Behaviour of Fluids in a Weightless Environment

8/3/2019 Behaviour of Fluids in a Weightless Environment

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t ens ion , and R and R a r e t h e r a d i i of c u r v a t u r e o f t h e i n t e r f a c e . The r a d i i of curva-2t u r e a r e c on si Ae re d p o s i t i v e when t h e i r c e n t e r o f c u r v a t u r e i s w i t h i n t h e l i q u i d . The sumo f t h e r e c i p r o c a l s o f t h e r a d i i , ( l /R 1 + 1 / ~ ~ ) ,s r e f e r r e d t o a s t h e c u r v a t u re of t h ei n t e r f a c e 1 ,

Under s t a t i c c o n d i t i o n s , t h e p r e s s u r e of t h e g as i n c o n ta c t w it h t h e l i q u i d i s u n i -form ov e r t h e e n t i r e l i q u i d s u r f a c e . An e q u i l i b r i u m i n t e r f a c e w i l l b e e s t a b l i s h e d whent h e p r e s su r e i n t h e l i q u i d i s a l s o u ni fo rm a nd t h e p r e s s u r e d i f f e r e n t i a l d e fi n e d by e qu a-t i o n ( 1) i s a c o n s ta n t . I f t h e p r e s s u r e d i f f e r e n c e i s c o n s t a n t, t h en t h e c u r va t u r e m usta l s o be u ni fo rm o v er t h e e n t i r e s u r f a c e .

C o ns i de r in g a g l o b u l e of l i q u i d i n l o w -g r a vi t y , n o t i n c o n t a c t w i t h an y s u r f a c e , t h er e qu i re m en t o f un i fo r m c u r v a t u r e c a n o n l y be s a t i s f i e d i f b o t h r a d i i o f c u r v a t u r e a r ee q u a l . The g l o b u l e t h en h a s t h e f or m o f a s p h er e , w i t h t h e r a d i i o f c u r v a t u r e e q u a l t oi t s r a d i u s . E qu ,a ti on ( 1 ) s i m p l i f i e s t o :

where r i s t h e r a d i u s o f t h e s p h e re . The p r e s s u r e w i t h i n t h e d r op o f l i q u i d i s g r e a t e rt h a n t h a t o f t h e s u r r o u n d i n g g a s by a n amount t h a t i s d i r e c t l y p ro p o rt i o na l t o t h e s u r f a c et e n s i o n and i n v e r s e l y p r o p o r t i o n a l t o t h e r a d i u s of t h e d r op . The same h o l d s t r u e f o r ab u bb l e s ur ro un de d b y l i q u i d e x c e pt t h a t t h e p r e s s u r e w i t h i n t h e g a s i s g r e a t e r t h a n t h a ti n t h e l i q u i d.

When th e l i q ui d i s i n c o n t a c t w i t h a s u r f a c e , t h e d et e r m i n a t i o n o f t h e i n t e r f a c ebecomes more complex. The cu rv at ur e of t he su rf ac e w i l l s t i l l b e a c o n s t a n t , b u t t h e twor a d i i of c u r v a t ur e w i l l v a r y o v e r t h e s u r f a c e . The l i q u i d - t o - s o l i d c o n t a c t a n g l e 0 i s ab ou nd ar y c o n d i t i o n t h a t m us t b e s a t i s f i e d . T h i s i s t h e a n g l e fo rm ed b e tw e en t h e l i q u i da nd t h e s u r f a c e , m ea su re d w i t h i n t h e l i q u i d .

V a r i a t i o ns i n 0 d ue t o t e m p e r a t u r e and i n 0 and 0 d ue t o c o n t a m i n a t i o n , d i s s o l v e dp r e s s u r a n t , and l i q u i d p u r i t y w i l l a f f e c t i n t e r f a c e s ha pe . The v a l u e of c o n t a c t a n gl ep r i m a r i l y d e p en ds on t h e l i q u i d s u r f a c e t e n s i o n a nd t h e s o l i d b ou nd ar y s u r f a c e e n e r g y .T he l a t t e r c a n b e e x pr e ss e d a s a s o - ca l le d " c r i t i c a l s u r f a c e t e ns i on . " I f t h e l i q u i d s u r -f a c e t e n si o n i s l e s s t h a n t h e c r i t i c a l v a l u e , t h e co n t ac t a n gl e i s z e ro . I f t h e s u r f a c et e n si o n i s a g r e a t e r t h a n t he c r i t i c a l v a l ue , t h e c o si n e of t h e c o n ta c t a n gl e i s a l i n e a rp r o p o r t i o n t o t h e d i f f e r e n c e be tw ee n t h e l i q u i d and t h e c r i t i c a l s u r f a c e t e n s i o n s . C l ea nm e t a l s u r f a c e s h a ve h i g h c r i t i c a l s u r f a c e t e n s i o n s and m ost l i q u i d s w i l l completely wetthem. Main ta in ing a con tamina nt - f ree sur f ace i s d i f f i c u l t t o a c hi e ve i n p r a c t i c e , h ow ev er .M ost m o no la ye r co n ta m in a nt f i l m s ( e x c e p t f l u o r o c a r b o n s ) ha v e c r i t i c a l s u r f a c e t e n s i o n sbetween 20 and 45 dynes/cm2y 3. T h i s i s a l s o t r u e o f p l a s t i c s . W at er , w i t h i t s h i g h s u r -fa ce t ens ion of 72 dynes /cm, w i l l h a ve h i g h c o n t a c t a n g l e s on t h e s e s u r f a c e s . A c o n t a c tan gl e of 33.5O i s shown i n F ig ure 1.

