Centrifugal Blower

Abstract

A centrifugal blower (1) comprises: a motor (3); a scroll casing (11) that has a bell mouth, a scroll part (6) having a width in the radial direction that increases toward the downstream side of an airflow, a tongue part (9) connecting the scroll part (6) and an outlet (10) such that the area of the opening of the outlet (10) expands, and a diffuser part (7); and an impeller (4) that has a disk-shaped main plate (4a) and a plurality of blades provided in a ring-like configuration at an outer circumferential part of the main plate (4a), and that forms an airflow from an inlet (8) to the outlet (10) in the scroll casing (11). Provided to the bell mouth, further downstream than the tongue part (9) in the rotation direction of the impeller (4), is a blocking part (8b) that blocks a secondary flow which flows out from the impeller (4) and flows back into the impeller (4). The impeller (4) has a tube-shaped protruding part (5) that protrudes from the back surface of the main plate (4a). The maximum width of the protruding part (5) is smaller than the outer diameter of the impeller (4).

Description

[Technical field]

[0001] The present disclosure relates to a centrifugal blower having an impeller that generates an air flow. [Background technology]

[0002] A centrifugal blower has a scroll casing that houses an impeller. The scroll casing has an air inlet and an air outlet, and constitutes a flow path for the airflow generated by the rotation of the impeller. Noise generated inside the scroll casing by the rotation of the impeller is released to the outside of the scroll casing through the air inlet or the air outlet. The flow speed of the airflow inside the centrifugal blower is fastest immediately after it leaves the impeller. Furthermore, the distance between the impeller and the wall surface of the scroll casing increases as the rotation proceeds, which makes it easy for fluctuations in the airflow to occur.

[0003] The airflow coming out of the impeller is roughly divided into a main flow and a secondary flow. The secondary flow is an airflow that flows perpendicular to the main flow. It is known that the secondary flow causes secondary loss and deteriorates the blowing performance.

[0004] In addition, centrifugal fans generate noise due to periodic pressure fluctuations on the blade surface flow caused by the rotation of the impeller. In addition, similar noise is generated when the flow around the impeller blades periodically changes due to the influence of the above-mentioned airflow fluctuations. This type of noise is called discrete frequency noise, and it deteriorates the acoustic characteristics of the centrifugal fan.

[0005] Patent Document 1 discloses a technique for installing a flow straightening block in a bell mouth to straighten the flow inside a scroll casing. This improves the blowing performance of the blower. Patent Document 2 discloses a technique for forming a cylindrical wall on the rear surface of the main plate of the impeller, with the outer diameter of the cylindrical wall being the same as the outer diameter of the impeller blades. This prevents the secondary flow formed by the rotation of the impeller from flowing into the rear surface of the main plate, mitigating secondary loss and improving blowing performance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6246434 [Patent Document 2] JP 2008-169826 A Summary of the Invention [Problem to be solved by the invention]

[0007] In a centrifugal blower, part of the flow blown out from the impeller becomes a secondary flow, which re-flows into the scroll casing or generates a circulating flow. Patent Document 1 presents a technique for reducing such secondary flows by providing a shielding section in the bellmouth, but when the technique described in Patent Document 1 is used, the provision of the shielding section strengthens the flow from the bellmouth side toward the main plate of the impeller, which tends to disturb the flow on the back side of the main plate of the impeller, resulting in an increase in noise.

[0008] Furthermore, the centrifugal blower disclosed in Patent Document 2 is expected to prevent the secondary flow formed by the rotation of the impeller from flowing into the back surface of the main plate, thereby reducing losses due to the secondary flow. However, it is unable to handle the flow from the bellmouth side toward the main plate of the impeller as described above, and this increases pressure fluctuations on the back surface of the main plate of the impeller, resulting in a problem of increased noise pollution.

[0009] Furthermore, discrete frequency noise generated around the blades of the impeller of a centrifugal blower is likely to occur due to interference between the impeller and the scroll casing in the area where the impeller and the scroll casing are close to each other. As a result, if the flow from the bellmouth side toward the main plate of the impeller described above becomes stronger and the pressure fluctuation on the back side of the main plate of the impeller increases, this may lead to an increase in discrete frequency noise of the centrifugal blower.

[0010] In particular, the distance between the impeller and the scroll casing is short on the rear side of the impeller main plate in order to suppress leakage flow, and therefore discrete frequency noise is easily transmitted on the rear side of the impeller main plate, resulting in deterioration of the acoustic characteristics of the centrifugal blower.

[0011] The present disclosure has been made in consideration of the above, and has an object to provide a centrifugal blower that suppresses pressure fluctuations caused by interference between the secondary flow and the impeller, and reduces discrete frequency noise generated around the impeller blades. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems and achieve the object, the centrifugal blower according to the present disclosure includes a motor, an inlet, an outlet, a bell mouth that guides an airflow toward the inlet, a scroll casing having a scroll section whose width in the radial direction increases toward the downstream side of the airflow, a tongue section that connects the scroll section and the outlet so as to increase the opening area of ​​the outlet, and a diffuser section that forms a flow path between the scroll section and the outlet, and an impeller that has a disk-shaped main plate that is fixed to the shaft of the motor and rotates, and a plurality of blades that are annularly installed on the outer periphery of the main plate, and is installed in the scroll casing to form an airflow from the inlet to the outlet in the scroll casing. The bell mouth is provided with a shielding section downstream of the tongue section in the rotation direction of the impeller to shield a secondary flow that flows out of the impeller and re-enters the impeller. The impeller has a cylindrical protrusion that protrudes from the back surface of the main plate. The maximum width of the protrusion is smaller than the outer diameter of the impeller. Effect of the Invention

