WO2022263629A1

WO2022263629A1 - A fan unit

Info

Publication number: WO2022263629A1
Application number: PCT/EP2022/066559
Authority: WO
Inventors: Damien BURATO; Ganna MOSTIPANENKO
Original assignee: Multi-Wing International A/S
Priority date: 2021-06-17
Filing date: 2022-06-17
Publication date: 2022-12-22
Also published as: KR20240000317U; JP3247021U; EP4356008A1

Abstract

The present disclosure provides a fan unit comprising a circumferential edge forming an aperture and an impeller arranged to establish a flow through the aperture. The impeller comprises a hub which is adapted for rotation about a rotation axis, and which carries a plurality of blades. Each blade extends in a radial direction between the hub and a ring element, where the ring element comprises an outer surface facing an inner surface of the circumferential edge. The circumferential edge comprises a plurality of diffusion elements arranged in the aperture off-set axially relative to the impeller, where the diffusion elements extend from a centre part non-rotational arranged about the rotation axis to the inner surface of the circumferential edge. One of the outer surface and the inner surface comprises a first wall portion projecting upwards from the corresponding one of the inner and outer surfaces, and terminates in a first free end. The first wall portion extends circumferentially around the ring element. The other one of the outer surface and the inner surface forms a first groove circumferentially along the corresponding one of the inner and outer surfaces, and the first wall portion and the first groove face each other.

Description

A FAN UNIT

Field of the disclosure

The present disclosure relates to a fan unit which comprises a circumferential edge forming an aperture and an impeller arranged at least partly in the aperture. The impeller comprises a hub being adapted for rotation about a rotation axis and carrying a plurality of blades, where each blade extends in a radial direction between the hub and a ring element.

Background of the disclosure

Traditionally, an impeller and a circumferential edge are mounted with a clearance between an outer surface of the impeller and the circumferential edge. The clearance is necessary due to vibrations and tolerances. However, this clearance distance increases aerodynamic losses, increases noise, and reduces performance.

Description of the disclosure

It is an object of embodiments of the disclosure to provide an improved fan unit.

It is a further object of embodiments of the disclosure to provide a fan unit which reduces aerodynamic losses and noise.

The disclosure provides in a first aspect a fan unit comprising a circumferential edge forming an aperture and an impeller arranged to establish a flow through the aperture, the impeller comprising a hub being adapted for rotation about a rotation axis and carrying a plurality of blades, each blade extending in a radial direction between the hub and a ring element, wherein the ring element comprises an outer surface facing an inner surface of the circumferential edge, and the circumferential edge comprising a plurality of diffusion elements arranged in the aperture off-set axially relative to the impeller, the diffusion elements extending from a centre part non-rotational arranged about the rotation axis to the inner surface of the circumferential edge, wherein one of the outer surface and the inner surface comprises a first wall portion projecting upwards from the corresponding one of the inner and outer surfaces, and terminating in a first free end, the first wall portion extending circumferentially around the ring element, and wherein the other one of the outer surface and the inner surface forms a first groove circumferentially along the corresponding one of the inner and outer surfaces, the first wall portion and the first groove facing each other. Thus, in a first embodiment the disclosure provides a fan unit comprising a circumferential edge forming an aperture and an impeller arranged to establish a flow through the aperture, the impeller comprising a hub being adapted for rotation about a rotation axis and carrying a plurality of blades, each blade extending in a radial direction between the hub and a ring element, wherein the ring element comprises an outer surface facing an inner surface of the circumferential edge, and the circumferential edge comprising a plurality of diffusion elements arranged in the aperture off-set axially relative to the impeller, the diffusion elements extending from a centre part non-rotational arranged about the rotation axis to the inner surface of the circumferential edge, wherein the outer surface comprises a first wall portion projecting upwards from the outer surface, and terminating in a first free end, the first wall portion extending circumferentially around the ring element, and wherein the inner surface forms a first groove circumferentially along the inner surface, the first wall portion and the first groove facing each other.

And in a second embodiment, the disclosure provides a fan unit comprising a circumferential edge forming an aperture and an impeller arranged to establish a flow through the aperture, the impeller comprising a hub being adapted for rotation about a rotation axis and carrying a plurality of blades, each blade extending in a radial direction between the hub and a ring element, wherein the ring element comprises an outer surface facing an inner surface of the circumferential edge, and the circumferential edge comprising a plurality of diffusion elements arranged in the aperture off-set axially relative to the impeller, the diffusion elements extending from a centre part non-rotational arranged about the rotation axis to the inner surface of the circumferential edge, wherein the inner surface comprises a first wall portion projecting upwards from the inner surface, and terminating in a first free end, the first wall portion extending circumferentially around the ring element, and wherein the outer surface forms a first groove circumferentially along the outer surface, the first wall portion and the first groove facing each other.