V er y few l i q u i d s ( o t h e r t h a n l i q u i d m e ta l s ) h a ve a h i g h e r s u r f a c e t e n s i o n t h a n w a t e r .S i n c e w a te r h a s s u ch a h i g h s u r f a c e e n e r g y , i t i s r e a d i l y c o n t a m i n a t e d a n d a c o n s i d e r a b l el o w e ri n g of t h e s u r f a c e t e n s i o n t a k e s p l a ce 4 . C on ta m in an ts t h a t l o we r t h e s u r f a c e t e n s i o no f a l i q u i d a r e r e f e r r e d t o a s s u r f a c e a c t i v e ag e nt s. A smal l amount o f a su r fa ce ac t i vea g e n t ' w i l l i m po se i t s l ow s u r f a c e t e n s i o n o n a l i q u i d o f much h i g h e r s u r f a c e t e n s i o n . A st h e c o n c e n tr a t i o n of t h e i m p u ri t y i s i n c r e a se d t h e s u r f a c e t e n s i o n of t h e s o l u t i o n d e c r e a s e su n t i l i t becomeg sa t ura ted . Fur t her a dd i t io n of th e impur i ty does no t cause any change i nsur face tens ion ' . For example , add ing soap of th e type used on Skylab to wate r , i n approx-i m a t e l y t h e c o n c e n t r a t i o n s u se d i n t h e d e m o ns t r at i o ns , r ed uc ed t h e s u r f a c e t e n s i o n t o20 dynes/cm6.


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While th e Young-Laplace eq uat i on can de f i ne more than one in te r f ac e fo r any g iven s e to f c o n d i t i o n s , t h e e q u a t io n f o r s u r f a c e e n er g y d e f i n e s t h e p r e f e r r e d c o n f i g u r a t i o n . As t a b l e i n t e r f a c e s ha pe i s achieved when th e s ur fa ce energy i s a minimum. I n s i m p l i f i e df o r m , s u r f a c e e n er gy i s d e f i n e d by

S . E . = QLV (ALV - ASL COSS ) ( 3 )where A i s a rea and LV deno te s l i qu id -vap o r and SL deno te s so l i d l i qu id . A c a p i l l a r y a r e acan be de f ined a s

Ac = Pt" - COS8f o r a ny g iv e n i n t e r f a c e . Then su r f ac e ene rgy can be fu r th e r s i mp l i f i ed t o

S.E. = =LV *c (5)A ga in co n s i d e r a f r e e - f l o a t i n g g l o b u le o f l i q u i d . The s u r f a c e e n er g y w i l l be a mini -

mum when th e ca p i ll a r y ar ea , which i s t h e a r e a of t h e l i qu i d- v ap o r i n t e r f a c e i n t h i s c a s e ,i s a minimum. This occu rs when th e g lobule assumes a s ph er ic a l shape , aga i n conf irmingt h e s t a t i c s ha pe f o r a l i q u i d d ro p.

The c a p i l l a r y a r e a i s r educed when a l i qu id d rop con t ac t s a so l i d su r f a ce . The re fo re ,c o n t a c t w i t h a s u r f a c e i s a p r e f e r r e d e q u i l i b r i u m c o n f i g u r a t i o n f o r a l i q u i d d ro p. T h i sd e c r e a s e i n s u r f a c e e n er g y ac c o u nt s f o r t h e a d h e si v e ne s s o f a l i q u i d o n a s u r f a c e . Energymust be added t o remove a drop from a surf ace .

The e q u i l i b r i u m i n t e r f a c e c o n f i g u r a t i o n i s es ta b l i s he d by t he mechanism of s ur f ace -t e n s i o n - d r iv e n f lo w , t er med c a p i l l a r y pumping. L i q u id w i l l p r e f e r e n t i a l l y o r i e n t w i t h i na con t a ine r by ca p i l l a r y pumping i f t h e sys t em i s i n a low-g environment7 . The geometrycan be mod i f i ed by compar tmenta ti on i n one a r ea o f t h e t ank such t h a t a l ower cha rac t e r i s -t i c d imension r e su l t s . Th i s reduced l eng th can be p rov ided by a vane s t ru c t u r e ( su r f a cet e n s i o n d e v i c e ) . The d e vi c e ca n r e du c e t h e p r e s s u r e o f l i q u i d a d j a c e n t t o and w i t h i n t h edev ice t o a va lu e l ower t han t h e p r e s s u re o f l i q u id l oca t ed away from the dev i ce . Thelow-pre s su re r eg ion w i l l be c r ea t e d when the dev i ce causes t he cu rva tu re of t he i n t e r f a cea b ou t t h e d e v i c e t o b e l a r g e ( s m a l l r a d i u s of c u r v a t u r e ) i n co mp ar is on t o t h e c u r v a t u r eo f t h e l i q u i d e l se w he r e i n t h e t an k. The p r e s s u r e d i f f e r e n c e w i l l h av e an e f f e c t o n l y i fth e two l i qu id volumes ar e i n communicat ion . Under near -zero-g cond i t ion s , spreading oft h e l i q u i d a s i t w e ts t h e t a n k w a l l s w i l l u s u a l l y b r i n g t h e l i q u i d i n t o c om mu nic at io n w i tht h e s u r f a c e t e n s i o n d e vi c e. I f t h i s i s not po ss i b le , some so r t of communicat ion channelmust be provided.

With a communicat ion pa th provided, l i qu id w i l l b e t r a n s f e r r e d , a s s hown i n F i g u r e 2 ,u n t i l t h e c u r v a tu r e o f t h e i n t e r f a c e t h r ou gh ou t t h e t a n k i s t h e s ame, i . e . , p r e s su re i sun ifo rm. The su r f ac e t ens ion dev i ce i s d e si g ne d s o t h e c u r v a t u r e o f t h e i n t e r f a c e r em ai nsh ig h u n t i l t h e d e vi ce h a s f i l l e d wi t h l i q u i d . I n c o mp ar is on , l i q u i d i n c o n t a c t w i t h o n l yt h e t a nk w a l l h a s a r e l a t i v e l y low c u rv a t u re .