[0013] The centrifugal blower according to the present disclosure has the advantage of being able to suppress pressure fluctuations caused by interference between the secondary flow and the impeller, and to reduce discrete frequency noise generated around the blades of the impeller. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a perspective view of a centrifugal blower according to a first embodiment; [Diagram 2] 1 is a cross-sectional view of a centrifugal blower according to a first embodiment; [Diagram 3] 1 is a top view of a centrifugal blower according to a first embodiment. [Figure 4] 1 is a cross-sectional view of a centrifugal blower according to a first comparative example of the first embodiment. [Diagram 5] FIG. 13 is a diagram showing the results of an actual test of the air volume-specific noise characteristics of the centrifugal blower according to the first embodiment and the centrifugal blower according to the first comparative example. [Figure 6] FIG. 1 is a diagram showing the position of a terminal end of a shielding portion provided in a bell mouth of a centrifugal blower according to embodiment 1. [Figure 7] FIG. 1 is a diagram showing the relationship between the position of a shielding portion provided in a bell mouth of a centrifugal blower according to embodiment 1 and the static pressure increase amount and noise. [Figure 8] 11 is a cross-sectional view of a centrifugal blower according to a second comparative example of the first embodiment. [Figure 9] FIG. 11 is a diagram showing a flow velocity distribution of a centrifugal blower according to a second comparative example of the first embodiment. [Figure 10] FIG. 13 is a diagram showing a distribution of effective values ​​of pressure fluctuations in a centrifugal blower according to a second comparative example of the first embodiment. [Figure 11] FIG. 11 is a diagram showing the distribution of pressure fluctuation levels at the bottom surface of a scroll section of a centrifugal blower according to a second comparative example of the first embodiment. [Figure 12] FIG. 1 is a diagram showing a flow velocity distribution of a centrifugal blower according to a first embodiment. [Figure 13] FIG. 1 is a diagram showing a distribution of effective values ​​of pressure fluctuations in a centrifugal blower according to a first embodiment. [Figure 14] FIG. 1 is a diagram showing the distribution of pressure fluctuation levels at the bottom surface of a scroll portion of a centrifugal blower according to a first embodiment. [Figure 15] FIG. 13 is a diagram showing the results of an actual machine test of the air volume-specific noise characteristics of the centrifugal blower according to the first embodiment, the centrifugal blower according to the second comparative example, and the centrifugal blower according to the third comparative example. [Figure 16] 1 is a cross-sectional view of a centrifugal blower according to a second embodiment. [Figure 17] FIG. 13 is a perspective view of an impeller of a centrifugal blower according to a third embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A centrifugal blower according to an embodiment will be described in detail below with reference to the drawings.

[0016] Embodiment 1 FIG. 1 is a perspective view of a centrifugal blower according to the first embodiment. FIG. 2 is a cross-sectional view of the centrifugal blower according to the first embodiment. FIG. 3 is a top view of the centrifugal blower according to the first embodiment. FIG. 2 shows a cross section taken along the line II-II in FIG. 1 and FIG. 3. FIG. 3 shows the entire impeller 4 with the bell mouth 8a omitted. The centrifugal blower 1 according to the first embodiment has a motor 3, an impeller 4 driven to rotate by the motor 3, and a scroll casing 11 that houses the impeller 4. The centrifugal blower 1 generates an airflow by rotating the impeller 4.

[0017] The scroll casing 11 has an air inlet 8 and an air outlet 10. The scroll casing 11 forms a flow path for the air flow generated by the rotation of the impeller 4. By rotating the impeller 4, the air outside the scroll casing 11 is sucked into the scroll casing 11 through the inlet 8. By rotating the impeller 4, the air inside the scroll casing 11 is blown out of the scroll casing 11 through the outlet 10. As shown in FIG. 1, outside the scroll casing 11, an air flow Y1 that flows from the outside of the scroll casing 11 toward the inlet 8 and an air flow Y2 that flows from the outlet 10 to the outside of the scroll casing 11 are generated. As shown in FIG. 2, inside the scroll casing 11, a main flow Y3 is generated, which is an air flow that flows from the inlet 8 through the impeller 4 to between the impeller 4 and the scroll casing 11. That is, as the impeller 4 rotates, an air flow is generated inside the scroll casing 11 from the suction port 8 to the air outlet 10 .

[0018] The scroll casing 11 has a scroll section 6, a diffuser section 7, and a tongue section 9. The scroll section 6 is a portion constituting a spiral flow passage whose radial width increases toward the downstream side of the air flow. The spiral flow passage is, for example, an Archimedes spiral. That is, the distance from the rotation shaft 2 of the impeller 4 to the scroll section 6 increases toward the rotation direction of the impeller 4, as shown in FIG. 3. The diffuser section 7 is a portion downstream of the scroll section 6 and constitutes a flow passage between the scroll section 6 and the outlet 10. The diffuser section 7 efficiently converts the dynamic pressure of the airflow flowing out of the impeller 4 into static pressure, while guiding the airflow to the outlet 10. The tongue section 9 is a portion that connects the scroll section 6 and the outlet 10 so as to enlarge the opening area of ​​the outlet 10. The tongue section 9 guides the airflow swirling inside the scroll casing 11 to the outlet 10.

[0019] The scroll casing 11 has a first wall 11a, a second wall 11b, and a third wall 11c. The first wall 11a and the second wall 11b face each other in the axial direction of the scroll casing 11. The third wall 11c connects the first wall 11a and the second wall 11b. The suction port 8 is formed in the first wall 11a. The first wall 11a is formed with a bell mouth 8a whose diameter increases as it moves away from the second wall 11b. The scroll casing 11 is made of a material such as resin. The bell mouth 8a is provided to surround the suction port 8 that draws in air.

