JP2019183738A - Axial flow blower - Google Patents

Abstract

To provide an axial flow blower capable of suppressing peeling of airflow at a bell mouth suction part, to reduce shaft power.SOLUTION: In an axial flow blower 70 provided with an axial flow fan 71 and a bell mouth 74, even if airflow in a suction part 76 of the bell mouth 74 is peeled off because the roundness of the suction-side opening part of the bell mouth 74 is small, with an air guider 75 provided upstream of the bell mouth 74, the peeling is suppressed by correcting the airflow in the direction of pressing against the suction part 76 of the bell mouth 74, so that the cross-sectional area of a ventilation passage of the axial flow blower 70 is prevented from narrowing to reduce air resistance, thereby reducing shaft power for obtaining a predetermined air volume.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an axial blower including an axial fan and a bell mouth.

  Conventionally, this type of axial blower reduces the axial power by expanding the suction portion of the bell mouth in a bell shape and suppressing the separation of the airflow at the suction portion of the bell mouth (see, for example, Patent Document 1). ).

  FIG. 5 shows a conventional axial blower described in Patent Document 1. In FIG. As shown in FIG. 5, the fan is composed of an axial fan 1 and a bell mouth 4 having a suction part 2 and a rounded part 3.

JP 2012-136951 A

  However, the conventional configuration has a problem that the airflow is peeled off when the bell mouth suction part is reduced in accordance with the downsizing of the device (for example, the refrigeration cycle device) in which the axial blower is used. The airflow that flows is closer to the center of the bellmouth than the bellmouth surface, and air stays in the area surrounded by the peeled airflow and the bellmouth surface, so the interface between the peeled airflow and the staying air However, despite the fact that the surface of the bell mouth was designed as a ventilation path, the cross-sectional area of the ventilation path of the axial flow fan was reduced and the air resistance was reduced. The axial power of the axial flow fan for producing a predetermined air volume has increased.

  The present invention solves the above-mentioned conventional problems, and suppresses the separation of the air flow in the suction portion of the bell mouth, and eliminates the boundary surface between the peeled air flow and the staying air, so that the surface of the bell mouth is An object of the present invention is to provide an axial blower that reduces air resistance and axial power by reducing the cross-sectional area of the ventilating passage of an axial blower due to air flow separation as a substantial ventilation passage. And

  In order to solve the above-described conventional problems, an axial fan of the present invention includes an axial fan and a bell mouth, and the axial fan includes a plurality of blades around a rotation axis of the axial fan. The bell mouth has a substantially cylindrical shape that surrounds the axial fan with a gap, and the suction portion of the bell mouth is the ridge line of the bell mouth. An axial blower having a configuration in which the bell mouth is arranged to overlap at least partially with the axial fan in the axial direction of the rotational axis of the axial fan. An air guider is disposed on the upstream side of the airflow with a gap, and the ridgeline on the downstream side of the airflower is smaller in diameter than the ridgeline of the suction portion of the bell mouth and toward the upstream side of the airflow Axis of axis of rotation of axial fan Headed is obtained by the axial flow fan having a Eagaida spread curved to be convex.

As a result, even if the bell mouth suction part is not shaped like a bell with sufficient roundness to prevent separation of the air flow, the air guider presses the air flow against the bell mouth suction part and corrects it along the surface. As a result, the separation of the airflow at the suction portion of the bell mouth can be suppressed.

  The axial blower of the present invention suppresses the separation of the air flow in the suction portion of the bell mouth and eliminates the boundary surface between the separated air flow and the staying air, so that the surface of the bell mouth becomes a substantial ventilation path. In addition, since the cross-sectional area of the ventilation passage of the axial blower due to the separation of the airflow can be suppressed, the air resistance can be reduced and the axial power of the axial blower can be reduced.

