Centrifugal fans and indoor units for air conditioners

Abstract

To suppress the generation of a turbulence, to improve blowing efficiency, and to reduce noise, in a centrifugal fan.SOLUTION: A centrifugal fan comprises: a main plate 21 having a boss part 21a in which a rotating shaft of a motor is connected to a center; a plurality of impellers 22 joined to the main plate 21, and arranged with equal intervals in a peripheral direction; a shroud 23 joined to the impellers 22 while opposing the main plate 21, formed into a circular ring shape, and having an opening at a center; and a bellmouth 24 arranged inside the shroud 23, and gradually expanded in an inside diameter toward an upstream side. In the impellers 22, a length Lh of a one-end joining part 22a which is joined to the main plate 21 is longer than a length Ls of the other end joining part 22b joined to the shroud 23, and in a shape projected on a flat surface vertical to a rotation axial line C of the rotating shaft 41, an angle βh which is formed of a first linear line L1 for connecting a front edge position and a rear edge position of the one-end joining part 22a, a middle point of the first linear line L1 and a linear line passing a rotation center is smaller than an angle βs which is formed of a second linear line L2 for connecting a front edge position and a rear edge position of the other end joining part 22b, a middle point of the second linear line L2 and the rotation center.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a centrifugal fan and an indoor unit for an air conditioner. [Background technology]

[0002] In conventional centrifugal fans, the blade shape is designed on the premise of blowing air only in the centrifugal direction, and although there are fans such as those in Patent Document 1 in which the leading edge shape of the blade is devised to deal with axial inflow, no fundamental blade shape has been designed.

[0003] This resulted in problems such as turbulence occurring when the axially flowing airflow entered the blades, and airflow resistance occurring because the leading edge shape of the blades was not adapted to the airflow direction. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-53803 Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, the present invention has been made to solve the above-mentioned problems, and its main object is to suppress the generation of turbulence in a centrifugal fan, thereby improving air blowing efficiency and reducing noise. [Means for solving the problem]

[0006] a centrifugal fan according to the present invention includes a main plate having a boss at the center to which a rotating shaft of a motor is connected; a plurality of blades joined to the main plate and arranged at equal intervals in the circumferential direction; a shroud facing the main plate and joined to the blades, the shroud being annular and having an opening in the center; and a bell mouth disposed inside the shroud and having an inner diameter gradually increasing toward the upstream side, wherein the length of one end joint of the blade joined to the main plate is longer than the length of the other end joint of the blade joined to the shroud, and when the blades are projected onto a plane perpendicular to the axis of rotation of the rotating shaft, the angle formed by a first line connecting the leading edge position and the trailing edge position of the one end joint and a line passing through the midpoint of the first line and the center of rotation is smaller than the angle formed by a second line connecting the leading edge position and the trailing edge position of the other end joint and a line passing through the midpoint of the second line and the center of rotation.

[0007] With a centrifugal fan configured in this way, axial flow blades that smoothly take in air flowing in the axial direction can be added to the front of the conventional centrifugal blades, closer to the center of rotation, so that the airflow flows smoothly into the blades. This makes it possible to suppress the generation of turbulence, improve airflow efficiency, and reduce noise.

[0008] In order to form the blades in a shape that actively draws in air at their outer peripheral portions, increase the outlet wind speed on the shroud side, and uniformize the wind speed distribution, it is desirable that the leading edges of the blades, when projected onto a plane perpendicular to the rotation axis, have a region that retreats in the direction of rotation from the rotation center side toward the outer peripheral side and then advances, or a region where the retreat in the direction of rotation gradually decreases as it moves from the rotation center side toward the outer peripheral side. This configuration makes it possible to equalize the fan outlet wind speed, which tends to be biased toward the main board, and to mitigate the collision of the airflow with heat exchangers and other devices located on the outlet side, thereby reducing resistance and noise.

