JP2009281198A - Multiblade centrifugal fan - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multiblade centrifugal fan achieving both securement of blast performance and a reduction in NZ noise. <P>SOLUTION: In this multiblade centrifugal fan 1, an impeller 10 having a number of blades 11 is rotatably provided in a scroll-shaped casing 2. A plurality of projections 12, 13 parallel to the direction of the rotating shaft of the impeller 10 are provided in a range of 0°<θ<45° and/or 90°<θ<180° on a side surface 5 in the casing 2 where θ presents the angle of the impeller 10 in a rotating direction based on a straight line connecting the center S1 of the rotating shaft of the impeller 10 to the rounded center S2 of the tongue portion 6 of the casing 2. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a multiblade centrifugal fan in which an impeller having a large number of blades is rotatably provided in a scroll-like casing.

  A multi-blade centrifugal fan with a structure in which an impeller having a large number of blades is rotatably provided in a scroll-like casing is widely used as a fan for ventilation or ventilation in refrigeration, air conditioning, ventilation equipment, etc. Yes. In such a multiblade centrifugal fan, it is well known that discrete frequency noise having a primary component of the product of the number of blades (Z) and the number of rotations (N) of an impeller is generated. This NZ sound is considered to be generated due to the airflow blown in the centrifugal direction from between the blades in the vicinity of the tongue portion of the casing and the secondary flow interfere with the tongue portion.

  In order to suppress the generation of NZ noise, various conventional methods have been used such as making the pitch of the blades non-uniform to break the periodicity, and preventing the tongue of the scroll casing from being parallel to the blades. Countermeasures are implemented. However, the current situation is that the NZ sound level has not been sufficiently reduced by these measures, and it is thought that there may be causes other than the tongue, but it has not been elucidated yet. .

  On the other hand, in Patent Document 1, in order to suppress energy loss when the airflow flows in the casing, a large number of protrusions including protrusions along the rotation axis direction of the impeller are provided on the inner surface of the casing, and the vicinity of the protrusions. There is provided a technology that suppresses energy loss by stabilizing the airflow flowing through the turbulent flow in the boundary layer and stabilizing it, thereby preventing the development of the boundary layer.

JP 2002-5090 A

  By the way, in the multiblade centrifugal fan, a large vortex is formed due to the turbulence of the flow that occurs each time the blade of the impeller passes with respect to the reverse flow that occurs near the tongue in the casing. In addition, the air volume passing through the meridional section (longitudinal section passing through the rotation axis) increases in the downstream direction (circumferential direction), and a vortex circulating in the opposite direction as the secondary flow of the vortex is formed on the lower side of the casing. Formed and grows downstream. The generation of NZ sounds other than the tongue occurs because these vortices have a pressure fluctuation component of NZ frequency, and the vortices propagate downstream of the casing and interfere with the side and bottom surfaces of the casing. it seems to do.

  As a result of detailed analysis of the flow of airflow inside the casing, the turbulent kinetic energy increases at low load and large airflow (location where turbulence is increased) This is supported by the fact that the peak frequency of the pressure fluctuation generated on the side and bottom surfaces of the casing coincided with the frequency of the NZ sound. Thereby, in order to reduce NZ sound, it became clear that it was necessary to reduce NZ sound which generate | occur | produces on the side surface and bottom face of a casing other than a tongue part.

  Patent Document 1 does not touch on the reduction of NZ sound. However, even if the protrusion provided on the inner surface of the casing has a function of dispersing the pressure fluctuation component having a frequency of NZ, the protrusion has a pressure loss due to the protrusion provided on the entire inner surface of the casing. Increasing and lowering airflow are inevitable. Therefore, it has been difficult for the technique disclosed in Patent Document 1 to ensure both the air blowing performance and the reduction of the NZ sound.

  This invention is made | formed in view of such a situation, Comprising: It aims at providing the multiblade centrifugal fan which can make airflow performance ensuring and reduction of NZ sound compatible.

In order to solve the above problems, the multiblade centrifugal fan of the present invention employs the following means.
That is, the multiblade centrifugal fan according to the present invention is a multiblade centrifugal fan in which an impeller having a large number of blades is rotatably provided in a scroll-like casing, and the rotation shaft center of the impeller and the casing When the angle in the rotation direction of the impeller is Θ on the basis of a straight line connecting the center of the tongue of the tongue, 0 ° <Θ <45 ° and / or 90 ° <Θ <180 on the side surface in the casing A plurality of protrusions parallel to the rotational axis direction of the impeller are provided in a range of °.

