Patent Publication Number: US-10767658-B2

Title: Axial fan and refrigerator

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2016-220592 filed on Nov. 11, 2016. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to axial fans and refrigerators. 
     2. Description of the Related Art 
     Various structures of axial fans have been proposed in the related art. For example, Japanese Unexamined Patent Application Publication No. 2013-113128 discloses the following structure of an axial fan. 
     The axial fan disclosed in Japanese Unexamined Patent Application Publication No. 2013-113128 includes an impeller and a casing surrounding the outer circumference in the radial direction of the impeller and including an intake port and an ejection port. The inner surface of the casing includes an intake-side inclined portion in which the intake port expands outward in the radial direction of the impeller. The inner surface of the casing also includes an ejection-side inclined portion in which the ejection port expands outward in the radial direction of the impeller. 
     The axial fan disclosed in Japanese Unexamined Patent Application Publication No. 2013-113128 is capable of increasing the blast volume by drawing fluid around the intake port using the intake-side inclined portion. Furthermore, the axial fan disclosed in Japanese Unexamined Patent Application Publication No. 2013-113128 smoothly guides a discharge flow along the ejection-side inclined portion. This reduces or eliminates generation of a turbulent flow, allowing a large static pressure to be obtained. 
     In the case where the axial fan is mounted in a refrigerator or the like, there is a case where the wind direction has to be adjusted depending on the specification of the air flow channel in the apparatus. However, it is not easy for the axial fan disclosed in Japanese Unexamined Patent Application Publication No. 2013-113128 to adjust the wind direction. 
     SUMMARY OF THE INVENTION 
     An axial fan according to an exemplary embodiment of the present disclosure includes an impeller configured to rotate about a rotation axis extending in a vertical direction, a motor configured to rotationally drive the impeller, and a housing disposed radially outside the impeller and the motor. In a region overlapping in the vertical direction with the impeller, an inner wall surface of the housing includes at least one first wall having a narrow gap with a radially outer edge of the impeller and at least one second wall having a wide gap with the radially outer edge of the impeller. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of an axial fan according to a first embodiment of the present disclosure. 
         FIG. 2  is a perspective view of the axial fan according to the first embodiment of the present disclosure viewed from above. 
         FIG. 3  is a plan view of the axial fan according to the first embodiment of the present disclosure viewed from above. 
         FIG. 4  is a plan view of the axial fan according to the first embodiment of the present disclosure viewed from below. 
         FIG. 5  is a perspective view of a housing according to the first embodiment of the present disclosure viewed from above. 
         FIG. 6  is a perspective view of the housing according to the first embodiment of the present disclosure viewed from below. 
         FIG. 7  is a graph illustrating an example of the P-Q characteristics ([static pressure (P)/quantity (Q)] characteristics) of the axial fan according to the first embodiment of the present disclosure and an axial fan according to a comparative example. 
         FIG. 8  is a partial perspective view of a housing of an axial fan according to a second embodiment of the present disclosure. 
         FIG. 9  is a partial perspective view of a housing of an axial fan according to a third embodiment of the present disclosure. 
         FIG. 10  is a partial perspective view of a housing of an axial fan according to a fourth embodiment of the present disclosure. 
         FIG. 11  is a side sectional view of a refrigerator including an axial fan according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present disclosure will be described hereinbelow with reference to the drawings. In the following description on the configurations of axial fans, a direction in which the rotation axis of an impeller is referred to as “vertical direction”. A radial direction around the rotation axis is simply referred to as “radial direction”, and a circumferential direction around the rotation axis is simply referred to as “circumferential direction”. However, the vertical direction does not indicate a positional relationship and a direction when the axial fan is installed in an actual apparatus. In the drawings, the upper side is denoted by X 1 , and the lower side is denoted by X 2 . 
     First, the overall configuration of an axial fan according to a first embodiment of the present disclosure will be described with reference to  FIGS. 1 to 4 .  FIG. 1  is a longitudinal sectional view of an axial fan  50  according to the first embodiment of the present disclosure.  FIG. 2  is a perspective view of the axial fan  50  viewed from above.  FIG. 3  is a plan view of the axial fan  50  viewed from above.  FIG. 4  is a plan view of the axial fan  50  viewed from below. 
