Patent Publication Number: US-2023135727-A1

Title: Impeller, multi-blade air-sending device, and air-conditioning apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to an impeller, a multi-blade air-sending device including the impeller, and an air-conditioning apparatus including the multi-blade air-sending device. 
     BACKGROUND ART 
     Conventionally, an impeller of a multi-blade air-sending device includes a disk-shaped back plate, radially-arranged blades, and a boss provided in the central part of the back plate and connected to an output shaft of a motor or other devices (see, for example, Patent Literature 1). For an increase in strength, the impeller described in Patent Literature 1 includes a plurality of radially-arranged ribs molded integrally with the back plate. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Utility Model Registration Application Publication No. 59-96397 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, although it is conceivable that the multi-blade air-sending device of Patent Literature 1 may be configured to have high ribs along an axial direction of a rotation shaft of the impeller for an increase in strength of the impeller, having high ribs results in an increased loss during suction, leading to deterioration in air-sending efficiency. Further, since the multi-blade air-sending device of Patent Literature 1 is configured such that a surface of the back plate on which the ribs are mounted and a surface of the back plate on which blades are mounted are flush with each other, outer circumferential portions of the ribs aerodynamically act to cause turbulence in a flow of gas on the inner circumference of the blades, causing deterioration in air-sending efficiency of the impeller. 
     The present disclosure is intended to solve the aforementioned problem, and has as an object to provide an impeller configured to have improved air-sending efficiency, a multi-blade air-sending device including the impeller, and an air-conditioning apparatus including the multi-blade air-sending device. 
     Solution to Problem 
     An impeller according to an embodiment of the present disclosure is an impeller connected to a motor having a drive shaft. The impeller includes a back plate having a boss having a shaft hole through which the drive shaft is inserted, a ring-shaped rim provided to face the back plate, and a plurality of blades connected to the back plate and the rim and arranged along a circumferential direction of the back plate about the rotation shaft. The back plate includes a first surface portion on which the plurality of blades are formed, a second surface portion provided at a region between the boss and the first surface portion and depressed from the first surface portion in an axial direction of the rotation shaft, and a plurality of projections provided at the second surface portion and extending in the axial direction. 
     A multi-blade air-sending device according to an embodiment of the present disclosure includes the impeller thus configured and a scroll casing housing the impeller and having a peripheral wall formed into a volute shape and a side wall having a bellmouth forming an air inlet communicating with a space formed by the back plate and the plurality of blades. 
     An air-conditioning apparatus according to an embodiment of the present disclosure includes the multi-blade air-sending device thus configured. 
     Advantageous Effects of Invention 
     According to an embodiment of the present disclosure, the back plate includes a first surface portion on which the plurality of blades are formed and a second surface portion provided at a region between the boss and the first surface portion and depressed from the first surface portion in an axial direction of the rotation shaft. Further, the back plate also includes a plurality of projections provided at the second surface portion and extending in the axial direction of the rotation shaft. While the impeller is rotating, the projections draw in a flow of gas by generating negative pressure on a surface of the impeller facing in a direction opposite to a direction of rotation of the impeller, making it possible to increase the amount of air that is suctioned into the impeller. Further, the impeller includes the second surface portion depressed from the first surface portion, on which the plurality of blades are formed, in the axial direction of the rotation shaft, and the projections are provided at the second surface portion. This inhibits a flow of gas produced by the projections from flowing from the second surface portion into the first surface portion. Moreover, the flow of gas produced by the projections has its centrifugally-outward force of wind broken by a step between the first surface portion and the second surface portion, so that the impeller does not suffer from turbulence in the flow of gas on the inner circumference of the blades. This allows the impeller to have higher air-sending efficiency than in a case in which the impeller does not include the projections or the second surface portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view schematically showing a multi-blade air-sending device according to Embodiment 1. 
         FIG.  2    is an external appearance diagram schematically showing a configuration of the multi-blade air-sending device according to Embodiment 1 as viewed from an angle parallel with a rotation shaft. 
         FIG.  3    is a schematic cross-sectional view of the multi-blade air-sending device as taken along line A-A in  FIG.  2   . 
         FIG.  4    is a perspective view of an impeller of the multi-blade air-sending device according to Embodiment 1. 
         FIG.  5    is a plan view of a back plate of  FIG.  4    as seen from one side. 
         FIG.  6    is a plan view of the back plate of  FIG.  4    as seen from the other side. 
         FIG.  7    is a cross-sectional view of the impeller as taken along line B-B in  FIG.  5   . 
         FIG.  8    is a partially-enlarged view of the back plate in a region indicated by part E of  FIG.  4   . 
         FIG.  9    is a partially-enlarged view of the impeller in a region indicated by part F of  FIG.  7   . 
         FIG.  10    is a schematic partially-enlarged view of the back plate in a region indicated by part G of  FIG.  9   . 
         FIG.  11    is a side view of the impeller of  FIG.  4   . 
         FIG.  12    is a schematic view of blades in a cross-section of the impeller as taken along line C-C in  FIG.  11   . 
         FIG.  13    is a schematic view of the blades in a cross-section of the impeller as taken along line D-D in  FIG.  11   . 
         FIG.  14    is a schematic view showing a relationship between the impeller and bellmouths in a cross-section of the multi-blade air-sending device as taken along line A-A in  FIG.  2   . 
         FIG.  15    is a schematic view showing a relationship between the blades and a bellmouth in a second cross-section of the impeller as viewed from an angle parallel with the rotation shaft in the impeller in  FIG.  14   . 
         FIG.  16    is a schematic view showing a relationship between the impeller and the bellmouths in the cross-section of the multi-blade air-sending device as taken along line A-A in  FIG.  2   . 
         FIG.  17    is a schematic view showing a relationship between the blades and a bellmouth as viewed from an angle parallel with the rotation shaft in the impeller in  FIG.  16   . 
         FIG.  18    is a partially-enlarged view of an impeller of a multi-blade air-sending device according to Embodiment 2. 
         FIG.  19    is a partially-enlarged view of the impeller of the multi-blade air-sending device according to Embodiment 2. 
         FIG.  20    is a plan view of an impeller of a multi-blade air-sending device according to Embodiment 3. 
         FIG.  21    is a cross-sectional view of the impeller as taken along line E-E in  FIG.  20   . 
         FIG.  22    is a plan view schematically showing an impeller of a multi-blade air-sending device according to Embodiment 4, 
         FIG.  23    is a schematic view showing an example of the shape of projections of the impeller of  FIG.  22   . 
         FIG.  24    is a plan view schematically showing an impeller of a multi-blade air-sending device according to Embodiment 5, 
         FIG.  25    is a perspective view of an impeller of a multi-blade air-sending device according to Embodiment 6 as seen from one side. 
         FIG.  26    is a perspective view of the impeller of the multi-blade air-sending device according to Embodiment 6 as seen from the other side. 
         FIG.  27    is a plan view of the impeller shown in  FIG.  25    as seen from one side. 
         FIG.  28    is a plan view of the impeller shown in  FIG.  26    as seen from the other side. 
         FIG.  29    is a cross-sectional view of the impeller as taken along line F-F in  FIG.  27   . 
         FIG.  30    is a conceptual diagram explaining a relationship between the impeller and a motor in a multi-blade air-sending device according to Embodiment 7. 
         FIG.  31    is a perspective view of an air-conditioning apparatus according to Embodiment 8. 
         FIG.  32    is a diagram showing an internal configuration of the air-conditioning apparatus according to Embodiment 8. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, an impeller  10 , a multi-blade air-sending device  100  or other devices, and an air-conditioning apparatus  140  according to embodiments are described, for example, with reference to the drawings. In the following drawings including  FIG.  1   , relative relationships in dimension between constituent elements, the shapes of the constituent elements, or other features of the constituent elements may be different from actual ones. Further, constituent elements given identical signs in the following drawings are identical or equivalent to each other, and these signs are adhered to throughout the full text of the description. Further, the directive terms (such as “upper”, “lower” “right”, “left”, “front”, and “back”) used as appropriate for ease of comprehension are merely so written for convenience of explanation, and are not intended to limit the placement or orientation of a device or a component. 
     Embodiment 1 
     [Multi-Blade Air-Sending Device  100 ] 
       FIG.  1    is a perspective view schematically showing a multi-blade air-sending device  100  according to Embodiment 1.  FIG.  2    is an external appearance diagram schematically showing a configuration of the multi-blade air-sending device  100  according to Embodiment 1 as viewed from an angle parallel with a rotation shaft RS.  FIG.  3    is a schematic cross-sectional view of the multi-blade air-sending device  100  as taken along line A-A in  FIG.  2   . A basic structure of the multi-blade air-sending device  100  is described with reference to  FIGS.  1  to  3   . 
     The multi-blade air-sending device  100  is a multi-blade centrifugal air-sending device, and has an impeller  10  configured to generate a flow of gas and a scroll casing  40  housing the impeller  10  inside. The multi-blade air-sending device  100  is a double-suction centrifugal air-sending device into which air is suctioned through both sides of the scroll casing  40  in an axial direction of a virtual rotation shaft RS of the impeller  10 . 
     (Scroll Casing  40 ) 
     The scroll casing  40  houses the impeller  10  inside for use in the multi-blade air-sending device  100 , and rectifies a flow of air blown out from the impeller  10 . The scroll casing  40  has a scroll portion  41  and a discharge portion  42 . 
     (Scroll Portion  41 ) 
     The scroll portion  41  forms an air trunk through which a dynamic pressure of a flow of gas generated by the impeller  10  is converted into a static pressure. The scroll portion  41  has a side wall  44   a  covering the impeller  10  from an axial direction of a rotation shaft RS of a boss  11   b  of the impeller  10  and having formed therein an air inlet  45  through which air is taken in and a peripheral wall  44   c  surrounding the impeller  10  from a radial direction of the rotation shaft RS of the boss  11   b  of the impeller  10 . 
     Further, the scroll portion  41  has a tongue  43  located between the discharge portion  42  and a scroll start portion  41   a  of the peripheral wall  44   c  to constitute a curved surface and configured to guide the flow of gas generated by the impeller  10  toward a discharge port  42   a  via the scroll portion  41 . It should be noted that the radial direction of the rotation shaft RS is a direction perpendicular to the axial direction of the rotation shaft RS. An internal space of the scroll portion  41  constituted by the peripheral wall  44   c  and the side wall  44   a  serves as a space in which the air blown out from the impeller  10  flows along the peripheral wall  44   c.    
     (Side Wall  44   a ) 
     The side wall  44   a  is disposed at both sides of the impeller  10  in the axial direction of the rotation shaft RS of the impeller  10 . In the side wall  44   a  of the scroll casing  40 , the air inlet  45  is formed so that air can flow between the impeller  10  and the outside of the scroll casing  40 . 
     The inlet port  45  is formed in a circular shape, and is disposed so that the center of the air inlet  45  and the center of the boss  11   b  of the impeller  10  substantially coincide with each other. It should be noted that the shape of the air inlet  45  is not limited to the circular shape but may be another shape such as an elliptical shape. 
     The scroll casing  40  of the multi-blade air-sending device  100  is a double-suction casing having side walls  44   a  at both sides of a back plate  11  in the axial direction of the rotation shaft RS of the boss  11   b  with air inlets  45  formed in the side walls  44   a.    
     The multi-blade air-sending device  100  has two side walls  44   a  in the scroll casing  40 . The two side walls  44   a  are formed to face each other via the peripheral wall  44   c . More specifically, as shown in  FIG.  3   , the scroll casing  40  has a first side wall  44   a   1  and a second side wall  44   a   2  as the side walls  44   a . The first side wall  44   a   1  forms a first air inlet  45   a  facing a plate side of the back plate  11  on which the after-mentioned first rim  13   a  is disposed. The second side wall  44   a   2  forms a second air inlet  45   b  facing a plate side of the back plate  11  on which the after-mentioned second rim  13   b  is disposed. It should be noted that the aforementioned air inlet  45  is a generic name for the first air inlet  45   a  and the second air inlet  45   b.    
     The air inlet  45  provided in the side wall  44   a  is formed by a bellmouth  46 . That is, the bellmouth  46  forms an air inlet  45  communicating with a space formed by the back plate  11  and a plurality of blades  12 . The bellmouth  46  rectifies a flow of gas to be suctioned into the impeller  10  and causes the flow of gas to flow into an air inlet  10   e  of the impeller  10 . 
     The bellmouth  46  has an opening having a diameter gradually decreasing from the outside toward the inside of the scroll casing  40 . Such a configuration of the side wall  44   a  allows air near the air inlet  45  to smoothly flow along the bellmouth  46  and efficiently flow into the impeller  10  through the air inlet  45 . 
     (Peripheral Wall  44   c ) 
     The peripheral wall  44   c  guides the flow of gas generated by the impeller  10  toward the discharge port  42   a  along a curved wall surface. The peripheral wall  44   c  is a wall provided between side walls  44   a  facing each other, and constitutes a curved surface in a direction of rotation R of the impeller  10 . The peripheral wall  44   c  is for example disposed parallel with the axial direction of the rotation shaft RS of the impeller  10  to cover the impeller  10 . It should be noted that the peripheral wall  44   c  may be formed at a slant with respect to the axial direction of the rotation shaft RS of the impeller  10 , and is not limited to being formed to be disposed parallel with the axial direction of the rotation shaft RS. 
     The peripheral wall  44   c  constitutes an inner circumferential surface covering the impeller  10  from the radial direction of the boss  11   b  and facing the after-mentioned plurality of blades  12 . The peripheral wall  44   c  faces a side of each of the blades  12  through which air is blown out from the impeller  10 . As shown in  FIG.  2   , the peripheral wall  44   c  is provided along the direction of rotation R of the impeller  10  over an area from the scroll start portion  41   a , which is located at a boundary with the tongue  43 , to a scroll end portion  41   b  located at a boundary between the discharge portion  42  and the scroll portion  41  at a side away from the tongue  43 . 
     The scroll start portion  41   a  is an end portion of the peripheral wall  44   c , which constitutes a curved surface, situated on an upstream side of a flow of gas generated by rotation of the impeller  10 , and the scroll end portion  41   b  is an end portion of the peripheral wall  44   c  situated on a downstream side of the flow of gas generated by rotation of the impeller  10 . 
     The peripheral wall  44   c  is formed in a volute shape. An example of the volute shape is a shape based on a logarithmic spiral, a spiral of Archimedes, or an involute curve. An inner peripheral surface of the peripheral wall  44   c  constitutes a curved surface smoothly curved along a circumferential direction of the impeder  10  from the scroll start portion  41   a , at which the volute shape starts rolling, to the scroll end portion  41   b , at which the volute shape finishes rolling. Such a configuration allows air sent out from the impeller  10  to smoothly flow through the space between the impeller  10  and the peripheral wall  44   c  in a direction toward the discharge portion  42 . This effects an efficient rise in static pressure of air from the tongue  43  toward the discharge portion  42  in the scroll casing  40 . 
     (Discharge Portion  42 ) 
     The discharge portion  42  forms a discharge port  42   a  through which a flow of gas generated by the impeller  10  and having passed through the scroll portion  41  is discharged. The discharge portion  42  is constituted by a hollow pipe having a rectangular cross-section orthogonal to a flow direction of air flowing along the peripheral wall  44   c . It should be noted that the cross-sectional shape of the discharge portion  42  is not limited to a rectangle. The discharge portion  42  forms a flow passage through which air sent out from the impeller  10  and flowing through a gap between the peripheral wall  44   c  and the impeller  10  is guided to be exhausted out of the scroll casing  40 . 
     As shown in  FIG.  1   , the discharge portion  42  is constituted by an extension plate  42   b , a diffuser plate  42   c , a first side plate portion  42   d , a second side plate portion  42   e , or other components. The extension plate  42   b  is formed integrally with the peripheral wall  44   c  to smoothly continue into the scroll end portion  41   b  downstream of the peripheral wall  44   c . The diffuser plate  42   c  is formed integrally with the tongue  43  of the scroll casing  40  and faces the extension plate  42   b . The diffuser plate  42   c  is formed at a predetermined angle with respect to the extension plate  42   b  so that the cross-sectional area of the flow passage gradually increases along a flow direction of air in the discharge portion  42 . 