[0009] In order to make the outlet air speed of the fan even more uniform, it is desirable that the leading edge of the blade, when projected onto a plane perpendicular to the rotation axis, has a region on the outer periphery side of a region where it retreats in the direction of rotation from the rotation center side toward the outer periphery and then advances, or a region on the outer periphery side of a region where the retreat in the direction of rotation gradually decreases as it moves from the rotation center side toward the outer periphery side, where the retreat in the direction of rotation gradually increases.

[0010] The blades have a twisted shape so that the outer peripheral end of the other end joint portion joined to the shroud is positioned in the counter-rotational direction relative to the outer peripheral end of the one end joint portion joined to the main plate. With this configuration, the outer peripheral surfaces of the blades near the outlet are angled toward the shroud, so the airflow is pushed toward the shroud, making the outlet air speed more uniform.

[0011] It is desirable that the leading edge of the blade has a region where, in a shape projected onto a meridian plane passing through the rotation axis, the angle it forms with the rotation axis decreases and then becomes constant or increases, or does not decrease and then becomes constant or increases. With this configuration, the length of the blades to the outlet can be increased to accommodate the airflow that flows in along the bell mouth and concentrates near the outer periphery, due to the region where the angle with the rotation axis decreases and then increases, or increases without decreasing, and the pressure can be gradually increased to blow air from the outlet.

[0012] It is desirable that the leading edge of the blade, when projected onto a meridian plane passing through the rotation axis, has a region where the angle it forms with the rotation axis exceeds 90 degrees before it is joined to the shroud, and that the downstream end of the bell mouth is located at a position opposite to that region. With this configuration, the length of the blades near the shroud can be increased without the blades interfering with the bell mouth.

[0013] Near the leading edge, it is necessary to consider the recirculation flow that occurs when air blown out from the outlet of the centrifugal fan flows in through the gap between the shroud and the bell mouth. This recirculation flow has a strong local axial velocity component, which causes the problem that the blade inlet angle does not match the recirculation flow. In order to suitably solve this problem, it is desirable that the thickness of the leading edge portion of the blade in a region where the shroud and the bell mouth overlap in the radial direction is larger than the thickness of a region where they do not overlap, in a shape projected onto a meridian plane passing through the rotation axis. This configuration allows the pressure surface of the blade to be thickened, and the inlet angle of the blade camber line to be substantially changed, which results in the recirculation flow being directed along the blade, suppressing airflow separation and reducing noise.

[0014] In order to make the outlet air velocity uniform without reducing the amount of air processed on the shroud side, it is desirable that the trailing edge of the blade be inclined so that the shroud side faces in the opposite direction of rotation. Here, in order to suppress flow concentration at the connecting portion between the trailing edge of the blade and the shroud, it is desirable that the trailing edge of the blade have a region in which the angle between the trailing edge and the rotation axis decreases toward the shroud.

[0015] When the leading edge of the blade is projected onto a meridian plane passing through the rotation axis, it is desirable that the angle between the tangent from the rotation center (or the extension line if it is a straight line) and the rotation axis is between -60 degrees and 60 degrees (preferably between 5 degrees and 30 degrees).

[0016] It is also considered that the ratio A2 / A1 of the inlet area A1 of the shroud to the outlet area A2 of the centrifugal fan is 1.1 or more and 1.5 or less.

[0017] As described above, the air blown out from the outlet of the centrifugal fan becomes a recirculating flow and flows in through the gap between the shroud and the bell mouth. In order to reduce this recirculation flow, it is desirable that the centrifugal fan of the present invention further include an annular member provided on the outer periphery of the bell mouth and spaced downward from the inlet portion of the shroud with a gap therebetween. By providing the annular member in this manner, it becomes difficult for the recirculation flow to flow into the gap between the shroud and the bell mouth. As a result, the recirculation flow is reduced, and the entrance angle of the recirculation flow is tilted in the counter-rotation direction, making it easier for the recirculation flow to flow along the blades.

