Protection device for arranging downstream of a rotary wheel about a rotational axis

Abstract

The invention relates to a protective device (1) for arrangement downstream of an impeller (2) rotatable about an axis of rotation (R) with an impeller diameter (X), which is designed to generate an airflow (L) at a predetermined operating point, which flows through the protective device (1) in a flow direction enclosing a flow angle α with the axis of rotation (R) and has a flow velocity (u) varying orthogonally to the flow direction, wherein the protective device (1) has a plurality of grid rings arranged about the axis of rotation (R), each of which is formed by a wire (10, 11, 12) circumferentially around the axis of rotation (R) and wherein the wires (10, 11, 12) of the grid rings are arranged in longitudinal section on a common curve (20) and two immediately adjacent wires are arranged in longitudinal section on a common imaginary straight line (30),which forms an angle β with the airflow (L) and along which the two wires are spaced apart from each other by a distance d, wherein a ratio d / D of the distance d to the wire diameter D of two immediately adjacent wires of the wires (11, 12) in a predetermined area (32) along the common curve (20) around the point (P) of maximum flow velocity is less than 3 or greater than 4.

Description

Description:

[0001] The invention relates to a protective device for arrangement downstream of an impeller rotatable about an axis of rotation. Such protective devices can also be referred to as contact guards or protective grilles and usually form a component of a fan, which generates an airflow by means of the impeller.

[0002] Protective grilles are necessary to prevent contact with the rotating impeller. These grilles consist of closely spaced, usually wire, mesh rings arranged concentrically around the impeller's axis of rotation and often held in place by a supporting structure.

[0003] Pressure-side mounted protective grilles, which are arranged downstream of the flow generated by the fan or impeller, are located in the fan's outflow field, so that the rings of the protective grille are surrounded by flow.

[0004] As the flow passes around the rings, periodic vortices detach from the rings or wires, which are also known as Kármán vortex streets (after Theodore von Kármán).

[0005] The frequency of this separation depends on the flow velocity u and the diameter D of the wire. The phenomenon can be described by the dimensionless Strouhal number St, which is essentially constant and in the range of 0.2. St = fD u ≈ 0 , 2 → f ≈ 0 , 2 u D

[0006] This mechanism has been known for a long time. Furthermore, current technology already offers methods for avoiding unfavorable frequencies or signal dropouts.

[0007] For example, to reduce vortex formation, the wires around bodies exposed to airflow are structured perpendicular to the direction of airflow, twisted, or given a spiral-shaped projection. Such measures are used, for example, in vehicle antennas or industrial chimneys.

[0008] It has been found that these vortex sheddings in fans are particularly significant for the overall fan noise at high volume flows, and especially at a specific operating point where the maximum possible volume flow passes through the fan without pressure increase. At this operating point of maximum volume flow, the fan is in free-blowing mode. Since very high flow velocities occur at this operating point, generating corresponding noise, the greatest acoustic improvement can be achieved by reducing the vortices for free-blowing operation. However, acoustic improvement can also be achieved by reducing vortices at other operating points.

[0009] Therefore, different protective grilles, depending on their specific design, can lead to significantly different overall noise levels on the same fan and at the same operating point.

[0010] Tests have shown that acoustically better-suited protective grilles can be up to 4 dB quieter than acoustically less suitable protective grilles.

[0011] At the fan's free-blowing operating point, the outflow from the fan or impeller is essentially axial, with a maximum flow velocity in the radially outer region. Near a housing ring surrounding the impeller, the velocity decreases again. Therefore, at the free-blowing operating point, the airflow generated by the impeller exhibits virtually no radial velocity component. At pressurized operating points, the direction changes, as a radially outward velocity component is added.

[0012] Furthermore, it has been shown that protective grilles, and in particular stackable protective grilles, whose grille rings have an increasing diameter in the radial outer area, exhibit a high acoustic impact or poor acoustic behavior at the free-blowing operating point compared to other protective grilles.

[0013] Further protective devices for arrangement downstream of an impeller rotating about an axis of rotation, as well as aspects of such protective devices, are also known in the prior art, for example, through the disclosures in documents US 9 746 003 B2 and CN 212 003 771 U.

[0014] The invention therefore aims to overcome the aforementioned disadvantages and provide a protective device for downstream placement of an impeller generating an airflow, which exhibits favorable acoustic properties and is thus as quiet as possible during operation, and particularly at a predetermined operating point. At the same time, the protective device should exhibit the lowest possible pressure loss and also fulfill the function of contact protection.

