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Minerals Engineer ing Vol. 5 No. 7 pp. 779-794 1992 Pergamon Press

p l

Printed in Great Britain

T H E L E A K A G E M E C H A N IS M S I N T H E H Y D R O C Y C L O N E

L . M I L I N K . T . H S I E H a n d R . K . RAJAMANI

Comminution Center Universi ty of Utah Salt Lake City UT 84112 U.S.A.

( R e c e i v e d 1 9 N o v e m b e r 1 9 9 1 ; a c c e p te d 2 D e c e m b e r 1 9 9 1 )

A B S T R A C T

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

c l a s s i f i c a t io n , a r e c o m m o n l y a t tr i b u te d to t w o f l o w m e c h a n i s m s : t h e s h o r t -

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

e n t r a i n e d i n t h e s p i g o t d i s c h a r g e s tr e a m , b z t h e p r e s e n t w o r k a n a d d i t i o n a l

f l o w , t e r m e d s e c o n d a r y f l o w , i s i d e n t i f i e d . I t s in f l u e n c e o n t h e h y d r o c y c l o n e s

e f f i c i e n c y c a n b e d e s c r i b e d a s f o l l o w s : i t c a r r ie s c l a s s i f i e d c o a r s e p a r t i c le s

f ro m t h e sp i g o t r eg i o n t o t h e vo r t ex f i n d e r b u t, o n th e o t h e r h a n d , i t m a y h i n d e r

t h e s h o r t - c i r c u i t i n g f l o w , t h u s i m p r o v i n g t h e c l a s s i f ic a t i o n e f f i c i e n c y . F i r s t ,

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

f l o w m o d e l o f t h e h y d r o c y c l o n e . T h e n e x p e r i m e n t a l d a t a , o b t a i n e d w i t h s i x

d i f f e r e n t h y d r o c y c l o n e s , a r e u s e d to d e m o n s t r a t e t h e e x i s te n c e o f t h e s e c o n d a r y

f l o w a n d i t s i n te r a c ti o n w i th t he s h o r t - c i r c u i ti n g f l o w . I n th e e x p e r i m e n t s , a

t a n g e n t i a l i n l e t w a s u s e d t o a cc e n tu a t e t he s h o r t - c i r c u i ti n g f l o w a n d a n a n n u l a r

t a k e - o f f i n s i d e t h e v o r t e x f i n d e r w a s u s e d t o i n v e s t i g a t e t h e p a r t i c l e s i z e

d i s t r i b u t i o n i n t h i s r eg i o n . I t i s sh o w n t h a t t h e cen t r i f u g a l f i e l d m u s t b e

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

s l u r r y f o r e f f i c i e n t c l a s s i f i c a t i o n .

K e y w o r d s

Hydrocyclone classification effici ency short-c ircuit secondary flow model

INTRODUCTION

The mineral-proce ssing industry uses the hydrocycl one extensively as a classifying or

desliming unit to close the rod - SAG- and ball-mill grinding circuits. Its design

operational simplicity high throughput reliability and low installation cost are the reasons

for this popula rity. Despite the attractive feature s there is an inheren t shortcoming: it

cannot render a sharp cut in particle sizes between the fine and the coarse stream. It is

common ly believed that there are two flow mechanisms that cause the ineffi ciency of the

hydrocyclone operation. The first one is that a part of the feed stream passes over the outer

surface of the vortex finde r directly into the overflow or fine stream and the second is that

the fine particles are entrained in the spigot discharge or coarse stream. In the literature the

first mechanism is termed as the short-circuit ing flow. A better understanding of the

leakage flows can lead to design modifications for high- capaci ty hydrocyclones and hence

these mechanisms are analyzed here from a fluid-flow point of view.

LITERATURE

As early as 1952 Kelsall [1] reported large downward velocities near the outer wall of the

vortex finde r which he considered to be the short-circ uiting flow. The short-circu iting

779

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780 L MILIN et al

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

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

s w i r l in g f l o w w i t h i n t h e h y d r o c y c l o n e . I n a s u b s e q u e n t p u b l i c a t i o n , K e l s a ll [2 ] r e p o r t e d o n

a s im p l e e x p e r i m e n t w i t h a m o d i f i e d h y d r o c y c l o n e w h i c h h a d a n a n n u la r t a k e - o f f s i tu a t e d

a r o u n d t h e r o o t o f th e v o r t e x f i n d e r . T h e e x p e r i m e n t a l w o r k l e d to th e c o n c l u s i o n t h a t 1 5

o f th e t o t a l w a t e r i n f l o w s h o r t - c i r c u i t s d i r e c t l y f r o m t h e i n l e t t o t h e a n n u l u s , a n d t h a t 1 5

o f t h e s ol id p a r t ic l e s i n t h e f e e d s t r e a m , i n d e p e n d e n t o f t h e ir s i z es , al so s h o r t - c i r c u i t s f r o m

t h e i n l e t t o t h e a n n u l u s . B r a d l e y a n d P u l l i n g [3 ], u s i n g a d y e - i n j e c t i o n t e c h n i q u e ,

c o n f i r m e d t h e e x i s t e n c e o f t he s h o r t - c i r c u i t i n g f l o w . B l o o r a n d I n g h a m [ 4] a p p l i e d t he

a p p r o x i m a t e m e t h o d o f P o h l h a u s e n t o o b t a i n s o l u t io n s to t h e m o m e n t u m i n te g r a l e q u a t io n s

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

t o r o i d a l v o r t e x i n t h e u p p e r r e g i o n o f t h e h y d r o c y c l o n e , w h i c h g e n e r a t e s a n o u t w a r d r a d i a l

v e l o c i t y j u s t o u t s id e th e b o u n d a r y l a y e r o f t h e r o o f , t h e y c o n c l u d e d t h a t, t y p i c a l ly , 1 0 o f

t h e f e e d s t r e a m l e a k s d i r e c t l y t o t h e o v e r f l o w s t r e a m .