F or a s p h e r i c a l g a s - l i q u i d i n t e r f a c e w i t h l i q u i d i n c o n t a c t w i t h a s u r f a c e , e q u a t i on(2) becomes


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T h i s p r e s su r e d i f f e r e n t i a l c a n be r e l a t e d t o a di me ns io n ( o t h e r t h a n t h e r a d i u s o f c u r v a -t u r e ) s uc h a s t h e p o re r a d i u s R and a s ec on d p a r am e t e r, t h e l i q u i d - t o - s o l i d c o n t a c t a n g l e63. T h i s i s d on e by i n t r o d u c i n g t h e r e l a t i o n s h i p b e tw ee n R , 0, and r as shown i n F igure 3 .Then

A f i n a l c r i t e r i o n f o r d e t er m i n in g i n t e r f a c e s t a b i l i t y i s t h e Bond number ( B o) , ad i me n si o nl e ss r a t i o o f a c c e l e r a t i o n f o r c e s t o c a p i l l a r y f o r c e s8

where L i s t h e c h a r a c t e r i s t i c s y s te m d im e ns io n. The l i q u i d l g a s i n t e r f a c e i s s t a b l e i n ac y l i n d r i c a l t a n k o r c i r c u l a r p o re when B O G 0 .8 4. The c r i t i c a l Bo f o r s q u a r e w eav e s c r e e ni s 0 . 4 5 ~ . S u r f a c e t e n s i o n f o r c e s become s i g n i f i c a n t , p r od u ci n g h i g h l y c ur ve d i n t e r f a c e s ,f o r Bo i n t h e r a ng e o f f i v e o r b elo w. T h e i n t e r f a c e i s e s s e n t ia l l y f l a t f o r Bo 2 509.Other s ca l in g parameters , such as t he Weber, Froude, and Reynolds numbers, a r e app l iedwhen l i q u i d f l o w i s i n vo l ve d 8.

LOW-G FLUID MECKANICSThe a bo v e d e s c r i b e d c h a r a c t e r i s t i c s o f . f l u i d b e h a v i o r i n a l o w- g e n vi ro n m en t ha ve b ee n

demons t ra ted i n space and i n s imula ted low-g envi ronments on ear th . These exper imentsi l l u s t r a t e t h e b a s i c phenom ena o f l ow -g f l u i d me c ha n ic s . A number of f l u i d mechanicssc i enc e demons t r a t ions were per formed on Skylab . Whi le the se demons t ra t ions d id no t fo l lowr i g o r o u s e x p e r i me n t a l p r o t o c o l s , t h e y d i d p ro v i d e i n t e r e s t i n g de m o n st r a ti o n s of b a s i c p h e-nomena, some of which had n ot been p re vi ou sl y o bserved6 .

Skylab Fluid Mechanics Demonstrat ionsThe fo l low ing a r e examples o f f lu id mechanics demons t ra t io ns performed aboard Skylab

t o i l l u s t r a t e low-g f l u i d b e ha vi or .S t a t i c I n t e r f a c e S h ap e. - S u r f a c e t e n s i o n and c o n t a c t a n g l e w ork t o g e t h e r , a s d i s c u s s e d

p r e v io u s ly , t o y i e l d t h e s t a t i c s ha pe of a g a s l l i q u i d i n t e r f a c e i n low-g. I t was demon-s t r a t e d t h a t a f r e e f l o a t i n g d r o p of w a t e r ass um es a s p h e r i c a l s h a pe , a s s hown i n F ig u r e 4 .Due t o d i s t u r b a n c e s i nd uc ed i n f o rm in g t h e d r op a nd t h e r e l a t i v e a c c e l e r a t i o n o f t h e S ky -l a b and t h e dr o p, a c om p le te ly s t a t i c i n t e r f a c e was d i f f i c u l t t o form . B ef or e t h e l i g h t l ydamped o s c i l l a t i o n s c e a s e d , t h e d r op im p ac te d a s u r f a c e .

The a s t r o n a u t s f ou nd t h a t t h e s e d i f f i c u l t i e s c o u l d be o ve rc om e b y p l a c i n g t h e dr o pon a t h r e a d . F o r t h e d r o p s i z e s u se d ( 30 t o 1 00 c c ) , t h e t h r e a d r e t a i n e d a nd c e n t e r e d t h ed r o p . An e v a l u a t i o n o f t h e d a t a showed t h a t t h e t h r e a d ha d a s i g n i f i c a n t e f f e c t o n t h ed am ping o f t h e d ro p o s c i l l a t i o n s a nd c au s ed a d i s t o r t i o n o f t h e d r op s h ap e ( - 5% e l o n g a -t i o n a l o ng t h e t h r ea d a x i s ) 6.

When t h e d ro p was i n c o n t a c t w i t h a l a r g e r s u r f a c e l i k e a s t r a w , t h e s u r f a c e i n f l u e n c ewas s t r o n g e r a nd t h e e q u i l i b r i u m i n t e r f a c e s h a pe p o s it i o n e d t h e d r op t a n g e nt t o s u r f a c e .On a l a r g e f l a t s u r f a c e , t h e c o n t a c t a n g le became a s i g n i f i c a n t f a c t o r i n e s t a b l i s h i n g t h ei n t e r f a c e s h ap e, a s i l l u s t r a t e d i n Fi gu re 5. W at er was t h e l i q u i d f o r a l l t h e s e d e m on st ra -t i o n s , g i v i n g c o n t a c t a n g l e s r a n g i n g f ro m 30 t o 90 d e g r e e s , d e pe n di n g on t h e s u r f a c e m at e-r i a l . A l i qu id wi t h a lower su r fa ce te ns i on would wet these sur fac es and g ive somewhatd i f f e r e n t i n t e r f a c e s ha p es 6 .


