Patent Publication Number: US-11391295-B2

Title: Propeller fan

Description:
CROSS-REFERENCE 
     This application claims priority from Japanese Patent Application No. 2017-101028 and Japanese Patent Application No. 2017-101029 filed with the Japan Patent Office on May 22, 2017, the entire content of which is hereby incorporated by reference. 
     FIELD 
     The present disclosure relates to a propeller fan. 
     BACKGROUND ART 
     For example, an air conditioner has, in an outdoor unit thereof, a propeller fan. An air velocity at the propeller fan is faster at a blade outer peripheral portion, and decreases toward the center of rotation. In recent years, the volume of air from the propeller fan has been improved for improvement of energy saving performance of the air conditioner. Specifically, the size of the propeller fan has been increased, and the speed of rotation of the propeller fan has been increased, for example. 
     Note that the technique of this area is disclosed in Japanese Laid-open Patent Publication No. 2010-101223, PCT International Application Publication No. WO 2011/001890 A. Japanese Translation of PCT International Application Publication No. JP-T-2003-503643, and Japanese Laid-open Patent Publication No. 2004-116511, for example. 
     SUMMARY 
     A propeller fan includes: a hub having a side surface about a center axis; and a plurality of blades provided on the side surface of the hub. Each blade includes an inner peripheral portion positioned closer to a base portion of the each blade connected to the hub, and an outer peripheral portion positioned closer to an outer edge of the each blade. A ratio r/R between a radius r as a distance from the center axis to a boundary between the inner peripheral portion and the outer peripheral portion and a radius R as a distance from the center axis to the outer edge of each blade is equal to or lower than 0.4. A relational expression of V 1 &lt;V 2 ×1.3 is satisfied, where an air velocity at the outer peripheral portion is V 1  and an air velocity at the inner peripheral portion is V 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an outdoor unit having a propeller fan according to a first embodiment (second and third embodiments); 
         FIG. 2  is a schematic plan view of a fan according to the first embodiment (the second embodiment) as viewed from a positive pressure side; 
         FIG. 3  is a schematic perspective view of the propeller fan according to the first embodiment; 
         FIG. 4  is a schematic perspective view of the propeller fan according to the second embodiment; 
         FIG. 5  illustrates P-Q curves; 
         FIG. 6  is a plan view of the propeller fan according to the third embodiment as viewed from the positive pressure side; 
         FIG. 7  is a plan view of one of blades of the propeller fan according to the third embodiment as viewed from the positive pressure side; 
         FIG. 8  is a perspective view of the periphery of bases of the blades of the propeller fan according to the third embodiment as viewed from the positive pressure side; 
         FIG. 9  is a plan view of the propeller fan according to the third embodiment as viewed from a negative pressure side; 
         FIG. 10  is a perspective view of one of the blades of the propeller fan according to the third embodiment as viewed from the negative pressure side; 
         FIG. 11  is a side view of the propeller fan according to the third embodiment; 
         FIG. 12  is a perspective view of the propeller fan according to the third embodiment; 
         FIG. 13  is a perspective view of one of the blades of the propeller fan according to the third embodiment; 
         FIG. 14  schematically illustrates the chord length of each blade element and the total chord length of the blade elements; 
         FIG. 15  illustrates graphs of a relationship of a radius ratio with an air volume and an efficiency; and 
         FIG. 16  illustrates graphs of a relationship of a blade element minimum/total chord length with the air volume and the efficiency. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     In a typical technique, air velocity distribution in a radial direction at a blade is non-uniform. For this reason, a surging phenomenon such as suction of air from a downstream side occurs at an inner peripheral portion of the blade, leading to an abnormal operation state. In the case of using a propeller fan for an outdoor unit, there is probability that the surging phenomenon leads to noise and damage of the propeller fan. Moreover, each inner peripheral portion of the propeller fan with a lower air velocity does not much contribute to air blowing. For this reason, it can be said that the amount of blown air with respect to the size of the propeller fan is smaller and each blade surface is not effectively used. 
     An object of the present disclosure is to provide a propeller fan and an outdoor unit of an air conditioner which are configured so that a difference (an air velocity difference) between an air velocity at an outer peripheral portion and an air velocity at an inner peripheral portion in a blade can be reduced and the volume of air from the propeller fan can be improved. 
