temperature h is tory f o r both t e s t panels. It i s observed that the PCM test panel responded f a s t e r i n i t i a l l y t o heating and cooling during both cycles. t h e slow response of t h e unprotected t e s t panel is the increased p l a t e thickness from 4 inch during the first two t e s t s t o $ inch f o r t h e t h i r d t e s t (see subsection I11 B 1). It should be noted in Figure 20, however, tkzt as far a s the equipment temperature is concerned, t he increased mass of t h e unprotected t e s t panel did not reduce the r e l a t i v e merits of the PCM Fanel i n damping out the temperature excursions.

The reason f o r

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TABLE I1

SLYNARY OF PCM THERMAL-VACUUM TESTS

Test Number

Phase-change Material

FCM Core Configuration

Unprotected Core Configuration

Test Panel cover Insula t ion

pQ.I core cavi ty thickness, in.

Weight of Phase-Change Haterial, lbs.

Space Chamber Pressure, TORR

cryogenic wall TWP. ,OF

Duration of t e s t , hrs.

R u n Number

Number of Cycles

External Radiant H e a t Load, BTU/hr-ft2

In t e rna l Equipment Heat Load, wat ts / f t

Duration of run, hrs.

1

Technical Eisosane (C2#&)

Finned

Approximately half the weight of t h e PCM core

40 layers of super insula t ion

0.25

0.232

-6 3.2 x 10

-250

18.2

I 11 I11 I V

4 3 1 2

174 228 111, 130

6 6 6 6

7.19 4.16 2.38 3.12

2

Polyethylene Glycol (Carbowax 600)

Finned

Approximately half t h e weight of t h e PCM core

20 layers of super insulation

0.25

0.3h7

8 x

-250

11.8

I I1 I11 Iv

2 2 1 3

hb 111 195 111

24 2h 24 0

2.33 2.1 1.168 2.38

~

3

Technical Eicosane (c20H42)

Finless with Fiber. glas insula tor

The same weight as t h e E M core

40 layers of super insula t ion

I

2

0

0

0.50

0.LI.Q

2.2 x 10-6

-250

16.0

I1 I11

2 3

280 23b

8 8

.42 8.67 4.84

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B. TEST DATA EVALUATION

It i s convenient, i n evaluating the test r e su l t s , t o define the e q u i p e n t temperature f luc tuat ion r a t io , R f , as t h e r a t i o of the equip- m e n & temperature f luc tua t ion i n the P a t e s t panel t o the equipmat temperature f luc tuat ion i n t h e conventional (unprotected) t e s t panel. This parameter gives an indica t ion of t h e equipment temperature limits that can be maintained with a PCM thermal protection systein as compared t o an unprotected s a t e l l i t e . We thus define

I n order t o g e t an indica t ion of t he PCM system weight penalty, a weight index, W', will be defined. The weight index gives the weight of phase-change material needed per u n i t of absorbed heat and is given by -

To ge t an overa l l f i i p r e of merit f o r the PCM system performance, it i s log ica l t o combine Rf, which t e l l s w h a t the system does, and b i t ,

which tells w h a t the system requires ( i n terms of weight). We def ine the Figure of Merit as

I Figure of Merit = -

I?+ w' Better performance i s indicated by a l a r g e r rmmber. Table I11 summarizes t h e r e s u l t s of the three POf thermal-vacuum t e s t s i n terms of t h e above defined parameters.

Comparing t h e performance of the f inned core configuration with t h e f i n l e s s core configuration, it is noted i n Table I11 that t h e finned core con- f igu ra t ion has a consis tent ly higher Figure of Merit f o r all runs. This means that for a given weight of phase-change mater ia l arid a given heat flux h i s to ry the f inned core is more e f f ec t ive i n maintaining narrow equipnent temperature limits than t h e f i n l e s s core.

To g e t a performance comparison between Eicosane and Polyethylene Glycol, t he r e s u l t s of t h e first and second thermal-vacuum tests have t o be compared. From Table I11 it may be observed t h a t t h e da ta of the various runs is not consist- e n t (compare Equipment Fluctuation Ratio and Figure of Merit). However, the average Figure of Merit pe r test f o r Eicosane i s l a rge r than t h e average Figure of Merit per test f o r Polyethylene Glycol.

C. COM?AIZISON BETwElN ANALYTICAL AND EXPERI?lENT- AL RESULTS

A comparison between the analytical and t h e experimental r e s u l t s was made f o r three cases. one case, t he experimentally obtained temperature d i s t r ibu t ions within t h e PCM core, under external heat load only, was compared with t h e ana ly t i ca l predictions. I n the second case the temperature d i s t r ibu t ions within t h e PCN core were compared under simultaneous external and i n t e r n a l heat, load- ings. The th i rd comparison w a s made f o r t h e unpro- tec ted test panel.

