Patent Publication Number: US-11022128-B2

Title: Axial fan

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention claims priority under 35 U.S.C. § 119 to Japanese Application No. 2018-118971 filed on Jun. 22, 2018, the entire contents of which are incorporated herein by reference. 
     1. FIELD OF THE INVENTION 
     The present disclosure relates to an axial fan. 
     2. BACKGROUND 
     A counter-rotating axial flow fan, which is a conventional axial fan, is disclosed in JP 2012-219712 A. The counter-rotating axial flow fan disclosed in JP 2012-219712 A includes a casing including an air channel, a front impeller configured to rotate in the air channel, a rear impeller configured to rotate in the air channel in a direction opposite to the front impeller. As a result, air volume and static pressure characteristics can be improved, and power consumption and noise can be reduced. 
     The counter-rotating axial flow fan disclosed in JP 2012-219712 A does not take into consideration the case where a large circuit board is provided as the circuit board for controlling rotation of the impeller. As a result, the hub of the impeller is increased in size and narrows the air channel, whereby the pressure-air volume characteristic of air is reduced. 
     SUMMARY 
     In view of the above points, example embodiments of the present disclosure provide axial fans that each secure installation space of a circuit board even when the circuit board is large, and favorably maintain a pressure-air volume characteristic of air. 
     An axial fan according to an example embodiment of the present disclosure includes a housing that extends along a central axis extending vertically, and includes an air inlet at an upper end and an air outlet at a lower end, an upper impeller that is disposed in an axially upper portion of the housing and rotates about the central axis, an upper motor that causes the upper impeller to rotate about the central axis, and an upper circuit board that is disposed axially below the upper impeller. The upper impeller includes an upper impeller cup fixed to the upper motor, and a plurality of upper blades arranged in a circumferential direction on an outer surface of the upper impeller cup. The upper impeller cup includes an upper cylindrical portion facing the upper motor in a radial direction and extending along the central axis, and an upper lid extending radially at an axial upper end of the upper cylindrical portion. The axial fan also includes a lower impeller that is disposed in an axially lower portion of the housing and rotates about the central axis, a lower motor that causes the lower impeller to rotate about the central axis, and a lower circuit board that is disposed axially above the lower impeller. The lower impeller includes a lower impeller cup fixed to the lower motor, and a plurality of lower blades arranged in the circumferential direction on an outer surface of the lower impeller cup. The lower impeller cup includes a lower cylindrical portion facing the lower motor in the radial direction and extending along the central axis, and a lower lid extending radially at an axial lower end of the lower cylindrical portion. An axial upper end outer diameter of the upper cylindrical portion is smaller than an axial lower end outer diameter of the upper cylindrical portion. An axial lower end outer diameter of the lower cylindrical portion is smaller than an axial upper end outer diameter of the lower cylindrical portion. The axial lower end outer diameter of the lower cylindrical portion is smaller than the axial lower end outer diameter of the upper cylindrical portion. 
     According to an example embodiment of the present disclosure, installation space of a circuit board is able to be secured even when the circuit board is large, and the pressure-air volume characteristic of air is able to be favorably maintained. 
     The above and other elements, features, steps, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of the example embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall perspective view of an example of an axial fan of an example embodiment of the present disclosure. 
         FIG. 2  is a longitudinal section of the axial fan. 
         FIG. 3  is an overall perspective view of the axial fan from which a housing is omitted. 
         FIG. 4  is a side view of the axial fan from which the housing is omitted. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, example embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the specification, a direction in which a central axis of an axial fan extends is simply referred to as “axial direction”, a direction perpendicular to the central axis of the axial fan as the center is simply referred to as “radial direction”, and a direction extending along a circular arc centered on the central axis of the axial fan is simply referred to as “circumferential direction”. Moreover, in the specification, the axial direction is the vertical direction for the sake of convenience in description, and the shape and positional relationships among parts are described on the assumption that the vertical direction in  FIG. 2  is the vertical direction of the axial fan. The “upper side” of the axial fan is the “intake side” and the “lower side” of the axial fan is the “exhaust side”. It should be noted, however, that the above definition of the vertical direction is not meant to restrict the orientation of, or positional relationships among parts of, the axial fan during use. Additionally, in the specification, a section parallel to the axial direction is referred to as a “longitudinal section”. Additionally, the term “parallel” used in the specification does not mean parallel in a strict sense, but includes substantially parallel. 
