U.S. patent application number 11/611895 was filed with the patent office on 2007-06-21 for axial flow fan.
This patent application is currently assigned to NIDEC CORPORATION. Invention is credited to Yusuke Yoshida.
Application Number | 20070140844 11/611895 |
Document ID | / |
Family ID | 38173708 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070140844 |
Kind Code |
A1 |
Yoshida; Yusuke |
June 21, 2007 |
Axial Flow Fan
Abstract
An axial flow fan includes a motor placed on a frame and
including a rotor rotatable around a rotation axis. An impeller is
attached to an outer circumference of the rotor to rotate around
the rotation axis and includes blades for generating an air flow
when the rotor rotates. A housing surrounds the impeller to form a
passage for the air flow. Ribs extend from the frame to the
housing, thereby securing the frame to the housing. Each rib
includes an air guide face which faces the impeller. An angle of
average inclination of the air guide face with respect to the
rotation axis decreases in a direction away from the rotation axis.
The average inclination of the air guide face is defined as
inclination of a straight line approximately connecting both ends
of the air guide face on a plane perpendicular to a radial
direction.
Inventors: |
Yoshida; Yusuke; (Kyoto,
JP) |
Correspondence
Address: |
JUDGE & MURAKAMI IP ASSOCIATES
DOJIMIA BUILDING, 7TH FLOOR
6-8 NISHITEMMA 2-CHOME, KITA-KU
OSAKA-SHI
530-0047
JP
|
Assignee: |
NIDEC CORPORATION
338 Kuze Tonoshiro-cho, Minami-ku
Kyoto
JP
601-8205
|
Family ID: |
38173708 |
Appl. No.: |
11/611895 |
Filed: |
December 18, 2006 |
Current U.S.
Class: |
415/220 |
Current CPC
Class: |
F04D 25/0613 20130101;
F04D 29/544 20130101; F04D 29/582 20130101 |
Class at
Publication: |
415/220 |
International
Class: |
F04D 19/00 20060101
F04D019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 19, 2005 |
JP |
JP-2005-365585 |
Claims
1. An axial flow fan comprising: a motor including a rotor
rotatable around a rotation axis; an impeller attached to an outer
circumference of the rotor to rotate around the rotation axis
together with the rotor, the impeller including a plurality of
blades generating an air flow when the rotor rotates; a housing
surrounding an outer circumference of the impeller to form a
passage for the air flow; a frame on which the motor is placed; and
a plurality of ribs extending from the frame to the housing and
securing the frame to the housing; wherein each of the ribs
includes an air guide face facing the impeller, the air guide face
being an approximately flat face or a curved face having average
inclination with respect to an axial direction parallel to the
rotation axis, the average inclination being defined as inclination
of a straight line approximately connecting both ends of the air
guide face on a plane perpendicular to a radial direction
perpendicular to the axial direction at a position in the radial
direction, an angle of the average inclination becoming smaller in
a direction away from the rotation axis.
2. The axial flow fan according to claim 1, wherein a
cross-sectional area of each of the ribs when seen in a
longitudinal direction of that rib is approximately constant at any
position in a radial direction.
3. The axial flow fan according to claim 1, wherein an angle
between the average inclination of the air guide face of each of
the ribs and average inclination of one of the blades, which is
located at a closest position to that rib in the axial direction,
on a plane perpendicular to the radial direction is approximately
constant at any position in the radial direction, the average
inclination of each of the blades being defined as inclination of a
straight line approximately connecting both ends of the blade on
the plane perpendicular to the radial direction.
4. The axial flow fan according to claim 1, wherein an angle
between the average inclination of the air guide face of each of
the ribs and average inclination of one of the blades, which is
located at a closest position to that rib in the axial direction,
on a plane perpendicular to the radial direction is 100.degree. or
less at any position in the radial direction, the average
inclination of each of the blades being defined as inclination of a
straight line approximately connecting both ends of the blade on
the plane perpendicular to the radial direction.