[0018] An indoor unit for an air conditioner using the centrifugal fan described above is also one aspect of the present invention. [Effects of the Invention]

[0019] According to the present invention configured as described above, the generation of turbulence in the centrifugal fan can be suppressed, thereby improving the air blowing efficiency and reducing noise. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a cross-sectional view schematically showing the configuration of an indoor unit for an air conditioner in one embodiment of the present invention. [Figure 2] 2A and 2B are perspective views of the centrifugal fan of the embodiment and a perspective view emphasizing one blade. [Figure 3] 3 is a schematic diagram showing the shape of a blade projected onto a plane perpendicular to the rotation axis of the embodiment. FIG. [Figure 4] 3A and 3B are diagrams illustrating the airflow of a conventional centrifugal fan and the centrifugal fan of the present embodiment; [Figure 5] FIG. 2 is a schematic diagram showing the shape of a blade projected onto a meridian plane passing through the rotation axis of the embodiment. [Figure 6] 10 shows simulation results showing the air blowing efficiency depending on the angle of the first region a in the same embodiment. [Figure 7] 10A and 10B are diagrams schematically showing air flows with and without the second region b in the same embodiment. [Figure 8] 4 is a simulation result showing the effect of reducing shaft power of the centrifugal fan of this embodiment compared to a conventional centrifugal fan. [Figure 9] 10 is a schematic diagram showing the shape of a blade projected onto a plane perpendicular to the rotation axis of a modified embodiment. FIG. [Figure 10] 10 is a schematic diagram showing the shape of a blade projected onto a plane perpendicular to the rotation axis of a modified embodiment. FIG. [Figure 11] 10 is a schematic diagram showing the shape of a blade projected onto a plane perpendicular to the rotation axis of a modified embodiment. FIG. [Figure 12] FIG. 10 is a schematic diagram showing the shape of a blade projected onto a meridian plane passing through the rotation axis of a modified embodiment. [Figure 13] FIG. 1 is a schematic diagram showing a recirculation flow. [Figure 14] 10 shows simulation results showing the airflow in (a) a configuration in which the pressure surface is not thickened, and (b) a configuration in which the pressure surface is thickened. [Figure 15] FIG. 10 is a perspective view showing the configuration of the trailing edge of the blade in a modified embodiment. [Figure 16] 10 is a simulation result showing the outlet wind speed distribution of a centrifugal fan when the trailing edges of the blades are inclined in the counter-rotation direction. [Figure 17] FIG. 10 is a partially enlarged cross-sectional view schematically showing the configuration of a modified embodiment. [Figure 18] Simulation results showing (a) the inflow direction near the recirculation flow without the annular element, and (b) the inflow direction near the recirculation flow with the annular element. DETAILED DESCRIPTION OF THE INVENTION

[0021] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an indoor unit for an air conditioner using a centrifugal fan according to the present invention will be described below with reference to the drawings.

[0022] <1. Indoor unit for air conditioner> The air conditioner indoor unit 100 according to this embodiment is a ceiling-embedded type, and has an air inlet H1 and air outlets H2 formed on its underside. Here, the air inlet H1 is formed in the center of the underside, and four air outlets H2 are formed surrounding the air inlet H1. Note that these four air outlets H2 are formed to correspond to the four sides of a rectangle in plan view.

[0023] The air conditioner indoor unit 100 also houses a centrifugal fan 2 that draws in air through an air inlet H1 and blows out the air through an air outlet H2, and a heat exchanger 3 that is hit by the airflow generated by the centrifugal fan 2. The heat exchanger 3 is disposed so as to surround the periphery of the centrifugal fan 2. The air drawn in through the air inlet H1 by the centrifugal fan 2 undergoes heat exchange in the heat exchanger 3, and is then blown out into the room through the air outlet H2.

[0024] <2. Centrifugal fan 2> As shown in FIG. 1, the centrifugal fan 2 of this embodiment includes a main plate 21, a plurality of blades 22, a shroud 23, and a bell mouth 24.

[0025] The main plate 21 has a circular shape in a plan view and has a boss portion 21a at its center to which the rotating shaft 41 of the motor 4 is connected. A protrusion portion 21b is formed in the center of the main plate 21, giving the main plate 21 a mountain-like shape.

