EP4242469A1 - Protection device for arranging downstream of a rotary wheel about a rotational axis - Google Patents

Abstract

The invention relates to a protective device (1) for arranging downstream of an impeller (2) which can be rotated about an axis of rotation (R) and has an impeller diameter (X), which is designed to generate an air flow (L) at a predetermined operating point, which the protective device ( 1) flows in a flow direction that includes a flow angle α with the rotation axis (R) and has a flow velocity (u) that varies orthogonally to the flow direction, the protective device (1) having a plurality of grid rings arranged around the rotation axis (R), each of which are formed by a wire (10, 11, 12) with a wire diameter D running around the axis of rotation (R) in the circumferential direction and the wires (10, 11, 12) of the grid rings in longitudinal section on a common curve (20) and two immediately adjacent ones Wires are arranged in longitudinal section on a common imaginary straight line (30), which forms an angle β with the air flow (L) and along which the two wires are spaced apart from one another at a distance d, with a ratio d/D of the distance d to that 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

The invention relates to a protective device for arranging downstream of an impeller that can be rotated about an axis of rotation. Such protective devices can also be referred to as contact protection or protective grilles and usually form part of a fan, which generates an air flow using the impeller.

Protective grilles are necessary to prevent contact with the rotating impeller. Such protective grids consist of tightly spaced grid rings, usually made of wire, whereby the rings are arranged concentrically around the axis of rotation of the impeller and are often held by a support structure.

Protective grilles mounted on the pressure side, which are arranged downstream of the flow generated by the fan or the impeller, are in the outflow field of the fan, so that the flow flows around the rings of the protective grille.

As the flow flows around the rings, there are periodic separations of vortices on the rings or wires, which are also known as Kärmän vortex street (after Theodore von Kármán).

The frequency of this separation depends on the flow velocity u and the diameter D of the wire. The phenomenon can be described using the dimensionless Strouhal number St, which is essentially constant and in the range of 0.2. $$\mathit{St}=\frac{\mathit{fD}}{u}\approx 0.2\to f\approx \frac{0.2u}{D}$$

This mechanism has been known for a long time. The state of the art also basically already knows ways to avoid unfavorable frequencies or separations.

For example, bodies around which flow occurs, here the wires, are structured, twisted or formed with a projection that extends in a spiral shape around the body to reduce the vortex street transversely to the direction of flow. Such measures are used, for example, in vehicle antennas or industrial chimneys.

It has been found that these separations of vortices in fans are decisive for the overall noise of the fan, particularly at high volume flows and particularly at an operating point or operating point, in which a maximum possible volume flow flows through the fan without increasing the pressure. At this operating point of maximum volume flow, the fan is free-blowing. Since there are very high flow velocities at this operating point and therefore associated noises arise, the greatest acoustic improvement can be achieved by reducing the vortices for the free-blowing operation of the fan. However, an acoustic improvement can also be achieved at other operating points by reducing the vortices.

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

Tests have shown that protective grilles that are acoustically more suitable can, for example, be up to 4 dB quieter than protective grilles that are less acoustically suitable.

At the free-blowing operating point of the fan, the outflow from the fan or the impeller is essentially axial, with a maximum of the flow velocity in the radially outer region. The speed decreases again close to a housing ring surrounding the impeller in the circumferential direction. For the free-blowing operating point, the air flow generated by the impeller has almost no radial velocity component. The direction changes for working points in the pressure because a radially outward velocity component is added.

Furthermore, it has been shown that protective grilles and in particular stackable protective grilles, whose grille rings have an increasing diameter in the radially outer region, have a high acoustic impact or poor acoustic behavior in the free-blowing operating point in comparison to other protective grilles.

The invention is therefore based on the object of overcoming the aforementioned disadvantages and of providing a protective device for arrangement downstream of an impeller which generates an air flow has favorable acoustic behavior and is therefore as quiet as possible during operation and in particular at a predetermined operating point. At the same time, the protective device should have the lowest possible pressure loss and continue to fulfill the function of protection against contact.

This task is achieved by the combination of features according to claim 1.

