Patent Publication Number: US-2012045323-A1

Title: Fan

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fan and more specifically to a fan that is especially suitable for use in a narrow space, such as a cooling air path of a refrigerator, for example. 
     2. Description of the Related Art 
     Household refrigerators are typically configured to send air that has been cooled in a cooler into storage compartments (a freezing compartment and a chilling compartment, for example) to thereby cool the inside of the storage compartments. In this case, as disclosed in, e.g., U.S. Pat. No. 7,331,193, a fan is arranged within a duct which extends toward a cooler to circulate air through storage compartments. In order to make the storage compartments as large as possible, the duct is designed to have a narrow space within which the fan needs to be arranged. 
     Typically, a centrifugal fan or an axial flow fan is used as the fan arranged within the duct as mentioned above. The centrifugal fan includes an impeller, which has a circular main plate rotating about a center axis and a plurality of vanes fixed to the main plate in a circumferentially spaced-apart relationship, and a bell mouth arranged at the air intake side of the impeller to define an air inlet. The axial flow fan includes a hub rotating about a center axis, a plurality of propeller blades fixed to an outer circumferential surface of the hub in a circumferentially spaced-apart relationship to rotate about the center axis together with the hub to thereby draw air from one axial side and discharge the air toward the other axial side, and a housing having an inner circumferential surface surrounding the outer ends of the propeller blades. 
     In the case of the centrifugal fan, depending on the service space thereof, there is a need to increase the gap (tip clearance) between the bell mouth (especially, the portion of the air inlet closest to the vanes) and the vanes. For example, if a centrifugal fan having a narrow tip clearance is used in a refrigerator, there is a possibility that the vanes will become stuck to the bell mouth due to frost. The reason for increasing the tip clearance is to avoid this possibility. In case of the axial flow fan, if the gap (tip clearance) between the outer ends of the propeller blades and the inner circumferential surface of the housing is small, it is likely that the propeller blades are stuck to the inner circumferential surface of the housing by frost and are unable to feed air into the storage compartments. Thus, there is a need to increase the amount of tip clearance. 
     However, if the amount of tip clearance is increased in the centrifugal fan, a large space will exist between the inner end of the air inlet and the impeller. This space serves as a negative pressure space in which the flow of air becomes unstable. Due to the increase in the volume of this space, a problem is caused in which noises become greater during the operation of the centrifugal fan (during the rotation of the impeller). If the amount of tip clearance is increased in the axial flow fan, air tends to flow backwards from a discharge side to an intake side in the tip clearance between the outer ends of the propeller blades and the inner circumferential surface of the housing. This poses a problem of impairing the characteristic relationship between a discharge-side static pressure (P) and a flow rate (Q) in the axial flow fan and increasing the noises. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention preferably provide a configuration capable of suppressing impairment of the noise performance of a centrifugal fan installed in a narrow space even when the amount of tip clearance between a bell mouth and vanes is increased. 
     Preferred embodiments of the present invention also preferably provide a configuration capable of minimizing an impairment of the PQ characteristic (as mentioned above, P corresponds to static pressure and Q corresponds to a flow rate) and the noise performance of an axial flow fan installed in a narrow space. 
     In accordance with one preferred embodiment of the present invention, a centrifugal fan includes an impeller including a main plate rotating about a center axis and a plurality of vanes fixed to the main plate in a circumferentially spaced-apart relationship; and a bell mouth arranged at an air intake side of the impeller to define an air inlet including a portion whose inner diameter is decreased toward the impeller, each of the vanes including a protrusion protruding toward the bell mouth and extending into the air inlet. 
     With the centrifugal fan of this configuration, the protrusion of each of the vanes extends into the air inlet of the bell mouth. The drawing in of the air in the inner portion of the air inlet is accelerated by the protrusion. This works to stabilize the airflow in the inner portion of the air inlet even when the amount of tip clearance is set to be larger, consequently reducing the volume of a negative pressure space in which the airflow becomes unstable. Accordingly, it is possible to suppress an increase in noises generated during the operation of the centrifugal fan. 
     In accordance with another preferred embodiment of the present invention, an axial flow fan includes a hub rotating about a center axis; a plurality of propeller blades fixed to an outer circumferential surface of the hub in a circumferentially spaced-apart relationship to rotate about the center axis together with the hub to thereby draw air from an air intake side as one center axis direction side and discharge the air toward an air discharge side as the other center axis direction side, the propeller blades including outer peripheral portions interconnected by a ring member; and a bell mouth provided at the air intake side with respect to the propeller blades, the bell mouth including a portion whose inner diameter is decreased toward the air discharge side to guide the air drawn by the propeller blades toward the propeller blades. 
     With the axial flow fan of this configuration, even if a wall exists near the air intake side of the axial flow fan, the air drawn through the inside of the bell mouth is guided by the bell mouth toward the propeller blades so that the air can smoothly flow in a center axis (fan axis) direction. This works to reduce any separation of airflow caused by the increase in an attack angle of the propeller blades. In cooperation with the bell mouth, the ring member serves to guide the flow of the air drawn by the propeller blades so that the air passing through the ring member can flow in the center axis direction. 
     In a case where the outer peripheral ends of the propeller blades are surrounded by a cylindrical housing, the members arranged around the bell mouth to support the bell mouth serves to cover the air intake side end of the clearance space between the outer peripheral ends of the propeller blades and the inner circumferential surface of the housing. Therefore, even if the amount of tip clearance is made greater, it is possible to prevent the air from flowing backwards from the air discharge side toward the air intake side through the clearance space. 
     In accordance with the preferred embodiments of the present invention, it is also possible for the ring member to prevent the air flowing through the inside of the ring member from leaking radially outwards. Accordingly, even if the axial flow fan is arranged in a narrow space where the propeller blades risk being held stationary by frost and where walls are present near the air intake side and the air discharge side of the axial flow fan, it is possible to suppress impairment of the PQ characteristic and to also suppress any increase in noises. 
