Title:
CONTRA-ROTATING AXIAL FLOW FAN UNIT
Kind Code:
A1


Abstract:
An axial flow fan unit includes an intake side axial flow fan, an exhaust side axial flow fan, and at least one intermediate axial flow fan. Each of the axial flow fans includes an impeller having blades rotatable about a central rotation axis, a motor arranged to rotate the intake side impeller, and a housing having an inner circumferential surface that surrounds the impeller. The rotational direction of the impeller in the intermediate axial flow fan is different from that of the impellers adjacent thereto.



Inventors:
Takeshita, Hidenobu (Kyoto, JP)
Sugiyama, Tomotsugu (Kyoto, JP)
Takaoka, Tsukasa (Kyoto, JP)
Application Number:
12/328822
Publication Date:
06/18/2009
Filing Date:
12/05/2008
Assignee:
NIDEC CORPORATION (Minami-ku, JP)
Primary Class:
Other Classes:
415/68, 415/199.5, 417/360
International Classes:
F04D25/16; F04B17/03
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Primary Examiner:
ZOLLINGER, NATHAN C
Attorney, Agent or Firm:
NIDEC CORPORATION (Reston, VA, US)
Claims:
What is claimed is:

1. An axial flow fan unit comprising: an intake side axial flow fan including: an intake side impeller arranged to rotate about a central rotation axis and having intake side blades; an intake side motor arranged to rotate the intake side impeller; and an intake side housing having an inner circumferential surface that surrounds the intake side impeller; an exhaust side axial flow fan including: an exhaust side impeller arranged to rotate about the central rotation axis and having exhaust side blades; an exhaust side motor arranged to rotate the exhaust side impeller; and an exhaust side housing having an inner circumferential surface that surrounds the exhaust side impeller; and at least one intermediate axial flow fan arranged between the intake side axial flow fan and the exhaust side axial flow fan, each of the at least one intermediate axial flow fan including: an intermediate impeller arranged to rotate about the central rotation axis and having intermediate blades; an intermediate motor arranged to rotate the intermediate impeller; and an intermediate housing having an inner circumferential surface that surrounds the intermediate impeller; wherein a rotational direction of the intermediate impeller is different from that of the impellers adjacent thereto.

2. The axial flow fan unit of claim 1, wherein the intake side axial flow fan includes intake side support ribs arranged to interconnect the intake side motor and the intake side housing; the exhaust side axial flow fan includes exhaust side support ribs arranged to interconnect the exhaust side motor and the exhaust side housing; and the intermediate axial flow fan includes intermediate support ribs arranged to interconnect the intermediate motor and the intermediate housing.

3. The axial flow fan unit of claim 2, wherein the intake side support ribs are arranged between the intake side impeller and the intermediate impeller, and the intermediate support ribs are arranged between the intermediate impeller and the exhaust side impeller.

4. The axial flow fan unit of claim 3, wherein the number of the blades in each axial flow fan is different from the number of each of the support ribs axially adjacent to the blades.

5. The axial flow fan unit of claim 1, wherein the intermediate housing is successively arranged with the intake side housing and the exhaust side housing in an axial direction.

6. The axial flow fan unit of claim 5, wherein the intermediate housing is successively arranged with the intake side housing and the exhaust side housing in the axial direction with gaps arranged between each of the housings in the axial direction.

7. The axial flow fan unit of claim 2, wherein the intake side support ribs, the exhaust side support ribs, and the intermediate support ribs are substantially flat.

8. The axial flow fan unit of claim 2, wherein each of the intake side support ribs, the exhaust side support ribs, and the intermediate support ribs has an upper edge and a lower edge.

9. The axial flow fan unit of claim 8, wherein each of the intake side support ribs, the exhaust side support ribs, and the intermediate support ribs has an air receiving surface arranged axially between the upper edge and the lower edge to confront an air stream flowing from the intake side impeller toward the exhaust side impeller.

10. The axial flow fan unit of claim 9, wherein the air receiving surface axially faces an exhaust side of the respective axial fan.

11. The axial flow fan unit of claim 2, wherein each of the intake side support ribs, the exhaust side support ribs, and the intermediate support ribs is inclined with respect to the central rotation axis.

12. The axial flow fan unit of claim 11, wherein each of the intake side blades, the exhaust side blades, and the intermediate blades is inclined with respect to the central rotation axis, the inclination of each of the blades being substantially the same as the inclination of each of the support ribs axially positioned at an intake side of the blades.

13. The axial flow fan unit of claim 8, wherein the upper edge of each of the intake side support ribs is positioned upstream of the lower edge thereof with reference to a rotational direction of the intake side impeller; the upper edge of each of the exhaust side support ribs is positioned upstream of the lower edge thereof with reference to a rotational direction of the exhaust side impeller; and the upper edge of each of the intermediate support ribs is positioned upstream of the lower edge thereof with reference to a rotational direction of the intermediate impeller.

14. The axial flow fan unit of claim 2, wherein the intake side support ribs are arranged at an intake side of the intake side housing; the intermediate support ribs are arranged between the intake side impeller and the intermediate impeller; and the exhaust side support ribs are arranged between the intermediate impeller and the exhaust side impeller.

15. The axial flow fan unit of claim 1, wherein the at least one intermediate axial flow fan includes at least a first intermediate axial flow fan and a second intermediate axial flow fan.

16. The axial flow fan unit of claim 15, wherein the intake side axial flow fan includes intake side support ribs arranged to interconnect the intake side motor and the intake side housing; the exhaust side axial flow fan includes exhaust side support ribs arranged to interconnect the exhaust side motor and the exhaust side housing; and each of the intermediate axial flow fans includes intermediate support ribs arranged to interconnect the intermediate motor and the intermediate housing.

17. The axial flow fan unit of claim 16, wherein the intake side support ribs are arranged between the intake side impeller and the intermediate impeller; the intermediate support ribs in the first intermediate axial flow fan are arranged between the intermediate impeller in the first intermediate axial flow fan and the intermediate support ribs in the second intermediate axial flow fan; and the exhaust side support ribs are opposite to the exhaust side impeller away from the intermediate impeller in the second intermediate axial flow fan.

18. The axial flow fan unit of claim 17, wherein the number of the blades in each axial flow fan is different from the number of each of the support ribs axially adjacent to the blades.

19. The axial flow fan unit of claim 15, wherein the intermediate housing of the first intermediate axial flow fan is axially opposite to the intake side housing, the exhaust side housing, or the second intermediate housing.

20. The axial flow fan unit of claim 15, wherein the intermediate housing of the first intermediate axial flow fan is opposite to the intake side housing, the exhaust side housing, or the second intermediate housing with gaps arranged between each of the housings in the axial direction.

21. The axial flow fan unit of claim 16, wherein the intake side support ribs are arranged at an intake side of the intake side housing; the intermediate support ribs in the first intermediate axial flow fan are arranged between the intake side impeller and the intermediate impeller in the first intermediate axial flow fan, or between the intermediate impeller in the first intermediate axial flow fan and the intermediate impeller in the second intermediate axial flow fan; and the exhaust side support ribs are arranged between the exhaust side impeller and the intermediate impeller in the second intermediate axial flow fan.

Description:

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a contra-rotating axial flow fan unit.

2. Description of the Related Art

Conventionally, a cooling fan is installed in electronic devices such as a personal computer, a server and the like to cool electronic parts enclosed within a casing thereof. The electronic parts are densely arranged within the casing and, consequently, the heat emitted from the electronic parts tends to stay within the casing. This requires improvement in the performance of the cooling fan. In particular, a cooling fan capable of delivering an air under a high static pressure and with an increased volume is required in a large-sized electronic device such as a server or the like.

