Centrifugal fan

A centrifugal fan includes a scroll casing having first and second flat base walls, and a circumferential side wall. An air inlet is formed on a center portion of the first base wall and an exhaust port is formed on the circumferential side wall. An airflow correction mechanism that forms smooth airflow when an impeller rotates is provided. The mechanism has an annular rib that is formed on the inside surface of the first base wall so as to be jutted to the side of the second base wall and to be concentric with the rotating shaft, and a recess portion that is formed on every blade of the impeller so that the annular rib is inserted therein with a predetermined gap. The annular rib and the recess portion are configured to change the direction of airflow directed to the air inlet back to a space between the blades.

BACKGROUND OF THE INVENTION

The present invention relates to a centrifugal fan that collects airflow taken in from an air inlet formed at the center of one flat base wall of a scroll casing and discharges the airflow from an exhaust port formed on a circumferential side wall in a centrifugal direction. More particularly, the present invention relates to a mechanism to prevent backflow during fan operating.

Centrifugal fans, which use DC brushless motors especially, are widely used to cool electronic components of OA (office automatic) equipment such as a personal computer, a copying machine, a liquid crystal projector and a disk array because they can not only make the motors compact and light in weight but also control air quantity easily due to easy control of the motor.

A prior art of such a centrifugal fan will be described with reference toFIG. 6throughFIG. 8.FIG. 6is a sectional view in a plane parallel to a rotating shaft showing a construction of a conventional centrifugal fan,FIG. 7is a front view of the centrifugal fan shown inFIG. 6viewed from an air inlet, andFIG. 8is an enlarged sectional view of the upper half of the centrifugal fan shown inFIG. 6.

The illustrated centrifugal fan1has a scroll casing10, an impeller20that is rotatably mounted in the casing10, and a motor30that rotates the impeller20. The casing10is provided with first and second flat base walls that are parallel to each other and a circumferential side wall that covers the circumferences of these base walls. The casing10is constructed by combining a first casing10athat constitutes the first base wall and a second casing10bthat constitutes the second base wall and the circumferential side wall.

An air inlet11that opens in the axial direction is formed at the center portion of the first casing10aand an exhaust port12(seeFIG. 7) that opens in the circumferential direction is formed on one portion of the circumferential side wall. As shown inFIG. 6, the inner circumference of the air inlet11is inside to form a bell mouth13.

A cylindrical bearing box15is formed on the second casing10b. The bearing box15supports the rotating shaft31via bearings14in its inside. A stator32of the motor30is fixed to the outside of the bearing box15.

The motor30is an outer-rotor type DC brushless motor that consists of a stator32having a stator core32aand coils32bwound in slots of the stator core32a, and a rotor33having a cup-shaped hub33afixed on the tip of the rotating shaft31and a permanent magnet33battached to the inner circumferential surface of the hub33a.

The impeller20is fitted to the outer circumference of the hub33aof the rotor33. A great number of blades21are arranged around the outer circumference of the impeller20. During fan operating, the impeller20rotates in a predetermined direction, which discharges the air taken in from the air inlet11to the periphery of the impeller20as regular airflow A by the centrifugal force as shown inFIG. 6. The air is collected by the inner circumferential surface of the casing10, and is discharged from the exhaust port12.

In the meantime, when the above-described centrifugal fan1operates with low air quantity, backflow B that flows in a space between the impeller20and the inner surface of the first casing10aand is discharged from the air inlet11and recycling flow C that returns back to the impeller20are generated. The backflow B and the recycling flow C are generated because the regular flow A in radial direction tends to be concentrated to the side of the hub33aduring low air quantity operation. Particularly, the recycling flow C results from a velocity difference of airflow passing through a space between the impeller20and the inner surface of the first casing10a. That is, the airflow at the side of the impeller20is slower than that at the side of the first casing10aas shown inFIG. 8. Such backflow B and recycling flow C deteriorate the blowing performance of the centrifugal fan1and increase the noise.

Publications of Japanese unexamined patent applications No. Hei10-141291 and No. Hei10-054388 disclose techniques to prevent the deterioration of the blowing performance and the generation of the noise that are caused by the backflow B and the recycling flow C described above.

Namely, the publication of Japanese unexamined patent application No. Hei10-141291 discloses a centrifugal fan in which a screen-like guide plate is mounted on an outer portion of a casing at a periphery of an air inlet in order to return airflow discharged from the air inlet back to the air inlet. Further, an annular jutted portion is formed at a tip of an impeller so as to be inserted into a recess portion of a bell mouth formed having a U-shaped section.

However, since the guide plate is mounted on the outside of the casing in the construction of the publication, the size of the centrifugal fan in the axial direction (the axial size) becomes larger. Further, since the jutted portion is formed on the impeller, the inertial mass of the impeller becomes larger, which increases load on the motor.

