Effect of Volute Tongue on Unsteady Flow in a

©Freund Publishing House Ltd. London

International Journal of Turbo and Jet Engines, 21, 223 - 231 (2004)

Effect of Volute Tongue on Unsteady Flow in a Centrifugal Pump

Weng-Guang Li

Department of Energy and Engineering for Power, l.angongping Road, Lanzhou, 730050, China. E-Mail:

Lanzhou University of Technology, Wen-GuangLi

85

Abstract T h e u n s t e a d y f l o w s in t h e v o l u t e of a s i n g l e - s t a g e , c a n t i l e v e r e d c e n t r i f u g a l p u m p with s p e c i f i c s p e e d o f 9 3 h a v e b e e n m a p p e d by u s i n g L D V at both best e f f i c i e n c y a n d p a r t - l o a d i n g p o i n t s r e s p e c t i v e l y w h i l e p u m p i n g w a t e r . T h e r e s u l t s s h o w that the w a t e r velocity f l u c t u a t e s p e r i o d i c a l l y with the i m p e l l e r s w e e p a n g l e with respect to m e a s u r i n g p o i n t s w h e r e t h e flow field will b e i n v e s t i g a t e d . A s t h e d i s t a n c e b e t w e e n i m p e l l e r tip and m e a s u r i n g p o i n t s increases, t h e p e r i p h e r a l c o m p o n e n t o f w a t e r v e l o c i t y d a m p s rapidly, b e c o m e s m o r e and m o r e u n i f o r m , and its p e r i o d i c a l f l u c t u a t i o n is s u p p r e s s e d c o n t i n u o u s l y . T h e m a g n i t u d e in t h e f l u c t u a t i o n is a b o u t 3 0 % - 7 0 % o f the local m e a n velocity. The f l u c t u a t i o n m a g n i t u d e in t h e How a n g l e is larger t h a n t h e m a g n i t u d e in velocity by an o r d e r o f 1-2. T h e m o r e closely t h e m e a s u r i n g p o i n t a p p r o a c h e s t o the v o l u t e t o n g u e , t h e larger t h e f l u c t u a t i o n s in velocity a n d flow a n g l e b e c o m e , and t h i s r e s u l t s in a m o r e and m o r e h e a v y e f f e c t o n t h e u n s t e a d y f l o w . The f l u c t u a t i o n s in t h e velocity and How a n g l e at p a r t - l o a d i n g p o i n t arc larger than at b e s t e f f i c i e n c y p o i n t . T h e flow pattern is n o n - a x i s - s y m m e t r i c a l a l o n g t h e i m p e l l e r p e r i p h e r y : T h e flow is d i f f u s e d in t h e v o l u t e . H o w e v e r , t h e l l o w velocity is m o r e u n e v e n , its v e l o c i t y g r a d i e n t is m o r e s e v e r e and f l o w d i f f u s e s m o r e heavily at p a r t - l o a d i n g p o i n t than t h a t at best e f f i c i e n c y p o i n t .

Keywords: c e n t r i f u g a l p u m p , v o l u t e , internal flow, u n s t e a d y llow

1. Introduction

also results in an unbalanced pressure distribution along the impeller periphery, which will contribute

Based on fundamentals of fluids dynamics, the

to an additional radial force on the impeller. The two

essential features of fluid flow in turbomachinery

forces will introduce vibration on the shaft of pumps

impellers have been investigated (Dean, 1959). The

and has a negative effect on the pump operation.

results showed that fluid velocity pressure, etc.,

Furthermore, the unsteady flow in the volute or

varied from blade suction side to pressure side: the

diffuser of pumps will create noise. The problems of

flow out of the impeller was non-uniform, and led to

vibration and flow noise owing to unsteady flow

an unsteady flow in geometry elements behind the

must be addressed for centrifugal pumps with high

impeller, such as the diffuser or volute. On the other

head and speed.

hand, the geometry elements behind the impeller will have an influence on the fluid flow inside the impeller.

Therefore,

between

impellers

a

heavy

(rotors)

and

fluid the

rotor-stator

interaction

the

present

time,

the

impeller-diffuser

interaction

(Eisele et al., 1997; Akhras et al, 2002; Aysheshim,

geometry

2002; Wuibaut et al., 2002). However, the impeller-

elements (stators) should exist. The

At

interaction has been tackled by many researchers

volute interaction has been explored by only few also

occurs

in

scholars (Yuasa and Hinata, 1979; Parrondo et al.,

only

2002; Gonzalez et al., 2002). The unsteady pressures

generates an unsteady fluid dynamic force acting on

on the casing wall of the volute, the wall vibration

the impeller by liquid in the tangential direction, but

and noise generated by flow in a centrifugal pump

centrifugal

pumps.