     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 sectional view showing a centrifugal fan according to a first preferred embodiment of the present invention, which is arranged in a fan arrangement space of a refrigerator. 
         FIG. 2  is a front view of the centrifugal fan shown in  FIG. 1 , which is seen from an air intake side. 
         FIG. 3  is a perspective view of the centrifugal fan shown in  FIG. 1 , which is obliquely seen from the air intake side. 
         FIG. 4  is a front view of an impeller of the centrifugal fan shown in  FIG. 1 , which is seen from the air intake side. 
         FIG. 5  is a perspective view of the impeller of the centrifugal fan shown in  FIG. 1 , which is obliquely seen from the air intake side. 
         FIG. 6  is a perspective view showing a modified example of the impeller of the centrifugal fan shown in  FIG. 1 . 
         FIG. 7  is a graph representing the PQ characteristics and the noise characteristics of the centrifugal fan according to the first preferred embodiment and the centrifugal fan according to a comparative example. 
         FIG. 8  is a section view showing an axial flow fan according to a second preferred embodiment of the present invention, which is arranged in a fan arrangement space of a refrigerator. 
         FIG. 9  is a front view of an impeller (a hub and propeller blades) of the axial flow fan shown in  FIG. 8 , which is seen from an intake side. 
         FIG. 10  is a perspective view of a bell mouth defining member of the axial flow fan shown in  FIG. 8 , which is obliquely seen from an intake side. 
         FIG. 11  is a front view showing a modified example of the axial flow fan shown in  FIG. 8 , in which support legs are replaced by static vanes. 
         FIG. 12  is a section view showing a modified example of the ring member of the axial flow fan shown in  FIG. 8 . 
         FIG. 13  is a perspective view showing a modified example of the flow straightening members of the axial flow fan shown in  FIG. 12 . 
         FIG. 14  is a section view showing an axial flow fan according to a further preferred embodiment of the present invention, which is arranged in a fan arrangement space of a refrigerator. 
         FIG. 15  is a graph representing the PQ characteristics and the noise characteristics of the axial flow fan shown in  FIG. 8  and the axial flow fans according to comparative examples. 
         FIG. 16  is a graph representing the PQ characteristics and the noise characteristics of the axial flow fan shown in  FIG. 8  and the axial flow fan according to another comparative example. 
         FIG. 17  is a graph representing the PQ characteristics and the noise characteristics of the axial flow fan shown in  FIG. 8  and the axial flow fans according to further comparative examples. 
         FIG. 18  is a graph representing the PQ characteristics and the noise characteristics of the axial flow fan shown in  FIG. 8  and the axial flow fans according to still further comparative examples. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
       FIG. 1  shows a centrifugal fan  1  according to a first preferred embodiment of the present invention. In this preferred embodiment, the centrifugal fan  1  is arranged to send a cooling air toward a freezing compartment  51  of a refrigerator and is arranged in a fan arrangement space  52  defined inwards of the freezing compartment  51 . At the inner side of the fan arrangement space  52 , there is defined a cooler arrangement space  53  in which a cooler (not shown) and the like are arranged. The fan arrangement space  52  and the cooler arrangement space  53  are divided by a first partition wall  54  extending in the vertical direction. The freezing compartment  51  and the fan arrangement space  52  are divided by a second partition wall  55  extending parallel or substantially parallel to the first partition wall  54  in the vertical direction. 
     In the fan arrangement space  52 , an attachment member  57  for attachment of the centrifugal fan  1  is provided to extend in the vertical direction. A first path  58  through which the cooling air passes is defined between the attachment member  57  and the first partition wall  54 . A second path  59  through which the cooling air passes is defined between the attachment member  57  and the second partition wall  55 . The first path  58  and the second path  59  extend in the vertical direction perpendicular or substantially perpendicular to the center axis J of a main plate  3  to be set forth later. 
     The cooling air cooled in the cooler is fed from the cooler arrangement space  53  to the first path  58  of the fan arrangement space  52 . The cooling air flows along the first path in the vertical direction and reaches a position where the centrifugal fan  1  exists. Then, the cooling air is sent to the second path  59  of the fan arrangement space  52  by the centrifugal fan  1 . After flowing through the second path  59  in the vertical direction, the cooling air is blown into the freezing compartment through an air outlet (not shown) defined in the second partition wall  55 . Hereinafter, the cooling air will be just referred to as “air”. 
     As shown in  FIGS. 1 through 5 , the centrifugal fan  1  of the present preferred embodiment preferably includes a main plate  3  rotating about the center axis J extending in the horizontal direction or substantially in the horizontal direction and an impeller  2  having a plurality of vanes  4  (preferably eleven vanes in the present preferred embodiment) fixed to the main plate  3  in a circumferentially spaced-apart relationship. The impeller  2  preferably is an open impeller that does not have a shroud opposed to the main plate  3  across the vanes  4 . Hereinafter, the center axis J of the main plate  3  (or the center axis of the impeller  2 ) will be just referred to as “center axis J”. The direction in which the center axis J extends (the left-right direction in  FIG. 1 ) will be referred to as “center axis direction”. The direction perpendicular or substantially perpendicular to the center axis direction will be referred to as “radial direction”. 
     The vanes  4  rotate about the center axis J together with the main plate  3 . The vanes  4  draw the air from the air intake side (the left side in  FIG. 1 , i.e., the side of the first path  58 ), which is one side of the impeller  2  in the center axis direction (the opposite side from the main plate  3 ), and discharge the air radially outwards (into the second path  59 ) at the radial outer end of the impeller  2  (see the airflow indicated by arrows in  FIG. 1 ). 
     As shown in  FIGS. 4 and 5 , the vanes  4  are interconnected by a ring member  5  at the air intake side and at the radial outer ends thereof. The main plate  3 , the vanes  4  and the ring member  5  are preferably made of, e.g., a resin, and are provided as a single monolithic piece. 
