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H e a t R e c o c e r y S y s t e m s & Cl i P V o l . 8 , N o . 4 , p p . 2 9 9 - 3 0 8 , 1 9 8 8 0 8 9 0 4 3 3 2 / 8 8 $ 3 . 0 0 + .00Printed in Gre at Britain. Pergamon Prem pk

P E R F O R M A N C E O F S H E L L - A N D - D I M P L E D - T U B EH E A T E X C H A N G E R S F O R W A S T E H E A T R E C O V E R Y

V . H . MORCOS*School of Technical Educa tion, Univers i ty of Technology, P .O. Box 35010, Baghdad, l ra q

(Received 16 July 1987)Alm tract- -This pap er presents an exp er imental s tudy of waste heat recovery shel l-and- tube heatexchangers. The exchanger heat duty , overal l heat t ransfer ~ e n t , ef fect iveness and tubetide f r ic t ionfactor are investigated as functions o f the tu be sur face geo metry (plain or dimpled) , the f low type (counteror pa r a l le l ), the tube Reynolds number and the the lk ide hea t capac i ty r ate . Wate r and the exhaus t gaseso f a Diesel e ngine ar e p~___t~ed_ nside the tu be an d th e she ll, respectively.The heat t ransfer character is t ics increase with an increase in tub e Re ynolds num ber an d th e shells ideheat capac ity rate , for a l l the f low types and the sur face ~ examined. The counter -f low,shell-and-dimpled- tube heat ezchangar , co mp ared with tha t exchanger having a plain tube, increases theheat duty and the overal l heat tm mf er coef fgient by 80%, and the heat exch anger ef fectivenem increasesby 35% . Fo r the paral le l-f low, sh el l-and-dimp led- tube heat exchanger, the he at du ty, the overal l heattransfer coeff icient and the e f f e c f i ~ increase by 30, 55, and 25% , respectively. At the tame t ime thedimpled tube incr eases the tubes /de f ric t ion f ac tor by 600% ov ~ tha t o f the p la in tube . The r a te o f w asteheat recovered f rom th e exhaust gases of the D iesel engine b y the counter- flow, shal l-end-dimp led- tubehea t exchanger i s equa l to 10% o f the maximum br ake pow er of the engine r unning a t 1500 r pm, andthe tube R eynolds number equa l to 8875 .

A

Efgh fLLMTD

M ,M ,NPrqQRe,t t , t2r , , r2r~UV% W H R

N O M E N C L A T U R Eheat tramfer mrface area [ma]brake power of the Diesel e~Oae, equation (7) [kW]spec if ic hea t a t cons tan t p r emur e of exhaus t gu m [ J ( kg K ) - ' ]specific heat a t comttant premm~ of w ater [ J (k gK )- ' ]imide d iamete r o f tube [ m |enhanoement rat io, equation (4)tubeside f r ic t ion factor , equation (6) o2e__]eration du e to gra vit y [m s -a]frictional head loss, equation (6) [m N N "t]exchanger heat tmafer length [m]logarithmiomean-tempamturedifference Ig ]m a s s f low rate of exhau st gamin k g s - t ]m a s s f low rate of w a t e r [ k g s - I ]sh e l l~ e hea t capac i ty r a te ( m ats f low r a te t imes the spec if ic hea t o f ~e l l s ide f lu id) [ W K - t ]tubes ide hea t cap ac i ty r a te [ W K - ' ]speed of the Diesel engine [rpm]Pr andt l numberheat f lux per unit heat t ransfer sur face a rea, equ ation (2) [W m-a]exchanger heat duty, equation ( i) [W]tube Reynolds number, equation (5)tubeside inlet and outlet temperatures , respectively [K]shells /de inlet and o utlet te mp era tura , respectively [K]brake torque on the Diesel engine shaf t [ J ]over a l l hea t t r ans f e r coef fg ien t based on the ou te r d iamete r o f the tube [ W ( m ' K ) - t ]water veloci ty in the tube I res - t ]percentage of waste hea t recovered, equa tion { 8)

Greek syml~isexchanger heat transfer effectiveness, equation (3)d y n a m i c v i s co s i tyo f w a t e r [ k g ( s m ) - ' ]

S I d 3 $ c r i p tP refers to reference plain tub e exchanger design.

*On leave f rom the Facu lty of Engineer ing, A miut U nivers i ty, Am iut , Egypt.299

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300 V. H. MogcosI N T R O D U C T I O N