     A propeller fan according to one aspect of the present disclosure includes: a hub having a side surface about a center axis; and a plurality of blades provided on the side surface of the hub. Each blade includes an inner peripheral portion positioned closer to a base portion of the each blade connected to the hub, and an outer peripheral portion positioned closer to an outer edge of the each blade. A ratio r/R between a radius r as a distance from the center axis to a boundary between the inner peripheral portion and the outer peripheral portion and a radius R as a distance from the center axis to the outer edge of each blade is equal to or lower than 0.4. A relational expression of V 1 &lt;V 2 ×1.3 is satisfied, where an air velocity at the outer peripheral portion is V 1  and an air velocity at the inner peripheral portion is V 2 . 
     According to one aspect of the present disclosure, the difference between the air velocity at the blade outer peripheral portion and the air velocity at the blade inner peripheral portion (a blade center portion) can be reduced while the volume of air from the propeller fan can be improved. 
     Embodiments of the present disclosure will be described in detail below with reference to the drawings. Various embodiments described below are not intended to limit the technique of the present disclosure. Moreover, various embodiments described below may be, as necessary, implemented in combination within a consistent range. Note that description of already-described elements is not repeated. 
     First Embodiment 
     (Configuration of Outdoor Unit) 
       FIG. 1  is a schematic view of an outdoor unit having a propeller fan according to a first embodiment. As illustrated in  FIG. 1 , an outdoor unit  1  of the first embodiment is an outdoor unit of an air conditioner. The outdoor unit  1  has a housing  6 . The housing  6  houses therein a compressor  3  configured to compress refrigerant, a heat exchanger  4  coupled to the compressor  3  and configured such that the refrigerant flows, and a propeller fan  5 A configured to send air to the heat exchanger  4 . 
     The housing  6  has suction openings  7  for taking in ambient air, and a blowing opening  8  for discharging air from the housing  6 . The suction openings  7  are each provided at a side surface  6   a  and a back surface  6   c  of the housing  6 . The blowing opening  8  is provided at a front surface  6   b  of the housing  6 . The heat exchanger  4  is disposed across the side surface  6   a  and the back surface  6   c  facing the front surface  6   b  of the housing  6 . The propeller fan  5 A is disposed facing the blowing opening  8 , and is rotatably driven by a fan motor (not shown). In description below, a side in the direction of air discharged from the blowing opening  8  by rotation of the propeller fan  5 A is a positive pressure side, and the opposite side thereof is a negative pressure side. 
     (Propeller Fan of First Embodiment) 
       FIG. 2  is a schematic plan view of the propeller fan according to the first embodiment as viewed from the positive pressure side. As illustrated in  FIG. 2 , the propeller fan  5 A according to the first embodiment has a hub  11  with a circular columnar (or polygonal columnar) outer appearance, and a plurality of blades  12 A. The plurality of blades  12 A are provided on a side surface  11   a  provided about the center axis of the hub  11 . The hub  11  and the plurality of blades  12 A are integrally molded using a molding material such as a resin material. The blade will be also called a vane. The hub  11  is formed in a circular columnar shape. The hub  11  has, at a position on the center axis O, a boss (not shown) onto which a shaft (not shown) of the fan motor is fitted. The hub  11  rotates, in association with rotation of the fan motor, about the center axis O of the hub  11  as viewed in plane in the direction of “R” illustrated in the figure. The boss (not shown) is provided on the negative pressure side (see  FIG. 3 ). The plurality of (three in an example of  FIG. 2 ) blades  12 A are formed integrally with the hub  11  at predetermined intervals along a circumferential direction of the hub  11  on the side surface  11   a  of the hub  11 . Each blade  12 A is formed in a plate shape. 
     In  FIG. 2 , the propeller fan  5 A has, as viewed in plane, inner peripheral portions  12 Aa and outer peripheral portions  12 Ab of the blades  12 A. The inner peripheral portions  12 Aa are positioned within the circumference of a circle with a radius r 1  about the center axis O. The outer peripheral portions  12 Ab are positioned outside the circumference of the circle with the radius r 1  about the center axis O and within the circumference of a circle with a radius R 1  about the center axis O. As illustrated in  FIG. 2 , the outer peripheral portion  12 Ab extending in a radial direction of the hub  11  is formed with a blade area larger than that of the inner peripheral portion  12 Aa coupled to the hub  11 . A ratio r 1 /R 1  (hereinafter referred to as a “radius ratio”) between the radius r 1  and the radius R 1  as described herein satisfies Expression (1) below.
 
 r 1/ R 1≤0.4  (1)
 
     For example, a radius ratio r 1 /R 1  of 0.4 means that a boundary between the inner peripheral portion  12 Aa and the outer peripheral portion  12 Ab of the blade  12 A as defined by the radius r 1  from the center axis O is at a position with a length from the center axis O, the length being 0.4 times as long as the radius R 1 . Note that in the present embodiment, r 1 =88 [mm] (ϕ=176) and R 1 =220 [mm] (ϕ=440) are satisfied by way of example. 