In a multiblade centrifugal fan having a scroll-shaped casing, it is known that discrete frequency noise (NZ sound) having a primary component that is a product of the number of blades and the number of rotations of an impeller is generated. This NZ sound is thought to be caused by the airflow blown in the centrifugal direction from between the blades near the tongue portion of the casing and the secondary flow interfere with the tongue portion. However, as a result of analyzing the flow in the casing, the vortex generated by the blade passing through the reverse flow region near the tongue also grows as it goes downstream, and the frequency of NZ contained in the vortex is also determined. It was found that the pressure fluctuation component possessed interfered with the side surface and the bottom surface in the casing to generate NZ sound.
According to the present invention, when the angle in the rotation direction of the impeller is Θ based on a straight line connecting the rotation axis center of the impeller and the rounded center of the tongue of the casing, 0 ° <Θ on the side surface in the casing Occurs when the blade passes through the backflow area near the tongue, since a plurality of projections parallel to the rotation axis direction of the impeller are provided in the range of <45 ° and / or 90 ° <Θ <180 ° However, the pressure fluctuation component having the frequency of NZ contained in the vortex that grows and propagates toward the downstream can be disturbed and dispersed by the plurality of protrusions provided on the side surface in the casing. Therefore, the peak level of the NZ sound can be reduced. Moreover, since the protrusion is selectively provided in a range effective for reducing the NZ sound, the pressure loss due to the protrusion can be minimized, and the reduction in the air volume can be suppressed. Therefore, it is possible to achieve both ensuring the air blowing performance and reducing the NZ sound.

  Furthermore, the multiblade centrifugal fan according to the present invention is characterized in that, in the multiblade centrifugal fan, the protrusion has a gradually lower height and a smaller sectional area along a height direction of a side surface of the casing. And

  According to the present invention, the protrusion has a gradually lower height along the height direction of the casing side surface and the cross-sectional area is reduced, so that the interference between the protrusion and the airflow is changed in the height direction of the casing side surface, The pressure fluctuation component having the frequency of NZ contained in the vortex can be more effectively dispersed. Therefore, the NZ sound reduction effect can be further enhanced. In addition, since the gradient formed by gradually reducing the height of the protrusion and gradually reducing the cross-sectional area can be used as the draft when forming the casing, the casing provided with the protrusion can be easily manufactured.

  Furthermore, in the multiblade centrifugal fan of the present invention, in any one of the multiblade centrifugal fans described above, the circumferential interval between the protrusions is set so as not to have the least common multiple with respect to the average pitch of the blades. It is characterized by.

  According to the present invention, since the circumferential interval of the protrusions is set so as not to have the least common multiple with respect to the average pitch of the blades, the phase of the interference between the protrusions and the blades can be periodically shifted. Therefore, it is not strengthened by matching the frequency components of NZ, and the NZ sound can be effectively reduced.

  Furthermore, the multiblade centrifugal fan of the present invention is any one of the above-described multiblade centrifugal fans, wherein the casing is divided into upper and lower parts along a height direction of a side surface, and the projection is provided in each divided casing. It is characterized in that it is provided on each of the side surfaces so that its height gradually decreases and its cross-sectional area decreases toward the dividing surface.

  According to the present invention, the casing is divided into upper and lower parts, and the protrusions are provided on the side surfaces in the respective casings so that the height gradually decreases toward the dividing surface and the cross-sectional area decreases. The flow path resistance due to the protrusion in the vicinity of the central portion in the height direction of the casing where the flow velocity of the casing becomes high can be reduced. Therefore, the pressure loss due to the protrusions can be further reduced, and both the ensuring of the blowing performance and the reduction of the NZ sound can be achieved. Moreover, even if the casing is divided, it is not necessary to set a draft at the time of molding again, and a casing provided with the projection can be easily manufactured.

  Furthermore, the multiblade centrifugal fan of the present invention is any one of the above-described multiblade centrifugal fans, wherein the casing is divided into upper and lower parts along a height direction of a side surface, and the projection is provided in each divided casing. It is characterized in that they are provided on the side surfaces of the first and second sides with their positions shifted in the circumferential direction.

  According to the present invention, the casing is divided into upper and lower parts, and the protrusions are provided on the side surfaces in the divided casings so as to be displaced from each other in the circumferential direction. The pressure fluctuation component can be more effectively dispersed by changing the degree of interference with the pressure fluctuation component. Therefore, the NZ sound reduction effect can be further enhanced.