The she l l -and- tube hea t exchanger i s the mos t wide ly used type o f indus t r ia l hea t t r ans fe re q u i p m e n t . I n o r d e r t o c a r r y o u t t h e t h e r m a l - h y d r a u l i c d e s ig n o f a s h e l l - a n d -t u b e e x c h a n g e r ,p r e s s u r e d r o p a n d h e a t t r a n s f e r c o r r e l a t i o n s ( o r t a b u l a t e d d a t a ) m u s t b e a v a i l a b l e f o r b o t h t h etubesid e an d the shellside. In i t ia l ly , only pla in tube s were used in shel l-and -tube excha ngers .How ever , a s inc reas ing incen t ives fo r m ore e f f ic ien t hea t exchangers , cons ide rab le emphas is hasb e e n p la c e d o n t h e d e v e l o p m e n t o f v a r io u s a u g m e n t e d , o r e n h a n c e d , h e a t t r a n s f e r s u rf a ce s . Th euse o f enhanc ed su r faces a llows the des igne r to inc rease the hea t du ty fo r a g iven exchanger , tor e d u c e t h e s iz e o f t h e e x c h a n g e r fo r a g i v en h e a t d u t y , t o r e d u c e t h e p u m p i n g p o w e r , o r t o r e d u c et h e a p p r o a c h t e m p e r a t u r e d i f f er e n ce .Berg les and Webb [1 ] have de f ined 13 pass ive and ac t ive techn iques to p rov ide hea t t r ans fe ra u g m e n t a t i o n . Th e m a j o r i t y o f c o m m e r c i a l l y i n t er e s ti n g a u g m e n t a t i o n t e c h n iq u e s a r e l i m i te d t opass ive techn iques , wh ich employ spec ia l su r face geomet r ie s .W i r e c o i l a n d t w i s te d t a p i n s er t s h a v e b e e n c o m m e r c i a l l y e m p l o y e d f o r s o m e y e a r s. Th e h e a tt rans fe r coe f fic ien t inc reases by up to 30 -100% fo r wi re co i l in se r t s [2 ] and up to 90% fo r twis tedtapes [3 ] . Th e f low obs t ruc t ions imp osed b y wi re co il in se rt s o f fe r po ten t ia l fou l ing and tube w a l le r o s i o n p r o b l e m . Tw i s t e d t a p e s o f f e r h i g h e r h e a t t r a n s f e r p e r f o r m a n c e p e r u n i t o f f r i c t i o n i nlaminar f low [4 ] .In tu rbu len t f low, in te rna l roug hness a nd in te rna l f in s can p ro v ide a g iven enhanceanen t l evelwi th smal le r p ressu re lo ss than tha t g iven by wi re co i l and twis ted tap inse r t s . Webb [5 ] p resen tsd a t a a n d d e s ig n r e c o m m e n d a t i o n s f o r t w o i m p o r t a n t i n t e rn a l r o u g h n e s s g e o m e t ri e s . I n t e r n a l lyf i n n ed t u b e s g iv e p e r f o r m a n c e s c o m p a r a b l e t o t h a t o f in t e r n a l r o u g h n e s s f o r m o d e r a t e P r a n d f ln u m b e r s ( P r = 3 ) in tu rbu len t w a te r f low [6 ] . Th e h ea t exc hange r ove ra l l hea t t r an s fe r coe f fic ien tt i m e s th e h e a t t r a n s f e r s u r f ac e a r e a m a y b e i n c r e a s e d 3 5 - 4 0 % f o r e q u a l p u m p i n g p o w e r a n d t o t a ltub ing len g th [6 ] . Th e in te rna l f in pe r fo r ma nce i s d ram at ica l ly impro ved i f the fins a re p rov ideda t a he l ica l ang le [6 -9 ]. G ee and W ebb [10] have a l so show n tha t two-d im ens iona l r ib roughn essapp l ied a t 45 -50 he lica l ang le is p re fe rab le to t ransve rse r ib roughne ss . M arn er a nd Berg les [11 ]showed tha t in te rna l f in s y ie ld h igher pe r fo rmance than twis ted tapes .Of the va r ious augmenta t ion techn iques , roughness and in te rna l f in s o f fe r the g rea tes t pe r -f o r m a n c e p o t e n t i a l p e r u n it m a t e r i a l , a n d p e r u n i t p u m p i n g p o w e r . H o w e v e r , h i g h m a n u f a c t u r i n gcos t and the ava i lab i l i ty o f low mate r ia l have l imi ted the i r u se [12 ] .M o s t o f t h e p r e v i o u s c o m m e r c i a l e f f or t s h a v e f o c u s e d o n t h e o u t e r s u r f a c e o f t h e t u b e . V e r y l it tl ehas been c lone to deve lop tubes ide enhancemen t , wi th the excep t ion o f in te rna l ly f inned tubes .In te rna l roughness o f fe r s h igh po ten t ia l fo r s ing le -phase fo rced convec t ion ins ide tubes . Doub lya u g m e n t e d t u b e s ( in s id e a n d o u t s i d e e n h a n c e m e n t ) r e q u i r e ex t e n si v e d e v e lo p m e n t e f f o rt s a n d o f f erh i g h p r o m i s e f o r a d v a n c e d h e a t e x c h a n g e r t e c h n o l o g y [1 2] .M a r n e r e t a l . [ 1 3 ] g iv e a c o m p r e h e n s i v e li st o f c o m m e r c i a l a u g m e n t e d t u b e s w h i c h m a y b ec o n s i d e r e d f o r u s e in s h e l l - a n d - tu b e e x c h a n g e r s, a l o n g w i t h a s u r v e y o f t h e p e r f o r m a n c e d a t a w h i c ha re ava i lab le in the l i t e ra tu re .M e n d e s a n d S p a r r o w [ 1 4 ] h a v e m a d e a p e r f o m i a n c e a n a l y si s c o m p a r i n g p e r i o d ic a l lyc o n v e r g i n g - d i v e r g i n g t u b e s a n d c o n v e n t i o n a l st r a ig h t t u b e s u s i n g t h e e x p e r i m e n t a l l y d e t e r m i n e dhea t t r an s fe r coe ff ic ien ts and f r ic t ion fac to rs a s inpu t . F o r eq ua l m ass f low ra te and equ a l t r ans fe rsu r face a rea , the re a re la rge enh ance me n ts o f the hea t t r an s fe r coe f fic ien t fo r pe r iod ic tubes , wi tha c c o m p a n y i n g l a rg e p r e ss u r e d ro p s . F o r e q u a l p u m p i n g p o w e r a n d e q u a l t r a n s f e r s u r f a c e a r e a ,e n h a n c e m e n t s in t h e 3 0 - 6 0 % r a n g e w e r e e n c o u n t e r e d . Tb e s c f in d i n gs in d i c a te t h a t p e r i o d icc o n v e r g i n g - d i v e r g i n g t u b e s p o s se s s f a v o r a b l e e n h a n c e m e n t c h a r a c te r i st i cs .S p a r r o w a n d Ch a b o k i [ 15 ] h a v e p e r f o r m e d e x p e r i m e n t s t o s t u d y t h e f l u id f lo w a n d h e a t t r a n s f e rcha rac te r i s t i c s fo r tu rbu len t a i r f low in a tube in wh ich th e re i s a deca y ing ax isymm ctr ic swir l. T heswirl gave r i se to subs tan t ia l h ea t t r ans fe r enhan cem en t in the in i ti a l po r t ion o f the tube .

