a Study of Radiator Cooling Fan With Labyrinth Seal

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JSAE Review 24 (2003) 431–439

A study of radiator cooling fan with labyrinth seal

Kota Shimada, Kazuhide Kimura, Hiroshi Watanabe

Product Development Center, Toyo Radiator Co., Ltd., 4-14, Shioya-cho, Minami-ku, Nagoya-shi, Aichi 457-8560, Japan

Received 25 September 2001; received in revised form 15 May 2003

Abstract

The performance of a conventional axial flow fan and other fans, which have various types of rings fixed at the blade tips of the

same fan, were compared. Then, the effects of these rings were investigated in detail. The fans displayed higher performance with a

fixed ring. The authors also showed that a labyrinth seal is able to seal the tip clearance between the ring and the fan shroud of thefan with a fixed ring, especially when the fan operates in a low static pressure difference. As a result, the fan with a fixed ring and the

labyrinth seal prevented the effect of the tip clearance, and achieved higher fan performance and lower fan noise than the

conventional fan in any practical tip clearances.

r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier B.V. All rights reserved.

1. Introduction

Two different types of motor fans are generally used

for radiator cooling of automobiles. One is the conven-

tional fan, which does not have a ring at the blade tips,and the other is the type of fans with such a ring. The

latter fans generally have numerous forward- or back-

ward-skewed blades with a short chord length. With

these fans, it is sometimes difficult to maintain the

strength of the blades, because of the difference in the

center of gravity of each cylindrical section of a blade.

These fans maintain strength by the ring which is fixed

at the blade tips. These rings are generally called bands.

Therefore, the main reason to attach a ring is to

improve the strength. However, it is known that the

reverse flow or the turbulence at the tip clearance affects

the fan performance and the noise level. Therefore, there

is a high possibility that the existence of the ring affects

not only strength but also performance. However, there

are few reports which have researched the influence of

the ring in the past.

Here, the authors investigated the effect of the ring

fixed at the blade tips. Then, a labyrinth seal was

provided between the ring and fan shroud in order to

decrease the reverse flow at the tip clearance. The

performance and effectiveness of the labyrinth seal were

studied. As a result, the fan with a fixed ring and the

labyrinth seal achieved higher performance and a lower

noise level.

2. The performance of the fan with a fixed ring

Fig. 1 shows the fundamental fan that was used in this

experimental study. The fan diameter is 340 mm and the

hub diameter is 125 mm. It has seven blades, and therotational direction is clock-wise. The blades are skewed

forward in order to decrease broad band noise. The

design flow coefficient is f ¼ 0:18 at the operating point

when the fan was mounted in an engine compartment.

The fan was designed quasi-three-dimensionally, with

two-dimensional NACA65-series cascade data, which

was added with some corrections [1,2]. The difference in

fan performance with or without several kinds of rings,

which are fixed at the blade tips, was investigated. Fig. 2

shows the model with a fixed ring. In spite of the

difference in ring types, the diameter of all these rings is

362 mm.

Fig. 3 shows the shape of the shroud and the relative

location between the blade tip and the shroud of the

conventional fan. The shroud has a bell-mouth which is

formed with Radius R=11 mm at both the inlet and the

outlet. The full length of the shroud is L0=49.7 mm, and

the relative distance of the trailing edge of the blade tip

and the inlet of the shroud is L1=38.7 mm. Standard tip

clearance e is 3 mm. Fig. 4 shows the model with a fixed

ring, which only has a bell-mouth at the inlet side. Type

A is the most popular commercially available form. The

shape of the ring is exactly the same as the fan shroud

shown in Fig. 3, except for the non-existence of the

ARTICLE IN PRESS

0389-4304/$30.00 r 2003 Society of Automotive Engineers of Japan, Inc. and Elsevier B.V. All rights reserved.

doi:10.1016/S0389-4304(03)00076-6 JSAE20034529



bell-mouth at the outlet. Here, the cover rate of the

shroud to the ring at the outlet is L2=3 mm. Fig. 5

shows the model with a bell-mouth added to the outlet

of the shroud of type A. The bell-mouth radius R of this

is the same as R of Figs. 3 and 4. This is also a

commonly seen fan model.