Configuration diagram of a refrigeration cycle apparatus in Embodiment 1 of the present invention The perspective view of the axial blower in Embodiment 1 of this invention The meridional view of the axial blower in Embodiment 1 of the present invention Front view of axial blower in Embodiment 1 of the present invention Meridian view of a conventional axial fan

  A first invention includes an axial fan and a bell mouth, and the axial fan includes a plurality of blades around a rotation axis of the axial fan, and generates an air flow by rotating the axial fan. The bell mouth has a substantially cylindrical shape that surrounds the axial fan with a gap, and the suction portion of the bell mouth opens at the ridge line of the bell mouth, and the bell mouth is the axial fan. An axial blower configured to be at least partially overlapped with an axial fan in the axial direction of the rotary shaft, and is disposed with a gap on the upstream side of the air flow with respect to the bell mouth. An air guider is provided, and the ridge line on the downstream side of the airflow of the air guider is smaller in diameter than the ridgeline of the suction portion of the bell mouth, and protrudes toward the axis of the rotation axis of the axial fan toward the upstream side of the airflow. Air that curves and expands Even if the bell mouth suction part is not round enough to prevent the air current from peeling off, the air guider presses the air current against the bell mouth suction part. Then, by correcting along the surface, it is possible to suppress the separation of the airflow in the suction portion of the bell mouth, and it is possible to suppress the narrowing of the cross-sectional area of the ventilation path of the axial flow fan due to the separation of the airflow, The air resistance can be reduced, and the axial power of the axial blower can be reduced.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the present embodiment.

(Embodiment 1)
FIG. 1 shows a configuration diagram of a refrigeration cycle apparatus according to a first embodiment of the present invention.

   1, the refrigeration cycle apparatus 10 includes a main circuit 11, a compressor 20, an outdoor heat exchanger 31, an indoor heat exchanger 32, a four-way valve 40, an outdoor expansion valve 51, and an indoor expansion valve 52. The refrigerant storage tank 53, the outdoor air blower 61, and the indoor air blower 62 are provided and radiate heat by the outdoor heat exchanger 31 and absorb heat by the indoor heat exchanger 32 or absorb heat by the outdoor heat exchanger 31. In addition, the operation of radiating heat with the indoor heat exchanger 32 can be switched.

  A product using the refrigeration cycle apparatus 10 for the purpose of heating or cooling air is called an air conditioner or the like, and a product used for the purpose of heating or cooling water is called a chiller or the like.

  Moreover, as a form of the refrigeration cycle apparatus 10, the outdoor unit 12 including the compressor 20, the outdoor heat exchanger 31, the four-way valve 40, the outdoor expansion valve 51, the refrigerant storage tank 53, and the outdoor air blower 61. In addition, the indoor unit 13 including the indoor heat exchanger 32, the indoor expansion valve 52, and the indoor air blower 62 may be separately configured, or the outdoor unit 12, the indoor unit 13, May be configured as an integral unit. Further, in the configuration in which the outdoor unit 12 and the indoor unit 13 are separated, there are a case where the number of the outdoor units 12 and the indoor units 13 are the same, and a case where the number of the indoor units 13 is larger than that of the outdoor units 12.

  In the present embodiment, an example of the configuration of an air conditioner in which the outdoor unit 12 and the indoor unit 13 are separated and the outdoor unit 12 and the indoor unit 13 are one each, which is often seen in home air conditioners and store air conditioners. Show.

  When the main circuit 11 performs an operation of radiating heat with the outdoor heat exchanger 31 and absorbing heat with the indoor heat exchanger 32, the compressor 20, the first path 41 of the four-way valve 40, the outdoor heat exchanger 31, and the outdoor expansion. The valve 51, the refrigerant storage tank 53, the indoor expansion valve 52, and the indoor heat exchanger 32 are connected in this order, and the circuit returns from the indoor heat exchanger 32 to the compressor 20 via the second path 42 of the four-way valve 40. It is. The first path 41 of the compressor 20 and the four-way valve 40 flows through the flow path 91, the first path 41 of the four-way valve 40 and the outdoor heat exchanger 31 flow through the flow path 92, and the outdoor heat exchanger 31 and the outdoor expansion valve 51 flow. The passage 93 allows the outdoor expansion valve 51 and the refrigerant storage tank 53 to pass through the flow path 94, the refrigerant storage tank 53 and the indoor expansion valve 52 pass through the flow path 95, and the indoor expansion valve 52 and the indoor heat exchanger 32 pass through the flow path 96. The indoor heat exchanger 32 and the second path 42 of the four-way valve 40 are connected by a flow path 97, and the second path 42 of the four-way valve 40 and the compressor 20 are connected by a flow path 98.