     Moreover, in  FIG. 2 , the propeller fan  5 A has, in each inner peripheral portion  12 Aa of the blades  12 A, blade elements  12 A- 11 ,  12 A- 12  as viewed in plane. Further, in  FIG. 2 , the propeller fan  5 A has, as viewed in plane, a hole  12 A- 21  between the blade element  12 A- 11  and the blade element  12 A- 12  in each inner peripheral portion  12 Aa of the blades  12 A. The hole  12 A- 21  is provided in abutting contact with the boundary (the position with the radius r 1  from the center axis O) between the inner peripheral portion  12 Aa and the outer peripheral portion  12 Ab. That is, each blade  12 A is connected to the hub  11  such that a base portion  12 A- 11   a  of the blade element  12 A- 11  and a base portion  12 A- 12   a  of the blade element  12 A- 12  form the hole  12 A- 21  in the inner peripheral portion  12 Aa. Each outer peripheral portion  12 Ab is formed continuously from the blade element  12 A- 11  and the blade element  12 A- 12 . The inner peripheral portion  12 Aa and the outer peripheral portion  12 Ab form a single blade surface. In the present embodiment, the base portion  12 A- 11   a  and the base portion  12 A- 12   a  are a base portion described in the CLAIMS. That is, the base portion  12 A- 11   a  and the base portion  12 A- 12   a  are portions of the blade  12 A, the portions being connected to the hub  11 . 
     In other words, the two blade elements  12 A- 11 ,  12 A- 12  are formed in such a manner that the blade  12 A is branched from the outer peripheral portion  12 Ab of the blade  12 A while extending toward the inner peripheral portion  12 Aa of the blade  12 A. The hole  12 A- 21  between the blade element  12 A- 11  and the blade element  12 A- 12  serves as a flow passage of an air current passing through the propeller fan  5 A. 
       FIG. 3  is a schematic perspective view of the propeller fan according to the first embodiment.  FIG. 3  is a schematic enlarged perspective view of one of the plurality of blades  12 A illustrated in  FIG. 2 . As illustrated in  FIG. 3 , the blade element  12 A- 12  positioned on an upstream side (a back edge side) in a rotation direction (the direction of “R” in the figure) is, in each blade  12 A, connected to the hub  11  on the positive pressure side with respect to the blade element  12 A- 11  positioned on a downstream side (a front edge side). Moreover, the hole  12 A- 21  of each blade  12 A is positioned between the blade element  12 A- 12  and the blade element  12 A- 11  in a center axis O direction and the circumferential direction. 
     In a case where a maximum air velocity at the outer peripheral portion  12 Ab upon rotation of the propeller fan  5 A is V 1  [m/s] and a maximum air velocity at the inner peripheral portion  12 Aa upon rotation of the propeller fan  5 A is V 2  [m/s], Expression (2) below is satisfied.
 
 V 1&lt; V 2×1.3  (2)
 
     In other words, an air velocity ratio V 1 /V 2  as the ratio of the air velocity V 1  at the outer peripheral portion  12 Ab to the air velocity V 2  at the inner peripheral portion  12 Aa satisfies Expression (3) below. Expression (3) is obtained by deformation of Expression (2).
 
 V 1/ V 2&lt;1.3  (3)
 
     Note that the number of blade elements  12 A- 11 ,  12 A- 12  and the number of holes  12 A- 21  in the blade  12 A of the first embodiment are not limited to those illustrated in  FIGS. 2 and 3 . The blade  12 A may have three or more blade elements and two or more holes. That is, the outer peripheral portion  12 Ab may be formed (configured) as a single blade surface (e.g., a blade surface with no hole), and the inner peripheral portion  12 Aa may include plurality of blade elements disposed at predetermined intervals. 
     Second Embodiment 
     (Propeller Fan of Second Embodiment) 
       FIG. 4  is a schematic perspective view of a propeller fan according to a second embodiment. A propeller fan  5 B according to the second embodiment is housed in an outdoor unit  1  illustrated in  FIG. 1  as in the propeller fan  5 A according to the first embodiment. Moreover, a schematic plan view of the propeller fan  5 B as viewed from the positive pressure side is similar to the equivalent plan view of the propeller fan  5 A according to the first embodiment illustrated in  FIG. 2 . Thus, in  FIG. 2 , each reference numeral of the propeller fan  5 B and components according to the second embodiment is described in parentheses. 