5. The axial flow fan according to claim 1, wherein an angle
between the average inclination of the air guide face of each of
the ribs and inclination of a trailing edge of one of the blades,
which is located at a closest position to that rib in the axial
direction, on a plane perpendicular to the radial direction is
100.degree. or less at any position in the radial direction.
6. The axial flow fan according to claim 1, wherein a
cross-sectional shape of each of the ribs seen in the radial
direction is different at different positions in the radial
direction.
7. The axial flow fan according to claim 1, wherein each of the
ribs is arranged at an angle to a radial direction, when seen in
the axial direction.
8. The axial flow fan according to claim 1, wherein the ribs are
curved toward one of a rotating direction of the impeller and a
direction opposite to the rotating direction.
9. The axial flow fan according to claim 1, wherein each of the
ribs includes a bottom face which is approximately parallel to a
face of the housing and arranged in the same plane as the face of
the housing.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to an axial flow fan, and more
particularly, relates to a shape of ribs in the axial flow fan.
[0003] 2. Description of the Related Art
[0004] At present, electronic devices are provided with many
cooling fans for radiating a heat generated in the electronic
devices. The generated heat amount has been increasing with
enhancement of the performance of the electronic devices, and
therefore a required cooling performance of fans has become higher.
In order to improve the cooling performance of fans, it is
necessary to improve flow rate characteristics and static pressure
characteristics of the fans. The improvement of both the
characteristics requires the fans to rotate at high speeds. On the
other hand, demands for reduction of noises in many electronic
devices have increased with increase in the use of the electronic
devices at home or offices.
[0005] Usual fans include a motor, an impeller having a plurality
of blades attached to a rotor of the motor, and a housing which
supports a stator of the motor and surrounds an outer circumference
of the impeller. The housing includes a cavity which forms a
passage for an air flow generated by rotation of the impeller, a
frame which supports the stator, and a plurality of ribs which
connect the cavity and the frame to each other. The ribs are
arranged to cross the passage. Thus, windage loss at the ribs
causes energy loss, lowering both a flow rate and a static pressure
of the air flow. Moreover, the air flow interferes with the ribs to
cause an interference noise which is one noise source in the
fans.
[0006] In order to overcome the above problems, a rib having a
streamlined cross section has been proposed. The rib is arranged in
such a manner that its principal axis of its cross-sectional shape
is parallel to the air flow.
[0007] In order to actually design the proposed rib, however, it is
necessary to measure a direction of the air flow generated by
rotation of the impeller. This direction is changed not only by the
shape of the impeller but also by the revolution speed of the
impeller. In addition, the shape of the cavity of the housing, the
surface condition of the impeller, a state of placement of the fan
in an electronic device, a temperature, and a humidity can also
change the direction of the airflow. Since the direction of the
airflow is changed with a small change in surroundings as described
above, the proposed shape of the ribs can be designed only for a
specific fan structure, a specific revolution speed, a specific
condition of use, and the like. However, fans can have various
structures, operate at various revolution speeds, and be used under
various conditions. Considering those, the shape of the rib has to
be designed.
BRIEF SUMMARY OF THE INVENTION
[0008] In order to overcome the above problems, it is an object of
the present invention to provide a shape of ribs of a fan, which
can improve an air-blowing performance without degrading noise
characteristics, irrespective of a structure and a condition of use
of the fan.
[0009] According to an aspect of the present invention, an axial
flow fan includes: a motor including a rotor rotatable around a
rotation axis; an impeller attached to an outer circumference of
the rotor to rotate around the rotation axis together with the
rotor, the impeller including a plurality of blades generating an
air flow when the rotor rotates; a housing surrounding an outer
circumference of the impeller to form a passage for the air flow; a
frame on which the motor is placed; and a plurality of ribs
approximately radiating from the frame and securing the frame to
the housing. Each of the ribs includes an air guide face facing the
impeller. The air guide face is an approximately flat face or a
curved face having average inclination with respect to an axial
direction parallel to the rotation axis. The average inclination is
defined as inclination of a straight line approximately connecting
both ends of the air guide face on a plane perpendicular to a
radial direction perpendicular to the axial direction at a position
in the radial direction. An angle of the average inclination
becomes smaller in a direction away from the rotation axis.