The

interaction

not

Brought to you by | Glasgow University Library (Glasgow University Library) Authenticated | 172.16.1.226 Download Date | 3/18/12 5:31 PM

223

224

were

measured

(Yuasa

and

Hinata,

1979).

The

2. Experimental Set-Up

results showed that the fluctuating pressure due to the

interaction

between

impeller

and

volute,

2.1. Test Pump

especially the tongue of the volute, has a close

The test p u m p is the single-stage, cantilevered

relation to flow noise and casing wall vibration. T h e

centrifugal pump whose f l o w rates are 25mVh, head

pressure fluctuation with blade passing frequency

8m, rotating speed I485r/min and specific speed 93*,

was observed by using sensors. T h e pressures with

respectively. The impeller and blade were designed

respect to t w o impellers of different diameter have

by means of one-dimensional flow theory, the eye

been recorded and simple acoustic models have been

diameter

established ( P a r r o n d o et αϊ, 2002). The properties of

diameter 180mm, the n u m b e r of blades 4 and the

acoustic sources were established after fitting the

discharge

available experimental data of pressure fluctuations

rectangular shape with a width of 4 0 m m and throat

in volute by m e a n s of a least-square error procedure.

area of I 4 4 0 m m : . Figure 1 illustrates the geometry

T h e interaction between impeller and volute tongue

of

plays an important role in pressure fluctuations and

measuring points as well as the three glass w i n d o w s

flow noise

for LDV measurement.

generation.

The

pressure

fluctuations

the

of

the

impeller

angle

20°.

impeller,

is

The

62mm,

section

volute

and

the

of

the

outlet

volute

locations

is

of

o w i n g to tongue in the volute of a centrifugal p u m p were measured by use of both piezo-resistive and piezo-electric pressure sensors installed around the volute wall ( G o n z a l e z et al2002). demonstrated

that

the

effect of

The tongue

results on

the

pressure oscillation is transmitted both upstream and d o w n s t r e a m of the volute tongue. Obviously, the research issues mainly focus on the relation between the volute tongue and the flow noise, as well as pressure fluctuation properties due 270"

to the tongue in the impeller-volute interaction in centrifugal p u m p s : this has led to quite a few current experimental

observations,

whose

aim

is

Fig. I: Impeller, volute and measuring points

to

investigate the features of unsteady fluid flow in the volute

of

measurements

centrifugal of

pumps.

unsteady

flow

The

LDV

have

been

p e r f o r m e d at various locations in three sections in

2.2. LDV System T h e apparatus used is a four-beam, two color

the volute of a single-stage, cantilevered centrifugal

(blue

p u m p w h o s e flow rates are 25m 7h. head 8m and

dimensional

rotating speed I485r/min respectively. T h e working

system is composed of nine parts: (1) Argon ion

liquid is tap water and working conditions for L.DV

laser source, 2 . 5 W ,

tests are both best efficiency and part-loading points.

model 9 2 0 1 , probe and optical fibers, (3) ColorLink|j

T h e experimental data have shown that the more

model 9230, (4) Digital data processor. IFA750, (5)

closely the measuring points approach to the volute

turbomachinery

tongue, the m o r e the flow fluctuates, and the more

encoder, (7) Personal c o m p u t e r ( P C ) model 4 8 6 / 2 5 .

clearly the unsteady feature is demonstrated.

(8) Oscilloscope, (9) Three-rectangular

The

farther the points f r o m the tongue, the more uniform

and

green),

back-scattered

LDV system produced

mode,

two-

by TSI.

The

I N N O V A 7 0 , (2)

resolver,

model

1989,

ColorBurst,

(6)

coordinate

traverser.

the velocity distribution. «V

3.65n[r/min]

to m


!H[ m

shaft

i() 75

225

The cannot

measuring rotate

with

volume the

of the

impeller

LDV shaft,

system and

extracted from the data files by means of a program developed in-house.

is

stationary. Hence, a shaft encoder has been installed on the end of the motor shaft to record the impeller sweep angle past the measuring volume and to set up

2.4. Working Liquid and LDV Test Conditions

an exact one-to-one relation between data sampled and the impeller sweep angle. After the ensemble

The working liquid is tap water, in which there

averages for the data sampled are performed by data

are a lot of sojid particles. These particles, never-

processing software PHASE provided by TS1, the

theless, can be used as seeding particles of LDV.

unsteady

Figure 2 demonstrates the hydraulic performance

flow velocities

in the

volute

will

be

curves of the test centrifugal pump under a rotating

available. The LDV probe can be moved to measuring

speed of 1485 r/min. The symbols Q, Η, Ρ and η

points arranged in the flow field by means of the

stand for the flow rate, head, shaft power and effi-

three-rectangular coordinate traverser, then the data

ciency of the pump respectively. The words BEP

sampling and processing, etc., can be conducted by

and PLP denote the working condition for the LDV

applying the PC.