       FIG. 4  is a schematic enlarged perspective view of one of a plurality of blades  12 B illustrated in  FIG. 2 . As illustrated in  FIG. 4 , each blade  12 B has an inner peripheral portion  12 Ba, an outer peripheral portion  12 Bb, a blade element  12 B- 11 , a blade element  12 B- 12 , a base portion  12 B- 11   a , a base portion  12 B- 12   a , and a hole  12 B- 21  similar to the inner peripheral portion  12 Aa, the outer peripheral portion  12 Ab, the blade element  12 A- 11 , the blade element  12 A- 12 , the base portion  12 A- 11   a , the base portion  12 A- 12   a , and the hole  12 A- 21  of the blade  12 A. Note that in the blade  12 B, the blade element  12 B- 12  positioned on an upstream side in a rotation direction (the direction of “R” in the figure) and the blade element  12 B- 11  positioned on a downstream side are connected to the same height position of a hub  11  in a center axis O direction. 
     As in the blade  12 A according to the first embodiment, the blade  12 B according to the second embodiment also satisfies Expressions (1) to (3) as described above. 
     Note that the number of blade elements  12 B- 11 ,  12 B- 12  and the number of the holes  12 B- 21  in the blade  12 B according to the second embodiment are not limited to those illustrated in  FIGS. 2 and 4 . The blade  12 B may have three or more blade elements and two or more holes. That is, the outer peripheral portion  12 Bb may be formed (configured) as a single blade surface (e.g., a blade surface with no hole), and the inner peripheral portion  12 Ba may include a plurality of blade elements disposed at predetermined intervals. 
     (Relationship Between Air Volume and Static Pressure and Relationship Between Radius Ratio and Air Velocity Ratio) 
       FIG. 5  illustrates P-Q curves.  FIG. 5  illustrates the basis of a radius ratio of equal to or lower than 0.4 and an air velocity ratio V 1 /V 2  of equal to or lower than 1.3 in the propeller fans of the first and second embodiments. In  FIG. 5 , an air volume Q [m 3 /h] is the horizontal axis, and an air pressure P [Pa] is the vertical axis. 
       FIG. 5  illustrates the P-Q curves in the case of air velocity ratios V 1 /V 2  of 1.1, 1.2, 1.24, 1.3, and 1.5. In  FIG. 5 , the P-Q curve in the case of an air velocity ratio V 1 /V 2  of 1.5 corresponds to a typical propeller fan having no blade element in each inner peripheral portion. The P-Q curves in the case of air velocity ratios V 1 /V 2  of 1.1, 1.2, 1.24, and 1.3 correspond to the propeller fan  5 A ( 5 B) having the plurality of blade elements  12 A- 11 ,  12 A- 12  ( 12 B- 11  and  12 B- 12 ) in each inner peripheral portion  12 Aa ( 12 Ba). In the propeller fan corresponding to each type of data, the chord length (the length of a straight line connecting one end and the other end of the blade element in a longitudinal direction of a section) of each of the blade elements  12 A- 11 ,  12 A- 12  ( 12 B- 11  and  12 B- 12 ) is adjusted such that the air velocity ratio V 1 /V 2  reaches the above-described numerical value. The propeller fan with an air velocity ratio V 1 /V 2  of 1.5 exhibits P-Q curve properties with a local minimum value and a local maximum value of a cubic curve. This means occurrence of a surging phenomenon (see a portion surrounded by a dashed circle in  FIG. 5 ). 
     The surging phenomenon described herein occurs when a blowing capacity at the inner peripheral portion  12 Aa reaches lower than that of the outer peripheral portion  12 Ab and a difference (an air velocity difference) between an air velocity at the inner peripheral portion  12 Aa and an air velocity at the outer peripheral portion  12 Ab increases in the blade  12 A. The surging phenomenon occurs within such a flow rate range that the P-Q properties of the propeller fan show the local minimum value and the local maximum value of the cubic curve. The surging phenomenon is a phenomenon that the pressure and flow rate of air become instable and greatly change within the above-described flow rate range. When the propeller fan is operated within the flowrate range leading to such a phenomenon, vibration and/or a backflow occur. As a result, it is, due to occurrence of noise and/or pressure pulsation, difficult to perform normal operation. 
     On the other hand, in the case of the air velocity ratio V 1 /V 2 ≤1.3, a lower air velocity ratio V 1 /V 2  results in a gentler P-Q curve. Thus, the surging phenomenon does not occur, and the air volume can be improved. 
     From the above, it has been found that a surging region is caused depending on a blade shape in the case of an air velocity ratio V 1 /V 2  of equal to or higher than 1.3. On the other hand, it has been found that occurrence of the surging region can be reduced regardless of the blade shape in the case of an air velocity ratio V 1 /V 2  of lower than 1.3. 