     The first side plate portion  42   d  is formed integrally with the first side wall  44   a   1  of the scroll casing  40 , and the second side plate portion  42   e  is formed integrally with the opposite second side wall  44   a   2  of the scroll casing  40 . Moreover, the first side plate portion  42   d  and the second side plate portion  42   e  are formed between the extension plate  42   b  and the diffuser plate  42   c . Thus, the discharge portion  42  has a rectangular cross-section flow passage formed by the extension plate  42   b , the diffuser plate  42   c , the first side plate portion  42   d , and the second side plate portion  42   e.    
     (Tongue  43 ) 
     In the scroll casing  40 , the tongue  43  is formed between the diffuser plate  42   c  of the discharge portion  42  and the scroll start portion  41   a  of the peripheral wall  44   c . The tongue  43  is formed with a predetermined radius of curvature, and the peripheral wall  44   c  is smoothly connected to the diffuser plate  42   c  via the tongue  43 . 
     The tongue  43  reduces inflow of air from the scroll start to the scroll end of a volute flow passage. The tongue  43  is provided in an upstream part of a ventilation flue, and has a role to effect diversion into a flow of air in the direction of rotation R of the impeller  10  and a flow of air in a discharge direction from a downstream part of the ventilation flue toward the discharge port  42   a . Further, a flow of air flowing into the discharge portion  42  rises in static pressure during passage through the scroll casing  40  to be higher in pressure than in the scroll casing  40 . Therefore, the tongue  43  has a function of separating such different pressures. 
     [Impeller  10 ] 
       FIG.  4    is a perspective view of the impeller  10  of the multi-blade air-sending device  100  according to Embodiment 1.  FIG.  5    is a plan view of a back plate  11  of  FIG.  4    as seen from one side.  FIG.  6    is a plan view of the back plate  11  of  FIG.  4    as seen from the other side.  FIG.  7    is a cross-sectional view of the impeller  10  as taken along line B-B in  FIG.  5   . It should be noted that  FIG.  5    is a diagram of the impeller  10  as viewed from a point of view V 1  indicated by an outline arrow in  FIG.  4   , and is a plan view as viewed from an angle parallel with the axial direction of the rotation shaft RS.  FIG.  6    is a diagram of the impeller  10  as viewed from a point of view V 2  indicated by an outline arrow in  FIG.  4   , and is a plan view as viewed from an angle parallel with the axial direction of the rotation shaft RS. The impeller  10  is described with reference to  FIGS.  4  to  7   . 
     The impeller  10  is a centrifugal fan. The impeller  10  is connected to a motor (not illustrated) having a drive shaft. The impeller  10  is driven into rotation, for example, by the motor. The rotation generates a centrifugal force with which the impeller  10  forcibly sends out air outward in a radial direction. The impeller  10  is rotated, for example, by the motor in a direction of rotation R indicated by an arrow. As shown in  FIG.  4   , the impeller  10  has a disk-shaped back plate  11 , a circular-ring-shaped rim  13 , and a plurality of blades  12  arranged radially along a circumferential direction of the back plate  11  on a peripheral edge of the back plate  11 . 
     (Back Plate  11 ) 
     The back plate  11  needs only be in the shape of a plate, and may for example have a non-disk shape such as a polygonal shape. The back plate  11  has in the central part thereof a boss  11   b  to which the drive shaft of the motor is connected. The boss  11   b  has formed therein a shaft hole  11   b   1  through which the drive shaft of the motor is inserted. The boss  11   b  is formed in a circular cylindrical shape, although the shape of the boss  11   b  is not limited to a circular cylindrical shape. The boss  11   b  needs only be formed in a columnar shape and, as one example, may be formed, for example, in a polygonal columnar shape. The back plate  11  is driven into rotation by the motor via the boss  11   b . It should be noted that the back plate  11  is not limited to being constituted by one plate-like element but may be constituted by a plurality of plate-like elements fixed in an integrated fashion. 
       FIG.  8    is a partially-enlarged view of the back plate  11  in a region indicated by part E of  FIG.  4   .  FIG.  9    is a partially-enlarged view of the impeller  10  in a region indicated by part F of  FIG.  7   .  FIG.  10    is a schematic partially-enlarged view of the back plate  11  in a region indicated by part G of  FIG.  9   . A configuration of the back plate  11  is described in more detail with reference to  FIGS.  8  to  10   . 
     (First Surface Portion  11   a  and Second Surface Portion  11   c ) 
     The back plate  11  has a first surface portion  11   a  on which the plurality of blades  12  are formed and a second surface portion  11   c  provided at a region between the boss  11   b  and the first surface portion  11   a  and depressed from the first surface portion  11   a  in an axial direction of the rotation shaft RS. The first surface portion  11   a  is located closer to the rim  13  than the second surface portion  11   c.    
     The first surface portion  11   a  is formed closer to an outer circumference than the second surface portion  11   c  about the rotation shaft RS. The first surface portion  11   a  is formed in a ring shape in a plan view as viewed in the axial direction of the rotation shaft RS, and the second surface portion  11   c  is formed at an inner circumferential side of the first surface portion  11   a.    
     In a plan view as viewed in the axial direction of the rotation shaft RS, the second surface portion  11   c  is provided at a circular-ring-shaped region about the boss  11   b . That is, the second surface portion  11   c  is depressed in a circular ring shape about the boss  11   b . It should be noted that when the second surface portion  11   c  is depressed, the second surface portion  11   c  is not limited to being depressed in a circular ring shape about the boss  11   b . As one example, the second surface portion  11   c  may be depressed in a radial fashion about the boss  11   b . The back plate  11  needs only include, at the inner circumferential side of the first surface portion  11   a , a second surface portion  11   c  depressed from the first surface portion  11   a.    
     As shown in  FIGS.  5  to  7   , the back plate  11  has its first and second surface portions  11   a  and  11   c  on both plate sides of the back plate  11  in the axial direction of the rotation shaft RS. In the back plate  11 , the second surface portion  11   c  is constituted by a plate whose thickness is thinner than the thickness of a plate constituting the first surface portion  11   a . As mentioned above, the second surface portion  11   c  is depressed from the first surface portion  11   a . Therefore, as shown in  FIG.  10   , the back plate  11  has a step  11   f  formed between the first surface portion  11   a  and the second surface portion  11   c.    
     In the back plate  11  of Embodiment 1, the step  11   f  forms an outer circumferential edge  11   c   1  of the second surface portion  11   c . As shown in  FIGS.  5  and  6   , the length of a depression outside diameter PO constituted by the outer circumferential edge  11   c   1  of the second surface portion  11   c  is greater than the magnitude of a difference PS between an inside diameter ID 1  of the blades  12  constituted by an inner circumferential end  14 A of each of the plurality of blades  12  and the depression outside diameter PO. That is, the back plate  11  is configured such that the relationships “Depression Outside Diameter PO&gt;(Inside DiameterID 1 −Depression Outside Diameter PO)” and “Depression Outside Diameter PO&gt;Difference PS” hold. Accordingly, the second surface portion  11   c  is formed close to a blade inside diameter of the blades  12  in a radial direction about the rotation shaft RS. It should be noted that the depression outside diameter PO is the diameter of a circle CR constituted by the outer circumferential edge  11   c   1  of the second surface portion  11   c  about the rotation shaft RS. Further, the inside diameter ID 1  is the diameter of a circle C 1  passing through the inner circumferential ends  14 A of the plurality of first blades  12 A about the rotation shaft RS. 
     (Projection  20 ) 
     As shown in  FIGS.  4  to  10   , the back plate  11  includes a plurality of projections  20  provided at the second surface portion  11   c  and extending in the axial direction of the rotation shaft RS. The plurality of projections  20  are provided in a radial fashion about the rotation shaft RS, and each of the plurality of projections  20  extends in the radial direction about the rotation shaft RS. As shown in  FIGS.  5  and  6   , the back plate  11  has its first and second surface portions  11   a  and  11   c  on both plate sides of the back plate  11 , and each of the second surface portions  11   c  formed on both plate sides of the back plate  11  includes the plurality of projections  20 . As shown in  FIG.  8   , the back plate  11  includes nine projections  20 . However, the number of projections  20  that are formed is not limited to 9. 
     As shown in  FIG.  8   , each of the plurality of projections  20  is a rib formed in the shape of a plate rising from the second surface portion  11   c . More specifically, the projection  20  is formed in the shape of a four-cornered plate. Note, however, that the projection  20  needs only be a structure projecting from the second surface portion  11   c  and is not limited to the four-cornered plate-like configuration. 
     As shown in  FIG.  8   , the projection  20  includes a base  24  connected to the second surface portion  11   c  and serving as a root portion of the projection  20  and a ridge  26  constituting a leading end portion in a direction of projection from the second surface portion  11   c  and forming a ridge line of the projection  20 . It should be noted that the ridge line is constituted by leading end portions of the projection  20  in the direction of projection, and refers to a series of leading end portions of the projection  20  opposite the second surface portion  11   c  and a series of highest portions of the projection  20  with the second surface portion  11   c  being a bottom surface portion. The ridge  26  is configured such that a ridge line constituted by the leading end portion in the direction of projection is formed in a linear fashion in a side view as viewed from a direction perpendicular to the axial direction of the rotation shaft RS. It should be noted that ridge  26  is not limited to being configured such that the ridge line is formed in a linear fashion in a side view as viewed from a direction perpendicular to the axial direction of the rotation shaft RS. 
     Further, the projection  20  includes a projection inner circumferential end  23  serving as an inner circumferential end portion located beside the rotation shaft RS in the radial direction about the rotation shaft RS and a projection outer circumferential end  21  serving as an outer circumferential end portion beside the plurality of blades  12  in the radial direction. The projection inner circumferential end  23  constitutes an inner circumferential end portion of the projection  20 , and the projection outer circumferential end  21  constitutes an outer circumferential end portion of the projection  20 . 
     As shown in  FIG.  8   , each of the plurality of projections  20  is connected to an outer circumferential wall  11   b   2  of the boss  11   b . That is, the projection inner circumferential end  23  of the projection  20  is connected to the boss  11   b . Note, however, that the projection  20  is not limited to being configured such that the projection inner circumferential end  23  is connected to the outer circumferential wall  11   b   2  of the boss  11   b . In the radial direction about the rotation shaft RS, a space may be formed between the projection inner circumferential end  23  of the projection  20  and the outer circumferential wall  11   b   2  of the boss  11   b.    
     Each of the plurality of projections  20  is connected to the step  11   f . That is, the projection outer circumferential end  21  of the projection  20  is connected to the step  11   f . Note, however, that the projection  20  is not limited to being configured such that the projection outer circumferential end  21  is connected to the step  11   f . In the radial direction about the rotation shaft RS, a space may be formed between the projection outer circumferential end  21  of the projection  20  and the step  11   f.    
     In a case in which a height direction is a direction parallel with the axial direction of the rotation shaft RS and a direction of projection from the second surface portion  11   c , the plurality of projections  20  have their heights formed at the same height. Note, however, that the back plate  11  is not limited to being configured such that the plurality of projections  20  have their heights formed at the same height. The plurality of projections  20  may be formed at different heights, or may form a group of the same height based on certain regularity. 
     In a case in which the height direction is the direction parallel with the axial direction of the rotation shaft RS and the direction of projection from the second surface portion  11   c , the projection outer circumferential end  21 , which serves as an outermost circumferential portion of the projection  20 , corresponds in height to the first surface portion  11   a . Alternatively, as shown in  FIG.  10   , the height of the projection outer circumferential end  21 , which serves as the outermost circumferential portion of the projection  20 , is lower than the height of the first surface portion  11   a , and the projection outer circumferential end  21  has an upper end portion  21   a  located closer to the second surface portion  11   c  than the first surface portion  11   a . In  FIG.  10   , a virtual surface extension of the first surface portion  11   a  is expressed as a surface of extension FL. As shown in  FIG.  10   , the upper end portion  21   a  of the projection outer circumferential end  21  is located closer to the second surface portion  11   c  than the surface of extension FL. In other words, the projection outer circumferential end  21 , which serves as the outermost circumferential portion of the projection  20 , is formed not to project from the first surface portion  11   a  in the direction parallel with the axial direction of the rotation shaft RS. 
     The height of the projection inner circumferential end  23  of the projection  20  is equal to or lower than the height of a leading end portion of the boss  11   b . It should be noted that the height of the leading end portion of the boss  11   b  is greater than the height of the first surface portion  11   a . For example, in the axial direction of the rotation shaft RS, the thickness of a plate constituting the boss  11   b  is greater than the thickness of the plate constituting the first surface portion  11   a . Note, however, that the height of the leading end portion of the boss  11   b  is not limited to being greater than the height of the first surface portion  11   a  but may be equal to the height of the first surface portion  11   a.    
     In a case in which the height of the leading end portion of the boss  11   b  is greater than the height of the first surface portion  11   a , each of the plurality of projections  20  has an inclined portion  26   a  on the ridge  26 . The inclined portion  26   a  is a portion of the ridge  26  whose ridge line is inclined such that the height of the inclined portion  26   a  in the axial direction of the rotation shaft RS decreases from the inner circumference toward the outer circumference. The inclined portion  26   a  of the projection  20  is formed to be higher beside the projection inner circumferential end  23  than beside the projection outer circumferential end  21 , and the ridge  26 , which constitutes the inclined portion  26   a , is inclined to increase in distance from the back plate  11  from the projection outer circumferential end  21  toward the projection inner circumferential end  23 . It should be noted that the configuration of the inclined portion  26   a  is not limited to this configuration. The inclined portion  26   a  may be a portion of the ridge  26  whose ridge line is inclined such that the inclined portion  26   a  increases in height of projection from the boss  11   b  toward the plurality of blades  12 . In this case, the inclined portion  26   a  of the projection  20  is formed to be higher beside the projection outer circumferential end  21  than beside the projection inner circumferential end  23 , and the ridge  26 , which constitutes the inclined portion  26   a , is inclined to increase in distance from the back plate  11  from the projection inner circumferential end  23  toward the projection outer circumferential end  21 . 
     As shown in  FIGS.  5  and  6   , the length of a projection outside diameter QO constituted by the projection outer circumferential end  21  of each of the plurality of projections  20  is greater than the magnitude of a difference QS between the inside diameter ID 1  of the blades  12  constituted by the inner circumferential end  14 A of each of the plurality of blades  12  and the projection outside diameter 00. That is, the back plate  11  is configured such that the relationship “Projection Outside Diameter QO&gt;(Inside Diameter ID 1 −Projection Outside Diameter QO)” or “Projection Outside Diameter QO&gt;Difference QS” holds. Accordingly, the projection  20  is formed close to the blade inside diameter of the blades  12  in the radial direction about the rotation shaft RS. It should be noted that the projection outside diameter QO is the diameter of a circle DR passing through the projection outer circumferential ends  21  of the plurality of projections  20  about the rotation shaft RS. In a case in which the projection outer circumferential end  21  of the projection  20  is connected to the step  11   f , the depression outside diameter PO and the projection outside diameter QO are equal (Depression Outside Diameter PO=Projection Outside Diameter QO), and the difference PS and the difference QS are equal (Difference PS=Difference QS). Further, the circle CR constituted by the outer circumferential edge  11   c   1  of the second surface portion  11   c  about the rotation shaft RS and the circle DR passing through the projection outer circumferential ends  21  of the plurality of projections  20  are equal (Circle CR=Circle DR). 
     As shown in  FIG.  8   , the back plate  11  includes a depression  34  in front of and behind a projection  20  along the circumferential direction. In other words, the depression  34  is formed between adjacent projections  20  along the circumferential direction. The depression  34  is formed by the second surface portions  11   c . More specifically, the depression  34  is formed by the second surface portion  11   c , adjacent projections  20 , the boss  11   b , and the step  11   f . The depression  34  is formed in a radial fashion with respect to the boss  11   b . A plurality of the depressions  34  are formed along the circumferential direction. 