As a cooling fan for increasing the static pressure of the discharged air, there is known a serially arranged axial flow fan unit in which two axial flow fans are connected in series. In the serially arranged axial flow fan unit, an air stream discharged from an upstream fan is introduced into a downstream fan. Due to this construction, the serially arranged axial flow fan unit is capable of efficiently delivering air under a high static pressure and with an increased volume.

In the serially arranged axial flow fan unit, however, the volume and the static pressure of the discharged air are not increased by merely connecting two axial flow fans in series. Since the number of fans is increased in the serially arranged axial flow fan unit, the amount of the electric current needed to rotate the impellers is also increased as compared to a unitary axial flow fan. For that reason, there is a need to efficiently rotate the impellers of the serially arranged axial flow fan unit, thereby reducing the amount of the supplied current.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, a preferred embodiment of the present invention includes a contra-rotating axial flow fan unit having an intake side axial flow fan and an exhaust side axial flow fan. The intake side axial flow fan includes an intake side impeller having intake side blades rotatable about a central rotation axis, an intake side motor arranged to rotate the intake side impeller, and an intake side housing having an inner circumferential surface that surrounds the intake side impeller. The exhaust side axial flow fan includes an exhaust side impeller having exhaust side blades rotatable about the central rotation axis, an exhaust side motor arranged to rotate the exhaust side impeller, and an exhaust side housing having an inner circumferential surface that surrounds the exhaust side impeller. Further, at least one intermediate axial flow fan is arranged between the intake side axial flow fan and the exhaust side axial flow fan. There may be a plurality (two or more) intermediate axial flow fans.

The intermediate axial flow fan includes an intermediate impeller having intermediate blades rotatable about the central rotation axis, an intermediate motor arranged to rotate the intermediate impeller, and an intermediate housing having an inner circumferential surface that surrounds the intermediate impeller. A rotational direction of the intermediate impeller is different from that of the impellers adjacent thereto. Therefore, air is drawn into the contra-rotating axial flow fan unit through the intake side axial flow fan and is discharged to the outside through the exhaust side axial flow fan. This makes it possible to increase the volume and the static pressure of the air drawn therein and discharged therefrom.

The intake side axial flow fan may include intake side support ribs arranged to interconnect the intake side motor and the intake side housing, the exhaust side axial flow fan may include a plurality of exhaust side support ribs arranged to interconnect the exhaust side motor and the exhaust side housing, and the intermediate axial flow fan may include intermediate support ribs arranged to interconnect the intermediate motor and the intermediate housing. Due to this feature, a portion of the swirling-direction velocity component of an air stream impinges against the respective support ribs and is changed to an axial velocity component. This increases the static pressure of the air.

It is preferable that the intake side support ribs are arranged between the intake side impeller and the intermediate impeller, the intermediate support ribs are arranged between the intermediate impeller and another intermediate impeller or between the intermediate impeller and the exhaust side impeller, and the exhaust side support ribs are opposite to the intermediate support ribs with the intermediate impeller lying therebetween.

In the above construction, it is preferable that the number of the respective blades differs from the number of the respective support ribs within a respective axial fan. Due to this feature, the frequency of a wind noise generated by the rotation of the respective impellers becomes different from the frequency of an interfering noise generated when the air stream impinges against the respective support ribs, thereby preventing sympathetic vibration of the wind noise and the interfering noise. As a result, it is possible to reduce the noise generated in the contra-rotating axial flow fan unit.

In a preferred embodiment of the present invention, the intermediate housing may be axially opposite to the intake side housing, the exhaust side housing, or another intermediate housing. The respective housings may make contact with one another or may be opposite to one another via gaps therebetween. The respective axial flow fans may be fixed to one another by screws, engaging structures or other suitable fixing mechanisms or materials.

It is preferable that the intake side support ribs, the exhaust side support ribs, and the intermediate support ribs are substantially flat. Each of the intake side support ribs, the exhaust side support ribs, and the intermediate support ribs may have an upper edge and a lower edge. Each of the intake side support ribs, the exhaust side support ribs, and the intermediate support ribs may have an air receiving surface defined between the upper edge and the lower edge to confront an air stream flowing from the intake side impeller toward the exhaust side impeller. It is preferable that the air receiving surface is inclined with respect to the central rotation axis to axially face toward an exhaust side.

It is preferable that the inclination of each of the blades relative to the central rotation axis is substantially the same as the inclination of each of the support ribs with respect to the central rotation axis. This ensures that the air drawn into the contra-rotating axial flow fan unit can smoothly flow through the vicinity of the respective blades and the respective support ribs.

In the respective support ribs of the axial flow fans, it is preferred that the upper edge of each of the support ribs is positioned upstream of the lower edge thereof with reference to the rotational direction of the respective impellers. This allows the air stream to smoothly flow through the vicinity of the respective support ribs with a minimal loss of energy of the air stream.

The respective support ribs may be positioned in an intake side of each of the housings. This ensures that a straightened air stream is smoothly drawn into the contra-rotating axial flow fan unit thereby reducing the noise generated in the contra-rotating axial flow fan unit.

Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a contra-rotating axial flow fan unit in accordance with a first preferred embodiment of the present invention.

FIG. 2 is a sectional view of the contra-rotating axial flow fan unit in accordance with the first preferred embodiment of the present invention.

FIG. 3 is a sectional view illustrating some of the blades and ribs included in the contra-rotating axial flow fan unit in accordance with the first preferred embodiment of the present invention.

FIG. 4 is a perspective view showing a modified example of the contra-rotating axial flow fan unit in accordance with the first preferred embodiment of the present invention.

FIG. 5 is a sectional view of the modified example of the contra-rotating axial flow fan unit in accordance with the first preferred embodiment of the present invention.

FIG. 6 is a perspective view showing a contra-rotating axial flow fan unit in accordance with a second preferred embodiment of the present invention.

FIG. 7 is a sectional view of the contra-rotating axial flow fan unit in accordance with the second preferred embodiment of the present invention.

FIG. 8 is a sectional view illustrating some of the blades and ribs included in the contra-rotating axial flow fan unit in accordance with the second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 through 8, preferred embodiments of the present invention will be described in detail. It should be noted that in the explanation of the preferred embodiments of the present invention, when positional relationships among and orientations of the different components are described as being up/down or left/right, ultimately positional relationships and orientations that are in the drawings are indicated; positional relationships among and orientations of the components once having been assembled into an actual device are not indicated. Meanwhile, in the following description, an axial direction indicates a direction parallel or substantially parallel to a rotation axis, and a radial direction indicates a direction perpendicular or substantially perpendicular to the rotation axis.

As shown in FIGS. 1 and 2, the contra-rotating axial flow fan unit 1 in accordance with a first preferred embodiment of the present invention preferably is a triple contra-rotating axial flow fan including three axial flow fans, i.e., an intake side axial flow fan 2, a first intermediate axial flow fan 4 and an exhaust side axial flow fan 3. The respective axial flow fans are fixed to one another by screws, engaging structures (not shown) or other suitable fixing mechanisms or materials.

As illustrated in FIG. 2, an intake side impeller 21, an intermediate impeller 41, and an exhaust side impeller 31 are respectively arranged inside the intake side axial flow fan 2, the first intermediate axial flow fan 4, and the exhaust side axial flow fan 3.

The intake side impeller 21 and the intermediate impeller 41 are rotated about a central rotation axis J1 in different directions. Likewise, the intermediate impeller 41 and the exhaust side impeller 31 are rotated about the central rotation axis J1 in different directions. In the present preferred embodiment, when seen from the axially upper side in FIG. 1, the intake side impeller 21 rotates clockwise, the intermediate impeller 41 rotates counterclockwise, and the exhaust side impeller 31 rotates clockwise. Consequently, air is drawn into the intake side axial flow fan 2 and delivered to the exhaust side axial flow fan 3. This generates an air stream flowing along the central rotation axis J1.