     Note that in a relationship between the air volume [m 3 /h] and an input [W], input power (power applied to a not-shown fan motor for driving the propeller fan) for outputting the same air volume is smaller in the propeller fans according to the first and second embodiments with air velocity ratios V 1 /V 2  of 1.1, 1.2, 1.24, and 1.3 as compared to the typical propeller fan with an air velocity ratio V 1 /V 2  of 1.5. Moreover, in the case of the same input power, a higher air velocity ratio V 1 /V 2  results in a greater air volume. In a relationship between the air volume [m 3 /h] and the number of rotations [rpm], the number of rotations for the same air volume is smaller in the propeller fans according to the first and second embodiments with air velocity ratios V 1 /V 2  of 1.1, 1.2, 1.24, and 1.3 as compared to the propeller fan with an air velocity ratio V 1 /V 2  of 1.5. Moreover, a higher air velocity ratio V 1 /V 2  results in a greater air volume. 
     As described above, as long as the propeller fans  5 A,  5 B satisfy two conditions of the radius ratio r 1 /R 1 ≤0.4 and V 1 &lt;V 2 ×1.3 (or V 1 /V 2 &lt;1.3) in the first and second embodiments, occurrence of surging can be reduced. 
     Third Embodiment 
       FIG. 6  is a plan view of a propeller fan according to a third embodiment as viewed from the positive pressure side.  FIG. 7  is a plan view of one of blades of the propeller fan according to the third embodiment as viewed from the positive pressure side.  FIG. 8  is a perspective view of the periphery of bases of the blades of the propeller fan according to the third embodiment as viewed from the positive pressure side. Moreover,  FIG. 9  is a plan view of the propeller fan according to the third embodiment as viewed from the negative pressure side.  FIG. 10  is a perspective view of one of the blades of the propeller fan according to the third embodiment as viewed from the negative pressure side. 
     Moreover,  FIG. 11  is a side view of the propeller fan according to the third embodiment.  FIG. 12  is a perspective view of the propeller fan according to the third embodiment.  FIG. 13  is a perspective view of one of the blades of the propeller fan according to the third embodiment.  FIG. 14  is a schematic view of the chord length of each blade element and the total chord length of the blade elements. Note that a propeller fan  5 C according to the third embodiment is housed in an outdoor unit  1  illustrated in  FIG. 1  as in the propeller fan  5 A according to the first embodiment and the propeller fan  5 B according to the second embodiment. 
     As illustrated in  FIGS. 6 to 14 , the propeller fan  5 C according to the third embodiment has a circular columnar hub  11  and a plurality of blades  12 C provided on a side surface of the hub  11 . The hub  11  and the plurality of blades  12 C are integrally molded using a molding material such as a resin material. The plurality of (five in an example of the third embodiment) blades  12 C are formed integrally with the hub  11  at predetermined intervals along a circumferential direction of the hub  11  on the side surface  11   a  of the hub  11 . Each blade  12 C is formed in a plate shape. 
     In  FIG. 6 , the propeller fan  5 C has, as viewed in plane, inner peripheral portions  12 Ca and outer peripheral portions  12 Cb of the blades  12 C. The inner peripheral portions  12 Ca are positioned within the circumference of a circle with a radius r 3  about the center axis O. The outer peripheral portions  12 Cb are positioned outside the circumference of the circle with the radius r 3  about the center axis O and within the circumference of the circle of the propeller fan  5 C with a radius R 3 . As illustrated in  FIG. 6 , the outer peripheral portion  12 Cb extending in a radial direction of the hub  11  is formed with a blade area larger than that of the inner peripheral portion  12 Ca coupled to the hub  11 . In each blade  12 C, a back edge portion  12 C- 1  on an upstream side in a rotation direction (the direction of “R” illustrated in  FIG. 6 ) of the blade  12 C is formed to curve toward a front edge portion  12 C- 2  positioned on the opposite side of the back edge portion  12 C- 1  (also see  FIG. 11 ). The back edge portion  12 C- 1  curves as viewed from the direction of the center axis O as a rotation axis. 
     A surface (a blade surface) of each blade  12 C is formed to gently curve from the negative pressure side to the positive pressure side of the propeller fan  5 C while extending from the back edge portion  12 C- 1  to the front edge portion  12 C- 2  in the circumferential direction of the hub  11  (see, e.g.,  FIG. 9 ). When the propeller fan  5 C provided with the above-described blades  12 C rotates in the R-direction (the direction of “R” illustrated in  FIG. 6 ), air flows from the negative pressure side to the positive pressure side. The volume of air flowing from the negative pressure side to the positive pressure side increases as the number of rotations of the propeller fan  5 C increases. 