In the context of the present disclosure, the term 'impeller' should be understood as, an element which is adapted to move a fluid, typically a gas, such as air, in a direction substantially parallel with the rotation axis of the impeller. The direction of the air flow may comprise a radial component, so that the impeller may provide an air flow with an axial and a radial component. Herein 'air' should be understood as covering any fluid, so that 'air flow' in the context of the present disclosure covers a fluid flow, such as a flow of gas.

The impeller may be used for cooling of an engine. The engine may e.g., form part of a tractor, or a combine harvester, or any similar kind of agricultural machinery, or an excavator, a bulldozer, a crane, or any similar kind of construction equipment for on-highway or off-highway usage. Additionally, the impeller may be used for ventilation, e.g., in relation to maritime, windmill, livestock, and building ventilation.

The hub is adapted for rotation about a rotation axis and will typically be connected to a shaft which is rotated by a motor, such as an electrically driven motor. The rotation causes rotation of the hub and thereby movement of air.

The hub carries a plurality of blades. The blades may be formed in one part with the hub, or each blade may be formed as separate components which are attached to a hub. In the latter case, the blades may be detachably attached to the hub.

Each blade extends in a radial direction between the hub and a ring element, which ring element is arranged circumferentially around the hub, whereby each blade extends from an outer surface of the hub to an inner surface of the ring element.

In one embodiment, the hub and the blades may be designed such that the pitch of the blades may be set in connection with the attachment of the blade to the hub. In this embodiment, the pitch of the blades may be changed by disconnecting the blades from the hub or by loosening the joint between the hub and the blades, and subsequently attaching or fastening the blades to the hub at a new pitch angle.

In an alternative embodiment, in which the blades and the hub are formed as a single element, the pitch of the blades is fixed.

The impeller comprises a plurality of blades such as three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen etc. The blades will typically be equidistantly spaced circumferentially along an outer surface of the hub. The blades may, however, also be arranged asymmetrically along an outer surface of the hub. The length of the blades may be in the range 50-1200 millimetres, such as 100 millimetres, such as 250 millimetres, such as 500 millimetres, such as 750 millimetres, such as 1000 millimetres, such as 1200 millimetres. The size of the impeller along the axial direction may be in the range of 10-1000 millimetres.

The length of the blade should be understood as the distance from the tip end fixed to the ring element end to the root end where the blade is fixed to the hub; i.e. the distance between the inner surface of the ring element and the outer surface of the hub. The circumferential edge forming an aperture may as an example be a non-rotating channel. The direction of the air flow may dependent on the size the channel along the rotation axis relative to the diameter of the impeller.

It should be understood that the circumferential edge may alternatively be an inlet, such as bell-mouth, such as orifice plate, such as a metal plate, in which the aperture is e.g., cut or stamped.

The impeller is arranged to establish a flow through the aperture and is arranged at least partly in the aperture. In embodiments, where the circumferential edge is formed by a channel, a majority of the impeller may be arranged in the aperture.

As the impeller is arranged at least partly in the aperture, at least a part of the outer surface of the ring element faces the inner surface of the circumferential edge. To avoid collision between the outer surface of the ring element and the inner surface of the circumferential edge, the outer diameter of the impeller including the ring element may be smaller than the inner diameter of the circumferential edge. The distance between the outer surface of the ring element and the inner surface of the circumferential edge may provide an air passage from one side of the fan unit to the other side hereof. It may be an advantage to provide the fan unit with an air passage as small as possible, while still avoiding collision, as the performance of the fan unit is decreased with an increased air flow through this air passage.

To reduce the velocity of the air flow leaving the impeller, the circumferential edge comprises a plurality of diffusion elements arranged in the aperture off-set axially relative to the impeller, where the diffusion elements extend from a centre part non-rotationally arranged about the rotation axis to the inner surface of the circumferential edge. The diffusion element may further convert kinematic energy of the moving air into potential energy of the pressure. The number of diffusion element may be equal to the number of blades. Alternatively, the number of diffusion element may be different from the number of blades. Typically, the number of diffusion elements are higher than the number of blades. As an example, the number of diffusion element depend on required sound level, required level of efficiency of the diffusion elements, etc.