     The central portion of the main plate  3  protrudes toward the air intake side. As a consequence, a space in which a motor  11  arranged to rotate the main plate  3  can be arranged is defined at the opposite side of the main plate  3  from the air intake side (at the right side in  FIG. 1 ). The motor  11  preferably includes a rotor  12  rotating about the center axis J and a stator  13  arranged inside the rotor  12 . The rotor  12  preferably includes a cylindrical rotor holder  15  closed only at the air intake side of the axially opposite sides thereof, field magnets  16  fixed to the inner surface of a sidewall portion of the rotor holder  15 , and a shaft  17  fixed to the central area of the air intake side end portion of the rotor holder  15  to rotate together with the rotor holder  15 . 
     The shaft  17  extends from the air intake side end portion of the rotor holder  15  toward the opposite side to the air intake side and is rotatably supported, preferably by a couple of bearings  20 . The air intake side end portion of the rotor holder  15  is fixed to the portion of the main plate  3  protruding toward the air intake side. Accordingly, the main plate  3  rotates about the center axis J together with the rotor holder  15  (the rotor  12 ). 
     A substantially disc-shaped flat base member  21  is arranged at the opposite side of the rotor holder  15  from the air intake side. A cylindrical bearing support portion  22  extends toward the air intake side from the center portion of the air intake side surface of the base member  21 . The bearings  20  are fixed to the inner surface of the bearing support portion  22  at two points spaced apart in the center axis direction. The base member  21  is held by a below-described bell mouth support member  32  through connecting members  35 , which are also described later. 
     The stator  13  preferably includes a substantially cylindrical stator core  25  installed on the outer circumferential surface of the bearing support portion  22  and defined by steel plates laminated one above another along the center axis direction and coils  26  wound around the stator core  25 . A board support portion  27  arranged to support a circuit board  28  is installed on the opposite surface of the stator  13  from the air intake side. When a drive current is supplied to the coils  26  via the circuit board  28 , rotation torque is generated between the field magnets  16  and the stator core  25  whereby the impeller (the main plate  3  and the vanes  4 ) will be caused to rotate about the center axis J. The rotating direction of the impeller  2  is indicated by an arrow R in  FIGS. 4 and 5  (This also holds true in  FIG. 6  showing a modified example). 
     In the present preferred embodiment, the centrifugal fan  1  is preferably a turbo fan, which means that the vanes  4  of the impeller  2  have a shape of turbo fan vanes. More specifically, when seen in the center axis direction, each of the vanes  4  is curved against the rotating direction R of the impeller  2  (the main plate  3 ) as it extends from a radial inner end thereof toward a radial outer end thereof. 
     At the air intake side of the impeller  2 , there is arranged a bell mouth  31  including a portion whose inner diameter is decreased toward the impeller  2 . The bell mouth  31  preferably includes a bell mouth support member  32  that supports the bell mouth  31  in the outer periphery thereof as a single monolithic piece. The bell mouth  31  includes an air inlet  31   a  whose center coincides with the center axis J. In the present preferred embodiment, the air inlet  31   a  is arranged such that the inner diameter thereof becomes smaller toward the impeller  2  over the entire center axis direction portion. Alternatively, a portion of the air inlet  31   a  in the center axis direction (especially, the end portion of the air inlet  31   a  near the impeller  2 ) may have an inner diameter kept constant or increased toward the impeller  2 , and the remaining portion of the air inlet  31   a  may have an inner diameter decreased toward the impeller  2 . 
     The base member  21  and the bell mouth support member  32  are interconnected by a plurality of connecting members  35  (preferably four connecting members in the present preferred embodiment, for example) arranged at the opposite side of the impeller  2  from the air intake side and at the radial outer side of the impeller  2  in a circumferentially spaced-apart relationship with one another. Thus, the base member  21  is supported on the bell mouth support member  32  through the connecting members  35 . In the outer edge portion of the bell mouth support member  32 , there is provided a flange portion  32   a  through which the centrifugal fan  1  is attached to the attachment member  57 . When the centrifugal fan  1  is in this attachment state, the bell mouth support member  32  (the bell mouth  31 ) is positioned in the first path  58  while the air-discharging radial outer end of the impeller  2  is situated in the second path  59 . In the air intake side radial inner end portions of the vanes  4  (in the portions of the vanes  4  opposed to the air inlet  31   a ), protrusions  4   a  protruding toward the bell mouth  31  and extending into the air inlet  31   a  are provided. 
     In this regard, the size of the gap (tip clearance) between the bell mouth  31  and the vanes  4  including the protrusions  4   a  is set at such a dimension that the vanes  4  will not become stuck to the bell mouth  31  due to frost. In a hypothetical case in which the protrusions  4   a  are absent with the tip clearance set at this dimension, noises are increased during the operation of the centrifugal fan  1  (during the rotation of the impeller  2 ). In the present preferred embodiment, the protrusions  4   a  preferably are defined in the vanes  4 . Therefore, even if the tip clearance is set at the dimension noted above, it is possible to suppress the increase in noises during the operation of the centrifugal fan  1 . In other words, the drawing of the air in the inner portion of the air inlet  31   a  is accelerated by providing the protrusions  4   a . This helps stabilize the airflow in the inner portion of the air inlet  31   a  even when the tip clearance is set at the dimension noted above, consequently reducing the volume of a negative pressure space in which the airflow becomes unstable. Accordingly, it is possible to suppress the increase in noises during the operation of the centrifugal fan  1 . 