     (Reinforcing Portion  30 ) 
     As shown in  FIGS.  8  and  9   , the back plate  11  includes a reinforcing portion  30  provided at the second surface portion  11   c  and extending in the axial direction of the rotation shaft RS. The reinforcing portion  30  is a reinforcing rib formed in the shape of a plate rising from the second surface portion  11   c . The reinforcing portion  30  is formed in a circular arc shape in a plan view as viewed in the direction parallel with the axial direction of the rotation shaft RS, and connects the plurality of projections  20  to each other along the circumferential direction. Accordingly, the reinforcing portion  30  is formed in a circular ring shape in a plan view as viewed in the direction parallel with the axial direction of the rotation shaft RS. The reinforcing portion  30  is connected to the projection  20 . The reinforcing portion  30  constitutes a wall that is equal in height to a wall of a projection  20  in a location where the reinforcing portion  30  is connected to the projection  20 . 
     A plurality of the reinforcing portions  30  are provided in the radial direction about the rotation shaft RS. In a case in which the plurality of reinforcing portions  30  are provided in the radial direction, the back plate  11  is formed such that in the radial direction about the rotation shaft RS, a reinforcing portion  30  located beside the inner circumference is higher in wall height than a reinforcing portion  30  located beside the outer circumference. As shown in  FIG.  8   , the back plate  11  includes reinforcing portions  30  forming two circles. However, the number of reinforcing portions  30  that are formed is not limited to 2. 
     As shown in  FIG.  8   , the back plate  11  forms a depression  35  formed in a depressed shape by projections  20 , the reinforcing portions  30 , and the second surface portion  11   c . Similarly, the back plate  11  forms a depression  36  formed in a depressed shape by projections  20 , a reinforcing portion  30 , the step  11   f , and the second surface portion  11   c . Similarly, the back plate  11  forms a depression  37  formed in a depressed shape by projections  20 , a reinforcing portion  30 , the outer circumferential wall  11   b   2  of the boss  11   b , and the second surface portion  11   c.    
     (Blade  12 ) 
     As shown in  FIG.  4   , the plurality of blades  12  are arranged along a circumferential direction about a virtual rotation shaft RS of the back plate  11 . One end of each of the plurality of blades  12  is connected to the back plate  11 , and the other end of each of the plurality of blades  12  is connected to the rim  13 . Each of the plurality of blades  12  is disposed between the back plate  11  and the rim  13 . The plurality of blades  12  are provided on both sides of the back plate  11  in the axial direction of the rotation shaft RS of the boss  11   b . The blades  12  are placed at regular spacings from each other on the peripheral edge of the back plate  11 . A configuration of the blades  12  will be described in detail later. 
     (Rim  13 ) 
     The ring-shaped rim  13  of the impeller  10  is attached to ends of the plurality of blades  12  opposite to the back plate  11  in the axial direction of the rotation shaft RS of the boss  11   b . The rim  13  is provided in the impeller  10  to face the back plate  11 . The rim  13  couples the plurality of blades  12  with each other, thereby maintaining a positional relationship between the tip of each blade  12  and the tip of the other blade  12  and reinforcing the plurality of blades  12 . 
       FIG.  11    is a side view of the impeller  10  of  FIG.  4   . As shown in  FIGS.  4  and  11   , the impeller  10  has a first blade group  112   a  and a second blade group  112   b . The first blade group  112   a  and the second blade group  112   b  are constituted by the plurality of blades  12  and the rim  13 . More specifically, the first blade group  112   a  is constituted by a ring-shaped first rim  13   a  disposed to face the back plate  11  and a plurality of blades  12  disposed between the back plate  11  and the first rim  13   a.    
     The second blade group  112   b  is constituted by a ring-shaped second rim  13   b  disposed on a side of the back plate  11  opposite to the first rim  13   a  to face the back plate  11  and a plurality of blades  12  disposed between the back plate  11  and the second rim  13   b . It should be noted that the rim  13  is a generic name for the first rim  13   a  and the second rim  13   b , and the impeller  10  has the first rim  13   a  on one side of the back plate  11  in the axial direction of the rotation shaft RS, and has the second rim  13   b  on the other side. 
     The first blade group  112   a  is disposed on one plate side of the back plate  11 , and the second blade group  112   b  is disposed on the other plate side of the back plate  11 . That is, the plurality of blades  12  are provided on both sides of the back plate  11  in the axial direction of the rotation shaft RS, and the first blade group  112   a  and the second blade group  112   b  are provided back to back with each other via the back plate  11 . In  FIG.  3   , the first blade group  112   a  is disposed on the left side of the back plate  11 , and the second blade group  112   b  is disposed on the right side of the back plate  11 . However, the first blade group  112   a  and the second blade group  112   b  need only be provided back to back with each other via the back plate  11 . The first blade group  112   a  may be disposed on the right side of the back plate  11 , and the second blade group  112   b  may be disposed on the left side of the back plate  11 . In the following description, those blades  12  which constitute the first blade group  112   a  and those blades  12  which constitute the second blade group  112   b  are collectively referred to as “blades  12 ” unless otherwise noted. 
     The impeller  10  is constituted in a tubular shape by the plurality of blades  12  disposed on the back plate  11 . Moreover, the impeller  10  has an air inlet  10   e  formed at a side of the rim  13  opposite to the back plate  11  in the axial direction of the rotation shaft RS of the boss  11   b  and configured to cause gas to flow into a space surrounded by the back plate  11  and the plurality of blades  12 . The impeller  10  has its blades  12  and rims  13  disposed on both plate sides, respectively, of the back plate  11 , and has its air inlets  10   e  formed at both plate sides, respectively, of the back plate  11 . 
     The impeller  10  is driven into rotation about the rotation shaft RS by driving of the motor (not illustrated). The rotation of the impeller  10  causes gas outside the multi-blade air-sending device  100  to be suctioned into the space surrounded by the back plate  11  and the plurality of blades  12  through the air inlet  45  formed in the scroll casing  40  shown in  FIG.  1    and the air inlet  10   e  of the impeller  10 . Moreover, the rotation of the impeller  10  causes air suctioned into the space surrounded by the back plate  11  and the plurality of blades  12  to be sent out outward in a radial direction of the impeller  10  through a space between a blade  12  and an adjacent blade  12 . 
     (Configuration of Blades  12  in Detail) 
       FIG.  12    is a schematic view of the blades  12  in a cross-section of the impeller  10  as taken along line C-C in  FIG.  11   .  FIG.  13    is a schematic view of the blades  12  in a cross-section of the impeller  10  as taken along line D-D in  FIG.  11   . In  FIG.  11   , a middle point MP of the impeller  10  indicates a middle point in the axial direction of the rotation shaft RS in the plurality of blades  12  constituting the first blade group  112   a.    
     In the plurality of blades  12  constituting the first blade group  112   a , a region from the middle point MP in the axial direction of the rotation shaft RS to the back plate  11  is a back-plate-side blade region  122   a  serving as a first region of the impeller  10 . Further, in the plurality of blades  12  constituting the first blade group  112   a , a region from the middle point MP in the axial direction of the rotation shaft RS to an end portion of the rim  13  is a rim-side blade region  122   b  serving as a second region of the impeller  10 . That is, each of the plurality of blades  12  has a first region located closer to the back plate  11  than the middle point MP in the axial direction of the rotation shaft RS and a second region located closer to the rim  13  than the first region. 
     As shown in  FIG.  12   , the cross-section taken along line C-C in  FIG.  11    is a cross-section of the plurality of blades  12  beside the back plate  11  of the impeller  10 , that is, in the back-plate-side blade region  122   a  serving as the first region. This cross-section of the blades  12  beside the back plate  11  is a first cross-section of the impeller  10  made by cutting through a portion of the impeller  10  close to the back plate  11  along a first plane  71  perpendicular to the rotation shaft RS. Note here that the portion of the impeller  10  close to the back plate  11  is for example a portion of the impeller  10  closer to the back plate  11  than a middle point of the back-plate-side blade region  122   a  in the axial direction of the rotation shaft RS or a portion of the impeller  10  in which end portions of the blades  12  facing the back plate  11  are located in the axial direction of the rotation shaft RS. 
     As shown in  FIG.  13   , the cross-section taken along line D-D in  FIG.  11    is a cross-section of the plurality of blades  12  beside the rim  13  of the impeller  10 , that is, in the rim-side blade region  122   b  serving as the second region. This cross-section of the blades  12  beside the rim  13  is a second cross-section of the impeller  10  made by cutting through a portion of the impeller  10  close to the rim  13  along a second plane  72  perpendicular to the rotation shaft RS. Note here that the portion of the impeller  10  close to the rim  13  is for example a portion of the impeller  10  closer to the rim  13  than a middle point of the rim-side blade region  122   b  in the axial direction of the rotation shaft RS or a portion of the impeller  10  in which end portions of the blades  12  facing the rim  13  are located in the axial direction of the rotation shaft RS. 
     A basic configuration of the blades  12  in the second blade group  112   b  is similar to a basic configuration of the blades  12  in the first blade group  112   a . That is, in  FIG.  5   , a middle point MP of the impeller  10  indicates a middle point in the axial direction of the rotation shaft RS in the plurality of blades  12  constituting the second blade group  112   b.    
     In the plurality of blades  12  constituting the second blade group  112   b , a region from the middle point MP in the axial direction of the rotation shaft RS to the back plate  11  is a back-plate-side blade region  122   a  serving as a first region of the impeller  10 . Further, in the plurality of blades  12  constituting the second blade group  112   b , a region from the middle point MP in the axial direction of the rotation shaft RS to an end portion of the second rim  13   b  is a rim-side blade region  122   b  serving as a second region of the impeller  10 . 
     Although the foregoing description assumes that a basic configuration of the first blade group  112   a  and a basic configuration of the second blade group  112   b  are similar to each other, a configuration of the impeller  10  is not limited to such a configuration but may be a configuration in which the first blade group  112   a  and the second blade group  112   b  are different from each other. Both or either the first blade group  112   a  and/or the second blade group  112   b  may have the configuration of the blades  12  to be described below. 
     As shown in  FIGS.  11  to  13   , the plurality of blades  12  include a plurality of first blades  12 A and a plurality of second blades  12 B. The plurality of blades  12  include an alternate arrangement of a first blade  12 A and or more second blades  12 B along the circumferential direction of the impeller  10 . 
     As shown in  FIGS.  4  and  12   , the impeller  10  has two second blades  12 B disposed between a first blade  12 A and a first blade  12 A disposed adjacent to the first blade  12 A in the direction of rotation R. Note, however, that the number of second blades  12 B that are disposed between a first blade  12 A and a first blade  12 A disposed adjacent to the first blade  12 A in the direction of rotation R is not limited to 2 but may be 1 or larger than or equal to 3. That is, at least one of the plurality of second blades  12 B is disposed between two of the plurality of first blades  12 A adjacent to each other along the circumferential direction. 
     As shown in  FIG.  12   , in the first cross-section of the impeller  10  as taken along the first plane  71  perpendicular to the rotation shaft RS, each of the first blades  12 A has an inner circumferential end  14 A and an outer circumferential end  15 A. The inner circumferential end  14 A is located closer to the rotation shaft RS in the radial direction about the rotation shaft RS, and the outer circumferential end  15 A is located closer to the outer circumference than the inner circumferential end  14 A in the radial direction. In each of the plurality of first blades  12 A, the inner circumferential end  14 A is disposed in front of the outer circumferential end  15 A in the direction of rotation R of the impeller  10 . 
     As shown in  FIG.  4   , the inner circumferential end  14 A serves as a leading edge  14 A 1  of the first blade  12 A, and the outer circumferential end  15 A serves as a trailing edge  15 A 1  of the first blade  12 A. As shown in  FIG.  12   , the impeller  10  has fourteen first blades  12 A disposed therein. However, the number of first blades  12 A is not limited to 14 but may be smaller or larger than 14. 
     As shown in  FIG.  12   , in the first cross-section of the impeller  10  as taken along the first plane  71  perpendicular to the rotation shaft RS, each of the second blades  12 B has an inner circumferential end  14 B and an outer circumferential end  158 . The inner circumferential end  14 B is located closer to the rotation shaft RS in the radial direction about the rotation shaft RS, and the outer circumferential end  15 B is located closer to the outer circumference than the inner circumferential end  14 B in the radial direction. In each of the plurality of second blades  128 , the inner circumferential end  14 B is disposed in front of the outer circumferential end  15 B in the direction of rotation R of the impeller  10 . 
     As shown in  FIG.  4   , the inner circumferential end  14 B serves as a leading edge  1481  of the second blade  12 B, and the outer circumferential end  158  serves as a trailing edge  1581  of the second blade  128 . As shown in  FIG.  12   , the impeller  10  has twenty-eight second blades  128  disposed therein. However, the number of second blades  128  is not limited to 28 but may be smaller or larger than 28. 
     The following describes a relationship between the first blades  12 A and the second blades  12 B. As shown in  FIGS.  4  and  13   , the blade length of each of portions of each of the first blades  12 A closer to the first rim  13   a  and the second rim  13   b  than the middle points MP in a direction along the rotation shaft RS is equal to the blade length of each of portions of each of the second blades  12 B closer to the first rim  13   a  and the second rim  13   b  than the middle points MP in the direction along the rotation shaft RS. 
     Meanwhile, as shown in  FIGS.  4  and  12   , the blade length of a portion each of the first blades  12 A closer to the back plate  11  than the middle point MP in the direction along the rotation shaft RS is greater than the blade length of a portion of each of the second blades  12 B closer to the back plate  11  than the middle point MP in the direction along the rotation shaft RS, and increases toward the back plate  11 . Thus, in the present embodiment, the blade length of at least a portion of each of the first blades  12 A in the direction along the rotation shaft RS is greater than the blade length of at least a portion of each of the second blades  12 B in the direction along the rotation shaft RS. It should be noted that the term “blade length” here means the length of each of the first blades  12 A in the radial direction of the impeller  10  and the length of each of the second blades  12 B in the radial direction of the impeller  10 . 
     Let it be assumed that as shown in  FIG.  12   , in the first cross-section closer to the back plate  11  than the middle point MP shown in  FIG.  11   , the diameter of a circle C 1  passing through the inner circumferential ends  14 A of the plurality of first blades  12 A about the rotation shaft RS, that is, the inside diameter of the first blades  12 A, is an inside diameter ID 1 . Let it be assumed that the diameter of a circle C 3  passing through the outer circumferential ends  15 A of the plurality of first blades  12 A about the rotation shaft RS, that is, the outside diameter of the first blades  12 A, is an outside diameter OD 1 . One-half of the difference between the outside diameter OD 1  and the inside diameter ID 1  is equal to the blade length L 1   a  of each of the first blades  12 A in the first cross-section (Blade Length L 1   a =(Outside Diameter OD 1 −Inside Diameter ID 1 )/2). 
     Note here that the ratio of the inside diameter to the outside diameter of the first blades  12 A is lower than or equal to 0.7. That is, the plurality of first blades  12 A are configured such that the ratio of the inside diameter ID 1  constituted by the inner circumferential end  14 A of each of the plurality of first blades  12 A and to the outside diameter OD 1  constituted by the outer circumferential end  15 A of each of the plurality of first blades  12 A is lower than or equal to 0.7. 
     It should be noted that in a common multi-blade air-sending device, the blade length of a blade in a cross-section perpendicular to a rotation shaft is shorter than the width dimension of a blade in a direction parallel with the rotation shaft. In the present embodiment too, the maximum blade length of each of the first blades  12 A, that is, the blade length of an end portion of each of the first blades  12 A close to the back plate  11 , is shorter than the width dimension W (see  FIG.  11   ) of each of the first blades  12 A in the direction parallel with the rotation shaft. 
