U.S. patent application number 12/921828 was filed with the patent office on 2011-02-03 for centrifugal fan.
Invention is credited to Shingo Ohyama.
Application Number | 20110023526 12/921828 |
Document ID | / |
Family ID | 41318788 |
Filed Date | 2011-02-03 |
United States Patent
Application |
20110023526 |
Kind Code |
A1 |
Ohyama; Shingo |
February 3, 2011 |
CENTRIFUGAL FAN
Abstract
A centrifugal fan 1 includes a plurality of blades 44 having a
three-dimensional shape, a main plate 43, an annular side plate 45,
and a motor 41 for rotating the blades 44 via the main plate 43.
One end face of each blade 44 in the span direction is fixed to the
main plate 43, such that the blades 44 are spaced at predetermined
intervals along the circumferential direction. The annular side
plate 45 is provided at the other end faces of the blades 44 in the
span direction. The radial length of a trailing edge 44b of each
blade 44 at a part close to the main plate 43 is set longer than
the radial length of the trailing edge 44b of each blade 44 at a
part close to the side plate 45.
Inventors: |
Ohyama; Shingo; (Sakai-shi,
JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
41318788 |
Appl. No.: |
12/921828 |
Filed: |
May 13, 2009 |
PCT Filed: |
May 13, 2009 |
PCT NO: |
PCT/JP2009/058938 |
371 Date: |
September 10, 2010 |
Current U.S.
Class: |
62/426 ;
416/223R |
Current CPC
Class: |
F04D 29/30 20130101;
F04D 29/281 20130101; F05D 2240/303 20130101 |
Class at
Publication: |
62/426 ;
416/223.R |
International
Class: |
F25D 17/06 20060101
F25D017/06; F04D 29/38 20060101 F04D029/38 |
Foreign Application Data
Date |
Code |
Application Number |
May 14, 2008 |
JP |
2008-126940 |
Claims
1. A centrifugal fan comprising a plurality of blades having a
three-dimensional shape, a main plate, an annular side plate, and
blade actuating means for rotating the blades via the main plate,
one end face of each blade in the span direction being fixed to the
main plate, such that the blades are spaced at predetermined
intervals along the circumferential direction, the annular side
plate being provided at the other end faces of the blades in the
span direction, the radial length of an outer end portion of each
blade at a part close to the main plate being set longer than the
radial length of an outer end portion of the blade at a part close
to the side plate.
2. The centrifugal fan according to claim 1, wherein the radial
length of the outer end portion of each blade close to the main
plate is set longer than the radial length of the outer end portion
of each blade close to the side plate, so that air flow flowing
past each blade effectively receives work from the blade at a part
of the blade close to the main plate, and that the velocity of the
air flow along the span direction of each blade is effectively
developed at a part of the blade close to the main plate.
3. The centrifugal fan according to claim 1 or 2, wherein the
radial length of the outer end portion of each blade close to the
main plate is set longer than the radial length of the outer end
portion of each blade close to the side plate by extending the
trailing edge of the blade rearward with respect to the air
flow.
4. The centrifugal fan according to claim 3, wherein the trailing
edge is extended such that its length is gradually increased toward
the main plate.
5. The centrifugal fan according to claim 4, wherein the trailing
edge is extended in a curved manner, and the trailing edge has a
bulge in a part close to the main plate such that there is at least
one inflection point in the curved part.
6. The centrifugal fan according to claim 3, wherein the trailing
edge is extended rearward by a length that corresponds to the
velocity distribution of the main flow of the air flow at the
trailing edge.
7. The centrifugal fan according to claim 6, wherein the amount of
extension of the trailing edge is less than or equal to 10% of the
outer diameter of the side plate.
8. The centrifugal fan according to 7 claim 1, wherein a step that
is extended forward by a predetermined length is formed at a part
of the leading edge of the blade close to the main plate.
9. The centrifugal fan according to claim 1, wherein the diameter
of the main plate is increased in accordance with the extension of
the blades.
10. The centrifugal fan according to claim 1, wherein the
centrifugal fan is configured as an air blower of an indoor unit of
an air conditioner.
Description
TECHNICAL FIELD
[0001] The present invention relates to a centrifugal fan, and more
particularly, to the structure of impeller blades for a centrifugal
fan.
BACKGROUND ART
[0002] In general, centrifugal fans such as backwardly curved blade
fans generate undesirable noise. Accordingly, reduction of noise in
centrifugal fans has been sought. Thus, various techniques have
been developed for reducing air flow noise from centrifugal fans. A
typical design approach for reducing air flow noise is to increase
the outer diameter of the impeller of a fan.
[0003] When the obtained air volume is the same, an increased
diameter of the fan impeller reduces the number of rotations of the
fan impeller. This reduces the flow velocity of the air flow
discharged by the fan impeller. The level of the air flow noise is
proportional to the sixth power of the flow velocity.
[0004] However, a simple increase of the outer diameter of the fan
impeller increases the size of the fan as a whole including an air
passageway. Also, an increase in the outer diameter of the impeller
will require an increase in the manufacturing costs for the fan,
and readjustment of the parameters of the blades (the inlet angle,
the outlet angle) and the layout in the apparatus.
[0005] In this respect, some techniques have been introduced that
use thick blades, that is, airfoil blades to reduce flow separation
around the blades, in an attempt to reduce noise at the lowest
costs possible (for example, refer to Patent Document 1).
[0006] However, the use of the airfoil blades does not necessarily
eliminate the flow separation over the entire surfaces of the
blades. That is, noise caused by trailing vortices at the trailing
edges of the blades is not reduced.
[0007] The flow velocity of the discharged air flow varies along
the span direction of each blade. That is, an uneven velocity
distribution of air flow exists between the main plate and the side
plate.
[0008] According to some other techniques, dimples or serrations
are formed over the entire surface of each blade to reduce noise
(for example, refer to Patent Document 2). In such a configuration,
separation of air flow is further efficiently reduced, so that
turbulence due to separation will be segmented. This further
suppresses the development of trailing vortices. As a result, the
noise caused by any trailing vortex is reduced.
[0009] However, this configuration can neither prevent an uneven
velocity distribution of air flow on each blade between the main
plate and the side plate along the span direction of the blade, nor
suppress the generation of trailing vortices. Therefore, the noise
caused by trailing vortices cannot be effectively prevented.
Further, the configuration disadvantageously complicates the
manufacturing process for surface machining and drawing of the
blades.
[0010] Meanwhile, various techniques are known that improve the
velocity distribution of air flow at the outlet of an impeller as
described above. According to one of these techniques, a part of
each blade adjacent thereto is cut out to reduce the length of the
blade, thereby reducing the flow separation at this part. This
equalizes the velocity distribution at the outlet of the impeller
(refer to Patent Document 3).
