Burley 1985 Aquacultural-Engineering

7/28/2019 Burley 1985 Aquacultural-Engineering

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Aquacu l tura l Eng ineer ing 4 (1985) 113-134

Flow Di s t r ibut ion Studies in F i sh Rear ing Tanks . Par t 2A n a l y s is o f H y d r a u l ic P e r f o r m a n c e o f l m S q u a r eT a n k sR. Bu rley and A . Klapsis

Department of Chemical and Process Engineering, Heriot-Watt University,Chambers Street, Edinburgh EH1 1HX, UK

A B S T R A C TT h e f i r s t p a r t o f t h is s t u d y o n t h e d eve l o p m en t o f f i s h r ea rin g ta n ks w a sconc erned wi th rev i ewing prev ious des igns and t ech n iques (K laps is andBur ley , 1984) . Fro m th is s tu dy i t was ma de c lear t ha t severa l ob / ec t i veshave to be s imu l taneous l y sa t i s fi ed or a t leas t a t t e m pte d w i th in sucha design and these have been clearly enumerated.In t he se cond par t o f t h is s tud y an inves tiga tion is repor t ed on a s imp lere-design of a square tank which of fers a 21% increase in useable surfacearea over an equ ivalent circular tank. W i th an asym m etrica l square tank, theax ia l sy m m et r y o f a circu lar t ank no longer app l ies and severa l imp or ta n tdesign changes need to be in troduced.

The des ign mod i f i ca t ions proposed and imp lemen ted in t h i s ease werefo u n d to g ive fa ir ly c on stan t f lu id veloci t ies a t a l l cross-sect ions, togethe rw ith e xc el le nt m ixing , sel f-cleaning an d aera tion characterist ics in theb o d y o f t h e f lu i d .

NOMENCLATUREDDsdEE ( O )L sQ , q

Length of square tank (1 m) (L)Dispersion coe ff ic ien t (cm 2 s -1) (L) 2 (T) -1Depth of water in tank (cm) (L)Exit age dis tribu tion (T) -tDimensionless exit age distributionCharacteristic length for hydraulic system (LIVolumetric flow rate (litres min-~) (L) 3 (T) -1

113Aquacu l tura l Eng ineer ing 0144-8609/85/$03.30 - Elsevier Applied SciencePublishers Ltd, England, 1985. Printed in Great Britain


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1 1 4 R . B u r l e y , A . K l a p s i s

t

l )1) s0P/ . t

TimeIdeal mean residence time (T)Calculated mean residence time (T)Velocity in tank (L) (T) -~Equivalent velocity for dispersion number (L) (T) -~? c / i = Dimensionless residence timeFluid density (M) (L) -3Fluid viscosity (M) (L) -1 (T) -~

INTRODUCTIONIn order to make the most efficient use of a given volume of fluid forfish rearing purposes it is necessary to understand that fluid deadvolumes, bypass flows and local recirculation need to be avoided.

This can be done by correct design of the fluid injection and drainagesystem which is particular to the tank shape and the rate of supply ofwater.

A study of the development of fish rearing tanks and ponds has beenmade (Klapsis and Burley, 1984), and this study was instigated afterobservation of the operation of several fish rearing tank facilitiespresently in operation at Heriot-Watt University. The principles ofoperation are quite general as far as the hydraulic characteristics of thesystem are concerned; however, the observations reported will need tobe applied intelligently in different fish rearing situations to maximisetheir benefit.

APPARATUS DESCRIPTIONThe tank under investigation was constructed from fibre-glass givinga smooth and inert surface to salt water. Figure 1 gives its essentialdetails and dimensions.

The flow inlets consisted of four submerged ~ in (9.5 ram) diameterPVC pipes, placed at each corner of the tank. These injected water fromseveral 3 mm diameter holes drilled at equal intervals along the inletpipe length. The depth of water was controlled by the height of theinternal standpipe which was fitted with a venturi bell-mouthed entranceso that turbulent effects were alleviated and steady exit vortex motionencouraged.


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F l o w d i s t r i b u t i o n s t u d i e s i n f i s h r e a r i n g t a n k s - 2 1 1 5

of %

I ~ " 1 0 0

PLANVIEW

FLUIDINJECTIONPIPE

4 B ~

SECTION VIEW

L F L u i d i n j e c t i o np ipe

A l l f i g u r e s i n cms.

F i g . 1. 1 m square tank.Around the standpipe a 22 cm diameter screen was placed. This had

eight 3-ram vertical slits equally spaced around its perimeter, eachhaving a height of 200 mm.

The screen sat on an annular disc, positioned on the lip of the conicalsump. The water was forced to pass through the slits and over the bell-mouthed standpipe. The space between screen and standpipe in con-junction with the conical sump acted as a sedimentation chamber.

One side o f one corne r o f the base of the tank was replaced by clearPerspex so that during the experi men tal work flow visualisationtechniques could be used and fish behaviour observed.

The sediment could be inte rmi tte ntl y siphoned off. The tank feeding,flow and flushing system was capable of total auto mati on.


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1 1 6 R . B u r l e y , A . K l a p s i s

W A T E RROTAMETER

0 L Sump T ~ Q n k l(11 ~

PUMPOVERFLOWPIPE

F ig . 2 .

12 )

l . B e r t J a ro r No .C to t u f i o n

A IR ROTAMETERA IRLINE

(,4.) DRAIN

---"~CONDUCTIVITYt. .-- .~DRAIN

CONDUCTIVITYETER ~ _ jCHA RTRECORDER

Diagram of tank showing two-way valve.

HYDRAULIC AND ANCILLARY SYSTEMThe tank was ere cte d on a steel frame. A ~ h.p. Beresford pump typeOV 41 was used to p ump the w ater from the 50-1itre sump tank to themain tank. At the out let o f the t ank a two-wa y valve (Fig. 2 (4)) wasplaced so that the water could ei ther flow to the drain or retur n to thesump tank. The closed system was desirable to cut down water runningcosts whilst velocity flow maps were being constructed.


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Fl ow dist ribu tion studies in fish rearing tanks - 2 1 17

T h e w a t e r f l o w t o t h e m a i n t a n k w a s c o n t r o l l e d b y a b y p a s s v a lv e ( 1 )a n d a f l o w v a lv e ( 2 ) w h il e t h e r at e w a s r e c o r d e d b y t h e R o t a m e t e r( t y p e F i s h e r 2 0 0 0 ) .

F i g u r e 2 a ls o s h o w s th e s y m m e t r i c a l p i p e w o r k e n s u r i n g a u n i f o r mf l o w r a t e t o e a c h i n j e c t i o n p i p e .