     Further, let it also be assumed that in the first cross-section, the diameter of a circle C 2  passing through the inner circumferential ends  14 B of the plurality of second blades  12 B about the rotation shaft RS, that is, the inside diameter of the second blades  12 B, is an inside diameter ID 2  that is larger than the inside diameter ID 1  (Inside Diameter ID 2 &gt;Inside Diameter ID 1 ). Let it be assumed that the diameter of the circle C 3  passing through the outer circumferential ends  15 B of the plurality of second blades  12 B about the rotation shaft RS, that is, the outside diameter of the second blades  12 B, is an outside diameter OD 2  that is equal to the outside diameter OD 1  (Outside Diameter OD 2 =Outside Diameter OD 1 ). One-half of the difference between the outside diameter OD 2  and the inside diameter ID 2  is equal to the blade length L 2   a  of each of the second blades  12 B in the first cross-section (Blade Length L 2   a =(Outside Diameter OD 2 −Inside Diameter ID 2 )/2). The blade length L 2   a  of each of the second blades  12 B in the first cross-section is shorter than the blade length L 1   a  of each of the first blades  12 A in the same cross-section (Blade Length L 2   a &lt;Blade Length L 1   a ). 
     Note here that the ratio of the inside diameter to the outside diameter of the second blades  12 B is lower than or equal to 0.7. That is, the plurality of second blades  12 B are configured such that the ratio of the inside diameter ID 2  constituted by the inner circumferential end  14 B of each of the plurality of second blades  12 B to the outside diameter OD 2  constituted by the outer circumferential end  15 B of each of the plurality of second blades  12 B is lower than or equal to 0.7. 
     Meanwhile, let it be assumed that as shown in  FIG.  13   , in the second cross-section closer to the rim  13  than the middle point MP shown in  FIG.  11   , the diameter of a circle C 7  passing through the inner circumferential ends  14 A of the first blades  12 A about the rotation shaft RS is an inside diameter ID 3 . The inside diameter ID 3  is larger than the inside diameter ID 1  of the first cross-section (Inside Diameter ID 3 &gt;Inside Diameter ID 1 ). Let it be assumed that the diameter of a circle C 8  passing through the outer circumferential ends  15 A of the first blades  12 A about the rotation shaft RS is an outside diameter OD 3 . One-half of the difference between the outside diameter OD 3  and the inside diameter ID 1  is equal to the blade length Lib of each of the first blades  12 A in the second cross-section (Blade Length L 1   b =(Outside Diameter OD 3 −Inside Diameter ID 3 )/2). 
     Further, let it be assumed that in the second cross-section, the diameter of the circle C 7  passing through the inner circumferential ends  14 B of the second blades  12 B about the rotation shaft RS is an inside diameter ID 4 . The inside diameter ID 4  is equal to the inside diameter ID 3  in the same cross-section (Inside Diameter ID 4 =Inside Diameter ID 3 ). Let it be assumed that the diameter of the circle C 8  passing through the outer circumferential ends  15 B of the second blades  12 B about the rotation shaft RS is an outside diameter OD 4 . The outside diameter OD 4  is equal to the outside diameter OD 3  in the same cross-section (Outside Diameter OD 4 =Outside Diameter OD 3 ). One-half of the difference between the outside diameter OD 4  and the inside diameter ID 4  is equal to the blade length L 2   b  of each of the second blades  12 B in the second cross-section (Blade Length L 2   b =(Outside Diameter OD 4 −Inside Diameter ID 4 )/2). The blade length L 2   b  of each of the second blades  12 B in the second cross-section is equal to the blade length L 1   b  of each of the first blades  12 A in the same cross-section (Blade Length L 2   b =Blade Length L 1   b ). 
     When viewed from an angle parallel with the rotation shaft RS, the first blades  12 A in the second cross-section shown in  FIG.  13    overlap the first blades  12 A in the first cross-section shown in  FIG.  12    so as not to extend off the contours of the first blades  12 A. For this reason, the impeller  10  satisfies the relationships “Outside Diameter OD 3 =Outside Diameter OD 1 ”, “Inside Diameter ID 3 &gt;Inside Diameter ID 1 ”, and “Blade Length L 1   b ≤Blade Length L 1   a”.    
     Similarly, when viewed from an angle parallel with the rotation shaft RS, the second blades  12 B in the second cross-section shown in  FIG.  13    overlap the second blades  12 B in the first cross-section shown in  FIG.  12    so as not to extend off the contours of the second blades  12 B. For this reason, the impeller  10  satisfies the relationships “Outside Diameter OD 4 =Outside Diameter OD 2 ”, “Inside Diameter ID 4 ≥Inside Diameter ID 2 ”, and “Blade Length L 2   b ≤Blade Length L 2   a”.    
     Note here that as mentioned above, the ratio of the inside diameter ID 1  to the outside diameter OD 1  of the first blades  12 A is lower than or equal to 0.7. Since the blades  12  are configured such that Inside Diameter ID 3 ≥Inside Diameter ID 1 , Inside Diameter ID 4 ≥Inside Diameter ID 2 , and Inside Diameter ID 2 &gt;Inside Diameter ID 1 , the inside diameter of the first blades  12 A can be the blade inside diameter of the blades  12 . Further, since the blades  12  are configured such that Outside Diameter OD 3 =Outside Diameter OD 1 , Outside Diameter OD 4 =Outside Diameter OD 2 , and Outside Diameter OD 2 =Outside Diameter OD 1 , the outside diameter of the first blades  12 A can be the blade outside diameter of the blades  12 . Moreover, in a case in which the blades  12  constituting the impeller  10  are seen as a whole, the blades  12  are configured such that the ratio of the blade inside diameter to the blade outside diameter of the blades  12  is lower than or equal to 0.7. 
     It should be noted that the blade inside diameter of the plurality of blades  12  is constituted by the inner circumferential end of each of the plurality of blades  12 . That is, the blade inside diameter of the plurality of blades  12  is constituted by the leading edges  14 A 1  of the plurality of blades  12 . Further, the blade outside diameter of the plurality of blades  12  is constituted by the outer circumferential end of each of the plurality of blade  12 . That is, the blade outside diameter of the plurality of blades  12  is constituted by the trailing edges  15 A 1  and  15 B 1  of the plurality of blades  12 . 
     (Configuration of First Blades  12 A and Second Blades  12 B) 
     In a comparison between the first cross-section shown in  FIG.  12    and the second cross-section shown in  FIG.  13   , each of the first blades  12 A has the relationship “Blade Length L 1   a &gt;Blade Length L 1   b ”. That is, each of the plurality of blades  12  is formed such that a blade length in the first region is longer than a blade length in the second region. More specifically, each of the first blades  12 A is formed such that its blade length decreases from the back plate  11  toward the rim  13  in the axial direction of the rotation shaft RS. 
     Similarly, in a comparison between the first cross-section shown in  FIG.  12    and the second cross-section shown in  FIG.  13   , each of the second blades  12 B has the relationship “Blade Length L 2   a &gt;Blade Length L 2   b ”. That is, each of the second blades  12 B is formed such that the blade length decreases from the back plate  11  toward the rim  13  in the axial direction of the rotation shaft RS. 
     As shown in  FIG.  3   , the leading edges of the first blades  12 A and the second blades  12 B are inclined such that the blade inside diameter increases from the back plate  11  toward the rim  13 . That is, the plurality of blades  12  are formed such that the blade inside diameter increases from the back plate  11  toward the rim  13 , and form an inclined portion  141 A inclined such that the inner circumferential ends  14 A constituting the leading edges  14 A 1  extend away from the rotation shaft RS. Similarly, the plurality of blades  12  are formed such that the blade inside diameter increases from the back plate  11  toward the rim  13 , and form an inclined portion  141 B inclined such that the inner circumferential ends  14 B constituting the leading edges  14 B 1  extend away from the rotation shaft RS. 
     (Sirocco Blade Portion and Turbo Blade Portion) 
     As shown in  FIGS.  12  and  13   , each of the first blades  12 A has a first sirocco blade portion  12 A 1  being forward-swept and including the outer circumferential end  15 A and a first turbo blade portion  12 A 2  being swept-back and including the inner circumferential end  14 A. In the radial direction of the impeller  10 , the first sirocco blade portion  12 A 1  constitutes an outer circumference of the first blade  12 A, and the first turbo blade portion  12 A 2  constitutes an inner circumference of the first blade  12 A. That is, each of the first blades  12 A is configured such that the first turbo blade portion  12 A 2  and the first sirocco blade portion  12 A 1  are arranged in this order from the rotation shaft RS toward the outer circumference in the radial direction of the impeller  10 . 
     In each of the first blades  12 A, the first turbo blade portion  12 A 2  and the first sirocco blade portion  12 A 1  are integrally formed. The first turbo blade portion  12 A 2  constitutes the leading edge  14 A 1  of the first blade  12 A, and the first sirocco blade portion  12 A 1  constitutes the trailing edge  15 A 1  of the first blade  12 A. In the radial direction of the impeller  10 , the first turbo blade portion  12 A 2  linearly extends from the inner circumferential end  14 A constituting the leading edge  14 A 1  toward the outer circumference. 
     In the radial direction of the impeller  10 , a region constituting the first sirocco blade portion  12 A 1  of each of the first blades  12 A is defined as a first sirocco region  12 A 11 , and a region constituting the first turbo blade portion  12 A 2  of each of the first blades  12 A is defined as a first turbo region  12 A 21 . Each of the first blades  12 A is configured such that the first turbo region  12 A 21  is larger than the first sirocco region  12 A 11  in the radial direction of the impeller  10 . 
     In both the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region, the impeller  10  has the relationship “First Sirocco Region  12 A 11 &lt;First Turbo Region  12 A 21 ” in the radial direction of the impeller  10 . The impeller  10  and each of the first blades  12 A are configured such that in both the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region, a ratio of the first turbo blade portion  12 A 2  is larger than a ratio of the first sirocco blade portion  12 A 1  in the radial direction of the impeller  10 . 
     Similarly, as shown in  FIGS.  12  and  13   , each of the second blades  12 B has a second sirocco blade portion  12 B 1  being forward-swept and including the outer circumferential end  15 B and a second turbo blade portion  12 B 2  being swept-back and including the inner circumferential end  14 B. In the radial direction of the impeller  10 , the second sirocco blade portion  12 B 1  constitutes an outer circumference of the second blade  12 B, and the second turbo blade portion  12 B 2  constitutes an inner circumference of the second blade  12 B. That is, each of the second blades  12 B is configured such that the second turbo blade portion  12 B 2  and the second sirocco blade portion  12 B 1  are arranged in this order from the rotation shaft RS toward the outer circumference in the radial direction of the impeller  10 . 
     In each of the second blades  12 B, the second turbo blade portion  12 B 2  and the second sirocco blade portion  12 B 1  are integrally formed. The second turbo blade portion  12 B 2  constitutes the leading edge  14 B 1  of the second blade  12 B, and the second sirocco blade portion  12 B 1  constitutes the trailing edge  15 B 1  of the second blade  12 B. In the radial direction of the impeller  10 , the second turbo blade portion  12 B 2  linearly extends from the inner circumferential end  14 B constituting the leading edge  14 B 1  toward the outer circumference. 
     In the radial direction of the impeller  10 , a region constituting the second sirocco blade portion  12 B 1  of each of the second blades  12 B is defined as a second sirocco region  12 B 11 , and a region constituting the second turbo blade portion  12 B 2  of each of the second blades  12 B is defined as a second turbo region  12 B 21 . Each of the second blades  12 B is configured such that the second turbo region  12 B 21  is larger than the second sirocco region  12 B 11  in the radial direction of the impeller  10 . 
     In both the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region, the impeller  10  has the relationship “Second Sirocco Region  12 B 11 &lt;Second Turbo Region  12 B 21 ” in the radial direction of the impeller  10 . The impeller  10  and each of the second blades  12 B are configured such that in both the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region, a ratio of the second turbo blade portion  12 B 2  is larger than a ratio of the second sirocco blade portion  12 B 1  in the radial direction of the impeller  10 . 
     According to the foregoing configuration, the plurality of blades  12  are configured such that in both the back-plate-side blade region  122   a  and the rim-side blade region  122   b , a region of a turbo blade portion is larger than a region of a sirocco blade portion in the radial direction of the impeller  10 . That is, the plurality of blades  12  are configured such that in both the back-plate-side blade region  122   a  and the rim-side blade region  122   b , a ratio of the turbo blade portion is larger than a ratio of the sirocco blade portion in the radial direction of the impeller  10 , and have the relationship “Sirocco Region&lt;Turbo Region”. In other words, each of the plurality of blades  12  is configured such that in the first region and the second region, a ratio of the turbo blade portion in the radial direction is larger than a ratio of the sirocco blade portion in the radial direction. 
     The plurality of blades  12  are not limited to being configured such that in both the back-plate-side blade region  122   a  and the rim-side blade region  122   b , a ratio of the turbo blade portion is larger than a ratio of the sirocco blade portion in the radial direction of the impeller  10 , or to having the relationship “Sirocco Region&lt;Turbo Region”. Each of the plurality of blades  12  may be configured such that in the first region and the second region, a ratio of the turbo blade portion in the radial direction is equal to or smaller than a ratio of the sirocco blade portion in the radial direction. 
     (Blade Outlet Angle) 
     Let it be assumed that as shown in  FIG.  12   , a blade outlet angle of the first sirocco blade portion  12 A 1  of each of the first blades  12 A in the first cross-section is a blade outlet angle α 1 . The blade outlet angle α 1  is defined as an angle formed by a tangent line TL 1  and a center line CL 1  of the first sirocco blade portion  12 A 1  at the outer circumferential end  15 A at an intersection of a segment of the circle C 3  about the rotation shaft RS and the outer circumferential end  15 A. This blade outlet angle α 1  is an angle of larger than 90 degrees. 
     Let it be assumed that a blade outlet angle of the second sirocco blade portion  12 B 1  of each of the second blades  12 B in the same cross-section is a blade outlet angle α 2 . The blade outlet angle α 2  is defined as an angle formed by a tangent line TL 2  and a center line CL 2  of the second sirocco blade portion  12 B 1  at the outer circumferential end  15 B at an intersection of a segment of the circle C 3  about the rotation shaft RS and the outer circumferential end  15 B. The blade outlet angle α 2  is an angle of larger than 90 degrees. 
     The blade outlet angle α 2  of the second sirocco blade portion  12 B 1  is equal to the blade outlet angle α 1  of the first sirocco blade portion  12 A 1  (Blade Outlet Angle α2=Blade Outlet Angle α 1 ). The first sirocco blade portion  12 A 1  and the second sirocco blade portion  12 B 1  are formed in arcs to curve out in a direction opposite to the direction of rotation R when viewed from an angle parallel with the rotation shaft RS. 
     As shown in  FIG.  13   , the impeller  10  is configured such that in the second cross-section, too, the blade outlet angle α 1  of the first sirocco blade portion  12 A 1  and the blade outlet angle α 2  of the second sirocco blade portion  12 B 1  are equal to each other. That is, each of the plurality of blades  12  has a sirocco blade portion being forward-swept and extending from the back plate  11  to the rim  13  and having a blade outlet angle of larger than 90 degrees. 
     Further, let it be assumed that as shown in  FIG.  12   , a blade outlet angle of the first turbo blade portion  12 A 2  of each of the first blades  12 A in the first cross-section is a blade outlet angle β 1 . The blade outlet angle β 1  is defined as an angle formed by a tangent line TL 3  and a center line CL 3  of the first turbo blade portion  12 A 2  at an intersection of a segment of a circle C 4  about the rotation shaft RS and the first turbo blade portion  12 A 2 . This blade outlet angle β 1  is an angle of smaller than 90 degrees. 
     Let it be assumed that a blade outlet angle of the second turbo blade portion  12 B 2  of each of the second blades  12 B in the same cross-section is a blade outlet angle β 2 . The blade outlet angle β 2  is defined as an angle formed by a tangent line TL 4  and a center line CL 4  of the second turbo blade portion  12 B 2  at an intersection of a segment of the circle C 4  about the rotation shaft RS and the second turbo blade portion  12 B 2 . The blade outlet angle β 2  is an angle of smaller than 90 degrees. 
     The blade outlet angle β 2  of the second turbo blade portion  12 B 2  is equal to the blade outlet angle β 1  of the first turbo blade portion  12 A 2  (Blade Outlet Angle β2=Blade Outlet Angle β 1 ). 
     Although not illustrated in  FIG.  13   , the impeller  10  is configured such that in the second cross-section, too, the blade outlet angle β 1  of the first turbo blade portion  12 A 2  and the blade outlet angle β 2  of the second turbo blade portion  12 B 2  are equal to each other. Further, the blade outlet angle β 1  and the blade outlet angle β 2  are angles of smaller than 90 degrees. 