[0020] The impeller 4 is a multi-blade impeller. The impeller 4 is fixed to the shaft 3a of the motor 3. The impeller 4 rotates around the shaft 3a. The impeller 4 includes a disk-shaped main plate 4a, a plurality of blades 4b arranged in an annular shape on the outer periphery of the front side surface of the main plate 4a, and a boss portion 4c fixed to the shaft 3a of the motor 3. In the centrifugal blower 1 according to the first embodiment, the number of blades 4b of the impeller 4 is 43. The impeller 4 also includes a rib 4d, which is a blade reinforcing member, on the outer diameter edge on the upstream side of the blade 4b. A protrusion 5 for rectifying the airflow is provided on the rear surface of the main plate 4a of the impeller 4.

[0021] The impeller 4 has a shape in which the protrusion 5 branches off from the middle of the main plate 4a. From the viewpoints of preventing leakage flow occurring on the back surface of the main plate 4a and reducing noise, the protrusion 5 is preferably shaped to be rotationally symmetric or axially symmetric with respect to the rotation shaft 2 of the impeller 4. In FIG. 2, the protrusion 5 has an axially symmetric cylindrical shape. A part of the motor 3 is housed in the space surrounded by the cylindrical wall of the protrusion 5. By housing a part of the motor 3 in the space surrounded by the cylindrical wall of the protrusion 5, the distance between the motor 3 and the back surface of the impeller 4 is minimized, and the centrifugal blower 1 can be made smaller overall.

[0022] Furthermore, the main plate 4a of the impeller 4 has a flat surface in the radial direction of the rotating shaft 2 from the outer periphery of the main plate 4a to the portion where the protrusion 5 is provided. The reason for making the region from the outer periphery of the main plate 4a to the portion where the protrusion 5 is provided a flat surface is to suppress a sudden turn of the airflow flowing out of the impeller 4 and to allow the main flow Y3 to reach the air outlet 10 with as little loss as possible. The effect of this will be described later.

[0023] As shown in Fig. 3, a plurality of ventilation holes 4e are provided in the main plate 4a of the impeller 4. The ventilation holes 4e are provided for the purpose of suppressing a temperature rise in the motor 3. In other words, the ventilation holes 4e are provided for promoting an air-cooling effect of the motor 3. Note that the ventilation holes 4e do not necessarily have to be provided in the main plate 4a. Furthermore, the number of ventilation holes 4e does not have to be six as shown in Fig. 3, and is not limited to a specific number.

[0024] The positional relationship and shape of the impeller 4 and the scroll casing 11, the presence or absence of the ventilation holes 4e, and the shape of the ventilation holes 4e are not limited to those shown in Figures 1 to 3. The positional relationship of each part and the shape of each part may be appropriately determined at the time of design.

[0025] The centrifugal blower 1 is structured so that air drawn into the scroll casing 11 from the suction port 8 along the axial direction of the rotary shaft 2 of the impeller 4 is blown out in the radial direction of the rotary shaft 2 of the impeller 4, and therefore inertial force acts on the airflow. For this reason, the airflow blown out through the impeller 4 into the scroll casing 11 forms a velocity distribution in which the velocity increases as the air approaches the second wall 11b of the scroll casing 11. Then, a part of the airflow blown out into the scroll casing 11 forms a secondary flow Y4 that rolls up from the main plate 4a of the impeller 4 toward the tip of the blade 4b or from the tip of the blade 4b toward the main plate 4a along the third wall 11c.

[0026] The bellmouth 8a has a structure in which the curved surface is partially removed in a portion around the rotation shaft 2 of the impeller 4 so as to prevent the outlet flow from the impeller 4 from re-flowing into the scroll casing 11 without flowing to the outlet 10. The curved surface of the bellmouth 8a is partially removed so as to block the secondary flow Y4, which is part of the outlet flow from the impeller 4, and this makes it possible to prevent collision or interference between the secondary flow Y4 and the airflow blown out from the impeller 4, thereby improving the blowing performance and suppressing noise of the centrifugal blower 1. Hereinafter, the part where the curved surface of the bellmouth 8a is partially removed is referred to as the shielding portion 8b.

[0027] Bellmouth 8a has a shape that is rotationally symmetrical with respect to rotation shaft 2 of impeller 4, excluding shielding portion 8b, and the shape of a cross section perpendicular to rotation shaft 2 of impeller 4 is an arc shape that is interrupted at shielding portion 8b. In a cross section passing through rotation shaft 2 of impeller 4 shown in Fig. 2, bellmouth 8a has a shape that becomes continuously smaller in the direction in which air is sucked in along the axial direction of rotation shaft 2, that is, the diameter of the circular opening becomes continuously smaller from upwind to downwind of the airflow.

[0028] The curved surface of the bellmouth 8a opens toward the outside of the scroll casing 11 on the upstream side. In addition, the curved surface of the bellmouth 8a is aligned along the axial direction on the downstream side. For this reason, the curved surface of the bellmouth 8a has a shape that is convex toward the air suction port 8. The curved surface of the bellmouth 8a that faces the suction port 8 is the front surface 8f, and the curved surface that does not face the suction port 8 is the back surface 8r.

[0029] The scroll casing 11 and the bellmouth 8a are integral and formed by processing a single metal plate through sheet metal processing. Therefore, a bellmouth back space 82 is formed between the curved surface of the bellmouth 8a and the first wall 11a of the scroll casing 11.

[0030] The shielding portion 8b circumferentially divides the bellmouth back space 82. The shielding portion 8b has two shielding portion side portions 81a formed on two planes that are parallel to the rotation shaft 2 of the impeller 4 and face each other across the rotation shaft 2, and a shielding portion bottom portion 81b that is on a plane intersecting the rotation shaft 2 and connects the downstream ends of the two shielding portion side portions 81a.

[0031] An upper end portion on the upstream side of the shielding side portion 81a is connected to the bell mouth 8a. An outer peripheral end portion of the shielding side portion 81a is connected to the scroll casing 11. A lower end portion on the downstream side of the shielding side portion 81a is connected to the shielding bottom portion 81b. An outer peripheral end portion of the shielding bottom portion 81b is connected to the scroll casing 11.

