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LS VI R

P I h S 0 1 4 0 - 7 0 0 7 ( 9 7 ) 0 0 0 3 3 -9

Int J. Refrig. Vol. 20, No. 8, pp. 575-582, 1997

1998 Elsevier Science Ltd and IIR

Printed in Great Britain. All rights reserved

0140-7007/97/ 17.00+00

P o o l b o i l i n g h e a t t r a n s f e r a t f i n n e d t u b e s : i n f l u e n c e o f

s u r f a c e r o u g h n e s s a n d s h a p e o f t h e fin s

P e t e r H i i b n e r a n d W o l f g a n g K i i n s tl e r

L a b o r a t o r i u m f i i r W a r m e - u n d K ~ i l t e t e ch n i k , U n i v e r s i t ~ i t - G H P a d e r b o r n ,

W a r b u r g e r S t r. 1 0 0 , D - 3 3 0 9 8 P a d e r b o r n , G e r m a n y

R e c e i v e d 3 0 A p r i l 1 9 97 ; a c c e p t e d 1 4 M a y 1 9 9 7

P o o l b o i l i n g h e a t t r a n s f e r f r o m f i n n e d t u b e s w i t h d i f f e r e n t s h a p e s o f f i n s (t r a p e z o i d - s h a p e d , T -

s h a p e d , o r Y- s h a p e d ) t o v a r i o u s h y d r o c a r b o n s a n d p a r t l y f l u o r i n a t e d h y d r o c a r b o n s h a s b e e n

inve s t iga te d a t t he Lab ora to r ium f i i r W~i rme- und K~ i l t et echn ik , Un ive r s i t~ i t -GH Pade rbo rn

d u r i n g t h e r e c e n t p as t . Co m p a r e d t o c o r r e s p o n d i n g m e a s u r e m e n t s o n p l a i n t u b e s, h e a t t r a n s f e r

o n t r a d i t i o n a l l y f i n n e d t u b e s w i t h t r a p e z o i d - s h a p e d f i n s i s c o n s i d e r a b l y i m p r o v e d , a n d e v e n

b e t t e r r e s u lt s a r e a c h i e v e d w i t h T - s h a p e d o r Y- s h a p e d f in s . Th e i n f l u e n c e s o f t h e

m a c r o s t r u c t u r e ( i .e . f in g e o m e t r y ) o r m i c r o s t r u c t u r e ( i . e . s u r f a c e r o u g h n e s s ) o n t h e h e a t

t r a n s f e r c o e f f i c i e n t h a v e b e e n s t u d i e d s e p a r a t e l y , i n o r d e r t o e v a l u a t e t h e i m p r o v e m e n t o f h e a t

t r a n s f e r b y e i t h e r i n f lu e n c e . 1 9 9 8 E l s e v i e r S c i e n c e L t d a n d I I R .

Keywords: he at transfer; refrigerant; pool bo il ing; geom etry; roughness)

Transfert de chaleur ~ 6bul l i t ion l ibre sur des tubes ai letrs:

inf luence de la rugos i t6 des surfaces e t de la forme

des ai let tes

Au Labo r a t o r i um f i i r W i i r me - und K i i l t e chn i k , de l Un i v e r s i t d GH Pade r bor n , on s e s t

r ~c e m me n t a t t ac h~ a I ~ t ude du t r ans f e r t de c h a l e u r f i l dbu l l it i on l i b r e g t par t i r de t ube s

a i l e t~s de d i f f~r e n t e s f o r m e s ( t r apk ze , T ou Y ) dans c e r t a i n s hy dr o c ar bur e s e t de s

hy dr oc a r bur e s par t i e l l e me n t f l uor ~s . Pa r r appor t aux me s u r e s c or r e s pondan t e s e f f e c t ude s

s ur de s t ube s s i mp l e s , l e t r ans f e r t de c ha l e ur e s t c ons i d~r ab l e m e n t am l i o r ~ s u r l e s t ube s ?J

a i l e t t e s c l a s s i que s t rap~zo i da le s ; on pe u t o b t e n i r de s r d s u l ta t s e nc or e me i l l e u r s a v e c de s

a i l e t t e s e n f o r m e de T ou de Y . On t ud i e s ~par ~m e n t l e s i n f l ue nc e s de l a ma c r os t r uc t u r e

( gdombt r i e de s a i l e t t e s ) ou de l a m i c r os t r uc t u r e ( r ugos i t g de s s u r f ac e s ) s u r l e c oe f f ic i e n t de

t r ans f e r t de c ha l e ur , a f i n d dv a l ue r l e u r r f i le dans l am l i o r a t i on du t r ans f e r t de e ha l e ur .