J u n k h a n e t a l . [ 16 ] h a v e s u m m a r i z e d a n e x p e r i m e n t a l s t u d y o f t h r e e p o p u l a r " t u r b u l a t o r " i n se r tsfo r f i re tube bo i le r s . Two commerc ia l tu rbu la to rs , cons i s t ing o f na r row, th in me ta l s t r ip s ben t andtwis ted in z ig -zag fa sh ion to a l low pe r iod ic con tac t wi th the tube wa l l , d i sp layed 135 and 175%increases in hea t t r a ns fe r coe f fic ien t a t a R eyno lds num ber o f 10 ,000. A th i rd co mm erc ia ltu rbu la to r , c ons i s t ing o f a twis ted s tr ip wi th wid th s l igh tly le ss than tube d iam ete r , p rov ided a 65%

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Shel l -and-dimpled-tube hea t exchangers 30 1i n c r e a s e i n h e a t t r a n s f e r o e f f l ~ n t . T h e f ri ~t i6 fi a e t0 f 'i i ic r ea s es c c o m p a n y i n g t h e s e h e a t t r a n s f e rc oe f f i c ie n t n c r e a s e s a r e I 1 0 , I 0 0 0 , a n d 1 6 0 % , r e sp e ct i ve l y, o r t h e s a m e R e y n o l d s n u m b e r .

W e b b a n d B l a n c o [ 1 7 ] h a v e s t u d i e d e n h a n c e m e n t o f h e a t a n d m a s s t r a n s f e r b e t w e e n ac o u n t e r c u r r e n t , g r a v i t y - d r a i n e d , a t e r f i l m a n d a i r f l o w i n g i n a v e r ti c a l u b e . T h e e n h a n c e m e n tt e c h n i q u e e m p l o y e d i s o n e o f s p a c e d , t r a n s v e r s e i r e s p l a c e d i n t h e a i r b o u n d a r y l a y er , n e a r t h ea i r - w a t e r i n te r f a ce . 3 8 % e n h a n c e m e n t o f m a s s t r a n s f e r o e f f ic i en t a s b e e n o b t a i n e d w i t h t h ep r e f e r r e d e n h a n c e m e n t g e o m e t r y ( D e s i g n I ) .

A l t h o u g h p e r f o r m a n c e d a t a f o r a n u m b e r o f c o m m e r c i a l l y a v a i l a b l e n h a n c e d t u b e s h a v e b e e nr e p o r t e d i n t h e li t er a tu r e, d d i t i o n a l a t a a r e b a d l y n e e d e d i n s e v e r a l r e a s . I n p a r t ic u l a r, a t a f o rs h el l -s i de l o w a n d h e a t t r a n s f e r r e v e r y l i m i t e d , s p e c i a ll y o r p a r al l e l l o w w h e r e d a t a a r e v ir t ua l l yn o n e x i s t e n t [ 13 ].