  The multi-blade centrifugal fan according to the present invention is a multi-blade centrifugal fan in which an impeller having a large number of blades is rotatably provided in a scroll-like casing. When the angle in the rotation direction of the impeller is Θ with reference to a straight line connecting the center of the tongue of the tongue, 0 ° <Θ <45 ° and / or 90 ° <Θ <140 on the bottom surface in the casing A plurality of vortex generators protruding in the direction of the rotation axis of the impeller are provided in a range of 0 °.

  According to the present invention, when the angle in the rotation direction of the impeller is Θ with respect to a straight line connecting the rotation axis center of the impeller and the rounded center of the tongue of the casing, 0 ° <Θ on the bottom surface in the casing When the blade passes through the backflow area near the tongue, since there are a plurality of vortex generators protruding in the direction of the rotation axis of the impeller in the range of <45 ° and / or 90 ° <Θ <140 ° The pressure fluctuation component having the frequency of NZ included in the vortex generated and propagated downstream is propagated and dispersed by the plurality of vortex generators provided on the bottom surface in the casing. Therefore, the peak level of the NZ sound can be reduced. In addition, since the vortex generator is selectively provided in a range effective for reducing the NZ sound, the pressure loss due to the vortex generator can be minimized and the decrease in the air volume can be suppressed. Therefore, it is possible to achieve both ensuring the air blowing performance and reducing the NZ sound.

  Moreover, the multiblade centrifugal fan of the present invention is characterized in that, in the multiblade centrifugal fan, the vortex generators are arranged in a staggered manner with respect to the airflow direction.

  According to the present invention, since the vortex generators are arranged in a staggered manner with respect to the airflow direction, the pressure fluctuation component having the frequency of NZ contained in the vortex is more effectively generated by the vortex generator group arranged in a staggered manner. Can be dispersed. Therefore, the NZ sound can be effectively reduced.

  Furthermore, the multiblade centrifugal fan of the present invention is the above-described multiblade centrifugal fan, wherein the vortex generator is protruded in the direction of the rotation axis of the impeller and has a columnar or conical shape with a gradually reduced cross-sectional area. It is characterized by being a protrusion.

  According to the present invention, since the vortex generator is protruded in the direction of the rotation axis of the impeller and is formed into a columnar or conical projection having a gradually reduced cross-sectional area, the vortex generator is integrated with the casing by injection molding or the like. Can be easily molded. Therefore, even if a vortex generator is provided, there is no particular increase in cost.

  According to the present invention, a pressure fluctuation component having a frequency of NZ included in a vortex that is generated when the blade passes through the backflow region near the tongue and grows and propagates downstream is applied to the side surface or the bottom surface in the casing. Dispersed by a plurality of projections or vortex generators provided. Therefore, the peak level of the NZ sound can be reduced. In addition, since the protrusions or vortex generators are selectively provided in an effective range for reducing the NZ sound, the pressure loss due to the protrusions or vortex generators can be minimized, and the decrease in the air volume can be suppressed. Therefore, it is possible to achieve both ensuring the air blowing performance and reducing the NZ sound.

Embodiments according to the present invention will be described below with reference to the drawings.
[First Embodiment]
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 1 to 6 and FIGS. 8 to 10.
FIG. 1 is a perspective view schematically showing the flow of airflow in the casing of the multiblade centrifugal fan according to the first embodiment of the present invention, FIG. 2 is a cross-sectional view thereof, and FIG. A longitudinal section (meridional section) passing through the rotation axis of the impeller is shown. The multiblade centrifugal fan 1 includes a scroll-shaped resin material casing 2.

  The casing 2 is composed of an upper surface 3, a bottom surface 4, and a side surface 5, and is formed in a spiral shape with the tongue 6 as a base point. An air suction port 8 having a bell mouth 7 is provided on the upper surface 3 of the casing 2, and a blowout port 9 is extended in a tangential direction from the upstream side of the tongue 6. An impeller 10 having a large number of blades 11 is rotatably supported inside the casing 2 via a motor (not shown) provided on the bottom surface 4 side.

  In the multiblade centrifugal fan 1 described above, the flow of the airflow in the casing 2 is analyzed in detail. As a result, the turbulent kinetic energy increases when the airflow is low and when the airflow is high, that is, the airflow caused by vortices. Where the turbulence is large and the vorticity is a certain value or more, the rotation direction of the impeller 10 is based on a straight line connecting the rotation axis center S1 of the impeller 10 and the round center S2 of the tongue 6 of the casing 2. 8 (see FIG. 2), as shown in FIGS. 8 (A) and 8 (B), there is a slight difference between the low load large air flow and the high load small air flow. The inner side surface 5 has a range of approximately 0 ° <Θ <45 ° and 90 ° <Θ <180 °, and the bottom surface 4 in the casing 2 has a range of approximately 0 ° <Θ <45 ° and 90 ° <Θ. It was revealed that the range was <140 °.