[0015] This problem is solved by the combination of features according to claim 1.

[0016] According to the invention, a protective device for arrangement downstream of an impeller rotatable about an axis of rotation is proposed, which is particularly designed to be stackable. The impeller has an impeller diameter and is configured to generate an airflow at a predetermined operating point, which flows through the protective device in a flow direction that includes a flow angle α with the axis of rotation. This airflow has a flow velocity that varies orthogonally to the flow direction, such that the flow velocity of the airflow relative to the axis of rotation, for example, in a free-blowing operating point of the fan where the flow direction is essentially parallel to the axis of rotation, increases from radially inward to radially outward to a maximum and then decreases again.Especially during free-blowing operation, the flow angle is typically 0°, allowing the airflow to run parallel to the axis of rotation. This maximum flow, therefore, is independent of the operating point and is typically annular and concentric to the axis of rotation. Viewed in a longitudinal section through the protective device, the airflow passes through it at a predetermined point, perpendicular to the flow direction, with a maximum flow velocity. This maximum, or point of maximum flow, can be defined for a specific operating point, such as the free-blowing operation of the fan. Alternatively, the absolute maximum for all or several possible operating points of the fan can also be defined as the maximum, or point of maximum flow.The protective device, which can also be referred to as a contact guard or protective grid, comprises a plurality of grid rings arranged around the axis of rotation. These rings are preferably arranged concentrically to the axis of rotation and each is formed by a wire with a diameter D circumferentially around the axis of rotation. The wires preferably merge into one another and are thus "endless." Furthermore, the wires preferably each have a circular cross-section, but can also have a different cross-section, for example, an oval one. The wire diameter D refers to the cross-section of a single wire. The wires can also be spaced radially apart from the axis of rotation. In longitudinal section through the protective device, the wires of the grid rings, or rather their cross-sections, lie on a common curve.Two immediately adjacent wires of two grid rings, or rather their cross-sections, lie together in longitudinal section on an imaginary straight line, in particular one passing through the center of the wires, which forms an angle β with the airflow. If the airflow is parallel to the axis of rotation, the imaginary straight line forms an angle β with the axis of rotation. Thus, at the point of free blowing, the geometric angle between the imaginary straight line and the axis of rotation is approximately equal to angle β. Furthermore, the two immediately adjacent wires are spaced apart from each other along the common straight line by a distance d, which is determined by the respective center point of the wire cross-sections.

[0017] Experiments have shown that not all grid rings or wires of the grid rings influence the noise behavior equally, and that the noise behavior is particularly influenced by a ratio d / D of the distance d to the wire diameter D of two immediately adjacent wires.

[0018] Therefore, according to the invention, it is proposed that the ratio d / D of the distance d to the wire diameter D of two immediately adjacent wires of the wires in a predetermined area along the common curve around the point of maximum flow velocity dh around the maximum of the flow velocity and in particular of all wires in this area and preferably exclusively in this preferred area is less than 3, in particular less than 2.6, or greater than 4.

[0019] Therefore, according to the invention, the following applies to the predetermined area: d D < 3 oder d D > 4

[0020] A favorable ratio can be achieved, for example, by smaller distances between the rings or wires or their cross-sections, i.e., more rings in the predetermined area, and / or by a larger diameter of the wires.

[0021] This predetermined area along the common curve is an area of ​​unfavorable flow conditions, in which particularly unfavorable separations occur with regard to acoustic behavior, resulting in high noise levels.

[0022] Such a region of unfavorable flow conditions arises in particular because the area is subjected to a high or maximum flow velocity at a specific operating point. Accordingly, an advantageous embodiment provides that the protective grid is subjected to a maximum flow velocity in the region predetermined along the common curve, and in particular for a predetermined operating point, and preferably for the free-blowing operating point.

[0023] An advantageous further development provides that the predetermined area along the common curve extends ±10%, in particular ±5% of the impeller diameter around the point of maximum flow velocity, which is further preferably located in the middle of the area.

[0024] Not viewed in longitudinal section, but from a top-down perspective of the protective device, the area runs in a ring shape around the axis of rotation and is concentric to it.

[0025] For a purely exemplary impeller diameter of 10 cm, the area would therefore be 2 cm (±10%), in particular 1 cm (±5%) wide, with the point of maximum flow being located in the middle of this area.