     It is preferred that the tip end surfaces of the protrusions  4   a  of the vanes  4  be located substantially in the same positions in the center axis direction as the diameter reduction start position where the inner diameter of the air inlet  31   a  begins to decrease toward the impeller  2  (the air intake side end of the air inlet  31   a  in the present preferred embodiment). This makes it possible to reduce the volume of a negative pressure space in which the airflow becomes unstable. Alternatively, the deviation amount in the center axis direction between the positions of the tip end surfaces of the protrusions  4   a  and the diameter reduction start position may be greater than zero and equal to or smaller than a specified value. The specified value may preferably be, e.g., about 10% of the center axis direction length of the air inlet  31   a . More preferably, the tip end surfaces of the protrusions  4   a  are arranged beyond the diameter reduction start position, and the deviation amount in the center axis direction between the positions of the tip end surfaces of the protrusions  4   a  and the diameter reduction start position is greater than zero and equal to or smaller than the specified value (see  FIG. 1 ). In other words, it is particularly preferable that the protrusions  4   a  protrude beyond the air inlet  31   a  into the air intake side external space (the first path  58 ) by the specified value or less. 
     As shown in  FIG. 6 , serrations  4   b  arranged to suppress the generation of a vortex of the air on the tip end surfaces of the protrusions  4   a  of the vanes  4  may be provided on the tip end surfaces of the protrusions  4   a . The serrations  4   b  are preferably a series of notches continuously defined on the tip end surfaces of the protrusions  4   a  along the longitudinal direction of the vanes  4 . Since the flow of the air drawn by the impeller  2  is straightened by the serrations  4   b , the airflow in the inner portion of the air inlet  31   a  becomes stable. This enhances the noise performance of the centrifugal fan  1 . 
     While the first preferred embodiment of the present invention has been described above, the shape of the vanes  4  of the impeller  2  is not limited to the shape of the turbo fan vanes and could instead be provided in any desirable shape. For example, as an alternative example, the vanes  4  may extend straight in the radial direction (all the vanes  4  may be arranged in a radial shape). Additionally, the impeller  2  is not limited to the open impeller but may be a closed impeller including a casing provided with a shroud. In the preferred embodiment described above, the centrifugal fan  1  is used to blow the cooling air into the freezing compartment  51  of a refrigerator. However, the present invention is not limited thereto. As an alternative example, the centrifugal fan  1  could instead be used as a blower arranged to cool electronic devices. If the centrifugal fan  1  is used in an application where there is a need to increase the tip clearance as in the preferred embodiment described above, it is possible to produce a great silencing effect. 
     In the preferred embodiment described above, the first partition wall  54  preferably is present near the air intake side of the centrifugal fan  1  (this will be referred to as “with wall” in the following examples and comparative examples). However, the present invention is also applicable to a case where the first partition wall  54  (and even the second partition wall  55 ) does not exist (This will be referred to as “without wall” in the following examples and comparative examples). In this case, even if the tip clearance is made greater, it is possible to suppress the increase in noises during the operation of the centrifugal fan  1 . 
     A centrifugal fan like the one described above in respect of the foregoing preferred embodiment was prepared to evaluate the change of the static pressure against the flow rate (air flow rate), i.e., the PQ characteristic, at the air discharge side and the change of the noise level against the flow rate (air flow rate), i.e., the noise characteristic, at the air discharge side. In the centrifugal fan thus prepared, the vanes  4  are provided with the protrusions  4   a . The tip end surfaces of the protrusions  4   a  is arranged beyond the diameter reduction start position. The deviation amount in the center axis direction between the positions of the tip end surfaces of the protrusions  4   a  and the diameter reduction start position is equal to or smaller than the specified value. The tip clearance is set at the same dimension as in the foregoing preferred embodiment. The PQ characteristic and the noise characteristic were evaluated with respect to a case (an test example with wall) where the centrifugal fan is arranged in a space between two walls like the first partition wall  54  and the second partition wall  55  of the foregoing preferred embodiment and a case (an test example without wall) where the centrifugal fan is arranged in a space having no wall. 
     For the purpose of comparison, a centrifugal fan in which the protrusions  4   a  are not provided in the vanes  4  (other configurations of which remain the same as those of the test example mentioned just above) was prepared. The PQ characteristic and the noise characteristic were evaluated with respect to a case (a comparative example with wall) where the centrifugal fan is arranged in a space between two walls and a case (a comparative example without wall) where the centrifugal fan is arranged in a space having no wall. 
     The results of evaluation are shown in  FIG. 7 . Comparison of the test example (with wall) and the comparative example (with wall) reveals that, over the entire flow rate sections, the noise level in the test example (with wall) is lower than that in the comparative example (with wall). It can be seen that little difference in the PQ characteristic exists between the test example (with wall) and the comparative example (with wall), which means that the centrifugal fans of the test example (with wall) and the comparative example (with wall) exhibit desired PQ performance. Comparison of the test example (without wall) and the comparative example (without wall) reveals that, over most of the flow rate sections, the noise level in the test example (without wall) is lower than that in the comparative example (without wall). The centrifugal fans of the test example (without wall) and the comparative example (without wall) exhibit desired PQ performance. Accordingly, it can be appreciated that, even if the tip clearance is made greater, the noises generated during the operation of the centrifugal fan can be reduced by providing the protrusions  4   a  in the vanes  4 . 
     Second Preferred Embodiment 
     Next, an axial flow fan  101  according to a second preferred embodiment of the present invention will be described with reference to  FIGS. 8 through 11 . In the present preferred embodiment, the axial flow fan  101  is preferably arranged to send a cooling air toward a freezing compartment  151  of a refrigerator and is arranged in a narrow fan arrangement space  152  defined inwards of the freezing compartment  151 . At the inner side of the fan arrangement space  152 , there is defined a cooler arrangement space  153  in which a cooler (not shown) and the like are arranged. The fan arrangement space  152  and the cooler arrangement space  153  are divided by a first partition wall  154  extending substantially in the vertical direction. The freezing compartment  151  and the fan arrangement space  152  are divided by a second partition wall  155  extending parallel or substantially parallel to the first partition wall  154  in the vertical direction. 
     In the fan arrangement space  152 , an attachment member  157  arranged to attach the axial flow fan  101  is provided to extend in the vertical direction or substantially in the vertical direction. A first path  158  through which the cooling air passes is defined between the attachment member  157  and the first partition wall  154 . A second path  159  through which the cooling air passes is defined between the attachment member  157  and the second partition wall  155 . The first path  158  and the second path  159  extend in the vertical direction perpendicular or substantially perpendicular to the fan axis (the center axis J to be set forth later). 
