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