As can be seen in FIG. 2, the intake side axial flow fan 2 includes an intake side impeller 21, an intake side motor 22, an intake side housing 23, and a plurality (for example, eight, in the present preferred embodiment) of intake side support ribs 24.

The intake side impeller 21 includes a plurality of (for example, seven, in the present preferred embodiment) intake side blades 211 and a substantially cylindrical closed-top hub 212. On the outer surface of the hub 212, the intake side blades 211 extend radially outward about the central rotation axis J1 and are circumferentially arranged preferably at an equal pitch. The hub 212 and the intake side blades 211 are preferably made of a synthetic resin and are preferably integrally formed by injection molding. When seen from the axially upper side in FIG. 2, the intake side impeller 21 is rotated clockwise about the central rotation axis J1 by the intake side motor 22. This generates an air stream axially flowing from the intake side blades 211.

The intake side housing 23 is a hollow member preferably made of a synthetic resin and has an inner circumferential surface having a substantially cylindrical shape designed to surround the intake side impeller 21. Within the intake side housing 23 (namely, on the inner circumferential surface of the intake side housing 23), there is defined a flow path through which the air stream generated by the rotation of the intake side impeller 21 flows. The intake side housing 23 is provided with an upper end portion and a lower end portion each having an inner circumferential surface arranged so that the distance between the central rotation axis J1 and the inner circumferential surface can be increased axially upward or downward. This ensures that the air is smoothly drawn into and discharged from the intake side housing 23 as the intake side blades 211 rotate.

The intake side support ribs 24 are preferably made of a synthetic resin and substantially flat. The intake side support ribs 24 are arranged below the intake side impeller 21 (namely, between the intake side impeller 21 and the first intermediate axial flow fan 4) and extend radially outward from the intake side motor 22 so that they can be connected to the intake side housing 23 to thereby support the intake side motor 22.

As shown in FIG. 2, the intake side motor 22 includes a stator portion 221 and a rotor portion 222. The rotor portion 222 is supported by a bearing mechanism so that the rotor portion 222 can rotate relative to the stator portion 221.

The stator portion 221 is provided with a base portion 2211 that is substantially disk-shaped when seen in a plan view. The base portion 2211 is fixed to the inner circumferential surface of the intake side housing 23 through the intake side support ribs 24 to thereby hold the stator portion 221 in place. The base portion 2211 is preferably made of a synthetic resin and is preferably integrally formed with the intake side support ribs 24 and the intake side housing 23 by injection-molding the synthetic resin. However, the material and the method used in producing the base portion 2211, the intake side support ribs 24, and the intake side housing 23 are not limited to synthetic resin and injection molding. Alternatively, the base portion 2211, the intake side support ribs 24 and the intake side housing 23 may be formed by, e.g., die-casting aluminum with an aluminum alloy or other suitable materials and/or processes.

As illustrated in FIG. 2, a substantially cylindrical bearing holder portion 2212 is fixed to the substantially central region of the base portion 2211 so that it can protrude upward from the base portion 2211. The bearing holder portion 2212 is preferably made of a metal and is preferably integrally fixed to the base portion 2211 by injection-molding (e.g., insert-molding) a resin. The bearing holder portion 2212 may be either formed from a synthetic resin or integrally formed with the base portion 2211 by injection-molding a resin or die-casting aluminum. The material and the method used in producing the bearing holder portion 2212 are not particularly limited. Ball bearings 2213 and 2214 are provided in the axial upper and lower regions inside the bearing holder portion 2212, thus defining a portion of the bearing mechanism. An elastic member (e.g., a spring) is preferably arranged to pre-compress the ball bearing 2214 from above.

The stator portion 221 is further provided with a stator 2215 and a circuit board 2216. The stator 2215 is attached to the outer surface of the bearing holder portion 2212. The circuit board 2216 has a substantially annular flat shape and is attached to the lower side of the stator 2215. The circuit board 2216 is provided with a circuit arranged to control the rotation of the rotor portion 222 and is electrically connected to the stator 2215 through a conductive pin (not shown) and the like. An electric current and a control signal are sent from an external power source (not shown) to the circuit board 2216 via a lead line group (not shown) having a plurality of lead lines tied together.

The rotor portion 222 includes a yoke 2221, a field magnet 2222, and a shaft 2223.

The yoke 2221 is preferably made of a magnetic metal and has a substantially cylindrical closed-top shape. The field magnet 2222 preferably has a substantially cylindrical shape and is fixed to the inner surface of the cylindrical portion of the yoke 2221 by an adhesive agent or fixing materials or mechanisms. The field magnet 2222 is radially opposite to the stator 2215. An axially downward protruding cylinder portion is provided in the substantially central region of the cover portion of the yoke 2221 and may be formed by burring or other suitable process. The shaft 2223 is press-fitted into the cylinder portion in a coaxial relationship with the central rotation axis J1. The shaft 2223 is inserted into the bearing holder portion 2212 and is supported by the ball bearings 2213 and 2214 for rotation relative to the stator portion 221. That is, in the intake side axial flow fan 2, the shaft 2223 and the ball bearings 2213 and 2214 define a bearing mechanism arranged to support the yoke 2221 so that the yoke 2221 can rotate about the central rotation axis J1 with respect to the base portion 2211.

As a driving current is supplied from the external power source to the stator 2215 through the lead line group and the circuit board 2216, the torque acting about the central rotation axis J1 is generated between the stator 2215 and the field magnet 2222. The rotor portion 222 and the intake side impeller 21 are rotated by the torque thus generated. The driving current supplied to the stator 2215 is controlled by the circuit in the circuit board 2216. This makes it possible to rotate the intake side impeller 21 at a predetermined rotation speed.

The first intermediate axial flow fan 4 includes a first intermediate impeller 41, a first intermediate motor 42, a first intermediate housing 43, and a plurality of (for example, eight, in the present preferred embodiment) first intermediate support ribs 44.

The first intermediate impeller 41 includes first intermediate blades 411 and a substantially cylindrical closed-top first hub 412. The first intermediate blades 411 extend radially outward and are circumferentially arranged at an equal pitch. The first intermediate blades 411 and the first hub 412 are all preferably made of a synthetic resin and integrally formed by injection molding.

The first intermediate motor 42 is arranged to rotate the first intermediate impeller 41 counterclockwise about the central rotation axis J1 when seen from the upper side in FIG. 2. This generates an air stream flowing along the central rotation axis J1.

The first intermediate housing 43 is a hollow member preferably made of a synthetic resin and has an inner circumferential surface having a substantially cylindrical shape surrounding the first intermediate impeller 41. Within the first intermediate housing 43 (namely, on the inner circumferential surface of the first intermediate housing 43), there is defined a flow path through which the air stream generated by the rotation of the intermediate impeller 41 flows. The first intermediate housing 43 is provided with an upper end portion and a lower end portion each having an inner circumferential surface arranged so that the distance between the central rotation axis J1 and the inner circumferential surface can be increased axially upward or downward. This ensures that the air is smoothly drawn into and discharged from the first intermediate housing 43 as the first intermediate blades 411 rotate.

The first intermediate support ribs 44 are preferably made of a synthetic resin and substantially flat. The first intermediate support ribs 44 are arranged below the intermediate impeller 41 (namely, between the intermediate impeller 41 and the exhaust side axial flow fan 3) and extend radially outward from the first intermediate motor 42 so that they can be connected to the first intermediate housing 43 to thereby support the first intermediate motor 42.

As shown in FIG. 2, the first intermediate motor 42 includes a stator portion 421 and a rotor portion 422. The rotor portion 422 is supported by the below-mentioned bearing mechanism so that it can rotate relative to the stator portion 421.