In one embodiment, the ring element comprises a first wall portion projecting upwards from the outer surface; i.e. a first outwardly extending wall portion on an outer surface facing an inner surface of the circumferential edge. The first outwardly extending wall portion terminates in a first free end and is arranged circumferentially along the ring element. Thus, the first outwardly extending wall portion may form first ring-shaped element along the outer surface of the ring element. In this embodiment, the circumferential edge forms a first groove circumferentially along the inner surface facing the impeller. By providing the ring element with a first outwardly extending wall portion and the circumferential edge with a first groove, an air flow in the air passage between the impeller and the circumferential edge is partly trapped, and the air flow deviates from a flow along a straight line. This increases the pressure resistance in the air passage for the flow by increasing turbulences and consequently increases the overall airflow performance and sound performance. Further, the reduced flow reduces noise. A further advantage of the outwardly extending wall portion is that it may increase the stiffness of the ring element and thereby the overall rigidity of the impeller which may further increase overall mechanical and durability performances.

In an alternative embodiment, the first wall portion projects upwards from the inner surface of the circumferential edge, whereas the outer surface of the ring element forms a first groove circumferentially along the outer surface.

The first wall portion and the first groove may be arranged so that they face each other with an overlap, as a projection of the first outwardly extending wall portion on a rotation plane along the rotation axis at least partly overlap a projection of the first groove on the rotation plane.

In one embodiment, the projection of the first groove may be larger than the projection of the first wall portion, and the projection of the first wall portion may be within the projection of the first groove.

By providing the first groove wider than the wall portion; i.e. the projection of the first groove is larger than the projection of the first outwardly extending wall portion, the risk of collision between the first groove and the first outwardly extending can be considerably reduced or even avoided despite vibrations and/or deformations on impeller and vanes. If the impeller hits the circumferential edge, it may be damaged due to high-speed rotations.

The following describes an embodiment in which the first wall portion is provided on the outer surface of the ring element, and in which the first groove is provided on the inner surface of the circumferential edge. It should be understood, that the disclosure is equally applicable in relation to embodiments where the first wall portion is provided on the inner surface of the circumferential edge and the first groove is provided on the outer surface of the ring element.

The ring element may on the outer surface comprise a second wall portion circumferentially along the ring element, and the circumferential edge may form a second groove circumferentially along the inner surface facing the impeller. A second wall portion and a second groove may deviate an air flow in the air passage between the impeller and the circumferential edge further from a straight air flow due to the further obstructions and due to the air flow being trapped which may increases flow resistance in this passage further and may increase the overall system performance; i.e. airflow, sound, and mechanical performance, even further.

Additionally, the stiffness of the ring element may be further increased whereby the overall rigidity of the impeller may be increased which may further increase mechanical and durability performances even further.

The second wall portions may be arranged substantially parallel to the first wall portion, thereby providing two substantially parallel ring-shaped element along the outer surface of the ring element.

The size and shape of the first and second wall portions projecting upwards form the surface in question may in one embodiment be substantially identical. Thus, a cross-sectional shape of the first wall portion and a cross-sectional shape the section wall portion in a radial cross- section may be substantial identical.

It should however be understood, that the size and/or the shape of the first and second wall portions may be different.

In a further alternative embodiment, the ring element may on the outer surface comprise more than two wall portions circumferentially along the ring element to thereby deviate and trap the air flow path even further from a straight air flow path.

The first wall portion may have a height being a distance from the outer surface of the ring element to the first free edge of the first wall portion, and a width being transverse to the height, where the width may be substantially uniform along the height to thereby provide first wall portion have a substantially square-shape cross-section.

The width of the first wall portion may be in the range of 50-150% of the height, such as in the range of 75-125%.

The height of the first wall portion may be in the range of 0.1-3.0% of a diameter of the impeller, where the diameter is the radial dimension of the impeller which is measured from an outer surface of the ring element. In an alternative embodiment, the cross-sectional shape of the first wall portion may be triangular, or of another shape. It should be understood, that at least one of the outer surfaces of the first and/or second and/or additional wall portion may form a surface being at least partly arch-shaped.