T h e s e t f l o w r a t e s u s e d f o r i n v e s ti g a ti n g t h e b e h a v i o u r o f t h e s y s t e mw e r e 8 , 1 0 , 1 2 a n d 1 4 l i tr e s m i n -1 a t t h r e e d i f f e r e n t t a n k w a t e r l e v e ls ,1 0 , 1 5 a n d 2 5 c m . T h i s g a v e f l u id v e l o c i t i e s o f a r o u n d 6 - 8 c m s - l: th e s ev a l u e s w e r e f o u n d t o b e a p p r o p r i a t e f r o m a ll p o i n t s o f v i e w in t hi s s t u d y .D i m e n s i o n l e s s r a t i o s o f v a r i a b le s a r e u s e d i n a n t i c i p a t i o n o f r e s i d e n c et i m e a n d m i x i n g s t u d i e s a n d t o e n a b l e t h e v a l u e s t o b e u s e f u l l y tr a n s-f e r r e d t o d i f f e r e n t s i z e d e q u i p m e n t o f s im i l ar g e o m e t r y b u t a l te r n a ti v es ca l e .

I N L E T I N J E C T I O N P IP E D E S I G NT h e o b j e c t i v e w a s to h a v e u n i f o r m f l o w d i s tr i b u t i o n f r o m e a c h i n j e c t i o no r i f ic e . T h i s w a s a c h i e v e d b y t h e c o r r e c t s iz i ng o f th e i n j e c t i o n h o l e s t og iv e a c c e p t a b l e h y d r a u l i c c h a r a c t e r i s t ic s a n d t h e c o r r e c t an g le o f f l u idi n j e c t i o n r e l a ti v e t o t h e w a l l t o g iv e e f f i c i e n t m i x i n g a n d s y m m e t r i c a lf l o w p a t t e rn s .

O n e s i m p l e a n d e f f i c i e n t w a y , w i d e l y u s e d in t h e c h e m i c a l i n d u s t r y ,is t o u s e p e r f o r a t e d d i s t r i b u t o r s . T h e c r i te r i a f o r t h e i r d e s ig n h a v e b e e ni n v e s t ig a t e d t h o r o u g h l y . S e n e c a l ( 1 9 5 7 ) r e p o r t e d t h a t th e r e a re t w oi m p o r t a n t r a ti o s: t h e r a ti o o f t h e k i n e t i c e n e r g y o f t h e in l e t st r e a m t ot h e p r e s s u r e d r o p a c r o s s t h e o u t l e t ; a n d t h e r a t i o o f t h e f r i c ti o n lo s s i nt h e p i p e t o t h e p r e s s u r e d r o p a c r o s s th e o u t l e t . F i g u r e 3 g iv e s a v i su a li n t e r p r e ta t i o n o f t h e p r o b l e m .I n t h is h y d r a u l i c s t u d y i n fi sh t a n k s t h e p r o b l e m w a s m u c h s i m p l e r ;t h e l e n g th o f t h e i n j e c ti o n p i p e w a s o n l y 3 6 c m a n d t h e n u m b e r o fi n j e c t io n h o l e s u s e d w a s e i t h e r f o u r o r fi ve d e p e n d i n g o n t h e d e p t h o fw a t e r u s e d , a s s h o w n i n F ig . 4 .

T h i s d e s i g n w a s f i r st t e s t e d b y f e e d i n g w a t e r f r o m t h e ta p w h i l s t t h ep i p e l ay i n a h o r i z o n t a l p o s i t i o n . I t w a s f o u n d t h a t t h e h e i g h t o f t h ej e t s w e r e e q u a l f o r e a c h i n j e c t i o n h o l e . I t w a s f u r t h e r t e s t e d w i t h allf o u r p i p e s c o n n e c t e d . A S t r e a m f l o v e l o c i t y m e t e r g a ve t h e i n s ta n t a -n e o u s d i sc h a r ge v e l o c i t y w h i c h w a s f o u n d t o b e t h e s a m e f o r e a c hi n j e c t i o n h o l e i r r e s p e c t i v e o f t h e i n j e c t i o n p i p e . T h e v e l o c i t y p r o b e


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1 1 8 R . B u r l e y , A . K l a p s is

F i g . 3 .

t t t t t tINLET PIPE IDEAL DISTRIBUTION, t , t t t t l

INLET PIPE MO ME NT UM ond KINETIC[NE ROY PREDOMINATE

T t t tf INLET PIPE t IFRICTION PREDOMINATESTypical fluid distribution (after Senecal , 1957).

W A T E Ri i , , " LEVEL

~r..

d/D= . 5F i g . 4 .

Al l f igures in cms.=

W A T E RJ L E V E L W A T E R

~ I "--- I ~ o ' ~ " LEVEL, --...

d / D = '15 d / D : 0 "10Arrangement o f injection orifice.

c o n s i s t e d o f a p r o p e l l e r w h i c h r o t a t e d i n p r o p o r t i o n t o t h e l o ca lv e l o c i t y . A s m a l l m a g n e t m o u n t e d in e a c h b l a d e , i n d u c e d a n o s c il la t in gc u r r e n t i n th e p r o b e w h i c h w a s a m p l i fi e d t o g iv e a v e l o c i t y r e ad i ng . T h eS t r e a m f l o h a d t w o r a ng e s w h i c h g a ve it li n ea r v e l o c i t y m e a s u r i n gc h a r a c t e r i s t i c s i n t h e r a n g e 6 - 3 0 0 c m s -1.

D E T E R M I N A T I O N O F S IZ E O F I N J E C T I O N H O L E S O N IN L E TP I P E S

A t ri al a n d e r r o r m e t h o d w a s u s e d h e r e b e c a u s e t h e j e t v e l o c i t y c o u l do n l y b e p r e d i c t e d w i t h d i s p r o p o r t i o n a t e e f f o r t b y t h e o re t ic a l m e a n s( D e N e v e r s , 1 9 7 3 ) a n d , i n a n y c a se , m o s t o f t h e k i n e ti c e n e r g y o f t h ei n j e c t e d f l u id w a s l o s t o n i m p a c t w i t h t h e b u l k o f t h e f l u id a l r e a d y i n


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F l o w d i s t r i b u t i o n s t u d i e s i n f i s h r e a ri n g t a n k s - 2 1 1 9

the tank. At first, 8-ram dia meter holes were used, however, for lowflowrates and high d i d ratios; this gave bulk fluid velocities which weretoo low. It was found that 3-ram diameter injection holes gave rise toacceptable velocities in the tank. The resulting velocity flow maps arepresented later.

DETERMINATION OF INJECTION ANGLESThe most important factor that affected the flow patterns in the tankwas the angle of water injection relative to the wall. At first the direc-tion of flow was set approximately parallel to the wall but most of thekinetic energy of the fluid was lost due to the fact that at steady-stateconditi ons the rotating fluid inside the tank tended to drive the jetsagainst the wall.