The stator portion 421 is provided with a base portion 4211 that is substantially disk-shaped when seen in a plan view. The base portion 4211 is fixed to the inner circumferential surface of the first intermediate housing 43 through the first intermediate support ribs 44 to thereby hold the stator portion 421 in place. The base portion 4211 is preferably made of a synthetic resin and is preferably integrally formed with the first intermediate support ribs 44 and the first intermediate housing 43 by injection-molding the synthetic resin. However, the material and the method used in producing the base portion 4211, the first intermediate support ribs 44, and the first intermediate housing 43 are not limited to synthetic resin and injection molding. Alternatively, the base portion 4211, the first intermediate support ribs 44, and the first intermediate housing 43 may be formed by, e.g., die-casting aluminum with an aluminum alloy or other suitable processes or materials.

As illustrated in FIG. 2, a substantially cylindrical bearing holder portion 4212 is fixed to the substantially central region of the base portion 4211 so that it can protrude upward from the base portion 4211. The bearing holder portion 4212 is preferably made of a metal and is preferably integrally fixed to the base portion 4211 by injection-molding (e.g., insert-molding) with a resin. The bearing holder portion 4212 may be either formed from a synthetic resin, aluminum, or aluminum alloy or integrally formed with the base portion 4211 and the first intermediate housing 43 by injection-molding a resin or die-casting aluminum. The material and the method used in producing the bearing holder portion 4212 are not particularly limited. Ball bearings 4213 and 4214 are provided in the axial upper and lower regions inside the bearing holder portion 4212, thus defining a portion of the bearing mechanism. An elastic member (e.g., a spring) is preferably arranged to pre-compress the ball bearing 4214 from above.

The stator portion 421 is provided with a stator 4215 and a circuit board 4216. The stator 4215 is attached to the outer surface of the bearing holder portion 4212. The circuit board 4216 has a substantially annular flat shape and is attached to the lower side of the stator 4215. The circuit board 4216 is provided with a circuit arranged to control the rotation of the rotor portion 422 and is electrically connected to the stator 4215 through a conductive pin (not shown) and the like. An electric current and a control signal are sent from an external power source (not shown) to the circuit board 4216 via a lead line group (not shown) having a plurality of lead lines tied together.

The rotor portion 422 includes a yoke 4221, a field magnet 4222, and a shaft 4223. The yoke 4221 has a substantially cylindrical closed-top shape and is preferably made of a magnetic metal. The field magnet 4222 preferably has a substantially cylindrical shape and is fixed to the inner surface of the cylindrical portion of the yoke 4221 by an adhesive agent or other suitable fixing materials or mechanisms. The field magnet 4222 is radially opposite to the stator 4215. An axially downward protruding cylinder portion is provided in the substantially central region of the cover portion of the yoke 4221 and may be formed by burring or other suitable process. The shaft 4223 is press-fitted into the cylinder portion in a coaxial relationship with the central rotation axis J1.

The shaft 4223 is inserted into the bearing holder portion 4212 and is supported by the ball bearings 4213 and 4214 for rotation relative to the stator portion 421. That is, in the first intermediate axial flow fan 4, the shaft 4223, and the ball bearings 4213 and 4214 define a bearing mechanism arranged to support the yoke 4221 so that it can rotate about the central rotation axis J1 with respect to the base portion 4211.

As a driving current is supplied from the external power source to the stator 4215 through the lead line group and the circuit board 4216, the torque acting about the central rotation axis J1 is generated between the stator 4215 and the field magnet 4222. The rotor portion 422 and the first intermediate impeller 41 are rotated by the torque thus generated. The driving current supplied to the stator 4215 is controlled by the circuit in the circuit board 4216. This makes it possible to rotate the first intermediate impeller 41 at a predetermined rotation speed.

The exhaust side axial flow fan 3 includes an exhaust side impeller 31, an exhaust side motor 32, an exhaust side housing 33 and a plurality of (for example, eight, in the present preferred embodiment) exhaust side support ribs 34.

The exhaust side impeller 31 includes a plurality of (for example, seven, in the present preferred embodiment) exhaust side blades 311 and a substantially cylindrical closed-top hub 312. On the outer surface of the hub 312, the exhaust side blades 311 extend radially outward and are circumferentially arranged at an equal pitch. When seen from the axially upper side in FIG. 2, the exhaust side impeller 31 is rotated clockwise about the central rotation axis J1 by the exhaust side motor 32. This generates an air stream axially flowing from the exhaust side impeller 31.

The exhaust side housing 33 is a hollow member preferably made of a synthetic resin and has an inner circumferential surface of substantially cylindrical shape designed to surround the exhaust side impeller 31. Within the exhaust side housing 33 (namely, on the inner circumferential surface of the exhaust side housing 33), there is defined a flow path through which the air stream generated by the rotation of the exhaust side impeller 31 flows. The exhaust side housing 33 is provided with an upper end portion and a lower end portion each having an inner circumferential surface arranged so that the distance between the central rotation axis J1 and the inner circumferential surface can be increased axially upward or downward. This ensures that the air is smoothly drawn into and discharged from the exhaust side housing 33 as the exhaust side blades 311 rotate.

The exhaust side support ribs 34 are arranged below the exhaust side impeller 31 and extend radially outward from the exhaust side motor 32 about the central rotation axis J1 so that they can be connected to the exhaust side housing 33 to thereby support the exhaust side motor 32.

As shown in FIG. 2, the exhaust side motor 32 includes a stator portion 321 and a rotor portion 322. The rotor portion 322 is supported by a bearing mechanism so that it can rotate about the central rotation axis J1 relative to the stator portion 321.

The stator portion 321 is provided with a base portion 3211 that is substantially disk-shaped when seen in a plan view. The base portion 3211 is fixed to the inner circumferential surface of the exhaust side housing 33 through the exhaust side support ribs 34 to thereby hold the stator portion 321 in place. The base portion 3211 is preferably made of a synthetic resin and is preferably integrally formed with the exhaust side support ribs 34 and the exhaust side housing 33 by injection-molding the synthetic resin. However, the material and the method used in producing the base portion 3211, the exhaust side support ribs 34, and the exhaust side housing 33 are not limited to synthetic resin and injection molding. Alternatively, the base portion 3211, the exhaust side support ribs 34, and the exhaust side housing 33 may be formed by, e.g., die-casting aluminum or other materials.

As illustrated in FIG. 2, a substantially cylindrical bearing holder portion 3212 is fixed to the substantially central region of the base portion 3211 so that it can protrude upward from the base portion 3211. The bearing holder portion 3212 is preferably made of a metal and is preferably integrally fixed to the base portion 3211 by injection-molding (e.g., insert-molding) with a resin. The bearing holder portion 3212 may be either formed from a synthetic resin, aluminum, or aluminum alloy or integrally formed with the base portion 3211 and the exhaust side housing 33 by injection-molding a resin or die-casting aluminum. The material and the method used in producing the bearing holder portion 3212 are not particularly limited. Ball bearings 3213 and 3214 are provided in the upper and lower regions inside the bearing holder portion 3212, thus defining a portion of the bearing mechanism. An elastic member (e.g., a spring) is preferably arranged to pre-compress the ball bearing 3214 from above.

The stator portion 321 is provided with a stator 3215 and a circuit board 3216. The stator 3215 is attached to the outer surface of the bearing holder portion 3212. The circuit board 3216 has a substantially annular flat shape and is attached to the lower side of the stator 3215. The circuit board 3216 is provided with a circuit arranged to control the rotation of the rotor portion 322 and is electrically connected to the stator 3215 through a conductive pin and the like. An electric current and a control signal are sent from an external power source (not shown) to the circuit board 3216 via a lead line group (not shown) having a plurality of lead lines tied together.