     The axial fan  50  includes an impeller  1 , a motor  2 , a motor base unit  3 , a housing  4 , ribs  5 , and a ring-shaped rib  6 . 
     The motor base unit  3 , the housing  4 , the ribs  5 , and the ring-shaped rib  6  are formed of the same resin material. The housing  4  houses the impeller  1  and the motor  2  and is disposed radially outside the impeller  1  and the motor  2 . 
     The motor  2  rotationally drives the impeller  1  about a rotation axis C 1 . The motor  2  includes a bearing portion  21 , a shaft  22 , a stator  23 , a rotor  24 , and a circuit board  25 . 
     The motor base unit  3  supports the motor  2 . The motor base unit  3  includes a base  31  extending in the radial direction on the lower surface side and a bearing holding portion  32  protruding upward from the center of the base  31 . The bearing holding portion  32  holds the cylindrical bearing portion  21  therein. The bearing portion  21  includes a sleeve bearing. The bearing portion  21  may include a pair of ball bearings disposed vertically. 
     The shaft  22  is a columnar member extending in the vertical direction and is formed of metal, such as stainless steel. The bearing portion  21  rotatably holds the shaft  22  about the rotation axis C 1 . 
     The stator  23  is fixed to the outer circumferential surface of the bearing holding portion  32 . The stator  23  includes a stator core  231 , an insulator  232 , and a coil  233 . The stator core  231  includes a laminated steel plate in which electromagnetic steel sheets, such as silicon steel sheets, are laminated in the vertical direction. The insulator  232  is formed of insulating resin. The coil  233  is wound around the stator core  232  in the vertical direction, with the insulator  232  therebetween. 
     The circuit board  25  is disposed below the stator core  232 . The circuit board  25  is a substrate on which an electronic circuit for applying a driving current to the coil  233  is mounted. The lead wire of the coil  233  is electrically connected to the circuit board  25 . 
     The rotor  24  includes a rotor yoke  241  and a magnet  242 . The rotor yoke  241  is a substantially cylindrical member having a cover on the top and is formed of a magnetic material. The rotor yoke  241  is fixed to the shaft  22 . The cylindrical magnet  242  is fixed to the inner circumferential surface of the rotor yoke  241 . The magnet  242  is disposed radially outside the stator  23 . The N-pole and the S-pole are alternately arranged in the circumferential direction on the pole face of the magnet  242 . A magnetic circuit is formed between the rotor yoke  241  and the magnet  242 . This reduces leakage of magnetic flux from the magnet  242  to the outside of the axial fan  50 . 
     The impeller  1  includes an impeller cup  11  and a plurality of blades  12  and is formed of a resin material. The impeller cup  11  is a substantially cylindrical member having a cover on the top. The rotor yoke  241  is fixed to the inside of the impeller cup  11 . The plurality of blades  12  are formed radially outside the impeller cup  1 . In the present embodiment, three blades  12  are disposed at regular intervals in the circumferential direction, as illustrated in  FIG. 3 , by way of example. 
     In the thus-configured axial fan  50 , when a driving current is applied to the coil  233  of the stator  23 , a magnetic flux in the radial direction is generated in the stator core  231 . The magnetic flux between the stator core  231  and the magnet  242  causes a circumferential torque. This causes a rotary unit including the rotor  24  and the impeller  1  to rotate about the rotation axis C 1 . The impeller  1  rotates counterclockwise in the top view of  FIG. 3 . 
     When the impeller  1  rotates, an air current is generated by the plurality of blades  12 . In other words, an air current in which the upper side of the axial fan  50  is on the air intake side and the lower side is on the exhaust side is generated to allow blowing. 
     Next, the configuration of the housing  4  will be described in detail.  FIG. 5  is a perspective view of the housing  4  viewed from above.  FIG. 6  is a perspective view of the housing  4  viewed from below. 
     The housing  4  includes a bottom plate  41  at the lower part. The bottom plate  41  includes a vent  411  which is a circular opening. 
     An outer wall surface  4 W 1  of the housing  4  extends upward from the outer edge of the bottom plate  41  and has a substantially square shape in a cross-sectional view perpendicular to the vertical direction. The outer wall surface  4 W 1  may have a shape other than the square shape, such as a rectangular shape. An inner wall surface  4 W 2  is disposed inside the outer wall surface  4 W 1 . The four sides of the inner wall surface  4 W 2  each have a thick-wall portion  42  and thin-wall portions  43 . The thick-wall portion corresponds to a first wall, and the thin-wall portions  43  correspond to a second wall. 