[0032] That is, the shielding portion 8b is continuously connected without branching to the bellmouth 8a, the shielding portion side portion 81a, the shielding portion bottom surface portion 81b, the shielding portion side portion 81a, and the bellmouth 8a in the circumferential direction of the rotating shaft 2. With this configuration, the bellmouth 8a and the shielding portion 8b can be integrally molded from a single metal plate.

[0033] Furthermore, by providing the shielding portion 8b, the bell-mouth back space 82 is divided in the circumferential direction, so that the secondary flow Y4 can be suppressed from flowing in the bell-mouth back space 82 in the circumferential direction.

[0034] Fig. 4 is a cross-sectional view of a centrifugal blower according to a first comparative example of the first embodiment. The bellmouth 8a of the centrifugal blower 1 according to the first comparative example shown in Fig. 4 has a structure in which there is no missing portion and the curved surface is continuous all around the axis of the rotating shaft 2 of the impeller 4. Moreover, in the centrifugal blower 1 according to the first comparative example, the bellmouth back space 82 is not partially filled, and the bellmouth back space 82 is connected without being divided in the circumferential direction of the rotating shaft 2 of the impeller 4. In Fig. 4, the structures and shapes of the components other than the bellmouth 8a are the same as those of the first embodiment.

[0035] As shown in FIG. 4, the centrifugal blower 1 according to the first comparative example has a structure in which the bellmouth 8a exists around the entire circumference of the rotating shaft 2 of the impeller 4. This causes the secondary flow Y4 to collide or interfere with the airflow blown out from the impeller 4, resulting in a decrease in blowing performance and an increase in noise.

[0036] Fig. 5 is a diagram showing the results of an actual machine test of the air volume-specific noise characteristics of the centrifugal blower according to the first embodiment and the centrifugal blower according to the first comparative example. In Fig. 5, the horizontal axis represents the air volume, and the vertical axis represents the specific noise. In Fig. 5, the black diamonds are plots representing the centrifugal blower 1 according to the first comparative example, and the white circles are plots representing the centrifugal blower 1 according to the first embodiment.

[0037] 5, the noise level in the operating air volume band of the centrifugal blower 1 is 12.5 dB for the centrifugal blower 1 according to the first comparative example, whereas it is 11.7 dB for the centrifugal blower 1 according to the first embodiment, confirming a noise reduction of 0.8 dB. The difference between the centrifugal blower 1 according to the first embodiment and the centrifugal blower 1 according to the first comparative example is whether or not the shielding portion 8b is provided in the bellmouth 8a, and therefore it can be determined that the noise reduction is achieved by the shielding portion 8b being able to effectively suppress the collision or interference between the secondary flow Y4 and the airflow blown out from the impeller 4.

[0038] In this way, the centrifugal blower 1 according to the first embodiment has a structure in which the curved surface of the bellmouth 8a is partially removed so as to block the secondary flow Y4, which is a part of the flow blown out from the impeller 4. This makes it possible to effectively prevent collision or interference between the secondary flow Y4 and the airflow blown out from the impeller 4, thereby achieving improved blowing performance of the centrifugal blower 1 and reduced noise.

[0039] The structure of the bellmouth 8a is not limited to a structure in which part of the curved surface is partially missing, as in the centrifugal blower 1 according to embodiment 1, but may be substituted by a structure in which the bellmouth back-side space 82 is partially filled or a protrusion that protrudes into the bellmouth back-side space 82 is partially provided on the back surface 8r of the bellmouth 8a so as to block the secondary flow Y4 of the blown flow from the impeller 4 re-entering the impeller 4.

[0040] FIG. 6 is a diagram showing the position of the shielding part end part of the shielding part provided in the bell mouth of the centrifugal blower according to the first embodiment. FIG. 7 is a diagram showing the relationship between the position of the shielding part provided in the bell mouth of the centrifugal blower according to the first embodiment, the static pressure rise amount, and the noise. The shielding part end part 8c is the downstream end part of the shielding part 8b. FIG. 7 shows the result of verifying the blocking effect of the shielding part 8b with respect to the secondary flow Y4 of the centrifugal blower 1 according to the first embodiment by an actual machine test. The horizontal axis of FIG. 7 represents the position of the shielding part end part 8c. The vertical axis of FIG. 7 represents the static pressure rise amount and the noise level of the centrifugal blower 1 according to the first embodiment. The angle α shown on the horizontal axis of FIG. 7 indicates the rotation angle from the position of the reference line P including the line segment connecting the rotating shaft 2 of the impeller 4 and the tip of the tongue part 9 to the position of the shielding part end part 8c, with the direction of rotation of the impeller 4 indicated by the arrow R as the positive direction around the rotating shaft 2.

[0041] As shown in FIG. 7, in the measurement results where the angle α from the reference line P to the shielding end 8c is between 60° and 150°, the noise tends to increase and the static pressure increase tends to decrease as the angle α increases. Since the static pressure increase is a positive value up to about 140°, the centrifugal blower 1 according to the first embodiment can obtain the effect of increasing the static pressure when the shielding end 8c is disposed at a position where the angle α is 140° or less. Even if the shielding end 8c is set at an angle α within a range of 60° to 120° in consideration of the deterioration of noise, a static pressure increase of approximately 4% or more can be obtained. Furthermore, when the angle α is 100° or less, the static pressure increases and the deterioration of noise is suppressed. In particular, when the angle α is around 70°, for example, when the angle α is in the range of 60° to 90°, the static pressure increase is large and the increase in noise is small compared to when the angle α is outside this range. When the angle α is increased, the influence of the reduction in the cross-sectional area of ​​the flow path becomes greater, and the above-mentioned effect obtained by providing the shielding portion 8b is offset. Therefore, in the mounting range of the shielding portion 8b, the angle α of the shielding portion end portion 8c, which is the rotation angle from the reference line P to the shielding portion end portion 8c, is preferably set to 120° or less, and more preferably, the angle α is set to 100° or less.