     1. Overall Configuration of Axial Fan 
       FIG. 1  is an overall perspective view of an example of an axial fan of an example embodiment of the present disclosure.  FIG. 2  is a longitudinal section of the axial fan. An axial fan  1  has a housing  2 . The housing  2  extends along a vertically extending central axis C and has an air flow passage  3  inside. The air flow passage  3  has an air inlet  31  at its upper end and an air outlet  32  at its lower end. That is, the housing  2  extends along the vertically extending central axis C, has the air flow passage  3  having the air inlet  31  at the upper end and the air outlet  32  at the lower end. 
     The axial fan  1  also has an upper fan  4  and a lower fan  5 . The upper fan  4  has an upper housing  41 , an upper impeller  42 , an upper motor  43 , and an upper circuit board  44 . The lower fan  5  has a lower housing  51 , a lower impeller  52 , a lower motor  53 , and a lower circuit board  54 . That is, the axial fan  1  has the housing  2 , the upper impeller  42 , the upper motor  43 , the upper circuit board  44 , the lower impeller  52 , the lower motor  53 , and the lower circuit board  54 . Note that the housing  2  includes the upper housing  41  and the lower housing  51 . 
     1-1. Configuration of Upper Fan 
     The upper housing  41  is disposed outside the upper impeller  42 , the upper motor  43 , and the upper circuit board  44 . The upper housing  41  has an upper motor base portion  411 , an upper peripheral wall  412 , and an upper rib  413 . 
     The upper motor base portion  411  is disposed axially below the upper motor  43 . The upper motor base portion  411  has a base  4111  and a bearing holder  4112 . The base  4111  is disposed axially below the upper motor  43 , and has a disk shape that spreads in the radial direction around the central axis C. The bearing holder  4112  protrudes axially upward from an upper surface of the base  4111 . The bearing holder  4112  has a cylindrical shape centered on the central axis C. Upper bearings  432  arranged in an upper and lower pair in the axial direction are accommodated and held inside the bearing holder  4112 . The upper motor  43  is fixed to a radially outer surface of the bearing holder  4112 . 
     The upper peripheral wall  412  is disposed radially outward of the upper impeller  42 . The upper peripheral wall  412  has a cylindrical shape extending to upper and lower sides in the axial direction. The air flow passage  3  is disposed radially inward of the upper peripheral wall  412 . That is, the air inlet  31  which is a circular opening is disposed at the upper end of the upper peripheral wall  412  in the axial direction. 
     The upper rib  413  is disposed radially outward of the base  4111  of the upper motor base portion  411  and radially inward of the upper peripheral wall  412 . The upper rib  413  extends radially to connect the base  4111  and the upper peripheral wall  412 . A plurality of upper ribs  413  are arranged in the circumferential direction. Air flowing through the air flow passage  3  passes between the adjacent upper ribs  413 . 
     The upper impeller  42  is disposed radially inward of the upper housing  41 , and axially above and radially outward of the upper motor  43 . The upper impeller  42  is rotated about the central axis C by the upper motor  43 . That is, the upper impeller  42  is disposed in an axially upper portion of the housing  2  and rotates about the central axis C. The upper impeller  42  has an upper impeller cup  421  and a plurality of upper blades  422 . 
     The upper impeller cup  421  is fixed to the upper motor  43 . The upper impeller cup  421  is a substantially cylindrical member having a lid on the upper side in the axial direction. A rotor yoke  4341  of the upper motor  43  is fixed to the inside of the upper impeller cup  421 . The plurality of upper blades  422  are circumferentially arranged on an outer surface of the upper impeller cup  421 . A detailed configuration of the upper impeller  42  will be described later. 