     The thick-wall portion  42  is disposed in the center of one side of the inner wall surface  4 W 2 . The thick-wall portion  42  includes a pair of first thick-wall portion  421  and second thick-wall portion  422 . The first thick-wall portion  421  and the second thick-wall portion  422  are disposed adjacent to each other along one side of the inner wall surface  4 W 2 . 
     The first thick-wall portion  421  and the second thick-wall portion  422  are each formed of a wall extending upward from the bottom plate  41 . The wall has a closed shape in a cross-sectional view perpendicular to the vertical direction. Thus, the first thick-wall portion  421  and the second thick-wall portion  422  respectively have cavities  421 A and  422 A inside thereof. These cavities  421 A and  422 A reduce or eliminate generation of sink marks during molding of the housing  4  using a mold. 
     The first thick-wall portion  421  and the second thick-wall portion  422  are both formed from the bottom plate  41  to the upper end of the housing  4  and overlap in the vertical position with the impeller  1 . An inner surface  421 B of the first thick-wall portion  421  and an inner surface  422 B of the second thick-wall portion  422  both constitute part of the substantial cylindrical shape centered on the rotation axis C 1 . The thick-wall portion  42  has a groove  423  (described later) disposed between the first thick-wall portion  421  and the second thick-wall portion  422 . 
     The thin-wall portions  43  are disposed on both sides of the thick-wall portion  42 . In other words, the thin-wall portions  43  are disposed at positions nearer to the four corners of the inner wall surface  4 W 2  than the thick-wall portion  42 . The gap between the radially outer edge  121  (see  FIG. 3 ) of each blade  12  of the impeller  1  and the thick-wall portion  42  is smaller than the gap between the radially outer edge  121  and the thin-wall portions  43 . 
       FIG. 7  is a graph illustrating an example of the P-Q characteristics ([static pressure (P)/quantity (Q)] characteristics) of the axial fan  50  according to the present embodiment and an axial fan according to a comparative example. In  FIG. 7 , the solid line indicates the present embodiment, and the broken line indicates the comparative example. The comparative example has a configuration in which the housing of the axial fan  50  according to the present embodiment does not include the thick-wall portion  42  and the thin-wall portions  43 . In other words, the thicknesses of the walls of the four sides of the housing are constant in a direction in which the sides extend. 
     As shown in  FIG. 7 , the present embodiment has a higher static pressure in a low air-volume region than the comparative example because of the configuration of the thick-wall portion  42  and the thin-wall portions  43 . The comparative example has a surge region R in which the static pressure does not change with respect to the blast volume, causing unstable blowing. In contrast, the present embodiment allows region corresponding to such a surge region to be a region in which the static pressure changes with respect to the blast volume, allowing stable blowing. 
     Furthermore, in the present embodiment, exhaust air exhausted downward through the vent  411  of the housing  4  flows directly below in the vicinity of the thick-wall portion  42 , whereas in the vicinity of the thin-wall portions  43 , the exhaust air flows relatively outward in the radial direction. Thus, the direction of the exhaust flow can be adjusted by the design of the thick-wall portion  42  and the thin-wall portions  43 . 
     Providing the thick-wall portion  42  increases the rigidity of the housing  4 , thereby reducing or eliminating vibrations generated when the axial fan  50  is in operation. 
     Furthermore, since the inner surface  421 B of the first thick-wall portion  421  and the inner surface  422 B of the second thick-wall portion  422  constitute part of the substantial cylindrical shape centered on the rotation axis C 1 , the gap between the radially outer edge  121  of each blade  12  and the thick-wall portion  42  is decreased to improve the static pressure. Furthermore, generation of noise can be reduced or eliminated by decreasing a turbulent flow. 
     As illustrated in  FIG. 3 , in the present embodiment, a circumferential length L 1  between one end of the first thick-wall portion  421  and one end of the second thick-wall portion  422  is smaller than a distance L 2  between forward ends of the radially outer edge  121  in the rotational direction of adjacent blades  12 . This prevents the adjacent blades  12  from crossing both ends of the thick-wall portion  42  at the same time, thereby reducing or eliminating generation of noise. Even if the length L 1  is larger than the distance L 2 , the same effect is exerted. 