[0011] However, because of the cut out portion in the part of each
blade adjacent to the side plate, the length of the blade is less
than a blade having no cut out portion. This reduces the amount of
work that each blade applies to an air flow.
[0012] This technique is effective in a centrifugal fan in which
the blade width is sufficiently small in relation to the outer
shape, as described in Patent Document 3. Specifically, cutting out
of the blades prevent separation of drawn-in air flow in a part of
each blade in the vicinity of the side plate, and the air flow is
allowed to flow along the part. However, the technique is not
necessarily effective in a normal centrifugal fan, which has wide
blades, that is, not in a case where the influence of separation
near the side plate is not dominant.
[0013] Also, if a part of the blade near the side plate is cut out,
air flow that passes through the part interferes with corners of a
part of the blade near the main plate. This can generate noise
having discretely-distributed frequency spikes.
[0014] Another technique has been known in which the blades are
perpendicular to the main plate and the side plate, and the
thickness of each blade is changed from the end face adjacent to
the main plate to the end face adjacent to the side plate.
Accordingly, fluctuation of the flow velocity distribution at the
outlet of the blades is suppressed. In this case, the outer
diameter of the main plate of the impeller and the outer diameter
of the side plate of the impeller are made differently at the
outlet of the impeller (outer end faces of the blades), and the
size of each blade is increased toward the main plate (for example,
refer to Patent Document 4), while maintaining the shape. This
reduces the velocity fluctuation of the flow at the blade wake.
[0015] However, at the outlet of the impeller, the ratio of the
outer diameter of the main plate of the impeller to the outer
diameter of the side plate of the impeller is set to 1.2 to 1.6.
Therefore, when the blades are enlarged with a small increase in
the length, a noise reduction effect cannot be achieved. In
contrast, when the length is excessively increased, the flow
characteristics will be degraded.
[0016] As a result, the outer diameter of the main plate of the
impeller is twenty percent greater than the outer diameter of the
side plate. This structure is not advantageous over the technique
in which the size of the fan is simply increased. Enlargement of
the size by twenty percent or more goes entirely against desired
goals that have been sought in the prior art, which are the size
reduction and lowered noise, in the first place.
[0017] In the meantime, an impeller has been proposed that uses
blades having a three-dimensional shape (for example, refer to
Patent Document 2). Specifically, each blade extends in the
rotation direction of the fan while being twisted from the main
plate toward the side plate.
[0018] Compared with the impellers having the above described
blades with two-dimensional shapes, an impeller having blades with
three-dimensional shapes improves the load distribution on the
blade surface and pressure fluctuations of air flow through between
the blades.
[0019] A centrifugal fan employing blades having a
three-dimensional shape and an air conditioner using the
centrifugal fan will now be described with reference to FIGS. 25 to
32.
[0020] FIG. 25 illustrates an air conditioner 1 using a centrifugal
fan having an impeller. The air conditioner 1 is a ceiling embedded
type, and includes a casing 2 accommodating various components, and
a decorative panel 3 provided underneath the casing 2. More
specifically, the casing 2 of the air conditioner 1 is fitted in an
opening formed in a ceiling U of an air conditioned room, and the
decorative panel 3 is arranged along the ceiling U.
[0021] The casing 2 is a box-like body having a lower opening. When
viewed in the plan view, the casing 2 has an octagonal shape with
short sides and long sides arranged alternately. The casing 2 has a
substantially octagonal top plate 21 with alternately arranged long
sides and short sides, and a side wall plate 22 extending downward
from the periphery of the top plate 21.
[0022] The decorative panel 3 is shaped as a rectangular plate in
the plan view. The panel 3 has an air inlet 31 and air outlets 32.
The air inlet 31 is substantially located at the center of the
decorative panel 3 and draws air from the air conditioned room. The
air outlets 32 blows out air from the casing 2 into the air
conditioned room. The decorative panel 3 is arranged such that each
of its sides corresponds to one of the long sides of the top plate
21 of the casing 2.
[0023] The air inlet 31 is a substantially square opening. In
contrast, each air outlet 32 is an elongated rectangular opening
along a side of the decorative panel 3. The air inlet 31 includes
an air intake grill 33 and a filter 34 for removing dust in the air
drawn into the air inlet 31.
[0024] Further, a horizontal flap 35 is provided at each air outlet
32. The horizontal flap 35 can be swung about an axis extending
along the longitudinal direction of the air outlet 32. The
horizontal flap 35 is a rectangular blade-like member extending
along the longitudinal direction of the air outlet 32. Each
horizontal flap 35 is supported at both ends in the longitudinal
direction with a support pins (not shown) so as to be rotatable.
The horizontal flap 35 is thus capable of changing the direction of
flow of air blown out of the air outlet 32.
[0025] An air blower 4 and a heat exchanger 6 are arranged in the
casing 2. The air blower 4 draws air into the casing 2 from the air
conditioned room through the air inlet 31 of the decorative panel
3. The heat exchanger 6 is arranged to surround the air blower
4.
[0026] The air blower 4 is a backwardly curved blade fan, which is
a centrifugal fan to which the present invention is applied. The
air blower 4 includes a fan motor (impeller driving means) 41 and
an impeller 42 coupled to the shaft (rotary shaft) of the fan motor
41. The fan motor 41 is provided at the center of the top plate 21
of the casing 2 and is oriented downward.
[0027] The heat exchanger 6 is a cross-inner-tube type heat
exchanger, which is formed by bending a tube to surround the air
blower 4. The heat exchanger 6 is connected to an outdoor unit (not
shown) installed outside the house through refrigerant piping. The
heat exchanger 6 functions as an evaporator during the cooling
operation, and as a condenser during the heating operation. The
heat exchanger 6 performs heat exchange with air that is drawn into
the casing 2 by the air blower 4 through the air inlet 31, so as to
cool the air during the cooling operation, and heat the air during
the heating operation.
[0028] A drain pan 7 is provided below the heat exchanger 6. The
drain pan 7 receives drain water, which is produced when water in
air is condensed on the surface of the heat exchanger 6. The drain
pan 7 is attached to the lower part of the casing 2. The drain pan
7 has an air inlet portion 71 communicating with the air inlet 31
of the decorative panel 3, air outlet portions 72 corresponding to
the air outlets 32 of the decorative panel 3, and a drain water
receiving groove 73 that is formed to surround the lower part of
the heat exchanger 6 to receive drain water.
[0029] A bellmouth 5 is provided at the air inlet portion 71 of the
drain pan 7. The bellmouth 5 guides air drawn in through the air
inlet 31 of the decorative panel 3 to the impeller 42 of the air
blower 4.
[0030] The structure of the impeller 42 of the centrifugal air
blower 4 will now be described with reference to FIGS. 26 and 27.