[0010] A cross-sectional area of each of the ribs when seen in a
longitudinal direction of that rib may be approximately constant at
any position in a radial direction.
[0011] The angle of the average inclination of the air guide face
of each of the ribs with respect to average inclination of one of
the blades, which is located at a closest position to that rib in
the axial direction, on a plane perpendicular to the radial
direction may be approximately constant at any position in the
radial direction. The average inclination of each of the blades is
defined as inclination of a straight line connecting both ends of
the blade on the plane perpendicular to the radial direction.
[0012] The angle of average inclination of the air guide face of
each of the ribs with respect to average inclination of one of the
blades, which is located at a closest position to that rib in the
axial direction, on a plane perpendicular to the radial direction
may be 100.degree. or less at any position in the radial direction.
The average inclination of each of the blades is defined as
inclination of a straight line approximately connecting both ends
of the blade on the plane perpendicular to the radial
direction.
[0013] The angle of average inclination of the air guide face of
each of the ribs with respect to inclination of a trailing edge of
one of the blades, which is located at a closest position to that
rib in the axial direction, on a plane perpendicular to the radial
direction may be 100.degree. or less at any position in the radial
direction.
[0014] A cross-sectional shape of each of the ribs seen in the
radial direction may be different at different positions in the
radial direction.
[0015] Each of the ribs may be arranged at an angle to the radial
direction, when seen in the axial direction.
[0016] The ribs may be curved toward one of a rotating direction of
the impeller and a direction opposite to the rotating
direction.
[0017] Each of the ribs may include a bottom face which is
approximately parallel to a lower face of the housing and arranged
in the same plane as the lower face of the housing.
[0018] Other features, elements, advantages and characteristics of
the present invention will become more apparent from the following
detailed description of preferred embodiments thereof with
reference to the attached drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0019] FIG. 1 is a cross-sectional view of an axial flow fan
according to an embodiment of the present invention.
[0020] FIG. 2 is a perspective view of a housing of the axial flow
fan of FIG. 1.
[0021] FIGS. 3A to 3C show cross-sectional shapes of a rib and a
blade seen at a given position in a radial direction.
[0022] FIG. 4 is a plan view of the axial flow fan of FIG. 1.
[0023] FIG. 5 is a plan view of a modified example of the axial
flow fan of the embodiment of FIG. 1.
[0024] FIG. 6 is a plan view of the housing of the axial flow fan
of FIG. 1.
[0025] FIGS. 7A to 7C are cross-sectional views of an exemplary rib
according to the present invention, taken along line A-A, B-B, and
C-C in FIG. 6, respectively.
[0026] FIGS. 8A to 8C are cross-sectional views of another
exemplary rib according to the present invention, taken along line
A-A, B-B, and C-C in FIG. 6, respectively.
[0027] FIGS. 9A to 9C are cross-sectional views of still another
exemplary rib according to the present invention, taken along line
A-A, B-B, and C-C in FIG. 6, respectively.
[0028] FIGS. 10A to 10C are cross-sectional views of further
another exemplary rib according to the present invention, taken
along line A-A, B-B, and C-C in FIG. 6, respectively.
[0029] FIGS. 11A to 11C are cross-sectional views of further
another exemplary rib according to the present invention, taken
along line A-A, B-B, and C-C in FIG. 6, respectively.
[0030] FIGS. 12A to 12C are cross-sectional views of further
another exemplary rib according to the present invention, taken
along line A-A, B-B, and C-C in FIG. 6, respectively.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Referring to FIGS. 1 through 12C, preferred embodiments of
the present invention will be described in detail. It should be
noted that in the explanation 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 to a rotation axis, and a
radial direction indicates a direction perpendicular to the
rotation axis.