     The number of the thick-wall portions  42  is four, whereas the number of the blades  12  is three, and the numbers are prime to each other. Furthermore, both the thick-wall portions  42  and the blades  12  are disposed at regular intervals in the circumferential direction. This prevents the three blades  12  from crossing the thick-wall portions  42  at the same time, thereby reducing or eliminating generation of noise. The number of the thick-wall portions and the number of the blades may be other than the above provided that they are prime to each other. 
     All of the circumferential lengths L 1  of the four thick-wall portions  42  are set equal. This makes the static pressure distribution symmetrical about the rotation axis, thereby reducing generation of a turbulent flow. 
     Both circumferential ends of the thick-wall portion  42  are disposed on the inner wall surface  4 W 2  of the same side. This increases the rigidity of the housing  4 . 
     As illustrated in  FIG. 3 , a rounded portion R 422  at a circumferential end of the second thick-wall portion  422  has a lager diameter than the diameter of a rounded portion R 421  at a circumferential end of the first thick-wall portion  421 . In other words, the rounded portion of the circumferential end of the second thick-wall portion  422  that the blade  12  crosses first is formed large. This reduces or eliminates generation of noise. In the above configuration, the first thick-wall portion  421  and the second thick-wall portion  422  are asymmetrical. Alternatively, they may be line-symmetrical. 
     The area of the inner surface of the thick-wall portion  42  facing the blades  12  in the radial direction affects the static pressure. Therefore, if the same area is secured, the thick-wall portion  42  can also be disposed off the center of one side of the inner wall surface  4 W 2 . 
     The thick-wall portion  42  may not be provided on all of the four sides of the inner wall surface  4 W 2 . For example, the thick-wall portion  42  may not be provided on opposing two sides of the four sides, and a thick-wall portion  42  having a larger circumferential length may be provided on the remaining two sides to improve the static pressure. 
     The thick-wall portion  42  may not be formed of two thick-wall portions as in the above. For example, the thick-wall portion  42  may be formed of three thick-wall portions. In this case, the groove  423  (described later) may be formed at a position between the thick-wall portions. In other words, two grooves  423  are provided. 
     In the present embodiment, as illustrated in  FIGS. 4 and 5 , first fixing portions  44  are provided at three corners of the housing  4 , and a second fixing portion  45  is provided at the remaining one corner. The first fixing portions  44  and the second fixing portion  45  are used to fix the housing  4  to an apparatus. The first fixing portions  44  extend upward from the bottom plate  41  and each include a portion having a through-hole  44 A for screw fixing and a projecting rib  441  projecting radially inward from a corner of the portion. The second fixing portion  45  extends upward from the bottom plate  41  and includes a portion having a through-hole  45 A for screw fixing and a projecting rib  451  projecting radially inward from a corner of the portion. Unlike the first fixing portions  44 , the second fixing portion  45  includes a first hole  452  and a second hole  453  formed in the bottom plate  41 . 
     The gap between the projecting ribs  441  and  451  and the blades  12  is small. This improves the static pressure. This also improves the rigidity of the corners of the housing  4 . However, since the present embodiment is configured to improve the static pressure with the thick-wall portions  42 , the projecting ribs  441  and  451  described above are not absolutely necessary. Without the projecting ribs  441  and  451 , noise can be reduced. 
     A configuration for drainage provided in the housing  4  of the present embodiment will be described in detail. The thick-wall portion  42  described above has the groove  423  for drainage between the first thick-wall portion  421  and the second thick-wall portion  422 . 
     The groove  423  is recessed radially outward and extends in the vertical direction. The upper end of the groove  423  extends to the upper end of the housing  4 . This allows moisture adhering to the inner wall surface  4 W 2  to be collected into the groove  423  and to be discharged through the upper end of the housing  4 . 
     The groove  423  radially faces each blade  12  of the impeller  1 . This allows moisture collected to a portion of the groove  423  facing the blade  12  to be discharged. For example, in the case where the axial fan is applied to a cold environment, such as a refrigerator, even if moisture adheres to the inner wall surface  4 W 2  of the housing  4 , a sufficient gap can be provided between the inner wall surface  4 W 2  of the housing  4  inner wall surface and the impeller  1 . 