[0026] The blades 22 are joined to one surface of the main plate 21 where the protruding portion 21b is formed, and are arranged at equal intervals in the circumferential direction. The blades 22 are formed from the outer peripheral surface of the protruding portion 21b formed in the center of the main plate 21 to the outer periphery of the main plate 21. The specific configuration of the blades 22 will be described later.

[0027] The shroud 23 is disposed opposite the main plate 21 and has an annular shape with an opening in the center, joined to the other ends of the blades 22. Air is drawn in through the opening in the center of the shroud 23. Air is also blown out from openings formed between the shroud 23 and the main plate 21.

[0028] The bell mouth 24 has a downstream end 24a provided inside the shroud 23, and the inner diameter gradually increases toward the upstream side.

[0029] 3. Specific Configuration of Blade 22 As shown in Figure 3, the blade 22 of this embodiment is configured so that the length Lh of the one end joint 22a joined to the main plate 21 is greater than the length Ls of the other end joint 22b joined to the shroud 23.

[0030] Furthermore, the blade 22 is configured so that, in a shape projected onto a plane perpendicular to the rotation axis C of the rotation shaft 41, the angle βh formed by a first straight line L1 connecting the leading edge position p1 and the trailing edge position p2 of the one end joint 22a and a line passing through the midpoint of the first straight line L1 and the rotation center X is smaller than the angle βs formed by a second straight line L2 connecting the leading edge position p3 and the trailing edge position p4 of the other end joint 22b and a line passing through the midpoint of the second straight line L2 and the rotation center X.

[0031] The surface of the inlet (suction side) of the blades 22 facing backward in the direction of rotation faces the suction side and counter-rotational direction, just like an axial fan. As a result, as shown in Figure 4, the change in flow direction at the point where the airflow enters the blades 22 is smaller than in a conventional centrifugal fan, and the airflow flows into the blades 22 smoothly.

[0032] 3, the leading edge LE of the blade 22 has a region 22x that, in a shape projected onto a plane perpendicular to the rotation axis C, retreats in the rotation direction from the rotation center X side toward the outer periphery and then changes toward the forward side. In this embodiment, the region 22x of the leading edge LE of the blade 22, in a shape projected onto a plane perpendicular to the rotation axis C, retreats in the rotation direction from the rotation center X side toward the outer periphery and then advances.

[0033] 5, the leading edge LE of the blade 22 has a shape projected onto a meridian plane passing through the rotation axis C, which has a first region a where the angle θ1 with the rotation axis C is constant from the end closest to the rotation center toward the outer periphery, and a second region b on the outer periphery continuous with the first region a where the angle θ1 with the rotation axis C decreases and then increases. Here, the angle θ1 with the rotation axis C in the shape projected onto the meridian plane passing through the rotation axis C is the angle between a tangent from the rotation center side (an extension line in the case of a straight line) and the rotation axis C, and is the angle looking toward the suction side with respect to the rotation axis C, that is, the angle formed on the suction side with respect to the rotation axis C.

[0034] In the first region a, the angle θ1 between a tangent from the rotation center side and the rotation axis C is between -60 degrees and 60 degrees, and preferably between 5 degrees and 30 degrees. FIG. 6 shows the airflow efficiency when the angle θ1 of the first region a is changed from 0 degrees to 90 degrees (for the blade configuration of FIG. 5). As can be seen from FIG. 6, the airflow efficiency becomes significant when the angle θ1 is between 5 degrees and 30 degrees. In addition, the ratio A2 / A1 of the inlet area A1 of the shroud 23 (the opening area at the center of the shroud) to the outlet area A2 of the centrifugal fan 2 (the opening area formed between the shroud 23 and the main plate 21) is between 1.1 and 1.5.

[0035] Furthermore, by providing the second region b, as shown in FIG. 7, the length of the blades 22 to the outlet can be increased for the airflow that flows in along the bell mouth 24 and concentrates near the outer periphery, compared to a configuration that does not have the second region b, and the pressure can be gradually increased and the air can be sent out from the outlet.