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

K e l s a l l [1] p r o p o s e d a n a n n u l a r t a k e - o f f t u b e o f a s u i ta b l e r a d iu s s u r r o u n d i n g t h e r o o t o f

t h e v o r t e x f i n d e r t o r e c y c l e t h e d i s c h a r g e f r o m t h e a n n u lu s b a c k t o t h e f e e d s u m p . T h e r e

w e r e s o m e a t t e m p t s t o d i v e r t t h e s h o r t - c i r c u i t i n g f l o w b a c k i n to t h e m a i n c i r c u l a ti o n z o n e

f o r c l a s s i f i c a t io n . B r a d l e y [5 ] v a r i e d t h e s h a p e a n d le n g t h o f t h e v o r t e x f i n d e r ; V a n D u i j n

a n d R i e t e m a [ 6] f i t t e d a d i s k t o t h e t ip o f t h e v o r t e x f i n d e r ; a n d R a j a m a n i [ 7] f i t t e d a n

i m p e l l e r s u r r o u n d i n g t h e v o r t e x f i n d e r to d e f l e c t t h e s h o r t - c i r c u i t i n g f l o w . H o w e v e r , t o

d a t e , n o n e o f t h e a b o v e d e s ig n m o d i f i c a t io n s h a v e r e d u c e d t h e s h o r t - c i r c u i t i n g f l o w

c o n v i n c i n g l y .

A r e c i r c u l a t i n g f l o w i n th e u p p e r s e c t i o n o f th e h y d r o c y c l o n e i s c a u s e d b y t h e i n a b il i ty o f

t h e o v e r f l o w o p e n i n g t o c o p e w i t h t h e b r o a d c o l u m n o f t h e c e n tr a l u p w a r d f l o w ; t h e r e f o r e ,

a p a r t o f it m u s t r e c i r c u l a t e . B r a d l e y a n d P u l l i n g [3 ] a d d r e s s e d t h e e x i s t e n c e o f s u c h a f l o w

f r o m o b s e r v a ti o n s o f t he m o v e m e n t s o f a d y e i n tr a n s p a r en t h y d r o c y c l o n e s a n d c o n c l u d e d

t h a t p a r t o f t h e c e n t r a l u p w a r d f l o w r e c ir c u l a te s d o w n w a r d u p o n m e e t i n g t h e r o o f . T h e y

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

A l t h o u g h t h e e x i s te n c e o f t h e s h o r t - c i r c u i t i n g f l o w is k n o w n , n o o n e to d a te h a s b e e n a b l e

t o q u a n t i f y t h is s t r e a m c o n v i n c i n g l y . I n t h is p a p e r t h i s f l o w i s a n a l y z e d w i t h t h e h e lp o f

a m a t h e m a t i c a l m o d e l . A n a d d it i o n a l f l o w , t e r m e d s e c o n d a r y f l o w , w h i c h a ls o c a r r ie s

c o a r s e p a r t i c l e s i n t o t he o v e r f l o w s t r e a m , i s i d e n t i f i e d .

F L O W F I E L D A N D L E A K A G E M E C H A N I S M S

I n t e r a c t i o n o f t h e S e c o n d a r y F l o w w i t h t h e S h o r t C i r c u i t i n g F l o w

A n a c c u r a t e m o d e l o f t h e f lu i d f l o w is n e e d e d t o e x a m i n e t h e l e a k a g e m e c h a n i s m s . S u c h

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

t h e h y d r o c y c l o n e . T h i s m o d e l s o l v es t h e g o v e r n i n g N a v i e r - S t o k e s e q u a t i o n s n u m e r i c a l l y b y

i n c o r p o r a t i n g a m o d i f i e d P r a n d t l m i x i n g - l e n g t h m o d e l f o r th e t u r b u l e n c e c l o s u r e [ 8, 9, 10 ] .

T h e m o d e l h a s b e e n v e r i f i e d w i t h e x te n s i v e l a s e r - D o p p l e r v e l o c i t y m e a s u r e m e n t s i n a 7 5 -

m m h y d r o c y c l o n e . F u r t h e r w o r k [ 11 ] p r o v e d t h a t th is f l u i d - f l o w m o d e l s o l u t io n h e l d w e l l

f o r v a r ia t i o n s i n s p i g o t - a n d v o r t e x f i n d e r s i ze s a n d c o n e a n g le . T h e r e f o r e , t h e a b o v e

s i m u l a t i o n m o d e l h as b e e n u s e d h e r e f o r f l u i d - f l o w c a l c u la t io n s . F o r t h e s a k e o f b r e v i t y ,

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

b e f o u n d i n c i t e d r e f e r e n c e s . N e v e r t h e l e s s , a b r i e f d e s c r i p t i o n o f t h e m o d e l f r a m e w o r k is

p r o v i d e d i n th e A p p e n d i x . T h e m o d e l a s s u m e s t h a t t h e i n le t f l o w e n t e r s t h r o u g h a r i n g

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

f i t t e d w i t h a n i n v o l u t e i n le t . I n t h e n u m e r i c a l s o l u t i o n o f t h e N a v i e r - S t o k e s e q u a t i o n s , t h e

t h r e e v e l o c it y c o m p o n e n t s a r e k n o w n a t e v e r y n o d a l p o i n t o f th e h y d r o c y c l o n e . T h e r e f o r e ,

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

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Hydrocyclone leakage mechanisms

781

Six 75- mm diameter h ydrocyclo nes, whose dimensions are listed in Table 1, were the

subject of experim ental investigation. Each hydro cyclone is referr ed to by its numb er, and

the relevant dimensions, applicable in the context are mentioned in parenthesis.