  Moreover, when performing the operation | movement which absorbs heat with the outdoor heat exchanger 31 and thermally radiates with the indoor heat exchanger 32, the compressor 20, the 3rd path | route 43 of the four-way valve 40, the indoor heat exchanger 32, the indoor expansion valve 52, The refrigerant storage tank 53, the outdoor expansion valve 51, and the outdoor heat exchanger 31 are connected in this order, and return from the outdoor heat exchanger 31 to the compressor 20 via the fourth path 44 of the four-way valve 40.

  The third path 43 of the compressor 20 and the four-way valve 40 flows through the flow path 91, the third path 43 of the four-way valve 40 and the indoor heat exchanger 32 flows through the flow path 97, and the indoor heat exchanger 32 and the indoor expansion valve 52 flow. By the path 96, the indoor expansion valve 52 and the refrigerant storage tank 53 are connected by the flow path 95, the refrigerant storage tank 53 and the outdoor expansion valve 51 are connected by the flow path 94, and the outdoor expansion valve 51 and the outdoor heat exchanger 31 are connected by the flow path 93. The outdoor heat exchanger 31 and the fourth path 44 of the four-way valve 40 are connected by a flow path 92, and the fourth path 44 of the four-way valve 40 and the compressor 20 are connected by a flow path 98.

  Switching of the main circuit 11 according to the operation of the refrigeration cycle apparatus 10 is performed by the four-way valve 40. Inside the main circuit 11, a refrigerant represented by R32 and R410A and a compressor oil for lubricating the sliding portion of the compressor 20 are enclosed.

  The compressor 20 is slidably accommodated in a rotary compressor, that is, a cylinder having a cylindrical inner space, a rotor arranged eccentrically with respect to the central axis inside the cylinder, and a slit provided on the cylinder wall surface. The gate valve is configured such that the tip is always in contact with the cylindrical surface of the rotor, and the passage 91 and the communication hole to the passage 98 are provided on both sides of the gate in the cylinder.

  The outdoor heat exchanger 31 and the indoor heat exchanger 32 are fin-and-tube heat exchangers, that is, a plurality of round holes having a diameter of about 5 mm to 8 mm are opened in an aluminum plate having a thickness of about 0.1 mm, and It is equipped with fins with round holes bent in a collar shape and copper or aluminum tubes, and several hundreds of fins are arranged side by side, and the tubes are inserted into the round holes, so that the tubes are spread and closely attached to the fins. It is composed.

  The four-way valve 40 has a configuration in which a combination of the first path 41 and the second path 42 or the combination of the third path 43 and the fourth path 44 can be switched using a valve provided therein.

  The outdoor expansion valve 51 and the indoor expansion valve 52 are configured to partially reduce the flow of the refrigerant by reducing the cross-sectional area of the flow path through which the refrigerant flows relative to the main circuit 11 or switching between closing and opening. ing.

  The refrigerant storage tank 53 includes a container and two communication holes for connecting to the flow path 94 and the flow path 95. The pipe extends from the communication hole to the lower part inside the container. The accumulated liquid phase refrigerant is returned to the main circuit 11.

  The indoor air blower 62 is generally a turbo fan, a sirocco fan, or a cross flow fan, but may be an axial fan.

  The outdoor blower 61 includes an axial blower 70 including an axial fan 71 and a bell mouth 74, and an electric motor 79. In the direction from the upstream side to the downstream side of the air flow generated by the axial fan 71, the outdoor heat exchanger 31, the electric motor 79, the axial fan 71, and the bell mouth 74 are arranged in this order. . The axial fan 71 is fixed to the rotating shaft 101 of the electric motor 79. The electric motor 79 is fixed to the outdoor unit 12.

  The bell mouth 74 surrounds the axial fan 71 with a gap, and has a substantially cylindrical shape concentric with the rotating shaft 101 of the axial fan 71. The bell mouth 74 and the axial fan 71 are arranged at least partially overlapping in the direction of the rotating shaft 101, and the bell mouth 74 is fixed to the outer plate of the outdoor unit 12.

  FIG. 2 shows a perspective view of the axial-flow fan in the first embodiment of the present invention. In FIG. 2, the axial fan 70 includes an axial fan 71 and a bell mouth 74.

  The axial fan 71 includes a plurality of blades 72 and a hub 73. The blades 72 have a fan shape that expands toward the outer periphery. The front ridge line 106 in the rotational direction of the blades 72 is positioned forward in the rotational direction toward the outer periphery.