  FIG. 8 shows a region where the vorticity due to the turbulence on the inner surface of the casing of the multiblade centrifugal fan is a certain value or more, and the rotation axis center S1 of the impeller 10 and the radius center of the tongue 6 of the casing 2 are 2 The angle (deg) in the rotation direction of the impeller 10 with reference to the straight line connecting the blades and Θ is represented as Θ (see FIG. 2), and the height position (dimensionless value) of the blade (blade) 11 is represented as H (see FIG. 3). It is the figure which showed the state at the time of low load large air volume (A) and high load small air volume (B).

  In the present embodiment, the side surface 5 in the casing 2 is shown in FIG. 4 or FIG. 5 in the above range of Θ, that is, in the range of 0 ° <Θ <45 ° and / or 90 ° <Θ <180 °. Thus, it is set as the structure which provided the some protrusion 12 or 13 parallel to the rotating shaft direction of the impeller 10. As shown in FIG. As shown in the drawing, the protrusions 12 and 13 have a shape in which the height gradually decreases and the cross-sectional area decreases gradually or stepwise along the height direction of the side surface 5.

  Further, a plurality of the protrusions 12 or 13 are provided on the side surface 5 in the casing 2 at a predetermined interval (pitch) along the circumferential direction. The interval is relative to the average pitch of the blades 11 of the impeller 10. Are set to have no least common multiple.

  Further, on the bottom surface 4 in the casing 2, as shown in FIG. 6, the impeller is in the above-described range of Θ, that is, in the range of 0 ° <Θ <45 ° and / or 90 ° <Θ <140 °. 10 is provided with a plurality of vortex generators 14 protruding in the direction of the rotation axis. The vortex generators 14 are formed by columnar or conical protrusions whose cross-sectional areas are gradually reduced, and are arranged in a staggered manner with respect to the airflow direction.

With the configuration described above, according to the present embodiment, the following operational effects can be obtained.
The air sucked in the axial direction from the air suction port 8 through the bell mouth 7 by the rotation of the impeller 10 passes between the plurality of blades 11 of the impeller 10 in the centrifugal direction from the inner peripheral side to the outer peripheral side. Pumped. This air is sent in the circumferential direction along the inner surface of the scroll-like casing 2 and blown out from the outlet 9 to the outside.

  While the above air flow is generated by the rotation of the impeller 10, the flow disturbance caused by the blade 11 of the impeller 10 passing through the reverse flow generated near the tongue 6 of the casing 2 is the cause. It is known that a large vortex is formed and a turbulent region having a large turbulence is generated. The pressure fluctuation due to the blade 11 passing through this turbulent flow region has a frequency component of NZ, propagates as it is in the casing 2 to the downstream side of the tongue 6, and interferes with the inner surface of the casing 2, thereby causing NZ sound. Is generated.

  As shown in FIGS. 8 (A) and 8 (B), this pressure fluctuation generation region is such that the angle Θ from the tongue 6 is approximately 0 ° <Θ <45 ° and 90 ° on the side surface 5 of the casing 2. In the range of ° <Θ <180 °, the bottom surface 4 in the casing 2 is generally in the ranges of 0 ° <Θ <45 ° and 90 ° <Θ <140 °. Accordingly, the protrusion 12 or 13 is provided on the side surface 5 of the region of 0 ° <Θ <45 ° and / or 90 ° <Θ <180 °, and 0 ° <Θ <45 ° and / or 90 ° < By providing the vortex generator 14 on the bottom surface 4 in the Θ <140 ° region, pressure fluctuation components having a frequency of NZ are respectively generated by the protrusions 12 and 13 and the vortex generator 14 on the side surface 5 and the bottom surface 4 in the casing 2. Disturbed and dispersed. Note that the difference in the pressure fluctuation generation region between the bottom surface 4 and the side surface 5 in the casing 2 is caused by the flow that flows out radially outward from the blade 11 and the static pressure distribution that increases as the casing 2 moves radially outward. It is generated by the interference effect between the secondary flow and the main flow flowing in the rotation direction in the casing 2.