[0032] FIG. 1 is a cross-sectional view of an axial flow fan
according to an exemplary embodiment of the present invention. FIG.
2 is a perspective view of a housing of the axial flow fan of FIG.
1. FIG. 3A shows a cross-sectional shape of an impeller's blade
seen in a radial direction, together with a rotating direction of
an impeller and an air flow. FIGS. 3B and 3C are cross-sectional
views of a rib and an impeller's blade located at a closest
position to that rib, when seen at a given position in the radial
direction. FIG. 4 is a plan view of the axial flow fan of FIG. 1.
FIG. 5 is a plan view of a modified example of the axial flow fan
of FIG. 1.
[0033] An axial flow fan A includes a motor placed on a frame 12.
The motor includes a rotor in which an approximately cylindrical
rotor yoke 31 with a cover is included. The rotor yoke 31 is driven
by a current supplied from the outside of the axial flow fan A to
rotate. An impeller 2 having a plurality of blades 21 is attached
to an outer circumference of the rotor, i.e., an outer
circumferential surface of the rotor yoke 31, and can rotate
together with the rotor yoke 31 when the rotor yoke 31 rotates. The
rotor yoke 31 includes a shaft 32 which has an end fixed at a
center of the rotor yoke 31 by fastening. The shaft 32 serves as a
rotation axis.
[0034] At a center of the frame 12, an approximately cylindrical
bearing housing 12a having a bottom is formed. In the bearing
housing 12a, a radial bearing 34 is press-fitted and supported. The
radial bearing 34 includes an insertion hole for the shaft 32. The
shaft 32 is inserted into the insertion hole to be rotatable. The
radial bearing 34 is an oil-retaining bearing formed of porous
material such as sintered material, with lubricating oil contained
therein. Since the radial bearing 34 contains the lubricating oil,
the radial bearing 34 can rotatably support the shaft 32 via the
lubricating oil. However, the radial bearing 34 is not limited to a
sliding bearing which rotatably supports the shaft 32 via the
lubricating oil as described above. Instead of the sliding bearing,
a roller bearing such as a ball bearing may be used. The type of
bearing to be used is chosen in an appropriate manner, considering
the required performance and cost of the axial flow fan A.
[0035] The axial flow fan A also includes a stator 3 as a part of
the motor. The stator 3 is supported on an outer circumference of
the bearing housing 12a. The stator 3 includes a stator core 35, a
coil 37, an insulator 36, and a circuit board 38. The stator core
35 is surrounded by the insulator 36 formed of insulating material
so that an upper and a lower ends of the stator core 35 and each
tooth are insulated. The coil 37 is wound around the teeth with the
insulator 36 interposed therebetween. The circuit board 38 which
controls driving and rotation of the impeller 2 is arranged at a
lower end of the stator 3. In the circuit board 38, electronic
components (not shown) are mounted on a printed circuit board to
form circuitry. An end of the coil 37 is electrically connected to
the electronic components on the circuit board 38 which is bonded
and fixed to a lower part of the insulator 36. When a current
supplied from the outside of the axial flow fan A is made to flow
through the coil 37 via the electronic components including an IC
and a hole element, a magnetic field is generated around the stator
core 35.
[0036] On an inner circumferential surface of the impeller 2, the
rotor yoke 31 which can reduce leakage magnetic flux to the outside
of the axial flow fan A is provided. Moreover, a rotor magnet 33 as
a part of the motor, which is magnetized to achieve multipole
magnet, is attached to an inner circumference of the rotor yoke 31
inside the impeller 2. The rotor magnet 33 and the stator core 35
are opposed in the radial direction by inserting the shaft 32 fixed
by fastening to the center of the rotor yoke 31 into the radial
bearing 34. When a current flows through the coil 37, a rotating
torque is generated in the impeller 2 by interaction of the
magnetic field generated by the stator core 35 and a magnetic field
formed by the rotor magnet 33 magnetized to achieve multipole
magnet, thereby rotating the impeller 2 around the shaft 32 as a
rotation axis. A change in magnetic flux from the rotor magnet 33
which is rotating is detected by the hole element. Based on this
detection, an output voltage is switched by a drive IC. In this
manner, rotation of the impeller 2 is controlled to be stable.