Fig. 7 shows the performance of these types. There is

no difference in performance at a higher flow coefficient

from the design flow coefficient f ¼ 0:18; between the

conventional fan and type A. The pressure coefficient c

decrease, which seems to be due to blade tip stall, is

shown at a lower flow coefficient from f ¼ 0:18 on the

conventional fan. However, it is not shown on type A.

Consequently, the existence of the ring has the effect of

creating a slight stall or a delay of stall in performance.

Type B shows the highest performance in the region

above f ¼ 0:15: This is due to the effect of rectification

of outlet flow and block of reverse flow stream by the

fixed bell-mouth on the outlet side. However, Type B

shows a decrease in c with a lower f; starting from

f ¼ 0:13; compared to type A. This observation means

that the outlet bell-mouth interfered with the main flow,

which became strong mixed flow or radial flow. There-

fore, this lower f region seems to be the radius flow

region [3].

Fig. 6 shows the fan with a fixed ring, which has a

bell-mouth at both sides. Although the shape of the ring

ARTICLE IN PRESS

Fig. 4. Fixed ring type A.

Fig. 5. Fixed ring type B.

Fig. 1. Conventional fan.

Fig. 2. Fan with a fixed ring.

Fig. 3. Conventional type.

Fig. 6. Fixed ring type C.

S

Type A

Type B

Conventional Type

=32000rpm

0.05 0.1 0.15 0.2 0.25 0.3

Flow Coefficient

0

0.1

0.2

0.3

0.4

P r e s s u r e C o e f f i c i e n t

0

30

60

S t a t i c P r e s s u r e E f f i c i e n c y

S

( % )

Fig. 7. Fan performance of fixed ring types.

K. Shimada et al. / JSAE Review 24 (2003) 431–439432



of type C is exactly the same as the shroud of the

conventional fan, the fixed ring rotates with the fan.

Fig. 8 shows its performance(s). Type C shows a little

higher c in the region of f=0.1–0.17, but almost the

same performance as type B. Generally, there is the

danger of deterioration in fan efficiency due to air

friction loss at the surface of the ring. However, in the

case of the fan that has a large tip clearance, it seems

that the effect of the restraint of reverse flow and

turbulence at the blade tips by the rings compensates for

that friction loss. Therefore, these fans with a fixed ring

have a better performance.

However, the tip clearance between the ring and the

fan shroud exists once more, though reverse flow and

turbulence are restrained at the blade tips. This tip

clearance has reverse flow and becomes one of the

causes of deterioration in performance. Therefore, the

authors focused on improving performance by adding a

seal structure between the ring and the fan shroud toprevent the reverse flow.

3. The labyrinth seal between the ring and the fan shroud

3.1. Operating pressure and clearance of the labyrinth

seal

It is problematic for a mechanical contact seal to be

used for the clearance between a ring and a shroud, as it

is necessary to maintain some clearance there. Accord-

ingly, the authors directed their attention to the

labyrinth seal as a clearance seal. This device contains

expansion grooves and throttles called fins, and

exchanges fluid pressure into velocity in order to

maintain the differential pressure. This is called a

labyrinth seal. Labyrinth seals are frequently used for

compressors and turbines in order to seal their shafts.

Generally, the maximum operating static pressure

difference of these seals is 1.0 102 –3.0 104 kPa, and

the clearance e of the seal section is about 0.1–0.6 mm

[4]. Therefore, most research concerning the use of

labyrinth seals has been conducted within these limits in

the past. However, in the case of these fans mentioned

above, the practical operating static pressure difference

is P s=50–250 Pa and the clearance e of the seal section

or tip clearance is 3 mm. There is room for further

research about labyrinth seals in these extremely low

static pressure difference P s and large clearance e;because their performance and the phenomenon is still

unclear. The authors chose type C because enough roomis available to construct a labyrinth seal within the ring.

3.2. Performance of labyrinth seals of straight-through

type and combined-staggered type

In order to discover the effectiveness of a labyrinth

seal, which has a large clearance e and operates in an

extremely low static pressure difference P s, the seal

performance was investigated and the result was

compared to the ideal labyrinth. The velocity, which

appears at the fin edge, converts to a static pressure

constantly in a static temperature in an expansiongroove and then the velocity is completely lost just

before the next fin. This is called the ideal labyrinth.