The rotor portion 322 includes a yoke 3221, a field magnet 3222, and a shaft 3223. The yoke 3221 has a substantially cylindrical closed-top shape and is preferably made of a magnetic metal. The field magnet 3222 has a substantially cylindrical shape and is fixed to the inner surface of the cylindrical portion of the yoke 3221 by an adhesive agent or other suitable fixing materials or mechanisms. The field magnet 3222 is radially opposite to the stator 3215. An axially downward protruding cylinder portion is provided in the substantially central region of the cover portion of the yoke 3221 and may preferably be formed by burring or other suitable process. The shaft 3223 is press-fitted into the cylinder portion in a coaxial relationship with the central rotation axis J1.

The shaft 3223 is inserted into the bearing holder portion 3212 and is supported by the ball bearings 3213 and 3214 for rotation relative to the stator portion 321. That is, in the exhaust side axial flow fan 3, the shaft 3223, and the ball bearings 3213 and 3214 define a bearing mechanism arranged to support the yoke 3221 so that it can rotate about the central rotation axis J1 with respect to the base portion 3211.

As a driving current is supplied from the external power source to the stator 3215 through the lead line group and the circuit board 3216, the torque acting about the central rotation axis J1 is generated between the stator 3215 and the field magnet 3222. The rotor portion 322 and the intake side impeller 31 are rotated by the torque thus generated. The driving current supplied to the stator 3215 is controlled by the circuit in the circuit board 3216. This makes it possible to rotate the exhaust side impeller 31 at a predetermined rotation speed.

The upper and lower end surfaces of the first intermediate housing 43 axially coincide with the lower end surface of the intake side housing 23 and the upper end surface of the exhaust side housing 33, respectively. When observing the contra-rotating axial flow fan unit 1 as a whole, the respective flow paths (i.e., the respective inner circumferential surfaces) of the intake side housing 23, the first intermediate housing 43, and the exhaust side housing 33 are successively and smoothly connected to one another along the axial direction, thereby defining a single flow path (i.e., a single inner circumferential surface). As shown in FIG. 2, the intake side impeller 21, the intake side support ribs 24, the first intermediate impeller 41, the first intermediate support ribs 44, the exhaust side impeller 31, and the exhaust side support ribs 34 are arranged in the single flow path one below another from the upper side (i.e., the intake side) in FIG. 2.

The number of the intake side support ribs 24 differs from the number of the blades (the intake side blades 211 and the first intermediate blades 411) of the impellers axially adjacent thereto (i.e., the intake side impeller 21 and the first intermediate impeller 41). Furthermore, the number of the first intermediate blades 411 differs from the number of the support ribs axially adjacent thereto (i.e., the intake side support ribs 24 and the first intermediate support ribs 44). Moreover, the number of the first intermediate support ribs 44 differs from the number of the blades (i.e., the first intermediate blades 411 and the exhaust side blades 311) of the impellers axially adjacent thereto (i.e., the first intermediate impeller 41 and the exhaust side impeller 31). In addition, the number of the exhaust side blades 311 of the exhaust side impeller 31 differs from the number of the first intermediate support ribs 44 and the exhaust side support ribs 34. That is, in the case of the contra-rotating axial flow fan unit 1, the axially adjacent blades and support ribs differ in number from one another.

The air stream generated by the rotation of the respective blades impinges on the respective support ribs axially adjacent to the blades, thus generating noise. If the number of the respective blades and the respective support blades are set as mentioned above, the frequency of a wind noise generated by the rotation of the respective impellers does not coincide with the frequency of an interfering noise generated when the air stream impinges against the respective support ribs. Therefore, it is possible to prevent an increase in noise which would otherwise occur by the sympathetic vibration of the wind noise and the interfering noise. This makes it possible to minimize the noise generated from the contra-rotating axial flow fan unit 1.

As described above, a single flow path defined by the inner circumferential surfaces of the respective housings successively connected to one another in the axial direction is formed within the contra-rotating axial flow fan unit 1. The air stream introduced from the intake side axial flow fan 2 is allowed to smoothly flow along the single flow path (i.e., the inner circumferential surfaces of the respective housings). Then, the air stream is discharged from the exhaust side axial flow fan 3 to the outside with a minimal loss of energy due to frictional contact of the air stream with the flow path.

FIG. 3 is a sectional view illustrating only the cross-sections of the intake side blades 211, the intake side support ribs 24, the first intermediate blades 411, the first intermediate support ribs 44, the exhaust side blades 311, and the exhaust side support ribs 34 taken along a cylindrical plane having an arbitrary radius measured from the central rotation axis J1 in FIG. 2.

Each of the intake side support ribs 24 preferably is substantially flat and has an upper edge 241 positioned adjacent to each of the intake side blades 211 and a lower edge 242 positioned adjacent to the intermediate axial flow fan 4. The upper edge 241 is positioned upstream of the lower edge 242 with reference to the rotational direction R2 of each of the intake side blades 211. Each of the intake side support ribs 24 has an air receiving surface 243 inclined relative to the central rotation axis J1 to face toward the exhaust side. Thus, the air receiving surface 243 confronts the air stream generated by the rotation of the intake side blades 211.

The air stream generated by the intake side impeller 21 can be divided into different directional components, including a radially outward centrifugal velocity component, a central rotation axis direction velocity component acting parallel to the central rotation axis J1, and a swirling velocity component acting tangential to the rotation of the impeller.

The swirling-direction velocity component acts substantially in the same direction as the rotational direction R2. A portion of the swirling-direction velocity component impinges against each of the intake side support ribs 24 and the air receiving surface 243 thereof and, therefore, is changed to a velocity component acting in the same direction as the central rotation axis J1. This increases the static pressure of the air.

The air stream passed through the vicinity of the air receiving surface 243 of each of the intake side support ribs 24 is introduced toward each of the first intermediate blades 411 that rotates in the rotational direction R4. As shown in FIG. 3, each of the first intermediate blades 411 is inclined with respect to the central rotation axis J1 substantially in the same direction as is each of the intake side support ribs 24. Therefore, the air stream whose velocity component has been changed by the intake side support ribs 24 is smoothly introduced toward the first intermediate blades 411, which makes it possible to minimize the reduction of energy of the air stream.

Each of the first intermediate support ribs 44 has an upper edge 441 positioned adjacent to each of the first intermediate blades 411 and a lower edge 442 positioned adjacent to the exhaust side axial flow fan 3. The upper edge 441 is positioned upstream of the lower edge 442 with reference to the rotational direction R4 of each of the first intermediate blades 411. Each of the first intermediate support ribs 44 has an air receiving surface 443 inclined relative to the central rotation axis J1 to face toward the exhaust side. Thus, the air receiving surface 443 confronts the air stream generated by the rotation of the first intermediate blades 411.

The swirling-direction velocity component generated by the rotation of the first intermediate blades 411 acts substantially in the same direction as the rotational direction R4. A portion of the swirling-direction velocity component impinges against each of the first intermediate support ribs 44 and, therefore, is changed to a velocity component acting in the same direction as the central rotation axis J1. This increases the static pressure of the air.

The air stream passed through the vicinity of the air receiving surface 443 is introduced toward each of the exhaust side blades 311. As shown in FIG. 3, each of the exhaust side blades 311 is inclined with respect to the central rotation axis J1 substantially in the same direction as is each of the first intermediate support ribs 44. Therefore, the air stream whose velocity component has been changed by the first intermediate support ribs 44 is smoothly introduced toward the exhaust side blades 311. As a result, it is possible to minimize the reduction of energy of the air stream.