     The cooling air cooled in the cooler is fed from the cooler arrangement space  153  to the first path  158  of the fan arrangement space  152 . The cooling air flows along the first path  158  in the vertical direction and reaches a position where the axial flow fan  101  exists. Then, the cooling air is sent to the second path  159  of the fan arrangement space  152  by the axial flow fan  101 . After flowing through the second path  159  in the vertical direction, the cooling air is blown into the freezing compartment  151  through an air outlet (not shown) defined in the second partition wall  155 . Hereinafter, the cooling air will be just referred to as “air”. 
     In the present preferred embodiment, the axial flow fan  101  preferably includes a cylindrical hub  102  rotating about the center axis J extending in the horizontal direction or substantially in the horizontal direction and a plurality of propeller blades  103  (preferably seven propeller blades in the present preferred embodiment as shown in  FIG. 7 ) fixed to the outer circumferential surface of the hub  102  at a regular interval in the circumferential direction. The propeller blades  103  are preferably defined by a single monolithic piece with the hub  102 . The hub  102  and the propeller blades  103  are collectively referred to as “impeller”. Hereinafter, the center axis J of the hub  102  will be just referred to as “center axis J”. The direction in which the center axis J extends (the left-right direction in  FIG. 8 ) will be referred to as “center axis direction”. The direction perpendicular to the center axis direction will be referred to as “radial direction”. 
     The propeller blades  103  rotate about the center axis J together with the hub  102 . The propeller blades  103  draw the air from the air intake side (the left side in  FIG. 8 , i.e., the side of the first path  158 ), which is one side in the center axis direction, and discharge the air toward the air discharge side (the right side in  FIG. 8 , i.e., the side of the second path  159 ), which is the other side in the center axis direction. The hub  102  preferably has a cup shape opened only at the air discharge side of the center axis direction opposite sides thereof. A shaft  105  is fixed to the central area of the air intake side end portion of the hub  102  to rotate together with the hub  102 . The shaft  105  extends from the air intake side end portion of the hub  102  toward the air discharge side along the center axis J. 
     A base member  110  is arranged at the air discharge side of the hub  102 . The base member  110  is provided into a flat disc shape having substantially the same outer diameter as that of the hub  102 . A cylindrical bearing support portion  111  extending toward the air intake side is preferably defined by provided as a single monolithic piece with the central portion of the air intake side surface of the base member  110 . A sleeve beating  112  is preferably fixed to the inner surface of the bearing support portion  111 . The shaft  105  is inserted into, and rotatably supported by, the sleeve bearing  112 . This enables the base member  110  to rotatably support the hub  102 . The base member  110  is held by a below-mentioned housing  130  through support legs  131  to be set forth below. 
     A motor  115  arranged to rotate the hub  102  is mounted between the inner circumferential surface of the hub  102  and the outer circumferential surface of the bearing support portion  111 . The motor  115  preferably includes a rotor  116  installed on the inner circumferential surface of the hub  102  and a stator  120  installed on the outer circumferential surface of the bearing support portion  111 . The rotor  116  preferably includes a magnet holder  117  fixed to the inner circumferential surface of the hub  102  and rotor magnets  118  held by the magnet holder  117 . The stator  120  preferably includes a substantially cylindrical stator core  121  defined by steel plates laminated one above another along the center axis direction and coils  122  wound around the stator core  121 . If a drive current is supplied to the coils  122 , rotation torque is generated between the rotor magnets  118  and the stator core  121  whereby the hub  102  and the propeller blades  103  (the impeller) can rotate about the center axis J. The rotating direction of the hub  102  and the propeller blades  103  is indicated by an arrow R in  FIGS. 9 through 11  and  13 . 
     As shown in  FIG. 9 , the propeller blades  103  are provided by forward-swept blades in the present preferred embodiment. In other words, when the propeller blades  103  are seen in the center axis direction, the intersection point P 1  between the leading edge  103   a , i.e., the front end in the rotating direction R, of each of the propeller blades  103  and the outer circumferential edge  103   b  of each of the propeller blades  103  is positioned ahead of the intersection point P 2  between the leading edge  103   a  and the outer circumferential surface of the hub  102  in the rotating direction R. 
     The outer peripheral portions of the propeller blades  103  are interconnected by a cylindrical ring member  125 . In the present preferred embodiment, the ring member  125  is attached to the outer peripheral ends of the propeller blades  103  in a coaxial relationship with the hub  102  (The center of the ring member  125  lies on the center axis J). The ring member  125  is preferably defined by a single monolithic piece with the propeller blades  103 . In cooperation with a below-mentioned bell mouth  135 , the ring member  125  serves to guide the flow of the air drawn by the propeller blades  103  so that the air passing through the ring member  125  can flow in the center axis direction. Moreover, the ring member  125  serves to prevent the air flowing inside thereof from leaking radially outwards from the outer peripheral ends of the propeller blades  103 . 
     The air intake side end of the ring member  125  is substantially flush with the air intake side ends of the propeller blades  103 . On the other hand, the air discharge side end of the ring member  125  is spaced apart in the center axis direction from the air intake side ends of the propeller blades  103  by about 50% through about 80% of the center axis direction length of the propeller blades  103 . In other words, the ring member  125  is installed near the air intake side ends of the propeller blades  103  in the center axis direction. 