     The upper motor  43  is disposed radially inward of the upper housing  41 . The upper motor  43  is supported by the upper motor base portion  411  of the upper housing  41 . The upper motor  43  rotates the upper impeller  42  about the central axis C. The upper motor  43  has an upper shaft  431 , the upper bearings  432 , an upper stator  433  and an upper rotor  434 . 
     The upper shaft  431  extends along the central axis C. The upper shaft  431  is a columnar member which is made of metal such as stainless steel and extends to upper and lower sides in the axial direction. The upper shaft  431  is rotatably supported about the central axis C by the upper bearings  432 . 
     The upper bearings  432  are arranged in at least an upper and lower pair in the axial direction. The upper bearing  432  is held inside the cylindrical bearing holder  4112  of the upper motor base portion  411 . The upper bearing  432  is configured of a ball bearing, or may be configured of a sleeve bearing, for example. The upper and lower pair of upper bearings  432  in the axial direction support the upper shaft  431 , so that the upper shaft  431  is rotatable about the central axis C relative to the upper housing  41 . 
     The upper stator  433  is fixed to an outer peripheral surface of the bearing holder  4112  of the upper motor base portion  411 . The upper stator  433  has a stator core  4331 , an insulator  4332 , and a coil  4333 . 
     The stator core  4331  is configured by laminating electromagnetic steel plates such as silicon steel plates on top of one another, for example. The insulator  4332  is made of an insulating resin. The insulator  4332  surrounds an outer surface of the stator core  4331 . The coil  4333  is configured of a conducting wire wound around the stator core  4331  through the insulator  4332 . 
     The upper rotor  434  is disposed axially above and radially outward of the upper stator  433 . The upper rotor  434  rotates about the central axis C relative to the upper stator  433 . The upper rotor  434  has the rotor yoke  4341  and a magnet  4342 . 
     The rotor yoke  4341  is a substantially cylindrical member that is made of a magnetic material and has a lid on the upper side in the axial direction. The rotor yoke  4341  is fixed to the upper shaft  431 . The magnet  4342  has a cylindrical shape, and is fixed to an inner peripheral surface of the rotor yoke  4341 . The magnet  4342  is disposed radially outward of the upper stator  433 . 
     The upper circuit board  44  is disposed axially below the upper impeller  42 . More specifically, the upper circuit board  44  is disposed axially below the upper impeller  42  and the upper motor  43  and axially above the base  4111  of the upper motor base portion  411 . The upper circuit board  44  has a disk shape that spreads in the radial direction around the central axis C, for example. A lead of the coil  4333  is electrically connected to the upper circuit board  44 . An electric circuit for supplying a drive current to the coil  4333  is mounted on the upper circuit board  44 . 
     In the upper fan  4  configured as described above, when a drive current is supplied to the coil  4333  of the upper motor  43  through the upper circuit board  44 , a radial magnetic flux is generated in the stator core  4331 . A magnetic field generated by the magnetic flux of the stator core  4331  and a magnetic field generated by the magnet  4342  act to generate torque in the circumferential direction of the upper rotor  434 . The torque causes the upper rotor  434  and the upper impeller  42  to rotate about the central axis C. As the upper impeller  42  rotates, the plurality of upper blades  422  generate an air flow. That is, in the upper fan  4 , air can be blown by generating an air flow where the upper side is the intake side and the lower side is the exhaust side. 
     1-2. Configuration of Lower Fan 
     The lower housing  51  is disposed outside the lower impeller  52 , the lower motor  53 , and the lower circuit board  54 . The lower housing  51  has a lower motor base portion  511 , a lower peripheral wall  512 , and a lower rib  513 . 