[0042] In addition, in Fig. 3, a circle 4f connecting the outer diameter side ends of the multiple ventilation holes 4e, a circle 4g ​​connecting the outer diameter side ends of the multiple blades 4b of the impeller 4, and a circle 4h connecting the inner diameter side ends of the multiple blades 4b of the impeller 4 are each illustrated by a dashed line. As shown in Figs. 2 and 3, the inner diameter of the impeller 4 is defined as D1, the outer diameter of the impeller 4 is defined as D2, the outer diameter of the motor 3 is defined as Dc, and the outer diameter of the protrusion 5 is defined as Da. In the centrifugal blower 1 according to the first embodiment, since the protrusion 5 is cylindrical, the outer diameter of the protrusion 5 is the maximum width of the protrusion 5. The diameter of the circle 4h connecting the inner diameter side ends of the multiple blades 4b of the impeller 4 is defined as the inner diameter D1 of the impeller 4. The diameter of the circle 4g ​​connecting the outer diameter side ends of the multiple blades 4b of the impeller 4 is defined as the outer diameter D2 of the impeller 4. As shown in Figs. 2 and 3, the diameter of a circle 4f connecting the outer peripheral ends of the multiple ventilation holes 4e with the rotation shaft 2 of the impeller 4 as its central axis is defined as the outer diameter Db of the ventilation hole 4e.

[0043] In the centrifugal blower 1 according to the first embodiment, the outer diameter Da of the protruding portion 5, which is a reference for defining the mounting position of the protruding portion 5, satisfies D2>Da>Dc. For example, in the centrifugal blower 1 according to the first embodiment, Da=0.9×D2. There are two reasons why the outer diameter Da is made to satisfy the above condition. The first reason is to secure the distance between the protruding portion 5 and the scroll casing 11 and enhance the effect of suppressing interference between them, as described later. The secondary flow Y4 has a larger flow rate toward the second wall 11b side than toward the bell mouth 8a side due to the structural cause that the inertial force acts on the airflow. Therefore, even in the centrifugal blower 1 according to the first embodiment in which the curved surface of the bell mouth 8a is partially missing, the effect of suppressing interference between the protruding portion 5 and the secondary flow Y4 is further enhanced by securing the distance between the protruding portion 5 and the scroll casing 11.

[0044] The second reason is to effectively suppress the propagation of discrete frequency noise, as described later. Discrete frequency noise is generated when the suction flow of the impeller 4 changes periodically, or when the impeller 4 sucks in large turbulence and the flow around the blades 4b changes periodically. For example, discrete frequency noise generated around the blades 4b of the impeller 4 of the centrifugal blower 1 is likely to occur when the secondary flow and the impeller 4 interfere with each other in a portion where the distance between the impeller 4 and the scroll casing 11 is small. For this reason, when the flow from the bell mouth 8a side toward the main plate 4a side of the impeller 4 becomes stronger, the pressure fluctuation on the back side of the main plate 4a of the impeller 4 increases, which may lead to an increase in the discrete frequency noise of the centrifugal blower 1. Due to such generation factors, the discrete frequency noise often becomes louder when it propagates from the back of the impeller 4 to the back of the scroll casing 11. For this reason, the discrete frequency noise can be effectively suppressed by providing the protrusion 5 especially near the back of the blades 4b.

[0045] Moreover, it is desirable that the outer diameter Da of the protrusion 5 is larger than the outer diameter Db of the ventilation hole 4e. Therefore, by arranging the protrusion 5 so as to satisfy the condition D2>Da>Db, it is possible to suppress the noise generated by the collision of the secondary flow Y4 with the protrusion 5, and also to suppress the generation of discrete frequency noise.

[0046] 2, it is desirable that the gap Hb between the protrusion 5 and the scroll casing 11 in the direction along the rotation axis 2 of the impeller 4 be as small as possible from the viewpoint of preventing leakage of the secondary flow Y4. However, if the gap Hb is too small, there is a risk that the protrusion 5 and the scroll casing 11 will collide with each other during rotation of the impeller 4. For this reason, it is necessary to strike a balance between preventing leakage of the secondary flow Y4 and avoiding collisions, and in practice, it is appropriate to provide the gap Hb of about 3 mm to 7 mm.

[0047] 8 is a cross-sectional view of a centrifugal blower according to a second comparative example of the first embodiment. In the centrifugal blower 1 according to the second comparative example, a protruding portion 20 is provided on the rear surface of the main plate 4a at the same position as the outer diameter D2 of the impeller 4. The protruding portion 20 is cylindrical like the protruding portion 5 of the centrifugal blower 1 according to the first embodiment. Therefore, in the centrifugal blower 1 according to the second comparative example, the outer diameter Da of the protruding portion 20 is the same as the outer diameter D2 of the impeller 4. The structures and shapes of the components other than the protruding portion 20 are the same as those of the centrifugal blower 1 according to the first embodiment.

[0048] The centrifugal blower 1 according to the third comparative example is provided with a protruding portion 20 that is 80% of the outer diameter D2 of the impeller 4 on the back surface of the main plate 4a. The structure and shape of the components other than the protruding portion 20 are the same as those of the centrifugal blower 1 according to the first embodiment. Therefore, in the centrifugal blower 1 according to the third comparative example, the outer diameter Da of the protruding portion 20 is equal to 0.8 × (outer diameter D2 of the impeller 4). Note that, except for the difference in the outer diameter of the protruding portion 20, the centrifugal blower 1 according to the third comparative example is the same as the centrifugal blower 1 according to the second comparative example, and therefore is not shown in the drawings.

[0049] Fig. 9 is a diagram showing the flow velocity distribution of a centrifugal blower according to a second comparative example of embodiment 1. Fig. 9 shows the flow velocity distribution in a cross section at a position corresponding to line II-II in Fig. 3 for centrifugal blower 1 according to the second comparative example. Fig. 9 shows the flow velocity distribution using a monotone image that becomes darker as the flow velocity increases.