The first groove may have a width being a dimension of the groove along the rotation axis, wherein the width of the first groove may be larger than the width of the first wall portion. In one embodiment, the first groove may form a substantially square-shaped cross-section, whereby the width may be uniform towards the bottom of the first groove. In an alternative embodiment, the width may decrease towards the bottom of the first groove thereby providing a groove forming the shape of a trapezium. The first and/or second groove may alternative have other cross-sectional shapes. It should further be understood, that the cross-sectional shape of the first groove may be different from the cross-sectional shape of the second groove.

To limit the amount of air passing the fan unit via the air passage between the circumferential edge and the ring element, a clearance ratio being a distance between the outer surface of the ring element and the inner surface of the circumferential edge may be below 3.0% of a diameter of the impeller, such as below 2.5%, such as below 2.0%, such as below 1.5%, such as below 1.0%, where the diameter is measured from an outer surface of the ring element.

The fan unit may further comprise brushes extending from the outer surface of the ring element, such as from the wall portion, towards the circumferential edge, whereby the brushes may reduce leakage via the air passage between ring element and the circumferential edge, or even seal this air passage. As an example, the brushes may block 30-100 percent, such as 50-95 percent of the air passage, whereby aerodynamic losses can be minimized, and a higher performance may be ensured. As an alternative, brushes may extend from the inner surface of the circumferential edge, such as from the groove, toward the outer surface of the ring element.

It should be understood, that the term "brushes extending form the ring element towards the circumferential edge" not only covers brushes extending radially at an angle of 0 degrees where the brushes point directly towards the circumferential edge, but also covers brushes extending at an angle different from zero. As an example, the brushes may extend at an angle in the range of +/- 30 degrees, such as in the range of +/- 20 degrees, such as +/- 15 degrees. The brushes may be formed of bundles of hair which may be arranged in a row. The hair may include fibres of a plastic material, such as different polyamides, e.g., PA6, Nylon, or Kevlar, or from polyester. As an alternative, the hair may include fibres of carbon or various natural fibres, e.g., fibres from plants.

The brushes may at a first end be attached to the wall portion at a point of attachment at a distance from the free end, whereby an overlap may be created between the wall portion and the brushes along a part of the length of the brushes. As the brushes are not attached to the wall portion in this overlap, the brushes are movable in this overlap and may be bended in case of impact between the brushes and the circumferential edge or alternatively impact between the brushes and the ring element.

Alternatively, or additionally, the ring element may in a radial cross-section form an archshaped edge portion terminating in a free edge extending along a periphery of the circumferential edge. The arch-shaped edge portion may limit the amount of air passing the fan unit in the air passage between the circumferential edge and the ring element, as the arch-shaped edge portion may partly obstruct the entrance into the area between the circumferential edge and the ring element. Additionally, the arch-shaped edge portion may facilitate an air flow into the impeller as the arch-shaped edge portion may substantially form a funnel.

The periphery of the circumferential edge may form an arch-shaped edge portion matching the arch-shaped portion of the ring element, thereby minimising the distance between the circumferential edge and the ring element along the periphery of circumferential edge.

To improve sound performance of the fan unit, the number of diffusion element may in one embodiment be at least twice the number of blades. This may additionally improve the efficiency of the fan unit.

To increase the efficiency of the fan unit, a pitch angle of each of the plurality of blades may in one embodiment be in the range of 1-89 degrees, such as in the range of 7-70 degrees, such as in the range of 15-50 degrees.

The fan unit may further comprise a motor being arranged about the rotation axis. The centre part may in this embodiment be arranged circumferentially around the motor.

In one embodiment, the plurality of blades and the ring element may be formed in one piece using polymer materials, such as thermoplastic and thermoset polymer materials. As an example, the material may be one of the following: PA, PP, PAG, PPG, PAC, etc. It should, however, be understood, that other materials may also be applicable. In the embodiment, where the first wall portion it arranged on the outer surface of the ring element, the plurality of blades, the ring element, and the first wall portion may be formed in one piece using polymer materials.

The ring element, and the first outwardly extending wall portion may be formed in one piece by moulding, by a 3D printing process, or by CNC machining process.

The fan unit may further comprise a rim portion releasably attached to the outer surface of the circumferential edge, where the rim portion and the outer surface comprising matching engagement elements. In one embodiment, the matching engagement element may be configured for attachment of the rim portion from both the pressure side of the impeller and the suction side of the impeller to allow for installation of the fan unit in a configuration for pulling air and in a configuration for pushing air.