FIG. 26 is a perspective view showing the appearance of the
impeller 42. FIG. 27 is a side view of the impeller 42 shown in
FIG. 3.
[0031] The impeller 42 is mainly formed by a disk-shaped main plate
43, an annular side plate 45 separated from the main plate 43, and
a plurality of blades 44 arranged between the main plate 43 and the
side plate 45. The main plate 43 is coupled to a shaft 41a of the
fan motor 41. The blades 44 are arranged along the main plate 43 at
predetermined angular intervals about the shaft 41a of the fan
motor 41.
[0032] The main plate 43 is a synthetic resin member. A
substantially truncated cone shaped bulge 43a is formed at the
center of the main plate 43. The bulge 43a protrudes toward the air
inlet 31. A main plate cover 46 is fixed at a position spaced from
the lower surface of the main plate 43. The main plate cover 46
covers a cooling air hole. Radially arranged guide blades 46a are
provided on the side of the main plate cover 46 that faces the main
plate 43. The guide blades 46a cause part of the air blown out of
the impeller 42 to flow about the main plate 43 by means of the
difference between the static pressure in the space between the
main plate 43 and the casing 2 and the static pressure in the space
between the main plate 43 and the side plate 45. This part of the
blown out air passes through the space about the fan motor 41 to
cool the fan motor 41. Thereafter, the air is blown toward the
space inside the impeller 42 through the cooling air hole of the
main plate 43 and the guide blades 46a of the main plate cover
46.
[0033] The side plate 45 has a diameter that gradually decreases
from the outer circumference toward the center opening. The side
plate 45 is a bell shaped resin member that protrudes toward the
air inlet 31.
[0034] The structure of the blades 44 of the impeller 42 will be
described with reference to FIGS. 28 to 32. FIG. 28 is a
perspective view of the blade 44 as seen from the left rear. FIG.
29 is a projection view of the blade 44 of FIG. 28, as viewed from
above. FIG. 30 is a side view of the blade 44 of FIG. 28 with
section lines 31A-31A to 31E-31E. FIGS. 31(a) to 31(e) are
cross-sectional views taken along lines 31A-31A to 31E-31E. FIG. 32
is a schematic view illustrating an operation of the blade 44.
[0035] The blades 44 are resin members that are molded separately
from the main plate 43 and the side plate 45. One end face of each
blade 44 is fixed to the main plate 43, and another end face is
fixed to the side plate 45. When viewed from the side of the
impeller 42, the end of the blade 44 that faces the side plate 45
is inclined rearward than the end close to the main plate 43 as
shown in FIG. 28. The blade 44 is formed such that these ends cross
each other to substantially form an X as shown in FIG. 29. In other
words, the blade 44 has a three-dimensional shape that extends
parallel to the rotary shaft while being twisted between the main
plate 43 and the side plate 45.
[0036] The edge at the leading side in the rotation direction of
the blade 44, which is a three-dimensional blade, or a leading edge
44a extends from the end close to the main plate 43 to a
predetermined position close to the side plate 45 with
substantially a constant radius. The blade 44 as an inclined edge
from the predetermined position close to the side plate 45, while
retreating outward with a gradually decreasing radius. The edge of
the blade 44 on the trailing side of the rotation direction R, or
the trailing edge 44b, has a shape formed by a straight line that
extends parallel to the rotation from the position close to the
main plate 43 and the position close to the side plate 45.
[0037] Compared to blade shapes formed based on a two-dimensional
blade element as in Patent Documents 1, 3 and 4, the load
distribution on the surface of the blade 44 and the pressure
fluctuation of the air flow through between the blades 44 are
significantly improved. Accordingly, at least noise caused by
pressure fluctuation of air flow is effectively reduced.
[0038] However, since the trailing edge 44b of the blade 44 has a
shape formed by connecting the ends between the main plate 43 and
the side plate 45, noise is still generated by the influence of the
trailing vortex. As shown in FIG. 32, the amount of air flow
F.sub.2, which flows along the curvature of the side plate 45 is
small in the entire air flow drawn in from a part of the blade 44
in the vicinity of the side plate 45. In contrast, main flow
F.sub.1, which is drawn in from the center of the side plate 45,
has a great amount. The main flow F.sub.1 flows in an area close to
the main plate 43 in a concentrated manner because of its velocity
vector. Therefore, the flow velocity distribution of the blown out
air flow at the outlet of the blade 44 is not constant along the
span direction of the blade 44. Also, it is not possible to reduce
the trailing vortex generated at a position downstream of the
trailing edge 44b of the blade 44. As a result, noise is still
generated by the influence of the trailing vortex.
Patent Document 1: Japanese Laid-Open Patent Publication No.
2002-339897
Patent Document 2: Japanese Laid-Open Patent Publication No.
2005-155510
Patent Document 3: Japanese Laid-Open Patent Publication No.
5-60096
Patent Document 4: Japanese Laid-Open Patent Publication No.
2001-132687
DISCLOSURE OF THE INVENTION
[0039] Accordingly, it is an objective of the present invention to
provide a three-dimensional blade for a centrifugal fan, and a
centrifugal fan that further reduces noise.
[0040] To achieve the foregoing objectives, and in accordance with
a first aspect of the present invention, a centrifugal fan is
provided that includes a plurality of blades having a
three-dimensional shape, a main plate, an annular side plate, and
blade actuating means for rotating the blades via the main plate.
One end face of each blade in the span direction is fixed to the
main plate, such that the blades are spaced at predetermined
intervals along the circumferential direction. The annular side
plate is provided at the other end faces of the blades in the span
direction. The radial length of an outer end portion of each blade
at a part close to the main plate is set longer than the radial
length of an outer end portion of the blade at a part close to the
side plate.
[0041] This configuration significantly improves the velocity
distribution of a main flow portion in an air flow that flows in a
concentrated manner at a part of the blade close to the main plate.
Accordingly, the static pressure-flow rate characteristics of the
fan are improved in the entire flow rate range, which increases the
discharged air volume. Also, the specific noise characteristics of
the air blower are significantly improved, so that the influence of
a trailing vortex generated at the trailing edge of a part of each
three-dimensional blade close to the main plate is relatively
reduced. As a result, noise caused by trailing vortices is
effectively reduced.
[0042] It is preferable that the radial length of the outer end
portion of each blade close to the main plate be set longer than
the radial length of the outer end portion of each blade close to
the side plate, so that air flow flowing past each blade
effectively receives work from the blade at a part of the blade
close to the main plate, and that the velocity of the air flow
along the span direction of each blade is effectively developed at
a part of the blade close to the main plate.
[0043] This configuration significantly improves the velocity
distribution of a main flow portion in air flow that flows in a
concentrated manner at a part of the blade close to the main plate.