1998 E l s e v i e r Sc i e nc e L t d e t I I R .

(Mots cl~s: transfert de chaleur; frigorig~ne; ~bullition libre; g~om ~trie; rogosit~)

I n t r o duc t i o n

Ch l o r o f l u o r o c a r b o n s ( CF Cs ) a n d h y d r o c h l o r o f l u o r o -

c a r b o n s ( HCF Cs ) u s e d a s r e f r i g e r a n t s u n t i l n o w h a v e

b e e n p h a s e d o u t b e c a u s e o f t h e i r o z o n e d e p l e t i o n

p o t e n t i a l a n d t h e i r c o m p a r a t i v e l y h i g h g l o b a l wa r m i n g

p o t e n t i a l a n d h a v e t o b e r e p l a c e d b y n e w s u b s t a n c e s . Th e

s u b s t i t u t e s w i l l b e p a r t l y f l u o r i n a t e d h y d r o c a r b o n s

( HF C s ) o r , i f f l a m m a b i l i t y c a n b e t o l e r a t e d , p u r e

h y d r o c a r b o n s . En e r g y c o n s u m p t i o n a s a b a s i c f a c t o r

f o r g l o b a l wa r m i n g c a n b e r e d u c e d f u r t h e r i f f i n n e d t u b e s

* Dedicated to Prof. Dr.-lng D ieter Gorenflo on the occasion o f

his 60th birthday

i n s t e a d o f p l a i n t u b e s a r e u s e d f o r h e a t t r a n s f e r

a p p l i c a t i o n s . W i t h t h i s r e s p e c t , n u m e r o u s e x p e r i m e n t s

w i t h d i f f e r e n t e n h a n c e d t u b e s ( K 1 9, K3 6 , T 1 9 , TX 1 9 a n d

Y X 2 6 ) ( m a n u f a c t u r e r : W i e l a n d - W e r k e A G U l m , F R G ;

t r a d e n a m e : GEW A) a n d w i t h v a r i o u s b o i l i n g l i q u i d s

h a v e b e e n p e r f o r m e d ( s e e Tab l e I Tab l e 2 ) . Th e t u b e

s u r f a c e s a s m a n u f a c t u r e d c o n t a i n r e g i o n s w i t h c l e a r l y

d i s t i n c t r o u g h n e s s l e v e l s (Table 3) . Ad d i t i o n a l h e a t

t r a n s f e r m e a s u r e m e n t s h a v e b e e n c a r r i e d o u t w i t h

t wo t r a p e z o i d - s h a p e d f i n n e d t u b e s ( K1 9 o r K3 6 ,

r e s p e c t i v e l y ) u n d e r m o d i f i e d r o u g h n e s s c o n d i t i o n s . Th e

r e s u l t s a r e c o m p a r e d w i t h d a t a f o r p l a i n t u b e s .

F o r l o w n o r m a l i z e d s a t u r a ti o n p r e s su r e s 0 .0 1 1 - p * - 1 4 W / m 2 K h e a t c o n d u c t i o n

w i t h i n t h e f i n s - - e v e n w i t h c o p p e r a s w a l l m a t e r i a l - -

c a n n o t b e n e g l e c t e d a s it c a n f o r a < - 1 04 W / m 2 K . T h e

c o n t r i b u t i o n o f t h is e f f e c t i s d e m o n s t r a t e d b y c a l c u l a t i n g

t h e h e a t t r a n s f e r c o e f f i c i e n t a R r e l a t e d t o t h e m e a n

s u r f a c e t e m p e r a t u r e o f t h e f i n n e d t u b e , c a l c u l a t e d

a c c o r d i n g t o R e f . s i n t h e c a s e o f t h e t u b e w i t h e m e r i e d

f i n t o p s ( d o t t e d l i n e s i n F i g u r e 3 , b o t t o m ) . T h e n i t i s

e v i d e n t t h a t a t t h e h i g h e s t f l u x e s i n v e s t i g a t e d h e a t

t r a n s f e r o n t h e f i n n e d t u b e i s s o m e w h a t b e t t e r t h a n o n t h e

p l a i n , d u e t o t h e e f f e c t o f t h e v e r y m a n y t i n y b u b b l e s

s t r e a m i n g u p w a r d s b e t w e e n t h e f i n s .

A p o s s i b l e e x p l a n a t i o n f o r t h e r e s u l t s a t p * = 0 . 0 3 7 f o r

n - h e x a n e m a y b e t h e s a m e a s g i v e n a b o v e u n d e r t h e

s e c o n d r e a s o n ( 2 ) , b e c a u s e t h e b u b b l e s i z e s a t d e p a r t u r e

a r e e v e n c l o s e r t o t h e f r e e f i n d i s t a n c e ( 0 . 7 t o 0 . 9 m m ~ ) .