T h e p r e s e n t w o r k a i m s t o c o m p a r e t h e e x p e r i m e n t a l p e r f o r m a n c e d a t a o f s h e l l - a n d - d i m p l e d - t u b eh e a t e x c h a n g e r w i t h t h a t o f a s h e l l - a n d - pl a i n -t u b e e a t e x c h a n g e r . T h e h e a t e x c h a n g e r p e r f o r m a n c ed a t a exam ined a re : the h ea t du ty , the overa l l hea t t rans fe r coe f f ic ien t , e f fec tiveness and tube , idef r ic t ion fac to r . The e xhau s t gases o f a D iese l eng ine i s u sed as the ho t f lu id passing ins ide the she lla n d t h e w a t e r i s u s e d a s t h e c o ld f l u id p a s sin g i n t h e t u b e . Th e c o m p a r i s o n b e tw e e n t h e tw o t y p e so f t h e s e h e a t e x c h a n g e r s is s t u d i e d u n d e r t h e c o n d i t i o n s o f c o u n t e r a n d p a r a ll e l f lo w . Th e r a t e o fw a s t e h e a t r e c o v e r e d f r o m th e D ie s e l e n g in e e x h a u s t g a s e s b y m e a n s o f t h e s h e l l - a n d - d im p le d - tu b eh e a t e x c h a n g e r is d e t e r m in e d a t a c o n s t a n t s p e e d te s t ( 15 00 r p m ) o n t h e D ie se l e n g in e f r o m n o l o a dt o m a x i m u m l o a d a t a c e r t a in t u b e R e y n o l d s n u m b e r 0 ( 8 8 7 5 .

T H E O R E T I C A L A N A L Y S I SF o r a s h e l l - a n d - tu b e h e a t e x c h a n g e r , t h e h e a t d u ty ( Q ) m a y b e c a l c u l a t e d b y

Q - - K , w C ~ ( t 2 - , t , ) -~ M t ( t 2 - t , ) ( l a )..-- R , C ~ ( T ~ - 7 2 ) - - - - M , ( T ~ - 7 " 2 ) ( I b )= U A L M T D . ( I c )

T h e h e a t f l u x p e r u n i t a r e a ( q ) t r a n s f e r r e d f r o m t h e e x h a u s t g a s e s t o t h e w a t e r i s g i v e n b yq = Q / A = U L ~ I T D . ( 2 )

E x c h a n g e r h e a t t r a n s f e r f f e c t i v e ne s s E ) a s d e f i n e d b y S i n g h [ 1 8 ] i s g i v e n b y= T , - T 2 / T , - t , . ( 3 )

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

E ~ = q / q p ( 4 a )E u = - U / U , ( 4 b )E ~ = ~ / ~ p . ( 4 c )

T h e t u b e ( w a t e r ) R e y n o l d s n u m b e r ( R e t ) i s g i v e n b yR e t = 4 m w / p . w F I D . ( 5 )

T h e d y n a m i c v i s c o s i t y f w a t e r ( ~ ) i s c a l c u l a t e d t t h e a r i t h m e t i c e a n b u l k t e m p e r a t u r e b e t w e e ni n le t a n d o u t le t .

F o r p i p e f l o w c a l c u l a t i o n s h e D a r c y - W e i s h b a c k e q u a t i o n i s u s e d t o g e t t h e f r ic t io n a c t o r ( f ) :V 2

f = h r / L 2g" (6)Th e b r a k e p o w e r d e v e lo p e d o n t h e e n g in e s h a f t i s g iv e n b y

2 F I N T b ( 7 )b P = 6 0 0 0 0

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302 V .H . MoR cos

Rotometer

Exho,~tT = iJ . l w m o . t T,

t ~ P ~ ~ t e r ShlKta nd t ub e ~ e x ~ rVo;.ve s ~ , ~ " ~ u~y

F ig . l ( a ). F lo w d i ag ram o f t e s t l o o p .

~.]Exhoustgores in

!2 0 0 : l = 2 0 0 . [ _ L = 2 0 0 = l " 2 0 0I

- f - - - - . - t- +Score 13, d imens io t ls in mm , ~ / D O=0 . 60 , L = I O00m m , t /L==0 . 20

Sect ion A-AF ig . 1 b ). D im p led t u b e co n f ig u ra t i o n an d d im en s io n s .

T h e p e r c e n t a g e o f w a s t e h e a t r e c o v e r e d ( % WHR), defined as the r a t i o o f t h e e x c h a n g e r h e a t d u t yt o t h e b r a k e p o w e r d e v e l o p e d b y t h e D i e s e l e n g i n e , i s g i v e n b y