  As a result, the peak level of the NZ sound can be reduced and the blowing noise in the multiblade centrifugal fan 1 can be reduced. In addition, since the protrusions 12 and 13 and the vortex generator 14 are limited to the effective range for reducing the NZ noise, the pressure loss due to the provision of the protrusions 12 and 13 and the vortex generator 14 is minimized. , It is possible to suppress a decrease in the air volume.

  9 and 10 show the amount of reduction in the air volume and the amount of reduction in the NZ sound when the installation range of the protrusions 12 and 13 and the vortex generator 14 is limited as described above and when it is provided over the entire inner surface of the casing 2. Are shown in comparison. The difference in the amount of NZ sound reduction is as small as less than 1 dB, while the difference in the amount of airflow reduction is large. As is clear from this comparison result, according to the present embodiment, it is possible to achieve both of ensuring the blowing performance and reducing the NZ sound.

  Further, the protrusions 12 and 13 have a gradually lower height and a smaller sectional area along the height direction of the side surface 5 of the casing 2. For this reason, in the height direction of the side surface 5, the degree of interference between the projections 12 and 13 and the airflow can be changed, and the pressure fluctuation component having the frequency of NZ contained in the vortex can be more effectively dispersed. The reduction effect can be further enhanced. Further, the gradient formed by gradually decreasing the height of the protrusions 12 and 13 and gradually decreasing the cross-sectional area can be used as the draft when the casing is formed. For this reason, the casing 2 provided with the protrusions 12 and 13 can be easily manufactured.

  Furthermore, since the circumferential interval (pitch) of the plurality of projections 12 and 13 is set so as not to have the least common multiple with respect to the average pitch of the blades 11 of the impeller 10, The phase of interference with the blade 11 can be periodically shifted. For this reason, it is not strengthened by the coincidence of the frequency components of NZ, and the NZ sound can be reduced more effectively.

  Further, the vortex generators 14 provided on the bottom surface 4 of the casing 2 are arranged in a staggered manner with respect to the airflow direction, and the pressure having the frequency of NZ contained in the vortex by the group of vortex generators 14 arranged in a staggered manner. Since the fluctuation component can be more effectively dispersed, the effect of reducing the NZ sound can be further enhanced. Furthermore, since the vortex generator 14 is a columnar or conical projection whose cross-sectional area is gradually reduced, the vortex generator 14 can be easily molded integrally with the casing 2 by injection molding or the like. Providing the vortex generator 14 does not cause a cost increase.

[Second Embodiment]
Next, a second embodiment of the present invention will be described with reference to FIG.
This embodiment differs in the structure of a casing and protrusion from above-described 1st Embodiment. Since other points are the same as those in the first embodiment, description thereof will be omitted.
In the present embodiment, the casing 2 is divided into two casings 2A and 2B in the vertical direction near the center of the side surface 5 in the height direction, and the side surfaces 5A in the casings 2A and 2B divided into the upper and lower parts 2A and 2B. With respect to 5B, the projections 12A, 12B or 13A, 13B having a gradually lower height and a smaller sectional area toward the dividing surface are provided.

  As described above, the casing 2 is divided into the upper and lower casings 2A and 2B, and the protrusions 12A and 12B or 13A and 13B are divided into side surfaces 5A and 5B in the divided casings 2A and 2B, respectively, gradually increasing in height toward the dividing surface. By providing it so as to have a low cross-sectional area, the flow resistance by the projections 12A, 12B or 13A, 13B near the center in the height direction of the casing 2 where the flow velocity of the airflow is increased is reduced. The pressure loss due to 12B or 13A, 13B can be further reduced.

  Therefore, according to the present embodiment as well as the first embodiment, it is possible to ensure both the air blowing performance and the NZ noise reduction effect. Further, even if the casings 2A and 2B are divided, it is not necessary to set the draft angle of the mold at the time of molding again to the projections 12A, 12B or 13A, 13B, and the casing 2A provided with the projections 12A, 12B or 13A, 13B. , 2B can be easily manufactured.

  In the present embodiment, the protrusions 12A or 13A provided on the casing 2A side and the protrusions 12B or 13B provided on the casing 2B side can be provided with their positions shifted in the circumferential direction. In this way, by providing the protrusions 12A, 12B or 13A, 13B provided on the side surfaces of the casings 2A, 2B divided in the vertical direction with the positions shifted in the circumferential direction, the upper and lower protrusions 12A, 12B or 13A, 13B are provided. The degree of interference with the pressure fluctuation component having the frequency of NZ contained in the vortex can be changed depending on the group, and the pressure fluctuation component can be more effectively dispersed. For this reason, NZ sound can be reduced more effectively.