Accordingly, the static pressure-flow rate characteristics of the
fan are improved over the entire flow rate range, which increases
the discharged air volume. Also, the specific noise characteristics
of the air blower are significantly improved, so that the influence
of a trailing vortex generated at the trailing edge of a part of
each three-dimensional blade close to the main plate is relatively
reduced. As a result, noise caused by trailing vortices is
effectively reduced.
[0044] It is preferable that the radial length of the outer end
portion of each blade close to the main plate be set longer than
the radial length of the outer end portion of each blade close to
the side plate by extending the trailing edge of the blade rearward
with respect to the air flow.
[0045] Extending the trailing edge of the blade rearward with
respect to the air flow significantly improves the velocity
distribution in the main flow portion of air flow that flows in a
concentrated manner at a part of the blade close to the main plate.
Accordingly, the static pressure-flow rate characteristics of the
fan are improved in the entire flow rate range, which increases the
blown air volume. Also, the specific noise characteristics of the
air blower are significantly improved, so that the influence of a
trailing vortex generated at the trailing edge of a part of each
three-dimensional blade close to the main plate is relatively
reduced. As a result, noise caused by trailing vortices is
effectively reduced.
[0046] It is preferable that the trailing edge be extended such
that its length is gradually increased toward the main plate.
[0047] According to this configuration, the trailing edge of each
blade has a tapered shape that becomes wider from the side plate
toward the main plate. Therefore, the shape of the trailing edge of
each blade is a suitable shape that is adapted to changes in the
velocity distribution of the main flow that flows in a concentrated
manner at a part of the blade close to the main plate. The
substantially tapered shape, which becomes gradually wider from the
side plate toward the main plate, may be a shape that changes
linearly or a shape that changes in a curve.
[0048] It is preferable that the trailing edge be extended in a
curved manner, and that the trailing edge have a bulge in a part
close to the main plate such that there is at least one inflection
point in the curved part.
[0049] In the centrifugal fan, air flow in the vicinity of the main
plate forms a laminar flow shear layer due to the viscosity and the
influence of the wall surface of the main plate. This can narrow
the passage of the main flow and lower the fan performance.
However, the above configuration suppresses the development of the
shear layer, thereby improving the fan performance.
[0050] The trailing edge is preferably extended rearward by a
length that corresponds to the velocity distribution of the main
flow of the air flow at the trailing edge.
[0051] According to this configuration, the shape of the extended
trailing edge is further suitable for changes in the velocity
distribution of the main flow, so that the fan performance is
further improved.
[0052] It is preferable that a step that is extended forward by a
predetermined length be formed at a part of the leading edge of the
blade close to the main plate.
[0053] According to this configuration, when air flow drawn into
the impeller through the air inlet is blown outward from the
trailing edges of the blades, the air flow is effectively prevented
from separating from the negative pressure surfaces of the blades.
This further effectively reduces noise of the air blower.
[0054] The diameter of the main plate is preferably increased in
accordance with the extension of the blades.
[0055] By extending the radius of the main plate in accordance with
the extension of the trailing edge of each blade, the structural
strength of the impeller of the centrifugal fan is also
increased.
[0056] The centrifugal fan is preferably configured as an air
blower of the indoor unit of an air conditioner.
[0057] The air blower of an air conditioner indoor unit is required
to have a large air volume and to be silent because of its intended
use. Therefore, a centrifugal fan according to the present
invention is optimal as the air blower of an air conditioner indoor
unit because of its small size, high air flow performance, and low
noise.
[0058] As described above, the present invention provides a compact
and quiet centrifugal fan that generates a large air volume and is
therefore suitable as an air blower for an air conditioner indoor
unit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0059] FIG. 1 is a perspective view showing an air conditioner
using a centrifugal fan according to one embodiment of the present
invention;
[0060] FIG. 2 is a longitudinal cross-sectional view showing the
air conditioner of FIG. 1;
[0061] FIG. 3 is a perspective view showing the impeller of the
centrifugal fan of FIG. 2;
[0062] FIG. 4 is a side view showing the impeller of FIG. 3;
[0063] FIG. 5 is a bottom view showing the impeller of FIG. 3;
[0064] FIG. 6 is a side view showing a blade of FIG. 3;
[0065] FIG. 7 is a projection view of the blade of FIG. 6, as
viewed from the side corresponding to the side plate;
[0066] FIG. 8 is a perspective view of the blade of FIG. 6, as
viewed from above and the left rear;
[0067] FIG. 9 is a side view of the blade of FIG. 6, with section
lines 10A-10A to 10E-10E;
[0068] FIGS. 10(a) to 10(e) are cross-sectional views corresponding
to respective section lines of FIG. 9;
[0069] FIG. 11 is a cross-sectional view showing the comparison
between the shape of the trailing edge of the blade according to
the present embodiment and the shape of the trailing edge of a
prior art blade;
[0070] FIG. 12 is a diagram showing the operation (characteristics)
of the blade of FIG. 6;
[0071] FIG. 13 is a graph showing changes in the static pressure
coefficient of four examples using flow rate coefficient as a
parameter, to measure the effects of the blade according to the
present embodiment, where (a) represents a case of the prior art
fan of FIGS. 25 to 32, (b) represents a case where the outer
diameter of the prior art fan is increased as a whole by 5%, (c)
represents a case where the outer diameter of only a lower part
(part close to the main plate) of the blade of the present
embodiment is increased by 5%, and (d) represents a case where the
outer diameter of only a lower part (part close to the main plate)
of the blade of the present embodiment is increased by 10%;
[0072] FIG. 14 is a graph showing changes in air flow noise of the
four examples of FIG. 13 using the discharged air volume as a
parameter to measure the effects of the blade according to the
present embodiment;
[0073] FIG. 15 is a graph showing changes in specific noise of the
four examples of FIG. 13 using the blown air volume as parameter,
to measure the effects of the blade according to the present
embodiment;
[0074] FIG. 16 is a diagram showing operation of a blade according
to a first modification, in which the shape of the trailing edge of
the blade according to the present embodiment is changed in a
curved manner;
[0075] FIG. 17 is a diagram showing operation of a blade according
to a second modification, in which the shape of the trailing edge
of the blade according to the present embodiment is changed to a
shape that reduces shearing force;
[0076] FIG. 18 is a longitudinal cross-sectional view showing an
air conditioner that uses a centrifugal fan according to a third
modification, in which steps are provided at the leading edge of
each blade according to the present embodiment, in a part that
faces the main plate;
[0077] FIG. 19 is a side view showing a blade of the impeller of
the air conditioner;
[0078] FIG. 20 is a perspective view of the blade of FIG. 19, as
viewed from above and the left rear;
[0079] FIG. 21 is a diagram showing the operation of the blade of
FIG. 19;
[0080] FIG. 22 is a graph showing changes in specific noise of four
examples using a flow rate coefficient as a parameter, to measure
the effects of the blade of FIG. 19, where (a) represents a case of
the prior art fan of FIGS. 25 to 32 without steps, (b) represents a
case where steps are formed in the prior art blade, (c) represents
a case of the present embodiment of FIGS. 1 to 12, and (d)
represents the third modification of the present embodiment shown
in FIGS. 18 to 21;
[0081] FIG. 23 is a graph showing changes in static pressure
coefficient of the four examples using a flow rate coefficient as a
parameter, to measure the effects of the blade of FIG. 19;
[0082] FIG. 24 is a diagram showing operation of a blade according
to a fourth modification of the present embodiment;
[0083] FIG. 25 is a longitudinal cross-sectional view showing an
air conditioner using a prior art centrifugal fan;
[0084] FIG. 26 is a perspective view showing the impeller of the
centrifugal fan of FIG. 25;
[0085] FIG. 27 is a side view showing a blade of FIG. 26;
[0086] FIG. 28 is a perspective view of the impeller of FIG. 26, as
viewed diagonally from the rear;
[0087] FIG. 29 is a projection view of the blade of FIG. 28;
[0088] FIG. 30 is a side view of the blade of FIG. 28, with section
lines;
[0089] FIGS. 31(a) to 31(e) are cross-sectional views respectively
along lines 31A-31A to 31E-31E of FIG. 30; and
[0090] FIG. 32 is a diagram showing the operation of the blade of
FIG. 28.