% WHR ffi (Q/bp) 100. (S )E X P E R I M E N T A L P R O G R A M M E

Test apparatusF l o w d i a g r a m o f t es t l o o p a n d d i m p l e d t u b e c o n f i g u r a t i o n a r e s h o w n i n F ig . 1 . T h es h e l l- a n d - tu b e h e a t e x c h a n g e r i s m a d e o f m i l d s t ee l tu b e s ( s c h e d u l e n u m b e r 4 0 ) . T h e s h e ll a n d t u b einner d i am ete r s a r e 52 .5 . and 26 .64 r am, r e spec t ive ly , and the o u te r d i amete r s a r e 60 .33 and33 .4 mm , r e spec t ive ly . Th e excha nger he a t t r ans f e r l eng th (L ) i s 1000 mm . Th e ou t s id e su r f ace o fthe she l l i s we l l i n su l a t ed us ing g l ass woo l t o p r even t hea t l o ss t o su r round ing a tmosphere . Thee x h a u s t g a s e s o f t h e D i e se l e n g i n e p a s s i n t o t h e a n n u l u s a n d a w a t e r p u m p is u s e d t o p u m p t h ew a t e r i n t o t h e t u b e . A l l th e t e m p e r a t u r e s i n d i c a t e d o n F i g . l ( a ), a r e m e a s u r e d b y m e r c u r yt h e rm o m e t e r s h a v i n g a n a c c u r a c y o f + 0 . 2 0 % . T h e w a t e r m a s s fl o w r a te is m e u u r e d w i t h r o ta m e t e rt y p e S W 1 6.1 V E B M L W w i th m e a s u r i n g r a n g e o f 0 . 5 0 - 12 l m i n - s , a n d m e a s u r in g a c c u r a cy o f+ 2 . 5 % , r e fe r ri n g t o f in a l v a l u e o f g r a d u a t i o n c o n c e r n e d .

T h e m a s s f lo w r a te o f t h e e x h a u s t g a s e s is t h e s u m m a t i o n o f t h e a i r a n d f u e l m a s s f lo w ra t e s.T h e m a s s f l o w r a te o f a ir i s m e a s u r e d w i t h a n o r i fi c e m e t e r t y p e A D ( 6 0 m m d i a m e t e r ) m o u n t e do n t h e a i r b o x o f th e D i e s e l e n g in e . T h e m a s s f l o w r a t e o f t h e f u el is m e a s u r e d b y c o u n t i n g t h et im e r e q u i r e d t o c o n s u m e a c o n s t a n t m a s s o f D i e s e l f u e l e q u a l t o 0 . 04 2 k g . A l s o , th e m a s s f l o wra te o f t he exhaus t gases t imes the spec i fi c hea t (M , ) i s chec ked us ing equ a t ion ( lb ) .

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Shell-and-dim pled- tube heat exchangers 303Table I. Nom itml experinieatalonditimm

M, ( W K-")- I0 12.5 15 17.5T, (C) 85 110 155 180th. (kg m in-') 0.50-12tn (C) r 27Flow type Counter and parallel

20190

T h e s p e e d a n d t h e b r a k e to r q u e o f th e D i e s e l e n g i n e a re m e a s u r e d w i t h s p e e d o m e t e r a n d F r o u d ed y n a m o m e t e r , r e s p ec t iv e ly .

T h e f r i c t io n a l h e a d l o s s (h f ) f o r t h e tu b e l e n g t h ( L ) o f 1 0 0 0 m m i s m e a s u r e d w i t h U - t u b em a n o m e t e r .

T h e D i e s e l e n g i n e s p e c i f i c a t i o n s a r e : 4 - s t r o k e , 4 - c y l i n d e r , i n l i n e w i t h e n g i n e d i s p l a c e m e n t o f1 .76 1 .Data

D a t a a r e t a k e n f o r t h e p l a i n a n d d i m p l e d t u b e h e a t e x c h a n g e rs w i t h t h e e x p e r i m e n t a l c o n d i t i o n ss h o w n i n T a b l e 1 .Experimental results

F i g ures 2 -7 s ho w the m ea s ured v a l ues o f q , U a nd ~ v s R et fo r co unte r a nd pa ra ll el f l o w s . Ea chf i g ure s ho w s the ex per i m enta l re s u l t s o f the p l a i n a nd d i m pl ed tube hea t ex cha ng ers a t d i f f erent

1 2 = / a / ~ O _ 1 2

- j . . . - = ~ ~ )

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- d im p le d t u b e a ~ 6 ~ 2 0 _6... .. J M .IW K 't l

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IO0 2 4 6 8 I0

R e t ( x I 0 3 ) R e t ( x I O a )Fig. 2. Heat f lux vs tube R eynolds numb er for counter f low. Fig. 3. Heat f lux vs tube R eynolds num ber for paral le l flow.

FIRS 8/4.--m

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6 0 I I I I I0 2 4 6 8 I0R e ~ ( x I O s )

F i g . 4 . O v e r a l l h e a t t r a n s f e r c o e f f i c i e n t v s t u b e R e y n o l d sn u m b e r f o r c o u n t e r f l o w .

nO0

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P o m t t e t f l owp t Q I n t ube M= ( WK - I )

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/ , , ,12 0 e " " " " \ A 2 0

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R e ( x I 0 5 )F i g . 5 . O v e r a l l h e a t t r a n s f e r c o e f f i c i e n t v s t u b e R e y n o l d sn u m b e r f o r p a r a l l e l f l o w .