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.- D ro ps we re o s c i l l a t e d i n t h e i r b a s i c f i r s t an d se co nd modes.S u r f a c e t e n s i o n i s t h e r e s t o r i n g f o r c e t h a t s u s t a i n s t h e o s c i l l a t i o n . O pp os it e s i d e s o ft h e d ro p we r e p u l l e d by r o d s t o i n d uc e o s c i l l a t i o n , A t h r ea d was a g a i n u sed t o i n i t i a l l ys t a b i l i z e t h e d ro p. Measured o s c i l l a t i o n f r e q u e n c i e s w e re f o un d t o c o r r e l a t e w e l l w i t ht h e o r y . Damping of th e os c i l l a t io n was found t o b e a t l e a s t a n o r d er o f ma gn it ud e g r e a t e rt h a n p r e d i c t e d by t h e o r y d u e t o t h e p r e s e n c e of h i g h e r modes o f o s c i l l a t i n n (a r e s u l t oft h e m et ho d of i n d u c i n g o s c i l l a t i o n ) a n d t h e da mp in g e f f e c t of t h e t h r e a d 6 .

C o a l es c e n ce . - C o a l es c e n ce de p en d s u pon t h e i n t e r a c t i o n of two l i q u i d i n t e r f a c e s w henthey meet . Depending upon t he a ng le of i n c id e n ce and r e l a t i v e v e l o c i t i e s , t h e d r op s ca nbounce of f one anot her o r combine . Momentum e f f e c t s can cause them t o sep ara te ag a in .

Drops were impac ted by ho ld i ng one drop s ta t i on ar y on a th re ad and maneuver ing t hes ec on d d r o p o n t o a c o l l i s i o n c o u r s e . Drops ranging from 1.8- t o 5.2-cm dia me ter could beo b se r ve d a s t h ey c o a l e sc e d . The d r o p s w e re c o lo r e d d i f f e r e n t l y s o t h e r a t e of m i x i ng( found t o be fa i r l y s low) could be observed . The coa l esce nce observed i n Skylab was con-s i s e n t w i t h a v a i l a b l e t h eo ry 6.

R o t a t i n g D ro p.- When a l i q u i d d r op i s r o t a t e d , c e n t r i f u g a l and s u r f a c e t e n s i o n f o r c e sb a l a n c e t o pr o du c e t h e r e s u l t i n g i n t e r f a c e s h a pe . T h i s was t h e m ost u n i qu e d e m o n s t r a t i o nper formed on Skylab . Such a t e s t was no t p rev io us ly fe as ib le because of th e long low-gp e r i o d r e q u i r e d a nd t h e n ee d t o m a n i p u l a t e t h e d r o p . A v a i l a b l e t h e o r y p r e d i c t s a com-p l e t e l y d i f f e r e n t r e s u l t , b u t d oe s h i n t t h a t o t h e r r e s u l t s may b e p o s si b l e.

When r o t a t e d a t l ow r a t e s , t h e d r o p h a s a w at e rm e lo n s h a pe . A t h i g h e r ra t e s , i tp i n c h e s o f f , a ss um in g a p e an u t sh a pe , a s shown i n F i g u r e 6. I f t h e r a t e i s i n c r e a s e df u r t h e r , a n e q u i l i b r i u m s h ap e ca n n o t be a c h i ev e d a nd t h e d r o p d i v i d e s i n t o two d ro p s6 .

Immisc ib le L iqu ids . - A d i s p e r s i o n o f t wo i m mi s c ib l e l i q u i d s c an b e fo rm ed i f t h e y a r es t r on g l y mixed. I f t h e d e n s i t i e s of t h e two l i q u i d s a r e d i f f e r e n t , t h e d i s p e r s io n w i l lq u i c k l y s e p a r a t e i n o ne -g . When g r a v i t y f o r c e s a r e s m a l l , t h e m ec ha nis m f o r s e p a r a t i o n ofa d i s p e r s i o n i s v e r y d i f f e r e n t . One l i q u i d c a n s e p a r a t e f ro m t h e o t h e r on l y by c o a l e s ce n c eo f t h e f i n e l y d i v i d e d d r o p s. I f d r o p s o f o n e l i q u i d d o come i n t o c o n t a c t an d d o c o a l e s c e ,s e p a r a t i o n c a n p r oc e ed a t some slow r a t e .

An e x p e r im e n t u s i n g v a r i o u s p r o p o r t i o n s o f o i l a nd w a t e r was p e rf o rm e d t o e xa mi net h i s phenomena . The two l iq u i ds were sepa ra te d ce n t r i fu ga l l y and then mixed by shak ing .They were observed f o r a p e r i o d o f 1 0 h o u r s t o s e e i f a ny s e p a r a t i o n c o u ld b e o bs e rv e d.Only a "c e l l u l a r s t r u c t u r e t h a t g re w c o a r s e " c o u l d b e o b se r ve d 6. On Ear t h , th e two l i qu i dsc o mp l et e ly s e p a r a t e i n l e s s t h a n 1 0 s e co n ds l O.

I c e M e l t in g . - T h i s d e m o n s t r a t i o n p r o v id e d a n i n d i c a t i o n o f t h e i n f l u e n c e o f a low-genvi ronment on the mechanisms of h ea t t r an s f e r 6 . I n s t e a d o f d r a i n i n g away a s i t d o es i no ne- g, t h e l i q u i d s u rr ou nd ed t h e i c e , a c t i n g t o i n s u l a t e i t f r o m t h e s u r r o u n d i n g a i r . I tt o ok 1 90 m i n u te s f o r t h e i c e t o c o m p le t el y m e l t t o a l i q u i d dr o p i n low-g. The i c e m e l t edi n 130 minutes i n t he same exper iment on earth1 '.