Each of the exhaust side support ribs 34 has an upper edge 341 positioned adjacent to each of the exhaust side blades 311 and a lower edge 342 positioned adjacent to the exhaust side. The upper edge 341 is positioned upstream of the lower edge 342 with reference to the rotational direction R3 of each of the exhaust side blades 311. Each of the exhaust side support ribs 34 has an air receiving surface 343 inclined relative to the central rotation axis J1 to face toward the exhaust side. Thus, the air receiving surface 343 confronts the air stream generated by the rotation of the exhaust side blades 311.

The swirling-direction velocity component generated by the rotation of the exhaust side blades 311 acts substantially in the same direction as the rotational direction R3. A portion of the swirling-direction velocity component impinges against each of the exhaust side support ribs 34 and, therefore, is changed to a velocity component acting in the same direction as the central rotation axis J1. This increases the static pressure of the air.

The air stream passed through the air receiving surface 343 of each of the exhaust side support ribs 34 is discharged to the outside of the exhaust side housing 33 (namely, to the outside of the contra-rotating axial flow fan unit 1).

The construction described above ensures that the air drawn into the contra-rotating axial flow fan unit 1 passes through the vicinity of the respective blades and the respective support ribs with a minimal loss of energy and is discharged to the outside of the fan 1. Furthermore, the air is allowed to smoothly pass through the vicinity of the respective blades and the respective support ribs, which makes it possible to efficiently rotate the respective impellers. As a consequence, it is possible to reduce the amount of electric power consumed in the respective motors of the axial flow fans.

Furthermore, the end surfaces of the respective housings axially coincide with one another and the inner circumferential surfaces thereof are continuously joined to one another. This allows the air to smoothly flow through the contra-rotating axial flow fan unit 1. As a result, it is difficult for the air to flow backwards, which makes it possible to greatly increase the volume of air and the static pressure. Due to this feature, it is possible to discharge the air present in a device casing and to supply a sufficient quantity of air to electronic parts in the device casing even when the system impedance within the device casing (namely, the flow path resistance within the device casing during introduction of the air) remains high.

The respective axial flow fans of the contra-rotating axial flow fan unit need not to be fixed to one another. FIG. 4 shows one modified example of the contra-rotating axial flow fan unit in accordance with a preferred embodiment of the present invention. The respective axial flow fans are directly mounted to a casing of an electronic device or other suitable apparatus to be axially spaced apart from one another, thus forming a contra-rotating axial flow fan unit 1a. Referring to FIG. 4, there exist gaps between the intake side housing 23 and the intermediate housing 43 and between the intermediate housing 43 and the exhaust side housing 33. It may also be possible to provide gaps between the respective housings by coupling the respective axial flow fans together with spacers or other suitable gap-forming members or materials interposed therebetween.

With this construction, air is introduced through the gaps between the respective housings. This makes it possible to increase the volume of the air delivered from the contra-rotating axial flow fan unit 1a. Furthermore, the contra-rotating axial flow fan unit 1a can be constructed by adjusting the gap size between the respective housings in conformity with the size of a casing of an electronic device or the like in which the contra-rotating axial flow fan unit 1a is arranged. This makes it possible to apply the contra-rotating axial flow fan unit 1a to device casings of different sizes.

The support ribs of the respective axial flow fans need not to be arranged on the exhaust side. FIG. 5 is a sectional view of another modified example of the contra-rotating axial flow fan unit in accordance with a preferred embodiment of the present invention.

In this contra-rotating axial flow fan unit 1b, the support ribs of the respective axial flow fans are arranged on the intake side. In this case, if the impellers of the respective axial flow fans are rotatingly driven, the air is straightened by the intake side support ribs 24 and then drawn into the intake side axial flow fan 2. This ensures that the air is smoothly drawn into the contra-rotating axial flow fan unit, thereby reducing the noise.

The inclination of the intake side support ribs 24 relative to the central rotation axis J1 is substantially the same as the inclination of the intake side blades 211 with respect to the central rotation axis J1. The air receiving surface 243 of each of the intake side support ribs 24 is inclined with respect to the central rotation axis J1 so as to face toward the exhaust side. The inclination of the intermediate support ribs 44 relative to the central rotation axis J1 is substantially the same as the inclination of the intermediate blades 411 with respect to the central rotation axis J1. The air receiving surface 443 of each of the intermediate support ribs 44 faces toward the exhaust side, thus confronting the air stream generated by the rotation of the intake side blades 211. Therefore, the air passes through the vicinity of the respective blades and the respective support ribs with a minimal loss of energy and is discharged to the outside of the contra-rotating axial flow fan unit 1b. Furthermore, the air is allowed to smoothly pass through the vicinity of the respective blades and the respective support ribs, which makes it possible to efficiently rotate the respective impellers.

FIG. 6 is a perspective view showing a contra-rotating axial flow fan unit 1A in accordance with a second preferred embodiment of the present invention. FIG. 7 is a vertical sectional view of the contra-rotating axial flow fan unit 1A taken along a plane containing a central rotation axis. In the following description, the same components of the fan 1A as those of the contra-rotating axial flow fan unit 1 will be designated by like reference numerals and description thereof will be omitted.

As shown in FIGS. 6 and 7, the contra-rotating axial flow fan unit 1A is a fourfold contra-rotating axial flow fan including an intake side axial flow fan 2, a first intermediate axial flow fan 4, a second intermediate axial flow fan 5, and an exhaust side axial flow fan 3 arranged along a central rotation axis J1 in the sequence described above. The mutually adjoining axial flow fans are fixed to one another by screws (not shown), engaging structures (not shown) or other suitable fixing members or materials.

The intake side impeller 21 and the first intermediate impeller 41 are rotated about the central rotation axis J1 in different directions. Likewise, the first intermediate impeller 41 and the second intermediate impeller 51 of the second intermediate axial flow fan 5 are rotated about the central rotation axis J1 in different directions. Furthermore, the second intermediate impeller 51 and the exhaust side impeller 31 of the exhaust side axial flow fan 3 are rotated about the central rotation axis J1 in different directions.

In the present preferred embodiment, when seen from the axially upper side in FIG. 6, the intake side impeller 21 rotates clockwise, the first intermediate impeller 41 rotates counterclockwise, the second intermediate impeller 51 rotates clockwise, and the exhaust side impeller 31 rotates counterclockwise. Consequently, air is drawn into the intake side axial flow fan 2 and delivered to the exhaust side axial flow fan 3. This generates an axially flowing air stream.

The second intermediate axial flow fan 5 includes a second intermediate impeller 51, a second intermediate motor 52, a second intermediate housing 53, and a plurality of second intermediate support ribs 54.

The second intermediate impeller 51 includes a plurality of (for example, seven, in the present preferred embodiment) second intermediate blades 511 and a substantially cylindrical closed-top hub 512. On the outer surface of the hub 512, the second intermediate blades 511 extend radially outward about the central rotation axis J1 and are circumferentially arranged at an equal pitch. The second intermediate blades 511 and the hub 512 are preferably integrally formed by injection-molding a resin, for example. When seen from the upper side in FIG. 7, the second intermediate impeller 51 is rotated clockwise about the central rotation axis J1 by the second intermediate motor 52. This generates an air stream flowing along the central rotation axis J1.

The second intermediate housing 53 is a hollow member preferably made of a synthetic resin and has an inner circumferential surface of substantially cylindrical shape designed to surround the second intermediate impeller 51. Within the second intermediate housing 53 (namely, on the inner circumferential surface of the second intermediate housing 53), there is defined a flow path through which the air stream generated by the rotation of the second intermediate impeller 51 flows. The second intermediate housing 53 is provided with an upper end portion and a lower end portion each having an inner circumferential surface arranged so that the distance between the central rotation axis J1 and the inner circumferential surface can be increased axially upward or downward. This ensures that the air is smoothly drawn into and discharged from the second intermediate housing 53 as the second intermediate blades 511 rotate.