     A ratio r 3 /R 3  (a radius ratio) between the radius r 3  and the radius R 3  as described herein satisfies Expression (4) below.
 
 r 3/ R 3≤0.7  (4)
 
     For example, a radius ratio r 3 /R 3  of 0.7 means that a boundary between the inner peripheral portion  12 Ca and the outer peripheral portion  12 Cb of the blade  12 C as defined by the radius r 3  from the center axis O is at a position with a length from the center axis O, the length being 0.7 times as long as the radius R 3 . 
     As illustrated in  FIGS. 8 to 14 , the propeller fan  5 C has, in each inner peripheral portion  12 Ca of the blades  12 C, three blade elements  12 C- 11 ,  12 C- 12 ,  12 C- 13 . Further, as specifically illustrated in  FIG. 8 , the propeller fan  5 C has a hole  12 C- 21  between the blade element  12 C- 11  and the blade element  12 C- 12  in each inner peripheral portion  12 Ca of the blades  12 C, for example. In addition, the propeller fan  5 C has a hole  12 C- 22  between the blade element  12 C- 12  and the blade element  12 C- 13  in each inner peripheral portion  12 Ca of the blades  12 C. That is, each blade  12 C is connected to the hub  11  such that a base portion  12 C- 11   a  of the blade element  12 C- 11 , a base portion  12 C- 12   a  of the blade element  12 C- 12 , and a base portion  12 C- 13   a  of the blade element  12 C- 13  form the holes  12 C- 21 ,  12 C- 22  in the inner peripheral portion  12 Ca. Each outer peripheral portion  12 Cb is formed continuously from the blade elements  12 C- 11 ,  12 C- 12 ,  12 C- 13 . The inner peripheral portion  12 Ca and the outer peripheral portion  12 Cb form a single blade surface. In the present embodiment, the base portion  12 C- 11   a , the base portion  12 C- 12   a , and the base portion  12 C- 13   a  are a base portion described in the CLAIMS. That is, the base portion  12 C- 11   a , the base portion  12 C- 12   a , and the base portion  12 C- 13   a  are portions of the blade  12 C, the portions being connected to the hub  11 . 
     In other words, the three blade elements  12 C- 11 ,  12 C- 12 ,  12 C- 13  are formed in such a manner that the blade  12 C is branched from the outer peripheral portion  12 Cb of the blade  12 C while extending toward the inner peripheral portion  12 Ca of the blade  12 C. The hole  12 C- 21  between the blade element  12 C- 11  and the blade element  12 C- 12  and the hole  12 C- 22  between the blade element  12 C- 12  and the blade element  12 C- 13  serve as flow passages of an air current passing through the propeller fan  5 C. 
     For example, as illustrated in  FIGS. 7 and 8 , the base portion  12 C- 13   a  of the blade element  12 C- 13  positioned on the most upstream side (the most back edge side) in the rotation direction (the direction of “R” in the figure) is, in each blade  12 C, connected to the hub  11  on the positive pressure side in a center axis O direction as compared to the base portion  12 C- 12   a  of the blade element  12 C- 12  and the base portion  12 C- 11   a  of the blade element  12 C- 1  positioned on a downstream side (a front edge side). Moreover, the base portion  12 C- 12   a  of the blade element  12 C- 12  is connected on the positive pressure side in the center axis O direction of the hub  11  with respect to the base portion  12 C- 11   a  of the blade element  12 C- 11 . Further, the hole  12 C- 21  of the blade  12 C is positioned between the blade element  12 C- 12  and the blade element  12 C- 11  in the center axis O direction and the circumferential direction. The hole  12 C- 22  of the blade  12 C is positioned between the blade element  12 C- 13  and the blade element  12 C- 12  in the center axis O direction and the circumferential direction. 
     When the total chord length of the inner peripheral portion  12 Ca as the total of the chord lengths of the blade elements  12 C- 11  to  12 C- 13  is L 0  [mm] and the minimum one of the chord lengths (the length of a straight line connecting one end and the other end of the blade element in a longitudinal direction of a section) of the blade elements  12 C- 11  to  12 C- 13  is Lmin [mm], Expression (5) below is satisfied.
 
 L  min/ L 0≥0.1  (5)
 
     Suppose that as illustrated in  FIG. 14 , the chord lengths of the blade elements  12 C- 11  to  12 C- 13  are each L 1  [mm], L 2  [mm], and L 3  [mm] and a magnitude relationship of L 1 &lt;L 2 &lt;L 3  is satisfied. In this case, Lmin=L 1  and L 0 =L 1 +L 2 +L 3  are satisfied, and L 1 /(L 1 +L 2 +L 3 )≥0.1 is satisfied from Expression (5) as described above. 