During the rotation of the impeller 2, the blades 21 push air
downward, thus generating an air flow approximately along the axial
direction.
[0037] The frame 12 on which the motor is placed is disposed to be
opposed to the circuit board 38 in the axial direction and has a
shape of an approximately circular disk having approximately the
same diameter as an outer diameter of the circuit board 38. The
frame 12 is secured to the housing 1 with four ribs 13 which extend
from the frame to the housing 1. Please note that the number of the
ribs 13 for securing the frame 12 to the housing 1 is not limited
to four. Three or five ribs may be provided, for example. The
housing 1 is formed to surround an outer circumference of the
impeller 2 and includes a cavity 11 serving as a passage for an air
flow generated by rotation of the impeller 2. Outer circumferential
portions of an upper and a lower end faces of the housing 1 are
formed in an approximately square frame. Flange portions 14 are
formed at four corners of the square, respectively, which spread
radially outward. Each flange portion 14 has an attachment hole 14a
formed therein. When the axial flow fan A is mounted on a device in
which the axial flow fan A is to be used, an attaching component
such as a screw is inserted into the attachment hole 14a. The four
ribs 13 are arranged at regular angular intervals in the
circumferential direction.
[0038] When orthogonally projected onto a plane perpendicular to
the axial direction, the blades 21 of the impeller 2 are inclined
toward the rotating direction of the impeller 2 in the
circumferential direction. A cross-sectional shape of each blade 21
seen in the radial direction is an arc-like shape curved toward the
rotating direction of the impeller 2, as shown in FIG. 3B. A fan
used for cooling the inside of an electronic device is usually
chosen, considering system impedance in the electronic device
(i.e., a relationship between a static pressure and a flow rate in
the electronic device) and a flow rate and a static pressure of the
fan. In many electronic devices, electronic components, a power
source, and the like are concentrated in a narrow space and
therefore the system impedance is high. When the system impedance
is high, it is hard for fans having a low static pressure to
generate a sufficient air flow. For this reason, fans used for
cooling the inside of electronic devices are required to have a
high static pressure. In order to make the static pressure higher,
there is an approach in which an interval between adjacent blades
21 when seen in the axial direction is made smaller. This can be
achieved by making an arc length of an arc-like portion in the
cross-sectional shape of each blade 21 seen in the radial direction
longer radially outward. In this case, however, an axial height
(i.e., a height in the axial direction) of each blade 21 increases
radially outward. By making a difference in the axial height
between at a radially inner position and at a radially outer
position smaller, an effective volume of a space occupied by the
blades 21 in the cavity 11 (which is a product of an area of the
blade 21 when seen in the axial direction and the axial height of
the blade 21) increases. Thus, an axial flow fan A which is high in
both a flow rate and a static pressure can be designed. This can be
achieved by making inclination of the blade 21 with respect to the
axial direction larger radially outward.
[0039] A cross-sectional shape of the rib 13 seen in the radial
direction has an approximately triangular shape formed by a bottom
face 131, an air guide face 132, and a side face 133 connecting the
bottom face 131 and the air guide face 132 to each other, as shown
in FIG. 7A. The bottom face 131 is substantially perpendicular to
the axial direction, i.e., substantially parallel to a lower end
face of the housing 1 and a lower end face of the frame 12, and
forms the same plane as that formed by the lower end faces of the
housing 1 and the frame 12. The air guide face 132 guides an air
flow generated by rotation of the impeller 2 and is arranged at an
angle with respect to the axial direction. Although the air guide
face 132 formed by a flat face is shown in FIG. 7A, the air guide
face 132 may be a curved face. In a case of a curved face, average
inclination of the curved air guide face 132 is defined as
inclination of a straight line approximately connecting both ends
of the curved air guide face 132 on a cross section perpendicular
to the radial direction, and an angle of the air guide face 132
with respect to the axial direction is represented by the thus
defined average inclination.