An angle of between 25 and 30 relative to the wall gave the bestresults, with the incoming fluid travelling parallel to the wall. When theangle was increased to 45 it was fou nd tha t the incoming fluid wasby-passing the bulk of the fluid, going straight to the outlet. In a similarstud y (Munch, 1969) it was foun d tha t in a fer ment atio n vessel thenozzle inj ection angle of 20 -30 gave the best effi ciency for a whirlpoolaction in the vessel when measured relative to a tangent at the wall ofthe circular tank.

OUTLET DESIGNMost of the tanks discussed previously (Klapsis and Burley, 1984) usea central drain as a water outlet. The usual configuration is a screen atthe centre coupled with an external or internal standpipe for regulatingwater depth. Parker (1980) discussed the use of improved externalstandpipe drain systems. Kinghorn (1982) used a tube inside a tank forcontrolling the water flow.

INFLUENCE OF OUTLET STANDPIPE DESIGNThe square tank used internal standpipes for regulating the waterdep th in this study (Fig. 5). The s tandpipes were measured to therequired height so th at wate r was forced to flow effect ively over


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1 2 0 R. Bur ley , A . K laps i s

I I C E N T R A LWATER I

~ Solidsbull.d-up[---I~ Fluid lowand sotids i c k - u p J

Fig. 5. Fluid flow and solids pick-up.

F ig . 6 .

AU f i g u r e s i n c m s .

Bell-mouthed standpipe development.

a s h a r p - e d g e d n o t c h i n a s p ir al m a n n e r . T h i s i n d u c e d t u r b u l e n t f l o wb e h a v i o u r i n si d e th e s u m p , w h i c h m e a n t t h a t s u s p e n d e d s o l id s a l r e a d yt r a p p e d in th e s u m p w e r e c a r r ie d u p w a r d s a n d o v e r t h e s t a n d p i p em o u t h ( F ig . 5 ). T o r e d u c e t u r b u l e n c e a n d i m p r o v e t h e o v e r f l o w a t t h er im , a b e l l - m o u t h e d s t a n d p i p e w a s i n t r o d u c e d ( F i g . 6) .

T h e i m p r o v e m e n t o n s u s p e n d e d s o li d s e p a r a t io n w a s q u i te e v i d e n ta n d , d e p e n d i n g o n d e p t h a n d f l o w r a t e u s e d , a l l o w e d t h e m t o s e t t l e i nt h e c e n t r a l s u m p a r e a .

I M P R O V E D S C R E E N D E S I G NA m o d i f i e d o u t l e t s c r e e n w a s d e s i g n e d to o p e r a t e in c o n j u n c t i o n w i t ht h e i n d u c e d v o r t e x f lu i d m o t i o n in t h e ta n k . F i g u r e 7 s h o w s t h ed e v e l o p m e n t f r o m r a d i a l s l i t s t o m a c h i n e d / f i l e d s l i t s .


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Fl ow dist ribu tion studies in fish rearing tanks - 2 1 21

F i g . 7 .

/,'Omm

D e v e l o p m e n t o f c e n t r a l sc r e e n c o rn e r s .


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122 R. Burley, A. KlapsisT h i s r e d e si g n l ed to a c o n s i d e r a b l e i m p r o v e m e n t i n t h e c le a n i ng

e f f i c ie n c y w i t h in t h e t a n k a n d m u c h o f t h e m a t e r ia l n o w p a s s e d t h r o u g ht h e a n g l e d s l o t s i n t o t h e c e n t r a l s e p a r a t i o n a n d s e t t l e m e n t w e ll .A s y p h o n t u b e w a s u s e d i n t e r m i t t e n t l y t o r e m o v e t h e c o l l e c t e d s o li ds ,b u t c o u l d e a s il y b e i n c o r p o r a t e d i n t o a m o r e p e r m a n e n t d e si gn .

E X P E R I M E N T A L M I X IN G S T U D IE SI n o r d e r t o a s se ss t h e m o d i f i c a t i o n s i n t r o d u c e d i n t h e in l e t a n d o u t l e td e si gn , f l o w v e l o c i t y m a p s w e r e p r o d u c e d s h o w i n g l o c a l v e l o c it ie s in s id et h e t a n k . T r a c e r s t u d i e s w e r e a l so c a r r ie d o u t t o a ss e ss m i x i n g p e r f o r m -a n c e. S o d i u m c h l o r i d e s o l u t i o n w a s in j e c t e d as a n i m p u l s e f u n c t i o ni n t o t h e s y s t e m a n d t h e o u t p u t r e s po n s e c u r ve w a s m e a s u r e d w i t ha c o n d u c t i v i t y m e t e r c o n n e c t e d t o a c h a r t r ec o r d e r . I n d e e d , t h e sep r o c e d u r e s w e r e a n e c e s sa r y p a r t o f t h e e x p e r i m e n t a n d m o d i f i c a t io nw h i c h w e r e c a rr i ed o u t s e q u e n t i a l l y u n t i l o p t i m u m d e s ig n w a s r e a c h e d .

T h e t a n k w a s a ls o t e s t e d w i t h d i f f e r e n t s t o c k i n g d e n s i t ie s o f e e ls a n da ir i n j e c t i o n v i a a c i r c u la r p o r o u s p i p e f r o m t h e b a s e o f t h e t a n k . T h ee f f e c t o f s t o c k i n g d e n s i t y o f e lv e rs ( t y p i c a l l y 2 5 - 3 0 c m l o n g ) o n a iri n j e c t io n w a s i n v e s t ig a t e d o n t h e d e g r e e o f m i x i n g i n t h e ta n k i n t e r m so f t h e m e a n r e s i d e n c e t i m e , e x i t a g e d i s t r i b u t io n a n d d is p e r s io n n u m b e r ,e t c . , a s is n o r m a l p r a c t i c e i n s u c h s y s t e m s ( L e v e n s p i e l , 1 9 6 6 ) .

R E S U L T S O F V E L O C I T Y S T U D I E SF l o w v e l o c i ty s t u d ie s w e r e p e r f o r m e d b y m e a s u ri n g t h e l o ca l h o r i z o n t a lv e l o c i t i e s i n t h e t a n k a t d i f f e r e n t d e p t h s , d , a n d r a d i a l p o s i t i o n s . F o rt h e diD r a t io s o f 0 . 1 0 a n d 0 . 1 5 , f o u r d i f f e r e n t f l o w r a t e s w e r e u s e d , i .e .8 , 1 0 , 1 2 a n d 1 4 l i t re s m i n - 1. A l s o , ford/D = 0 . 2 5 , f l o w r a t e s o f 1 0 a nd1 2 l it r e s m i n -~ w e r e u s e d ; w h e r e d i s t h e f l u i d d e p t h a n d D t h e t a n kl e n g t h .