     The center axis direction section of the ring member  125  is not limited to the section near the air intake side ends of the propeller blades  103 . Alternatively, the ring member  125  may be arranged in the section near the air discharge side ends of the propeller blades  103 , in the axially middle portions of the propeller blades  103  or in the axially entire portions of the propeller blades  103 . However, if the axial flow fan  101  is arranged in the narrow fan arrangement space  152  as in the present preferred embodiment, it is preferred that the ring member  125  be installed near the air intake side ends of the propeller blades  103  in the center axis direction. More specifically, the flow of the air discharged by the propeller blades  103  is finally struck against the second partition wall  55  positioned near the air discharge side of the axial flow fan  101  and is sharply curved radially outwards. If the ring member  125  is not provided in the air discharge side end portions of the propeller blades  103 , the air is discharged toward the air discharge side in a direction inclined to the outwardly radial direction. Accordingly, the flow of the air discharged by the propeller blades  103  is gently curved radially outwards under the guidance of a guide portion  141  to be set forth later. 
     It is not always necessary to provide the ring member  125  at the outer peripheral ends of the propeller blades  103 . Alternatively, the ring member  125  may be installed in the outer peripheral portion (e.g., in the position spaced apart from the roots of the propeller blades  103 , i.e., the outer circumferential surface of the hub  102 , by about 70% or more and less than 100% of the length of the propeller blades  103 ). It is however preferred that the ring member  125  be provided at the outer peripheral ends of the propeller blades  103  as in the present preferred embodiment. This ensures that all the air drawn to the propeller blades  103  can flow toward the air discharge side without leaking radially outwards from the outer peripheral ends of the propeller blades  103 . 
     The axial flow fan  101  preferably further includes a substantially cylindrical housing  130  installed in a coaxial relationship with the hub  102  to surround the outer peripheral ends of the propeller blades  103 . The size of the gap (tip clearance) between the outer circumferential surface of the ring member  125  and the inner circumferential surface of the housing  130  is preferably set at such a dimension that the propeller blades  103  will not become stuck to the inner circumferential surface of the housing  130  due to frost. 
     A plurality of support legs  131  extending straight in the radial direction is arranged in the air discharge side opening of the housing  130  at a regular interval along the circumferential direction (The number of the support legs  131  may be equal to or may differ from the number of the propeller blades  103  although eight support legs are preferably included in the illustrated example). The radial outer ends of the support legs  131  are fixed to the air discharge side end portion of the inner circumferential surface of the housing  130 . The radial inner ends of the support legs  131  are secured to the outer circumferential surface of the base member  110 . 
       FIG. 11  shows a modified example of the housing  130  in which the support legs  131  are replaced by static vanes  131 ′. However, the shape of the housing  130  remains the same in the modified example and the present preferred embodiment. Referring now to  FIG. 11 , the contour of the housing  130  has a substantially rectangular shape when seen in the center axis direction. Near the four corner portions of the housing  130 , there are provided fixing portions  130   a  to which a below-mentioned bell mouth defining member  136  is fixed. The fixing portions  130   a  have thread holes  130   b  to which screws, for example, are fitted to fix the bell mouth defining member  136  in place. 
     A bell mouth  135 , by which the air drawn by the propeller blades  103  is guided toward the propeller blades  103 , is provided at the air intake side of the propeller blades  103  (more specifically, at the axially outer side of the housing  130  and near the air intake side opening of the housing  130 ). The bell mouth  135  includes a portion whose inner diameter becomes smaller toward the air discharge side. The bell mouth  135  is defined in the portion of the bell mouth defining member  136  other than the outer peripheral portion. The bell mouth defining member  136  is fixed to the housing  130  so that the bell mouth  135  can be coaxial with the hub  102 . 
     As shown in  FIG. 10 , the bell mouth defining member  136  has a substantially rectangular shape when seen in the center axis direction. Near the four corner areas of the air discharge side surface of the bell mouth defining member  136 , bosses  136   a  are arranged in a corresponding relationship with the four fixing portions  130   a  of the housing  130  to protrude toward the air discharge side (see  FIG. 8 ). The tip end surfaces of the bosses  136   a  make contact with the fixing portions  130   a . Through-holes  136   b  through which the screws can pass are defined in the center portions of the bosses  136   a . The bell mouth defining member  136  is fixedly secured to the housing  130  by inserting the screws through the through-holes  136   b  and fitting the screws to the thread holes  130   b . In this manner, the outer peripheral portion of the bell mouth defining member  136  serves to support the bell mouth  135 . 
     The bell mouth defining member  136  has a sidewall portion  136   c  extending toward the air discharge side from the entire peripheral edge portion of the air discharge side surface thereof. The air intake side end portion of the housing  130  is fitted to the inside of the sidewall portion  136   c . The sidewall portion  136   c  is fitted and secured to the opening of the attachment member  157 . The housing  130  is fixed to the attachment member  157  through the bell mouth defining member  136 . Alternatively, the housing  130  may be directly fixed to the attachment member  157 . 
     In the manner described above, the axial flow fan  101  is attached and fixed to the attachment member  157 . In the attached state, the bell mouth defining member  136  (the bell mouth  135 ) is positioned in the first path  158  and is opposed to the first partition wall  154 . On the other hand, the air discharge side opening of the housing  130  is positioned in the second path  159  and is opposed to the second partition wall  155 . 
     Despite the existence of the first partition wall  154  near the air intake side of the axial flow fan  101 , the air drawn through the inside of the bell mouth  135  (the inside of the portion having an inner diameter decreased toward the air discharge side) is guided by the bell mouth  135  toward the propeller blades  103  so that the air can smoothly flow in the center axis direction. This helps reduce separation of airflow caused by the increase in an attack angle of the propeller blades  103 . The outer peripheral portion of the bell mouth defining member  136  (including the sidewall portion  136   c ) serves to cover the air intake side end of the clearance space between the outer circumferential surface of the ring member  125  and the inner circumferential surface of the housing  130 . Therefore, even if the tip clearance is made greater, the air is prevented from flowing backwards from the air discharge side toward the air intake side through the clearance space. 
     In the present preferred embodiment, the minimum inner diameter of the bell mouth  135  (the inner diameter of the air discharge side end of the bell mouth  135 ) is preferably set to be equal to or smaller than the inner diameter of the air intake side end of the ring member  125 . This makes it possible to introduce all the air drawn through the inside of the bell mouth  135  into the ring member  125 . In order to smoothly draw the air into the ring member  125  as much as possible, it is preferred that the minimum inner diameter of the bell mouth  135  be equal to the inner diameter of the air intake side end of the ring member  125  or smaller than or approximate to the inner diameter of the air intake side end of the ring member  125 . 