Labyrinth seals are generally divided into two

categories: the straight-through type and the staggered

type. In this case, the labyrinth seal does not change

radius. Fig. 9 shows the shape of the straight-through

type and the state of the tests. This seal consists of three

fins and two expansion grooves, and includes the bell-

mouths as the fins. The seal disk is inserted into the

inside of the ring to measure the leakage rate only from

the seal section. The ring with the labyrinth seal is able

to rotate with the seal disk. The influence of the rotationwas also investigated.

Fig. 11 shows the performance of the straight-through

type. The performance of the ideal labyrinth, which is

n¼ 123; is also shown in Fig. 11. Here, n means the

number of fins, and n ¼ 1 means just a ring-shaped

orifice on the wall, which divides a room into two

different rooms of pressure. F is a dimensionless number

showing the leakage rate of the labyrinth seal and

named labyrinth function. In the case of the ideal

labyrinth, F is shown by Martin’s approximate equa-

tion, as shown [5] in Eq. (1). Here, the pressure ratio l is

the ratio of the outlet and the inlet absolute pressure,

shown as l ¼ P n=P 0: In this experiment, the differentialpressure is significantly small and l is nearly 1. In the

case of the experimental value, F is generally shown in

ARTICLE IN PRESS

S

Type C

Type B

0.05 0.1 0.15 0.2 0.25 0.3

Flow Coefficient

0

0.1

0.2

0.3

0.4


0

30

60

S t a t i c P

r e s s u r e E f f i c i e n c y

S ( % )

=32000rpm

Fig. 8. Fan performance of fixed ring type C.

ShroudAir

Fin1 Fin2

Fin3

PressureChamber

Seal Disc ExpansionGrooves

Fig. 9. Straight-through type.

K. Shimada et al. / JSAE Review 24 (2003) 431–439 433



Eq. (2) [5]. Here, G : leakage rate kg/s, F : opening section

of labyrinth seal m2, P 0: absolute inlet pressure kPa, u0:

specific volume m3/kg,

F ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 l2

n þ In 1=l

s ; ð1Þ

F ¼ G F

ffiffiffiffiffiffiP 0

u0

r 1

: ð2Þ

When the rotational speed is 0 rpm, the curve of the

straight-through type is located at a little lower F region

than the ideal labyrinth of n ¼ 1 in the whole pressure

region. In this state, the straight-through type is almost

the same as an orifice rather than a labyrinth seal. It is

well known that the drawback of the straight-through

type is deterioration in performance, which is caused by

the air flow to the axial direction at the clearance area

[5], otherwise known as carry-over. It seems that the

expansion grooves did not work effectively because of

the large clearance, and just worked like a pipe line or a

tube. However, when the rotational speed is 2500 rpm,

the leakage rate decreases significantly, especially in the

lower P s. When P s=100Pa, the leakage rate is the same

as the ideal labyrinth of n ¼ 2: When P s=50 Pa, the

leakage rate is the same as the ideal labyrinth of n ¼ 3:Fig. 10 shows the combined-staggered type. This type

has a counter-throttle added onto the shroud surface in

the upstream expansion groove of the mentioned

straight-through type. This counter-throttle intercepts

the carry-over flow. This type consists of both the

staggered type, with an expansion groove at the

upstream side, and the straight-through type, with an

expansion groove at the downstream side.

Fig. 12 shows the seal performance of the combined-

staggered type. When the rotational speed is 0 rpm, it

shows significantly higher performance than the

straight-through type, and indicates a little higher F

than the ideal labyrinth of n ¼ 3: This means that the

combined-staggered type has nearly the same perfor-

mance as the ideal labyrinth with the same number of

fins. When the rotational speed is 2500rpm, theperformance improves slightly, especially in the lower

P s region. At this rotational speed, the leakage rate

decreases more than the ideal labyrinth of n ¼ 3 in the

whole of the pressure region. Thus, as mentioned before,

the combined-staggered type shows the ideal perfor-

mance. However, this does not mean that it is working

as an ideal labyrinth without carry-over, but that it is

still working with considerable carry-over. The reason

for maintaining the same performance as the ideal

labyrinth is due to the fluid loss of the wind stream by

the counter-throttle [6].