  It has a depth of about 6% of the length of the cross section of the blade 72 so that it protrudes to the upstream side of the air flow generated by the blade 72 in all radial cross sections centering on the axis of the rotary shaft 101 of the axial fan 71. In the form of a plate bent in a substantially arc shape, the hub 73 has a predetermined angle with respect to a plane orthogonal to the rotation shaft 101 at equal intervals around the rotation shaft 101. The blade 72 is rotated by the electric motor 79 to generate an airflow.

  The bell mouth 74 has a substantially cylindrical shape concentric with the rotation shaft 101, the suction portion 76 opens at the suction side ridgeline 102, and the blowout portion 85 of the outdoor unit 12 orthogonal to the blowout side ridgeline 103 or the axis of the rotation shaft 101. An opening is connected to the outer plate. The bell mouth 74 is fixed to the outer plate of the outdoor unit 12 so as to be at least partially located inside the refrigeration cycle apparatus 10.

As for the roundness of the suction part 76 of the bell mouth 74, the radius 113 generally has about 30 millimeters and is preferably about ¼ arc. However, the outdoor unit 12 and the bell mouth are reduced by downsizing the outdoor unit 12. The gap 114 between the bell mouth 74 and the suction port 76 of the bell mouth 74 is arranged so as to have a sufficient roundness around the entire circumference of the rotating shaft 101 and the space is insufficient. The roundness is small. The bell mouth 74 is made of resin.

  FIG. 3 is a meridional view of the axial blower according to the first embodiment of the present invention. In particular, in FIG. 3, the axial blower 70 includes an air guider 75.

  The air guider 75 is disposed with a gap of 10 to 50 millimeters upstream of the airflow with respect to the bell mouth 74. The outlet side ridge line 108 of the air guider 75 has a smaller radius than the suction side ridge line 102 of the bell mouth 74.

  The air guider 75 is curved and spreads toward the upstream side of the airflow so as to protrude toward the axis of the rotary shaft 101. In FIG. 3, the air guider 75 has an angle formed between the suction side tangent 104 and a plane orthogonal to the rotation shaft 101 set to about 10 degrees, and an angle formed between the blowout side tangent 105 and the rotation shaft 101 set to about 20 degrees. The distance between the suction side ridgeline 107 and the blowout side ridgeline 108 of the air guider 75 is about 10 percent of the diameter of the bell mouth 74.

  FIG. 4 is a front view of the axial blower according to the first embodiment of the present invention. In FIG. 4, the air guider 75 is held by seven fixing ribs 78 extending from the bell mouth 74 and having a plate thickness of about 10 mm and a length of about 20 mm as a cross-sectional shape in a plane perpendicular to the axis of the rotation axis. is doing.

  The fixing rib 78 has a shape that is called a wing shape (NACA-4330) and has a generally rounded shape with a small air resistance, and is partially cut. The center line of the airfoil has an arc shape, and the fixing rib 78 is arranged in a direction that is convex opposite to the rotational direction of the axial fan 71.

  Further, a straight line connecting the rotation shaft 101 and the end point closer to the rotation axis of the center line of the fixed rib 78, and a straight line connecting the end points of the center line in the longitudinal direction of the fixed rib 78 in the cross section of the fixed rib 78. The angle is set to about 10 degrees.

  When the bell mouth 74 and the air guider 75 are integrated, it becomes a shape that cannot be removed from a molding die called an undercut in resin molding. Therefore, the shape in which the bell mouth 74 and the fixing rib 78 are integrated, and the air guider 75, After individual resin molding, they are integrated by ultrasonic welding. In the present embodiment, the following modes may be used.

  The bell mouth 74 may be formed by pressing or drawing the outer plate of the refrigeration cycle apparatus 10. Any material and processing method can be used as long as the airflow can be rectified.

  The bell mouth 74 is rounded so that the suction portion 76 is convex toward the axis of the rotary shaft 101, and the suction portion 76 is partially cut away to avoid contact with peripheral parts. It may be. Separation caused by cutting out the suction portion 76 by the air guider 75 can be reduced.

  The angle formed between the suction side tangent 104 of the air guider 75 and the plane orthogonal to the rotating shaft 101 differs depending on the use environment (mainly air volume) of the axial blower 70 and the distance between the bell mouth 74 and the air guider 75. Is generally in the range of 0 to 20 degrees.