In an additional aspect of the disclosure, use of the fan unit according to any of the first aspect may be provided, wherein the impeller is rotated with a rotational speed in the range of -6000 to 6000 rotations per minute, where the negative value represents backflow of the impeller. Such a backflow may be used to clean the fan unit.

It should be understood, that a skilled person would readily recognise that any feature described in combination with the first aspect of the disclosure could also be combined with the second aspect of the disclosure, and vice versa. The remarks set forth above in relation to the fan unit are therefore equally applicable in relation to the use hereof.

Brief description of the drawings

Embodiments of the disclosure will now be further described with reference to the drawings, in which:

Fig. 1 illustrates a section through a fan unit;

Fig. 2 illustrates an exploded view of the fan unit of Fig. 1;

Fig. 3 schematically illustrates an air volume being moved by the fan unit;

Fig. 4 illustrates two different views of the fan unit with the impeller in front; Fig. 5 illustrates two different views of the fan unit with the diffusion elements in front;

Fig. 6 illustrates an enlarged view of a first and a second outwardly extending wall portion; and

Figs. 7A-7C illustrate different embodiments of outwardly extending wall portions.

Detailed description of the drawings

It should be understood that the detailed description and specific examples, while indicating embodiments of the disclosure, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure will become apparent to those skilled in the art from this detailed description.

Fig. 1 illustrates a section through an embodiment of a fan unit 1. The fan unit 1 comprises a circumferential edge 2 forming an aperture 3 and an impeller 4 arranged at least partly in the aperture 3.

The impeller 4 comprises a hub 5 being adapted for rotation about a rotation axis (not shown) extending through the centre of the fan unit 1. The hub 5 carries a plurality of blades 6, in the illustrated embodiment a total of five blades 6 (see Fig. 4).

Each blade 6 extends in a radial direction between the hub 5 and a ring element 7. The outer surface of the ring element 7 faces the inner surface of the circumferential edge 2. On the outer surface of the ring element 7, the ring element comprises a first outwardly extending wall portion 8 terminating in a first free end 9 (see Fig. 6). The first outwardly extending wall portion 8 is arranged circumferentially along the ring element 7. In the illustrated embodiment, the ring element 7 additionally comprises a second outwardly extending wall portion 10. The second outwardly extending wall portion 10 is likewise arranged circumferentially along the ring element 7.

The circumferential edge 2 forms a first groove 11 circumferentially along the inner surface facing the impeller. Additionally, a second groove 12 is formed along the inner surface parallel to the first groove 11.

In the illustrated embodiment, the wall portions 8, 10 are arranged on the outer surface of the ring element 7, whereas the grooves 11, 12 are formed on the inner surface of the circumferential edge 2. It should be understood, that in alternative embodiments (not shown), the wall portions 8, 10 are arranged on the inner surface of the circumferential edge 2, whereas the grooves 11, 12 are formed on the outer surface of the ring element 7.

A motor 13 is mounted centrally in the fan unit 1 along the rotation axis. The motor 13 is attached to the impeller at the hub 2 and configured for rotation of the impeller 4 with the plurality of blades 6.

To reduce the velocity of the air flow leaving the impeller 4, the circumferential edge 2 comprises a plurality of diffusion elements 14 arranged in the aperture 3 off-set axially relative to the impeller 4. In the illustrated embodiment, the circumferential edge 2 comprises a total of fourteen diffusion element 14 (see Fig. 5). The diffusion elements 14 extend from a centre part to the inner surface of the circumferential edge 2.

Fig. 2 illustrates an exploded view of the fan unit 1 illustrated in Fig. 1. The fan unit 1 comprises a circumferential edge 2 forming an aperture 3 and an impeller 4. The impeller 4 comprises a hub 5 and carries a plurality of blades 6. Additionally, the illustrated embodiment of a fan unit comprises a circumferential edge 2 comprising a plurality of diffusion elements 14 arranged in the aperture 3 off-set axially relative to the impeller 4. The illustrated embodiment of the fan unit 1 additionally comprises a motor 13 for rotation of the impeller 4.

In the illustrated embodiment, the fan unit 1 further comprises a guard 15 arranged in front of the impeller 4 to prevent accidental contact with the rotating blades 6 by fingers other parts of the body, and to prevent contact by other objects that could lead to the deterioration and even failure of the fan unit.