I n

I n Figure 22, be t t e r corre la t ion could have been obtained i f not f o r the f o l l m h g two rea- sons. In t h e analysis it was assumed t h a t heat w a s t ransferred within t h e core only by conduction. I n ac tua l i t y convective currents were present which account f o r the crowding of t he experimental l ines . The other reason is that i n t h e analysis the-sink temperature was taken a s -250%' ( t h e measured tem- perature of the cryopanels during t h e tests (see Table 111). The ac tua l sink temperature was t h e r e su l t an t e f fec t ive temperature due t o t h e cryo- panel temperature and the -radiant lamp filarrent temperature (see Figure lh). temperature could not be measured d i rec t ly .

This ef fec t ive sink

I n the order t o v e r i f y the last contention, a comparison was made between t h e analy t ica l and t h e experimental r e su l t s on t h e unprotected test panel as shown i n Figur t 23. done f o r a sink temperature of -250oF and lS°F (ca lcula ted e f f ec t ive sink temperature on a weighted area - temperature bas is ) . ved t h a t on a basis of ef fec t ive sink temperature, be t t e r agreement &s obtained. temperature of 1.5 F would a l so s h i f t the analy t i- c a l curves t o the l e f t on the time scale i n F i y r e 22. Thus tending t o coincide with the experimen- tal curves.

The ana ly t i ca l results were

It i s obser-

An effec t ive s ink

It should be noted i n both comparison figures, that during cool-down, when the radiant lamps were shu t off , and the effec t ive space temperature was equal t o the cryopanel temperature, the analy t ica l and experimental s lopes a r e almost identical . Based on the above, it i s concluded t h a t the analy- t ica l approach presented, modified f o r convective currents, and coupled with a configuration f ac to r program t o ca lcula te e f f ec t ive sink temperatures, can be used ef fec t ive ly to predic t the performance of PCM thermal control systems. t o predic t t he performance of PCM thermal control systems under ac tua l satel l i te o r b i t conditions (including zero g) the convective current modifica- t i o n is not necessary.

Note that i n order

V . ECOiWEN DATIONS

I n view of the experimentally determined f e a s i b i l i t y of thermal contro l by phase-change materials , fu r the r ana ly t i ca l and experimental inves t iga t ions in the following areas are warranted:

1. Basic material research and experiments t o explore, uncover, and formulate substances especia l ly su i table f o r PO1 thermal control systems. These substances (compounds or mixtures) should possess most o r a11 of the proper t ies l i s t e d a t t he end of subsection I11 A.

2. A more sophist icated ane ly t i ca l approach, one that would include convection terms jn addi- t i o n t o other possible improvements (see l a s t subsection).

3. Improvement i n PCM core design with emphasis on b e t t e r heat t r ans fe r charac ter i s t ics .

b . A systems-application oriented test pro- gram. t e s t ing of PCM thermal cont ro l systems for speci- f i c mission applications.

This program would include the design and

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c u r l I 4

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cu

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V I . REFERENCES AND ACKNObLEKMENTS

A . REFERENCES

1.

2.

3.

4.

5.

6.

Goodman, T.R., ud J.J, Shea, "The Melt- ing of F i n i t e Slabs,tt AFOSR-TN-58-824, AD 202-908, August 1958

Leatheman, R.A., ttComponent Th-1 Control Via Heat-of-Fusion Radiator", ASME Paper No. 63-AHGT-12, 1963

IBM Users Group, SHARE Routine ZO MLFTHAN-LMSC Thermal Netuork Analyzer, 7090 FORTRAN, J.L. Fick, Lockheed Missile and Space Company

Fixlar , S .Z. ,ttThermal Control of a Synchronous Equatorial Sa t e l l i t e " , presented a t the IAS 31stAnnual Meeting, New York, N.Y., January 21-23, 1963.

Tanzi l l i , R., t tDevelopent of a Stable White Coating System", A I M Journal, Vol. I, No. 4, Apr i l 1963.

Vehrencamp, J.E., tlOrbiting Geophysical Observatory Temperature Control System Designtt, Reprint No. 218 - presented a t the M E - A I C H E Heat Transfer Con- ference, August 1963

B. ACKNOWLEMiMENTS

The author wishes t o express h i s apprecia- t ion t o D r . Maria Talkes of Cryo-Them Inc., f o r supplying advice and information on Transit- Heet materials , a n d t o Messrs. W. Gadzuk and K. McIlroy of Republic Aviation Corporation fo r t h e i r valuable contributions.

D. Linzer,

c = k *

L =

Q =

3 -

Of =

R f "

R -

s - S' - T -

t =

i ; j =

W' =

APPENDIX

LIST OF SYMBOLS

capacitance

thermal conductivity, BTU/hr-ft--

length of s a t e l l i t e , f t .