[0050] 9, the region with a high flow velocity is a region through which the main flow Y3 flowing out from the impeller 4 passes. A secondary flow Y4 is formed that flows perpendicular to the main flow Y3. If the outer diameter Da of the protruding portion 20 is made the same as the outer diameter D2 of the impeller 4 as in the centrifugal blower 1 according to the second comparative example, the distance between the protruding portion 20 and the secondary flow Y4 will inevitably be short, and the protruding portion 20 and the secondary flow Y4 will be more likely to interfere with each other.

[0051] Fig. 10 is a diagram showing the distribution of the effective value of pressure fluctuation in a centrifugal blower according to a second comparative example of embodiment 1. Fig. 10 shows the distribution of the effective value of pressure fluctuation in a monotone image that becomes darker as the pressure fluctuation becomes larger.

[0052] As shown in Fig. 10, the pressure fluctuation on the surface of the protrusion 20 provided on the impeller 4 is approximately 30 Pa or more. The cause of the pressure fluctuation is considered to be that the secondary flow Y4 interferes with the protrusion 20, as shown in Fig. 9, and the airflow around the protrusion 20 is disturbed.

[0053] FIG. 11 is a diagram showing the distribution of pressure fluctuation levels at the bottom surface of the scroll section of the centrifugal blower according to the second comparative example of the first embodiment. Here, the pressure fluctuation level is the time fluctuation average of pressure expressed by the root mean square (RMS) of pressure. As shown in FIG. 11, the pressure fluctuation level near the tongue section 9 at the bottom surface of the scroll section 6 is 104 to 112. The reason why the pressure fluctuation level near the tongue section 9 becomes large is considered to be that in addition to the pressure fluctuation due to the secondary flow Y4, the fluctuation due to the circulating flow generated near the tongue section 9 is also added. Near the tongue section 9, not only the main flow Y3 toward the outlet 10 but also a circulating flow that tries to return to the upstream of the flow is constantly generated. Therefore, near the tongue section 9, the circulating flow and the secondary flow Y4 are mixed, and discrete frequency noise is particularly likely to occur.

[0054] FIG. 12 is a diagram showing a flow velocity distribution of the centrifugal blower according to the first embodiment. FIG. 12 shows the flow velocity distribution in the cross section of the position of the line II-II in FIG. 3 for the centrifugal blower 1 according to the first embodiment. FIG. 12 shows the flow velocity distribution in a monotone image that becomes darker as the flow velocity increases. FIG. 13 is a diagram showing the distribution of the effective value of the pressure fluctuation of the centrifugal blower according to the first embodiment. FIG. 13 shows the distribution of the effective value of the pressure fluctuation in a monotone image that becomes darker as the pressure fluctuation increases. In the centrifugal blower 1 according to the first embodiment, the outer diameter Da of the protruding portion 5 is not the same as the outer diameter D2 of the impeller 4, and the entire cylindrical wall of the protruding portion 5 is provided on the inner diameter side of the outer diameter D2 of the impeller 4.

[0055] As shown in FIG. 12, the centrifugal blower 1 according to the first embodiment also generates a secondary flow Y4, similar to the centrifugal blower 1 according to the second comparative example. However, in the centrifugal blower 1 according to the first embodiment, the protruding portion 5 is provided on the inner diameter side of the outer circumferential end of the impeller 4, so that the distance between the secondary flow Y4 and the protruding portion 5 can be secured, and interference between the secondary flow Y4 and the protruding portion 5 is suppressed. As a result of investigating to what extent the secondary flow Y4 is prominent, it was found to be most prominent between the inner diameter D1 and the outer diameter D2 of the impeller 4. Therefore, by providing the protruding portion 5 on the inside of the inner diameter D1 of the impeller 4, it is possible to suppress the collision of the secondary flow Y4 with the protruding portion 5 and suppress the generation of noise due to the secondary flow Y4.

[0056] Furthermore, since the main plate 4a of the impeller 4 has a flat surface from the outer periphery of the main plate 4a on which the blades 4b are arranged to the portion where the protrusion 5 is provided, it can be seen that the main flow Y3 flows out almost parallel to the main plate 4a of the impeller 4. Since the main flow Y3 flows out almost parallel to the main plate 4a, no sudden turning of the airflow occurs, and the air can reach the air outlet 10 with little loss. Furthermore, there is also an effect of effectively separating the secondary flow Y4 from the main flow Y3, and interference between the main flow Y3 and the secondary flow Y4 can also be effectively suppressed.

[0057] 13, in the centrifugal blower 1 according to the first embodiment, the magnitude of the pressure fluctuation at the protruding portion 5 is reduced by about 15 Pa to 30 Pa, compared to the centrifugal blower 1 according to the second comparative example. This is considered to be because the interference between the secondary flow Y4 and the protruding portion 5 is effectively suppressed, as shown in FIG.

[0058] Noise and pressure fluctuations are closely related, and points with large pressure fluctuations are noise sources. Therefore, it goes without saying that reducing pressure fluctuations on the cylindrical wall surface of the protruding portion 5 leads to noise reduction. In other words, as shown in Figure 12, installing the protruding portion 5 on the inner diameter side of the impeller 4 is effective in achieving noise reduction.

[0059] Fig. 14 is a diagram showing the distribution of pressure fluctuation levels at the bottom surface of the scroll section of the centrifugal blower of the first embodiment. As shown in Fig. 14, the centrifugal blower 1 of the first embodiment has a smaller area where the pressure fluctuation level is high, particularly near the tongue section 9, compared to the centrifugal blower 1 of the second comparative example. The reason for this is considered to be that, as described above, by setting the outer diameter Da of the protrusion 5 between the inner diameter D1 and the outer diameter D2 of the impeller 4, the mutual interference between the secondary flow Y4, the circulating flow generated near the tongue section 9, and the protrusion 5 is effectively suppressed, and by providing the protrusion 5 on the back surface of the impeller 4, the distance from the bottom surface of the scroll section 6 is increased, and the propagation of discrete frequency noise is also suppressed.