3.3. Influence of rotation

Fig. 13 shows the change in the leakage rate of the

straight-through type with the ring tip speed U and the

specific leakage rate G: G is the ratio of the leakage rate

in the rotational state G r and the leakage rate in the

stationary state G s, thus shown as G=G r/G s. Here, the

peripheral velocity U =47.4 m/s corresponds with the

fan speed at 2500 rpm. G decreases when there is an

increase in U . This is particularly evident when P s is low.

When P s=50 Pa and U =47.4 m/s, G reaches G ¼ 0:58:However, with previous research concerning the

ARTICLE IN PRESS

Air Shroud

Seal Disc

PressureChamber

Counter-Throttle

h

Fig. 10. Combined-staggered type.

0.06

0.05

0.04

0.03

0.02

0.0150 100 150 200 250

Static Pressure Difference Ps (Pa)

L a

b y r i n t h F u n c t i o n Φ 0 rpm

n = 1 ( O r i f i c

e )

n = 2

n = 3

=3

Experimental(n=3)

Calculated IdealLabyrinth

2 5 0 0

r p m

Fig. 11. Seal performance of straight-through type.

0.06

0.05

0.04

0.03

0.02

0.0150 100 150 200 250

Static Pressure Difference Ps (Pa)

L a b y r i n t h F

u n c t i o n Φ

0 r p m

2500rpm

n = 2

n = 3

=3

h =3

a =3 n = 1 ( O r i f i c

e )

Experimental(n=3)

Calculated IdealLabyrinth

Fig. 12. Seal performance of combined-staggered type.




straight-through type labyrinth seal [7,8], G is about

G=0.8–0.9 when U =250 m/s and l ¼ 0:5: In addition,

they report that there is no significant decrease in the

leakage rate when under U =50 m/s. Therefore, it was

discovered that G is seriously affected by U when P s is

extremely low. In spite of the case that the performance

is almost the same as the ideal labyrinth of n ¼ 1 due to

the large carry-over by the large e; it is possible to

further improve the performance closer to the ideal

labyrinth of n=2–3, by increasing the rotational speed

to approximately U =50m/s.

Fig. 14 shows the change in the leakage rate with

rotation in the combined-staggered type. It shows

almost the same tendency as the straight-through type,

yet G is comparatively larger. Notably, when

P s=250 Pa, G does not change significantly even when

U increases. In the case of P s=150 Pa, G starts to

decrease more sharply from about U =40 m/s. In the

case of P s=50 Pa, G starts to decrease drastically from

about U =20 m/s. Generally, the staggered type brings

about a significant decrease in the leakage rate in lower

U compared to the straight-through type [4]. Therefore,

the results shown in Fig. 14 are different in comparison

to the past results. The reason for the decrease in G with

U is due to the increase in fluid loss in the labyrinth. It

has been considered that the fluid loss increases as theeddy, which already existed when the labyrinth is

stationary, accelerated as a spiral eddy due to the

rotation [7]. However, the details are still not fully clear,

providing the impetus for further research to be done.

4. The fan performance with a labyrinth seal

4.1. Fan performance of the straight-through type and the

combined-staggered type

As mentioned in the last section, these labyrinth sealsare able to achieve a similar performance to that of the

ideal labyrinth, even though it has large e with carry-over.

Therefore, the authors tested and investigated the effect

of these labyrinth seals on the fan performance, shown in

Figs. 9 and 10, when these labyrinth seals are applied to

type C as shown in Fig. 6. In Fig. 15, the results of the

tests are shown. The combined-staggered type shows a

higher c2f curve for the most part and Zs curve over the

region of f ¼ 0:16; compared to the straight-through

type. However, in the region of extreme low c; the

difference in performance from the straight-through type

is very slight, as the performance of the labyrinth seal of

the straight-through type improves significantly. In

addition, both of the fans achieve a higher performance

in most regions than the conventional fan. On the other

hand, there was no large difference in the fan perfor-

mance between the straight-through type which is shown

in Fig. 9 and type C in Fig. 6.