     As shown in  FIG. 10 , a plurality of flow straightening members  137  extending in the radial direction to straighten the flow of the air drawn by the propeller blades  103  is arranged inside the bell mouth  135  at a regular interval along the circumferential direction (The number of the flow straightening members  137  may be equal to or may differ from the number of the propeller blades  103  although nine flow straightening members are preferably employed in the illustrated example). The radial outer ends of the flow straightening members  137  are fixed to the inner circumferential surface of the bell mouth  135 . The radial inner ends of the flow straightening members  137  are secured to the outer circumferential surface of a flat disc-shaped central portion  138  having an outer diameter substantially equal to that of the hub  102 . The flow straightening members  137  and the central portion  138  are preferably defined by a single monolithic piece together with the bell mouth  135  (the bell mouth defining member  136 ). When seen in the center axis direction, each of the flow straightening members  137  is curved against the rotating direction R as it extends from a radial inner end thereof toward a radial outer end thereof. The flow straightening members  137  act to reduce turbulence of the airflow drawn through the inside of the bell mouth  135 . 
     The guide portion  141 , by which the air discharged toward the air discharge side by the propeller blades  103  is guided to flow radially outwards, is arranged at the air discharge side of the propeller blades  103  (between the air discharge side surface of the base member  110  and the second partition wall  155 ). The guide portion  141  is defined by the conical surface of a conical guide member  142  fixed to the air discharge side surface of the base member  110 . The guide portion  141  obliquely extends radially outwards toward the air discharge side. The guide portion  141  is arranged to smoothly guide the air discharged by the propeller blades  103  to flow radially outwards. 
     A flow path defining member  143  cooperating with the guide portion  141  to define a flow path of the air discharged by the propeller blades  103  is arranged along the full perimeter of the air discharge side end portion of the outer circumferential surface of the housing  130 . The flow path is defined so that the cross-sectional area thereof can be gradually increased in the flow direction of the air. In other words, the guide portion  141  and the flow path defining member  143  serves as a diffuser, thereby increasing the static pressure at the air discharge side. 
     As mentioned earlier, the first partition wall  154  and the second partition wall  155  are respectively installed near the air intake side and the air discharge side of the axial flow fan  101 . At the air intake side, therefore, the air flows into the axial flow fan  101  from the radial direction. In the present preferred embodiment, however, the air drawn through the inside of the bell mouth  135  is guided by the bell mouth  135  to smoothly flow in the center axis direction. This makes it possible to reduce any separation of airflow caused by the increase in an attack angle of the propeller blades  103 . In cooperation with the bell mouth  135 , the ring member  125  serves to guide the drawn airflow. Thus, the air passing through the ring member  125  flows in the center axis direction. 
     Despite the fact that the tip clearance is increased to prevent the propeller blades  103  from becoming fixed to the inner circumferential surface of the housing  130  due to frost, it is possible to prevent the air from flowing backwards from the air discharge side toward the air intake side through the clearance space between the outer circumferential surface of the ring member  125  and the inner circumferential surface of the housing  130 . This is because the outer peripheral portion of the bell mouth defining member  136  serves to cover the air intake side end of the clearance space. 
     It is also possible for the ring member  125  to prevent the air flowing through the inside of the ring member  125  from leaking radially outwards from the outer peripheral ends of the propeller blades  103 . Since the ring member  125  does not exist in the air discharge side end portions of the propeller blades  103 , the air is leaked radially outwards from the outer peripheral ends of the propeller blades  103 . Due to the presence of the guide portion  141 , however, the airflow discharged by the propeller blades  103  is smoothly curved radially outwards. 
     Accordingly, even if the axial flow fan  101  is arranged in the narrow fan arrangement space  152  and even when the tip clearance is made greater, it is possible to suppress an impairment of the PQ characteristic of the axial flow fan  101  and to suppress the increase in noises. 
     The present invention is not limited to the preferred embodiments described above but may be modified without departing from the scope of the invention defined in the claims. For instance, as shown in  FIG. 11 , a plurality of static vanes  131 ′ may be provided at a regular interval along the circumferential direction, instead of the support legs  131  extending in the radial direction. In this case, it becomes possible to increase the static pressure at the air discharge side. When seen in the center axis direction, each of the static vanes  131 ′ is curved against the rotating direction R as it extends from a radial inner end thereof toward a radial outer end thereof. 
     While the ring member  125  preferably extends straight in the center axis direction in the preferred embodiment described just above, the diameter of the air intake side portion of the ring member  125  may be increased toward the air intake side. This makes it easy to introduce the air drawn through the bell mouth  135  into the ring member  125 . In cooperation with the outer peripheral portion of the bell mouth defining member  136 , the increased diameter portion of the ring member  125  serves to cover the air intake side end of the clearance space between the outer circumferential surface of the ring member  125  and the inner circumferential surface of the housing  130 . Accordingly, it is possible to effectively prevent the air from flowing backwards through the clearance space. 
     While the flow straightening members  137  are provided preferably inside the bell mouth  135  in the preferred embodiment described just above, the flow straightening members  137  are not essential and may be omitted if so desired. In case of providing the flow straightening members  137 , the shape of the flow straightening members  137  is not limited to the shape illustrated and described in the preferred embodiments. For example, as shown in  FIG. 13 , it would also be possible to provide four flow straightening members  137  in a cross shape (in which case the central portion  138  included in the foregoing preferred embodiment does not exist). 