BEST MODE FOR CARRYING OUT THE INVENTION
[0091] Hereinafter, a centrifugal fan according to one embodiment
of the present invention and an air conditioner using the
centrifugal fan will be described with reference to the
drawings.
(1) Overall Structure of Air Conditioner
[0092] FIG. 1 is a perspective view showing the appearance of an
air conditioner 1 using a centrifugal fan according to one
embodiment of the present invention (ceiling part is omitted). The
air conditioner 1 is a ceiling embedded type, and includes a casing
2 accommodating various components, and a decorative panel 3
provided underneath the casing 2. More specifically, the casing 2
of the air conditioner 1 is fitted in an opening formed in a
ceiling U of an air conditioned room as shown in FIG. 2 (a
longitudinal cross-sectional view of the air conditioner 1), and a
decorative panel 3 is arranged along the ceiling U.
[0093] The casing 2 is a box-like body having a lower opening. When
viewed in the plan view, the casing 2 has an octagonal shape with
short sides and long sides arranged alternately. The casing 2 has a
substantially octagonal top plate 21 with alternately arranged long
sides and short sides, and a side wall plate 22 extending downward
from the periphery of the top plate 21.
[0094] The decorative panel 3 is shaped as a rectangular plate in
the plan view. The panel 3 has an air inlet 31 and air outlets 32.
The air inlet 31 is substantially located at the center of the
decorative panel 3 and draws air from the air conditioned room. The
air outlets 32 discharges air from the casing 2 into the air
conditioned room. The decorative panel 3 is arranged such that each
of its sides corresponds to one of the long sides of the top plate
21 of the casing 2.
[0095] The air inlet 31 is a substantially square opening. In
contrast, each air outlet 32 is an elongated rectangular opening
along a side of the decorative panel 3. The air inlet 31 includes
an air intake grill 33 and a filter 34 for removing dust in the air
drawn into the air inlet 31.
[0096] Further, a horizontal flap 35 is provided at each air outlet
32. The horizontal flap 35 can be swung about an axis extending
along the longitudinal direction of the air outlet 32. The
horizontal flap 35 is a rectangular blade-like member extending
along the longitudinal direction of the air outlet 32. Each
horizontal flap 35 is supported at both ends in the longitudinal
direction with a support pins (not shown) so as to be rotatable.
The horizontal flap 35 is thus capable of changing the direction of
flow of air discharged from the air outlet 32.
[0097] An air blower 4 and a heat exchanger 6 are arranged in the
casing 2. The air blower 4 draws air into the casing 2 from the air
conditioned room through the air inlet 31 of the decorative panel
3. The heat exchanger 6 is arranged to surround the air blower
4.
[0098] The air blower 4 is a backwardly curved blade fan, which is
a centrifugal fan to which the present invention is applied.
[0099] The air blower 4 includes a fan motor (impeller driving
means) 41 and an impeller 42 coupled to the shaft (rotary shaft) of
the fan motor 41. The fan motor 41 is provided at the center of the
top plate 21 of the casing 2 and is oriented downward. The detailed
structure of the impeller 42 will be discussed below.
[0100] The heat exchanger 6 is a cross-inner-tube type heat
exchanger, which is formed by bending a tube to surround the air
blower 4. The heat exchanger 6 is connected to an outdoor unit (not
shown) installed outside the house through refrigerant piping. The
heat exchanger 6 functions as an evaporator during the cooling
operation, and as a condenser during the heating operation. The
heat exchanger 6 performs heat exchange with air that is drawn into
the casing 2 by the air blower 4 through the air inlet 31, so as to
cool the air during the cooling operation, and heat the air during
the heating operation.
[0101] A drain pan 7 is provided below the heat exchanger 6. The
drain pan 7 receives drain water, which is produced when water in
air is condensed on the surface of the heat exchanger 6. The drain
pan 7 is attached to the lower part of the casing 2. The drain pan
7 has an air inlet portion 71 communicating with the air inlet 31
of the decorative panel 3, air outlet portions 72 corresponding to
the air outlets 32 of the decorative panel 3, and a drain water
receiving groove 73 that is formed to surround the lower part of
the heat exchanger 6 to receive drain water.
[0102] A bellmouth 5 is provided at the air inlet portion 71 of the
drain pan 7. The bellmouth 5 guides air drawn in through the air
inlet 31 of the decorative panel 3 to the impeller 42 of the air
blower 4.
[0103] As described above, the air conditioner 1 of the present
embodiment has an air passage from the air inlet 31 of the
decorative panel 3 to the four air outlets 32 via the filter 34,
the bellmouth 5, the drain pan 7, the air blower 4, and the heat
exchanger 6. The air conditioner 1 draws in air in the air
conditioned room and performs heat exchange between the drawn in
air and the refrigerant at the heat exchanger 6. Thereafter, the
air conditioner 1 discharges air, the temperature of which has been
adjusted, to the four sides of the air conditioned room through the
air passage.
[0104] (2) Structure of Impeller of Centrifugal Fan
[0105] The structure of the impeller 42 of the centrifugal fan 4
will be described with reference to FIGS. 2 to 5. FIG. 3 is a
perspective view showing the appearance of the impeller 42. FIG. 4
is a side view of the impeller 42 shown in FIG. 3. Further, FIG. 5
is a diagram showing the impeller 42 installed as shown in FIG. 4,
as viewed from above.