The fan unit 1 may further comprise a rim portion 16 releasably attached to the outer surface of the circumferential edge 2. The rim portion 16 and the outer surface comprise matching engagement elements 17A, 17B to releasably attached the rim portion 16 to the circumferential edge 2. In the illustrated embodiment, the matching engagement elements 17A, 17B are configured for attachment of the rim portion 16 from both the pressure side of the impeller 4 and the suction side of the impeller 4 to allow installation of the fan unit 1 in a configuration for pulling air and in a configuration for pushing air. The rim portion 16 can be mounted in combination with the guard 15.

Fig. 3 schematically illustrates an air volume being moved by the fan unit 1. The air volume is illustrated by the shaded area 20 forming a cylinder. The air volume 20 is moved through the fan unit 1 via the aperture 3. Fig. 4 illustrates two different views of the fan unit 1 with the impeller 4 in the front part of the figures. The fan unit 1 is identical to the fan unit 1 illustrated in Figs. 1 and 2. The impeller 4 comprises a hub 5 and carries five blades 6.

Fig. 5 illustrates two different views of the fan unit 1 with the diffusion elements 14 in the front part of the figures. The fan unit 1 is identical to the fan unit 1 illustrated in Figs. 1 and 2. The circumferential edge 2 forms an aperture 3 and comprises a plurality of diffusion elements 14 arranged in the aperture 3 off-set axially relative to the impeller 4.

Fig. 6 illustrates an enlarged view of a part of the fan unit 1 comprising a first and a second outwardly extending wall portion 8, 10.

The ring element 7 comprises a first outwardly extending wall portion 8 and a second outwardly extending wall portion 10 on the outer surface which faces the inner surface of the circumferential edge 2. The first and second outwardly extending wall portion 8, 10 terminate in a free end 9 and is arranged parallel and circumferentially along the ring element 7.

The circumferential edge 2 forms a first groove 11 and a second groove 12 circumferentially along the inner surface facing the impeller. The first and second groves 11, 12 are arranged parallel to each other.

By providing the ring element 7 with first and second outwardly extending wall portions 8, 10 and the circumferential edge with a first and a second groove 11, 12, a straight air path between the impeller 4 and the circumferential edge 2 is partly obstructed; i.e. the air flow deviates from a flow along a straight line. This increases flow resistance and increases the performance. Further, the reduced flow reduces noise.

The ring element 7 forms an arch-shaped edge portion 7A, such as a spline, a bell-shape, or a bell-mouth, terminating in a free edge 7B extending along a periphery of the circumferential edge 2. The arch-shaped edge portion 7A limits the amount of air passing the fan unit 1 between the circumferential edge 2 and the ring element 7, as the arch-shaped edge portion 7A partly obstructs the entrance into the area between the circumferential edge 2 and the ring element 7.

Figs. 7A-7C illustrate different embodiments of outwardly extending wall portions 8, 10. Each of Figs. 7A-7C illustrates a part of an embodiment of a fan unit 1. The fan unit 1 is similar to the embodiments of fan units as illustrated in the previous figures and comprises a circumferential edge 2 forming an aperture 3 and an impeller 4 arranged at least partly in the aperture 3. The impeller 4 comprises a hub 5 (see e.g. Fig. 1) carrying a plurality of blades 6 (see Fig.

4). Each blade 6 extends in a radial direction between the hub 5 and a ring element 7. The outer surface of the ring element 7 faces the inner surface of the circumferential edge 2.

On the outer surface of the ring element 7, the ring element comprises a first outwardly extending wall portion 8 and a second outwardly extending wall portion 10. The first and second outwardly extending wall portions 8, 10 are arranged circumferentially along the ring element 7.

The circumferential edge 2 forms a first groove 11 and a second groove 12 circumferentially along the inner surface facing the impeller.

In the embodiment illustrated in Fig. 7A, both the first and second outwardly extending wall portions 8, 10 and the first and second grooves 11,12 are substantially square-shaped in a cross-section in the radial direction, as also illustrated in the embodiments illustrated in Figs.

I, 2, and 6.

In the embodiment illustrated in Fig. 7B, the first and second outwardly extending wall portions 8, 10 and the first and second grooves 11,12 form a substantially triangular shape in a cross-section in the radial direction.