Hydrocyclone 1 was the standard unit that was compared with the others; it had a 16-mm

spigot and an involute inlet. A smaller spigot 12.5 mm) was the diffe rin g featur e of

hydr ocycl one 2, which was otherwise identical to the first one. Likewise, hydro cyclon e 3

was similar to the first one except that it had a shorter cyli ndri cal section 25 mm in stead

of 75 mm). Hyd ro cyc lon e 4 was also similar to the firs t one but it had a smaller spigot I 1

mm) and tangential inlet. The first four were copies of the standard hydrocy clone but the

last two had a concen tric tube inside the vortex finder. The concen tric tube exten ded 10

mm below the tip of the vortex finder. Thro ugh the annulus, between the vortex finder and

the concent ric tube, a thir d stream was collected for size analysis. Late r, m uch will be said

about this stream. Hydroc yclones 5 and 6 were identical in dimensions to hydrocy clones

1 and 4, respectively, with the exception of the extra concentric tube fitting. Thus,

hydr ocyc lon es 4 and 6 were the only ones with a tangential inlet.

TABLE 1

i m e n s i o n s f o r t h e e x p e r i m e n t a l

hydrocyclones

Unit 1 2 3 4 5 6

No.

Hydrocyclone 75 75 75 75 75 75

Diameter mm)

Inlet Type a a a b a b

Inlet 25 25 25 22 25 22

Diameter mm)

Vortex finder 22 22 22 22 22 22

Diameter mm) 16)c 16)c

Vortex finder 50 50 50 50 50 50

Length mm) 60)d 60)d

Cylindrical 75 75 25 75 75 75

Section

Length mm)

Spigot 16 12.5 16 11 16 11

Diameter mm)

Included Cone 20 20 20 20 20 20

Angle degree)

a. Involute Type

b. TangentialType

c. Annulusvortex finder, I.D. of inner tube = 16 mm

d. Annulusvortex finder, length of inner tube = 60 mm

The experiments were done in the usual sump-pump recirculation system to determine the

particle size-classification effi cienc y of each hydrocyclone. All of the experiments were

perf orme d with feed limestone slurry of the same size distribution. A Microtrac Particle

Anal yzer was used for particle size analysis. In all the experime nts, the volu metr ic feed

flow was maintain ed at around 80 liter/ min, so that comparisons could be made at the same

level of cent rif ugal field. Table 2 lists the operating condi tions and the flow rates of spigot

and vortex finder streams.

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7 8 2

L MILIN e t a l

T A B L E 2 E x p e r i m e n t a l o p e r a ti n g c o n d i t i o n s

Run Unit Volumetric Flow Rate ( l/ra in) W eight Sol ids

N o . N o . F e e d U n d e r f l o w O v e r f l o w F e e d U n d e r f l o w O v e r f l o w

1 1 80 .0 18 .0 61 .9 wa te r wa te r wa te r

2 2 80 .0 8 .7 71 .2 wa te r wa te r wa te r

3 3 80 .0 16 .7 63 .2 wa te r wa te r wa te r

4 1 76.3 14.4 61.9 12.8 37.7 5 .5

5 1 76.8 14.2 62.9 22.6 57.4 11.1

6 1 80.2 15.4 64.7 31.3 65.5 19.0

7 1 79.3 16.5 62.8 37.1 67.6 25.4

8 1 82.7 17.3 65.3 46.9 70.5 38.3

9 2 74.4 7 .3 67.0 12.5 57.0 5 .1

10 2 70.6 8 .6 62.0 20.8 66.4 10.6

11 2 75.5 11.3 64.2 31.9 69.9 21.5

12 2 74.2 11.9 62.3 39.8 71.5 30.9

13 2 76.7 12.2 64.5 48.8 73.1 42.5

14 3 73.3 16.9 56.3 48.2 69.7 39.5

15 3 72.8 15.2 57.6 32.1 63.7 19.9

16 1 74.3 16.2 58.1 31.6 61.9 19.4

17 4 64.9 3 .4 61.4 14.4 40.3 12.6

18 4 75.3 6 .1 69.1 30.6 70.9 25.2

19 4 66.3 5 .5 60.8 35.0 71.2 30.1

20 4 43 .5 6 .2 37 .3 41 .4 60 .2 37 .5

21 4 76.8 9 .7 67.1 55.8 68.0 53.7

22 5 77.6 18.3 19.3 40.0) 28 .9 52.7 10.9 18.9)

23 6 66.1 9 .1 21.9 35.1) 32 .9 69.9 13.3 29.9)

So l id dens i ty : 2 .7 g / cc

N u m b e r s i n b r a c k e t s a r e f o r a n n u l u s f l o w

I n s i de a n y h y d r o c y c l o n e t h e f l o w f i e l d d e v e l o p s as f o l lo w s : f r e s h i n c o m i n g f l u i d f o r c e s t h e

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

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

d i s c h a r g e o f t h e s w i r l in g f l u i d ; h e n c e , a p a r t o f t h e f l u i d r e v e r s e s i t s d i r e c t i o n a n d s p i r a l s

u p w a r d t o t h e o t h e r o u t l e t, th e v o r t e x fi n d e r . H o w e v e r , th is c e n t r a l u p w a r d f l o w r e f e r r e d

t o a s s e c o n d a r y f l o w f o r b r e v i t y ) i s g e n e r a l l y b r o a d e r i n s iz e t ha n t h e v o r t e x - f i n d e r

o p e n i n g . A p a r t o f th is f l o w m i ss e s t h e v o r t e x f i n d e r a n d c o n t i n u e s t o m o v e u p t o w a r d t h e

r o o f . A s it a p p r o a c h e s t h e r o o f , it m u s t r e v e r s e i ts d i r e c t i o n d o w n w a r d a g a i n s in c e t h e

r o o f is a s o li d w a ll . T h i s s e c o n d a r y d o w n w a r d f l o w m o v e s t o w a r d t h e v o r t e x f i n d e r d u e t o

t h e p r e s s u r e g r a d i e n t . B e s i d e s, p a r t o f th e f e e d f l o w h a s a t e n d e n c y t o m o v e a l o n g t h e

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

a n d j o i n s th e f l o w m o v i n g u p w a r d t h r o u g h t h e v o r t e x f i n d e r . T h i s f l o w is s e v e r e i f t h e

r a d i a l c o m p o n e n t o f th e in l e t f l o w is l a rg e . T h e i n v o l u t e i n le t m i n i m i z e s t h e r a d i a l

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

T h e s e c o n d a r y f l o w m e e t s t h e s h o r t - c i r c u i t i n g f l o w i n th e v i c i n i ty o f t h e v o r t e x f i n d e r ; i f

i ts v o l u m e t r i c f l o w r a t e is h i g h e n o u g h i t c a n b l o c k t h e s h o r t - c i r c u i t i n g f l o w c o m p l e t e l y .