A s e x p e c t e d , t h e h i g h e r t h e v o l u m e t r i c f l o w r a t e ( 1 2 l it re s m i n - 1) f o rt h e s a m e n u m b e r a n d s iz e o f i n j e c t io n o r if ic e s , t h e h ig h e r t h e v e lo c i t i e sin t h e t a n k w i ll b e . A h ig h e r f l ui d m o m e n t u m is b e i n g d is s i p a te d i n t ot h e s a m e h o l d - u p v o l u m e o f f lu i d i n t h e t a n k .A s t h e d i s t a n c e f r o m t h e c e n t r e d e c r e a s e s t h e r a d i c a l v e l o c i t i e s a l sod e c r e a s e ( F i g s 8 a n d 9 ) .


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Flo w distribution studies in fish rearing tanks - 2

REPRESEN TATION OF HOR IZONTAL VELOC ITIES IN CMISEC IN FRON T OF THEINJECTION PIPE S AT A FLOWRATE OF 10 L /M IN .A N G LE OF F L OW 30 d / D = e ' - 1 5

123

t l l t/ z z / / / / / / / / / / / / / / / / / ' / / / / / / . ' . ' z ' z ' / / / ' ,9"1 8"1 7"6 7"1 6"1 5"5 5"6 5"9 7-2 7"6 ~,'2 9"29"3 B-2 7-5 7 5"8 5"5 5 3 6 71 7"7 8. t~ 9 29 B'3 7.1 6"1 5.6 5- 5.3 5-8 6"3 7.2 ~'1 9.1Fig . 8. F lu id ve loc i t ies i n f ron t o f t he i n j ec t i on p ipes .R E P R E S E N T A T I O N O F H O R I Z O N T A L V E L OC I T I ES I N C M / SE E . B E H I N D T H EI N J E CT I O N P I P ES A T A F L O W R A T E O F 1 0 L / M I N .A N G L E O F F L O W 3 0 , d / D : g . 1 5

6-7 6"5 6.2 5,6 5" 5 2 S '2 5.6 6 3 6 7 7.17.1 6.6 6-5 s-6 s-~ ~.9 s.1 s.~ 5e 6-~, 6-7 6.9s-8 6 .3 6 .2 6 s .2 s.1 s.2 s.6 6.1 6 . 3 6 . ~ 6.9

Fig. 9 . Flu id veloci t ies beh ind the in ject io n p ipes .

L o o k i n g a t F ig . 9 it c a n b e d e d u c e d t h a t l o c a l v e l o c i t ie s d o n o tc h a n g e s i g n i f i c a n t ly t h r o u g h o u t t h e d e p t h o f th e t a n k f o r a s p e c i f i cr a d ia l p o s i t i o n , i .e . t h e v e l o c i t y a t t h e b o t t o m o f t h e t a n k is w i t h i n1 0% o f t h a t in t h e c e n t r e a n d a t th e t o p , i n d i c a t i n g a r e l a t iv e l y s m a l le n e r g y l os s d u e t o w a l l s u r f a c e f r i c t i o n d r a g .


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124 R. Bur ley , A . K laps i sD u r i n g t h e t i m e t h a t e lv e rs w e r e k e p t in t h e t a n k a u n i f o r m

d i s t r i b u t i o n w a s a c h i e v e d s h o w i n g t h a t t h e f l o w d i s t r i b u t io n s y s t e mw a s s u f f i c i e n t l y e f f i c i e n t i n p r o d u c i n g a n a c c e p t a b l y c o n s t a n t le v elo f fl u id v e l o c i t y t h r o u g h o u t t h e t a n k .

V e l o c i t y m a p s w i t h e lv e rs p r e s e n t w e r e v e r y d if f i c u lt t o o b t a i n ;i n e v i ta b l y t h e y s w a m a r o u n d t h e p r o p e l l e r o f t h e v e l o c i ty p r o b e , o rt o u c h e d i t, w h i c h l e d t o s o m e w h a t e r r a t ic r e a d in g s . A s m a l l d r a f t t u b ew a s p l a c e d a r o u n d t h e p r o p e l l e r t o t r y a n d a ll ev i at e t h e p r o b l e m b u tl it tl e s i g n i fi c a n t i m p r o v e m e n t w a s re c o r d e d . D i r e c t o b s e r v a t i o n int h i s c a s e s h o w e d g o o d e l v e r d i s t r i b u t i o n a t a ll s t o c k i n g d e n s i t i e s u s e d ,b u t i n s y s t e m s w h e r e t h e s t o c k i n g d e n s i t y i s l o w , f u r t h e r i n v e s t i g a t i o no f t h e f l u i d v e l o c i t y w o u l d b e a d v is a b le .

O n e m a y a s s u m e , h o w e v e r , t h a t a r e d u c t i o n i n l o c al f lu i d v e l o c i t yw o u l d o c c u r si n c e a m a s s o f w e ll d i s tr i b u t e d e lv e rs w o u l d t e n d t od is s i pa te t h e m o m e n t u m o f t h e o n c o m i n g f lo w .

A E R A T I O N O F T A N KW h e n a ir w a s i n t r o d u c e d v i a a p o r o u s p i p e i n t o t h e ta n k t h e r e w a s a ls on o a p p a r e n t r e d u c t i o n i n v e l o c it ie s . T h e p i p e w a s a t t a c h e d t o t h e b a s ei n t h e f o r m o f a c i r cl e , d i s t r i b u t i n g b u b b l e s o f ai r o v e r a l ar g e v o l u m e .T h i s m e t h o d o f air d i s t r ib u t i o n c o m p a r e d f a v o u r a b l y w i t h a n a ir s t o n ef r o m w h i c h b u b b l e s a re d i s t r i b u t e d f r o m a r e l at iv e l y s m a l l c o n c e n t r a t e dv o l u m e . T h e c i rc u l a r g e o m e t r y w a s c h o s e n t o b e . in c o n c e r t w i t h t h ei n d u c e d c i r c u l a r f l o w .

M I X I N G / T R A C E R S T U D I E ST h e r e q u i r e d m i x i n g i n t h e t a n k w a s a c h i e v e d b y t h e a c t i o n o f t h ec o m e r j e t s p r o d u c i n g a v o r t e x f l o w p a t te r n . F o s s e t t (1 9 5 1 ) d e s c r ib e dt h e a c t i o n o f j e t s i n t h e m i x i n g o f f lu i d s a n d s t a t e s t h a t t h is c a n b ec o m p a r e d v e r y fa v o u r a b l y w i t h a c e n t r a l p r o p e l l e r a g it a to r .