[0036] 5, the leading edge LE of the blade 22 has, on the outer circumferential side continuous with the second region b, a third region c in which the angle θ1 with respect to the rotation axis C exceeds 90 degrees, in a shape projected onto a meridian plane passing through the rotation axis C, until it is joined to the shroud 23. The downstream end 24a of the bell mouth 24 is located at a position facing the third region c.

[0037] Next, the simulation results of the axial power reduction effect of the centrifugal fan of this embodiment compared to a conventional centrifugal fan are shown in Fig. 8. The horizontal axis of Fig. 8 represents air volume [CMM], and the vertical axis represents axial power [W]. As can be seen from Fig. 8, the centrifugal fan of this embodiment reduces axial power by 7.5% compared to the conventional centrifugal fan.

[0038] <4. Effects of this embodiment> With the air conditioner indoor unit 100 configured in this way, it is possible to add an axial flow impeller to the front of the conventional centrifugal impeller, closer to the center of rotation, that smoothly takes in air flowing in in the axial direction, and the airflow flows smoothly into the impeller 22. This makes it possible to suppress the generation of turbulence, improve airflow efficiency, and reduce noise.

[0039] The leading edges of the blades 22, when projected onto a plane perpendicular to the rotation axis C, have a region that retreats and then advances in the direction of rotation from the rotation center toward the outer periphery, so that the outer periphery of the blades 22 can actively draw in air, increasing the outlet wind speed on the shroud 23 side and making the wind speed distribution uniform. This reduces the collision of the airflow with the heat exchanger 3 and other devices disposed on the outlet side of the centrifugal fan 2, thereby reducing resistance and noise.

[0040] The leading edge LE of the blade 22 has a first region a where the angle θ1 with the rotation axis is constant from the end closest to the rotation center toward the outer periphery, and a second region b where the angle θ1 decreases on the outer periphery side continuous with the first region a and then increases or does not decrease and increases. Therefore, the second region b makes it possible to increase the length of the blade 22 to the outlet for the airflow that flows in along the bell mouth 24 and concentrates near the outer periphery, and air can be blown with a gradual increase in pressure.

[0041] The leading edge LE of the blade 22 has a third region c, where the angle θ1 exceeds 90 degrees, on the outer circumferential side continuing from the second region b until it is joined to the shroud 23, and the downstream end 24a of the bell mouth 24 is located at a position opposite to the third region c, so that the length of the blade 22 near the shroud 23 can be increased without the blade 22 interfering with the bell mouth 24.

[0042] <5. Other Modified Embodiments> The present invention is not limited to the above-described embodiment.

[0043] For example, as shown in Figure 9, the blade 22 has a twisted shape so that the outer peripheral end of the other end joint 22b, which is joined to the shroud 23, is positioned in the counter-rotation direction relative to the outer peripheral end of the one end joint 22a, which is joined to the main plate 21. With this configuration, the outer peripheral surfaces of the blades 22 near the outlet are angled toward the shroud 23, so that the airflow is pushed toward the shroud 23, making the outlet wind speed more uniform.

[0044] 10, the leading edge LE of the blade 22 may have a plurality of regions 22x that recede in the direction of rotation from the rotation center X side toward the outer periphery and then change toward the forward direction in a shape projected onto a plane perpendicular to the rotation axis. Fig. 10 shows an example in which regions 22x that recede in the direction of rotation and then change toward the forward direction are formed on both the rotation center side and the outer periphery side of the leading edge LE of the blade 22.

[0045] Furthermore, in the above embodiment, the region 22x at the leading edge LE of the blade 22 is configured to retreat in the rotation direction from the rotation center X side toward the outer periphery and then change toward the forward moving side, but it may also be configured so that the retreat (i.e., the amount of retreat) in the rotation direction gradually decreases as it moves from the rotation center X side toward the outer periphery and then the retreat (i.e., the amount of retreat) increases again, as shown in Fig. 11. Also, it may be configured so that the region 22x retreats in the rotation direction from the rotation center X side toward the outer periphery and then changes toward the forward moving side, and then retreats again.