A s a r e s u lt , t h e d i s c h a r g e s t r e a m t h r o u g h t h e v o r t e x f i n d e r is m a d e u p o f b o t h t h e s h o r t -

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

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Hydrocyclone leakage mechanisms 78

F i r s t, h y d r o c y c l o n e s 1 a n d 2 f it t e d w i t h a n i n v o l u t e in l e t w e r e t h e s u b j e c t o f t h e f l o w f i e ld

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

h y d r o c y c l o n e 1 h a d a l a r g e r s p i g o t 1 6 m m ) t h a n h y d r o c y c l o n e 2 1 2 .5 ra m ) . T h e c o m p u t e d

f l o w f i e ld s o f h y d r o c y c l o n e s I a n d 2 a re s h o w n i n F i g u r e s l a a n d 2 , r e s p e c t i v e ly . F i g u r e

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F i g . l a F l o w f i e ld a n d ta n g e n t i a l - v e l o c i t y p r o f i l e o f h y d r o c y c l o n e 1 .

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784 L MILIN et al

l b s h ow s i n g r e a t e r d e t a i l t he s e c o n d a r y f l o w i n th e v i c i n i t y o f t h e v o r t e x f i n d e r . F o r

h y d r o c y c l o n e I , t h e b o u n d a r y - l a y e r f l o w w a s a b o u t 3 - m m t h i c k at th e v o r t e x - f i n d e r t ip ,

a n d t h e v o l u m e t r i c f l o w ra t e o f t h is st r e a m w a s a b o u t 22 o f t h e i n l e t f l o w . U p o n f u r t h e r

a n a ly s i s, i t w a s f o u n d t h a t t h e s h o r t - c i r c u i t i n g f l o w c o n t r i b u t e s o n e - t h i r d t o th i s b o u n d a r y -

l a y e r f l o w a n d t h e s e c o n d a r y f l o w m a k e s u p t h e re m a i n d e r . I n t e r e s t i n g l y , h y d r o c y c l o n e 2

e x h i b i t e d t h e s a m e p r o p o r t i o n f o r e a c h o f t h e c a te g o r i e s e v e n t h o u g h i t h a d a s m a l l e r sp i g o t

o p e n i n g . W h a t t hi s i m p l i e s i s t h a t t h e m i s p l a c e m e n t o f t h e c o a r s e p a r t ic l e s t o t h e o v e r f l o w

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

f l o w . T h e m i s p l a c e m e n t is p a rt l y d u e t o t h e s e c o n d a r y f l o w w h i c h c a r r i es c l a s s if i e d c o a r se

p a r ti c le s . T h e s e c o a r s e pa r t ic l e s c o u l d be t h r o w n o u t w a r d b y t h e c e n t r i f u g a l f o r c e a n d t h u s

g e t r e c la s s i f ie d , o r t h e y c o u l d m o v e u p w a r d t o j o i n t h e s h o r t - c i r c u i t i n g f l o w a n d e x i t

t h r o u g h t h e v o r t e x f i n d e r .

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Hyd rocyclone leakage mechanisms 78

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

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

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

o f th e v o r t e x f in d e r . T o i l lu s t ra t e th i s p h e n o m e n o n , t h e f lo w f i e l d f o r h y d r o c y c l o n e 3 ,

w h i c h w a s i d e n t i c a l i n al l d i m e n s i o n s t o h y d r o c y c l o n e 1 e x c e p t t h a t it h a d a s h o r t e r

c y l i n d r i c a l s e c t i o n 2 5 - r a m i n s t e a d o f 7 5 r a m ) , is s h o w n i n F i g u r e 3 a . F i g u r e 3 b s h o w s i n

g r e a t e r d e t a il t h e t o p h a l f o f F i g u r e 3 a . A s c a n b e s e e n in F i g u r e 3 b , th e s e c o n d a r y f l o w

w a s u n a b l e t o ri s e a s h i g h a s i t d i d i n h y d r o c y c l o n e 1 s e e F i g u r e l b ) . I n t hi s c a s e , th e

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Hydrocyclone leakage mechanisms 787

contributi on of the secondary flow to the bounda ry-la yer flow around the vortex find er was

insignificant. In fact, model calculations indicate that the bound ary -la yer flow is composed

of 95 short-c ircuit ing flow and a mere 5 of secondary flow. As a consequence, the flow

rate of the short-circuiting stream is twice as much as that for hydrocyclone 1 (75-mm

cylindrical section length).

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Finally, the tangential -velocity profiles were very similar for all three hydrocycl ones (1, 2

and 3) despite the diffe rence s in their geometry. This implies that the centrifugal field is

relative ly identical in magnitude as long as the inlet volumetric flow is main tained constant.

However, the axial and radial components induced by the swirling flow were not the same

in the three hydrocyclones. Hence, the flow fields are diff ere nt in Figures la , 2 and 3a.

It is the nonswirling flow components, which develop according to hydrocyclone

dimensions, that determine the classification efficiency.