F o x a n d G e x ( 1 9 5 6 ) h a ve c o m p a r e d i n j e c ti o n j e ts w i t h m e c h a n i c a la g i t a to r s . T h e a i m o f t h e s e s t u d i e s w a s t o i n v e s t ig a t e t h e m i x i n g a n df l o w c h a r a c t e ri s ti c s o f t h e t a n k p r o d u c e d b y t h e j e t s , t o g e t h e r w i t h a ni n v e s t i g a t i o n o f t h e e f f e c t s t h a t s t o c k i n g d e n s i t y a n d a ir i n j e c t i o n h a v eo n t h e m . I n e s s e n ce , a p e r t u r b a t i o n i n t h e in l e t c o n c e n t r a t i o n w a s


13/22

Flo w d istribution studies in fish rearing tank s - 2 125caused by injecting salt solution. The outlet concentration from thetank was the n mon itor ed by use of a condu ctivi ty mete r and theresults interpreted using dimensionless concentration and time units.Levenspiel (1966) gives a clear acc oun t of the calcula tion of meanresidence time (MRT) from such data.

DISCUSSION OF RESULTSComparing the ratio f c / t (Table 1), it can be seen that for d /D = 0.10,at an y f lowrate, the ratio is ~ 1 meaning t hat some recirculation existswith the total elimination of dead areas. For d iD = 0.15 the ratio of{ c / f is between 0.86 and 0.93 depending on the flowrate. At lowerflowrates the dead areas increase due to velocity decrease comparedwith higher flowrates. For d iD = 0.25 the f c / f ratio is between 0.83and 0.79, i.e. there is an evident increase in dead areas compared withthe other d iD ratios. We may deduce that with the same flowrate andwith an increase in the volume of water in the tank, the dead areasincrease as well. This was to be expec ted as a similar flow of liquid hasto dissipate its momentum in a larger volume of water. A decrease inthe size of the injection holes for the higher d iD ratios would increasethe jet velocities, at the same flowrate, and increase the mixing, thusincreasing the tc/t ratio and reducing the dead areas.

Burrows and Chenoweth (1955) performed similar tracer studies forthree different ponds and fo und typically that:

T y p e o f p o n d f c /{Foster Lucas 0.83Circular 0.88Raceway 1.11

The above results show that Foster Lucas had more dead areas whencompared with the circular pond and that in the Raceway pond recircu-lation occurred since the f c / f r a t i o was above 1.

The range for the dispersion number* ( Ds / v Ls ) was between thevalues of 0.192 for d /D = 0.10 and 14 litres rain -~ and 0-156 ford /D = 0.25 and 10 litres rain -1. These two configur ati ons have, respec-tively, f e l t ratios of 0.13 and 0.75, the highest and lowest from thewhole range. These values of the dispersion numbers show a relatively* See Appendix for definition.


14/22

b,3

TABLE1

dD=O1owrae(rmin-1

8

1

1

1

dD=O1owrae(rmin-

8

1

1

1

dD=02owrae(rran-1

8

1

1

1

Ccaeme

14

94

79

66

red

me

tc(min

Idme

18

95

79

67

red

me

t(min

Roc

11

09

10

09

Dvume

-

09

-

19

Vd(e

Saddao94

63

50

44

s=X(c-~2n-1

Dmeoe

07

06

06

06

saddao

sP

Dsponmb

01

01

DsVLs

14

17

18

12

08

08

19

16

17

85

06

06

01

01

01

01

14

94

28

19

15

13

18

11

26

27

17

19

08

09

08

07

11

99

43

53

65

62

16

11

06

06

06

06

01

01

01

01

07

07

52

48

12

91

06

06

01

01


15/22

Flow distribution studies in fish rearing tanks - 2 127h ig h d e g r ee o f m i x in g . D u p l ic a t e t r a c e r s t u d ie s w e r e p e r f o r m e d u n d e rt h e s a m e c o n d i t i o n s a n d t h e c o n d u c t i v i t y v e r s u s t im e c u rv e s w e r ep r o d u c e d , w i t h t h e e x c e p t i o n o f d/D - - 0 - 2 5 , a t a l o w f l o w o f 8 l i tr e sm i n -~ , w h i c h s h o w e d t h a t g e n e r a l ly s t a b l e f l o w p a t t e r n s e x i st . F o rd/D = 0 . 1 0 a n d d/D = 0 . 1 5 , t h e m o s t c o m m o n l y u s e d ra t i o s, a n d w i t hf l o w r a t e s 1 0 , 1 2 a n d 1 4 l it r e s m i n -1 , t h e h y d r a u l i c c h a r a c t e r i s t i c s o f t h et a n k w e r e c o m p a r e d w h e n o t h e r c o n d i t i o n s p r e v a i l e d . T h e s e i n c l u d e d :( i) l o w s t o c k i n g d e n s i t y o f e l v e r s ; ( ii ) h i g h s t o c k i n g d e n s i t y o f e l v e rs ;( ii i) i n j e c t i o n o f a ir ; a n d ( i v ) i n j e c t i o n o f ai r a n d h i gh s t o c k i n g d e n s i t yo f e l v er s s i m u l t a n e o u s l y .

E F F E C T O F E L V E R S O N M I X I N GL o o k i n g a t T a b l e 2 , w h e r e t h e r e s u l ts o f m i x i n g s tu d i e s f o r diD = O. 10a n d 1 0 l i tr e s m i n -~ a r e p r e s e n t e d , i t c a n b e s e e n t h a t t h e c a l c u l a t e dm e a n r e s i d e n c e t i m e o f t h e f lu i d i n t h e t a n k i n c r e a s e d w h e n t h e e lv e rsw e r e i n t r o d u c e d a n d t h e h i g h e r t h e s t o c k i n g d e n s i t y t h e l ar ge r t h ei n cr e a se . T h i s i n c r e as e d e p e n d e d u p o n s e v e r a l f a c t o rs : ( 1 ) w h e t h e r t h ee l v e rs a t t h e t i m e o f t h e t r a c e r s t u d i e s w e r e a c t i v e o r n o t , i .e . p r i o r t o o ra f t e r fe e d in g ; ( 2 ) w h e t h e r t h e y w e r e e v e n l y d i s t r i b u t e d t h r o u g h o u t t h ev o l u m e ; ( 3 ) w h a t s iz e t h e e lv e rs w e r e , g i ve n t h e y w e r e w e l l d i s t r i b u t e da n d q u i e s c e n t . I t m u s t b e e m p h a s i s e d h e r e th a t w i t h a d i f f e r e n t s p e c ie so f f is h , e v e n w i t h t h e s a m e s t o c k i n g d e n s i t y , t h e h y d r a u l i c c h ar a c -t e r is t ic s c o u l d b e d i f f e r e n t , f o r e x a m p l e , w i t h f l a t f is h .