[0040] Since the ribs 13 are arranged to cross the passage for the
air flow, the ribs 13 have to have such a shape that energy loss in
the air flow when the air flow passes by the ribs 13 is minimized.
If the ribs 13 have a streamlined shape in which a cross-sectional
shape of each rib 13 seen in the radial direction is parallel to
the air flow, energy loss in the air flow caused by hitting of the
air flow against the ribs 13 becomes smaller as the thickness of
the ribs 13 decreases. In a case where the ribs 13 are thin,
however, the axial height of the ribs 13 has to be increased in
order to obtain a sufficient level of strength of the ribs 13. The
ribs 13 which are high in the axial height are not preferable,
because the ribs 13 are close to the blades 21 and a noise
generated by interference of the air flow with the ribs 13 becomes
loud. Such a loud interference noise makes a noise level higher.
Based on the above consideration, in the present embodiment, the
ribs 13 are formed to have an approximately triangular
cross-sectional shape when seen in the radial direction. This
cross-sectional shape can increase both the thickness and strength
of the ribs 13 while suppressing energy loss in the air flow caused
by the ribs 13.
[0041] The air flow generated by rotation of the impeller 2 flows
along the air guide faces 132 of the ribs 13 when passing by the
ribs 13, and flows out of the cavity 11 to the outside of the axial
flow fan A. The blades 21 are inclined toward the rotating
direction of the impeller 2 as described above. Average inclination
of each blade 21 on a cross section perpendicular to the radial
direction is defined as inclination of a straight line
approximately connecting both ends of the blade 21 on that cross
section. The air flow does not flow parallel to the axial
direction. Instead, an angle of the air flow with respect to the
axial direction depends on the average inclination of the blade 21
and the air flow is discharged at an angle of 90.degree. or less
with respect to the average inclination of the blade 21. However,
this angle is changed by the cross-sectional shape of the blades
21, the shape of the cavity 11, the revolution speed of the
impeller 2, and an outside temperature at which the axial flow fan
A is used. The average inclination of each blade 21 is different at
different positions in the radial direction. Therefore, the angle
of the air flow from each blade 21 is different at different
positions in the radial direction.
[0042] In order to design the shape of the ribs 13 that can reduce
energy loss in the air flow caused by the ribs 13, it is necessary
to change average inclination of the air guide face 132 of each rib
13 depending on a position in the radial direction. Please note
that average inclination of the air guide face 132 of each rib 13
on a plane perpendicular to the radial direction is defined as
inclination of a straight line connecting both ends of the air
guide face 132. By changing the average inclination of the air
guide face 132 of each rib 13 in accordance with the angle of the
air flow flowing from the blades 21, the energy loss in the air
flow can be minimized. Since the angle of the air flow flowing from
the blade 21 changes depending on the cross-sectional shape of the
blades 21, the shape of the cavity 11, the revolution speed of the
impeller 2, and the outside temperature at which the axial flow fan
A is used, according to the present invention, the shape of the
ribs 13 is designed in such a manner that an angle of the average
inclination of the air guide face 132 of each rib 13 with respect
to average inclination of one of the blades 21 that is located at a
closest position to that rib 13 in the axial direction is
100.degree. or less, considering the above change. In the present
embodiment, the angle of the average inclination of the air guide
face 132 of each rib 13 with respect to the blade 21 located at a
closest position to that rib 13 in the axial direction is set to
90.degree., as shown in FIG. 3B. Moreover, the energy loss in the
air flow can be made constant at any position in the radial
direction by designing the shape of the ribs 13 in such a manner
that the angle of the average inclination of the air guide face 132
of each rib 13 with respect to the inclination of the blade 21
located at a closest position to that rib 13 in the axial direction
is the same at any position in the radial direction. In particular,
when the angle of the average inclination of the air guide face 132
of each rib 13 with respect to the inclination of the blade 21
located at a closest position to that rib 13 in the axial direction
is set to 90.degree., the energy loss can be reduced. In a case
where a curvature of a curved portion of a cross-sectional shape of
each blade 21 seen in the radial direction is relatively small with
respect to an arc length thereof, the angle of the air flow from
that blade 21 does not depend on average inclination of that blade
21 on a cross section perpendicular to the radial direction but
depends on an angle of a trailing edge 211 of the blade 21 with
respect to the axial direction. In this case, the cross-sectional
shape of the ribs 13 is designed based on the angle of the trailing
edge 211 in place of the inclination of the blade 21.