[0106] The impeller 42 is mainly formed by a disk-shaped main plate
43, an annular side plate 45 separated from the main plate 43, and
a plurality of blades 44 arranged between the main plate 43 and the
side plate 45. The main plate 43 is coupled to a shaft 41a of the
fan motor 41. The blades 44 are arranged along the main plate 43 at
predetermined angular intervals about the shaft 41a of the fan
motor 41. The rotation direction of the impeller 42 is shown by
arrow R in FIG. 5.
[0107] The main plate has an outer diameter Db'. The main plate 43
is a synthetic resin member. A substantially truncated cone shaped
bulge 43a is formed at the center of the main plate 43. The bulge
43a protrudes toward the air inlet 31. A main plate cover 46 is
fixed at a position spaced from the lower surface of the main plate
43. The main plate cover 46 covers a cooling air hole. Radially
arranged guide blades 46a are provided on the side of the main
plate cover 46 that faces the main plate 43. The guide blades 46a
cause part of the air blown out of the impeller 42 to flow about
the main plate 43 by means of the difference between the static
pressure in the space between the main plate 43 and the casing 2
and the static pressure in the space between the main plate 43 and
the side plate 45. This part of the discharged air passes through
the space about the fan motor 41 to cool the fan motor 41.
Thereafter, the air is blown to the space inside the impeller 42
through the cooling air hole of the main plate 43 and the guide
blades 46a of the main plate cover 46.
[0108] The side plate has an outer diameter Da. The side plate 45
has a shaper that gradually decreases from the outer circumference
toward the center opening. The side plate 45 is a bell shaped resin
member that protrudes toward the air inlet 31.
(3) Structure of Impeller Blade
[0109] The structure of the blades 44 of the impeller 42 will be
described with reference to FIGS. 6 to 11. FIG. 6 is a side view of
the blade 44 as seen from the direction of the negative pressure
surface. FIG. 7 is a projection view of the blade 44 of FIG. 6, as
viewed from above. FIG. 8 is a perspective view of the blade 44 of
FIG. 6, as viewed from above and the left rear. FIG. 9 is a side
view of the blade 44 of FIG. 6, with section lines 10A-10A to
10E-10E from a lower part to an upper part (from the end close to
the main plate 43 to the end close to the side plate 45). FIGS.
10(a) to 10(e) are cross-sectional views taken along lines 10A-10A
to 10E-10E of FIG. 9, respectively. FIG. 11 is a cross-sectional
view showing a characteristic of the blade 44 (difference from the
prior art shape shown in FIGS. 25 to 32). FIG. 11 is a comparison
between FIG. 10 and the cross-sectional view of FIG. 31 taken along
line 31B-31B.
[0110] The blades 44 are resin members that are molded separately
from the main plate 43 and the side plate 45. One end face of each
blade 44 is fixed to the main plate 43, and another end face is
fixed to the side plate 45. When viewed from the side of the
impeller 42, the end of the blade 44 that faces the side plate 45
is inclined rearward than the end close to the main plate 43 as
shown in FIG. 28. The blade 44 is formed such that these ends cross
each other to substantially form an X as shown in FIG. 29. In other
words, the blade 44 has a three-dimensional shape that extends
parallel to the rotary shaft while being twisted between the main
plate 43 and the side plate 45.
[0111] The edge at the leading side in the rotation direction of
the blade 44, which is a three-dimensional blade, or a leading edge
44a extends from the end close to the main plate 43 to a
predetermined position close to the side plate 45 with
substantially a constant radius. The blade 44 as an inclined edge
from the predetermined position close to the side plate 45, while
retreating outward with a gradually decreasing radius. The end of
the blade 44 on the opposite side of the rotation direction R, or
the trailing edge 44b, has a shape different from that of the prior
art blade. Unlike the prior art shown in FIGS. 25 to 32, the
trailing edge 44b does not have a shape formed by a straight line
(perpendicular to the main plate 43) that extends parallel to the
rotation axis from the end close to the main plate 43 and the end
close to the side plate 45. For example, as shown in FIGS. 6 to 11,
the trailing edge 44b is extended rearward with respect to air flow
such that the degree of extension increases from the end close to
the side plate 45 toward the main plate 43. Accordingly, the radial
length (Rb' in FIG. 5) of the trailing edge 44b of the main plate
43 of the blade 44 through which main flow F.sub.1 of the
centrifugal fan flows is set longer than the radial length (Ra in
FIG. 5) of the trailing edge 44b at a part close to the side plate
45 of the blade 44. Thus, the velocity distribution of air flow
along the span direction of the blade 44 effectively develops in a
part close to the main plate 43, at which the flow rate is
greater.
[0112] The trailing edge 44b is extended without changing the basic
shape of the three-dimensional blade, but rather in conformity with
the shape of the three-dimensional blade. In this case, using the
original outer diameter Db of the main plate 43 (the outer diameter
Da of the side plate 45) as a reference, the outer diameter Db'
(FIG. 12) of the expanded main plate 43 is preferably determined
such that the amount of extension of the trailing edge 44b is less
than or equal to 10% of the original diameter Db. In other words,
the amount of extension of the trailing edge 44b is preferably less
than or equal to 10% of the outer diameter Da of the side plate
45.
[0113] In the case of the configuration of FIGS. 6 to 12, the outer
diameter Db' of the main plate 43 is enlarged by approximately 5%
compared to the outer diameter Db of the prior art main plate 43
shown in FIGS. 28 to 32.
[0114] When extending the trailing edge 44b, the inlet angle, the
outlet angle, the attaching angle, and the skew angle of a blade
element of the blade 44 that is close to the main plate 43 are
maintained to the same values as those in the original blade 44
shown in FIGS. 28 to 32. Therefore, as shown in, for example, the
perspective view of FIG. 8, the blade 44 of the present embodiment
is enlarged at the attachment position on the side corresponding to
the trailing edge 44b bulges in the direction of rotation.
[0115] Also, in accordance with the extension of the radius at the
end of the blade 44 close to the main plate 43, the outer diameter
of the main plate 43 is increased.
[0116] As described above, since the radius of the main plate 43 is
increased in accordance with the extension of the trailing edge 44b
of each blade 44, the structural strength of the impeller 42 of the
centrifugal fan is also increased.
[0117] In accordance with the above described configuration, the
velocity distribution of the main flow F.sub.1, which flows in a
concentrated manner in the vicinity of the main plate 43, is
significantly improved compared to the flow F.sub.2 in the vicinity
of the side plate 45. Accordingly, the static pressure-flow rate
characteristics (P-Q characteristics) of the blade 44 is improved
over the entire flow rate range, which increases the discharged air
volume. Also, the specific noise characteristics are significantly
improved, so that the influence of a trailing vortex generated at
the trailing edge 44b of a part of each three-dimensional blade 44
close to the main plate 43 is relatively reduced. As a result, the
noise caused by trailing vortices is effectively reduced.