F u r t h e r m o r e , i n r e s p o n s e c u r v e s t u d i e s o f t h is s o r t it is n o t d i r e c t lyp o s s i b l e t o c o m p a r e e x a c t l y s i t u a t io n s o f s e v er a l s t o c k i n g d e n s i t ie so w i n g t o t h e f a c t t h a t f is h d i s p l a c e a c e r t a i n v o l u m e o f f lu i d . T h u s , t h eh i g h e r t h e s t o c k i n g d e n s i t y t h e l o w e r t h e v o l u m e o f f r ee fl u id a v ai la b lea n d in g e n e ra l t h e s h o r t e r t h e m e a n r e s i d e n c e t im e . T h is o f c o u r s ew o u l d b e m o d i f i e d b y o t h e r f a c t o r s s u c h a s t h e s t a te o f a c ti v i ty ,m e t h o d o f f lu i d d e l iv e r y , sh a p e o f fi sh , e t c ., a s n o t e d a b o v e .

A s e x p e c t e d , t h e g e n e r a l t r e n d f o u n d w a s t h a t w i t h in c r e a s e ins t o c k i n g d e n s i t y , a n i n c re a s e i n t h e m e a n r e s i d e n c e t im e o c c u r r e d . T h ed i sp e r si o n n u m b e r w a s f o u n d t o i n cr e as e w i th t h e i n t r o d u c t i o n o fe lv e rs . T h i s w a s e x p e c t e d s in c e t h e r a n d o m s w i m m i n g a c t i o n w o u l di n c re a s e t h e d i sp e r s iv e m i x i n g o f t r a c e r f lu i d (s e e A p p e n d i x ) .

I n F i g. 1 0 , t h e e x i t a g e d i s t r i b u t i o n is p l o t t e d a g a i n st 0 ( d i m e n s i o n l e s st im e ) , w h e r e t h e e x i t ag e d i s t r i b u t i o n r e p r e s e n t s t h e v a r ia t io n o f


16/22

1 2 8 R. Burley, A. KlapsisT A B L E 2

d iD = 0 .10 , f l ow ra te = 1 0 l itres m in -1E m p t y 2 .1 kg o f e lver s, 7 5 kg o f e lver s,

t a n k s t o ck i n g d en s i t y s t o ck i n g d en s i t y2 2 .1 k g m -a 78. 9 kg m -3

T a n k w i th T a n k w it hair 7.5 kg

injec t ion elvers +air

in ject ionCalc ula ted m ean 9-46 10.37 11-11 9-71 11.41

re s idence t imet c ( m i n )Idea l m ean 9 .5 9 .5 9 .5 9 .5 9 .5

res idencet i m e a s s u m i n gw e l l d i s t r i b u t e dqu ie scen t f i sh ,? ( m i n )

Ra t io t -c / t 0 .99 1 .09 1 .16 1-02 1 .2D e a d v o l u m e , 0 . 9 5 - - -Vd ( l i t res )S tan da rd 6 .34 6 .59 7 .48 6 -81 7 .35

d e v i a t i o n ,s = : C ( c - ~ ) 2 / n - 1

Dim ens ion le s s 0 .67 0 .64 0 .67 0 .7 0 .64s t a n d a r dd e v i a t i o n , s / i 2

Dispe rs ion 0 -18 0 .16 0 .18 0 -19 0 .17n u m b e r , Dsv /Ls

d i m e n s i o n l e s s c o n c e n t r a t i o n o f t r a c e r f l u id i n th e e x i t s t r e a m - E c u rv e s.T h e s e c u r v e s a re d i r e c t ly r e l a t e d to t h e h y d r a u l i c m i x i n g c h a r a c te r i s t i c so f t h e t a n k u n d e r g i v en c o n d i t io n s . C o m p a r i n g t h e se c u r v e s i t i s e v i d e n tt h a t w i t h a n i n c re a s e i n th e s t o c k i n g d e n s i t y t h e p e a k s o f t h e E c u r v esa re r e d u c e d s h o w i n g a n i n c re a s e in th e d e g r e e o f m i x i n g . T h e s a m et r e n d o c c u r s w h e n a ir is i n t r o d u c e d i n t o t h e s y s t e m v i a t h e c ir c u l a rp o r o u s t u b e .


17/22

Fl ow distrib ution studies in fish rearing tanks - 2d i D = 0 " 1 F l ow r a t e = 1 / , I /r a i n .

129

0"1

i---m

b -X J

6

+ E m pt y t an k[ ] Tank wi th 2.1kg of eels Tank wi th 7.5kg of eets0 Tank wi th ai r in ject ion* Tank w, th ai r in jec t ion

DIMENSIONLESS TIME,t/t,(O)Fig. IO. Exit age distribution versus 0 (= fo/t-).

DISSOLVED OXYGEN STUDIESThe main aims of these studies were to investigate dissolved oxygen(DO) concentration variations within the tank so that the existence, ifany, of areas of low oxygen concentration could be identified, and toevaluate the introduction of air in the tank by the porous tubingreferred to previously.

The tube was placed on the bottom of the tank at a distance of10 cm from the screen. The 10 cm distance was det ermine d fromprevious studies which showed that dead areas usually occurred at thatposition (Klapsis and Burley, 1984).

OXYGEN STUDIES WITH LOW STOCKING DENSITYWhen 2.1 kg of elvers were kept in the tank the dissolved oxygen valueswere maintained at a relatively high percentage saturation without theneed for injecting air from the porous pipe. This was achieved by


18/22

130 R. Bur ley , A . K laps i sintroducing fresh water daily, extra oxygenation occurring at the watersurface as fluid entered from the bypass and the main tank outlet pipe(see Fig. 2) in the sump tank. The dissolved oxygen con centr ati on wasmeasured at mid-depth in the tank.

The results shown in Fig. 11 give values of dissolved ox ygen in ppmor mg litre -] in the tank afte r 24-h use of the same recirculating water.Near the injection pipes the DO concentrations were a little higher thanthose nearer the screen, which is likely to be due to the water whicharrives from the sump tank undergoing extra aeration provided by thesplashing action on entry.

It was f ound that wit hou t the porous tube injecting air, the DOconcentration in the tank was reduced, after a 24-h operation, to 6.3ppm by comparison with the porous tube injecting air in which theconc entr atio n was mai ntain ed around 8-2 ppm. A roun d the porouspipe an increase in the dissolved oxygen concentration was evident, asshown in Fig. 12. Even greater improvement could have been achievedif the porous tubing had been placed at a distance of 15-20 cm fromthe screen since the quantity of air released would have been greaterand the oxyge nate d water w ould have remained longer in the tankbefore reaching the outlet. Introduction of air showed that secondary

REPRESENTATION OF DISSOLV ED OXY GEN CONCENTRATION IN MG /LAT MID DEPTH, d/D =0 "IO , FLOWRATE 10L /H IN .STOCKING DENSITY 2 2"1 KS /M 3

9-3 9.1 9"2

9"2 9"191

,; 9-2 92 9"~. 2 9 . 1 9 - I flI

9 " I ~ . 9 . 1~',, ,, 9. 1~ ~ /9-1 9"2

9"2 9"2 9"2~ , O 9 - 3

Fig. 11. Dissolved oxygen concentration at mid depth - low stocking density.