4.2. Fan performance and noise level when tip clearance is

different

It is well known that the tip clearance seriously affects

the fan performance [3]. The authors investigated the

ARTICLE IN PRESS

1.0

0.9

0.8

0.7

0.6

0.50 10 20 30 40 50

Peripheral Velocity U (m/s)

S p e c i f i c L e a k a g e R a t e

Ps=250Pa

Ps=150Pa

Ps=50Pa

=3

Fig. 13. Effect of rotation in straight-through type.

1.0

0.9

0.8

0.7

0.6

0.5 0 10 20 30 40 50

Peripheral Velocity U (m/s)

S p e c i f i c L e a k a g e R a t e

Ps=250Pa

Ps=150Pa

=3

h =3

a =3

Ps=50Pa

Fig. 14. Effect of rotation in combined-staggered type.

S

ConventionalType

Straight-ThroughType

Combined-Staggered

Type

0.05 0.1 0.15 0.2 0.25 0.3

Flow Coefficient

0

0.1

0.2

0.3

0.4


0

30

60


S

( % )

=32000rpm

Fig. 15. Fan performance of combined-staggered type.




fan performance of the fans with a fixed ring, when the

tip clearance was changed. It is easy to predict that

the seal performance of the straight-through type

decreases when e increases, because of the increase in

the carry-over. However, in the case of the combined-

staggered type, the carry-over is always intercepted

when the axial clearance a remains constant and theheight of the counter-throttle h is always the same as e ;as shown in Fig. 10. Therefore, it seems that the decrease

in performance is comparatively smaller than the

straight-through type.

Fig. 16 shows the performance(s) of the conventional

fan shown in Fig. 3, when the tip clearance is different.

Both c and Zs decrease substantially with an increase in

the tip clearance. Fig. 17 shows the case of the straight-

through type under the same conditions as above. Both

c and Zs decrease with an increase in e: However, the

decrease in performance is slight compared to the

conventional type, especially in the low c region.

Fig. 18 shows the performance in the case of the

combined-staggered type under the same conditions. A

large decrease in performance is not observed, even

though e is increased from 1 to 7 mm.

Fig. 19 shows the noise level of each type of fan when

e was changed. Each fan was mounted on the same

radiator under the same conditions, and the noise level

was measured at 1 m upstream from the radiator core

surface on the rotational axis. In the case of the

conventional fan, the noise level increases continuously

when e increases from 1 to 5 mm. However, the noise

level becomes saturated and there is no change when e

was increased from 5 to 7 mm.

In the case of the straight-through type, the noise level

is significantly small compared to the conventional fan,

even though the noise level increased continuously when

e was increased from 1 to 7 mm. The fixed ring prevents

leakage of the air from the pressure side to the suction

side of a blade through the tip clearance of theconventional fan. As a result, the noise level decreased

due to the lower prominence of the stall or the eddies at

the blade tips [3].

In the case of the combined-staggered type, the noise

level is the lowest in all tip clearances, and a large

increase in the noise level is not observed in the region

where e is from 3 to 7 mm. The main difference between

the straight-through type and that of the combined-

staggered type is the difference in the leakage rate. Thus,

it is clear that the increase in the leakage rate made the

noise level higher. The cause seems to be the suction of

the turbulence component or eddies contained therein to

the leakage flow of the fan.

Generally, it is extremely difficult to control the tip

clearance when the fan shroud is plastic molded by an

injection molding machine, because of the contraction

of the resin. Therefore, a large plus tolerance is usually

given to e for safety reasons, and the designers have to

pre-consider the performance decrease due to this.

However, in the case of the fan with a combined-

staggered type labyrinth seal, there is no considerable

effect on performance and it is possible to keep the

fan noise lower, even though the tip clearance is

increased.