     The groove  423  increases in depth in the entire vertical direction toward the upper end of the housing  4 . This allows the moisture collected to the groove  423  to be guided upward for drainage. The depth of the groove  423  may be constant partly in the vertical direction. 
     An end of the groove  423  extending to the upper end of the housing  4  is disposed on the air intake side. If the end of the groove extending to the end of the housing  4  is disposed at the exhaust side, the moisture is diffused widely far away by the discharged air. However, the above configuration avoids such diffusion. 
     The groove  423  has vertically extending edges  423 A positioned on both sides of the groove  423  in the circumferential direction and connected to the inner wall surface  4 W 2 . The edges  423 A are rounded. In other words, the edges  423 A are curved. This makes it easy to guide moisture adhering to the inner wall surface  4 W 2  into the groove  423 . 
     Furthermore, the end of the groove  423  at the upper end of the housing  4  has an edge  423 B. The edge  423 B is rounded. In other words, the edge  423 B is curved. This makes it easy to efficiently discharge the moisture collected in the groove  423 . 
     Only the lower end of the groove  423  may extend to the lower end of the housing  4 , or alternatively, the upper and lower ends of the groove  423  may extend to the upper and lower ends of the housing  4 , respectively. 
     The thin-wall portions  43  are inclined so as to decrease in thickness toward the above. In other words, the gap between the thin-wall portions  43  and the blades  12  increases toward the above. This allows moisture adhering to the thin-wall portions  43  to be guided upward for drainage. 
     The motor base unit  3  is disposed in the center of the vent  411 . Four ribs  5  are formed in such a manner as to extend from the outer circumferential surface of the base  31  of the motor base unit  3  toward the four corners of the housing  4 . The ribs  5  connect the lower surface of the bottom plate  41  and the outer circumferential surface of the base  31 . Providing the ribs  5  improves the rigidity of the axial fan  50 . 
     As illustrated in  FIG. 4 , of the four ribs  5 , the rib  5  extending toward the second fixing portion  45  has a recess  51  that is recessed upward from the lower surface. A through-hole  33  is formed in the lower surface of the base  31 . The through-hole  33  and the recess  51  are connected. 
     A cable (not shown) that is electrically connected to the circuit board  25  is passed through the through-hole  33  from above to below, is routed in the recess  51 , is passed through the second hole  453  from below to above, and is then passed through the first hole  452  from above to below. 
     As illustrated in  FIG. 5 , the upper surface of each rib  5  has an inclined surface  52  that is inclined downward toward the forward end of the impeller  1  in the rotating direction. This allows an air current to be guided downward along the inclined surface  52 . 
     The ring-shaped rib  6  connects the four ribs  5  to form a ring shape centered on the rotation axis C 1 . As illustrated in  FIG. 5 , the upper surface of the ring-shaped rib  6  has an inclined surface  61  that is inclined radially outward. This allows an air current to be guided radially outward along the inclined surface  61 . 
     Next, a second embodiment, which is a modification of the first embodiment, will be described.  FIG. 8  is a partial perspective view of a housing  401  of an axial fan according to the second embodiment of the present disclosure. 
     The housing  401  has not the thick-wall portion  42  in the center of each side of the inner wall surface, as in the first embodiment, but has a thick-wall portion  4011  at each of the corners of the rectangular shape. 
     The inner surfaces of the thick-wall portions  4011  constitute part of a cylinder centered on the rotation axis. In other words, both circumferential ends of each thick-wall portion  4011  are disposed on the inner wall surfaces of different sides of the rectangular shape. 
     Thus, the thick-wall portion is not provided on the inner wall surface of each side. This allows the radially outer edge of the impeller to be extended toward the inner wall surface, allowing the diameter of the impeller to be increased. This improves the rigidity of the housing  401  and the static pressure as in the first embodiment. 
     Each thick-wall portion  4011  has a groove  4012  in the circumferential center of the inner surface. The groove  4012  may have a configuration similar to that of the groove  423  of the first embodiment and exerts a drainage effect similar to that in the first embodiment. 