  The angle formed between the blowout side tangent 105 of the air guider 75 and the rotary shaft 101 differs depending on the usage environment (mainly the air volume) of the axial blower 70 and the distance between the bell mouth 74 and the air guider 75, but is generally 0. It is in the range of degrees to 30 degrees.

Note that the number of fixing ribs 78 is not limited to seven. The number varies depending on vibration countermeasures and required rigidity. The cross-sectional shape of the fixing rib 78 by a plane orthogonal to the rotation shaft 101 may be a cylindrical shape, an elliptical shape, a rectangular shape, or an arc shape.

  The fixing rib 78 may have any cross-sectional shape or attachment angle as long as the air guider 75 can be held by the bell mouth 74. In particular, when the bell mouth 74 and the air guider 75 are formed of sheet metal, it is possible to reduce the size while maintaining the rigidity of the bell mouth 74 and the air guider 75 by using an arc shape and applying hemming bending to the end portion. The adverse effect on the flow can be reduced.

  About the refrigerating-cycle apparatus 10 and the axial blower 70 which were comprised as mentioned above, the operation | movement and an effect | action are demonstrated below.

  First, when the refrigeration cycle apparatus 10 performs the operation of radiating heat with the outdoor heat exchanger 31 and absorbing heat with the indoor heat exchanger 32, in the main circuit 11, the refrigerant sealed in the main circuit 11 is in a low-temperature and low-pressure gas phase state. Is sucked into the compressor 20 and compressed by the compressor 20 into a high-temperature and high-pressure gas phase. The direction of flow of the refrigerant is selected by the four-way valve 40 and flows to the outdoor heat exchanger 31, and heat is radiated by the outdoor heat exchanger 31 to be in a liquid phase state of intermediate temperature and intermediate pressure.

  After the refrigerant is stored in the refrigerant storage tank 53, the amount of refrigerant flowing is adjusted and discharged by the indoor expansion valve 52, and the refrigerant absorbs heat from the outside air and evaporates in the indoor heat exchanger 32 to be in a low-temperature and low-pressure gas phase state. The compressor 20 is compressed again by the compressor 20 into a high-temperature and high-pressure gas phase.

  With this series of operations, the indoor heat is moved to the outside through the refrigerant, and thus the cooling operation in the air conditioner is performed.

  Further, when the refrigeration cycle apparatus 10 performs an operation of absorbing heat in the outdoor heat exchanger 31 and dissipating heat in the indoor heat exchanger 32, in the main circuit 11, the refrigerant sealed in the main circuit 11 is a low-temperature and low-pressure gas phase. In this state, the air is sucked into the compressor 20 and is compressed by the compressor 20 into a high temperature and high pressure gas phase. The direction of the flow of the refrigerant is selected by the four-way valve 40 and flows to the indoor heat exchanger 32, and the heat is radiated by the indoor heat exchanger 32 to become a liquid-phase refrigerant of medium temperature and medium pressure.

  After the refrigerant is stored in the refrigerant storage tank 53, the amount of refrigerant flowing is adjusted and discharged by the outdoor expansion valve 51, and is radiated and evaporated to the outside air in the outdoor heat exchanger 31, resulting in a low-temperature and low-pressure gas phase state. The compressor 20 is compressed again by the compressor 20 into a high-temperature and high-pressure gas phase.

  By this series of operations, the outdoor heat is moved into the room through the refrigerant, so that the heating operation in the air conditioner is performed.

  In addition, when heat is radiated or absorbed by the outdoor heat exchanger 31, the efficiency of the outdoor unit 12 is improved by using the axial fan 71 together.

  That is, when the axial flow fan 71 is not used together, the outdoor heat exchanger 31 dissipates heat with a natural air flow in which hot air moves vertically upward or cold air moves vertically downward, so that the efficiency of the outdoor unit 12 is small. On the other hand, when the axial fan 71 is used in combination, the airflow is forcibly generated, so that the air is frequently replaced, and the efficiency of the outdoor unit 12 can be improved.