M ,. Figures 8 and 9 show the enhancement rat io E for the data o f Figs 2-7 for counter and paralle lflow , respectively. F igure 10 show s the tubeside friction facto r f vs Ret for the plain and d impledtubes, under isothermal con ditio ns o f water flow as stated in [13]. Figure 11 sho ws the heat dutyand % WH R o f the shell-and-dimpled-tube, cou nter-flo w heat exchanger vs bp at a constan t speedof 1500 rpm and an Ret o f 8875.

D I S C U S S I O NFigures 2 and 3 sho w that the heat f lux of the heat exchanger increases with the increase of Ret

and M, . For the same Ret and M, the heat flux of the dimpled tube heat exchanger is greater than

0 .8

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R e t ( x 1 0 56. E x c h a n g e r h e a t t r a n s f e r e f f e c t i v e n e s s v s t u b eR e y n o l d s n u m b e r f o r c o u n t e r f l o w .F i g .

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I I I 1 10 2 4 6 8 I0Re ,( x 103 )

E x c h a n g e r h e a t t r a n s f e r e f f e c t i v e n e s s v s t u b eR e y n o l d s n u m b e r f o r p a r a l l e l f l o w .

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S h e l l -an d -d imp led - tu b e h ea t e x c h ~ B e n i 305

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o . o I I I I , J I I J I ,o 2 4 e , o o 2 4 i o

T u b e ~ n u m b e r , R e t ( x I 0 3 )F i g. 8 . E n h a n c e m e n t r a ti o v s t u b e R e y n o l d s n u m b e r f o r c o u n t e r fl o w , a n d P r : 5 .5 . ( 0 F ~ , O F ~ ,xE , ) .

t h a t o f t h e p l a i n t u b e h e a t e x c h a n g e r f o r b o t h c o u n t e r a n d p a r a l l e l f l o w . F i g u r e 8 s h o w s t h a t t h ee n h a n c e m e n t r a t io E q f o r t h e c o u n t e r f l o w h e a t e x c h a n g e r i s a m a x i m u m a t R e , = 4 0 0 0 - 6 0 0 0 ,a n d i t s v a l u e in t h e r a n g e f r o m 1 .2 t o 1 .7 5 f o r t h e ra n g e o f M , e x a m i n e d . F i g u r e 9 s h o w s t h a t t h ee n h a n c e m e n t r a t io E q fo r t h e p a r a l l e l- f lo w h e a t e x c h a n g e r i s a m a x i m u m a t R e t = 6 0 0 0 - 8 0 0 0 ,

1.6

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I I I I I i0 2 4 6 I 0T u b e R e y n o ~ n u m b e r , R e t ( x l O ~ )

F i g . 9 . F - - n h a n c ~ r n e n t r a t i o v s t u b e R e y n o l d s n u m b e r f o r p a r a l l e l f l o w , a n d P r == 5 .5 . ( 0 ~ , C ) ~ ,a n d x E , ) .

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a nd i t s v a lue ra ng es f ro m 1 .0 5 t o 1 .3 0 f o r t he ra ng e o f M , ex a mined . Co mpa r ing F ig s 8 a nd 9 f o rt h e s a m e M , s h o w s t h a t t h e e n h a n c e m e n t r a t io E q o f t h e c o u n t e r f l o w h e a t e x c h a n g e r i s 8 -- 64 %g rea t er t ha n t ha t o f t he pa ra l l e l - f l o w hea t ex cha n# r .

Figures 4 and 5 show that the overal l heat transfer coef f ic ient increases with the increase of Ret ,a nd i t decreases a s M , increa ses fro m 1 0 t o 1 2 .5 W K - ~ a s sho w n by t he da sh ed l ine curv es, t heni t increases w i t h t he increa se o f M , f ro m 1 5 t o 2 0 W K - ' . Fo r t he sa m e Re, a nd M , t he o v era l l hea tt ra nsfer co e f f i c ient o f t he d im pled- t ube hea t ex cha ng er i s g rea ter t h a n t ha t o f t he p la in- t ube hea tex cha ng er f o r bo t h co unt er a nd pa ra l l e l f l o w. F ig ure 8 sho ws t ha t t he enha ncement ra t io E , f o rt he co unt er - f lo w hea t ex cha ng er i s in t he ra ng e o f 1 .3 7 - 1 .8 2 , whi l e f o r t he pa ra l l e l - f l o w hea te x c h a n g e r (F i g . 9 ) , t h e en h a n c e m e n t r a ti o E , = 1 . 2 0 -1 . 5 5 . C o m p a r i s o n o f F i g s 8 a n d 9 s h o w st h a t th e e n h a n c e m e n t r a t io E , o f t h e c o u n t e r - fl o w h e a t e x c h a n g e r is 2 0 - 3 7 % g re at er t h a n t h a t o ft h e p a r a ll e l- f lo w h e a t e x c h a n g e r a t t h e s a m e M , .