       FIG. 9  is a partial perspective view of a housing  402  of an axial fan according to a third embodiment of the present disclosure. The housing  402  has a hole  4231  in the groove  423 , which is a configuration difference from the housing  4  according to the first embodiment. The hole  4231  is disposed at the bottom of the groove  423  and passes through the housing  402  in the radial direction. 
     This allows moisture adhering to the inner wall surface of the housing  402  and collected into the groove  423  to be discharged through the hole  4231 . 
     The hole  4231  is opposed to part of the blade of the impeller in the radial direction. This ensures a sufficient gap between the housing inner wall surface and the impeller even if moisture adheres to the housing inner wall surface. 
     A radially inside edge (an edge connecting to the bottom of the groove  423 ) of the hole  4231  is rounded. This makes it easy to guide moisture collected in the groove  423  into the hole  4231 . 
       FIG. 10  is a partial perspective view of a housing  403  of an axial fan according to a fourth embodiment of the present disclosure. The housing  403  includes a thick-wall portion  4201  disposed on each side of the inner wall surface, which is a configuration difference from the housing  4  according to the first embodiment. 
     The thick-wall portion  4201  does not include a plurality of thick-wall portions and grooves unlike the first embodiment. The inner surface of the thick-wall portion  4201  constitutes part of a cylinder centered on the rotation axis. The thick-wall portion  4201  has a hole  4201 A passing through the housing  403  in the radial direction at the circumferential center of the inner surface. The configuration of the hole  4201 A may be the same as the configuration of the hole  4231  of the third embodiment. 
     The hole  4201 A also allows moisture adhering to the inner wall surface of the housing  403  to be discharged. 
     Next, a case where an axial fan according to one of the above embodiments is used in a refrigerator, which is an example application, will be described.  FIG. 11  is a side sectional view of a refrigerator  100  including an axial fan  101  according to an embodiment of the present disclosure. Arrow S indicates the flow of cold air. The refrigerator  100  is installed on a floor surface F. A refrigerating compartment  102  (a storage room), which is opened and closed by a door  102 A, is disposed at the upper part of the refrigerator  100 . A freezer  103  (a storage room), which is opened and closed by a door  103 A, is disposed at the lower part of the refrigerator  100 . 
     The refrigerating compartment  102  is kept at a refrigeration temperature (for example, 3° C.) to refrigerate stored items. The refrigerating compartment  102  includes a plurality of trays  160  on which stored items are to be placed. The door  102 A of the refrigerating compartment  102  includes a plurality of storage pockets (not shown). 
     The freezer  103  is isolated from the refrigerating compartment  102  by an adiabatic wall  107  and is kept at a freezing point or below to keep stored items frozen. The freezer  103  includes a plurality of storage cases  170  for storing stored items. The storage case  170  is supported by rails (not shown) provided on both side walls of the freezer  103  so as to be movable in the front-to-back direction. 
     A machine room  150  is provided on the back of the freezer  103 . A compressor  157  is disposed in the machine room  150 . The compressor  157  connects to a condenser, an expander (both are not shown), and a cooler  111 . When the compressor  157  is driven, a refrigerant, such as isobutane, circulates to operate a refrigeration cycle. This brings the cooler  111  to the low temperature side of the refrigeration cycle. 
     A cold air passage  131  partitioned by a back plate  106 A is provided on the back of the freezer  103 . A cold air passage  132  partitioned by a back plate  106 B and communicating with the cold air passage  131  is provided on the back of the refrigerating compartment  102 . The cold air passage  131  is partitioned by a partition  135  into a front portion  131 A and a rear portion  131 B. A cooler  111  is disposed in the rear portion  131 B. The cooler  111  serving as the low temperature side of the refrigeration cycle and air circulating in the rear portion  131 B exchange heat to generate cold air. 
     In the cold air passage  131 , the axial fan  101  is disposed above the cooler  111 . The axial fan  101  draws cold air from the axial direction and exhausts it in the axial direction. In the case where the axial fan  101  is the axial fan  50  according to the first embodiment, the housing  4  is inclined so that, for example, one side of the outer wall surface of the housing  4  is directed downward, and the exhaust side is directed above the refrigerator  100 . 
     The back plate  106 A has an ejection port  109 A in the exhaust side in the axial direction of the axial fan  101 . The back plate  106 A also has an ejection port  109 B below the ejection port  109 A and a freezer return port  122  below the ejection port  109 B. 