[0060] Fig. 15 is a diagram showing the results of an actual machine test of the air volume-specific noise characteristics of the centrifugal blower according to the first embodiment, the centrifugal blower according to the second comparative example, and the centrifugal blower according to the third comparative example. The horizontal axis indicates the air volume, and the vertical axis indicates the specific noise. In Fig. 15, the black diamonds indicate the plots of the centrifugal blower 1 according to the second comparative example, the black triangles indicate the plots of the centrifugal blower 1 according to the third comparative example, and the white circles indicate the plots of the centrifugal blower 1 according to the first embodiment.

[0061] 15, the noise value of the minimum specific noise is 13.1 dB for the centrifugal blower 1 according to the second comparative example, 13.0 dB for the centrifugal blower 1 according to the third comparative example, and 12.8 dB for the centrifugal blower 1 according to embodiment 1, confirming a noise reduction of 0.2 dB or more. The reason that such a reduction in noise was confirmed is believed to be that the protruding portion 5 is provided on the inner diameter side of the impeller 4, thereby suppressing interference between the protruding portion 5 and the secondary flow Y4 and effectively suppressing discrete frequency noise generated from the back surface of the impeller 4.

[0062] In this way, in the centrifugal blower 1 according to the first embodiment, the entire circumference of the protruding portion 5 is disposed offset radially inward from the multiple blades 4b of the impeller 4, so that it is possible to suppress interference between the protruding portion 5 and the secondary flow Y4 and effectively reduce pressure fluctuations on the surface of the protruding portion 5. This effect also makes it possible to suppress discrete frequency noise generated by the rotation of the impeller 4, thereby achieving low noise levels.

[0063] Furthermore, in a multi-blade impeller, if the number of blades is reduced, the momentum per gap between each blade increases, and there is a concern that the fluctuation component of the flow becomes larger, making discrete frequency noise more likely to occur at the rear surface of the impeller. Since the centrifugal blower 1 according to the first embodiment is expected to have the effect of suppressing the generation of discrete frequency noise, it is expected that the number of blades can be reduced by 10% or more compared to the impeller 4 of the centrifugal blower 1 according to the second comparative example in which the protrusion 20 is provided at the same position as the outer diameter of the impeller 4 on the rear surface of the main plate 4a, and it is possible to realize reductions in material costs and processing costs of the impeller 4.

[0064] Embodiment 2 FIG. 16 is a cross-sectional view of a centrifugal blower according to the second embodiment. In FIG. 16, the scroll casing 11 is omitted. In the second embodiment, the protruding portion 5 in the first embodiment is replaced with a protruding portion 30. The centrifugal blower 1 according to the second embodiment has the same configuration as the centrifugal blower 1 according to the first embodiment except for the protruding portion 30, so that the overlapping description will be omitted. The protruding portion 30 does not branch out from the main plate 4a, and is formed by bending the main plate 4a into a V-shaped cross section. The impeller 4 of the centrifugal blower 1 according to the second embodiment can be formed by resin molding. Unlike the centrifugal blower 1 according to the first embodiment, the protruding portion 30 does not branch out from the main plate 4a, so that the productivity by resin molding is high.

[0065] As with the centrifugal blower 1 of embodiment 1, when the outer diameter of the impeller 4 is defined as D2, the inner diameter of the impeller 4 as D1, the outer diameter of the motor 3 as Dc, and the outer diameter of the ventilation hole 4e as Db, the mounting position of the protrusion 30 is within the range that satisfies D2>Da>Dc and D1≧Da>Db.

[0066] In the centrifugal blower 1 according to the second embodiment, the protruding portion 30 has a V-shaped cross section, which allows the impeller 4 to be mass-produced by resin molding. Also, as described above, the protruding portion 5 is provided so as to satisfy the condition D1≧Da>Db, which not only provides the same effects as the centrifugal blower 1 according to the first embodiment, but also makes it possible to reduce the cost of materials used for the protruding portion 5.

[0067] Embodiment 3 17 is a perspective view of an impeller of a centrifugal blower according to embodiment 3. In embodiment 3, protruding portion 5 of embodiment 1 is replaced with protruding portion 40. The configuration of embodiment 3 other than protruding portion 40 is the same as embodiment 1, and therefore repeated explanations will be omitted.

[0068] The protrusion 40 has a rotationally symmetric shape, for example, an elliptical cylindrical shape. The elliptical cylindrical protrusion 40 has a central axis on the rotation shaft 2 of the impeller 4. The major axis of the elliptical cylinder of the protrusion 40 is equal to or smaller than the outer diameter D2 of the impeller 4, and the minor axis of the elliptical cylinder is equal to or larger than the outer diameter Db of the ventilation hole 4e. The major axis and the minor axis of the elliptical cylinder are measured, for example, by the outer diameter of the protrusion 40. In this way, the entire elliptical cylindrical protrusion 40 is disposed in the range from the outer diameter D2 of the impeller 4 to the outer diameter Db of the ventilation hole 4e. In the centrifugal blower 1 according to the third embodiment, since the protrusion 40 has an elliptical cylindrical shape, the major axis of the elliptical cylinder is the maximum width of the protrusion 40.

[0069] In the centrifugal blower 1 according to the third embodiment, the impeller 4 does not necessarily have to be provided with the ventilation holes 4e. When the impeller 4 is not provided with the ventilation holes 4e, the protrusion 40 is provided around the entire circumference without protruding into a region radially outward from the outer diameter of the impeller 4, and at least a portion of the entire circumference of the protrusion 40 is provided radially inward from the outer diameter of the impeller 4. In Fig. 17, the long diameter portion of the protrusion 40 is provided on the outer diameter portion of the impeller 4, and the portion other than the long diameter portion is provided radially inward from the outer diameter of the impeller 4.