     The lower motor base portion  511  is disposed axially above the lower motor  53 . The lower motor base portion  511  has a base  5111  and a bearing holder  5112 . The base  5111  is disposed axially above the lower motor  53 , and has a disk shape that spreads in the radial direction around the central axis C. The bearing holder  5112  protrudes axially downward from a lower surface of the base  5111 . The bearing holder  5112  has a cylindrical shape centered on the central axis C. Lower bearings  532  arranged in an upper and lower pair in the axial direction are accommodated and held inside the bearing holder  5112 . The lower motor  53  is fixed to a radially outer surface of the bearing holder  5112 . 
     The lower peripheral wall  512  is disposed radially outward of the lower impeller  52 . The lower peripheral wall  512  has a cylindrical shape extending to upper and lower sides in the axial direction. The air flow passage  3  is disposed radially inward of the lower peripheral wall  512 . That is, the air outlet  32 , which is a circular opening is disposed at the lower end of the lower peripheral wall  512  in the axial direction. 
     The lower rib  513  is disposed radially outward of the base  5111  of the lower motor base portion  511  and radially inward of the lower peripheral wall  512 . The lower rib  513  extends radially to connect the base  5111  and the lower peripheral wall  512 . A plurality of lower ribs  513  are arranged in the circumferential direction. Air flowing through the air flow passage  3  passes between the adjacent lower ribs  513 . 
     The lower impeller  52  is disposed radially inward of the lower housing  51  and axially below and radially outward of the lower motor  53 . The lower impeller  52  is rotated about the central axis C by the lower motor  53 . That is, the lower impeller  52  is disposed in an axially lower part of the housing  2  and rotates about the central axis C. The lower impeller  52  has a lower impeller cup  521  and a plurality of lower blades  522 . 
     The lower impeller cup  521  is fixed to the lower motor  53 . The lower impeller cup  521  is a substantially cylindrical member having a lid on the lower side in the axial direction. A rotor yoke  5341  of the lower motor  53  is fixed to the inside of the lower impeller cup  521 . The plurality of lower blades  522  are circumferentially arranged on an outer surface of the lower impeller cup  521 . A detailed configuration of the lower impeller  52  will be described later. 
     The lower motor  53  is disposed radially inward of the lower housing  51 . The lower motor  53  is supported by the lower motor base portion  511  of the lower housing  51 . The lower motor  53  causes the lower impeller  52  to rotate about the central axis C. The lower motor  53  has a lower shaft  531 , the lower bearings  532 , a lower stator  533 , and a lower rotor  534 . 
     The lower shaft  531  extends along the central axis C. The lower shaft  531  is a columnar member which is made of metal such as stainless steel and extends to upper and lower sides in the axial direction. The lower shaft  531  is rotatably supported about the central axis C by the lower bearings  532 . 
     The lower bearings  532  are arranged in at least an upper and lower pair in the axial direction. The lower bearing  532  is held inside the cylindrical bearing holder  5112  of the lower motor base portion  511 . The lower bearing  532  is configured of a ball bearing, or may be configured of a sleeve bearing, for example. The upper and lower pair of lower bearings  532  in the axial direction support the lower shaft  531 , so that the lower shaft  531  is rotatable about the central axis C relative to the lower housing  51 . 
     The lower stator  533  is fixed to an outer peripheral surface of the bearing holder  5112  of the lower motor base portion  511 . The lower stator  533  includes a stator core  5331 , an insulator  5332 , and a coil  5333 . 
     The stator core  5331  is configured by laminating electromagnetic steel plates such as silicon steel plates on top of one another, for example. The insulator  5332  is made of an insulating resin. The insulator  5332  surrounds an outer surface of the stator core  5331 . The coil  5333  is configured of a conducting wire wound around the stator core  5331  through the insulator  5332 . 
     The lower rotor  534  is disposed axially below and radially outward of the lower stator  533 . The lower rotor  534  rotates about the central axis C relative to the lower stator  533 . The lower rotor  534  has the rotor yoke  5341  and a magnet  5342 . 