[0046] 12, the leading edge LE of the blade 22 may have a second region where the angle θ1 between the blade 22 and the rotation axis C is constant and does not decrease, or where the angle θ1 increases, when projected onto a meridian plane passing through the rotation axis C. The leading edge LE of the blade 22 has a third region c on the outer circumferential side continuous with the second region b, where the angle θ1 between the blade 22 and the rotation axis C exceeds 90 degrees, before being joined to the shroud 23. The downstream end 24a of the bell mouth 24 is located at a position facing the third region c.

[0047] 13, the air blown out from the outlet of the centrifugal fan 100 becomes a recirculation flow and flows in through the gap between the shroud 23 and the bell mouth 24. This recirculation flow has a strong local axial velocity component, which causes a problem that the inlet angle of the blades 22 does not match the recirculation flow.

[0048] For this reason, it is desirable that the thickness of the leading edge portion of the blade 22, in a shape projected onto a meridian plane passing through the rotation axis C, in a region where the shroud 23 and the bellmouth 24 overlap in the radial direction be greater than the thickness of a region where they do not overlap. Specifically, as shown in FIG. 14(b), it is desirable to thicken the pressure surface at the leading edge portion of the blade 22. As can be seen from a comparison of FIGS. 14(a) and 14(b), by thickening the pressure surface at the leading edge portion of the blade 22, the inlet angle of the camber line of the blade can be substantially changed. As a result, the recirculation flow can be made to follow the blade, which suppresses airflow separation and reduces noise.

[0049] In order to uniformize the outlet air velocity distribution without reducing the treated air volume on the shroud side, it is desirable that the trailing edge 22z of the blade 22 be inclined so that the shroud side faces in the counter-rotation direction, as shown in FIG. 15 . This allows the airflow to be guided toward the shroud side, thereby improving the treated air volume on the shroud side. Here, in order to suppress the concentration of airflow at the connecting portion of the trailing edge 22z of the blade 22 with the shroud 23, it is desirable that the trailing edge 22z of the blade 22 have a region 22z1 in which the angle A between the trailing edge 22z and the rotation axis C decreases toward the shroud side. This configuration makes it possible to prevent airflow from concentrating at the connecting portion between the trailing edge 22z and the shroud 23, as can be seen from FIG. 16 . This allows the treated air volume to be increased overall near the shroud, while improving power consumption and airflow noise.

[0050] As described above, the air blown out from the outlet of the centrifugal fan 100 becomes a recirculating flow and flows in through the gap between the shroud 23 and the bell mouth 24 (see FIG. 13). In order to reduce this recirculating flow, it is desirable to further provide an annular member 25 on the outer periphery of the bell mouth 24, provided with a gap downward from the lower end of the inlet portion of the shroud 23, as shown in FIG.

[0051] This annular member 25 is attached to the bell mouth 24, and has an annular flat plate portion 251 provided facing and spaced apart from the lower end of the inlet portion of the shroud 23, and a convex rib portion 252 formed on the outer periphery of the surface of the annular flat plate portion 251 facing the shroud. The outer surface of the convex rib portion 252 is a smoothly curved surface, and has, for example, a partially circular cross section.

[0052] By providing the annular member 25 in this manner, it becomes difficult for the recirculation flow to flow into the gap between the shroud 23 and the bellmouth 24. As a result, as shown in Fig. 18, the recirculation flow decreases, the inflow angle of the recirculation flow is tilted in the counter-rotation direction, and the recirculation flow is more likely to flow along the blades. In addition, since the outer surface of the ridge portion 252 is a smoothly curved surface, the flow path through which the recirculation flow flows is roughly S-shaped, which prevents the flow from becoming turbulent in the flow path and reduces noise.

[0053] Furthermore, in the above embodiment, an air conditioner indoor unit using a centrifugal fan has been described, but the centrifugal fan of the present invention can also be used in other types of blowers.