     (Radial Blade Portion) 
     As shown in  FIGS.  12  and  13   , each of the first blades  12 A has a first radial blade portion  12 A 3  serving as a portion of connection between the first turbo blade portion  12 A 2  and the first sirocco blade portion  12 A 1 . The first radial blade portion  12 A 3  is a portion configured to be a radial blade linearly extending in the radial direction of the impeller  10 . 
     Similarly, each of the second blades  12 B has a second radial blade portion  12 B 3  serving as a portion of connection between the second turbo blade portion  12 B 2  and the second sirocco blade portion  12 B 1 . The second radial blade portion  12 B 3  is a portion configured to be a radial blade linearly extending in the radial direction of the impeller  10 . 
     The first radial blade portion  12 A 3  and the second radial blade portion  12 B 3  each have a blade angle of 90 degrees. More specifically, an angle formed by a tangent line at an intersection of a center line of the first radial blade portion  12 A 3  and a circle C 5  about the rotation shaft RS and the center line of the first radial blade portion  12 A 3  is 90 degrees. Further, an angle formed by a tangent line at an intersection of a center line of the second radial blade portion  12 B 3  and the circle C 5  about the rotation shaft RS and the center line of the second radial blade portion  12 B 3  is 90 degrees. 
     (Inter-Blade Distance) 
     When a spacing between two of the plurality of blades  12  adjacent to each other along the circumferential direction is defined as an inter-blade distance, the inter-blade distance between a plurality of blades  12  widens from the leading edges  14 A 1  toward the trailing edges  15 A 1  as shown in  FIGS.  12  and  13   . Similarly, the inter-blade distance between a plurality of blades  12  widens from the leading edges  14 B 1  toward the trailing edges  15 B 1 . 
     Specifically, an inter-blade distance in the turbo blade portion constituted by the first turbo blade portion  12 A 2  and the second turbo blade portion  12 B 2  widens from the inner circumference toward the outer circumference. Moreover, an inter-blade distance in a sirocco blade portion constituted by a first sirocco blade portion  12 A 1  and a second sirocco blade portion  12 B 1  is wider than the inter-blade distance in the turbo blade portion and widens from the inner circumference toward the outer circumference. 
     That is, an inter-blade distance between a first turbo blade portion  12 A 2  and a second turbo blade portion  12 B 2  or an inter-blade distance between adjacent second turbo blade portions  12 B 2  widens from the inner circumference toward the outer circumference. Further, an inter-blade distance between a first sirocco blade portion  12 A 1  and a second sirocco blade portion  12 B 1  or an inter-blade distance between adjacent second sirocco blade portions  12 B 1  is wider than the inter-blade distance in the turbo blade portion and widens from the inner circumference toward the outer circumference. 
     (Relationship Between Impeller  10  and Scroll Casing  40 ) 
       FIG.  14    is a schematic view showing a relationship between the impeller  10  and bellmouths  46  in a cross-section of the multi-blade air-sending device  100  as taken along line A-A in  FIG.  2   .  FIG.  15    is a schematic view showing a relationship between blades  12  and a bellmouth  46  as viewed from an angle parallel with the rotation shaft RS in a second cross-section of the impeller  10  in  FIG.  14   . 
     As shown in  FIGS.  14  and  15   , a blade outside diameter OD constituted by the outer circumferential end of each of the plurality of blades  12  is larger than the inside diameter BI of a bellmouth  46  constituting the scroll casing  40 . It should be noted that the blade outside diameter OD of the plurality of blades  12  is equal to the outside diameters OD 1  and OD 2  of the first blades  12 A and the outside diameter OD 3  and OD 4  of the second blades  12 B (Blade Outside Diameter OD=Outside Diameter OD 1 =Outside Diameter OD 2 =Outside Diameter OD 3 =Outside Diameter OD 4 ). 
     The impeller  10  is configured such that the first turbo region  12 A 21  is larger than the first sirocco region  12 A 11  in the radial direction with respect to the rotation shaft RS. That is, the impeller  10  and each of the first blades  12 A are configured such that the ratio of the first turbo blade portion  12 A 2  is larger than the ratio of the first sirocco blade portion  12 A 1  in the radial direction with respect to the rotation shaft RS, and have the relationship “First Sirocco Blade Portion  12 A 1 &lt;First Turbo Blade Portion  12 A 2 ”. The relationship between the ratio of the first sirocco blade portion  12 A 1  and the ratio of the first turbo blade portion  12 A 2  in the radial direction of the rotation shaft RS holds in both the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region. 
     It should be noted that the impeller  10  and each of the first blades  12 A are not limited to being configured such that the ratio of the first turbo blade portion  12 A 2  is larger than the ratio of the first sirocco blade portion  12 A 1  in the radial direction with respect to the rotation shaft RS, or to having the relationship “First Sirocco Blade Portion  12 A 1 &lt;First Turbo Blade Portion  12 A 2 ”. The impeller  10  and each of the first blades  12 A may be formed such that the ratio of the first turbo blade portion  12 A 2  is equal to or smaller than the ratio of the first sirocco blade portion  12 A 1  in the radial direction with respect to the rotation shaft RS. 
     Furthermore, a region of portions of the plurality of blades  12  situated closer to the outer circumference than the inside diameter BI of the bellmouth  46  in the radial direction with respect to the rotation shaft RS when viewed from an angle parallel with the rotation shaft RS is defined as an outer circumferential region  12 R. It is desirable that the impeller  10  be configured such that in the outer circumferential region  12 R, too, the ratio of the first turbo blade portion  12 A 2  is larger than the ratio of the first sirocco blade portion  12 A 1 . That is, in the outer circumferential region  12 R of the impeller  10  situated closer to the outer circumference than the inside diameter BI of the bellmouth  46  when viewed from an angle parallel with the rotation shaft RS, a first turbo region  12 A 21   a  is larger than the first sirocco region  12 A 11  in the radial direction with respect to the rotation shaft RS. 
     The first turbo region  12 A 21   a  is a region of the first turbo region  12 A 21  situated closer to the outer circumference than the inside diameter BI of the bellmouth  46  when viewed from an angle parallel with the rotation shaft RS. Moreover, in a case in which a first turbo blade portion  12 A 2  constituting the first turbo region  12 A 21   a  is a first turbo blade portion  12 A 2   a , it is desirable that the outer circumferential region  12 R of the impeller  10  be configured such that a ratio of the first turbo blade portion  12 A 2   a  is larger than the ratio of the first sirocco blade portion  12 A 1 . The relationship between the ratio of the first sirocco blade portion  12 A 1  and the ratio of the first turbo blade portion  12 A 2   a  in the outer circumferential region  12 R holds in both the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region. 
     Similarly, the impeller  10  is configured such that the second turbo region  12 B 21  is larger than the second sirocco region  12 B 11  in the radial direction with respect to the rotation shaft RS. That is, the impeller  10  and each of the second blades  12 B are configured such that the ratio of the second turbo blade portion  12 B 2  is larger than the ratio of the second sirocco blade portion  12 B 1  in the radial direction with respect to the rotation shaft RS, and have the relationship “Second Sirocco Blade Portion  12 B1&lt;Second Turbo Blade Portion  12 B 2 ”. The relationship between the ratio of the second sirocco blade portion  12 B 1  and the ratio of the second turbo blade portion  12 B 2  in the radial direction of the rotation shaft RS holds in both the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region. 
     It should be noted that the impeller  10  and each of the second blades  12 B are not limited to being configured such that the ratio of the second turbo blade portion  12 B 2  is larger than the ratio of the second sirocco blade portion  12 B 1  in the radial direction with respect to the rotation shaft RS, or to having the relationship “Second Sirocco Blade Portion  12 B 1 &lt;Second Turbo Blade Portion  12 B 2 ”. The impeller  10  and each of the second blades  12 B may be formed such that the ratio of the second turbo blade portion  12 B 2  is equal to or smaller than the ratio of the second sirocco blade portion  12 B 1  in the radial direction with respect to the rotation shaft RS. 
     Furthermore, it is desirable that the impeller  10  be configured such that in the outer circumferential region  12 R, too, the ratio of the second turbo blade portion  12 B 2  is larger than the ratio of the second sirocco blade portion  12 B 1 . That is, in the outer circumferential region  12 R of the impeller  10  situated closer to the outer circumference than the inside diameter BI of the bellmouth  46  when viewed from an angle parallel with the rotation shaft RS, a second turbo region  12 B 21   a  is larger than the second sirocco region  12 B 11  in the radial direction with respect to the rotation shaft RS. 
     The second turbo region  12 B 21   a  is a region of the second turbo region  12 B 21  situated closer to the outer circumference than the inside diameter BI of the bellmouth  46  when viewed from an angle parallel with the rotation shaft RS. Moreover, in a case in which a second turbo blade portion  12 B 2  constituting the second turbo region  12 B 21   a  is a second turbo blade portion  12 B 2   a , it is desirable that the outer circumferential region  12 R of the impeller  10  be configured such that a ratio of the second turbo blade portion  12 B 2   a  is larger than the ratio of the second sirocco blade portion  12 B 1 . The relationship between the ratio of the second sirocco blade portion  12 B 1  and the ratio of the second turbo blade portion  12 B 2   a  in the outer circumferential region  12 R holds in both the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region. 
       FIG.  16    is a schematic view showing a relationship between the impeller  10  and the bellmouths  46  in the cross-section of the multi-blade air-sending device  100  as taken along line A-A in  FIG.  2   .  FIG.  17    is a schematic view showing a relationship between the blades  12  and a bellmouth  46  as viewed from an angle in parallel with the rotation shaft RS in the impeller  10  in  FIG.  16   . In  FIG.  16   , the outline arrow L indicates a direction from which the impeller  10  is viewed from an angle parallel with the rotation shaft RS. 
     As shown in  FIGS.  16  and  17   , a circle passing through the inner circumferential ends  14 A of the plurality of first blades  12 A about the rotation shaft RS at connecting locations between the first blades  12 A and the back plate  11  when viewed from an angle parallel with the rotation shaft RS is defined as a circle C 1   a . Moreover, let it be assumed that the diameter of the circle C 1   a , that is, the inside diameter of the first blades  12 A at the connecting locations between the first blades  12 A and the back plate  11 , is an inside diameter ID 1   a.    
     Further, a circle passing through the inner circumferential ends  14 B of the plurality of second blades  12 B about the rotation shaft RS at connecting locations between the second blades  12 B and the back plate  11  when viewed from an angle parallel with the rotation shaft RS is defined as a circle C 2   a . Moreover, let it be assumed that the diameter of the circle C 2   a , that is, the inside diameter of the second blades  12 B at the connecting locations between the first blades  12 A and the back plate  11 , is an inside diameter ID 2   a . The inside diameter ID 2   a  is larger than the inside diameter ID 1   a  (Inside Diameter ID 2   a &gt;Inside Diameter ID 1   a ). 
     Further, let it be assumed that the diameter of a circle C 3   a  passing through the outer circumferential ends  15 A of the plurality of first blades  12 A and the outer circumferential ends  15 B of the plurality of second blades  12 B about the rotation shaft RS when viewed from an angle parallel with the rotation shaft RS, that is, the outside diameter of the plurality of blades  12 , is a blade outside diameter OD. 
     Further, a circle passing through the inner circumferential ends  14 A of the plurality of first blades  12 A about the rotation shaft RS at connecting locations between the first blades  12 A and the rim  13  when viewed from an angle parallel with the rotation shaft RS is defined as a circle C 7   a . Moreover, let it be assumed that the diameter of the circle C 7   a , that is, the inside diameter of the first blades  12 A at the connecting locations between the first blades  12 A and the rim  13 , is an inside diameter ID 3   a.    
     Further, a circle passing through the inner circumferential ends  14 B of the plurality of second blades  12 B about the rotation shaft RS at connecting locations between the second blades  12 B and the rim  13  when viewed from an angle parallel with the rotation shaft RS is the circle C 7   a . Moreover, let it be assumed that the diameter of the circle C 7   a , that is, the inside diameter of the second blades  12 B at the connecting locations between the second blades  12 B and the rim  13 , is an inside diameter ID 4   a.    
     As shown in  FIGS.  16  and  17   , the inside diameter BI of the bellmouth  46  is located in a region of the first turbo blade portions  12 A 2  and the second turbo blade portions  12 B 2  between the inside diameter ID 1   a  of the first blades  12 A beside the back plate  11  and the inside diameter ID 3   a  of the first blades  12 A beside the rim  13  when viewed from an angle parallel with the rotation shaft RS. More specifically, the inside diameter BI of the bellmouth  46  is larger than the inside diameter ID 1   a  of the first blades  12 A beside the back plate  11  and smaller than the inside diameter ID 3   a  of the first blades  12 A beside the rim  13 . 
     That is, the inside diameter BI of the bellmouth  46  is larger than the blade inside diameter of the plurality of blades  12  beside the back plate  11  and smaller than the blade inside diameter of the plurality of blades  12  beside the rim  13 . In other words, an opening  46   a  forming the inside diameter BI of the bellmouth  46  is located in a region of the first turbo blade portions  12 A 2  and the second turbo blade portions  12 B 2  between the circle C 1   a  and the circle C 7   a  when viewed from an angle parallel with the rotation shaft RS. 
     Further, as shown in  FIGS.  16  and  17   , the inside diameter BI of the bellmouth  46  is located in a region of the first turbo blade portions  12 A 2  and the second turbo blade portions  12 B 2  between the inside diameter ID 2   a  of the second blades  12 B beside the back plate  11  and the inside diameter ID 4   a  of the second blades  12 B beside the rim  13  when viewed from an angle parallel with the rotation shaft RS. More specifically, the inside diameter BI of the bellmouth  46  is larger than the inside diameter ID 2   a  of the second blades  12 B beside the back plate  11  and smaller than the inside diameter ID 4   a  of the second blades  12 B beside the rim  13 . 
     That is, the inside diameter BI of the bellmouth  46  is larger than the blade inside diameter of the plurality of blades  12  beside the back plate  11  and smaller than the blade inside diameter of the plurality of blades  12  beside the rim  13 . More specifically, the inside diameter BI of the bellmouth  46  is larger than a blade inside diameter constituted by the inner circumferential end of each of the plurality of blades  12  in the first region and smaller than a blade inside diameter constituted by the inner circumferential end of each of the plurality of blades  12  in the second region. In other words, the opening  46   a  forming the inside diameter BI of the bellmouth  46  is located in a region of the first turbo blade portions  12 A 2  and the second turbo blade portions  12 B 2  between the circle C 2   a  and the circle C 7   a  when viewed from an angle parallel with the rotation shaft RS. 
     Let it be assumed that as shown in  FIGS.  16  and  17   , in the radial direction of the impeller  10 , a radial length of each of the first and second sirocco blade portions  12 A 1  and  12 B 1  is a distance SL. Further, let it be assumed that in the multi-blade air-sending device  100 , the shortest distance between the plurality of blades  12  of the impeller  10  and the peripheral wall  44   c  of the scroll casing  40  is a distance MS. In this case, the multi-blade air-sending device  100  is configured such that the distance MS is more than twice as long as the distance SL (Distance MS&gt;Distance SL x  2 ). Although the distance MS is shown in the A-A section of the multi-blade air-sending device  100  in  FIG.  16   , the distance MS is the shortest distance from the peripheral wall  44   c  of the scroll casing  40  and is not necessarily shown on the A-A section. 