ARTICLE IN PRESS

0.05 0.1 0.15 0.2 0.25 0.3

Flow Coefficient

0

0.1

0.2

0.3

0.4

P r e s s u r e C o e f f i c i e n t S

=1

=3 =5

=7

0

30

60


S

( % )

2000rpm

Fig. 16. Performance of conventional type with different e :

S

0.05 0.1 0.15 0.2 0.25 0.3

Flow Coefficient

0

0.1

0.2

0.3

0.4


0

30

60


S

( % )

=1

=3

=5

=7 =h2000rpm

Fig. 17. Fan performance of straight-through type with different e :

S

=1

=3

=5

=7

0.05 0.1 0.15 0.2 0.25 0.3

Flow Coefficient

0

0.1

0.2

0.3

0.4

P r e s

s u r e C o e f f i c i e n t

0

30

60

S t a t i c P

r e s s u r e E f f i c i e n c y

S

( % )

=h2000rpm

Fig. 18. Fan performance of combined-staggered type with different e:

Conventional Type

Straight-ThroughTypeCombined-

Staggered Type

2000rpm

Tip Clearance (mm)

65

70

75

80

S o u n d P r e s s u r e L e v e l

S P

L d B ( A )

1 5 72 3 4 6

Fig. 19. Noise of the fans with different e:




5. Form of fan noise occurrence

The authors demonstrated the decrease in the fan

noise levels and the possible causes of this in the case of

the fans with a fixed ring. Next, the authors observed the

difference of fan noise occurrence between the conven-

tional fan and the fan with straight-through typelabyrinth seal.

Generally, the fan noise of a simple axial flow fan,

which has a single stage, is divided into two categories,

the turbulent noise (broad band noise) and the revolu-

tion noise (discrete frequency noise). A reduction in

both noise types is essential. The revolution noise

frequency is generally expressed by Eq. (3). Here, F r:

revolution noise frequency (Hz), ni: integer (ni=1,2,3y),

N f : rotational speed (rpm), Z: number of blade:

F r ¼ niN f Z =60: ð3Þ

The fan noise is frequently evaluated with an A-weighted value correction. However, in the case of

radiator fans for automobiles, the revolution noise does

not often affect the corrected noise level, because the

frequency of revolution noise is comparatively low, which

is located out of the high sensitivity region of the A-

weighted value. In spite of this, the revolution noise is

very annoying. This is because the revolution noise is at

the specific frequency of noise that is pure-sound, against

the broad band random-sound, and it is known that

people feel pure-sound as excessive tonal annoyance [9].

Therefore, the authors decided to discuss these two noises

independently.

Fig. 20 shows the results of the sound pressure

distribution of the fans with and without a ring. This

experimental condition is suitable for the comparison in

Fig. 19 at e ¼ 3 between the conventional fan, and the

fan with straight-through type labyrinth seal. The fan

with the straight-through type labyrinth seal shows

lower turbulence noise at the region of 500–5 kHz, and

this is especially significant around 1 kHz. From Eq. (3),

the revolution noise frequency is F r=233 Hz, in ni =1.

As shown in Fig. 20, the primary peak is observed

around 233 Hz, and the secondary peak is observed

around 466 Hz. Both of them improve in the case of the

fan with the straight-through type labyrinth seal.

In order to discover the phenomenon of noise

occurrence of these two fans in detail, the sound sources

were identified. Specifically the three-dimensional sound

intensity of the revolution noise and the turbulence noisewas measured on the surface of the radiator core. The

fan, shroud, radiator and operating conditions are the

same as in the case of Figs. 20 and 19, with e ¼ 3:Moreover, the measuring positions to discover the

sound sources are close together at 62.3 mm intervals

upstream from the radiator core surface. The shape of

the core is a 374 mm square. There are 49 measurement

points, with a distance of 62.3 mm between each point.

These seven points exist in each X (abscissa) and Y

(ordinate) directions, dividing both sides equally into 6.

The sound intensity was measured by a pair of two

different microphones, facing each other, at all of the 49

measurement points in the three directions that are

shown in X , Y , Z . The center of a fan’s axis is located at

the center of the radiator core. The axial length from the

upstream end of the ring or the fan shroud to the

downstream core surface is 27mm. Between this

distance, the skirt, which has a depth of 27 mm and

which is thin and cubically shaped, seals the whole of the

radiator core surface.

Fig. 21 shows the results of the measurement of the

sound intensity. First, turbulent noise was compared.

The reference broad band noise is 1 kHz. The axial

direction constituent of the intensity, which is radiated

from the core surface, is shown on the contour maps.There is no large difference in the height, and the maps

show a quite gentle unevenness. Thus, the whole of the

core surface is a nearly uniform sound source. However,

the intensity level tends to be higher at the core center

than the four corners. There is a possibility that the

sound of the air flow which is passing through in the

core is being measured as well as the sound of eddies

from the blades, because the portion seems the same as

the portion of high air velocity on the core. The fan with

a fixed ring shows a lower and gentler unevenness in

intensity level as a whole than the conventional fan. The

X–Y direction constituent is shown on the vector maps.