[0026] As previously explained, the flow velocity varies orthogonally to the flow direction, i.e., radially. At the free-blowing operating point, it typically rises to a maximum from the axis of rotation outwards and then decreases again. This maximum, or rather its position, depends on the specific fan and impeller being considered. Therefore, a specific range can only be defined in relation to a particular fan. However, for some fans, the maximum flow velocity occurs at approximately 90% of the impeller diameter or 90% of the impeller radius. To achieve favorable acoustic behavior for such a fan, especially at the free-blowing operating point, the radially predetermined range may be, for example, 80% to 100%, and particularly 85% to 95%, of the impeller diameter.that the predetermined area borders this area of ​​the impeller in the axial direction.

[0027] Depending on the impeller or fan and the operating point under consideration, the point of maximum flow can also lie radially outside the impeller.

[0028] Tests have also shown that stackable safety grilles, in particular those with a truncated cone shape to allow stacking of multiple grilles, exhibit particularly unfavorable noise characteristics.

[0029] The poor acoustic properties can be explained by the fact that, viewed in the longitudinal section of the protective device, the wires are arranged almost one behind the other in the radially outer area in the direction of flow or in the axial direction, so that they act as flow-enclosed tandem cylinders, especially at the free-blowing operating point.

[0030] It is known from the literature that two closely spaced cylinders, i.e., tandem cylinders, influence each other depending on the angle and distance. Measurements on such cylinders have shown that, particularly in the range of approximately 20° and especially when the ratio d / D of the distance d and the wire diameter D is greater than or equal to 3, excessively high force fluctuations occur.

[0031] The angle at which the cylinders are positioned relative to each other is the angle at which a downstream cylinder or wire is arranged behind an upstream cylinder or wire relative to the flow direction or airflow. The flow direction or airflow can be parallel to the axis of rotation, particularly at the free-blowing operating point. However, depending on the fan and the operating point under consideration, the flow direction or airflow can also have a radial component, so that the airflow is oblique to the axis of rotation and forms an angle α with it.

[0032] Another advantageous embodiment of the invention therefore provides that the angle β is greater than 20°, in particular greater than 30° and preferably less than or equal to 90°, at least in the predetermined range.

[0033] Since the angle at which the wires or cylinders are arranged relative to each other also depends on a radial velocity component and thus on the operating point of the fan, the ratio d / D, unlike a specific angle, has an influence on the noise behavior at a large number of operating points.

[0034] Given the suitability as touch protection and the stability of the wires, a ratio d / D < 3 is particularly advantageous in the predetermined area.

[0035] As mentioned, the ratio d / D of the wire diameter D to the distance d between two immediately adjacent wires is particularly relevant for the noise behavior. However, with regard to the entire protective device, care must be taken to ensure that the pressure loss across the device is not too high and that the effectiveness of the touch protection is not compromised. Since a small distance d and a large diameter D increase the obstruction caused by the protective grid and thus the pressure loss, the wire diameters D of each grid ring or section of the protective device should not be "too large," and the spacing between the wires should also not be "too wide" or "too narrow."

[0036] Accordingly, the ratio d / D of the distance d to the wire diameter D of two immediately adjacent wires, and in particular of all wires, in a second region adjacent to the predetermined (first) region along the common curve is, according to the invention, different from the ratio in the predetermined region and is in particular greater than 2.6, in particular greater than 3 and less than 4. For example, the ratio in such a second region can be 3.3.

[0037] Viewed from the predetermined area, a second area can be adjacent both radially outside and radially inside, whereby the ratios d / D in the two second areas may also differ.

[0038] By using such second areas with a d / D ratio that differs from that in the predetermined area, both the material usage and the influence on the aerodynamic characteristics can be reduced or minimized.

[0039] The proposed protective device can also be designed to be stackable. For this purpose, the common curve transitions from a steep first section to a flatter second section from the radial outer to the radial inner, so that the protective device is designed in a particularly pot- or frustoconical shape with a diameter that widens radially outwards, and several protective devices can be stacked inside one another.

[0040] Furthermore, the common curve can transition radially inwards from the second region into a third region and, for example, bend sharply, which is inclined in the opposite direction to the first region. The curve is then convex in the direction of flow.

[0041] In addition to the inventive ratio d / D, further measures to improve noise behavior may also be provided.

[0042] For example, the surface roughness of the wires in the predetermined area may be increased compared to the wires outside the predetermined area.