     The rotor yoke  5341  is a substantially cylindrical member that is made of a magnetic material and has a lid on the lower side in the axial direction. The rotor yoke  5341  is fixed to the lower shaft  531 . The magnet  5342  has a cylindrical shape, and is fixed to an inner peripheral surface of the rotor yoke  5341 . The magnet  5342  is disposed radially outward of the lower stator  533 . 
     The lower circuit board  54  is disposed axially above the lower impeller  52 . More specifically, the lower circuit board  54  is disposed axially above the lower impeller  52  and the lower motor  53  and axially below the base  5111  of the lower motor base portion  511 . The lower circuit board  54  has a disk shape that spreads in the radial direction around the central axis C, for example. A lead of the coil  5333  is electrically connected to the lower circuit board  54 . An electric circuit for supplying a drive current to the coil  5333  is mounted on the lower circuit board  54 . 
     In the lower fan  5  configured as described above, when a drive current is supplied to the coil  5333  of the lower motor  53  through the lower circuit board  54 , a radial magnetic flux is generated in the stator core  5331 . A magnetic field generated by the magnetic flux of the stator core  5331  and a magnetic field generated by the magnet  5342  act to generate torque in the circumferential direction of the lower rotor  534 . The torque causes the lower rotor  534  and the lower impeller  52  to rotate about the central axis C. As the lower impeller  52  rotates, the plurality of lower blades  522  generate an air flow. That is, in the lower fan  5 , air can be blown by generating an air flow where the upper side is the intake side and the lower side is the exhaust side. 
     2. Detailed Configuration of Upper Impeller and Lower Impeller 
     Next, detailed configurations of the upper impeller  42  and the lower impeller  52  will be described with reference to FIGS. and  4  in addition to  FIGS. 1 and 2 .  FIG. 3  is an overall perspective view of the axial fan  1  from which the housing  2  is omitted.  FIG. 4  is a side view of the axial fan  1  from which the housing  2  is omitted. For convenience of explanation, in  FIG. 4 , an axial lower end outer diameter D 422  of an upper cylindrical portion  4211  and an axial upper end outer diameter D 522  of a lower cylindrical portion  5211  are shown in both upper and lower parts of each of the upper impeller  42  and the lower impeller  52 . 
     The upper impeller cup  421  has the upper cylindrical portion  4211  and an upper lid  4212 . The upper cylindrical portion  4211  and the upper lid  4212  are a single member. 
     The upper cylindrical portion  4211  is disposed radially outward of the upper motor  43 , and includes the upper motor  43  in the radial direction. The upper cylindrical portion  4211  extends vertically along the central axis C. That is, the upper cylindrical portion  4211  faces the upper motor  43  in the radial direction, and extends along the central axis C. 
     The upper lid  4212  is disposed in an axial upper end portion of the upper cylindrical portion  4211 . The upper lid  4212  has a disk shape that spreads in the radial direction around the central axis C. An outer edge portion of the upper lid  4212  is connected to the axial upper end portion of the upper cylindrical portion  4211 . That is, the upper lid  4212  spreads in the radial direction at the axial upper end of the upper cylindrical portion  4211 . 
     The lower impeller cup  521  has the lower cylindrical portion  5211  and a lower lid  5212 . The lower cylindrical portion  5211  and the lower lid  5212  are a single member. 
     The lower cylindrical portion  5211  is disposed radially outward of the lower motor  53 , and includes the lower motor  53  in the radial direction. The lower cylindrical portion  5211  extends vertically along the central axis C. That is, the lower cylindrical portion  5211  faces the lower motor  53  in the radial direction, and extends along the central axis C. 
     The lower lid  5212  is disposed in an axial lower end portion of the lower cylindrical portion  5211 . The lower lid  5212  has a disk shape that spreads in the radial direction around the central axis C. An outer edge portion of the lower lid  5212  is connected to the axial lower end portion of the lower cylindrical portion  5211 . That is, the lower lid  5212  spreads in the radial direction at the axial lower end of the lower cylindrical portion  5211 . 