test. The flow rate is Q n E P = 5.933 L/s at best efficiency point (BEP), Q = 0.58 Q w v

at part-

loading point (PLP). Reynolds number Re = D2U2/v

2.3. Measuring Points Distribution

2.5 χ I0 6 , where D2 is diameter of impeller, (J2 is

The unsteady velocity measurements take place

impeller speed tip, v, the kinematic viscosity

in three sections of the volute, i.e., sections IV. VI and VIII. These sections are located at angle 0 =

mental results confirmed that the uncertainty

180°, 270°, 360°, which are marked in Fig. I. The

of

efficiency is 0.76% ~ 1.15%. The uncertainty of

measuring points have been distributed, as shown in

LDV in velocity is 1.8%, and the uncertainty of

Fig. I. on the central line of the section, that is,

measuring point location is 5.0%. Therefore, the

through the middle of the impeller blade span. The

total uncertainty of the LDV system will be 5.3%.

impeller sweep angle φ past measuring points will be determined automatically by the turbomachinery

100

resolver based on its selected work mode, impulse

9»)

80

numbers per revolution for sampling, number of

70

sectors and windows opened. Table I illustrates the

Μ

£CL·,

geometrical locations of the measuring points.

50 ξ. Γ40 30

The data flies in ASCII format, which include

20 10

the absolute velocity and other information about

υο

flow at each measuring point, can be worked out by using program PHASE equipped in LDV system.

2: Pump performance curves

The velocity and other useful information can be

Table I Geometrical locations of measuring points in LDV measurements Κ (mm)

2

5

10

15

20

25

30

35

R/R2

1.022

1.056

I 1 11

1.167

1.222

1.278

1.333

1.389

Y coordinate from measuring point to impeller tip R

of

water at 20°, is ν = 1.06 χ I0" 6 m 2 /s. The experi-

distance from the point to geometrical center of impeller

R2 radius of impeller


226

3. Results and Discussion

impeller s w e e p angle at measuring points in the volute. T h e r e are f o u r periodicities, which are equal

J. 1. Velocity Variation along Radius Figure

3

illustrates

the

to the n u m b e r of blades of the impeller, at the angle the

interval o f 360°. T h e positions o f Vr » 0 or α χ in

tangential c o m p o n e n t V,„ radial c o m p o n e n t Vr and

variations

of

abscissa, where the tangential c o m p o n e n t of velocity

flow angle a o f absolute velocity o f water at the

is m i n i m u m , should correspond to the trailing e d g e s

measuring points of Y = 2 m m , 10 m m , 2 0 mm in

of blades at the m e a s u r i n g point Y = 2 m m . T h e

section VI (Θ = 2 7 0 ° ) at best e f f i c i e n c y point. T h e

intervals c o v e r i n g these positions should correspond

f l o w angle is defined as the angle between

the

to the flow p a s s a g e s of the impeller. T h e figure

absolute velocity and the impeller rotating direction.

illustrates that the absolute velocity distribution is

T h e abscissa stands f o r impeller s w e e p angle φ past

uneven at impeller discharge, and leads the velocity

measuring points. T h e

entering the volute to depend upon the sweep angle

ordinate

denotes

the

two

velocity c o m p o n e n t s and flow angle, respectively. In fact, the ensemble revolutions

two

components

average of

of

the

are

data

impeller

obtained

sampled via

the

or time elapsed; that is, the flow is unsteady.

through

Nevertheless, the magnitude of absolute velocity ,

in

many

especially the tangential c o m p o n e n t , d a m p s rapidly

LDV

data

and b e c o m e s m o r e and m o r e uniform; its periodical

processing program P H A S E . T h e increase direction

fluctuation is suppressed gradually with the increase

o f the impeller sweep angle φ is also the increase

in

diretion o f sampling time. T h e d e c r e a s e direction is

impeller tip. T h i s p h e n o m e n o n implies that, because

the rotating direction of the impeller. T h e r e f o r e , Fig.

of the turbulent m i x i n g o f low. the w a k e fluctuation

3 demonstrates essentially the variation of velocity

behind the impeller gradually w e a k e n s and b e c o m e s

with time elapsed at m e a s u r i n g point in the volute.

uniform.

e=27«·

Y=10mm Y*20nim Y*20inm

2

-

_

120

1B0

240

300

illustrates

measuring

the

points

variations

^ r

I ι .

60

4

between

Q=1.0Qssb 1*270·

Qsl.OQ.,

0

distance

Figure

T h e absolute velocity varies periodically with the

A

the

360

0

60

120

180

240

Y=2mm Y=10mm Y=20mm

300

360

0

60

120