The plurality of (for example, eight, in the present preferred embodiment) second intermediate support ribs 54 are preferably made of a synthetic resin. The second intermediate support ribs 54 are arranged below the second intermediate impeller 51 (namely, between the second intermediate impeller 51 and the exhaust side axial flow fan 3) and extend radially outward from the second intermediate motor 52 so that they can be connected to the second intermediate housing 53 to thereby support the second intermediate motor 52.

As shown in FIG. 7, the second intermediate motor 52 includes a stator portion 521 and a rotor portion 522. The rotor portion 522 is supported by a bearing mechanism so that it can rotate relative to the stator portion 521.

The stator portion 521 has a base portion 5211 that is substantially disk-shaped and arranged about the central rotation axis J1 when seen in a plan view. The base portion 5211 is fixed to the inner circumferential surface of the second intermediate housing 53 through the second intermediate support ribs 54 to thereby hold the stator portion 521 in place. The base portion 5211 is preferably made of a synthetic resin and is preferably integrally formed with the second intermediate support ribs 54 and the second intermediate housing 53 by injection-molding the synthetic resin. However, the material and the method used in producing the base portion 5211, the second intermediate support ribs 54, and the second intermediate housing 53 are not limited to synthetic resin and injection molding. Alternatively, the base portion 5211, the second intermediate support ribs 54, and the second intermediate housing 53 may be formed by, e.g., die-casting an aluminum material or other suitable material.

As illustrated in FIG. 7, a substantially cylindrical bearing holder portion 5212 is fixed to the substantially central region of the base portion 5211 so that it can protrude upward from the base portion 5211. The bearing holder portion 5212 is preferably made of a metal and is preferably integrally fixed to the base portion 5211 by injection-molding (e.g., insert-molding) with a resin. The bearing holder portion 5212 may be either formed from a synthetic resin, aluminum, aluminum alloy or other suitable materials or integrally formed with the base portion 5211 and the second intermediate housing 53 by injection-molding a resin or die-casting aluminum. The material and the method used in producing the bearing holder portion 5212 are not particularly limited. Ball bearings 5213 and 5214 are provided in the axial upper and lower regions inside the bearing holder portion 5212, thus defining a portion of the bearing mechanism. An elastic member (e.g., a spring) is preferably arranged to pre-compress the ball bearing 5214 from above.

The stator portion 521 is provided with a stator 5215 and a circuit board 5216. The stator 5215 is attached to the outer surface of the bearing holder portion 5212. The circuit board 5216 preferably has a substantially annular flat shape and is attached to the lower side of the stator 5215. The circuit board 5216 is provided with a circuit arranged to control the rotation of the rotor portion 522 and is electrically connected to the stator 5215 through a conductive pin and the like. An electric current and a control signal are sent from an external power source (not shown) to the circuit board 5216 via a lead line group (not shown) having a plurality of lead lines tied together.

The rotor portion 522 includes a yoke 5221, a field magnet 5222, and a shaft 5223.

The yoke 5221 has a substantially cylindrical closed-top shape and is made of a magnetic metal. The field magnet 5222 preferably has a substantially cylindrical shape and has an outer circumferential surface fixed to the inner surface of the cylindrical portion of the yoke 5221 by an adhesive agent or suitable fixing materials or members. The field magnet 5222 has an inner circumferential surface radially opposite to the stator 5215. An axially protruding cylinder portion is provided in the substantially central region of the cover portion of the yoke 5221 and may be formed by burring or other suitable process. The shaft 5223 is press-fitted into the cylinder portion in a coaxial relationship with the central rotation axis J1.

The shaft 5223 is inserted into the bearing holder portion 5212 and is supported by the ball bearings 5213 and 5214 for rotation relative to the stator portion 521. That is, in the second intermediate axial flow fan 5, the shaft 5223 and the ball bearings 5213 and 5214 define a bearing mechanism arranged to support the yoke 5221 so that it can rotate about the central rotation axis J1 with respect to the base portion 5211.

As a driving current is supplied from the external power source to the stator 5215 through the lead line group and the circuit board 5216, the torque acting about the central rotation axis J1 is generated between the stator 5215 and the field magnet 5222. The rotor portion 522 and the second intermediate impeller 51 are rotated by the torque thus generated. The driving current supplied to the stator 5215 is controlled by the circuit in the circuit board 5216. This makes it possible to rotate the second intermediate impeller 51 at a predetermined rotation speed.

The upper and lower end surfaces of the first intermediate housing 43 axially coincide with the lower end surface of the intake side housing 23 and the upper end surface of the second intermediate housing 53, respectively. Furthermore, the upper and lower end surfaces of the second intermediate housing 53 axially coincide with the lower end surface of the first intermediate housing 43 and the upper end surface of the exhaust side housing 33, respectively.

When observing the contra-rotating axial flow fan unit 1A as a whole, the respective flow paths (i.e., the respective inner circumferential surfaces) of the intake side housing 23, the first intermediate housing 43, the second intermediate housing 53, and the exhaust side housing 33 are successively and smoothly connected to one another along the axial direction, thereby defining a single flow path (i.e., a single inner circumferential surface). The intake side impeller 21, the intake side support ribs 24, the first intermediate impeller 41, the first intermediate support ribs 44, the second intermediate impeller 51, the second intermediate support ribs 54, the exhaust side impeller 31, and the exhaust side support ribs 34 are arranged in the single flow path one below another from the upper side (i.e., the intake side) in FIG. 7.

In the contra-rotating axial flow fan unit 1A, the axially adjacent blades and support ribs differ in number from one another. Specifically, the number of the intake side support ribs 24 differs from the number of the blades (the intake side blades 211 and the first intermediate blades 411) of the impellers axially adjacent thereto (i.e., the intake side impeller 21 and the first intermediate impeller 41). Furthermore, the number of the first intermediate blades 411 differs from the number of the support ribs axially adjacent thereto (i.e., the intake side support ribs 24 and the first intermediate support ribs 44). Moreover, the number of the first intermediate support ribs 44 differs from the number of the blades (i.e., the first intermediate blades 411 and the second intermediate blades 511) of the impellers axially adjacent thereto (i.e., the first intermediate impeller 41 and the second intermediate impeller 51). In addition, the number of the second intermediate blades 511 differs from the number of the support ribs axially adjacent thereto (i.e., the first intermediate support ribs 44 and the second intermediate support ribs 54). The number of the second intermediate support ribs 54 differs from the number of the blades (i.e., the second intermediate blades 511 and the exhaust side blades 311) of the impellers axially adjacent thereto (i.e., the second intermediate impeller 51 and the exhaust side impeller 31). The number of the exhaust side blades 311 differs from the number of the exhaust side support ribs 34.

The air stream generated by the rotation of the respective blades impinges on the respective support ribs axially adjacent to the blades, thus generating a noise. If the respective blades and the respective support blades adjacent to each other along the central rotation axis J1 differ in number from each other, the frequency of a wind noise generated by the rotation of the respective impellers does not coincide with the frequency of an interfering noise generated when the air stream impinges against the respective support ribs. Therefore, it is possible to prevent an increase in noise which would otherwise occur by the sympathetic vibration of the wind noise and the interfering noise. This makes it possible to reduce the noise generated from the contra-rotating axial flow fan unit 1A.

As described above, a single flow path defined by the inner circumferential surfaces of the respective housings successively connected to one another in the axial direction is formed within the contra-rotating axial flow fan unit 1A. The air stream introduced from the intake side axial flow fan 2 is allowed to smoothly flow along the single flow path (i.e., the inner circumferential surfaces of the respective housings). Then, the air stream is discharged from the exhaust side axial flow fan 3 to the outside with a minimal loss of energy due to frictional contact of the air stream with the flow path.