[0043] This increased surface roughness can be achieved, for example, by coating the wires with a lacquer. Using a textured lacquer, for instance, creates a turbulent boundary layer as the flow around the cylinder or wire. This reduces the size of the vortex region downstream of the wire and lowers the flow resistance.

[0044] Accordingly, in a particularly advantageous further development, it is provided that the wires in the predetermined area have a relative surface roughness k / D in the range of 1% to 10%. k represents the average depth of the roughness and D the diameter of the respective wire.

[0045] Furthermore, the surface of the wires in the predetermined area can be structured and, in particular, may have projections extending in the longitudinal direction of the wires and winding spirally around the longitudinal direction.

[0046] Alternatively or additionally to the aforementioned measures, it can also be provided that the protective device upstream of the grid rings has a pre-grid adjacent to at least the predetermined area in the direction of flow, which is formed in particular from a second wire or a plurality of second wires whose diameter is smaller than the diameter of the wires of the grid rings. The second wire forms, in particular, meshes so that turbulent vortices can be generated in the airflow upstream of the grid rings.

[0047] By means of an upstream pre-grid made of a multitude of second wires or by upstream meshes made of a second wire, the flow can be preconditioned, especially adjacent to the predetermined area, with an additional modification of the rings by at least the proposed d / D ratio.

[0048] The mesh of such a pre-grid should be arranged at an angle to the rings or their wires. Small turbulent vortices are generated behind the pre-grid and in front of the grid rings, influencing the boundary layer flow at the grid rings in a manner similar to increased surface roughness.

[0049] With a small diameter of the wire mesh, the additional pressure loss is small and no significant additional noise is to be expected.

[0050] Furthermore, a fan or ventilator can have a protective device according to the invention downstream of its impeller.

[0051] The features disclosed above can be combined in any way, provided that this is technically possible and they do not contradict each other.

[0052] Other advantageous embodiments of the invention are characterized in the dependent claims or are described in more detail below together with the description of the preferred embodiment of the invention with reference to the figures. The figures show: Fig. 1 a longitudinal section through a conventional protective device; Fig. 2 a longitudinal section through a first variant of a protective device; Fig. 3 a longitudinal section through a second variant of a protective device; Fig. 4 an enlarged view of the predetermined area of ​​the protective device according to Figure 3 Fig. 5 shows a longitudinal section through a third variant of a protective device.

[0053] The figures are schematic examples and each shows the cut surfaces or a cross-section of the wires of a longitudinally sectioned protective device, i.e., in the longitudinal section of the protective device. Identical reference symbols in the figures indicate identical functional and / or structural features. Figure 4 This corresponds to an enlarged representation of the predetermined area 32 according to Figure 3 , wherein the reference numerals shown also refer to an embodiment according to Figure 2 or 5 are transferable.

[0054] In Figure 1A protective device 3 according to a known configuration is shown. The protective device 3 is shown upstream of an impeller 2, which generates the airflow L by rotation. The airflow L flows through the protective device parallel to the axis of rotation R with a flow velocity u that varies in the radial direction r. As shown by way of example for a free-blowing operating point, this velocity increases from radially inward (i.e., from the axis of rotation R) to radially outward until reaching a maximum at point P of maximum flow in region 32, and then decreases again further radially outward, i.e., until it reaches a housing of the impeller 2 (not shown), which surrounds it circumferentially. In a different operating point, the airflow L can also run obliquely relative to the axis of rotation R and enclose a common flow angle with it, as is the case, for example, in Figure 5 is shown.

[0055] The in Figure 1The impeller shown has a plurality, and here 31, of grid rings, each of which is formed by an "endless" wire 10 that completely encircles the axis of rotation R in the circumferential direction, the cross-section of which is shown.

[0056] The wires 10 or their cross-sections lie in the longitudinal section of the protective device on a common curve 20 which has from radially outside to radially inside from a steep first area 21, i.e. an area with a positive slope, to a flat second area 22, i.e. an area with a lower slope, to a third area 23 inclined opposite to the first area 21, i.e. an area with a negative slope.

[0057] In particular, the first area 21 and the second area 22 give the protective device 3 a frustoconical shape with a radially expanding diameter, so that a plurality of similarly designed protective devices 3 can be stacked inside each other.