  In addition, this invention is not limited to the invention concerning the said embodiment, In the range which does not deviate from the summary, it can change suitably. For example, in the above-described embodiment, the example in which the protrusion 12 or 13 is provided on the side surface 5 in the casing 2 and the vortex generator 14 is provided on the bottom surface 4 is described. Of course, only one of them may be provided. It is. Further, the protrusions 12 and 13 provided on the side surface 5 in the casing 2 and the vortex generator 14 provided on the bottom surface 4 are not limited to those of the above-described embodiment, and various modifications can be made.

It is the perspective view which modeled the flow of the airflow in the casing of the multiblade centrifugal fan which concerns on 1st Embodiment of this invention. It is a cross-sectional view of the multiblade centrifugal fan shown in FIG. It is a longitudinal cross-sectional view (meridian surface cross section) which passes along the rotating shaft line of the impeller of the multiblade centrifugal fan shown in FIG. It is a perspective view which shows the installation state of an example of the protrusion provided in the casing side surface of the multiblade centrifugal fan shown in FIG. It is a perspective view which shows the installation state of the other example of the protrusion provided in the casing side surface of the multiblade centrifugal fan shown in FIG. It is a perspective view which shows the installation state of an example of the vortex generator provided in the casing bottom face of the multiblade centrifugal fan shown in FIG. It is a longitudinal cross-sectional view of the casing and protrusion of the multiblade centrifugal fan which concerns on 2nd Embodiment of this invention. It is a state figure at the time of the low load large air volume (A) and the high load small air volume (B) which shows the area | region where the vorticity by the disturbance of the flow in the casing inner surface of a multiblade centrifugal fan becomes a fixed value or more. It is a comparison figure which shows the fall amount of the airflow by the multiblade centrifugal fan of this invention. It is a comparison figure which shows the reduction amount of NZ sound by the multiblade centrifugal fan of this invention.

Explanation of symbols

1 Multiblade centrifugal fan 2, 2A, 2B Casing 4 Bottom surface 5, 5A, 5B Side surface 6 Tongue portion 10 Impeller 11 Blade 12, 12A, 12B, 13, 13A, 13B Protrusion 14 Vortex generator Θ Installation range

Claims (8)

In a multiblade centrifugal fan in which an impeller having a large number of blades is rotatably provided in a scroll-shaped casing,
When the angle in the rotation direction of the impeller is Θ with respect to a straight line connecting the rotation shaft center of the impeller and the rounded center of the tongue of the casing, 0 ° <Θ <45 on the side surface in the casing. A multiblade centrifugal fan characterized in that a plurality of projections parallel to the rotational axis direction of the impeller are provided in a range of ° and / or 90 ° <Θ <180 °.   The multi-blade centrifugal fan according to claim 1, wherein the protrusion has a gradually lower height and a smaller cross-sectional area along a height direction of a side surface of the casing.   The multiblade centrifugal fan according to claim 1 or 2, wherein the circumferential interval between the protrusions is set so as not to have a least common multiple with respect to an average pitch of the blades.   The casing is divided up and down along the height direction of the side surface, and the protrusions are gradually reduced in height and cross-sectional area toward the divided surface on the side surface in each of the divided casings. The multiblade centrifugal fan according to any one of claims 1 to 3, wherein the multiblade centrifugal fan is provided.   The casing is divided into upper and lower portions along a height direction of a side surface, and the protrusions are provided on the side surfaces in each divided casing so as to be shifted from each other in the circumferential direction. The multiblade centrifugal fan according to any one of 1 to 4. In a multiblade centrifugal fan in which an impeller having a large number of blades is rotatably provided in a scroll-shaped casing,
When the angle in the rotation direction of the impeller is Θ with respect to a straight line connecting the rotation shaft center of the impeller and the rounded center of the tongue of the casing, 0 ° <Θ <45 at the bottom surface in the casing. A multiblade centrifugal fan comprising a plurality of vortex generators protruding in the direction of the rotation axis of the impeller in a range of ° and / or 90 ° <Θ <140 °.   The multi-blade centrifugal fan according to claim 6, wherein the vortex generators are arranged in a staggered manner with respect to the airflow direction. The multi-blade centrifugal fan according to claim 6 or 7, wherein the vortex generator is a columnar or cone-shaped protrusion that protrudes in the direction of the rotation axis of the impeller and has a gradually reduced cross-sectional area. .

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