[0070] In the centrifugal blower 1 according to the third embodiment, the protruding portion 40 has an elliptical cylindrical shape instead of a cylindrical shape. In the elliptical cylindrical protruding portion 40, the distance from the central axis to the wall surface varies depending on the location, so that sound waves existing inside the protruding portion 40 and sound waves propagating in the opposite direction with the same frequency are unlikely to overlap. Therefore, in the centrifugal blower 1 according to the third embodiment, standing waves are unlikely to occur, and resonance noise can be effectively suppressed.

[0071] In order to make it difficult for standing waves to occur, it is preferable that the difference between the major axis and the minor axis of the elliptical cylindrical projection 40 is large. For example, it is preferable that the difference between the major axis and the minor axis is half or more of the difference between the outer diameter D2 of the impeller 4 and the outer diameter Db of the ventilation hole 4e. In other words, if the major axis of the projection 40 is Dx and the minor axis is Dn, it is preferable that (Dx-Dn)>(D2-Db) / 2.

[0072] In this way, in the centrifugal blower 1 according to the third embodiment, since the protruding portion 40 is formed in an elliptical cylindrical shape, standing waves are less likely to occur, resonance sounds can be effectively suppressed, and noise can be further reduced. Note that the protruding portion 40 may have any shape other than an ellipse, so long as it is provided without protruding into the radially outer region from the outer diameter of the impeller 4 over its entire circumference, and at least a part of the entire circumference of the protruding portion 40 is provided radially inward from the outer diameter of the impeller 4.

[0073] The configurations shown in the above embodiments are merely examples of the contents, and may be combined with other known technologies. Parts of the configurations may be omitted or modified without departing from the spirit of the invention. [Explanation of symbols]

[0074] 1 Centrifugal blower, 2 Rotating shaft, 3 Motor, 3a Shaft, 4 Impeller, 4a Main plate, 4b Blade, 4c Boss, 4d Rib, 4e Ventilation hole, 4f, 4g, 4h Circles, 5, 20, 30, 40 Protrusion, 6 Scroll, 7 Diffuse, 8 Inlet, 8a Bell mouth, 8b Shielding, 8c Shielding end, 8f Surface, 8r Back, 9 Tongue, 10 Outlet, 11 Scroll casing, 11a First wall, 11b Second wall, 11c Third wall, 81a Shielding side, 81b Shielding bottom, 82 Bell mouth back space, Y1, Y2 Air flow, Y3 Main flow, Y4 Secondary flow.

Claims

1. A motor; a scroll casing having an intake port, an exhaust port, a bell mouth that guides an airflow toward the intake port, a scroll section whose radial width increases toward the downstream side of the airflow, a tongue section that connects the scroll section and the exhaust port so as to increase the opening area of ​​the exhaust port, and a diffuse section that forms a flow path between the scroll section and the exhaust port; an impeller having a disk-shaped main plate fixed to a shaft of the motor and rotating, and a plurality of blades arranged in an annular shape on an outer periphery of the main plate, the impeller being installed within the scroll casing and forming an airflow in the scroll casing that flows from the suction port to the air outlet, the bell mouth is provided with a shielding portion downstream of the tongue portion in the rotation direction of the impeller, the shielding portion shielding a secondary flow flowing out of the impeller and re-entering the impeller, the shielding portion is a groove-shaped recess in the axial direction of the impeller, penetrates the bell mouth in the radial direction of the impeller, and is not provided in a diffuse portion of the bell mouth.

2. The impeller has a cylindrical protrusion protruding from the rear surface of the main plate, 2. The centrifugal blower according to claim 1, wherein a maximum diameter of the protrusion is smaller than an outer diameter of the impeller.

3. 3. The centrifugal blower according to claim 2, wherein a plurality of ventilation holes are formed in the main plate, and the plurality of ventilation holes are arranged in a circumferential direction of the rotation shaft of the impeller.

4. 4. The centrifugal blower according to claim 3, wherein an outer diameter of a circle connecting the outer peripheral ends of the ventilation holes is smaller than a maximum diameter of the protrusion.

5. 3. The centrifugal blower according to claim 2, wherein the protrusion has a shape rotationally symmetrical with respect to the rotation axis of the impeller.

6. 3. The centrifugal blower according to claim 2, wherein the maximum diameter of the protrusion is larger than an outer diameter of the motor.

7. 3. The centrifugal blower according to claim 2, wherein at least a portion of the protruding portion is provided on an inner diameter side of the impeller relative to an outer diameter side.

8. A motor; a scroll casing having an intake port, an exhaust port, a bell mouth that guides an airflow toward the intake port, a scroll section whose radial width increases toward the downstream side of the airflow, a tongue section that connects the scroll section and the exhaust port so as to increase the opening area of ​​the exhaust port, and a diffuse section that forms a flow path between the scroll section and the exhaust port; an impeller having a disk-shaped main plate fixed to a shaft of the motor and rotating, and a plurality of blades arranged in an annular shape on an outer periphery of the main plate, the impeller being installed within the scroll casing and forming an airflow in the scroll casing that flows from the suction port to the air outlet, the bell mouth is provided with a shielding portion downstream of the tongue portion in the rotation direction of the impeller, the shielding portion shielding a secondary flow flowing out of the impeller and re-entering the impeller, The impeller has an elliptical cylindrical protrusion that protrudes from the rear surface of the main plate and has a central axis on the rotation axis of the impeller, The maximum diameter of the protrusion is smaller than the outer diameter of the impeller, A centrifugal blower, characterized in that at least a portion of the protrusion is provided on an inner diameter side of the impeller relative to an outer diameter side.

9. 9. The centrifugal blower according to claim 2, wherein a region of the main plate from an outer periphery to the protruding portion in a radial direction is a flat surface.

Images