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788 L. MILINet al

T h e i n t e r a c t i o n b e t w e e n t h e s h o r t - c i r c u i t i n g a n d s e c o n d a r y f l o w s is i l lu s t r a te d i n F i g u r e

4 . H e r e t h e p e r f o r m a n c e o f h y d r o c y c l o n e 3 ( s h o r te r c y l i n d r ic a l s e c ti o n , 25 m m ) is

c o m p a r e d w i t h h y d r o c y c l o n e 1 ( 7 5 - m m c y l i n d r ic a l s e c t i o n l e n g th ) a t a c o n s t a n t f e e d f l o w

r a te o f 8 0 l i t e r /m i n , b u t a t t w o f e e d p e r c e n t s o l id s - 3 2 % a n d 4 8 % , r e s p e c t i v e l y . A t b o t h

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

p a r t ic l e s i n t o t h e o v e r f l o w w h i c h v e r i f i e s t h a t th e s h o r t - c i r c u i t i n g f l o w i s h i g h e r i n

h y d r o c y c l o n e 3 s i n c e t he c e n t r i f u g a l f i e ld i s t h e s a m e i n b o t h h y d r o c y c l o n e s . I n li g h t o f th e

m o d e l r e s u lt s d i s c u s s e d , t h e s e c o n d a r y f l o w i n h y d r o c y c l o n e 3 d i d n o t r is e h i g h e n o u g h t o

b l o c k , a t l e a s t p a r t l y , t h e s h o r t - c i r c u i t i n g f l o w .

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C o m p a r i s o n o f cl a ss i fi c a ti o n p e r f o r m a n c e b e t w e e n h y d r o c y c l o n e s 1 a n d 3.

I n f l u e n c e o f t h e S e c o n d a r y a n d S h o r t C i r c u i t in g F l o w s o n t h e C l a s s if i c a t io n E f f i c i e n c y

N e x t , t h e i n f l u e n c e o f t h e s e c o n d a r y f l o w o n t h e c la s s i f ic a t i o n e f f i c i e n c y is i n v e s t i g a t e d

e x p e r i m e n t a l l y . M o d e l c a l c u l a t i o n s i n d i c a t e th a t t h is f l o w i n c r e a s e s w h e n t h e s p i g o t i s

u n a b l e t o d i s c h a r g e a ll o f th e s w i r li n g d o w n w a r d f l o w . T h e s a m e c o n c l u s i o n i s r e f l e c t e d b y

t h e e x p e r i m e n t a l s i z e - c l a s s i fi c a t i o n r e s u lt s o f h y d r o c y c l o n e 1 ( 1 6 - m m s p i g o t) , s h o w n i n

F i g u r e 5 . I t i s c l e a r l y s e e n t h a t a s l o n g a s t h e f e e d s t r e a m c o n t a i n s l e ss t h a n 3 7 w t . % s o l i d s ,

t h e a m o u n t o f c o a r s e p a r t ic l e s r e p o r t in g t o t h e o v e r f l o w s t r e a m i s i n s i g n if i c a n t. I n d e e d , f o r

t h e s e ru n s , w h i l e t h e f e e d s t r e a m c o n t a i n e d p a r t i c l e s u p to 30 0 m i n si z e , t h e p a r t i c l e si z e

o f t h e o v e r f l o w s t re a m d i d n o t e x c e e d 3 8 m . T h i s im p l i e s th a t t he s e c o n d a r y f l o w w a s

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

t h e y g o t r e c l a s s if i e d as t h e y m o v e d u p w a r d s . F u r t h e r , t h is f l o w s u c c e s s f u l l y b l o c k e d o u t

m o s t o f t h e s h o r t - c i r c u i t i n g f l o w . F i n a l ly , t h e r e w a s s u f f i c i e n t c e n t r i f u g a l f o r c e w i t h i n t h e

b o u n d a r y - l a y e r f l o w t h a t c o a rs e p a rt i cl e s e n t e r in g t h e s h o r t - c i r c u i t i n g f l o w w e r e c l a s s if i ed .

I n c o n t r a s t , a t a f e e d c o n c e n t r a t i o n o f 4 7 w t . % s o l id s , th e s a m e h y d r o c y c l o n e e x h i b i t e d

c o n s i d e r a b l e d e g r a d a t i o n i n e f f i c i e n c y e s p e c i a l ly in t h e c o a rs e r si ze r a n ge . T h i s d e g r a d a t i o n

i n e f f i c i e n c y m u s t b e d u e to e x c e s s iv e s e c o n d a r y f l o w . A s th e s o li d s c o n t e n t i n t h e f e e d

i n c r e a s e d , t h e a m o u n t o f so l id s th a t r e p o r t e d t o t h e u n d e r f l o w s t r e a m w a s b e y o n d t h e

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

s p i g o t w h i c h l e d t o a b r e a k d o w n o f th e f o r c e d - f r e e v o r t e x f l o w p a t te r n . C o n s e q u e n t l y t h e

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

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

I t i s k n o w n t h a t t h e e f f e c t o f a h i g h e r c o n c e n t r a t i o n o f s o li d s in t h e f e e d i s th e s a m e a s th a t

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

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Hydrocyclone leakage mechanisms 89

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

e x p e r i m e n t a l w o r k d o n e w i t h h y d r o c y c l o n e 2 ( 1 2 . 5 - m m s p i g o t) . A s c a n b e s e e n i n F ig u r e

6 , th e d e g r a d a t i o n b e g i n s f o r i n l e t c o n c e n t r a t i o n g r e a t e r t h a n 2 0 w t . . T h i s i m p l i e s t h at

f l o w p a tt e r n s a r e c o n d u c i v e t o g o o d s iz e c l a s s i f i c a t i o n u p t o 2 0 w t . s o l id s . B e y o n d t h is

c o n c e n t r a t i o n t h e s e c o n d a r y f l o w e n t r a in s c l a s s i f i e d c o a r s e p a r t ic l e s t h a t ar e n o t r e c l a s s i f i e d

b e f o r e e x i t i n g t h r o u g h t h e v o r t e x f i n d e r .