[0118] In the present embodiment, the trailing edge 44b is extended
further rearward toward the main plate 43. As shown in FIG. 12, the
trailing edge 44b is also extended so that it becomes gradually
longer from the side plate 45 toward the main plate 43. That is,
the trailing edge 44b of the blade 44 has a tapered shape that is
linearly enlarged from the side plate 45 toward the main plate 43.
Accordingly, the velocity distribution at the outlet of the blade
44 effectively develops in the main flow F.sub.1. Therefore, the
influence of the wake flow generated at a part of the trailing edge
44b close to the main plate 43 is relatively weakened.
[0119] Thus, since the trailing edge 44b is enlarged into a tapered
shape, the shape of the trailing edge 44b is more suitable for
changes in the velocity distribution of the main flow F.sub.1, in
which the flow rate increases toward the main plate 43. That is,
the shape of the blade 44 is optimized for the velocity
distribution of air flow, so that the fan performance is
improved.
[0120] Therefore, by using a centrifugal fan according to the
present embodiment, a compact and low noise air conditioner having
a large air volume can be produced at low cost.
[0121] Particularly, in the impeller of the centrifugal fan
according to the present embodiment, each blade has the
three-dimensional shape as described above (see FIGS. 7 and 8).
Thus, compared to blade shapes formed based on two-dimensional
blade element as in the prior art, the load distribution on the
surface of the blade 44 and the pressure fluctuation of air flow
through between the blades 44 are significantly improved.
[0122] Therefore, if such a three-dimensional blade is combined
with the blade 44 of the present embodiment and operates as
described above, trailing vortices are further effectively
eliminated.
[0123] As a matter of course, several techniques have been known
that improve the velocity distribution of air flow at the outlet of
the blade 44, as described in the prior art section. For example,
Patent Document 3 discloses a blade that is shortened by cutting
out a part close to the side plate. This reduces the flow
separation at the part of the blade close to the side plate,
thereby equalizing the velocity distribution at the outlet of the
blade.
[0124] However, because of the cut out portion in the part of each
blade adjacent to the side plate, the length of the blade is less
than a blade having no cut out portion. This reduces the amount of
work that each blade applies to the air flow. According to the
present embodiment, the blade 44 itself is not shortened. On the
contrary, the area of the blade 44 is increased. The present
embodiment there does not have such a drawback and effectively
increases the work of the blade 44.
[0125] This technique is effective in a centrifugal fan in which
the blade width is sufficiently small in relation to the outer
shape, as described in Patent Document 3. Specifically, cutting out
of the blades prevent separation of drawn-in air flow in a part of
each blade in the vicinity of the side plate, and the air flow is
allowed to flow along the part. However, the technique is not
necessarily effective in a normal centrifugal fan, which has wide
blades, that is, not in a case where the influence of separation
near the side plate is not dominant.
[0126] Also, if a part of the blade near the side plate is cut out,
air flow that passes through the part interferes with corners of a
part of the blade near the main plate. This can generate noise
having discretely-distributed frequency spikes. According to the
present embodiment, the basic shape of the blade 44 itself is not
changed at all. Thus, the present embodiment does not have these
drawbacks of the prior art.
[0127] Another technique has been known in which the blades are
perpendicular to the main plate and the side plate, and the
thickness of each blade is changed from the end face adjacent to
the main plate to the end face adjacent to the side plate.
Accordingly, fluctuation of flow velocity distribution at the
outlet of the blades is suppressed. In some prior art
configurations, the outer diameter of a part of the blade close to
the main plate and the outer diameter of a part of the blade close
to the side plate are made different, so that the shape of each
blade is increased toward the main plate (for example, refer to
Patent Document 4). This reduces the velocity fluctuation of the
flow at the blade wake.
[0128] However, the ratio of the diameter of the impeller at the
side close to the main plate to the diameter of the impeller at the
side close to the side face is set to 1.2 to 1.6. Therefore, when
the blades are enlarged with a small increase in the length, noise
reduction effect cannot be achieved. In contrast, when the length
is excessively increased, the flow characteristics will be
degraded.
[0129] As a result, the diameter of the impeller at the end close
to the main plate is twenty percent greater than the diameter at
the side plate. This structure is not advantageous over the
technique in which the size of the fan is simply increased.
Enlargement of the size by twenty percent or more goes entirely
against desired goals that have been sought in the first place in
the prior art, which are size reduction and lowered noise.
[0130] However, such drawbacks are effectively eliminated by the
present embodiment.
[0131] In the present embodiment, the trailing edge 44b is extended
further rearward toward the main plate 43. As shown in FIG. 12, the
trailing edge 44b is also extended so that it becomes gradually
longer from the side plate 45 toward the main plate 43.
[0132] According to this configuration, the trailing edge 44b of
each blade 44 has a tapered shape that becomes wider from the side
plate 45 toward the main plate 43. As shown in FIG. 12, the
trailing edge 44b is more suitable for changes in the velocity
distribution of the main flow F.sub.1, which flows in a
concentrated manner in the vicinity of the main plate 43. In this
case, in accordance with the extension of the radius at a part of
the trailing edge 44b of the blade 44 close to the main plate 43,
the outer diameter of the main plate 43 is increased.
[0133] As described above, since the radius of the main plate 43 is
increased in accordance with the extension of the trailing edge 44b
of each blade 44, the structural strength of the impeller 42 of the
centrifugal fan is also increased.
EXAMPLES
[0134] The flow characteristics of two prior art Examples a, b and
two Examples c, d according to the present invention were measured.
Example c was an impeller having three-dimensional blades, or the
blades 44 of the present embodiment in which the enlargement factor
of the outer diameter Db' of the main plate 43 (extension factor of
the width of the trailing edge 44b rearward with respect to air
flow) was 5%. Example d was an impeller having three-dimensional
blades, or the blades 44 of the present embodiment in which the
enlargement factor of the outer diameter Db' of the main plate 43
was 10%. The prior art Example a, or comparison example, was an
impeller having the prior art blades shown in FIGS. 27 to 32. The
prior art Example b was equivalent to the fan of the prior Example
a in which the diameter of the fan was increased by 5%.
[0135] In the Examples c and d, a part of each blade 44 close to
the main plate 43 (the lower part as viewed in FIGS. 6 to 9) was
extended by 5% and 10% rearward with respect to the air flow,
respectively. The main flow F.sub.1, which dominantly determined
the fan performance, passed through this part. This increased the
amount of work applied to air flow by the blades 44, so that the
volume of air blown discharged from the impeller was effectively
increased. Thus, compared to the prior art Example a or the prior
art Example b, in which the diameter of the fan was increased by 5%
as a whole, the static pressure-flow rate characteristics (P-Q
characteristics) were increased over the entire flow rate range in
the Examples c and d as shown in FIG. 13. Also, in the Examples c
and d, the air flow noise was reduced except for a low air volume
range as shown in FIG. 14.