Further, the publication of Japanese unexamined patent application No. Hei10-054388 discloses a centrifugal fan having labyrinth seal, which consists of a cylindrical shield plate mounted on an outer tip of an impeller and a cylindrical rib formed on a housing side, in order to prevent the backflow.

However, since the fan of the publication is constructed to reduce the backflow by seal effect, a high manufacturing accuracy is required to satisfy the seal effect, which increases a manufacturing cost. Further, since the shield plate is formed on the impeller, both the axial size of the impeller and the inertial mass of the impeller become larger, which increases the axial size of the centrifugal fan and load on the motor.

SUMMARY OF THE INVENTION

The purpose of the present invention is to solve the above-mentioned problems by providing an improved centrifugal fan, which is capable of preventing deterioration of the blowing performance and generation of the noise that are caused by the backflow and the recycling flow during low air quantity operation without increasing the size in the axial direction, the size of the impeller and the inertial mass.

In order to accomplish the above-mentioned first purpose, a centrifugal fan according to the present invention includes:

a scroll casing that has first and second flat base walls, a circumferential side wall covering the circumferences of the base walls, an air inlet that is opened in an axial direction being formed on a center portion of the first base wall and an exhaust port that is opened in a circumferential direction being formed on one portion of the circumferential side wall;

a motor that is attached to a center portion of the second base wall at the inside of the casing so that a rotating shaft of the motor is perpendicular to the second base wall;

an impeller that is fixed to the rotating shaft, the impeller having many blades along the outer region thereof; and

an airflow correction mechanism that forms smooth airflow when the impeller rotates,

wherein the airflow correction mechanism has an annular rib that is formed on the inside surface of the first base wall so as to be jutted to the side of the second base wall and to be concentric with the rotating shaft, and a recess portion that is formed on every blade of the impeller so that the annular rib is inserted therein with a predetermined gap, the annular rib and the recess portion being configured to change the direction of airflow directed to the air inlet back to a space between the blades.

With this construction, since the insertion of the annular rib into the recess portion forms a wall in an airflow path directed to the air inlet, the airflow directed to the air inlet returns back to the space between the blades, which can prevent generation of backflow and recycling flow. This prevents deterioration of the blowing performance and generation of the noise during low air quantity operation.

Since the structure of the present invention returns airflow back to the space between the blades without using the seal effect used in the prior art, it does not require high manufacturing accuracy, which can reduce the manufacturing cost. Further, since the annular rib is formed on the housing, it does not increase the axial size of the impeller and the inertial mass thereof. Therefore, the annular rib has little effect on the axial size of the centrifugal fan and the load on the motor.

In addition, it is preferable that an outer bottom portion of the annular rib is formed to have a circular curve section so that the outer circumferential surface of the annular rib is smoothly connected to the inside surface of the first base wall. With this construction, the airflow passing through the space between the impeller and the casing is effectively redirected so as to merge with the regular flow that is taken in from the air inlet and flows in the radial direction.

Further, an inner tip portion of the annular rib is preferably formed to have a circular curve section. Although the regular airflow that is taken in from the air inlet and flows in the radial direction tends to be concentrated to the side of the second base wall during low air quantity operation, it flows not only at the side of the second base wall but also at the side of the first base wall at which the airflow correction mechanism is formed during high air quantity operation. If the inner tip of the annular rib has a rectangular section shape, the regular airflow would be interrupted. On the other hand, when the inner tip portion of the annular rib is formed to have a circular curve section as mentioned above, the regular airflow along this portion is not interrupted during high air quantity operation.

Still further, the depth of the recess portion is preferably larger than a gap formed between the inside surface of the first base wall and the impeller that are faced with each other. With this construction, enough airflow resistance can be obtained by the airflow correction mechanism that consists of the annular rib and the recess portion.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, an embodiment of a centrifugal fan according to the present invention will be described with reference to the drawings.

FIG. 1is a sectional view of the centrifugal fan2of the embodiment in a plane parallel to a rotating shaft,FIG. 2is a front view of the centrifugal fan2shown inFIG. 1when a first casing is removed,FIG. 3Ais a sectional view of the first casing of the centrifugal fan2shown inFIG. 1,FIG. 3Bis a sectional view of the first casing shown inFIG. 3Aalong a IIIB-IIIB line viewed from inside, andFIG. 4is an enlarged sectional view of the upper half of the centrifugal fan2shown inFIG. 1. Since the outward appearance and the generic construction at the inside of the centrifugal fan2of the embodiment are identical to that of the prior art, the same parts are described with the same reference numbers.