[0058] For the operating point shown here, which can be the free-blowing operating point of the fan or impeller 2, the maximum flow velocity u, primarily responsible for noise generation, is located at approximately 90% of the impeller diameter X. Therefore, it is proposed to implement noise reduction measures, especially for this operating point, within the predetermined range 32 ± 5% around the point P of maximum flow. Since the point P of maximum flow velocity is located at approximately 90% of the impeller diameter X, range 32 extends from approximately 85% to 95% of the impeller diameter. The exact range of maximum airflow can vary depending on the impeller 2 used and may also lie outside the impeller diameter, so this illustration is purely illustrative.

[0059] In this case, the stackability means that two immediately adjacent wires 10 are arranged in a flow-wise unfavorable position within this predetermined area 32. When considering two immediately adjacent wires 10 in the predetermined area 32, the downstream wire of the two wires 10 along the airflow L is arranged within an angle β of less than 30° relative to the upstream wire of the two wires 10. See also Figure 4 .

[0060] As a result, the two immediately adjacent wires 10 act as a tandem cylinder, so that in addition to the poor acoustic behavior due to the maximum flow velocity u in the predetermined area 32, further noise-generating separations and turbulences occur.

[0061] It follows that measures to reduce noise are particularly effective and useful for the free-blowing operating point of the fan or impeller 2 in this predetermined area 32.

[0062] Such an acoustically favorable, i.e., low-noise, protective device can be achieved by the in Figure 2 and Figure 3 The depicted variants can be achieved.

[0063] It has been found that the ratio d / D of the wire diameter D to the distance d between two immediately adjacent wires has a significant influence on the noise behavior of the protective device.

[0064] For the variant according to Figure 1 This results in an unfavorable d / D ratio of approximately 3.3 for all wires 10, which is detrimental to noise performance.

[0065] According to the claimed invention, the ratio d / D is less than three or greater than four, which can be achieved if, in the predetermined range 32 - as in Figure 2shown - the distance d between the wires is reduced or - as in Figure 3 shown - the wires in this area are 32 thicker or have a larger diameter D.

[0066] If the number of wires or their diameter were increased not only in the predetermined area 32, but across all areas 31, 32, 33 of the protective grid 1, other properties of the protective grid 1, such as the pressure loss, would deteriorate significantly, so the proposed modification of the protective grids 1 according to the Figures 2 and 3 The changes are only carried out in the predetermined area 32 and not in the adjacent areas 31 and 33. In these areas, the d / D ratio is left in a range between 3 and 4, i.e., no modifications are made compared to the variant according to... Figure 1 carried out.

[0067] In the variant according to Figure 2The acoustic optimization or noise reduction of the protective grille 1 is achieved by the fact that in the predetermined area 32, compared to the design according to Figure 1 More wires 11 are provided. By adding a wire 11 in this area 32, the distances d between the wires 11 are reduced.

[0068] The protective grille 1 in Figure 3 In contrast, it provides that the number of wires is 12 in the predetermined area 32 compared to the variant according to Figure 1 remains unchanged, but the diameters D of the wires 12 in this area are 32 compared to the original design according to Figure 1 are enlarged.

[0069] Both in Figure 2 as well as Figure 3 For example, a d / D ratio of 2.5 is achieved through the different measures, which can also be combined.

[0070] In Figure 4 is the predetermined area 32 according to Figure 3shown enlarged, with the representation analogous to the variant according to Figure 2 is transferable.

[0071] The grid rings, or rather their wires 10, 12 of the total n wires, each have a distance a1, a2, ... an from the axis of rotation R. Each of the wires 10, 12 has a diameter D, where the diameters D of the wires 12 in the predetermined region 32 are larger than the diameters D of the wires 10 in the adjacent regions 30, 31. The distances d between any two immediately adjacent wires 12 in the predetermined region 32 and the distances d between any two immediately adjacent wires 10 in the adjacent regions 31, 33 are essentially the same. The ratio d / D in the predetermined region is approximately 2.6 and in the adjacent regions approximately 3.3.

[0072] In Figure 5is a protective device 1 with a difference compared to the protective devices of the Figures 1 to 4The differing common curve 20 is shown, in which sections 21, 22, and 23 of the curve have different slopes. Simultaneously, the air flows obliquely to the axis of rotation R through the protective device, such that the airflow L forms an angle α with the axis of rotation R. As described, the flow velocity u varies orthogonally to the flow direction or to the airflow L and also passes through the protective device 1 at a point P with a maximum flow velocity. Thus, orthogonal to the flow direction, the flow velocity increases from radially inward to radially outward to its maximum and then decreases again further radially outward. Although the flow velocity profile is defined here orthogonally to the flow direction, it can, in principle, also be converted to a coordinate system in which the x-axis is parallel to the radial direction, as is the case in Figure 1and is particularly the case for the free-blowing operating point anyway. In the present case, this point P of maximum radial flow velocity lies outside an impeller, as is the case in Figure 1 is shown. Regardless of this, and also regardless of the fact that the common curve 20 in the radially outermost third region 23 runs parallel to the axis of rotation R, all wires in the area of ​​maximum flow velocity around the point P are formed with the ratio d / D proposed according to the invention, as is the case with the Figures 3 and 4 was described.