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F i g . 5 C l a s s if i ca t i o n e f f i c i e n c y c u rv e s o f h y d r o c y c l o n e 1

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

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s o l i d s

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F i g . 6 C l a s s if i ca t i o n e f f i c i e n c y c u r v es o f h y d r o c y c l o n e 2

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

J u s t as a n in c r e a s e i n s e c o n d a r y f l o w w a s b r o u g h t a b o u t b y a s m a l le r s p i g o t s i z e , a n i n c r e a s e

i n th e s h o r t - c i r c u i t i n g f l o w o c c u r r e d w i t h a t a n g e n t i a l - i n l e t t u b e . F o r th i s r e a s o n

h y d r o c y c l o n e 4 ( l 1 - r a m s p i g o t ) w a s e q u i p p e d w i t h s u c h a n i n l e t. I n s m a l l - d i a m e t e r

h y d r o c y c l o n e s t h e t a n g e n t i a l i n l e t g e n e r a t e s v e r y h i g h r a d i a l v e l o c i t y i n t h e i n l e t r e g i o n a s

8/18/2019 milin1992

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790 L Mll IN et al

c o m p a r e d w i t h a n i n v o l u t e in l et . A s a r e s u lt t h e c o a r s e p a r t ic l e s e n t e r i n g t h e h y d r o c y c l o n e

f r o m a ta n g e n t i a l i n le t h a v e a m u c h h i g h e r p r o b a b i l i t y o f r e a c h i n g t h e b o u n d a r y - l a y e r f l o w

a n d e x i ti n g v i a t h e v o r t e x f in d e r . T h e e x p e r i m e n t a l c l a s s i f i c a ti o n e f f i c i e n c y c u r v e s f o r

h y d r o c y c l o n e 4 , ( ta n g e n t i al i n le t ) o b t a i n e d a t a c o n s t a n t i n l e t f l o w b u t d i f f e r e n t f e e d

c o n c e n t r a t i o n s , a r e s h o w n i n F i g u r e 7 . T h e l a rg e s t p a rt i c le s i z e i n th e o v e r f l o w s t r e a m f o r

h y d r o c y c l o n e 4 ( ta n g e n t i a l i n l e t) a t 1 4 w t . % s o l id s w a s 6 5 /z m w h i c h i s m u c h h i g h e r t h a n

t h a t f o u n d i n a n i d e n t ic a l h y d r o c y c l o n e e q u i p p e d w i t h a n i n v o l u t e i n le t . W i t h h y d r o c y c l o n e

2 ( i n v o l u t e i n l e t ) a t a f e e d c o n c e n t r a t i o n o f 1 2 . 5 w t . % , t h e l a r g e s t p a r t i c l e s i z e i n t h e

o v e r f l o w w a s o n l y 2 2 /z m. I n f a c t t h e la r g e s t p a r t ic l e s i z e i n t h e o v e r f l o w s t r e a m o f

h y d r o c y c l o n e 4 (t a n g e n ti a l i n le t ) in e a c h r u n w a s m u c h l a rg e r t h a n t h a t o f th e c o r r e s p o n d i n g

e x p e r i m e n t d o n e w i t h h y d r o c y c l o n e 2 ( se e F i g u r e 5 ). T h e r e f o r e , i t is t o b e c o n c l u d e d t h a t

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

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F i g .7 C l a s s i f ic a t i o n e f f i c i e n c y c u r v e s o f h y d r o c y c l o n e 4

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

T h u s f a r , t he s e c o n d a r y f l o w a n d t h e s h o r t - c i r c u i t i n g f l o w w e r e e x p e r i m e n t a l l y e n h a n c e d

t o i ll u s tr a te t h e i r e f f e c t o n p a r t ic l e s iz e c l a s s if i ca t i on . N o w w e p r o v i d e f u r t h e r p r o o f b y

c o l l e c ti n g a s a m p l e o f t h e r o o f b o u n d a r y - l a y e r s t r e a m . K e l s a l l [2 ] p r o p o s e d t h e i n s t a ll a ti o n

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

a n n u l u s b e t w e e n t h e in n e r t u b e a n d th e v o r t e x f i n d e r a n d r e c y c l e d . P r e s u m a b l y t hi s s t r e a m

w o u l d c a r r y th e b o u n d a r y - l a y e r f l o w o n t h e o u t e r s u r f a c e o f t h e v o r t e x f in d e r . H o w e v e r ,

s o m e s e c o n d a r y f l o w w o u l d a l so pa s s t h r o u g h t h e a n n u lu s s i n ce t h e a n n u l u s s p a c i n g c a n n o t

b e f i x e d a p r i o r i t o m a t c h t h e th i c k n e s s o f t h e b o u n d a r y - l a y e r f l o w . T h e p r e s e n c e o f th e

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

f i n d e r o u t l e t . H y d r o c y c l o n e s 5 a n d 6 , w h o s e d i m e n s i o n s a r e s h o w n i n T a b l e 1, w e r e

i d e n t ic a l t o 1 a n d 4 w i t h t h e e x c e p t i o n o f t h e c o n c e n t r i c t u b e . F o r h y d r o c y c l o n e 5 ( i n v o l u t e

i n le t ), t h e d i s t r i b u t i o n o f th e f l o w s c o ll e c t e d i n th e u n d e r f l o w , o v e r f l o w , a n d t h r o u g h t h e

3 - m m a n n u l u s s p a c i n g i s s h o w n in F i g u r e 8 . T h i s fi g u r e s h o w s w e i g h t p e r c e n t a g e ( b a s e d

o n f e e d ) i n e a c h s iz e t h a t r e p o r t s t o t h e s t r e a m i n d i c a t e d . T h u s t h e c u r v e c o r r e s p o n d i n g t o

t h e u n d e r f l o w s t r e a m is a c t u a l ly t h e u n c o r r e c t e d e f f i c i e n c y c u r v e . F o r h y d r o c y c l o n e 5

( i n v o l u t e i n l e t ) t h e c u t s iz e w a s a r o u n d 1 5 /z m, t h e o v e r f l o w s i z e d i s t r i b u t i o n r a n g e d f r o m