heat r a t e

powcr density, watts/ft

latent heat o f fusion, BTlJ/lb

res is tance , a l s o r a t i o

temperature f luc tua t ion r a t i o

s o l a r constant, &2 .s BTU/hr-f t

integrated so la r f lux , BTU/hr-ft*

temper a t u r e

time

weight per square f o o t

weight index, lbs/BTU

2

2

= Stefan-Boltman constant, Em/iw-ft2 OR^ = f i n i t e time s t ep

i. therm01 d i f fus iv i ty

- so la r absorpt iv i ty

c: density, l b / f t

= infrared emissivity

E

3

angle between the surface normal and the sun-earth vector

Subscripts

d = dark

f P fusion, a l s o f luc tuat ion

i = node i, also i n t e rna l

il = first in ter face

i 2 - second in t e r f ace

j = node j

L = l i qu id

0 - a t time 0

P M - phase-change material

S * so l id

S ' = s u n l i t

L W - unprotected

EQUIPMENT SAY

PHASE-CHANGE MATERIAL CORE \

F I C U R E 1 . SCHEMATIC REPRESENTARON OF PCM T H E W A L CONTROL SCHEME

-150-

Figure Analytical Results of Temperatwe Distribution and Interface Propagation Durinf Melting and Sohdlficatmn of Technical E~cosane

I

NOTES I COMPONENT TEMPERATURE. FUSION

2 S"NCHR0NOUS "RBlT CONOlTlONS

I SUNLITE M O DARK P E R I W S

TEMPERATURE

ARE EQUAL

Figure 3 PCM Mdel Node Arrangement

INTERNAL POWER DISSIPATION-

P H A S E C W N G E MAlER14L

F L I T P L l T E W D I A T O R

F I G S R E 5 F L A I PLATE RADIATOR CONFIGURATIO!~ FOR OVERALL PCM P I E W A L CONTROL ANkLLYSIS

I . , I1 1 U I I

O I 0 8 SOLAS t X P 0 I " R L T l M l PET1 ossll. , "OUR3

Figure 7 . PCM Thermal Control System Performance, Weight

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TYPlCAL RESISTOR7

RESISTOR AND INSIDE SURFACE THERMOCOUPLE

OUTSIDE SURFACE 1 SPACE RADIATOR 1 I:::>. THERMOCOUPLE

I 1

RESISTOR AND INSIDE SURFACE THERMOCOUPLES

OUTSIDE SURFACE THERMOCOUPLES

SPACE RADIATOR I ~~~~. PCM TEMPERATURE MONITORING L THERMOCOUPLES 7

TYPICAL RESISTOR - /

Figure 11. Wiring and Instrumentation of the Pcnl Test Panel. Thermal-Vacuum Tests)

(1st and 2nd

Figure 1 2 . Wiring and Instrumentation of the Unprotected Test Panel Flkwrt. 13. Instrumentatian Block Diagram - PCM Thermal-Vacuum T e s t s

-1 52-

-~ ~

0 THERMOCOUPLE 3

0

T M E - MNUTES

Figure lti. First PCM Thermal-Vacuum Test Results , Run E Eicosane Temperatures

140

; I20

Y

100 4

Y z 80

60

0 20 40 60 80 loo 120 I40 IM 180 200 220 240 260 40

T ME-MINUTES

0 UNPROTECTED T E S T P A N E L I THERMOCOUPLES 19 20 21 22 231 PCM TEST PANEL I THERMOCOUPLES I 2 3 4 51

Figure 15, FWSt PCM ThermalYacuum Test Results, Run U Equipmeot Surface Temperawes

, , I 0 0 20 ,0 40 50 6C 70 BO 90 I O 0 110 ,20 I 3 C I40 I50

6 0

TIME - MlNUTES 3-HE' IUoCCUPLES 28 , 2 3 OTHERMOCOJPLE 22 }UNPROTECTED TEST P INEL E d e m a 1 Heir FLU\ 4 DTI hr I f P

3THERMDCOUPLES 4,2,3} PCH TEST PINEL CTHERMOCOUPLES 4 5

V,,lERHoCO"PLE 19 L q r l ~ , > , e , , f /,Eat L,>*d 2 1 r r t l b / l l Z

Figure ~ 7 . Second PCM Thermal-Vacuum Test Results. Run I Equipment Surface Temperatures

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~~

17,18,25.26,27 THERMOCOUPLES 24 THERMOCOUPLES 6,7,8.9,10.11 PCM TEST PANEL

7-7-77- 100

1 w

= 1

3 E 7 0 5 80

5 a. w a

3 * (r w

c 5 60

60

I I I I I 0 m 40 60 80 100 120 140

TIME. MINUTES

Figure 18 . Second PCM Thermal-Vacuum Test Results, Run I lnside and Outside Core Temperatures

130

120

I IO

80

70

0 THERMOCOUPLES 13 0 THERMOCOUPLES 14

50 0 20 40 60 80 100 120 140

TIME. MINUTES

Figure 19. Second PChl Thermal-Vacuum Test Results. Run I Polyethylene Glycol Temperalures

1 -- 1-

0-2 ' o 20 40 60 80 io0 120 IN 160 180 200 220 240 260 280

TIME-MINUTES

Figure 21. Third PCM Thermal-Vacuum Test Results, Run I Outside Surface Core Temperatures

Figure 23. Comparison of Analytical and Experlmental Results of the Unprotected Test Panel

60

Figure 22. Comparison of Analytical and Experimental Results of PCM Test Panel External Heat Load Only

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