[0073] Also for such a non-stackable protective device 1 according to Figure 5 , this results in a significant noise reduction and thus an acoustic improvement for the predetermined operating point with the described airflow L.

[0074] The invention is not limited in its implementation to the preferred embodiments specified above.

Claims

1. A protection device (1) for arranging downstream of an impeller (2) rotatable about a rotational axis (R) with an impeller diameter (X) adapted to generate an air flow (L) in a predetermined operating point flowing through the protection device (1) in a flow direction which encloses a flow angle α with the rotational axis (R) and having a flow velocity (u) which varies orthogonally to the flow direction so that the protection device (1), in the predetermined operating point of the impeller (2) as seen in a longitudinal section through the protection device (1), is flowed through orthogonally to the flow direction at a predetermined point P with a maximum through-flow velocity, wherein the protection device (1) has a plurality of grille rings arranged about the rotational axis (R) each formed by a wire (10, 11, 12) with a wire diameter D enveloping the rotational axis (R) in a circumferential direction, and wherein the wires (10, 11, 12) of the grille rings, in the longitudinal section, are arranged on a common curve (20), and two immediately adjacent wires, in the longitudinal section, are arranged on a common imaginary straight line (30) which encloses an angle β with the air flow (L) and along which the two wires are spaced from one another at a distance d, characterised in that a ratio d / D of the distance d to the wire diameter D of two immediately adjacent wires of the wires (11, 12) in a region (32) predetermined along the common curve (20) about the point P of maximum through-flow velocity is less than 3 or greater than 4, wherein the ratio d / D of the distance d to the wire diameter D of two immediately adjacent wires of the wires (10) in a second region (31, 33) adjacent along the common curve (20) deviates from the ratio d / D in the predetermined region (32).

2. The protection device of claim 1, wherein the protection device (1) is adapted to, in the predetermined operating point of the impeller (2) in the region (32) predetermined along the common curve (20) and in particular centrally of the predetermined region (32), be flowed through at the maximum through-flow velocity (u).

3. The protection device of claim 1 or 2, wherein the protection device (1) is adapted such that the region (32) predetermined along the common curve (20) extends ±10%, in particular ±5%, of the impeller diameter (X) in the predetermined operating point of the impeller (2) about the point P of maximum through-flow velocity.

4. The protection device of any one of the preceding claims, wherein the angle β, at least in the predetermined region (32), is greater than 20°, in particular greater than 30°.

5. The protection device of any one of the preceding claims, wherein the ratio d / D of the distance d to the wire diameter D of two immediately adjacent wires of the wires (10) in the second region (31, 33) is greater than 2.6 and less than 4.

6. The protection device of any one of the preceding claims, wherein the common curve (20) transitions from radially outwards to radially inwards from a steep first region (21) to a flat second region (22) so that multiple protection devices (1) are stackable within each other.

7. The protection device of the preceding claim, wherein the common curve (20) transitions from the second region (22) radially inwards to a third region (23) inclined with respect to the first region (21).

8. The protection device of any one of the preceding claims, wherein the wires (11, 12) in the region (32) predetermined along the common curve (20) have a relative surface roughness k / D in a range of 1% to 10%.

9. The protection device of any one of the preceding claims, wherein a surface of the wires (11, 12) in the region (32) predetermined along the common curve (20) is textured and in particular has protrusions extending in a longitudinal direction of the wires (11, 12) and further in particular helically winding about the longitudinal direction.

10. The protection device of any one of the preceding claims, wherein upstream of the grille rings, the protection device (1) has a pre-grille which in the flow direction abuts at least the region (32) predetermined along the common curve (20) and in particular is formed of at least one second wire having a diameter less than the diameter of the wires (10, 11, 12) of the grille rings and in particular forming meshes so that turbulent vortices are generatable in the air flow (L) in the predetermined operating point of the impeller (2) upstream of the grille rings.

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