     [Working Effects of Impeller  10  and Multi-Blade Air-Sending Device  100 ] 
     The back plate  11  includes a first surface portion  11   a  on which the plurality of blades  12  are formed and a second surface portion  11   c  provided at a region between the boss  11   b  and the first surface portion  11   a  and depressed from the first surface portion  11   a  in an axial direction of the rotation shaft RS. Further, the back plate  11  also includes a plurality of projections  20  provided at the second surface portion  11   c  and extending in the axial direction of the rotation shaft RS. While the impeller  10  is rotating, the projections  20  draw in a flow of gas by generating negative pressure on a surface of the impeller  10  facing in a direction opposite to a direction of rotation R of the impeller  10 , making it possible to increase the amount of air that is suctioned into the impeller  10 . Further, the impeller  10  includes the second surface portion  11   c  depressed from the first surface portion  11   a , on which the plurality of blades  12  are formed, in the axial direction of the rotation shaft RS, and the projections  20  are provided at the second surface portion  11   c . This inhibits a flow of gas produced by the projections  20  from flowing from the second surface portion  11   c  into the first surface portion  11   a . Moreover, the flow of gas produced by the projections  20  has its centrifugally-outward force of wind broken by a step  11   f  between the first surface portion  11   a  and the second surface portion  11   c , so that the impeller  10  does not suffer from turbulence in the flow of gas on the inner circumference of the blades  12 . This allows the impeller  10  to have higher air-sending efficiency than in a case in which the impeller  10  does not include the projections  20  or the second surface portion  11   c.    
     Further, the flow of gas produced by the projections  20  has its centrifugally-outward force of wind broken by the step  11   f  between the first surface portion  11   a  and the second surface portion  11   c , so that the impeller  10  does not suffer from turbulence in the flow of gas on the inner circumference of the blades  12 . This allows the impeller  10  to reduce noise caused by turbulence in the flow of gas. 
     Further, the second surface portion  11   c  is formed in a circular ring shape about the boss  11   b . This inhibits a flow of gas produced by the projections  20  from flowing from the second surface portion  11   c  into the first surface portion  11   a . Moreover, the flow of gas produced by the projections  20  has its centrifugally-outward force of wind broken by the step  11   f  between the first surface portion  11   a  and the second surface portion  11   c , so that the impeller  10  does not suffer from turbulence in the flow of gas on the inner circumference of the blades  12 . This allows the impeller  10  to have improved air-sending efficiency. Further, with the second surface portion  11   c  formed in a circular ring shape about the boss  11   b , the impeller  10  makes it possible to break the centrifugally-outward force of wind at any place along the circumferential direction about the boss  11   b . Further, since the second surface portion  11   c  is formed in a circular ring shape about the boss  11   b , the impeller  10  is more easily manufactured than in a case in which the second surface portion  11   c  is complex in structure. Further, since the second surface portion  11   c  is formed in a circular ring shape about the boss  11   b , the impeller  10  more easily keeps its balance and is more easily manufactured than in a case in which the second surface portion  11   c  is complex in structure. 
     Further, the length of a depression outside diameter PO constituted by the outer circumferential edge  11   c   1  of the second surface portion  11   c  is greater than the magnitude of a difference PS between an inside diameter ID 1  of the blades  12  constituted by an inner circumferential end  14 A of each of the plurality of blades  12  and the depression outside diameter PO. Therefore, the impeller  10  can be configured such that the projections  20 , which draw in a flow of gas, are formed to extend from the boss  11   b  to the vicinity of the inside diameter of the blades  12  in a radial direction. This results in allowing the impeller  10  to suction a larger amount of air with the projections  20  than in a case in which the impeller  10  does not include the projections  20  and to have improved air-sending efficiency. 
     The plurality of projections  20  are provided in a radial fashion about the rotation shaft RS, and each of the plurality of projections  20  extends in a radial direction about the rotation shaft RS. While the impeller  10  is rotating, the projections  20  draw in a flow of gas by generating negative pressure on the surface of the impeller  10  facing in a direction opposite to the direction of rotation R of the impeller  10 , making it possible to increase the amount of air that is suctioned into the impeller  10 . By being formed in this configuration, the plurality of projections  20  make it easier to manufacture the impeller  10  than in a case in which the projections  20  are complex in structure. Further, by being formed in this configuration, the plurality of projections  20  make it easier to keep the balance of the impeller  10  and make it easier to manufacture the impeller  10  than in a case in which the projections  20  are complex in structure. 
     Further, each of the plurality of projections  20  is formed in the shape of a plate rising from the second surface portion  11   c . While the impeller  10  is rotating, the projections  20  make it easy to generate negative pressure on the surface of the impeller  10  facing in a direction opposite to the direction of rotation R of the impeller  10  and make it even easier to draw in a flow of gas, thereby making it possible to further increase the amount of air that is suctioned into the impeller  10 . 
     Further, each of the plurality of projections  20  is connected to an outer circumferential wall  11   b   2  of the boss  11   b . Since the impeller  10  is configured such that the projections  20  are connected to the boss  11   b , the strength of the projections  20  can be improved. Further, since the impeller  10  is configured such that the projections  20  are connected to the boss  11   b , the strength of the impeller  10  can be improved. 
     Further, a projection outer circumferential end  21  of each of the projections  20  does not project from the first surface portion  11   a  in the axial direction of the rotation shaft RS. Therefore, even when the projections  20  are connected to the step  11   f , the flow of gas produced by the projections  20  has its centrifugally-outward force of wind broken by the step  11   f  between the first surface portion  11   a  and the second surface portion  11   c , so that the impeller  10  does not suffer from turbulence in the flow of gas on the inner circumference of the blades  12 . This allows the impeller  10  to have higher air-sending efficiency than in a case in which the impeller  10  does not include the projections  20  or the second surface portion  11   c.    
     Further, the length of a projection outside diameter QO constituted by the projection outer circumferential end  21  of each of the plurality of projections  20  is greater than the magnitude of a difference QS between the inside diameter ID 1  of the blades  12  constituted by the inner circumferential end  14 A of each of the plurality of blades  12  and the projection outside diameter QO. Therefore, the impeller  10  can be configured such that the projections  20 , which draw in a flow of gas, are formed to extend from the boss  11   b  to the vicinity of the inside diameter of the blades  12  in a radial direction. This results in allowing the impeller  10  to suction a larger amount of air with the projections  20  than in a case in which the impeller  10  does not include the projections  20  and to have improved air-sending efficiency. 
     Further, each of the plurality of projections  20  includes an inclined portion  26   a  whose ridge line is inclined such that the height of the inclined portion  26   a  in the axial direction of the rotation shaft RS decreases from the inner circumference toward the outer circumference. While the impeller  10  is rotating, the projections  20  draw in a flow of gas by generating negative pressure on the surface of the impeller  10  facing in a direction opposite to the direction of rotation R of the impeller  10 , making it possible to increase the amount of air that is suctioned into the impeller  10 . In so doing, the impeller  10  is higher in wind speed on the outer circumference than on the inner circumference, and an increase in height of projections  20  on the outer circumference leads to an increase in the amount of a flow of gas that is generated on the outer circumference of the projections  20 , which may cause turbulence in the flow of gas on the inner circumference of the blades  12 . On the other hand, since the impeller  10  is lower in wind speed on the inner circumference than on the outer circumference, an increase in the amount of a flow of gas that is generated on the inner circumference of the projections  20  does not cause turbulence in the flow of gas by the blades  12 . This allows the impeller  10  to suction a further increased amount of a flow of gas and to have improved air-sending efficiency by reducing turbulence in the flow of gas. Further, in a case in which the projections  20  are connected to the boss  11   b , making the projections  20  higher on the inner circumference than on the outer circumference makes it possible to increase an area of integration of the projections  20  and the boss  11   b , making it possible to further improve the strength of the impeller  10 . 
     Further, the back plate  11  includes a reinforcing portion  30  provided at the second surface portion  11   c  and extending in the axial direction of the rotation shaft RS, and the reinforcing portion  30  connects the plurality of projections  20  to each other along the circumferential direction. Since the impeller  10  is configured such that the reinforcing portion  30  and the projections  20  are connected to each other, the strength of the projections  20  can be improved. Further, since the impeller  10  is configured such that the reinforcing portion  30  and the projections  20  are connected to each other, the strength of the impeller  10  can be improved. Further, the reinforcing portion  30  makes it possible to reduce wind currents produced by the projections  20  and flowing in the radial direction and break the force of the wind blowing from the boss  11   b  toward the blades  12 . 
     Further, a plurality of the reinforcing portions  30  are provided in the radial direction about the rotation shaft RS. Since the impeller  10  is configured such that the projections  20  and the plurality of reinforcing portions  30  are connected to each other, the strengths of the projections  20  and the impeller  10  can be further improved. Further, the plurality of reinforcing portions  30  make it possible to further reduce wind currents produced by the projections  20  and flowing in the radial direction and further break the force of the wind blowing from the boss  11   b  toward the blades  12 . With the second surface portion  11   c  having a wide area in the radial direction, the impeller  10  increases in volume of air that is suctioned into the impeller  10 . Narrowing the area of the second surface portion  11   c  in the radial direction by providing the plurality of reinforcing portions  30  allows the impeller  10  to adjust the volume of air that is suctioned into the impeller  10 . 
     Further, the second surface portion  11   c  is constituted by a plate whose thickness is thinner than the thickness of a plate constituting the first surface portion  11   a . Varying plate thicknesses of the back plate  11  of the impeller  10  make it possible to form the first surface portion  11   a  and the second surface portion  11   c , making it easier to manufacture the impeller  10  than in a case in which a relationship between the first surface portion  11   a  and the second surface portion  11   c  is complex in structure. 
     Further, the back plate  11  has its first and second surface portions  11   a  and  11   c  on both plate sides of the back plate  11 , and each of the second surface portions  11   c  formed on both plate sides of the back plate  11  includes the plurality of projections  20 . This allows the impeller  10  to exert the aforementioned effects not only as a single-suction impeller  10  having a plurality of blades  12  formed only on one side of a back plate  11  but also as a double-suction impeller  10  having a plurality of blades  12  formed on both sides of a back plate  11 . 
     The impeller  10  is configured such that in the first and second regions of the impeller  10 , a ratio of the turbo blade portion in the radial direction is larger than a ratio of the sirocco blade portion in the radial direction. Since the impeller  10  is configured such that the ratio of the turbo blade portion is high in any region between the back plate  11  and the rim  13 , sufficient pressure recovery can be achieved through the plurality of blades  12 . This allows the impeller  10  to better improve pressure recovery than an impeller that does not include such a configuration. This results in allowing the impeller  10  to improve the efficiency of the multi-blade air-sending device  100 . Furthermore, by including the foregoing configuration, the impeller  10  can reduce leading edge separation of a flow of gas beside the rim  13 . 
     Further, a multi-blade air-sending device  100  includes the impeller  10  thus configured. The multi-blade air-sending device  100  includes a scroll casing  40  housing the impeller  10  and having a peripheral wall  44   c  formed into a volute shape and a side wall  44   a  having a bellmouth  46  forming an air inlet  45  communicating with a space formed by the back plate  11  and the plurality of blades  12 . The multi-blade air-sending device  100  can bring about effects similar to those of the aforementioned impeller  10 . 
     Embodiment 2 
     [Multi-Blade Air-Sending Device  100 B] 
       FIG.  18    is a partially-enlarged view of an impeller  10  of a multi-blade air-sending device  100 B according to Embodiment 2.  FIG.  19    is a partially-enlarged view of the impeller  10  of the multi-blade air-sending device  100 B according to Embodiment 2.  FIGS.  18  and  19    are different partially-enlarged view of the impeller  10  in a region indicated by part F of  FIG.  7   . The multi-blade air-sending device  100 B according to Embodiment 2 is described with reference to  FIGS.  18  and  19   . It should be noted that elements having identical configurations as those of the multi-blade air-sending device  100  or other devices of  FIGS.  1  to  17    are given identical signs and a description of such elements is omitted. The impeller  10  of the multi-blade air-sending device  100 B according to Embodiment 2 is intended to further specify the configuration of the ridge  26 . Accordingly, the following description is given with reference to  FIGS.  18  and  19    with a focus on the configuration of the ridge  26  of the impeller  10 . 
     While the ridge  26  of each of the projections  20  of the impeller  10  according to Embodiment 1 includes an inclined portion  26   a , the ridge  26  of each of the projections  20  of the impeller  10  according to Embodiment 2 includes a horizontal portion  26   b  as shown in  FIG.  18   . The horizontal portion  26   b  is a portion of the ridge  26  whose ridge line is formed parallel with a plane perpendicular to the rotation shaft RS. 
     Each of the plurality of projections  20  includes a horizontal portion  26   b  having a ridge line constituted by a leading end portion in a direction of projection and extending in a direction perpendicular to the axial direction of the rotation shaft RS in a side view as viewed from the direction perpendicular to the axial direction of the rotation shaft RS. The ridge  26  of each of the projections  20  of the impeller  10  according to Embodiment 2 may be constituted solely by a horizontal portion  26   b  or, as shown in  FIG.  18   , may include a horizontal portion  26   b  and an inclined portion  26   a.    
     The ridge  26  of each of the projections  20  of the impeller  10  according to Embodiment 1 has a ridge line constituted by a leading end portion in a direction of projection and formed in a linear fashion in a side view as viewed from the direction perpendicular to the axial direction of the rotation shaft RS. On the other hand, as shown in  FIG.  19   , the ridge  26  of each of the projections  20  of the impeller  10  according to Embodiment 2 may include a wavy portion  26   c  having a ridge line constituted by a leading end portion in a direction of projection and formed in a wavelike fashion in a side view as viewed from the direction perpendicular to the axial direction of the rotation shaft RS. 
     As shown in  FIG.  19   , each of the plurality of projections  20  includes a wavy portion  26   c , and is formed such that the height of the projection  20  in the axial direction of the rotation shaft RS decreases from the inner circumference toward the outer circumference. The ridge  26  of the projection  20  may be constituted solely by the wavy portion  26   c  or may have the wavy portion  26   c  as part thereof in a radial direction about the rotation shaft RS. Further, each of the plurality of projections  20  is not limited to being configured to be formed such that the height of the projection  20  in the axial direction of the rotation shaft RS decreases from the inner circumference toward the outer circumference. 
     [Working Effects of Impeller  10  and Multi-Blade Air-Sending Device  100 B] 
     As mentioned above, while the impeller  10  is rotating, the projections  20  draw in a flow of gas by generating negative pressure on a surface of the impeller  10  facing in a direction opposite to the direction of rotation R of the impeller  10 , making it possible to increase the amount of air that is suctioned into the impeller  10 . By having a horizontal portion  26   b , each of the plurality of projections  20  can adjust the area of the projection  20  in a cross-section taken along the radial direction of the impeller  10 , and can adjust the volume of air that is suctioned into the impeller  10 . This allows the impeller  10  and the multi-blade air-sending device  100 B to have improved air-sending efficiency. Further, the plurality of projections  20  include wavy portions  26   c . The impeller  10  and the multi-blade air-sending device  100 B can attenuate vibration, as they can have their strengths increased by the wavy portions  26   c  of the projections  20 . 
     Further, by having a wavy portion  26   c , each of the plurality of projections  20  can adjust an area to be formed by the projection  20  in a cross-section taken along the radial direction of the impeller  10 , and can adjust the volume of air that is suctioned into the impeller  10 . This allows the impeller  10  and the multi-blade air-sending device  100 B to have improved air-sending efficiency. 
     Embodiment 3 
     [Multi-Blade Air-Sending Device  100 C] 
       FIG.  20    is a plan view of an impeller  10  of a multi-blade air-sending device  100 C according to Embodiment 3.  FIG.  21    is a cross-sectional view of the impeller  10  as taken along line E-E in  FIG.  20   . The multi-blade air-sending device  100 C according to Embodiment 3 is described with reference to  FIGS.  20  and  21   . It should be noted that elements having identical configurations as those of the multi-blade air-sending device  100  or other devices of  FIGS.  1  to  19    are given identical signs and a description of such elements is omitted. The impeller  10  of the multi-blade air-sending device  100 C according to Embodiment 3 is intended to further specify the relationship between the projections  20  and the boss  11   b . Accordingly, the following description is given with reference to  FIGS.  20  and  21    with a focus on the relationship between the projections  20  and the boss  11   b.    
     In the impeller  10  according to Embodiment 1, as shown in  FIG.  8   , each of the plurality of projections  20  is connected to the outer circumferential wall  11   b   2  of the boss  11   b . On the other hand, in the multi-blade air-sending device  100 C according to Embodiment 3, the impeller  10  has a space GA formed between each of the plurality of projections  20  and the outer circumferential wall  11   b   2  of the boss  11   b . That is, the impeller  10  of the multi-blade air-sending device  100 C according to Embodiment 3 has a gap formed between the projection inner circumferential end  23  of the projection  20  and the boss  11   b . It should be noted that the projection  20  and the boss  11   b  are connected to each other via the back plate  11 . 