       FIGS. 6 to 14  illustrate such a form that the holes  12 C- 21 ,  12 C- 22  extend to the hub  11 . However, when Expressions (4) to (6) as described above are satisfied, the shapes, forms, and the like of the holes  12 C- 21 ,  12 C- 22  are changeable as necessary. For example, a form can be employed, in which the holes  12 C- 21 ,  12 C- 22  reach positions apart from the hub  11  with predetermined distances. 
     As will be described later, in the third embodiment, as long as the propeller fan  5 C satisfies conditions of the radius ratio r 3 /R 3 ≤0.7 and Lmin/L 0 ≥0.1, surging is less caused, and an air volume can be improved. 
     Note that the number of blade elements  12 C- 11  to  12 C- 13  and the number of holes  12 C- 21 ,  12 C- 22  in the blade  12 C of the third embodiment are not limited to those illustrated in  FIGS. 8 to 13 . The blade  12 C may have two blade elements and a single hole. Alternatively, the blade  12 C may has four or more blade elements and three or more holes. That is, the outer peripheral portion  12 Cb may be configured as a single blade surface, and the inner peripheral portion  12 Ca may include at least one hole and a plurality of blade elements formed to sandwich the hole. The holes  12 C- 21 ,  12 C- 22  may be formed within an area from the boundary between inner peripheral portion  12 Ca and the outer peripheral portion  12 Cb to the side surface of the hub  11  in the radial direction. Alternatively, the holes  12 C- 21 ,  12 C- 22  may be formed in abutting contact with both of the above-described boundary and the side surface of the hub  11 . 
     (Relationship of Radius Ratio with Air Volume and Efficiency and Relationship of Blade Element Minimum/Total Chord Length with Air Volume and Efficiency) 
       FIG. 15  illustrates graphs (curves) of a relationship of the radius ratio with the air volume and an efficiency.  FIG. 16  illustrates graphs (curves) of a relationship of the blade element minimum/total chord length with the air volume and the efficiency.  FIG. 15  shows the basis of a radius ratio of equal to or lower than 0.7 in the third embodiment. Moreover,  FIG. 16  shows the basis of a blade element minimum/total chord length of equal to or longer than 0.1 in the third embodiment. 
     In  FIG. 15 , the radius ratio is the horizontal axis, and the air volume Q [m 3 /h] and the efficiency η(=the air volume Q/an input) [m 3 /h/W] are the vertical axes. In  FIG. 15 , the air volume Q 11  and the efficiency η 11  correspond to an air volume and an efficiency when the propeller fan  5 C rotates with a rated load of an air conditioner. On the other hand, the air volume Q 12  and the efficiency η 12  correspond to an air volume and an efficiency when the propeller fan  5 C rotates with a higher load than the rated load of the air conditioner. In any of the rated load state and the high load state, it is preferable that the efficiencies η 11 , η 12  do not extremely decrease from peak values. 
     In  FIG. 15 , the efficiencies η 11 , η 12  show the peak values thereof at the radius ratio r 3 /R 3 ≤0.4 to 0.5. Thus, in the rated load state, when the radius ratio r 3 /R 3 ≤0.7 is satisfied, the efficiency η 11  of the propeller fan  5 C falls within a range from the peak value to equal to or smaller than about −10% of the peak value. Moreover, in the high load state, when the radius ratio r 3 /R 3 ≤0.5 is satisfied, the air volume Q 12  and the efficiency η 12  of the propeller fan  5 C are maximum. 
     In  FIG. 16 , the minimum cord length of the base portion of the blade element/the blade element total chord length (=Lmin/L 0 ) is the horizontal axis, and the air volume Q [m 3 /h] and the efficiency η[m 3 /h/W] are the vertical axes. In  FIG. 16 , the air volume Q 21  and the efficiency η 21  correspond to an air volume and an efficiency when the propeller fan  5 C rotates with the rated load of the air conditioner. On the other hand, the air volume Q 22  and the efficiency η 22  correspond to an air volume and an efficiency when the propeller fan  5 C rotates with the higher load than the rated load of the air conditioner. 
     Regarding the efficiency η 21  in the rated load state, the amount of decrease in the efficiency η 21  in the rated load state is a small value of 10% of the peak value across the entire range of the blade element minimum/total chord length (=Lmin/L 0 ) as illustrated in  FIG. 16 . Thus, there is no specific limitation on the blade element minimum/total chord length (=Lmin/L 0 ). On the other hand, in the high load state, the rate of decrease in the air volume Q 21  is equal to or higher than 40% of the peak value in the case of the blade element minimum/total chord length (=Lmin/L 0 )&lt;0.1 as illustrated in  FIG. 16 . For this reason, the blade element minimum/total chord length (=Lmin/L 0 )≥0.1 is set. 