     In the case where the axial fan  101  is the axial fan  50  according to the first embodiment, a duct  133  whose channel extends toward the thin-wall portion  43  positioned above from the rotation axis C 1  is disposed in the cold air passage  131 . In other words, the channel of the duct  133  is inclined in the upward direction and in the lateral direction when the refrigerator  100  is viewed from the front. 
     In the case where the axial fan  101  is the axial fan  50  according to the first embodiment, exhaust air is directed in the axial direction (downward in the above description on the axial fan  50 ) in the radial center of the housing  4  and in the vicinity of the thick-wall portion  42 , so that the exhaust cold air efficiently flows through the ejection port  109 A into the freezer  103 . The cold air exhausted by the driving of the axial fan  101  downward in the cold air passage  131  flows through the ejection port  109 B into the freezer  103 . The cold air that has flowed into the freezer  103  cools stored items in the storage case  170  and flows out through the freezer return port  122  back to below the cooler  111 . 
     In the case where the axial fan  101  is the axial fan  50  according to the first embodiment, the exhaust air around the thin-wall portion  43  is discharged radially outward, so that the exhaust air flows upward along the channel of the duct  133  into the cold air passage  132 . A plurality of ejection ports  108  through which the cold air is ejected are provided at the upper part of the cold air passage  132 . A return air duct (not shown) is led out from the lower part of the back surface of the refrigerating compartment  102 . The return air duct is connected to the lower part of the cold air passage  131 . The cold air flowing out of the refrigerating compartment  102  and passing through the return air duct returns to below the cooler  111 . 
     Since the axial fan  101  according to the present embodiment has the thick-wall portions and the thin-wall portions as described above, the cooling performance of the refrigerator  100  can be adjusted by adjusting the wind direction by the design of the thick-wall portions and the thin-wall portions. 
     Furthermore, since the axial fan  101  has a groove (for example, the groove  423 ) of the axial fan  50 ) at each thick-wall portion, moisture adhering to the inner wall surface of the housing can be discharged, reducing or eliminating freezing of the moisture on the inner wall surface of the housing. This is ditto for the case where the housing has no groove but has a draining hole as in the fourth embodiment. 
     The back plate  106 A may not have the ejection port  109 A but may have a protruding portion protruding toward the axial fan  101  on the exhaust side in the axial direction of the axial fan  101 . For example, the protruding portion has a conical shape. The protruding portion allows the air exhausted in the axial direction to be guided in the vertical direction. In this case, the protruding portion can cause part of the exhaust air to flow back to the axial fan. Therefore, with the configuration in which one end of the groove provided at the thick-wall portion extends to the exhaust end of the housing, moisture collected in the groove is pushed back to the side opposite to the discharge side by the backflow of air, and drainage is hindered. Therefore, it is desirable not to adopt the above configuration. 
     As described above, the axial fan  50  according to the first embodiment of the present disclosure includes the impeller  1  that rotates about the rotation axis C 1  extending in the vertical direction, the motor  2  that rotationally drives the impeller  1 , and the housing  4  disposed radially outside the impeller  1  and the motor  2 . In a region overlapping in the vertical direction with the impeller  1 , the inner wall surface  4 W 2  of the housing  4  includes a first wall (a thick-wall portion)  42  having a narrow gap with the radially outer edge of the impeller  1  and a second wall (a thin-wall portion)  43  having a wide gap with the radially outer edge of the impeller  1 . 
     This configuration decreases the gap between the impeller  1  and the inner surface of the housing  4  because of the presence of the first wall  42 , thereby improving the static pressure. The static pressure can be adjusted by adjusting the ratio of the length of the first wall  42  to the length of the second wall  43  along the inner wall surface  4 W 2 . Since the exhaust flow flows in the vertical direction around the first wall  42  and flows radially outward around the second wall  43 , the direction of the exhaust flow can be adjusted. 
     The inner surface of the first wall  42  is part of a cylindrical shape centered on the rotation axis C 1 . This further decreases the gap between the first wall  42  and the impeller  1 , thereby improving the static pressure. Since the gap between the inner surface of the first wall  42  and the radially outer edge of the impeller  1  in the circumferential direction is constant, a turbulent flow can be reduced, thereby reducing or eliminating generation of noise. The cylindrical shape includes a substantially cylindrical shape. 