[0043] FIG. 6 is a plan view of the housing in the present
embodiment. FIGS. 7A to 12C are cross-sectional views of the rib
13, taken along line A-A, line B-B, and line C-C, respectively. In
an example of FIGS. 7A to 7C, the bottom face 133 is always formed
in the same plane as the lower end face of the frame 12. Moreover,
the length of the bottom face 131 in the cross-sectional shape seen
in the radial direction becomes shorter radially outward, and the
height of the side face 133 in that cross-sectional shape becomes
higher radially outward. That is, the cross-sectional shape of the
ribs 13 in the example of FIGS. 7A to 7C changes in such a manner
that an angle .theta. of the air guide face 132 with respect to the
bottom face 131 gradually increases radially outward, i.e., an
angle of the average inclination of the air guide face 132 with
respect to the axial direction becomes smaller in a direction away
from the rotation axis. In this example, the ribs 13 are designed
have a constant cross-sectional area when seen in a longitudinal
direction. With this design, concentration of stress does not occur
even when a load is applied to the ribs 13, and lowering of the
strength of the ribs 13 can be suppressed. In addition, strength is
not high at a joint of the frame 12 and each rib 13. However, by
forming the bottom faces 131 of the ribs 13 in the same plane as
the lower end face of the frame 12, concentration of stress can be
suppressed and therefore lowering of the strength at the joint of
the frame 12 and each rib 13 can be suppressed. The same can be
applied to a joint of the housing 1 and each rib 13. That is,
lowering of the strength at the joint of the housing 1 and each rib
13 can be suppressed by forming the bottom face 131 of each rib 13
in the same plane as the lower end face of the housing 1.
[0044] FIGS. 8A to 12C illustrate modified examples of the
cross-sectional shape of the ribs 13 in the present embodiment. In
the example of FIGS. 7A to 7C, the side face 133 of each rib 13 is
parallel to the axial direction, whereas in an example of FIGS. 8A
to 8C the cross-sectional shape of the ribs 13 changes in such a
manner that an angle of the side face 133 with respect to the
bottom face 131 becomes smaller radially outward. Therefore, in the
example of FIGS. 8A to 8C, inclination of the side face 133 becomes
close to inclination of the air guide face 132, so that energy loss
in the air flow can be suppressed. However, the thickness of the
ribs 13 is thinner in the example of FIGS. 8A to 8C than in the
example of FIGS. 7A to 7C. Therefore, the strength of each rib 13
in the example of FIGS. 8A to 8C is lower than in the example of
FIGS. 7A to 7C. In an example of FIGS. 9A to 9C, the length of the
bottom face 131 in the cross-sectional shape seen in the radial
direction is kept constant, and an angle of the air guide face 132
with respect to the bottom face 131 is increased radially outward.