     The housing  130  may be omitted. In this case, as shown in  FIG. 14  by way of example, the base member  110  and the bell mouth defining member  136  are interconnected by a plurality of connecting members  147  arranged at the air discharge side and the radial outer side of the propeller blades  103  in a circumferentially spaced-apart relationship with one another. Only the connecting members  147  exist at the radial outer side of the propeller blades  103 . The base member  110  is held by the attachment member  157  through the bell mouth defining member  136  and the connecting members  147 . The portions  147   a  of the connecting members  147  positioned at the air discharge side of the propeller blades  103  may be allowed to serve as static vanes (like the static vanes  131 ′ shown in  FIG. 11 ). If the housing  130  is omitted in this manner, only the connecting members  147  exist at the radial outer side of the propeller blades  103 . There exists no flow path through which the air can flow backwards. Even if the housing  130  is omitted in this manner, the ring member  125  causes the air to flow toward the air discharge side without leaking radially outwards from the outer peripheral ends of the propeller blades  103 . 
     An axial flow fan like the one described above in respect of the foregoing preferred embodiment (not including the guide portion  141 , the flow path defining member  143  and the flow straightening members  137 ) was prepared to evaluate the change of the static pressure against the flow rate (air flow rate), i.e., the PQ characteristic, at the air discharge side and the change of the noise level against the flow rate (air flow rate), i.e., the noise characteristic, at the air discharge side. The results of evaluation are shown in  FIG. 15 . The fan (A) shown in  FIG. 15  is an axial flow fan like the one described above in respect of the foregoing preferred embodiment. The fan (A) is arranged in a narrow space (like the fan arrangement space  152 ) between walls existing at the air intake side and the air discharge side thereof. 
     For the purpose of comparison, axial flow fans (B) and (C) that do not include the ring member  125  and the bell mouth  135  (the bell mouth defining member  136 ) were prepared to evaluate the PQ characteristic and the noise characteristic thereof. The results of evaluation are all shown in  FIG. 15 . The tip clearance of the fan (B) is smaller than that of the fan (A) mentioned above. Other configurations of the fan (B) remain the same as those of the fan (A). The fan (B) is arranged in a broad space with no wall existing at the air intake side and the air discharge side thereof. The tip clearance of the fan (C) is substantially equal to that of the fan (A) mentioned above. Other configurations of the fan (C) remain the same as those of the fan (A). Just like the fan (A), the fan (C) is arranged in a narrow space between walls existing at the air intake side and the air discharge side thereof. 
     As can be seen in  FIG. 15 , if the fan (C) including an increased tip clearance but not provided with the ring member  125  and the bell mouth  135  is arranged in a narrow space, the PQ characteristic thereof is bitterly impaired as compared with the fan (B) including a reduced tip clearance and arranged in a broad space. The noise level of the fan (C) becomes greater than that of the fan (B) if the flow rate exceeds a certain value. In the fan (A) provided with the ring member  125  and the bell mouth  135 , the maximum flow rate thereof is smaller than that of the fan (C). However, it can be seen that, as compared with the fan (C), the static pressure of the fan (A) is increased and the noise level of the fan (A) is decreased in the flow rate sections other than maximum flow rate section. In particular, the static pressure and the noise level of the fan (A) are equal to those of the fan (B) in the low flow rate section excluding the zero flow rate point. Accordingly, it can be appreciated that the characteristics of a fan are greatly enhanced by the ring member  125  and the bell mouth  135 . 
     For evaluation of the effects provided by the ring member  125 , the fan (A) was compared with a fan (D) differing from the fan (A) only in that the fan (D) is not provided with the ring member  125 . The fans (A) and (D) are all arranged in narrow spaces like the fan arrangement space  152  mentioned above. The results of evaluation of the PQ characteristics and the noise characteristics of the fans (A) and (D) are shown in  FIG. 16 . It can be seen in  FIG. 16  that, as compared with the fan (D), the static pressure of the fan (A) is increased and the noise level of the fan (A) is decreased in most flow rate sections other than some sections. 
     For evaluation of the effects provided by the bell mouth  135 , the fan (A) was compared with fans (E) through (G), all of which are not provided with the bell mouth  135 . The fan (E) differs from the fan (A) only in that the fan (E) is not provided with the bell mouth  135  (the bell mouth defining member  136 ). The fan (F) differs from the fan (A) in that the fan (F) is not provided with the bell mouth  135 . Moreover, the fan (F) differs from the fan (A) in the position of the ring member  125  in the longitudinal direction of the propeller blades  103 . In the fan (F), the ring member  125  is installed in the position spaced apart from the roots of the propeller blades  103  by about 80% of the length of the propeller blades  103 . The fan (G) differs from the fan (A) in that the fan (G) is not provided with both the bell mouth  135  and the ring member  125 . All the fans (E) through (G) are arranged in narrow spaces like the fan arrangement space  152  mentioned above. 
     The results of evaluation of the PQ characteristics and the noise characteristics of the fan (A) and the fans (E) through (G) are shown in  FIG. 17 . It can be seen in  FIG. 17  that, in the fan (A), the maximum flow rate thereof is smaller than those of the fans (E) through (G). However, as compared with the fans (E) through (G), the static pressure of the fan (A) is increased and the noise level of the fan (A) is decreased in the flow rate sections other than maximum flow rate section. In particular, the static pressure of the fan (A) is sharply increased in the low flow rate section excluding the zero flow rate point. 
     For evaluation of the effects provided by the flow straightening members  137 , the fan (A) was compared with a fan (H) in which the flow straightening members  137  shown in  FIG. 13  are provided inside the bell mouth  135  of the fan (A) and a fan (I) in which the flow straightening members  137  shown in  FIG. 10  are provided inside the bell mouth  135  of the fan (A). The results of evaluation of the PQ characteristics and the noise characteristics of the fans (A), (H) and (I) are shown in  FIG. 18 . It can be seen in  FIG. 18  that, in the PQ characteristic, the static pressure at the zero flow rate point (cut-off point) is increased if the flow straightening members  137  are provided inside the bell mouth  135 . This is because the turbulence of the airflow drawn through the inside of the bell mouth  135  is mitigated by the flow straightening members  137 . 
     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.