     [Working Effects of Impeller  10  and Multi-Blade Air-Sending Device  100 C] 
     The back plate  11  includes a plurality of projections  20  provided at the second surface portion  11   c  and extending in the axial direction of the rotation shaft RS. By including the projections  20 , the impeller  10  and the multi-blade air-sending device  100 C make it possible to, while the impeller  10  is rotating, draw in a flow of gas by generating negative pressure on a surface of the impeller  10  facing in a direction opposite to a direction of rotation R of the impeller  10  and increase the amount of air that is suctioned into the impeller  10 . Since the projections  20  are lower in wind speed on the inner circumference than on the outer circumference, the projections  20  less contributes to the increase in the amount of air that is suctioned into the impeller  10  than on the outer circumference. This allows the impeller  10  and the multi-blade air-sending device  100 C to reduce the number of inner circumferential walls of the projections  20 , and reducing the number of inner circumferential walls of the projections  20  makes it possible to inhibit the deformation of a shaft portion during molding. Further, by reducing the number of inner circumferential walls of the projections  20 , the impeller  10  and the multi-blade air-sending device  100 C can reduce necessary cost through material reductions or other reductions. 
     Embodiment 4 
     [Multi-Blade Air-Sending Device  100 D] 
       FIG.  22    is a plan view schematically showing an impeller  10  of a multi-blade air-sending device  100 D according to Embodiment 4.  FIG.  23    is a schematic view showing an example of the shape of projections  20  of the impeller  10  of  FIG.  22   . The multi-blade air-sending device  100 D according to Embodiment 4 is described with reference to  FIGS.  22  and  23   . It should be noted that elements having identical configurations as those of the multi-blade air-sending device  100  or other devices of  FIGS.  1  to  21    are given identical signs and a description of such elements is omitted. The multi-blade air-sending device  100 D according to Embodiment 4 is intended to further specify the configuration of the projections  20 . Accordingly, the following description is given with reference to  FIGS.  22  and  23    with a focus on the configuration of the projections  20 . 
     The step  11   f  of the back plate  11  forms the outer circumferential edge  11   c   1  of the second surface portion  11   c . As shown in  FIG.  22   , a circle constituted by the outer circumferential edge  11   c   1  of the second surface portion  11   c  about the rotation shaft RS is defined as a circle CR. Moreover, as shown in  FIG.  22   , an outlet angle of each of the projections  20  is defined as a projection outlet angle θ. The projection outlet angle θ is defined as an angle formed by a tangent line DL and a center line EL of the projection  20  at the projection outer circumferential end  21  at an intersection between a segment of the circle CR about the rotation shaft RS and the projection outer circumferential end  21 . Each of the plurality of projections  20  is formed such that a projection outlet angle θ at an outer circumferential end portion is an angle smaller than or equal to 90 degrees. As shown in  FIG.  23   , the projection  20  extends backward in the direction of rotation R. The projection  20  is formed in an arc to curve out in the direction of rotation R in a plan view as viewed from an angle parallel with the axial direction of the rotation shaft RS. 
     [Working Effects of Impeller  10  and Multi-Blade Air-Sending Device  100 D] 
     By including the projections  20 , the impeller  10  and the multi-blade air-sending device  100 D make it possible to, while the impeller  10  is rotating, draw in a flow of gas by generating negative pressure on a surface of the impeller  10  facing in a direction opposite to a direction of rotation R of the impeller  10  and increase the amount of air that is suctioned into the impeller  10 . Further, each of the plurality of projections  20  is formed such that a projection outlet angle θ at an outer circumferential end portion is an angle smaller than or equal to 90 degrees. This allows the impeller  10  and the multi-blade air-sending device  100 D to have improved air-sending efficiency, as the load on the projections  20  during rotation is reduced. 
     Embodiment 5 
     [Multi-Blade Air-Sending Device  100 E] 
       FIG.  24    is a plan view of an impeller  10  of the multi-blade air-sending device  100 E according to Embodiment 5. The multi-blade air-sending device  100 E according to Embodiment 5 is described with reference to  FIG.  24   . It should be noted that elements having identical configurations as those of the multi-blade air-sending device  100  or other devices of  FIGS.  1  to  23    are given identical signs and a description of such elements is omitted. The multi-blade air-sending device  100 E according to Embodiment 5 includes other projecting portions other than the projections  20  at the second surface portion  11   c . Accordingly, the following description is given with reference to  FIG.  24    with a focus on a configuration of the other projecting portions formed at the second surface portion  11   c.    
     As shown in  FIG.  24   , the second surface portion  11   c  includes a plurality of second projections  25  projecting from the back plate  11 . Each of the second projections  25  is provided between ones of the projections  20  adjacent to each other along the circumferential direction, and is formed such that the length of the second projection  25  in a radial direction about the rotation shaft RS is shorter than the length of each of the projections  20 . 
     The plurality of second projections  25  are provided in a radial fashion about the rotation shaft RS, and each of the plurality of second projections  25  extends in a radial direction about the rotation shaft RS. As shown in  FIG.  24   , the back plate  11  includes twenty-seven second projections  25 . However, the number of second projections  25  that are formed is not limited to 27. 
     The plurality of second projections  25  are arranged on circumferences with different diameters about the rotation shaft RS, and the number of the plurality of second projections  25  that are arranged on the circumferences increases from the boss  11   b  toward the plurality of blades  12 . For example, in the impeller  10  shown in  FIG.  24   , nine second projections  25  are formed on a first circle EN 1  located on the inner circumference, and eighteen second projections  25  are formed on a second circle EN 2  located on the outer circumference of the first circle EN 1 . 
     Each of the plurality of second projections  25  is a rib formed in the shape of a plate rising from the second surface portion  11   c . More specifically, the second projection  25  is formed in the shape of a four-cornered plate. Note, however, that the second projection  25  needs only be a structure projecting from the second surface portion  11   c  and is not limited to the four-cornered plate-like configuration. 
     In a case in which a height direction is a direction parallel with the axial direction of the rotation shaft RS and a direction of projection from the second surface portion  11   c , the plurality of second projections  25  have their heights formed at the same height. Note, however, that the back plate  11  is not limited to being configured such that the plurality of second projections  25  have their heights formed at the same height. The plurality of second projections  25  may be formed at different heights, or may form a group of the same height based on certain regularity. 
     In a case in which the height direction is the direction parallel with the axial direction of the rotation shaft RS and the direction of projection from the second surface portion  11   c , a second projection  25  provided at an outermost circumferential portion within the second surface portion  11   c  is formed to correspond in height to the first surface portion  11   a  at an outer circumferential end portion serving as an outermost circumferential portion. Alternatively, the second projection  25  provided at the outermost circumferential portion within the second surface portion  11   c  is formed to be lower in height than the first surface portion  11   a  at the outer circumferential end portion serving as the outermost circumferential portion. In other words, the second projection  25  provided at the outermost circumferential portion within the second surface portion  11   c  is formed such that the outer circumferential end portion of the second projection  25  does not project from the first surface portion  11   a  in the direction parallel with the axial direction of the rotation shaft RS. 
     The impeller  10  includes a plurality of depressions  38 . Each of the depressions  38  is formed by being surrounded by any one or more of the second surface portion  11   c , the projections  20 , the second projections  25 , and the reinforcing portion  30 . The plurality of depressions  38  are formed along the circumferential direction about the rotation shaft RS of the back plate  11 . The number of depressions  38  that are formed along the circumferential direction increases from the boss  11   b  toward the plurality of blades  12 . 
     [Working Effects of Impeller  10  and Multi-Blade Air-Sending Device  100 E] 
     The impeller  10  and the multi-blade air-sending device  100 E include a second projection  25  provided between ones of the projections  20  adjacent to each other along the circumferential direction and formed such that the length of the second projection  25  in a radial direction about the rotation shaft RS is shorter than the length of each of the projections  20 . The second projection  25  makes it possible, while the impeller  10  is rotating, draw in a flow of gas by generating negative pressure on a surface of the impeller  10  facing in a direction opposite to a direction of rotation R of the impeller  10  and increase the amount of air that is suctioned into the impeller  10 . 
     Further, the number of a plurality of the second projections  25  that are arranged on the circumferences increases from the boss  11   b  toward the plurality of blades  12 . With the second surface portion  11   c  having a wide area in the radial direction, the impeller  10  increases in volume of air that is suctioned into the impeller  10 , making it easy to cause turbulence in the flow of air. Since the number of the plurality of second projections  25  that are arranged on the circumferences increases toward the outer circumference, the impeller  10  can be configured such that the second surface portion  11   c  has a narrow area in the radial direction. Moreover, with the second surface portion  11   c  having a narrow area in the radial direction, the impeller  10  makes it possible to break the force of the wind flowing in the radial direction and adjust the volume of air that is suctioned into the impeller  10 . 
     Further, the number of depressions  38  that are formed along the circumferential direction increases from the boss  11   b  toward the plurality of blades  12 . With the second surface portion  11   c  having a wide area in the radial direction, the impeller  10  increases in volume of air that is suctioned into the impeller  10 , making it easy to cause turbulence in the flow of air. Since the number of depressions  38  that are formed on the same circumference increases toward the outer circumference, the impeller  10  can be configured such that the second surface portion  11   c  has a narrow area in the radial direction. Moreover, with the second surface portion  11   c  having a narrow area in the radial direction, the impeller  10  makes it possible to break the force of the wind flowing in the radial direction and adjust the volume of air that is suctioned into the impeller  10 . 
     Embodiment 6 
     [Multi-Blade Air-Sending Device  100 F] 
       FIG.  25    is a perspective view of an impeller  10  of a multi-blade air-sending device  100 F according to Embodiment 6 as seen from one side.  FIG.  26    is a perspective view of the impeller  10  of the multi-blade air-sending device  100 F according to Embodiment 6 as seen from the other side.  FIG.  27    is a plan view of the impeller  10  shown in  FIG.  25    as seen from one side.  FIG.  28    is a plan view of the impeller  10  shown in  FIG.  26    as seen from the other side.  FIG.  29    is a cross-sectional view of the impeller  10  as taken along line F-F in  FIG.  27   . The multi-blade air-sending device  100 F according to Embodiment 6 is described with reference to FIGS.  25  to  29 . It should be noted that elements having identical configurations as those of the multi-blade air-sending device  100  or other devices of  FIGS.  1  to  24    are given identical signs and a description of such elements is omitted. The multi-blade air-sending device  100 F according to Embodiment 6 differs in configuration of the back plate  11  of the impeller  10  from that of Embodiment 1. Accordingly, the following description is given with reference to  FIGS.  25  to  29    with a focus on the configuration of the back plate  11 . 
     The back plate  11  includes an inner circumferential portion  31  inclined with respect to the rotation shaft RS and an outer circumferential portion  32  formed in a ring shape along an outer edge of the inner circumferential portion  31 . 
     The inner circumferential portion  31  is formed in a conical shape. In a case in which one surface of the inner circumferential portion  31  formed in a conical shape is an inner surface and the other surface is an outer surface, the inner surface is formed in a concave shape, and the outer surface is formed in a convex shape. 
     The inner surface of the inner circumferential portion  31  faces the rotation shaft RS. The inner surface of the inner circumferential portion  31  is formed in such a bowl shape that the depth of the concave shape increases from the outer circumference toward the inner circumference in the radial direction about the rotation shaft RS. This inner surface of the inner circumferential portion  31  constitutes the second surface portion  11   c . That is, one surface of the inner circumferential portion  31  in the axial direction of the rotation shaft RS constitutes the second surface portion  11   c.    
     The inner surface of the inner circumferential portion  31  constitutes the second surface portion  11   c , and at the inner surface of the inner circumferential portion  31  constituting the second surface portion  11   c , projections  20  are formed. Further, at the inner surface of the inner circumferential portion  31  constituting the second surface portion  11   c , a reinforcing portion  30  is formed. Furthermore, at the inner surface of the inner circumferential portion  31  constituting the second surface portion  11   c , second projections  25  may be formed. The outer surface of the inner circumferential portion  31  is formed in a convex shape, and at the outer surface of the inner circumferential portion  31 , the second surface portion  11   c , the projections  20 , the second projections  25 , and the reinforcing portion  30  are not formed. 
     In the impeller  10  according to Embodiment 1, the second surface portion  11   c  is depressed from the first surface portion  11   a  by using a difference in thickness of the back plate  11 , and in the impeller  10  according to Embodiment 6, the second surface portion  11   c  is formed by using the shape of the inner circumferential portion  31  formed in a conical shape. 
     The outer circumferential portion  32  is formed in a ring shape in a plan view as viewed from the direction parallel with the axial direction of the rotation shaft RS. The outer circumferential portion  32  is formed, for example, in a circular ring shape. On the inner circumference of the outer circumferential portion  32 , the inner circumferential portion  31  is formed. The outer circumferential portion  32  located on the outer circumference of the second surface portion  11   c  constitutes the first surface portion  11   a.    
     [Working Effects of Impeller  10  and Multi-Blade Air-Sending Device  100 F] 
     The back plate  11  includes a second surface portion  11   c  depressed from the first surface portion  11   a  in an axial direction of the rotation shaft RS and a plurality of projections  20  provided at the second surface portion  11   c  and extending in the axial direction of the rotation shaft RS. While the impeller  10  is rotating, the projections  20  draw in a flow of gas by generating negative pressure on a surface of the impeller  10  facing in a direction opposite to a direction of rotation R of the impeller  10 , making it possible to increase the amount of air that is suctioned into the impeller  10 . Further, the impeller  10  includes the second surface portion  11   c  depressed from the first surface portion  11   a , on which the plurality of blades  12  are formed, in the axial direction of the rotation shaft RS, and the projections  20  are provided at the second surface portion  11   c . This inhibits a flow of gas produced by the projections  20  from flowing from the second surface portion  11   c  into the first surface portion  11   a . Moreover, the flow of gas produced by the projections  20  has its centrifugally-outward force of wind broken by a step  11   f  between the first surface portion  11   a  and the second surface portion  11   c , so that the impeller  10  does not suffer from turbulence in the flow of gas on the inner circumference of the blades  12 . This allows the impeller  10  and the multi-blade air-sending device  100 F to have higher air-sending efficiency than in a case in which the impeller  10  and the multi-blade air-sending device  100 F do not include the projections  20  or the second surface portion  11   c.    
     The back plate  11  includes an inner circumferential portion  31  inclined with respect to the rotation shaft RS and an outer circumferential portion  32  formed in a ring shape along an outer edge of the inner circumferential portion  31 , and one surface of the inner circumferential portion  31  in the axial direction of the rotation shaft RS constitutes the second surface portion  11   c . Causing the inner circumferential portion  31  to have a long inclined surface in the axial direction of the rotation shaft RS allows the impeller  10  to secure the depth of the inner circumferential portion  31  on the inner surface. Therefore, the impeller  10  and the multi-blade air-sending device  100 F make it possible to increase the heights of the projections  20 , the reinforcing portion  30 , and the second projections  25  by using the depth of the inner circumferential portion  31  on the inner surface and improve the strength of the impeller  10 . Further, the impeller  10  and the multi-blade air-sending device  100 F make it possible to increase the heights of the projections  20 , the reinforcing portion  30 , and the second projections  25  by using the depth of the inner circumferential portion  31  on the inner surface and further increase the amount of air that is suctioned into the impeller  10 . 
     Further, consideration is given to a case in which when a double-suction impeller  10  is incorporated into a product, an obstacle that prevents the flow of air is placed on one suction side of the impeller  10  and a suction load is unevenly put on one side of the impeller  10 . In such a case, the impeller  10  and the multi-blade air-sending device  100 F make it possible to achieve a balance of amounts of suction between the two suction sides by placing the projections  20  and the second surface portion  11   c  so that the projections  20  and the second surface portion  11   c  face the obstacle and to bring about improvement in air-sending efficiency. 