0 to 2 0 /z m, t h e a n n u l u s d i s c h a r g e s iz e d i s t r i b u t i o n r a n g e d f r o m 0 to 4 5 /~ m , a n d t h e

u n d e r f l o w c a r r i e d a ll o f t h e f e e d p a r t i c l e s f o r si z es a b o v e 4 5 /z m. T h e i m p l i c a t i o n is t h a t

t h e a n n u l u s c o l l e c ts o n l y n e a r - c u t s i z e p a r ti c le s c a r r ie d u p w a r d b y t h e s e c o n d a r y f l o w . T h e

s h o r t - c i r c u i t i n g f l o w , i f t h e r e is a n y , w a s c l a s s if i ed . H a d t h e r e b e e n a s h o r t - c i r c u i t i n g f l o w

w e w o u l d o b s e r v e s o m e p e r c e n t a g e o f a ll s i ze s u p t o 3 0 0 m i n t h e a n n u l u s d i s c h a r g e ( s i n c e

f e e d c o n t a i n s p a r t i c l e s u p to 3 0 0 m ) . I n c o n t r a s t , a s s e e n in F i g u r e 9 , t h e a n n u l u s

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Hydro cyclone leakage mechan isms 791

d i s c h a r g e f o r h y d r o c y c l o n e 6 ( t a n g e n t i a l in l e t ) s h o w s t h e p r e s e n c e o f p a r t i cl e s u p t o 1 2 5 / zr n ,

w h e r e a s t h e o v e r f l o w st r e a m c o n t a i n s p a r t i cl e s i n t h e s i z e r a n g e 0 - 3 2 m o n l y . H e n c e ,

p a r ti cl e s o f s i z es b e t w e e n 3 2 a n d 1 2 5 m m u s t e i th e r c o m e t h r o u g h t h e r o o f b o u n d a r y -

l a y er f l o w o r t h e s e c o n d a r y f l o w . T h e f o r m e r i s m o r e l i k e ly s i n c e t h is h y d r o c y c l o n e w a s

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

f l o w i s p r e d o m i n a n t . I t i s t o b e c o n c l u d e d t h a t c o a r s e p a r t i cl e s i n t h e si z e ra n g e 1 2 5 - 3 0 0

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

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

p r e c l u d e d . O n l y i n g e n i o u s p a r t ic l e t r a ci n g e x p e r i m e n t s ca n d i f f e r e n t i a t e b e t w e e n s h o r t -

c i r c u i t in g f l o w a n d s e c o n d a r y f l o w i n t h e a n n u l u s d is c h a r g e . N e v e r t h e l e s s , t h e a n n u l u s

d i s c h a r g e s i z e d i s t r i b u t i o n i s q u i t e d i f f e r e n t f r o m t h e c e n t r a l f l o w i n t h e v o r t e x t u b e ,

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

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F i g . 8 W e i g h t p e r c e n t so l i d d i s c h a r g e d t h r o u g h r e s p e c t i v e

o u t l e t s t r e a m s o f h y d r o c y c l o n e 5 .

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F i g . 9 W e i g h t p e r c e n t s o l i d d i s c h a r g e d th r o u g h r e s p e c t i v e

o u t l e t s t r e a m s o f h y d r o c y c l o n e 6 .

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792 L MILIN e t a l

O N L U S I O N S

Ideally the hydrocyc lone is designed to make a perfec t separation betwe en coarse particles

and fine particles. However, separation occurs over a size range since the classified stream

must reverse its direction near the spigot, during which some coarse particles are carried

upward to the vortex finder . The extent of this misclassification depends on the spigot size

while the classification depends on the cen trifugal force field in the cylindrica l and conical

sections. Thus, for excellent classification, the allowable conc entration of solids in the

fee d is limited by the spigot size. Anot her stream that contribu tes to misclassification is

short-c ircuit ing flow. Experiments that compare the tangential inlet with the involute inlet

show that in the latter short -cir cui ting flow is very much reduced. It is not possible to

eliminate this flow since as long as there are solid surfaces a boundary-layer flow is

inevitable. However , the cent rifugal field can be increased to a certa in limit (subject to

the hydrocyc lone dimensions) to classify as much as possible the coarse particles in the

short-circuiting flow.

The mineral industries operate large hydrocyc lones at high feed per cent solids. Clearly, the

spigot canno t handle such high concentra tions. Howeve r, classification circuits are operated

in this manner to increase the concentration of solids in the overflow as otherwise a

dewatering unit would be needed ahead of flotation circuits. In fact the effic ienc y of size

classification and, hence, mineral liberation is compromised to gain this operating

advantage.

The analysis of the short-circuiting flow and the secondary flow done with the fluid-flow

model and the experimental work furt her paves the way to understanding the internal flow

mechanisms within the hydrocyclone.

ACKNOWLEDGMENT

This research has been supported by the Department of the Interior's Mineral Institute

program administered by the Bureau of Mines through the Generic Mineral Technology

Center for Comminution under grant number G1175149.

l

2.

3.

4.

5.

6.

7.

8.

9.

10.

R E F E R E N E S

Kelsall, D.F., A study of the motion of solid particles in a hydraulic cyclone. Trans.

Inst. Chem. Engrs. 30, 87 (1952).

Kelsall, D.F., A further study of the hydraulic cyclone.

Chem. Engng. Sci.

2, 254

(1953).

Bradley, D. & Pulling, D.J., Flow patterns in the hydraulic cyclone and their

interpretation in terms of performance. Trans. Inst. Chem. Engrs. 37, 34 (1959).

Bloor, M.I.G. & Ingham, D.B., The leakage effect in the industrial cyclone. Trans.

hzst. Chem. Engrs. 53, 7 (1975).

Bradley, D., Design and performance of cyclone thickeners. Int. Miner. Proc. Congr.

London, 129 (1960).

VanDuijn, G. & Rietema, K., Perfo rmance of large-co ne-ang le hydrocyclone .Chem.

Engng. Sci. 38(10), 1651 (1983).

Rajamani, K., Improvements in the classification efficiency of a hydroc yclone with

an impeller installation around the vortex finder. Particulate Sci. Tech. 5, 83 (1987).

Hsieh, K.T., Phenomenological model of the hydrocyclone. Ph.D. Dissertation,

University of Utah, Salt Lake City, Utah (1988).

Hsieh, K.T. & Rajaman i, K., Phenomenological model of the hydrocyclone: model

development and verification for single-phase flow. Int. J. Min. Proc. 22, 223

(1988).

Hsieh, K.T. & Rajamani, K., Mathematica l model of the hydrocyclone based on the

physics of fluid flow. AIChE Journal 37(5), 735 (1991).