[0136] Further, the improvement of the two properties significantly
improved the specific noise characteristics of the Examples c and
d, for example, as shown in FIG. 15. The properties of the Examples
c and d were clearly improved in comparison with the prior art
Example b, in which the diameter of the fan was evenly
increased.
[0137] In comparison with the blade of Patent Document 4, the size
of which was increased without changing the shape, the same
advantages were achieved in a range of the enlargement factor less
than or equal to 10% (5%). Therefore, the impeller according to the
present embodiment is superior in terms of increased air volume and
lowered noise, and is capable of being reduced in size.
First Modification
[0138] The shape of enlargement (extension) of the trailing edge
44b of the blade 44 is not limited to the linear shape (tapered
shape corresponding to a linear function) as described above. For
example, as shown in FIG. 16, the extension may be shaped as a
curve of a quadratic function for a parabola.
[0139] This curved shape of the trailing edge 44b is selected in
consideration of the deviation of the velocity distribution of air
flow discharged from the impeller. Specifically, the trailing edge
44b of the blade 44 is extended further in the radial direction
toward the main plate 43 from the side plate 45.
[0140] According to this configuration, the trailing edge 44b of
each blade 44 has a tapered shape of a quadratic function that
becomes wider in an arcuate manner from the side plate 45 toward
the main plate 43. The trailing edge 44b of the blade 44 is
suitable for the velocity distribution of the main flow F.sub.1,
which flows in a concentrated manner in the vicinity of the main
plate 43 as shown in FIG. 16.
[0141] That is, the shape of the trailing edge 44b of the blade 44
is further effectively optimized for the velocity distribution of
the main flow F.sub.1, so that the fan performance is improved.
Second Modification
[0142] In the centrifugal fan as described above, air flow in the
vicinity of the main plate 43 forms a laminar flow shear layer due
to the influence of the viscosity at the wall surface of the main
plate. This can narrow the passage of the main flow F.sub.1 and
lower fan performance.
[0143] To deal with such a problem, the trailing edge 44b may be
formed such that there is at least one inflection point in the
curved part as shown in FIG. 17. In this case, the trailing edge
44b has a bulge in the vicinity of the main plate 43. Taking into
consideration the velocity distribution of the air flow at the
outlet, the enlargement of the trailing edge 44b at the end face
close to the main plate 43 is slightly shorter than the maximally
extended part (but, at least greater than or equal to the end of
the trailing edge 44b close to the side plate 45).
[0144] This configuration suppresses the development of the shear
layer, thereby improving the fan performance.
Third Modification
[0145] As shown in FIGS. 18 to 21, the leading edge 44a of the
blade 44 may be formed stepwise (in the present embodiment, two
steps) toward the inside of the impeller 42. Specifically, the
leading edge 44a may have first and second steps 44c, 44d. When air
flow drawn into the impeller through the air inlet 31 and the
bellmouth 5 shown in FIG. 1 is blown to the outside by the blade
44, the first and second steps 44c, 44d prevent the air flow from
separating from the negative pressure surface of the blade 44. In
this manner, the first and second steps 44c, 44d contribute to
further reduction of air flow noise of the air blower 4.
[0146] The negative pressure surface refers to a surface of the
blade 44 that faces inward of the impeller 42. The surface opposite
to the negative pressure surface, that is, a surface of the blade
44 that faces outward of the impeller 42 is referred to as a
positive pressure surface.
[0147] The lengths LaLb, LcLd of the first and second steps 44, 44d
are set to be 0.09 to 0.18 times the original chord lengths
L.sub.1, L.sub.2, L.sub.3 of the blade 44 in the span direction.
That is, the length of the first step 44c, which is the lower step,
varies along the span direction in a range between 0.15 times the
length L.sub.1 (La) and 0.2 times the length L.sub.1 (Lb), and the
length of the second step 44d, which is the upper step, varies
along the span direction in a range between 0.08 times the length
L.sub.2 (Lc) and 0.1 times the length L.sub.3 (Ld).
[0148] The blade 44 according to the third modification with
trapezoid steps will be discussed as an Example d. The above
described example of FIGS. 1 to 12 with none of the first and
second steps 44c, 44c on the leading edge 44a will be expressed as
an example c (without trapezoid steps). A comparison Example b is
the one with the first and second steps 44c, 44d formed in the
prior art blade. When Example c and Example d are compared, the
specific noise characteristics are lowered in the entire flow
region. The Example c achieves a reduction of at least 1.1 [dBA] at
the minimum specific noise characteristic point. This is because
when the impeller is rotating, the protruding shapes of the first
and second steps 44c, 44d of the leading edge 44a generate a
longitudinal vortex in the air flow prior to flowing into the blade
44 as a reference, and the longitudinal vortex prevents the air
flow from separating from the leading edge 44a.
[0149] The centrifugal fan according to the embodiment shown in
FIGS. 1 to 12 is designed such that air flow that passes through
the impeller flows below the blade 44 compared to the prior art
Example a. Therefore, the longitudinal vortex generated by the
first and second steps 44c, 44d of the leading edge 44a further
effectively prevents separation.
[0150] As shown in FIG. 23, the Example c is inferior to the
comparison Example b in terms of the aerodynamic characteristics in
the low air volume region. In the Example d, the first and second
steps 44c, 44d improved the characteristics in the low air volume
region compared to the comparison Example b. Accordingly, in
comparison with the comparative Example b, the aerodynamic
characteristics of the Example d were improved in the entire air
volume range. Consequently, the centrifugal fan using the Example d
according to the third modification of the present invention was
capable of generating a greater amount of air volume at the same
static pressure.
[0151] As a result, the centrifugal fan according to the third
modification of the present embodiment generates lower noise than a
conventional centrifugal fan, and is capable of increasing the air
volume at the same static pressure. This enables development of a
compact and quiet fan.
[0152] Accordingly, the use of the centrifugal fan realizes a
compact air conditioner that runs quietly while maintaining greater
air volume.
Fourth Modification
[0153] As shown in FIGS. 24, the first and second steps 44c, 44d of
the third modification may be combined with the second
modification.
[0154] <Application of Centrifugal Fan of Present
Invention>
[0155] Generally, biased velocity distribution of the discharged
air flow at a part of each blade close to the main plate is
commonly seen in any type of centrifugal fan. Therefore, the
present invention can be applied to impellers in centrifugal fans
of various types (for example, backwardly-curved blade types,
sirocco types, and straight radial blade types). In these cases,
the fan performance is sufficiently improved.
* * * * *