The sound is radiated uniformly and radially from the

core, and there is no significant difference between either

of the fans.

Next, the revolution noise was compared. As the

primary frequency of the revolution noise, the intensity

at 250 Hz (1/3 oct.) was analyzed. The contour maps

show the axial direction constituent of the sound. The

solid lines show the positive intensity, which is radiated

to the upstream from the core surface, and the broken

lines show the negative intensity, which goes back the

other way to the downstream radiator core. In the case

of the conventional fan, the intensity level at the four

ARTICLE IN PRESS

Conventional Fan

63 125 250 500 1k 2k

1/3 Octave Frequency (Hz)

50

60

70

80

S o u n d P r e s s u r e L e v e l ( d B

)

=32000rpm

Fixed Ring Fan

4k 8k 16k

40

30

Straight-ThroughType, n=3( )

Fig. 20. Analysis of fan noise frequency.




corners is the highest, and these portions are sufficiently

high until they are connected to each other diagonally.

In the case of the fan with a fixed ring, the intensity level

at the four corners is also the highest. However, the

portion of the upper right and the lower left are

independent and are not high until they are connected

to each other. At this time, the positive portion is

comparatively smaller than the negative portion. The

revolution noise of an axial flow fan occurs because of

interference by the fixed structures, like the stays and the

stators, and the suction of the non-uniform flow [10]. In

this test, there are no fixed structures in the front and the

back of the fan. Therefore, in this case, the suction of the

centripetal flow from the four corners is one kind of

suction of the non-uniform flow, and it is one of the

main causes of the occurrence of the revolution noise.

The X–Y direction constituent is shown on the vector

maps. Here, the sound source of the revolution noise is

the pressure fluctuation on the blade surface because of

the suction of the non-uniform flow [11]. Thus, the fan

blades are sound sources and the sound is rotating with

the blades in the same direction. At this time, portions

that have irregular vectors are observed at four corners

of the core. There is a tendency that the portions where

the vectors face each other almost correspond with the

negative portions on the contour maps, and the portions

where the vectors are in the opposite direction of eachother almost correspond with the positive portions on

the contour maps. Thus, the revolution noise shows the

three-dimensional flow, where the noise appears from

the four corners of the core and rotates with the fan

blades, then is sucked to the negative portions on the

contour maps on the core surface. As a result, there is

not a large difference in the phenomenon of the noise

occurrence, but just a difference in the degree between

both of the fans, which are with or without a ring.

6. Conclusion

1. The rings which are fixed at the fan blade tips affect

not only the mechanical strength, but also the fan

performance.

2. It is possible that the straight-through type labyrinth

seal and the combined-staggered type labyrinth seal,

which are mentioned in this research, achieve a close

performance to the ideal labyrinth. This occurs when

they are operated with a static pressure difference

lower than 250 Pa, and rotated with a peripheral

velocity lower than 50 m/s. Even though the periph-

eral velocity was under 50 m/s, a significantly large

decrease in the leakage rate was observed, especially

in the straight-through type labyrinth seal, rather

than the combined-staggered type labyrinth seal.

3. The labyrinth seal, which is applied for tip clearance

between the ring fixed at the blade tips and the fan

shroud, improves both the fan performance and

noise. In the case when the combined-staggered type

labyrinth seal is used, it is possible to ensure a

minimal effect in the change of the tip clearance to

the fan performance and noise level.

4. The results of the three-dimensional analysis in the

sound intensity method show that there is not a

significant difference between the conventional fanand the fan with a fixed ring, which has a ring at the

blade tips. However just the intensity level is

different.

Acknowledgements

The authors would like to acknowledge the instruc-

tions and encouragement given to them in the research

by emeritus Dr. Koichi Ohyama of the National

Aerospace Laboratory of Japan.

ARTICLE IN PRESS

Fig. 21. Three-dimensional sound intensity of fan noise on the

radiator core surface.




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a Study of Radiator Cooling Fan With Labyrinth Seal

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