19/22

Fl ow distribution studies in fish rearing tanks - 2 131REPRESENTATION OF DISSOLV ED OXYGE N CONCENTRATION IN H fi /LAT HID DEPTH, d/D =0 .10 , FLOWRATE 10 L /H INAIR FLOWRATE 3 LIT/H IN, STOCKING DENSITY 7B -9 Kf i /H 3

7" 9 B ' 2 B

B.3 B - 2} . " ~ \I

#B-1 8"1 ~,'5 8-3 B l|, M _ J /

8-3 8- 281 8"4 8"2

8-1

Fig. 12. Dissolved oxygen concentration at mid depth - high stocking density.mot ion produ ced in the fluid increased the mixing efficiency in thetank, and maintained an acceptable oxygen level.

SELF-CLEANING EFFICIENCYThe cleaning efficiency was tested under real conditions for twomonths. For the first month 2-1 kg of elvers were kept in the tank andfor the second month this was increased to 7.5 kg. When experimentalruns were not being carried out the system was kept at a flowrate of10 litres min -1 with d i d ratios of 0.10 and 0.15.

The screen/standpipe configuration worked extremely well collectingall the uneaten food and detritus material. The solids on the bottomwere caused to roll along to the central outlet by the circular flowpattern induced by the water injection scheme. The suspended particlesat different depths rotated around the screen and were eventuallycollected in the sump tank via the redesigned slits (see Fig. 7(c)) runningdown the whole length of the central screen.When the air injection pipe was introduced in the system there wasno appare nt reducti on or improv emen t of the cleaning efficiency. Theinjection pipe had to be supported with four small plastic supports so


20/22

132 R. Burley, A. Klapsist h a t a g a p o f 3 - 4 m m w a s l e f t f r o m t h e b a s e o f t h e ta n k , la rg e e n o u g hf o r f o o d p a r ti c l e s a n d d e t r i t u s t o b e s w e p t t o w a r d s t h e o u t l e t .

F o r a v a l u e o f d i D = 0 - 2 5 , t h e c l ea n i n g e f f i c i e n c y o f th e t a n k w a st e s t e d w i t h f o o d p a r t ic l e s o n l y . A g a in , t h e cl e a n in g e f f i c i e n c y a p p e a r e dg o o d b u t t h is t i m e t h e p a r t ic l e s t o o k l o n g e r t o re a c h t h e o u t l e t a s t h ew a t e r v e l o c i t ie s i n th e t a n k w e r e v e r y l o w , s u i t a b l e f o r v e r y s m a ll f r y.I f f r y w e r e t o b e u s e d w i t h t h e p r e s e n t a r r a n g e m e n t t h e sl it w i d t h o ft h e s c r e e n w o u l d b e r e d u c e d b u t t h e i r n u m b e r s i n c r e a s e d .

I t h a s b e e n s t a t e d t h a t a m i n i m u m v e l o c i t y o f 6 c m s -1 s h o u l d b eu s e d f o r se l f -c le a n in g a c t io n ( B u r r o w s a n d C h e n o w e t h , 1 9 7 0 ) b u t f o o dp a r t ic l e s iz e a n d w e i g h t m u s t b e t a k e n i n t o a c c o u n t . W i th s m a l l f o o dp a r t ic l e s t h e v e l o c i t y c o u l d b e r e d u c e d t o a r o u n d 4 c m s -1 w i t h o u t a n ya p p a r e n t r e d u c t i o n i n c le a n in g e f f i c i e n c y . T h e r e m o v a l o f t h e s o li d s int h e s u m p t a n k w a s c a r ri e d o u t u s in g a n i n t e r m i t t e n t s y p h o n . A ll t h es u s p e n d e d s o li ds a r o u n d t h e h o le w e r e r e m o v e d b u t t h e in d u c e d c u r r e n tf r o m t h is a c t i o n , in s id e t h e s u m p t a n k , w a s n o t s u f f ic i e n t t o c a r r y a w a yt h e r e st o f t h e s u s p e n d e d s o l id s t h a t w e r e d e p o s i t e d a w a y f r o m t h eh o l e ; th i s p a r t o f th e s y s t e m is c a p a b l e o f f u r t h e r i m p r o v e m e n t .

H o w e v e r , a s w i t h a ll p r o c e s s e q u i p m e n t , t h e l aw o f d i m i n is h in gr e t u r n s s u g g e s ts t h a t t h e r e w i ll b e a n e c o n o m i c l i m i t t o t h e d e s ig nc o m p l e x i t y p o s s i b l e a n d w o r t h w h i l e .

C O N C L U S I O N SS t u d y o f t h e f l u id m i x i n g a n d f l o w b e h a v i o u r o f t h is t a n k r e v e a ls t h a t :

( i) T h e i n t r o d u c t i o n o f a n a p p r o p r i a t e i n le t d e si gn g a ve e a s y c o n t r o lo f v e l o c i t i e s w i t h i n t h e t a n k , t o su i t p a r t i c u l a r c o n d i t i o n s .

( ii ) T h e s y m m e t r i c a l i n t r o d u c t i o n o f w a t e r p r o v i d e d a u n i f o r mf l o w o f f lu i d , a n d e l v er s w e r e f o u n d t o d i s t r ib u t e t h e m s e l v e su n i f o r m l y i n t h e s y s t e m .

( ii i ) T h e m i x i n g e f f i c i e n c y f r o m t h e tr a c e r s t u d i e s s h o w e d t h a t d e a da r ea s w e r e m i n i m i s e d , w h i c h w e r e o t h e r w i s e i n c r e a se d b y i nc r ea s -i ng t h e v o l u m e o f w a t e r in t h e t a n k w h i l s t k e e p i n g t h e s a m ef l o w r a t e .

( iv ) M i x in g r a t e s w e r e i n c r e a s e d b y t h e s w i m m i n g a c t i o n o f t h ee l v er s ; t h e h i g h e r t h e s t o c k i n g d e n s i t y , t h e m o r e e f f i c i e n t t h em i x i n g .( v ) T h e e x t r a a e r a t i o n g iv e n b y i n j e c t i o n o f a ir v i a a c i r c u l a r p o r o u st u b e o n t h e b o t t o m o f th e t a n k g a ve a n i n c re a s e i n m i x i n gw i t h o u t r e d u c i n g v e l o c i t i e s .