     Thus, according to the above-described first to third embodiments, the air velocity at the inner peripheral portion  12 Aa,  12 Ba,  12 Ca is improved regardless of improvement of the air velocity at the outer peripheral portion  12 Ab,  12 Bb,  12 Cb of the blade  12 A,  12 B,  12 C. Consequently, the difference (the air velocity difference) between the air velocity at the outer peripheral portion  12 Ab,  12 Bb.  12 Cb and the air velocity at the inner peripheral portion  12 Aa,  12 Ba,  12 Ca can be reduced. With this configuration, air turbulence at the inner peripheral portion  12 Aa,  12 Ba,  12 Ca due to the air velocity difference and an abnormal operation state such as the surging phenomenon due to stalling of an air current can be reduced. As a result, the volume of air which can be generated by rotation of the propeller fan  5 A,  5 B,  5 C can be increased. 
     The embodiments have been described above. Note that the above-described contents are not intended to limit the technique disclosed in the present application. Moreover, the above-described components include those easily arrived by those skilled in the art, those substantially identical to the above-described components, and those within a so-called equivalent scope. Further, the above-described components can be combined as necessary. In addition, at least one of various omissions, replacements, and changes of the components can be made without departing from the gist of the embodiments. 
     Note that a radius ratio r 1 /R 1  of 0.4 may mean that the boundary between the inner peripheral portion  12 Aa and the outer peripheral portion  12 Ab is, in the blade  12 A, at such a position that the radius r 1  from the center axis O is 0.4 times as long as the radius R 1 , taking the radius R 1  from the center axis O as 1. A radius ratio r 3 /R 3  of 0.7 may mean that the boundary between the inner peripheral portion  12 Ca and the outer peripheral portion  12 Cb is, in the blade  12 C, at such a position that the radius r 3  from the center axis O is 0.7 times as long as the radius R 3 , taking the radius R 3  from the center axis O as 1. 
     The embodiments of the present disclosure may be the following first to sixth propeller fans. 
     The first propeller fan includes a hub having a side surface about a center axis, and a plurality of blades provided on the side surface of the hub. Each blade includes, in a portion from a base portion connected to the hub to an outer edge, an inner peripheral portion positioned on a base portion side, and an outer peripheral portion positioned on an outer edge side. A ratio r/R between a radius r as a distance from the center axis to a boundary between the inner peripheral portion and the outer peripheral portion and a radius R as a distance from the center axis to the outer edge is equal to or lower than 0.4. A relational expression of V 1 &lt;V 2 ×1.3 is satisfied, where an air velocity at the outer peripheral portion is V 1  and an air velocity at the inner peripheral portion is V 2 . 
     The second propeller fan is the first propeller fan in which the outer peripheral portion is formed as a single blade surface and the inner peripheral portion includes a plurality of blade elements disposed at predetermined intervals. 
     The third propeller fan includes a hub having a side surface about a center axis, and a plurality of blades provided on the side surface of the hub. Each blade includes, in a portion from a base portion connected to the hub to an outer edge, an inner peripheral portion positioned on a base side, and an outer peripheral portion positioned on an outer edge side. The outer peripheral portion is formed as a single blade surface. The inner peripheral portion includes at least one hole and a plurality of blade elements formed to sandwich the hole. The hole is provided in abutting contact with a boundary between the inner peripheral portion and the outer peripheral portion in a radial direction. A ratio r/R between a radius r as a distance from the center axis to the boundary between the inner peripheral portion and the outer peripheral portion and a radius R as a distance from the center axis to the outer edge is equal to or lower than 0.7. A relational expression of Lmin/L 0 ≥0.1 mm is satisfied, where the total of the chord lengths of the plurality of blade elements is L 0  [mm] and the minimum one of the chord lengths of the plurality of blade elements is Lmin [mm]. 
     The fourth propeller fan is the third propeller fan in which the hole is formed from the boundary between the inner peripheral portion and the outer peripheral portion to the side surface of the hub in the radial direction. 
     The fifth propeller fan is the third or fourth propeller fan in which a back-edge-side blade element of the plurality of blade elements is, in each blade, connected to the hub on a positive pressure side of the each blade as compared to a front-edge-side blade element of the plurality of blade elements. 
     The foregoing detailed description has been presented for the purposes of illustration and description. Many modifications and variations are possible in light of the above teaching. It is not intended to be exhaustive or to limit the subject matter described herein to the precise form disclosed. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims appended hereto.