F ig ures 6 a n d 7 sh o w t ha t t he hea t ex cha n g er e ff ec tiveness increa ses wi t h increa se in Re , a n ddecreases w ith increase in M , due to increase in 7"1 . Fo r the sam e Ret an d M , the ef fect iveness o ft he d impled- t ube hea t ex cha ng er i s g rea t er t ha n t ha t o f t he p la in- t ube hea t ex cha ng er f o r bo t hco unt er a nd pa ra l l e l f l o w. F ig ure 8 sho ws t ha t , f o r t he co unt er - f lo w hea t ex cha ng er , t heenha ncem ent ra t io E ~ = 1 .2 2 - 1 .3 5 , wh i l e f o r t he pa ra l l e l -f l o w hea t ex ch a ng er ( F ig . 9 ) , t heenha ncem ent ra t io E c = 1 .1 - 1 .2 5 .

IO O i . o oR o t = 8 8 7 5C o u n t o r f l o w

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4 0 0 . 4 0

2 0 ~ o . , ~ . _ . _ 1 4 fH R % ) 0 2 0

I I I I I0 2 4 6 0 i0 12B r o k e p o w e r , b p ( k W )

F i g . I I . W a s t e h e a t r e c o v e r e d b y t h e s h e l l - a n d - d i m p l e d - t u b e h e a t e x c h a n g e r v s b r a k e p o w e r .

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Shell-and-dimpled-tube heat exchang ers 307F i g u r e 1 0 s h o w s t h a t t h e f r i c ti o n f a c t o r o f t h e d i m p l e d t u b e s i d e i s a p p r o x i m a t e l y 3 0 0 - 6 0 0 %

g r e a t e r t h a n t h a t o f th e p l a in t u b e f o r t h e r a n g e o f R e , e x a m i n e d . A s P e t i n c r e a s e s , t h e r a t i o o ff / f p i n c r e a s e s a n d t e n d s t o b e c o n s t a n t a t 7 0 0 % . A l s o , i t s h o u l d b e n o t e d i n F i g . 1 0 t h a t t h ef r i c ti o n a l t r a n s i t i o n f r o m l a m i n a r t o t u r b u l e n t f l o w is t y p i c a l l y m u c h s m o o t h e r w i t h a d i m p l e d t u b et h a n w i t h a p l a i n t u b e .

F i g u r e 11 s h o w s t h a t , f o r a c o n s t a n t s p e e d o f 1 5 00 r p m a n d c o n s t a n t R e t o f 8 8 7 5 , t h e h e a t d u t yo f t he d i m p l e d - t u b e h e a t e x c h a n g e r i n c r ea s e s w i th i n c r e as e i n b r a k e p o w e r f r o m n o l o a d t om a x i m u m l o a d . T h e p e r c e n t a g e o f w a s t e h e a t r e c o v e r e d b y t h e c o u n t e r- f lo w , s h e l l- a n d - di m p l e d -t u b e h e a t e x c h a n g e r i s a b o u t 1 0 % a t t h e m a x i m u m p o w e r l im i t o f t h e D i es e l e n g in e .

C O N C L U S I O N ST h e r e s e a r c h d e s c r i b e d h e r e c o n s t i t u t e s a c o m p r e h e n s i v e s t u d y o f t h e h e a t t r a n s f e r a n d f l u i d f l o w

c h a r a c t e r i s t ic s o f s h e ll - a n d - t u b e h e a t e x c h a n g e r s u s e d f o r w a s t e h e a t r e c o v e r y . D u r i n g t h e c o u r s eo f t h e w o r k , f o u r p a r a m e t e r s h a v e s y s t e m a t ic a l ly b e e n v a r i e d , in c l u d in g t h e t u b e s u r fa c e g e o m e t r y( p l a in o r d i m p l e d ) , t h e f l o w t y p e ( c o u n t e r o r p a r a l l e l ), t h e s h e l ls i d e h e a t c a p a c i t y r a t e ( M , , W K - i ) ,a n d t h e t u b e R e y n o l d s n u m b e r .T h e i n v e s t i g a t io n h a s s h o w n t h e f o l lo w i n g c o n c l u s io n s .

1 . T h e h e a t t r a n s f e r c h a r a c t e r i s t i c s i m p r o v e g r e a t l y w i t h i n c r e a s e i n t u b e R e y n o l d s n u m b e r a n dt h e s h e l l si d e h e a t c a p a c i t y r a t e . -

2 . T h e c o u n t e r - f l o w , s h e l l - a n d - d i m p l e d - t u b e h e a t e x c h a n g e r , c o m p a r e d t o t h a t w h i c h h a s a p l a int u b e , e n h a n c e s t h e h e a t f l ux , t h e o v e r a l l h e a t t r a n s f e r c o e f f ic i e n t a n d t h e e x c h a n g e r h e a t t r a n s f e re f f e c t i v e n e s s b y 7 5 , 82 a n d 3 5 % , r e s p e c t i v e l y .

3 . T h e p a r a l le l - f lo w , s h e l l - a n d - d i m p l e d - t u b e h e a t e x c h a n g e r , c o m p a r e d t o t h a t w h i c h h a s a p l a i nt u b e , e n h a n c e s t h e h e a t f l ux , t h e o v e r a l l h e a t t r a n s f e r c o e f f ic i e n t a n d t h e e x c h a n g e r h e a t t r a n s f e re f f e c t i v e n e s s b y 3 0 , 55 a n d 2 5 % , r e s p e c t i v e l y .