That is, an angle of the average inclination of the air guide face
132 with respect to the axial direction becomes smaller in the
direction away from the rotation axis. In this case also, energy
loss in the air flow can be suppressed. Moreover, since the axial
height of the ribs 13 is kept constant in the example of FIGS. 9A
to 9C, a cross-sectional area of each rib 13 when seen in the
longitudinal direction of that rib 13 can be made constant. In an
example of FIGS. 10A to 10C, a corner connecting the bottom face
131 and the side face 133 to each other is rounded. With this
cross-sectional shape, it is possible to prevent the air flow which
is to flow along the ribs 13 from flowing away the ribs 13, thus
suppressing generation of turbulence. In the present invention, the
cross-sectional shape of each rib 13 seen in the radial direction
is not limited to an approximately triangular shape. For example,
the cross-sectional shape of each rib 13 seen in the radial
direction may be a shape of a static vane shown in FIGS. 11A to
11C, or a shape with both longitudinal ends rounded, as shown in
FIGS. 12A to 12C.
[0045] Assuming that the air flow flows approximately parallel to
the air guide face 132, an angle of the side face 133 with respect
to the air flow in FIG. 7C is smaller than in FIG. 7A. At a
radially outer position, the arc length in the cross-sectional
shape of the blade 21 seen in the radial direction is longer and a
circumferential velocity of the blade 21 is larger, as compared
with those at the radially inner position. Therefore, a flow rate
of the air flow generated by the blades 21 is higher at the
radially outer position than at the radially inner position. Thus,
by making an angle of the side face 133 of each rib 13 with respect
to the air flow on a plane perpendicular to the radial direction
smaller at the radially outer position, energy loss in the air flow
can be suppressed. At the radially inner position, even if the
angle of the side face 133 of each rib 13 with respect to the air
flow is large, an effect of the ribs 13 on energy loss in the air
flow is small because a flow rate is low. Therefore, it is possible
to provide an axial flow fan which can achieve a high flow
rate.
[0046] By setting an angle of average inclination of the air guide
face 132 of each rib 13 with respect to average inclination of a
blade 21 located at a closest position to that rib 13 in the axial
direction to 90.degree. on a plane perpendicular to the radial
direction, not only energy loss in the air flow but also an
interference noise generated by hitting of the air flow against the
ribs 13 can be reduced. If the trailing edges 211 of the blades 21
and the ribs 13 are arranged approximately parallel to each other
during rotation of the blades 21, the air flow pushed by the blades
21 hits the ribs 13 at the same time and therefore the interference
noise becomes large. In order to avoid this, each rib 13 is
arranged at an angle to the radial direction in such a manner that
a radially outer end thereof is located backward of a radially
inner end thereof in the rotating direction of the impeller 2, when
seen in the axial direction, as shown in FIG. 4. In this case, the
ribs 13 and the blades 21 cannot be parallel to each other because
the blades 21 are inclined toward the rotating direction of the
impeller 2 in the present embodiment. Moreover, as shown in FIG. 5
as a modified example, the thus arranged ribs 13 may be curved
toward a direction opposite to the rotating direction of the
impeller 2. Also in this case, the interference noise can be
suppressed. Alternatively, in a case where the blades 21 are
inclined toward the direction opposite to the rotating direction of
the impeller 2 when being orthogonally projected onto a plane
perpendicular to the axial direction, each of the ribs 13 is
arranged at an angle to the radial direction in such a manner that
a radially outer end thereof is located forward of a radially inner
end thereof in the rotating direction of the impeller 2.
[0047] In the present invention, when a cross-sectional shape of
each rib 13 seen at a given position in the radial direction is
determined, it is only necessary to change average inclination of
the air guide face 132 of each rib 13 in accordance with average
inclination of the blades 21. Therefore, the ribs 13 can be easily
designed.
[0048] According to the present invention, energy loss in an air
flow generated by rotation of an impeller, which is caused by ribs,
can be minimized. In addition, reduction in a flow rate and a
static pressure of the air flow can be suppressed. Moreover, it is
also possible to suppress an interference noise generated when the
air flow passes by the ribs.
[0049] 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.
* * * * *