S i n c e c o n v e c t i o n i s t h o u gh t o f a s b e i n g d r i v e n by b u oy a nt f o r c e s , c o n d u c t i o n a nd r a d -i a t i o n h e a t t r a n s f e r a r e u s u a l l y p resum ed t o b e t h e means o f h e a t t r a n s f e r i n l ow-g. Whi len o t e s t a b l i s h e d f ro m t h i s e x p er i m e nt , c o n v e c t i o n c o u l d s t i l l be a mechanism of h ea t t r an s -f e r i n l ow-g. C o n v ec t i o n c a n b e d r i v e n by s u r f a c e t e n s i o n f o r c e s ( M ar an go ni f l ow ) s i n c eg r a d i e n t s i n t e m pe r at u re a l o ng a n i n t e r f a c e a l s o p ro du ce g r a d i e n t s i n s u r f a c e t e n s i o n l 2 .T he rm oa co us ti c e f f e c t s , m e c ha n ic a l v i b r a t i o n s , e l e c t r i c a nd m a gn e ti c f i e l d s , c o n c e n t r a t i o ng r a d i e n t s a nd c h em i ca l p o t e n t i a l s a r e a l s o p o s s i b l e mechanisms f o r c o nv e c t iv e h e a t t r a n s f e ri n 1 0 w - ~ l 3.


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C a v i t a t i o n . - A b u b b le w i t h i n a l i q u i d c a n o s c i l l a t e i n much t h e same m an ne r a s al i q u i d d r op o s c i l l a t e s . B ot h h yd ro dy nam ic an d s u r f a c e t e n s i o n f o r c e s a c t a t t h e s u r f a c e

t h e bu b b l e. I f t h e m a g n it u d e o f t h e hy dr od yn am ic f o r c e s e x ce e d s t h e s u r f a c e t e n s i o nf o r c e s a t s ome p o i n t o n t h e s u r f a c e , t h e b u b b le c a n become u n s t a b l e and c o l l a p s e up on i t -s e l f . Each t i m e a n u n s t a b l e d r op c o l l a p s e s a j e t o f l i q u i d fo rm s t h a t s h o o t s a c r o s s t h e

This phenomena was demo nstrated on Skylab i n th e fol low ing manner. A bubble wasf or me d i n s i d e a d ro p s uc h t h a t o n l y a t h i n f i l m s e p a r a t e d them a t o ne a r e a o n t h e s u r f a c e .

hen the su r f ac e was touched wi t h a p lunger , th e bubble rup ture d . Whi le the sourc e of thei n s t a b i l i t y o f t h e b u b bl e w as n o t a h yd ro dy na m ic f o r c e , t h e r e s u l t i n g c o l l a p s e w as t h e sames c a v i t a ti o n . A j e t o f l i q u i d s h o t o u t of t h e d ro p a t t h e p o i n t t h e b ub b le h ad b ee n r up -

t u r e d , a s shown i n F i g u r e 7. As t h e v e l o c i t y o f t h e j e t d e c r ea s e d , s u r f a c e t e n s i o n f o r c e sa c t e d t o r e t r a c t p a r t of t h e j e t b ac k i n t o t h e d r o p , w h i l e some o f t h e j e t p in ch ed o f fi n t o a d d i t i o n a l d r op s 6.

Drop Tower TestingD ro p t o we r t e s t f a c i l i t i e s ha v e b ee n e x t e n s i v e l y em pl oy ed t o i n v e s t i g a t e some o f t h e

i c low-g f l u i d b e h av i o r , A lt ho ug h l i m i t e d by r e l a t i v e l y s h o r t t e s t ti m e s , on t h e o r d e rof 2 t o 5 s e c , t he y d o p r o v i d e a q u i t e a c c u r a t e l y c o n t r o l l e d a c c e l e r a t i o n en vi ro nm en t(a cc el er at io n range between 10-5 and 10-I g)14. E xa mp le s of i n t e r f a c e s h a p e s w i t h i n c o n-t a i n e r s and l i q u i d m o ti on r e s u l t i n g i n r e o r i e n t a t i o n w i t h i n a c o n t a i n e r a r e p r e se n t ed

e low.C a p i l l a r y P umping an d I n t e r f a c e S h ap e s. - L i q u i d s a r e u s u a l l y s t o r e d i n a c o n t a i n e r an d

t h e s h a p e a ss um ed by t h e l i q u i d / v a p o r i n t e r f a c e i s i mp or ta nt t o t h e d ra i n i n g o r f i l l i n g o ft h e c o n t a i n e r i n low-g. The t a n k sh a pe and an y i n t e r n a l s t r u c t u r e s i n f l u e n c e t h e i n t e r f a c es ha pe . D i f f e r e n ce s i n c a p i l l a r y p r e s s u r e w i l l c au s e l i q u i d t o be t r a n s f e r r e d f r om a r e g i o no f l ow c u r v a t u r e t o a r eg i o n o f h i g h e r c u r v a t u r e , a s d i s c u s s e d p r e v i o u s l y . T h i s c a p i l l a r y

um ping e s t a b l i s h e s t h e e q u i l i b r i u m i n t e r f a c e s ha p e s. I n t e r f a c e s h a p es a nd pumping r a t e sb e en e s t a b l i s h e d f o r n um ero us g e o me t r ie s 7 . As a n e xa mp le , o r i e n t a t i o n of s m a l l l i q u i d

volumes by var io us vane conf igu ra t ion s i s shown in F igu re 8. O r i e n t a t i o n of t h e l i q u i d i na b a r e t a n k i s shown i n t h e up p er l e f t h an d c o r n er . L i q u i d p o s i t i o n i n g w i t h e a ch o f t h ef i v e d i f f e r e n t v an e d e v i c e s , h ow ev er , i s s uc h t h a t g a s - f r e e l i q u i d c ou l d b e su p p l i e d t o a no u t l e t l o c a t ed a t t h e 6 o ' c l o c k p o s i t i on .