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Flo w d i s t r ibu t ion s tud ie s in fi sh r ear ing tank s - 2 133( vi ) T h e v o r t e x f l o w p a t t e r n t h a t d e v e l o p e d in t h e t a n k p r o v i d e d

a u n i f o r m d i s t r i b u t i o n o f d i s s o lv e d o x y g e n a n d i n c r e a s e d t h es e lf - cl e a n in g a c t i o n w i t h t h e i n t r o d u c t i o n o f t h e s c r e e n / s t a n d -p i p e a n d s u m p a r r a n g e m e n t .

( v ii ) T h e r e d e s i g n e d c e n t r a l s c r e e n a n d s ta n d p i p e a r r a n g e m e n t a l l o w e dt h e c a p t u r e o f s o l id m a t e r i a l s w h i l st a c t in g a s a s e d i m e n t a t i o nt a n k .

I t h a s b e e n s h o w n t h a t t h e a p p l i c a t i o n o f s im p l e p r o c e s s e n g i n e e ri n gp r i n c ip l e s t o g e t h e r w i t h a k n o w l e d g e o f fl u id m e c h a n i c s c a n l e a d t os ig n i fi c a nt i m p r o v e m e n t in th e p r o c e s s e q u i p m e n t o f i n te n s i v e fi shr e a ri n g . T h i s s t u d y i s o n e o f s e v e r a l b e i n g c a r r ie d o u t i n t h e H e r i o t - W a t tU n i v e r s i ty ' s D e p a r t m e n t o f C h e m i c a l a n d P r o c e s s E n g i n e e r in g a lo n gt h o s e l i n e s .

A C K N O W L E D G E M E N T ST h e a u t h o r s a re i n d e b t e d t o H e r i o t - W a t t U n i v e r s i t y w h o s u p p l ie d f u n d si n s u p p o r t o f t h is w o r k , t o M r D . K l a ps is , w h o p r o v i d e d M r A . K l a p s is 'b u r s a r y , a n d t o M e s s rs A . R . O s b o r n e a n d B . T . L i n f o o t , D e p a r t m e n t o fC iv il E n g i n e e r in g , w h o s e r e p o r t p r o v i d e d s o m e o f th e s t i m u l u s f o r t hi ss t u d y .

A P P E N D I X : D I S P E R S I O N N U M B E RF l u i d f l o w m a y b e d iv i d e d , f o r t h e p u r p o s e s o f m i x i n g s t u d ie s , i n t od i s p e rs i v e a n d p i s t o n o r p l u g f l o w s . T h e l a t t e r i s o f t e n r e f e r r e d t o a sb u l k f l o w a n d r e p r e s e n t s t h e g e n e ra l o v e ra ll m o v e m e n t o f f lu i d ina g i v en d i r e c t i o n . O n t h e o t h e r h a n d , d i s p e r si v e f l o w is a m e a s u r e o ft h e r a n d o m b e h a v i o u r o f th e f l u i d w h i c h is s u p e r i m p o s e d u p o n t h e b u lkf l o w a n d i s d u e t o m a n y c a u s es . T h e s e m a y i n c l u d e t u r b u l e n t e f f e c t s ,a g i t a t i o n , t h e p r e s e n c e o f s o li d m a t e r i a l a n d d i r e c t io n a l f l o w s , f o re x a m p l e . T h e d i sp e r si o n n u m b e r w il l b e a f u n c t i o n o f t h e R e y n o l d sn u m b e r f o r a g iv e n f lo w s y s t e m .

T h e d i s p e r si o n n u m b e r is a m e a s u r e o f t h e r a ti o o f d i sp e r si v e o rr a n d o m f l o w s t o t h e b u l k f l o w . T h e d i sp e r si v e f l o w is r e p r e s e n t e d b yt h e d i s p e rs i o n c o e f f i c i e n t , D s , a n d t h e b u l k f l o w b y t h e p r o d u c t o f t h ev e l o c i t y , v , a n d a c h a r a c t e r is t i c d i m e n s i o n , o f th e s y s t e m L s . T h u s ,D s / v L s is z e r o w h e n b u l k f l o w h a s a v e r y h i g h v a l u e a n d in f i n i t e w h e n


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134 R. Burley, A. Klapsisbulk flow has a low value. By c ontrast the second dimensionless groupkno wn as the Reyno lds numb er gives a measure of the ratio of fluidmomentum to viscous (retarding) forces, and in the case of flow ina tank would be characterised by the group L s v p / t z . Levenspiel (1966)gives an excellent a ccoun t of flow and mixing.

REFERENCESAcrivos, A., Babock, B. P. & Pigford, R. L. (1959). Flow distribution in manifolds.

Chem. Eng. Science, 10, 112-21.Burrows, R. & Chenoweth, H. (1955). Evaluation of three types of fish rearingponds. US Fish and Wildlife Service, Records Report 39.Burrows, R. & Chenoweth, H. (1970). The rectangular circulating rearing pond.Progressive Fish Culturist, April, 67-81.De Nevers, N. (1973). Bernoulli's equation with friction. Chemical EngineeringEduca t ion , Summer, 126-8.

Fossett, H. (1951). The action of free jets in the mixing of fluids. Trans. Inst. Chem .Eng . , 29,322-32.Fox, E. & Gex, V. (1956). Single-phase blending of liquids. A I C h E , 2 (4), 539-44.

Greskovich, E. J. & Obara, J. T. (1968). Perforated pipe distributors. 1 and ECProcess Design an d Deve lopm ent , 7 (4).Kinghorn, B. (1982). Water flow controller. Progressive Fish Culturist, January,48-9.Klapsis, A. & Burley, R. (1984). Flow distribution studies in fish rearing tanks.

Part 1 - Design constraints. Aquacultural Engineering, 3, 103-18.Levenspiel, O. (1966). Chem. Reaction Engineering, Wiley, New York, chapter 9.Munch, H. (1969). Practical aspects of the whirlpool. Brewers G uardian, November,46-8.Osborne, A. R. (1981). Final Year Re po rt , Dept. of Civil Engineering, Heriot-WattUniversity, Edinburgh. Supervisor B.T. Linfoot.Parker, N. (1980). External standpipe drain system for fish tanks. Progressive FishCulturist, 42 (1), 53-4.Senecal, V. E. (1957). Fluid distribution in process equipment. Industrial andEngineering Chemistry, 49 (6), 993-7.Van Der Hegge, Jizneu, B. C. (1951). Flow through uniformly tapped pipes .Appl iedScience Research, A3, 144-62.

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Burley 1985 Aquacultural-Engineering