The centrifugal fan2of the embodiment is provided with a scroll casing10that has first and second flat base walls101and102, a circumferential side wall103covering the circumferences of the base walls101and102. The resin-made casing10consists of a first casing10aand a second casing10b. The first casing10aconstitutes the first base wall101, and the second casing10bconstitutes the second base wall102and the circumferential side wall103. An air inlet11that is opened in an axial direction is formed on a center portion of the first casing10a, and an exhaust port12that is opened in a circumferential direction is formed on one position of the circumferential side wall103(seeFIG. 2).

Inside the casing10, an impeller20having many blades21along the outer region thereof is rotatably mounted. The inner circumferential surface of the casing10is formed like a scroll and the width of an airflow path, which is formed between the inner circumferential surface of the casing10and the outer circumference of the impeller20, in the radial direction gradually increases from a nose12a(seeFIG. 2) of the exhaust port12as a starting point in the rotating direction of the impeller20(the clockwise direction inFIG. 2).

A motor30that drives to rotate the impeller20is fixed to a bearing box15that is formed on the center portion of the second base wall102of the second casing10b. A rotating shaft31of the motor30is perpendicular to the base walls101and102.

The rotating shaft31of the motor30is rotatably supported by bearings14arranged inside the bearing box15. The motor30is an outer-rotor type DC brushless motor that consists of a stator32having a stator core32aand coils32bwound in slots of the stator core32a, and a rotor33having a cup-shaped hub33afixed on the tip of the rotating shaft31and a permanent magnet33battached to the inner circumferential surface of the hub33a. The stator32is fixed to the outer circumference of the bearing box15. Further, the impeller20is fitted to the outer circumference of the hub33aof the rotor33.

A bell mouth13is formed along the inner circumference of the air inlet11. The bell mouth13is formed by bending a tip whose thickness is the same as the other portion of the casing10inside.

During operation, the impeller20rotates in the clockwise direction inFIG. 2. As a result, the major portion of air taken in from the air inlet11is discharged to the periphery of the impeller20as regular airflow A by the centrifugal force as shown inFIG. 1andFIG. 4. The air is collected by the inner circumferential surface of the casing10, and is discharged from the exhaust port12.

The centrifugal fan2of the embodiment is provided with an airflow correction mechanism40that forms smooth airflow when the impeller20rotates. The airflow correction mechanism40has an annular rib41that is formed on the inside surface of the first casing10aso as to be jutted to the side of the second base wall102of the second casing10band to be concentric with the rotating shaft31, and a recess portion42that is formed on every blade21of the impeller20so that the annular rib41is inserted therein with a predetermined gap. The annular rib41and the recess portion42are configured to change the direction of airflow from the scroll space outside the impeller20to the air inlet11back to a space between the blades21during operation.

That is, since the insertion of the annular rib41into the recess portion42forms a wall in the path directed to the air inlet11and produces airflow resistance, the airflow directed to the air inlet11returns back to the space between the blades21as shown by an arrow D inFIG. 4, which prevents generation of backflow and recycling flow. In addition, since the annular rib41is inserted into the recess portion42while keeping a noncontact condition, it does not disturb the rotation of the impeller20.

Further, an outer bottom portion41aof the annular rib41is formed to have a circular curve section so that the outer circumferential surface of the annular rib41is smoothly connected to the inside surface of the first casing10a(the first base wall101). As a result, the airflow passing through the space between the impeller20and the casing10is effectively redirected so as to merge with the regular flow A that is taken in from the air inlet11and flows in the radial direction.

Still further, inner tip portion41bof the annular rib41is formed to have a circular curve section. This does not interrupt the regular airflow A along this portion during high air quantity operation.

As shown inFIG. 4, a depth d1of the recess portion42formed on the blades21is larger than a gap d2formed between the inside surface of the first casing10aand the impeller20faced with each other. As a result of examinations to keep enough airflow resistance to prevent backflow by the airflow correction mechanism40, it has been confirmed that enough airflow resistance can be obtained when the condition d1>d2is satisfied.

FIG. 5is a graph showing the performance of the embodiment in comparison with that of the prior art shown inFIG. 6throughFIG. 8. In the graph, solid lines represent the embodiment and dotted lines represent the prior art. The horizontal axis of the graph shows air quantity, the left vertical axis shows static pressure, and the right vertical axis shows noise. The upper two lines represent relationship between air quantity and noise, and the lower two lines represent relationship between air quantity and static pressure. This graph shows that there are almost no difference in the static pressure (blowing performance) between the embodiment and the prior art and that the noise of the embodiment drops 3 dB at the maximum as compared with the prior art.

As described above, the construction of the embodiment can prevent from generating the backflow directed to the air inlet and the recycling flow even when the fan operates with low air quantity, which can prevent reduction of the blowing performance and generation of noise.