[0054] Furthermore, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications are possible without departing from the spirit of the present invention. [Explanation of symbols]

[0055] 100... Indoor unit for air conditioner 2. Centrifugal fan 3...Heat exchanger 4 Motor 41 Rotation axis 21a Boss part 21...Main plate 22 Feather 23 Shroud 24 Bellmouth 22a...One end joint 22b... Other end joint 22x: The area that retreats and then advances in the direction of rotation from the center of rotation toward the outer periphery C: Rotation axis θ1: Angle with the rotation axis X: Center of rotation Lh: Length of one end joint Ls: Length of the joint at the other end βh: Angle of the straight line at one end of the joint βs: Angle of the straight line at the other end a: The region where the angle with the rotation axis is constant from the end closest to the center of rotation toward the outer periphery (first region) b: The region where the angle between the rotation axis and the axis of rotation decreases and then becomes constant, increases, or does not decrease and becomes constant or increases (second region). c. Area where the angle with the rotation axis exceeds 90 degrees (third area) 25 Circular member

Claims

1. a main plate having a boss portion at the center to which a rotating shaft of a motor is connected; a plurality of blades joined to the main plate and arranged at equal intervals in the circumferential direction; a shroud that faces the main plate and is joined to the blades, and that has an annular shape and an opening at its center; a bell mouth provided inside the shroud, the inner diameter of which gradually increases toward the upstream side, a length of one end joint portion of the blade joined to the main plate is longer than a length of the other end joint portion of the blade joined to the shroud, and an angle formed by a first line connecting a leading edge position and a trailing edge position of the one end joint portion and a line passing through a midpoint of the first line and the rotation center, in a shape projected onto a plane perpendicular to the rotation axis of the rotation shaft, is smaller than an angle formed by a second line connecting a leading edge position and a trailing edge position of the other end joint portion and a line passing through a midpoint of the second line and the rotation center; a leading edge of each blade, when projected onto a meridian plane passing through the rotation axis, has a region where the angle formed with the rotation axis on the suction side exceeds 90 degrees before the leading edge is joined to the shroud, and a downstream end of the bell mouth is located at a position opposite to this region.

2. 2. The centrifugal fan according to claim 1, wherein a leading edge of each blade, when projected onto a plane perpendicular to the rotation axis, has a region in which the leading edge recedes and then advances in the direction of rotation from the rotation center toward the outer periphery, or a region in which the degree of receding in the direction of rotation gradually decreases from the rotation center toward the outer periphery.

3. 3. The centrifugal fan according to claim 2, wherein the leading edges of the blades, when projected onto a plane perpendicular to the rotation axis, have a region where the leading edges retreat and then advance in the direction of rotation from the rotation center toward the outer periphery, or a region where the retreat angle with respect to the direction of rotation gradually decreases from the rotation center toward the outer periphery.

4. 4. The centrifugal fan according to claim 1, wherein a leading edge of each blade, when projected onto a meridian plane passing through the rotation axis, has a region in which an angle formed with the rotation axis on the suction side decreases and then remains constant or increases, or remains constant without decreasing or increases.

5. 5. The centrifugal fan according to claim 1, wherein a thickness of a region where the shroud and the bell mouth overlap in the radial direction of the leading edge of the blade is greater than a thickness of a region where the shroud and the bell mouth do not overlap in a shape projected onto a meridian plane passing through the rotation axis.

6. The trailing edge of the blade is inclined so that the shroud side faces in the counter-rotation direction, 6. The centrifugal fan according to claim 4, wherein the blade has a region in which an acute angle formed on the main plate side by the trailing edge and a line parallel to the rotation axis when viewed from the radially outside decreases toward the shroud side.

7. 7. The centrifugal fan according to claim 4, wherein, in a shape projected onto a meridian plane passing through the rotation axis, an angle formed on the suction side by a tangent from a rotation center side and the rotation axis is 60 degrees or less.

8. 8. The centrifugal fan according to claim 1, wherein a ratio A2 / A1 of an inlet area A1 of the shroud to an outlet area A2 of the centrifugal fan is 1.1 or greater and 1.5 or less.

9. The centrifugal fan according to claim 1 , further comprising an annular member provided on an outer periphery of the bell mouth, with a gap provided upstream from the inlet of the shroud.

10. An indoor unit for an air conditioner, comprising the centrifugal fan according to any one of claims 1 to 9.

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