4 . T h e e n h a n c e m e n t o f t h e h e a t t r a n s f e r c h a ra c t e r is t ic s o f t h e d i m p l e d t u b e i s a t t h e e x p e n s e o ft h e t u b e s i d e f r i c ti o n f a c t o r . T h e d i m p l e d t u b e s i d e f r i c t i o n f a c t o r is 6 0 0 % g r e a t e r t h a n t h a t o f th ep l a i n t u b e a t t h e s a m e R e , .

5 . T h e h e a t d u t y o f t h e c o u n t e r - f l o w , s h e l l - a n d - d i m p l e d - t u b e h e a t e x c h a n g e r is e q u a l t o 1 0 % o ft h e m a x i m u m p o w e r d e v e l o p e d b y t h e D i e se l e ng i n e a t c o n s t a n t s p e e d o f 1 5 00 r p m a n d R e t o f 8 8 7 5.

R E F E R E N C E S1. A. E. Bargles and R. L. W ebb, Energy conservation via heat transfer enhancem ent, Energy 4, 193-200 (1979).2. P. Kum ar and R. L. Judd, H eat transfer with coiled wire turbulence promoters, Can. J . chem. Engng& 378-383 (1970).3. R. F. Lopina and A. E. Bergles, Heat transfer in tape ge nerated swirl flow of single-phase water, J. Heat Transfer 91 ,434-442 (1969).4. S. W. H ong and A. E. Bargles, Augm entation of laminar flow hea t transfer in tubes by mean s of twisted-tape,J . HeatTransfer 98, 251-2 56 (1976).5. R. L. W ebb, Tow ard a comm on understanding of the performance and selection of roughness for forced convection,in A Festschrift for E. R. G. Eckert (edited by J. P. H artnett, T. F. Irvine Jr., E. P fend er and E. M . Sparrow).Hemisphere, Washington, D .C. (1979).6. R. L. W ebb and M. J. Sc ott, A parametric analysis of the pe rformance of internally finned tubes for hea t exchangerapplication, J. Heat Transfer 102, 38--43 (1980).7. R. L. Webb and E. R. G . Eckert, Application of rough surfaces o heat exchangerdesign, Int . J. H eat and Mass Transfer1 5 , 1647-1658 (1972).8. G. W. F enner and E. R asi, Enha nced tube inner surface heat transfer device and method, US Patent 4, 154, 293, (15May 1979).9. T. C. C arnavos, Heat transfer performance of internally finned tubes in turbulent flow, in Advances in Enhanced Hea tTransfer (edited by J. M. Chenow eth, J. Kaellis, J. Michel and S. Shenkma n), pp. 61-68. ASM E, New York (1979).10. D . L. G ee and R. L. W ebb, Forced convection heat transfer in helically rib-roughened tubes, Int . J. Heat and MassTransfer 23, 1127-1136 (1980).11. W. J. Ma rncr and A. E. lk'rgles, Augm entation of tube-side laminar flow heat transfer by mean s of twisted tape inserts,stat ic mixers inserts, and internally finned tubes, J. Heat Transfer 2, 583-58 8 (1978).12. R. L. W ebb, special surface geometries or heat transfer augm entation, in Developments in Hea t Exchanger T echnology(edited by I. D. Chisholm), Chapter 7, pp. 179-215. Applied Science Publishers. Barking, Essex (1980).13. W . J. Marn er et al ., On the presentation of pe rformancedata for enhance d tubes used in shell-and-tubeheat exchange~,J. Heat Transfer 105, 358-3 65 (1983).14. P. So uz a Men des and E. M. Sparrow, Periodically converging--diverging ubes and their turbulent heat transfer,pressure drop, fluid flow, and enhancem ent characteristics,J. Heat. Transfer 106, 55-63 (1984).

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3 0 8 V .H . M O RC OS1 5. E . M . S p a r r o w a n d A . C h a b o k i , S w i r l- a f fe c t e d t u r b u l e n t f l u id fl o w a n d h e a t t r a n s f e r i n a c i r c u l a r t u b e , J. Heat Transfer106 , pp . 766-773 (1984) .1 6 . G . H . J u n k h a n et al., I n v e s t i g a t i o n o f t u r b u i a t o r s f o r f i r e t u b e b o i l e r s , J. Heat Tran.vfer 107, 354-360 (1985).1 7. R . L . W e b b a n d H . P e r c z -B l a n c o , E n h a n c e m e n t o f c o m b i n e d h e a t a n d m a s s t r a n s f e r i n a v e r ti c a l t u b e h e a t a n d m a s se x c h a n g e r , J. Heat. Transfer 108 , 70-75 (1986) .1 8 . P . S i n g h , S o m e f u n d a m e n t a l r e l a t i o n s h i p s f o r t u b u l a r h e a t e x c h a n g e r t h e r m a l p e r f o r m a n c e , J. Heat Transfer 103,573-578 (1981) .