     As shown in  FIG. 4 , an axial upper end outer diameter D 421  of the upper cylindrical portion  4211  is smaller than the axial lower end outer diameter D 422  of the upper cylindrical portion  4211 . Furthermore, an axial lower end outer diameter D 521  of the lower cylindrical portion  5211  is smaller than the axial upper end outer diameter D 522  of the lower cylindrical portion  5211 . Furthermore, the axial lower end outer diameter D 521  of the lower cylindrical portion  5211  is smaller than the axial lower end outer diameter D 422  of the upper cylindrical portion  4211 . 
     According to the configuration of the example embodiment described above, an air flow space near the axial upper end of the upper cylindrical portion  4211  of the upper impeller  42  is wider than an air flow space near the axial lower end of the upper cylindrical portion  4211 . Hence, air can be efficiently transmitted to the exhaust side while suppressing or reducing increase in air flow disturbance on the radially outer side of the upper cylindrical portion  4211 . Further, the air flow space near the axial lower end of the lower cylindrical portion  5211  of the lower impeller  52  is wider than the air flow space near the axial upper end of the lower cylindrical portion  5211 . Hence, the air can be efficiently transmitted to the exhaust side while suppressing or reducing increase in the pressure of air on the radially outer side of the lower cylindrical portion  5211 . As a result, installation space for the upper circuit board  44  and the lower circuit board  54  can be secured in the radial direction, and the pressure-air volume characteristic of air can be favorably maintained. 
     The axial upper end outer diameter D 522  of the lower cylindrical portion  5211  is the same as the axial lower end outer diameter D 422  of the upper cylindrical portion  4211 . According to this configuration, air on the upper impeller  42  side where pressure rise is suppressed can be smoothly passed to the lower impeller  52  side. Hence, air can be blown efficiently. 
     The upper cylindrical portion  4211  has a first upper inclined portion  4211   a . The first upper inclined portion  4211   a  is disposed on an outer peripheral portion of the upper cylindrical portion  4211 . The first upper inclined portion  4211   a  has a conical shape with an outer diameter increasing toward the lower side in the axial direction. According to this configuration, the outer shape of the upper cylindrical portion  4211  is inclined along the axial direction and is conical. That is, the air flow space on the radially outer side of the upper cylindrical portion  4211  gradually narrows from the upper side in the axial direction toward the lower side in the axial direction. Hence, it is possible to increase the installation space of the upper circuit board  44  on the lower side in the axial direction of the upper cylindrical portion  4211 , while suppressing a rapid pressure rise of air on the radially outer side of the upper cylindrical portion  4211 . 
     The lower cylindrical portion  5211  has a first lower inclined portion  5211   a . The first lower inclined portion  5211   a  is disposed on an outer peripheral portion of the lower cylindrical portion  5211 . The first lower inclined portion  5211   a  has a conical shape with an outer diameter increasing toward the upper side in the axial direction. According to this configuration, the outer shape of the lower cylindrical portion  5211  is inclined along the axial direction and is conical. That is, the air flow space on the radially outer side of the lower cylindrical portion  5211  gradually widens from the upper side in the axial direction toward the lower side in the axial direction. Hence, it is possible to increase the installation space of the lower circuit board  54  on the upper side in the axial direction of the lower cylindrical portion  5211 , while gradually reducing the pressure of air on the radially outer side of the lower cylindrical portion  5211 . 
     The upper lid  4212  has a second upper inclined portion  4212   a . The second upper inclined portion  4212   a  is disposed on an outer peripheral portion of the upper lid  4212 . The second upper inclined portion  4212   a  has a conical shape extending axially downward toward the radially outer side. According to this configuration, in order to guide air on the upper side in the axial direction of the upper impeller cup  421  to the radially outer side of the upper cylindrical portion  4211 , the air flow space gradually narrows from the upper side in the axial direction toward the lower side in the axial direction. Hence, air can be fed toward the lower side in the axial direction of the upper impeller cup  421  while suppressing the resistance of the air sucked from the air inlet  31 . 