     The circumferential length of the first wall  42  differs from the circumferential distance between the radially outer ends of adjacent blades  12  of the impeller  1 . This prevents the adjacent blades  12  from crossing both ends of the first wall  42  at the same time, improving the noise-reduction performance. 
     An intake port is disposed at the upper part of the housing  4 , and an exhaust port is disposed at the lower part of the housing  4 . The axial fan further includes the motor base portion  3  that supports the motor  2 , as well as the ribs  5  connecting the motor base portion  3  and the housing  4  together. The upper surface of each rib  5  has the inclined surface  52  that is inclined downward toward the forward end of the impeller  1  in the rotating direction. This enhances the rigidity of the, axial fan. The ribs  5  allows the air flow to be guided downward on the exhaust side. 
     The axial fan further includes the ring-shaped rib  6  centered on the rotation axis C 1  and connected to the ribs  5 . The upper surface of the ring-shaped rib  6  includes the inclined surface  61  that is inclined downward toward the radially outside. This enhances the rigidity of the axial fan. The ring-shaped rib  6  allows the air flow to be guided radially outward. 
     The number of the first walls  42  and the number of the blades  12  of the impeller  1  are prime to each other. This prevents the plurality of blades  12  from crossing the first walls  42  at the same time, improving the noise-reduction performance. 
     All of the lengths of the plurality of the first walls  42  along the inner wall surface  4 W 2  are equal. This makes the static pressure distribution symmetrical about the rotation axis C 1 , thereby reducing or eliminating generation of a turbulent flow. 
     The outer wall surface of the housing  4  has a substantially rectangular shape in a cross sectional view perpendicular to the vertical direction. Both circumferential ends of the first wall  42  are provided on the inner wall surface of one side of the substantially rectangular shape. Thus, the first wall  42  is formed on the one side. This increases the radial width between the outer surface of the housing  4  and the inner surface of the first wall  42 . This increases the thickness of the outer wall surface of the housing  4 , thereby enhancing the rigidity of the housing  4 . 
     The outer wall surface of the housing  4  has a substantially rectangular shape in a cross sectional view perpendicular to the vertical direction. The both circumferential ends of the first wall  42  are disposed on the inner wall surface  4 W 2  of different sides of the substantially rectangular shape. In other words, the first wall  42  is formed in the vicinity of each corner of the substantially rectangular housing  4 . This allows the thickness of the housing  4  to be increased at the corner of the housing  4  while increasing the diameter of the impeller  1  on the inner surface of each side of the housing  4 , thereby enhancing the rigidity of the housing  4 . 
     The inner surface of the first wall  42  includes the groove  423  that is recessed radially outward and extending to an end of the housing  4  in the vertical direction. This allows moisture adhering to the inner surface of the housing  4  to be collected to the groove  423  for drainage. 
     The intake port is disposed at the upper part of the housing  4 , and the exhaust port is disposed at the lower part of the housing  4 . One end of the groove  423  is disposed adjacent to the intake port. The groove  423  decreases in depth toward the end of the housing  4  in the vertical direction. This allows moisture collected in the groove  423  to be guided to the end for drainage. This also prevents the moisture from being diffused widely far away. 
     The gap between the second wall  43  and the impeller  1  increases toward one end in the vertical direction. In other words, the radially inner surface of the second wall  43  is inclined radially outward toward the one end in the vertical direction. This allows moisture accumulated on the second wall  43  to be guided to the one end in the vertical direction for drainage. The radially inner surface of the second wall  43  may not be inclined but may be curved. 
     The refrigerator  100  according to an embodiment of the present disclosure includes the axial fan  101  with one of the above configurations. Thus allows the static pressure and the wind direction to be adjusted according to the specifications of the refrigerator  100 . 
     The refrigerator  100  includes the duct  133 . The channel of the duct  133  extends in a direction from the rotation axis C 1  to the second wall  43 . This allows the exhaust air to be flowed along the channel of the duct  133 , allowing efficient cooling. 
     The reference signs assigned to the above components of the embodiments are mere examples. Any other signs can be assigned unless there is a contradiction. 
     The present disclosure may be used for an axial fan mounted in a refrigerator, for example. 
     Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.