FIG. 8 is a sectional view illustrating only the cross-sections of the intake side blades 211, the intake side support ribs 24, the first intermediate blades 411, the first intermediate support ribs 44, the second intermediate blades 511, the second intermediate support ribs 54, the exhaust side blades 311, and the exhaust side support ribs 34 taken along a cylindrical plane having an arbitrary radius measured from the central rotation axis J1 in FIG. 7.

Each of the intake side support ribs 24 is substantially flat and has an upper edge 241 positioned adjacent to each of the intake side blades 211 and a lower edge 242 positioned adjacent to the first intermediate axial flow fan 4. The upper edge 241 is positioned upstream of the lower edge 242 with reference to the rotational direction R2. Each of the intake side support ribs 24 has an air receiving surface 243 inclined relative to the central rotation axis J1 to face toward the exhaust side. Thus, the air receiving surface 243 confronts the air stream generated by the rotation of the intake side blades 211.

The air stream generated by the rotation of the intake side impeller 21 passes through the vicinity of the air receiving surface 243 and is introduced toward each of the first intermediate blades 411. Each of the first intermediate blades 411 is inclined with respect to the central rotation axis J1 substantially in the same direction as is each of the intake side support ribs 24. Therefore, the air stream whose velocity component has been changed by the intake side support ribs 24 is smoothly introduced toward the first intermediate blades 411, which makes it possible to minimize the reduction of energy of the air stream.

Each of the first intermediate support ribs 44 is substantially flat and has an upper edge 441 positioned adjacent to each of the first intermediate blades 411 and a lower edge 442 positioned adjacent to the second intermediate axial flow fan 5. The upper edge 441 is positioned upstream of the lower edge 442 with reference to the rotational direction R4. Each of the first intermediate support ribs 44 has an air receiving surface 443 inclined relative to the central rotation axis J1 to face toward the exhaust side. Thus, the air receiving surface 443 confronts the air stream generated by the rotation of the first intermediate blades 411.

The swirling-direction velocity component generated by the rotation of the first intermediate blades 411 acts substantially in the same direction as the rotational direction R4. A portion of the swirling-direction velocity component impinges against each of the first intermediate support ribs 44 and, therefore, is changed to a velocity component acting in the same direction as the central rotation axis J1. This increases the static pressure of the air.

The air stream passed through the vicinity of the air receiving surface 443 is introduced toward each of the second intermediate blades 511. As shown in FIG. 8, each of the second intermediate blades 511 is inclined with respect to the central rotation axis J1 substantially in the same direction as is each of the first intermediate support ribs 44. Therefore, the air stream whose velocity component has been changed by the first intermediate support ribs 44 is smoothly introduced toward the second intermediate blades 511, which makes it possible to reduce the loss of energy of the air stream.

Each of the second intermediate support ribs 54 has an upper edge 541 positioned adjacent to each of the second intermediate blades 511 and a lower edge 542 positioned adjacent to the exhaust side axial flow fan 3. The upper edge 541 is positioned upstream of the lower edge 542 with reference to the rotational direction R5. Each of the second intermediate support ribs 54 has an air receiving surface 543 inclined relative to the central rotation axis J1 to face toward the exhaust side. Thus, the air receiving surface 543 confronts the air stream generated by the rotation of the second intermediate blades 511.

The swirling-direction velocity component generated by the rotation of the second intermediate blades 511 acts substantially in the same direction as the rotational direction R5. A portion of the swirling-direction velocity component impinges against each of the second intermediate support ribs 54 and, therefore, is changed to a velocity component acting in the same direction as the central rotation axis J1. This increases the static pressure of the air.

The air stream passed through the vicinity of the air receiving surface 543 is introduced toward each of the exhaust side blades 311. As shown in FIG. 8, each of the exhaust side blades 311 is inclined with respect to the central rotation axis J1 substantially in the same direction as is each of the second intermediate support ribs 54. Therefore, the air stream whose velocity component has been changed by the second intermediate support ribs 54 is smoothly introduced toward the exhaust side blades 311, which makes it possible to reduce the loss of energy of the air stream.

The swirling-direction velocity component generated by the rotation of the exhaust side blades 311 acts substantially in the same direction as the rotational direction R3. A portion of the swirling-direction velocity component impinges against each of the exhaust side support ribs 34 and, therefore, is changed to a velocity component acting in the same direction as the central rotation axis J1. This increases the static pressure of the air.

Each of the exhaust side support ribs 34 has an upper edge 341 positioned adjacent to each of the exhaust side blades 311 and a lower edge 342 positioned adjacent to the exhaust side. The upper edge 341 is positioned upstream of the lower edge 342 with reference to the rotational direction R3. Each of the exhaust side support ribs 34 has an air receiving surface 343 inclined relative to the central rotation axis J1 to face toward the exhaust side. Thus, the air receiving surface 343 confronts the air stream generated by the rotation of the exhaust side blades 311.

The air stream passed through the vicinity of the air receiving surface 343 is discharged to the outside of the exhaust side housing 33 (namely, the outside of the contra-rotating axial flow fan unit 1A).

The construction described above ensures that the air drawn into the contra-rotating axial flow fan unit 1A passes through the vicinity of the respective blades and the respective support ribs with a minimal loss of energy and is discharged to the outside of the fan 1A. Furthermore, the air is allowed to smoothly pass through the vicinity of the respective blades and the respective support ribs, which makes it possible to efficiently rotate the respective impellers. As a consequence, it is possible to reduce the amount of electric power consumed in the respective motors of the axial flow fans.

Furthermore, the end surfaces of the respective housings axially coincide with one another and the inner circumferential surfaces thereof are continuously joined to one another. This allows the air to smoothly flow through the contra-rotating axial flow fan unit 1A. As a result, it becomes difficult for the air to flow backwards, which makes it possible to greatly increase the volume of air and the static pressure. Due to this feature, it is possible to discharge the air present in a device casing and to supply a sufficient quantity of air to electronic parts in the device casing, even when the system impedance within the device casing (namely, the flow path resistance within the device casing during introduction of the air) remains high.

As in the modified examples described above, the respective axial flow fans may be spaced apart along the central rotation axis J1 or spacers may be arranged between the respective axial flow fans in the present preferred embodiment. The support ribs of the respective axial flow fans may be arranged on the intake side along the axial direction. In this case, the air receiving surface of each support rib of the intermediate axial flow fans and the exhaust side axial flow fan is oriented to face toward the exhaust side. The inclination of the air receiving surface relative to the central rotation axis J1 is substantially the same as the inclination of each blade of the respective exhaust side impellers with respect to the central rotation axis J1. The inclination of the intake side support ribs 24 is substantially the same as the inclination of the intake side blades 211 with respect to the central rotation axis J1. This construction ensures that the air is smoothly drawn into the contra-rotating axial flow fan unit, thereby reducing a noise.

The preferred embodiments of the contra-rotating axial flow fan units described above are merely non-limiting examples. The contra-rotating axial flow fan units are not limited to the aforementioned shapes and configurations insofar as the impellers of the adjoining axial flow fans differ from one another in their rotational direction.

The number of the axial flow fans used in the contra-rotating axial flow fan units is not limited and may be, e.g., five or more, insofar as the respective axial flow fans adjoining to one other along the central rotation axis J1 differ in their rotational direction.

The cross-sectional shape of the respective support ribs is not particularly limited but may be a substantially circular shape, a substantially elliptical shape, a substantially polygonal shape or other shapes. It is preferred that the respective support ribs are shaped to have an upper edge, a lower edge, and an air receiving surface.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.