     The lower lid  5212  has a second lower inclined portion  5212   a . The second lower inclined portion  5212   a  is disposed on an outer peripheral portion of the lower lid  5212 . The second lower inclined portion  5212   a  has a conical shape extending axially upward toward the radially outer side. According to this configuration, in order to guide air on the radially outer side of the lower cylindrical portion  5211  to the lower side in the axial direction of the lower impeller cup  521 , the air flow space gradually widens from the upper side in the axial direction toward the lower side in the axial direction. Hence, air can be discharged to the outside from the air outlet  32  while suppressing disturbance of air flowing toward the lower side in the axial direction of the lower impeller cup  521 . 
     Axially upper and lower portions of each of the plurality of upper blades  422  are curved in different directions as they extend axially upward and downward, respectively. Specifically, the radially outer end of the axially upper portion of each of the plurality of upper blades  422  curves axially upward toward the upper side in the axial direction. The radially outer end of the axially lower portion of each of the plurality of upper blades  422  curves axially downward toward the lower side in the axial direction. According to these configurations, the flow velocity on the inner side in the radial direction can be suppressed, and the air resistance on the downstream side of the air flow passage  3  can be reduced. Hence, it is possible to improve the pressure-air volume characteristic of air. 
     Axially upper and lower portions of each of the plurality of lower blades  522  are curved in different directions as the axially upper and lower portions extend axially upward and downward, respectively. Specifically, the radially outer end of the axially upper portion of each of the plurality of lower blades  522  curves axially upward toward the upper side in the axial direction. The radially outer end of the axially lower portion of each of the plurality of lower blades  522  curves axially downward toward the lower side in the axial direction. According to these configurations, the flow velocity on the inner side in the radial direction can be suppressed, and the air resistance on the downstream side of the air flow passage  3  can be reduced. Hence, it is possible to improve the pressure-air volume characteristic of air. 
     The upper impeller  42  is disposed axially below the air inlet  31 . That is, the upper impeller  42  does not protrude to the outside of the air flow passage  3 . The lower impeller  52  is disposed axially above the air outlet  32 . That is, the lower impeller  52  does not protrude to the outside of the air flow passage  3 . That is, the upper impeller  42  and the lower impeller  52  are accommodated in the air flow passage  3 . According to this configuration, it is possible to improve the pressure-air volume characteristic of air. Furthermore, since the upper impeller  42  and the lower impeller  52  do not protrude to the outside of the housing  2 , it is possible to attach the axial fan  1  easily to a device or the like. The upper impeller  42  and the lower impeller  52  can thus be protected. 
     As shown in  FIG. 4 , the outer diameter D 44  of the upper circuit board  44  is smaller than the axial lower end outer diameter D 422  of the upper cylindrical portion  4211  of the upper impeller  42 . That is, the outer diameter of the upper circuit board  44  is smaller than the outer diameter of the upper impeller  42 . According to this configuration, it is possible to suppress disturbance of air in the air flow passage  3  caused by the upper circuit board  44  projecting farther to the radially outer side than the upper impeller  42 . 
     As shown in  FIG. 4 , an outer diameter D 54  of the lower circuit board  54  is smaller than the axial upper end outer diameter D 522  of the lower cylindrical portion  5211  of the lower impeller  52 . That is, the outer diameter of the lower circuit board  54  is smaller than the outer diameter of the lower impeller  52 . According to this configuration, it is possible to suppress disturbance of air in the air flow passage  3  caused by the lower circuit board  54  projecting farther to the radially outer side than the lower impeller  52 . 
     3. Other 
     While example embodiments of the present disclosure have been described above, it will be understood that the scope of the present disclosure is not limited to the above-described example embodiments, and that various modifications may be made to the above-described preferred example embodiments without departing from the gist of the present disclosure. In addition, features of the above-described example embodiments and the modifications thereof may be combined appropriately as desired. 
     The present disclosure is applicable to an axial fan, for example. 
     While example embodiments of the present disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The scope of the present disclosure, therefore, is to be determined solely by the following claims.