     Embodiment 7 
     [Multi-Blade Air-Sending Device  100 G] 
       FIG.  30    is a conceptual diagram explaining a relationship between the impeller  10  and a motor  50  in a multi-blade air-sending device  100 G according to Embodiment 7. The multi-blade air-sending device  100 G according to Embodiment 7 is described with reference to  FIG.  30   . It should be noted that elements having identical configurations as those of the multi-blade air-sending device  100  or other devices of  FIGS.  1  to  29    are given identical signs and a description of such elements is omitted. The multi-blade air-sending device  100 G according to Embodiment 7 is intended to further describe an example of a relationship between the impeller  10  of the multi-blade air-sending device  100 F according to Embodiment 6 and an obstacle that prevents air from flowing into the impeller  10 . 
     As shown in  FIG.  30   , the multi-blade air-sending device  100 G may have, in addition to the impeller  10  and the scroll casing  40 , a motor  50  configured to rotate the back plate  11  of the impeller  10 . That is, the multi-blade air-sending device  100 G has an impeller  10 , a scroll casing  40  housing the impeller  10 , and a motor  50  configured to drive the impeller  10 . 
     The motor  50  is disposed adjacent to the side wall  44   a  of the scroll casing  40 . A motor shaft  51  serving as a rotation shaft of the motor  50  is inserted in the scroll casing  40  through a side surface of the scroll casing  40 . 
     The back plate  11  is disposed to be perpendicular to the rotation shaft RS along the side wall  44   a  of the scroll casing  40  facing the motor  50 . The back plate  11  has provided in a central part thereof a boss  11   b  to which the motor shaft  51  is connected, and the motor shaft  51  is fixed to the boss  11   b  of the back plate  11  while being inserted in the scroll casing  40 . The motor shaft  51  of the motor  50  is connected and fixed to the back plate  11  of the impeller  10 . 
     The multi-blade air-sending device  100 G is configured such that the motor  50  is disposed at and the motor shaft  51  is connected to a side of the back plate  11  at which the projections  20  and the second surface portion  11   c  are formed. Moreover, the multi-blade air-sending device  100 G is configured such that the motor  50  is not disposed at and the motor shaft  51  is not connected to a side of the back plate  11  at which the projections  20  and the second surface portion  11   c  are not formed. In other words, the projections  20  and the second surface portion  11   c  of the multi-blade air-sending device  100 G are disposed to face the motor  50 . 
     Let it be assumed that in the multi-blade air-sending device  100 G, the motor diameter of the motor  50  is a motor diameter MO and the inside diameter of the bellmouth  46  is an inside diameter BI. The motor diameter MO of the motor  50  is larger than the inside diameter BI of the bellmouth  46 . The multi-blade air-sending device  100 G is configured to satisfy the relationship “Motor Diameter MO&gt;Inside Diameter BI”. 
     The impeller  10  of the multi-blade air-sending device  100 G may be the impeller  10  of the multi-blade air-sending device  100  or other devices according to Embodiments 1 to 5, or may be the impeller  10  of the multi-blade air-sending device  100 F according to Embodiment 6. In a case in which the impeller  10  of the multi-blade air-sending device  100 G is the impeller  10  of the multi-blade air-sending device  100 F according to Embodiment 6, the back plate  11  of the impeller  10  includes an inner circumferential portion  31  and an outer circumferential portion  32  as shown in  FIG.  30   . 
     Once the motor  50  is brought into operation, the plurality of blades  12  rotate about the rotation shaft RS via the motor shaft  51  and the back plate  11 . This causes outside air to be suctioned into the impeller  10  through the air inlet  45  and blown out into the scroll casing  40  by a booster action of the impeller  10 . The air blown out into the scroll casing  40  recovers its static pressure by having its speed reduced in an expanded air trunk formed by the peripheral wall  44   c  of the scroll casing  40 , and is blown out to the outside through the discharge port  42   a  shown in  FIG.  1   . 
     [Working Effects of Impeller  10  and Multi-Blade Air-Sending Device  100 G] 
     At a side of the scroll casing  40  at which the motor  50  is disposed, the motor  50  becomes an obstacle to the flow of gas to narrow the air inlet  45  of the scroll casing  40  and the air inlet  10   e  of the impeller  10 , with the result that the amount of a flow of gas that is suctioned decreases in general. 
     On the other hand, the multi-blade air-sending device  100 G is configured such that the projections  20  and the second surface portion  11   c  are disposed to face the motor  50 . As mentioned above, the projections  20  and the second surface portion  11   c  increase the amount of a flow of gas that is suctioned and reduce turbulence in the flow of gas, thereby making it possible to achieve higher air-sending efficiency than in a case in which the multi-blade air-sending device  100 G do not include the projections  20  or the second surface portion  11   c . Therefore, even at the side of the scroll casing  40  at which the motor  50  is disposed, where the amount of a flow of gas that is suctioned decreases in general, the multi-blade air-sending device  100 G can have improved air-sending efficiency by increasing the amount of a flow of gas that is suctioned and reducing turbulence in the flow of gas. 
     In a case in which the multi-blade air-sending device  100 G includes an inner circumferential portion  31  and an outer circumferential portion  32 , the inner surface of the inner circumferential portion  31  makes it possible by having including the projections  20  and the second surface portion  11   c  to improve air-sending efficiency by increasing the amount of a flow of gas that is suctioned and reducing turbulence in the flow of gas. Moreover, the multi-blade air-sending device  100 G is configured such that the projections  20  and the second surface portion  11   c  are disposed to face the motor  50 . Therefore, even at the side of the scroll casing  40  at which the motor  50  is disposed, where the amount of a flow of gas that is suctioned decreases in general, the multi-blade air-sending device  100 G can have improved air-sending efficiency by increasing the amount of a flow of gas that is suctioned and reducing turbulence in the flow of gas. On the other hand, the outer surface of the inner circumferential portion  31  does not include the projections  20  or the second surface portion  11   c . Therefore, the multi-blade air-sending device  100 G makes it possible to achieve a balance between the amounts of air that are suctioned through both sides of a double-suction impeller  10  and to bring about improvement in air-sending efficiency. 
     Further, the motor diameter MO of the motor  50  is larger than the inside diameter BI of the bellmouth  46 . As mentioned above, the multi-blade air-sending device  100 G is configured such that the projections  20  and the second surface portion  11   c  are disposed to face the motor  50 . Therefore, even in a case in which the presence of the motor  50 , which becomes an obstacle to the flow of gas, causes a decrease in the amount of a flow of gas that is suctioned and a great loss in suction of the impeller  10 , the multi-blade air-sending device  100 G can have improved air-sending efficiency by increasing the amount of a flow of gas that is suctioned and reducing turbulence in the flow of gas. 
     Embodiments 1 to 7 have been described by taking as an example a multi-blade air-sending device  100  including a double-suction impeller  10  having a plurality of blades  12  formed on both sides of a back plate  11 . However, the present disclosure is also applicable to a multi-blade air-sending device  100  including a single-suction impeller  10  having a plurality of blades  12  formed only on one side of a back plate  11 . 
     Embodiment 8 
     [Air-Conditioning Apparatus  140 ] 
       FIG.  31    is a perspective view of an air-conditioning apparatus  140  according to Embodiment 8.  FIG.  32    is a diagram showing an internal configuration of the air-conditioning apparatus  140  according to Embodiment 8. As for a multi-blade air-sending device  100  used in the air-conditioning apparatus  140  according to Embodiment 8, elements having identical configurations as those of the multi-blade air-sending device  100  or other devices of  FIGS.  1  to  30    are given identical signs, and a description of such elements is omitted. To show the internal configuration of the air-conditioning apparatus  140 ,  FIG.  32    omits to illustrate an upper surface portion  16   a.    
     The air-conditioning apparatus  140  according to Embodiment 8 includes any one or more of the multi-blade air-sending devices  100  to  100 G according to Embodiments 1 to 7 and a heat exchanger  15  disposed in such a location as to face a discharge port  42   a  of the multi-blade air-sending device  100 . Further, the air-conditioning apparatus  140  according to Embodiment 8 includes a case  16  installed above a ceiling of a room to be air-conditioned. In the following description, the term “multi-blade air-sending device  100 ” indicates the use of any one of the multi-blade air-sending devices  100  to  100 G according to Embodiments 1 to 7. Further, although, in  FIGS.  31  and  32   , a multi-blade air-sending device  100  having a scroll casing  40  in the case  16  is shown, an impeller  10  having no scroll casing  40  may be installed in the case  16 . 
     (Case  16 ) 
     As shown in  FIG.  31   , the case  16  is formed in a cuboidal shape including an upper surface portion  16   a , a lower surface portion  16   b , and side surface portions  16   c . The shape of the case  16  is not limited to the cuboidal shape but may for example be another shape such as a circular columnar shape, a prismatic shape, a conical shape, a shape having a plurality of corner portions, or a shape having a plurality of curved surface portions. 
     One of the side surface portions  16   c  of the case  16  is a side surface portion  16   c  having a case discharge port  17  formed therein. The case discharge port  17  is formed in a rectangular shape as shown in  FIG.  31   . The shape of the case discharge port  17  is not limited to the rectangular shape but may for example be another shape such as a circular shape or an oval shape. 
     Another one of the side surface portions  16   c  of the case  16  is a side surface portion  16   c  having a case air inlet  18  formed therein and being opposite the side surface portion  16   c  having the case discharge port  17  formed therein. The case air inlet  18  is formed in a rectangular shape as shown in  FIG.  32   . The shape of the case air inlet  18  is not limited to the rectangular shape but may for example be another shape such as a circular shape or an oval shape. A filter configured to remove dust in the air may be disposed at the case air inlet  18 . 
     Inside the case  16 , the multi-blade air-sending device  100  and the heat exchanger  15  are housed. The multi-blade air-sending device  100  includes an impeller  10 , a scroll casing  40  having a bellmouth  46  formed therein, and a motor  50 . 
     The motor  50  is supported by a motor support  9   a  fixed to the upper surface portion  16   a  of the case  16 . The motor  50  has a motor shaft  51 . The motor shaft  51  is disposed to extend parallel to the side surface portion  16   c  having the case air inlet  18  formed therein and the side surface portion  16   c  having the case discharge port  17  formed therein. As shown in  FIG.  32   , the air-conditioning apparatus  140  has two impellers  10  attached to the motor shaft  51 . 
     The impellers  10  of the multi-blade air-sending device  100  forms a flow of air that is suctioned into the case  16  through the case air inlet  18  and blown out into an air-conditioned space through the case discharge port  17 . The number of impellers  10  that are disposed in the case  16  is not limited to 2 but may be 1 or larger than or equal to 3. 
     As shown in  FIG.  32   , the multi-blade air-sending device  100  is attached to a divider  19  configured to divide an internal space of the case  16  into a space S 11  facing a suction side of the scroll casing  40  and a space S 12  facing a blowout side of the scroll casing  40 . 
     The heat exchanger  15  is disposed in such a location as to face the discharge port  42   a  of the multi-blade air-sending device  100 , and is disposed in the case  16  to be on an air trunk of air to be discharged by the multi-blade air-sending device  100 . The heat exchanger  15  adjusts the temperature of air that is suctioned into the case  16  through the case air inlet  18  and blown out into the air-conditioned space through the case discharge port  17 . As the heat exchanger  15 , a heat exchanger of a publicly-known structure can be applied. The case air inlet  18  needs only be formed in a location perpendicular to the axial direction of the rotation shaft RS of the multi-blade air-sending device  100 . For example, the case air inlet  18  may be formed in the lower surface portion  16   b.    
     Rotation of the impeller  10  of the multi-blade air-sending device  100  causes the air in the air-conditioned space to be suctioned into the case  16  through the case air inlet  18 . The air suctioned into the case  16  is guided toward the bellmouth  46  and suctioned into the impeller  10 . The air suctioned into the impeller  10  is blown out outward in the radial direction of the impeller  10 . 
     The air blown out from the impeller  10  passes through the inside of the scroll casing  40 , blown out of the scroll casing  40  through the discharge port  42   a , and then supplied to the heat exchanger  15 . The air supplied to the heat exchanger  15  is subjected to temperature and humidity control by, during passage through the heat exchanger  15 , exchanging heat with refrigerant flowing through the inside of the heat exchanger  15 . The air having passed through the heat exchanger  15  is blown out to the air-conditioned space through the case discharge port  17 . 
     The air-conditioning apparatus  140  according to Embodiment 8 includes any one of the multi-blade air-sending devices  100  to  100 G according to Embodiments 1 to 7. Therefore, the air-conditioning apparatus  140  can bring about effects similar to those of any of Embodiments 1 to 7. 
     Each of Embodiment 1 to 8 may be implemented in combination with the other. Further, the configurations shown in the foregoing embodiments show examples and may be combined with another publicly-known technology, and parts of the configurations may be omitted or changed, provided such omissions and changes do not depart from the scope. For example, an embodiment describes an impeller  10  or other devices constituted by the back-plate-side blade region  122   a  serving as the first region and the rim-side blade region  122   b  serving as the second region. The impeller  10  is not limited to an impeller constituted solely by the first region and the second region. The impeller  10  may further have another region as well as the first region and the second region. 
     REFERENCE SIGNS LIST 
     
         
         
           
               9   a : motor support,  10 : impeller,  10   e : air inlet,  11 : back plate,  11   a : first surface portion,  11   b : boss,  11   b   1 : shaft hole,  11   b   2 : outer circumferential wall,  11   c : second surface portion,  11   c   1 : outer circumferential edge,  11   f : step,  12 : blade,  12 A: first blade,  12 A 1 : first sirocco blade portion,  12 A 11 : first sirocco region,  12 A 2 : first turbo blade portion,  12 A 21 : first turbo region,  12 A 21   a : first turbo region,  12 A 2   a : first turbo blade portion,  12 A 3 : first radial blade portion,  12 B: second blade,  12 B 1 : second sirocco blade portion,  12 B 11 : second sirocco region,  12 B 2 : second turbo blade portion,  12 B 21 : second turbo region,  12 B 21   a : second turbo region,  12 B 2   a : second turbo blade portion,  12 B 3 : second radial blade portion,  12 R: outer circumferential region,  13 : rim,  13   a : first rim,  13   b : second rim,  14 A: inner circumferential end,  14 A 1 : leading edge,  14 B: inner circumferential end,  14 B 1 : leading edge,  15  heat exchanger,  15 A: outer circumferential end,  15 A 1 : trailing edge,  15 B: outer circumferential end,  15 B 1 : trailing edge, 16 case,  16   a : upper surface portion,  16   b : lower surface portion,  16   c : side surface portion,  17 : case discharge port,  18 : case air inlet,  19 : divider,  20 : projection,  21 : projection outer circumferential end,  21   a : upper end portion,  23 : projection inner circumferential end,  24 : base,  25 : second projection,  26 : ridge,  26   a : inclined portion,  26   b : horizontal portion,  26   c : wavy portion,  30 : reinforcing portion,  31 : inner circumferential portion,  32 : outer circumferential portion,  34 : depression,  35 : depression,  36 : depression,  37 : depression,  38 : depression,  40 : scroll casing,  41 : scroll portion,  41   a : scroll start portion,  41   b : scroll end portion,  42 : discharge portion,  42   a : discharge port,  42   b : extension plate,  42   c : diffuser plate,  42   d : first side plate portion,  42   e : second side plate portion,  43 : tongue,  44   a : side wall,  44   a   1 : first side wall,  44   a   2 : second side wall,  44   c : peripheral wall,  45 : air inlet,  45   a : first air inlet,  45   b : second air inlet,  46 : bellmouth,  46   a : opening,  50 : motor,  51 : motor shaft,  71 : first plane,  72 : second plane,  100 : multi-blade air-sending device,  100 B: multi-blade air-sending device,  100 C: multi-blade air-sending device,  100 D: multi-blade air-sending device,  100 E: multi-blade air-sending device,  100 F: multi-blade air-sending device,  100 G: multi-blade air-sending device  112   a : first blade group,  112   b : second blade group,  122   a : back-plate-side blade region,  122   b : rim-side blade region,  140 : air-conditioning apparatus,  141 A: inclined portion,  141 B: inclined portion