In particular, since the roundness of the suction portion 76 of the bell mouth 74 is small with the downsizing of the outdoor unit 12, the airflow flowing from the outside in the radial direction of the rotating shaft 101 is caused from the surface of the suction portion 76 of the bell mouth 74 by the inertial force. The separation reduces the cross-sectional area of the air flow path 99 of the axial blower 70, increases the air resistance, and increases the axial power of the axial blower 70 at a predetermined air volume. Is corrected so as to be along the surface of the bell mouth 74 to suppress separation, reduce air resistance, and reduce the axial power of the axial blower 70 at a predetermined air volume.

  As described above, in the present embodiment, the air guider 75 is provided with a gap on the upstream side of the airflow with respect to the bell mouth 74, and the outlet side ridgeline 108 on the downstream side of the airflow of the air guider 75 is The air guider 75 has a diameter smaller than the suction side ridgeline 103 of the bell mouth 74 and is curved and spreads so as to protrude toward the axis of the rotary shaft 101 of the axial fan 71 toward the upstream side of the airflow. By using an axial blower, generally, if the bell mouth 74 does not peel off, the larger the diameter, the better the performance of the axial flow fan 71. The suction portion 76 of the bell mouth 74 has sufficient roundness to prevent air flow peeling. Even if it is not a bell shape, the air guider 75 presses the air current against the suction portion 76 of the bell mouth 74 and corrects it along the surface. Air flow separation can be suppressed, and the reduction of the cross-sectional area of the ventilation path of the axial blower 70 due to the air flow separation can be suppressed, so that an increase in air resistance can be suppressed, and the suction portion 76 of the bell mouth 74 can be suppressed. Since the roundness of the fan can be reduced, the diameter of the fan can be maintained, and the axial power of the axial blower 70 can be reduced.

  As described above, the bell mouth and the axial flow fan according to the present invention provide the axial power of the axial flow fan even when the suction portion of the bell mouth is not a bell shape having a roundness sufficient to prevent separation of the air flow. In addition to refrigeration cycle devices such as air conditioners and chillers, ventilation devices, spray devices, intake devices such as fan heater combustors, air circulation devices such as bio benches, etc. It can be applied to other uses.

DESCRIPTION OF SYMBOLS 1 Conventional axial fan 2 Conventional bell mouth 3 Conventional bell mouth suction part 4 Conventional bell mouth rounded part 10 Refrigeration cycle apparatus 11 Main circuit 12 Outdoor unit 13 Indoor unit 20 Compressor 31 Outdoor heat Exchanger 32 Indoor heat exchanger 40 Four-way valve 41 First path of four-way valve 42 Second path of four-way valve 43 Third path of four-way valve 44 Fourth path of four-way valve 51 Outdoor expansion valve 52 Indoor expansion valve 53 Refrigerant reservoir REFERENCE SIGNS LIST 61 outdoor fan 62 indoor fan 70 axial fan 71 axial fan 72 blades 73 hub 74 bell mouth 75 air guider 76 bell mouth suction part 78 fixed rib 79 motor 85 bell mouth outlet part 91-98 flow path 99 air passage DESCRIPTION OF SYMBOLS 101 Rotating shaft 102 of axial fan Bell mouth suction side ridge line 103 Bell mouth outlet side ridge line 104 Air guider Air guider blow side tangent line 106 Front ridge line 107 of blade rotation direction Air guider suction side ridge line 108 Air guider blow side ridge line 111 Airflow of prior art axial flow fan 112 Airflow of axial flow fan of the present invention Round radius of the suction part of the mouse 114 Gap between the heat exchanger and the bell mouth

Claims (1)

An axial fan and a bell mouth,
The axial fan includes a plurality of blades around a rotation axis of the axial fan, and the axial fan is configured to generate an airflow by rotating.
The bell mouth has a substantially cylindrical shape surrounding the axial fan with a gap, and the suction portion of the bell mouth opens at the ridge line of the bell mouth, and the bell mouth is connected to the axial fan. An axial flow fan configured to be disposed at least partially overlapping the axial flow fan in the axial direction of the rotating shaft,
An air guider disposed with a gap on the upstream side of the air flow with respect to the bell mouth, the ridge line of the air guider on the downstream side of the air flow is smaller in diameter than the ridge line of the suction portion of the bell mouth, and The axial flow blower comprising the air guider that curves and spreads toward the upstream side of the airflow so as to protrude toward the axis of the rotation axis of the axial flow fan