L i q ui d Mo ti on .- D i s t u r b i n g f o r c e s a c f i n g o n a c o n t a i n e r c a n c a u s e t h e l i q u i d w i t h i nt o r e o r i e n t. A s m a l l l a t e r a l a c c e l e r a t i o n com ponent w i l l make t h e l i q u i d f l ow a l o n g on es i d e of t h e t a n k a s i t r e o r i e n t s . I n a t a n k w i t h a sm ooth i n t e r i o r w a l l t h e f lo w a d h e r e st o t h e t a nk w a l l c o n t i n u i n g p a s t i t s f i n a l e q u il i b ri u m p o s i t i o n. A t y p i c a l e x am pl e i sshown i n F igu re 915. A s t h e l i q u i d be gan t o move, t h e l i q u i d i n t e r f a c e r em ai ne d r e l a t i v e l yf l a t s o t h e m ot io n a pp ea re d a s a r o t a t i o n o f t h e i n t e r f a c e ab o ut i t s c e n t e r . Very l i t t l es p l a s h i n g o f t h e l i q u i d o cc u r re d d u r i n g t h e r e o r i e n t a t i o n . Once t h e l e a d i n g e dg e o f t h ef l o w r e a ch e d t h e t a n k dome, t h e l i q u i d i n t e r f a c e b eg an t o a c q u i r e some c u r v a t u r e . Thel i q u i d o v e r s h o t i t s f i n a l e q u i l i b r i u m p o s i t i o n , c o n t i n u i ng ar ou nd t h e t a n k and r e c i r c u l a t -i n g some o f t h e l i q u i d . B a f f l e s o r s u r f a c e t e n s i o n d e v i c e s t r u c t u r e w i l l b re ak up t h i sr e c i r c u l a t i o n a nd s p ee d t h e a c hi e ve m en t o f i t s new s t a t i c p o s i t i o n . B a f f l e s d o i n d uc et u r b u l e n c e a nd p r o d uc e g a s b u b b l e s w i t h i n t h e b u l k l i q u i d h ow ev er .

STORAGE AND TRANSFERO p e r at i o n o f p a s s i v e d e v i c e s f o r t h e s t o r a g e and t r a n s f e r of l i q u i d i n low-g i l l u s -

t r a t e s a p r a c t i c a l a p p l i c a t i o n of t h e p r e v i o u s l y d e s c r i b e d , s u r f a c e t e n s i o n d o m i n a t e d ,b e h a vi o r . D e vi c es a r e a v a i l a b l e t o h a n d l e a w id e ra ng e of f l u i d s s u ch a s a l c o h o l s , f r e o n s ,p r o p e l l a n t s and c ry o ge n s1 6 ,1 7 . C o n f i g u r a ti o n s d i f f e r b ec a us e o f v a r i e d f u n c t i o n a l r e q u i r e -

; h ow ev er , t h e o p e r a t i o n a l p r i n c i p l e f o r ea ch s ys t em r e l i e s o n t h e r e l a t i v e l y s m a ll


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8/15

The p a r t i c u l a r d e v i c e s e l e c t e d d e pe nd s upon f a c t o r s s uc h a s f l u i d c o m p a t i b i l i t y , c y c l el i f e , e x p u ls i o n e f f i c i e n c y , c o s t . e t c .

P r e ss u r e c o n t r o l i n t h e r e c e i v e r t an k d ur i n g f i l l i n g i s a k ey d e s i g n c o n s i d e r a t i o n .When s m a l l q u a n t i t i e s o f l i q u i d and s m a l l c o n t a i n e r s a r e b e i n g u s e d , e v a c u a t i n g t h e c o n-t a i n e r t o a c h ie v e t r a n s f e r i s a r a t h e r s i m p le a p pr o ac h . On a l a r g e r s c a l e , s t r u c t u r a lc o n s i d e r a t i o n s a nd / o r v a p o r i z a t i o n o f t h e l i q u i d u s u a l l y r u l e o u t t h i s meth od o f t r a n s f e r .F o r a h i g h v a p o r p r e s s u r e f l u i d o r a c r y o ge n t h e r e c e i v e r c o n t a i n e r mu st b e v e n te d d u r i n gt h e t r a n s f e r , an d t h e l o c a t i o n o f t h e g a s m ust b e c a r e f u l l y c o n t r o l l e d .

An example of ev acua t ing one con ta i ne r to ach iev e t r an s f e r i n a low-g envi ronment wasd e m o n st r a te d d u r i n g t h e S k y l a b m i s s i o n s 6 . A s a mple o f b l ood was d ra wn i n a l a r ge s y r i ng ea nd t r a n s f e r r e d t o an ev a cu a te d b o t t l e . The p r e s s u r e w i t h i n t h e b o t t l e was s e l e c t e d s ot h a t t h e a d d i t i o n o f a g i v e n v ol um e o f b l o od r e du c ed t h e p r e s s u r e d i f f e r e n t i a l b et we en t h es y r i n g e and b o t t l e t o z er o.