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11.

12.

13.

14.

H y d r o c y c l o n e l e a k a g e m e c h a n i s m s

7 9 3

M o n r e d o n , T . ,

Hydrocyclone: Investigation of the flu id flow model

M. Sc.

T h e s i s , U n i v e r s i t y o f U t a h , S a lt L a k e C i t y , U t a h ( 19 9 0) .

R o a c h e , P . J ., Computational fluid dynamics H e r m o s a P u b l is h e r s, A l b u q u e r q u e , N e w

Mex i co ( 1 9 7 2 ) .

G o u r l a y , A . R . , H o p s c o tc h : a fa s t s e c o n d - o r d e r p a r t ia l d i f f e r e n t i a l e q u a t i o n s o lv e r .

J Inst Maths Applics 6, 375 (1970) .

Y o u n g , D . , I t e r a t iv e m e t h o d s f o r s o l v i n g p a r t ia l d i f f e r e n t i a l e q u a t i o n s o f e l l i p t i c ty p e .

Trans Amer Soc 76 , 92 (1954) .

A P P E N D I X

F L U I D F L O W M O D E L P R E S E N T A T I O N

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

c o n s t a n t p r o p e r ti e s c a n b e r e p r e s e n te d b y t h e c o n t i n u i t y e q u a t io n a n d t h e N a v i e r - S t o k e s

e q u a t i o n s . W e u s e th e f a c t t h a t t h e f l o w i n t h e h y d r o c y c l o n e i s a x i s y m m e t r i c , e x c e p t f o r

t h e i n l e t r e g i o n w h e n u s i n g a t a n g e n t i a l i n le t , t o s e t a l l d e r i v a t i v e s i n t h e a z i m u t h a l d i r e c t i o n

e q u a l to z e r o . I n t r o d u c i n g t h e v o r t i c i t y - s t r e a m f u n c t i o n a p p r o a c h [1 2], th e d i m e n s i o n l e s s

m o d e l e d t r a n s p o r t e q u a t i o n s i n c y l i n d r i c a l c o o r d i n a t e s a r e a s f o l l o w s [ 8 ] .

V o r t i c i t y •

OT1 = 1 0 g / 2 Or a l O w r l 1 ( O 2 r l 1 0 r l r l 0211 / ( 1 )

O t r 3 0 -- 7- - O r - O z + ~ ~ r 2 + r ~ - 7 ~ + ~ z 2

S t r e a m f u n c t i o n •

0 2 V _ 1 O V + 0 2 R * = - r r l

0 r 2 r 0 r 0 z 2

(2 )

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r 0 r = w ' - _ . = u , _ _ = v . ( 4 )

r z r

T h e n o r m a l i z a t i o n c o n s t a n t u se d h e r e i s b a s e d o n t h e a d v e c t i v e t i m e s c a le R e / U o.

S i n c e t u r b u l e n t c o n d i t i o n s e x i st in s i d e th e h y d r o c y c l o n e ( in l e t R e y n o l d s N u m b e r a s h i g h a s

1 0 5 t o 1 0 6 ) a t u r b u l e n c e c l o s u re m o d e l i s n e e d e d . A m o d i f i e d P r a n d t l m i x i n g - l e n g t h m o d e l

i s u s ed

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794 L MIuN et al

T h e n u m e r i c a l p r o b l e m i s t h e n a s e t o f c o u p l e d p a r a b o l i c 1 , 3) a n d e l l ip t i c 2 ) d i f f e r e n t i a l

e q u a t i o n s . E q u a t i o n s 1 a n d 3 a r e so l v e d u s i n g th e H o p s c o t c h m e t h o d [ 1 3] w h i l e e q u a t i o n

2 is s o l v e d b y t h e s u c c e s s iv e o v e r = r e l a x a ti o n m e t h o d [ 14 ]. A r e c t a n g u l a r m e s h s y s t e m i s

o v e r l a p p e d t o t h e h y d r o c y c l o n e a n d n u m e r i c a l v a l u e s a re s t o r e d a t e a c h n o d e o f th e g r id .

T h e c o m p u t a t i o n s a re c a r ri e d o u t o n a S P A R C w o r k s t a t io n S u n M i c r o s y s t e m s ) a n d t h e y

r e q u i r e a b o u t t h r e e h o u r s o f C P U t i m e f o r 2 ,0 0 0 i t e ra t io n s .

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= r a d ia l d i s t a n c e f r o m t h e a x is o f s y m m e t r y i n c y l i n d r i c a l c o o r d i n a t e s , c m

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

= R e y n o l d s n u m b e r o f t h e h y d r o c y c l o n e d e f i n e d as R e U 0 / v

= d i m e n s i o n l e s s r a d i a l d is t a n c e f r o m t h e ax i s o f s y m m e t r y i n c y l i n d r i c a l c o o r d i n a t e s

= d i m e n s i o n l e s s t i m e

-- m e a n i n l e t v e l o c i t y , c m / s

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

= t a n g e n t i a l v e l o c i t y o f t h e f l u i d , c m / s

= d i m e n s i o n l e s s t a n g e n t i a l v e l o c i t y o f th e f lu i d

= a x i a l v e l o c i t y o f t h e f l u i d , c m / s

= d i m e n s i o n l e s s a x i a l v e l o c i t y o f t h e f l u i d

= d i m e n s i o n l e s s a x i a l d i s t a n c e f r o m t h e r o o f o f t h e h y d r o c y c l o n e i n c y l i n d r i c a l

c o o r d i n a t e s

= P r a n d t l m i x i n g l e n g t h , c m

= d e n s i t y o f t h e s l u r r y , g / c m 3

-- t u r b u l e n t v i s c o s i t y , g / c m . s

= k i n e m a t i c v i s c o s it y , c m 2 / s

= d i m e n s i o n l e s s v o r t i c i t y

= d i m e n s i o n l e s s s t r e a m f u n c t i o n

= d i m e n s i o n l e s s a n g u l a r s p in v e l o c i ty