In the embodiment illustrated in Fig. 7C, the first and second outwardly extending wall portions 8, 10 are substantially square-shaped in a cross-section in the radial direction, whereas the first and second grooves 11,12 form a substantially triangular-shape in a cross- section in the radial direction. The width of the first and second outwardly extending wall portions 8, 10 are considerably smaller in the embodiment illustrated in Fig. 7C than in the embodiment illustrated in Fig. 7A.

It should be understood, that the illustrated wall portions 8, 10 and the illustrated grooves

II, 12 are different examples, and that the wall portions 8, 10 and/or the grooves 11, 12 may be differently shaped in alternative embodiments.

Claims

1. A fan unit comprising a circumferential edge forming an aperture and an impeller arranged to establish a flow through the aperture, the impeller comprising a hub being adapted for rotation about a rotation axis and carrying a plurality of blades, each blade extending in a radial direction between the hub and a ring element, wherein the ring element comprises an outer surface facing an inner surface of the circumferential edge, and the circumferential edge comprising a plurality of diffusion elements arranged in the aperture off-set axially relative to the impeller, the diffusion elements extending from a centre part non-rotational arranged about the rotation axis to the inner surface of the circumferential edge, wherein one of the outer surface and the inner surface comprises a first wall portion projecting upwards from the corresponding one of the inner and outer surfaces, and terminating in a first free end, the first wall portion extending circumferentially around the ring element, and wherein the other one of the outer surface and the inner surface forms a first groove circumferentially along the corresponding one of the inner and outer surfaces, the first wall portion and the first groove facing each other.

2. A fan unit according to claim 1, wherein a projection of the first wall portion on a rotation plane along the rotation axis at least partly overlap a projection of the first groove on the rotation plane.

3. A fan unit according to claim 2, wherein the projection of the first groove is larger than the projection of the first wall portion, and wherein the projection of the first wall portion in within the projection of the first groove.

4. A fan unit according to any of the preceding claims, wherein one of the outer surface and the inner surface comprises a second wall portion projecting upwards from the corresponding one of the inner and outer surfaces, and terminating in a first free end, the second wall portion extending circumferentially around the ring element, and wherein the other one of the outer surface and the inner surface forms a second groove circumferentially along the corresponding one of the inner and outer surfaces.

5. A fan unit according to claim 4, wherein the second wall portion is arranged substantially parallel to the first wall portion.

6. A fan unit according to claim 4 or 5, wherein a cross-sectional shape of the first wall portion and a cross-sectional shape of the second wall portion in a radial cross-section are substantial identical.

7. A fan unit according to any of the preceding claims, comprising more than two wall portions circumferentially around the ring element.

8. A fan unit according to any of the preceding claims, wherein the first wall portion has a height being a distance from the outer surface or the inner surface to the first free edge, and a width being transverse to the height, wherein the width is substantially uniform along the height.

9. A fan unit according to claim 8, wherein the width is in the range of 50-150% of the height, such as in the range of 75-125%.

10. A fan unit according to claim 8 or 9, wherein the first groove has a width being a dimension of the groove along the rotation axis, wherein the width of the first groove is larger than the width of the first wall portion.

11. A fan unit according to any of the preceding claims, wherein a clearance distance being a distance between the outer surface of the ring element and the inner surface of the circumferential edge is below 3.0% of a diameter of the impeller.

12. A fan unit according to any of the preceding claims, wherein the ring element in a radial cross-section forms an arch-shaped edge portion terminating in a free edge extending along a periphery of the circumferential edge.

13. A fan unit according to claim 12, wherein the periphery of the circumferential edge forms an arch-shaped edge portion matching the arch-shaped portion of the ring element.

14. A fan unit according to any of the preceding claims, wherein the plurality of blades and the ring element are formed in one piece of a polymer material.

15. A fan unit according to any of the preceding claims, wherein the number of diffusion element is at least twice the number of blades.

16. A fan unit according to any of the preceding claims, wherein a pitch angle of each of the plurality of blades is in the range of 1-89 degrees, such as in the range of 7-70 degrees, such as in the range of 15-50 degrees.

17. A fan unit according to any of the preceding claims, further comprising a motor being arranged about the rotation axis, and wherein the centre part is arranged circumferentially around the motor.

18. A fan unit according to any of the preceding claims, further comprising a rim portion releasably attached to the outer surface of the circumferential edge, the rim portion and the outer surface comprising matching engagement elements.

19. Use of the fan unit according to any of the preceding claims, wherein the impeller is rotated with a rotational speed in the range of -6000 to 6000 rotations per minute.