The n e e d l e o f t h e s y r i n g e was i n s e r t e d i n t o t h e b o t t l e , p i e r c i n g a diaphragm. I te x te n de d p a r t way i n t o t h e b o t t l e . B loo d im m ed i at e ly b eg an t o t r a n s f e r f ro m t h e s y r i n g et o t h e b o t t l e , s i n c e t h e b o t t l e was a t a p r e s s u r e somewhat b el ow a m bi e nt . A s t h e f l o w o fl i q u i d b e g a n , a dro p could be obse rved forming a t t h e t i p o f t h e s y r i n g e . No t u r b u l e n c eo r ge ys e r i ng o f t he bl ood was obse r ve d. The d r op c on t i nue d t o e xpand u n t i l i t c on t a c t e dt h e w a l l o f t h e b o t t l e . A t t h i s p o i n t t h e r e w as a volu me o f l i q u i d , l o c a t e d n e a r o n e en do f t h e b o t t l e , d i v i d i n g t h e g a s i n t o two s e p a r a t e v ol um es . T he i n t e r f a c e o n e ac h s i d e o ft h e l i qu i d vo lume moved t owar d t h e e nds o f t he b o t t l e c ompr e s si ng t h e t wo ga s vol umes a st he t r a ns f e r c on t i nue d . The vol ume o f ga s ne a r t he d i a phra gm o f t h e b o t t l e was t he s ma l l e rof th e two. Each volume was a t th e same p r e s s u r e w i th t h e l i q u i d a c t i n g as a p i s t o n b e -t we en t hem. Some p r e s s u r e was a pp l i e d t o t h e p l unge r o f t he s y r i n ge t o a c h i e ve compl e tet r a n s f e r of t h e l i q u id .

The t r a ns f e r o f l i q u i d f r om one t a nk t o a s ec ond ven t e d t a nk i n a w e i g h t l e s s e n v i r o n -m en t was de m on s tr at e d by t h e cr ew o f A p o ll o 1 4 ~ ~ .wo s u r f a c e - t e n s i o n b a f f l e d e s i g n s w e rei n c o r p o r a t ed i n s e p a r a t e t a n k s of a s c a le - mo d el l i q u i d - t r a n s f e r s y st e m w i t h e a ch t a n kb e i ng us ed a l t e r n a t i v e l y as t h e s upp ly and r e c i e ve r t a nk . A s k e t c h of t h e t a n k s i s p r e -s e n t e d i n F i gu r e 14 . One t a nk c on t a i ne d a s t a n d p i p e -l i n e r b a f f l e s t r u c t u re . c o n s i s t i n g o f ap e r f o r a t e d s t a n d p i p e l o c a t ed o v er t h e d r a i n l f i l l p o r t a nd a w a l l - l i n e r s p a c e d a f i x ed d i s -t an ce away f rom th e t ank wa l l . The second t ank cont a ine d a c ur ve d- we b b a f f l e s t r u c t u r ec o n s i s t i n g o f t h r e e c i r c u l a r p e r f o r a t e d p l a t e s n e s te d a ro un d a s m a ll f e e d e r c a p i l l a r y s e c -t io n . The curved web b a f f l e s a re a r r an g e d o f f - c e n t e r s uc h t h a t t h e c r o s s - s e c t i o n a l a r e ab et we en b a f f l e s i n c r e a s e s g r a d u a l l y f ro m t h e f e e d e r s e c t i o n t o w a rd s t h e o p p o s i t e e nd o ft h e t a n k . T h i s a r ra n ge m en t te n d s t o r e t a i n t h e b u l k l i q u i d a d j a c e n t t o t h e f e e d e r .

T e s t i n g w as pe rf or me d t o d e t e r m i n e t h e a b i l i t y t o a c h i e v e g a s - f r e e o u t f l o w f ro m t h es u p p ly t a n k an d o r d e r l y i n f l o w i n t o t h e r e c e i v e r t a n k w i t h g a s l o c a t e d a t t h e t a n k v e n ta n d l i q u i d a t t h e f i l l p o r t . G a s- f re e l i q u i d was t r a n s f e r r e d t o and fr om e i t h e r b a f f l e dt a n k t o w i t h i n 2 p e r ce n t o f t h e l i q u i d a v a i l a b l e f o r t r a n s f e r and t h e r e c e i v e r t a nk v e ntr ema ine d i n c on t a c t w i t h ga s . T r a n s f e r be t we e n unba f f l e d t a nk s was i nc l ude d f o r c ompa ri -s on a nd ga s i ng e s t i on oc c u r r e d when l e s s t ha n 12 pe r c e n t o f t he s upp l y t a nk vol ume had be e nd e l i v e r e d . A t t h e t e r m i n a t i o n o f t r a n s f e r l i q u i d had i n g e s t e d i n t o t h e r e c e i v e r t a n k v e n t.

CONCLUDING REMARKSThe pa rame te r s t h a t g o v er n f l u i d b e ha v io r i n a we i gh t l e s s e nv i r onme n t ha ve be e n pa r -

t i a l l y c h a r a c t e r i z e d an d v e r i f i e d by g ro un d a nd o r b i t a l t e s t i n g . B ec au se t h e e n vi r on m en t ,g eo me tr y, an d f l u i d s o f i n t e r e s t i n f l u e n c e t h i s b e h av i o r and a l s o d i f f e r fr om s ys te m t os y s te m , e a c h new a p p l i c a t i o n m us t b e e v a l u a t e d t o a s s u r e t h a t t h e d e s i r e d p e r fo r ma n ce i sa c h i e ve d . Ea r l y a s s e ss me n t o f f l u i d be ha v i o r a nd c on t r o l f o r e a ch e xpe ri me n t i s r e q u i r e d .One-g b en ch a n d / o r d r o p t ow er t e s t i n g c a n p r o v id e v e r i f i c a t i o n p r i o r t o o r b i t a t o p e r a t i o n .