H o w e v e r , t h e f o r m e r p l a i n t u b e u s e d f o r n - h e x a n e i n

1 9 9 0 h a d a s ig n i f i c a n t l y r o u g h e r s u r f a c e (R ~ = 0 . 5 8 m ,

T a b l e s 2 , a n d 4 ) , s o t h e c o m p a r i s o n i s n o t a s

s t r a i g h t f o r w a r d a s w i t h p r o p a n e a n d t h e t e s t s s h o u l d b e

r e p e a t e d w i t h t h e n e w p l a i n t u b e i n o r d e r t o g e t a

c o h e r e n t c o m p a r i s o n f o r t h e e n t i r e p r e s s u r e r a n g e .

C o m p a r i s o n o f t h e t u b e s u n d e r a s p e c t s o f t e c h n i c a l

a p p l i c a t i o n

F o r t h e a p p l i c a t i o n o f p l a i n o r f i n n e d t u b e s i n p r a c t i c e ,

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

m a d e a t th e s a m e h e a t l u x p e r t u b e l en g t h . T h e r e f o r e , t h e

r e s u l t s f o r p r o p a n e b o i l i n g a t t h e d i f f e r e n t t u b e s a r e

p l o t t e d i n F i g u r e 4 i n t e r m s o f ~ o n r e l a t e d t o t h e

a r e a A n n o f a p l a i n t u b e w i t h t h e o u t e r d i a m e t e r

o f t h e f i n n e d , a n d a t c o n s t a n t h e a t f lu x p e r t u b e l e n g t h ,

q o o = 2 0 0 0 0 W / m 2. T h e o u t e r d i a m e t e r d a o f t h e t u b e

h a s b e e n c h o s e n f o r c o m p a r i s o n , b e c a u s e m o d e r n f i n n e d

t u b e s a r e f i x e d i n t u b e b u n d l e s a t t h i s d i a m e t e r ( a n d n o t a t

t h e d i a m e t e r d k w i t h o u t f i n s , a s e a r l i e r ) . S i g n i f i c a n t

e n h a n c e m e n t o f h e a t t r a n s f e r o v e r t h e p l a i n t u b e i s

d e m o n s t r a t e d u n d e r t h i s c o n d i t i o n , w i t h t h e Y X t u b e

b e i n g b e s t . A t 7 % o f t h e c r i t i c al p r e s s u r e , a n n i s

a u g m e n t e d o v e r t h e p l a i n t u b e b y a f a c t o r o f 4 . 0 ( Y X ) ,

3 . 0 ( T X ) , 2 . 4 ( T ) , o r 1 . 6 ( K 3 6 ) , r e s p e c t i v e l y , w h i l e

a t 2 0 % o f p ~ t h e a u g m e n t a t i o n i s r e d u c e d t o f a c t o r s o f

2 . 7 / 2 . 0 / 1 . 9 / 1 . 6 . T h i s d e m o n s t r a t e s t h e a d v a n t a g e o f

8/10/2019 Pool Boiling Heat Transfer at Finned Tubes Influence of Surface Roughness and Shape of the Fins

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5 8 P H O b n e r a n d W K b n s t l e r

the finned tubes investigated when applied at low

normalized saturation pressures.

Conclusions

Pool boiling heat transfer measurements of various

new refrigerants and hydrocarbons at finned tubes

with different surface structures have shown the

following.

The enhancement of pool boiling heat transfer at

finned tubes with trapezoid-shaped fins over the plain

tube is mainly due to enhanced bubble formation at the

tops of the fins which are very rough as a result of the

manufacturing process. After sandblasting the whole

surface of the tube with fine grain particles and achievin g

a uniform surface roughness similar to that of the plain

tube, the enhancement is reduced, resulting in more or

less the same heat transfer coefficients for the plain and

finned tubes, if comparin g at ~ and q related to the total

outer area of the finned tube. Especially favourable

conditions seem to occur when the departure diameters of

the bubbles corre spond to the free distance between the fins.

Heat transfer from finned tubes with special shape of

the fins T-shape d or Y-shap ed) is consider ably

improved over the plain tube at low normalized

saturation pressures, especially if compared at the same

heat flux per tube length, being important for technical

applications. The relative increase of the heat transfer

coefficient with heat flux and pressure differs consider-

ably from the results for plain tubes and finned tubes with

traditionally shaped fins = trapezoid- shaped).

cknowledgements

Thanks are due to Deutsche Forschungsgemeinschaft,Bonn,

for financial support. The authors are also indebted to Dr

Andrea Luke for measuring the surface roughness.

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