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Col lo id Science, Vol . 2 , E . Mat i j ev i c , E d i to r , Joh n Wiley & Sons, N ew York, 1969.2. H. W . Fox and W . A . Zisman, Jou rna l of Collo id Scien ces, Vol. 5 , 1950, p . 514.3 . E . G. S h af r in an d W. A. Zisman, "Upper Limi ts t o the Contact Angles of Liqu ids onSo l i ds ," Contact Angle , We t t ab i l i t y and Adhesion , Advances i n C h em is tr y S e r i e s 4 3 ,

American Chemical Society, Washington, D. C . , 1965 , pp. 145-157.4. Wal ter Drost-Hansen, "Aqueous In te rf ac es ." Pa rt s I and I1 i n Chemist ry and P hysic s

of In te rf ac es , American Chemical Soc iet y , Washington, D . C . , 1965.5. J . J . Bikerman, S urf ace Chemistry , Theory and Ap pl ic at io ns , Academic Pr es s, In c. ,

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J . Sp ac ec ra ft , Vol. 12, No. 9, September 1975, pp. 544-551.8. H. L. Pa yn te r and T. R. B a r ks d a le , " C r i t e r i a f o r P a s s i v e P r o p e l l a n t C o n t r o l Schem es.J . Sp ac ec ra ft , Vol. 7, No. 6, June 1970, pp. 702-706.9. L. J . Hast ings and R. Ruthe rfor d, "Low Grav ity Liquid-Vapor In te rf ac e Shapes i n

Axisymm etric Co nt ai ne rs and a Computer So l ut io n, " NASA TMX-53790, George C. M a r s h a l lSpace F l ig h t Ce n ter , Hu nts v i l le , Alabama, October 7 , 1968.

10 . L. Lacy and H. Ott o , "The S t a b i l i ty o f L iqu id Dispers ions i n Low-Gravi ty ," A I A A PaperNo. 74-1242, AIAAIAGU C o nfe ren ce on S c ie n t i f i c E x per iment s o f S k y lab, H u n t sv i l l e ,Alabama, November 1974.

11. G. H. O t t o and L. L. L ac y, " O b se r va t io ns of t h e ~ i q u i d l ~ o l i dn t e r fa ce i n L ow -G ravi tyMel ti ng ," AIAA Pa pe r No. 74-1243, AIAAIAGU Conference on Sc ie n t i f ic Experiments o fSkylab, Huntsville, Alabama, November 1974.

12. J . L. McGrew, e t a l . , "The Ef fe ct of Temperature Induced Surfa ce Tension Gr adi ent s onBubble Mechanics I 1 Applied S cie nce Research, Vol. 29, June 1974, pp 195-210.

13. F. T. Dodge, e t a l . , "Flui d Physics , Thermodynamics, and Heat Tra nsf er Experiments i nSpac e: F i n a l Repo rt of th e Overstu dy Cormnittee," NASA CR-134742, Sout hwes t Res ear chI n s t i t u t e , San Anton io , Texas , January 1975 .

14 . H. L. Pay nte r, "The Marti n Company's Low-g Experime ntal Fa ci l i t y , " Proceedings of th eSymposium on Fl ui d Mechanics and Heat T ra ns fe r Under Low Grav ity , United S t a t e s A i rForce Of fi ce of S c i e n t i f i c Research and Lockheed Mi ss il es and Space Company, June1965, pp. 15-1 to 15-17.

15. J . R. T eg ar t , " P ro p e l l an t R eo r i en ta t io n w i th Of f - a x i s A c ce le ra t io n s , " AIAA Paper No.75-1195, Pres ente d a t t he AIAA/SAE 11th Propu lsi on Conference, Anaheim, Ca li fo rn ia ,September 29-October 1, 1975.


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16. D . A. F e s t e r , e t a l . , " A c q u i s i t i o n / E x p u l s i o n S y st em f o r E a r t h O r b i t a l P r o p u l s i o nSystem Stud y, F i na l Rep or t , Volume V - Ea rt h St or ab le Desig n," MCR-73-97 (Vol . V) ,M ar t i n M a r i e t t a C or po r a t i on , D enve r , C o l o r ado , O c t obe r 1973 .

17 . G. R. P a g e, e t a l . , " k c q u i s i t i o n / E x p u l s i o n S ys te m f o r E a r t h O r b i t a l P r o p u l s i o n S ys te mStudy, F i na l Repor t , Volume I11 - Cryoge nic Te sts ," MCR-73-97 (Vol . 1 11 ) , Mar t inM a r i e t t a C or po r a t i on , D enve r , C o l o r ado , O c t obe r 1973 .

18 . D . A. F e s t e r , R. N. E b e r h a r d t , a n d J . R. T eg a r t , " Space S hu t t l e R eac t i on C on t r o l Sub -s y s te m P r o p e l l a n t A c q u i s i t i o n T ec h no lo g y, " AIAA Pap er No. 74-1106, P res en ted a t t h eAIAA/SAE 1 0 t h P r o p u l s i o n C o n fe r en c e , S an ~ i e ~ o ,a l i f or n i a , October 21-22 , 1974.

19 . C . D. Brown, e t a l . , "Vik ing Or bi t e r 1975 Pr op el l a n t Management Device , " SE 010-47-01 ,Mar t in Ma r i e t t a Corpo ra t io n , Denver , Colorado , May 1973.

20. K. L. A bda l l a , e t a l . , "Liquid T ran sfe r Demonst r a t ion on Board Apol lo 14 Dur ing Trans-e a r t h Co as t," NASA TMX-2410, Lewis Resea rch Ce nt er , Cle ve la nd , Ohio , November 1971.


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ORIGBsra PAGE ISOF POOR QUALITY

Figure 1.- 33.5' cont act a ngle of a drop ona contaminated surface.

COMMUNICATION

OUTLET

Figure 2.- Capillary pumping with a vanesurface-tension device.


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,. ressurization

Figure 10.- Total communication screen Liquid Flowdevice. Annulus

Figure 11.- Schematic of a compartmental-ized sc reen device.

Figure 12.- Schematic of t h e Vikingor bi te r 1975 pro pel lan t managementdevice.Figure 13.- Photograph of the Vikingorbiker 1975 vane s t ru c tu re .


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Behaviour of Fluids in a Weightless Environment

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