According to the invention, a protective device is therefore proposed for arranging downstream of an impeller that can be rotated about an axis of rotation, which is in particular designed to be stackable. The impeller has an impeller diameter and is designed to generate an air flow at a predetermined operating point, which flows through the protective device in a flow direction including a flow angle α with the axis of rotation. This air flow has a flow velocity that varies orthogonally to the flow direction, so that the flow velocity of the air flow relative to the axis of rotation, for example in a free-blowing operating point of the fan, in which the flow direction is essentially parallel to the axis of rotation, increases from radially inside to radially outside to a maximum and then falls off again. Particularly for free-blowing operation, the flow angle is usually 0°, so that the air flow can run parallel to the axis of rotation. This maximum therefore runs, regardless of the operating point, in particular in a ring shape and concentric to the axis of rotation around the axis of rotation, with the protective device, seen in a longitudinal section through the protective device, being flowed through at a maximum flow rate at a predetermined point orthogonally to the flow direction. This maximum or the point of maximum flow can be determined for a predetermined operating point, such as the free-blowing operation of the fan. Alternatively, this can also be used as a maximum or as a point of maximum flow absolute maximum for all or several possible operating points of the fan. The protective device, which can also be referred to as a contact protection or protective grille, has a plurality of grid rings arranged around the axis of rotation, which are arranged in particular concentrically to the axis of rotation and are each formed by a wire with a wire diameter D that runs around the axis of rotation in the circumferential direction. The wires each preferably merge into themselves and are therefore “endless”. Furthermore, the wires preferably each have a round cross-section, but can also have a different and, for example, oval cross-section. The wire diameter D refers to the cross section of a wire. Furthermore, the wires can each be described as spaced apart in the radial direction at a distance from the axis of rotation. The wires of the grid rings or their cross sections lie on a common curve in the longitudinal section through the protective device. Two immediately adjacent wires of two grid rings of the plurality of grid rings or their cross sections lie together in longitudinal section on an imaginary straight line, in particular running through the center of the wires, which forms an angle β with the air flow. If the air flow runs parallel to the axis of rotation, the imaginary straight line encloses the angle β with the axis of rotation. At the free-blowing point, the geometric angle between the imaginary straight line and the axis of rotation is approximately equal to the angle β. Furthermore, the two immediately adjacent wires are spaced apart from one another along the common straight line at a distance d, which is preferably determined by the respective center of the wire cross sections.

Tests have shown that not all grid rings or wires of the grid rings influence the noise behavior to the same extent and that the noise behavior is determined in particular by a ratio d/D of the distance d to the wire diameter D of two immediately adjacent wires is influenced.

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, i.e. around the maximum of the flow velocity and in particular of all wires in this area and preferably exclusively in this preferred range is less than 3, in particular less than 2.6, or greater than 4.

According to the invention, the following therefore applies to the predetermined range: $$\frac{d}{D}<3\mathit{or}\frac{d}{D}>4$$

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.

This area predetermined along the common curve is an area of unfavorable flow conditions in which separations are particularly unfavorable for the acoustic behavior and therefore a high level of noise is generated.

Such an area of unfavorable flow conditions results in particular from the fact that the area is flowed through at a high or maximum flow rate at a certain operating point. Accordingly, an advantageous development provides that the protective grid is in the predetermined area along the common curve and in particular for a predetermined operating point and preferably for the free-blowing working point is flowed through with a maximum flow velocity.

An advantageous development provides that the area predetermined 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.

Viewed not in longitudinal section, but from a top view of the protective device, the area runs in a ring around the axis of rotation and concentric to it.

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

As already explained, the flow velocity varies orthogonally to the flow direction or in the radial direction, whereby in the free-blowing operating point it usually increases radially outwards from the axis of rotation to a maximum and then falls again. This maximum or the positioning of the maximum flow speed depends on the specific fan being considered or the impeller used in it. Therefore, a specific area can only be specified depending on a specific fan. However, for some fans the maximum flow speed occurs at approximately 90% of the impeller diameter or at approximately 90% of the impeller radius. In order to enable favorable acoustic behavior for such a fan, in particular for the free-blowing operating point, it can be provided, for example, that the range predetermined in the radial direction is 80% to 100% and in particular 85% to 95% of the impeller diameter or that the predetermined range in Axial direction adjacent to this area of the impeller.

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

Tests have also shown that, in particular, stackable protective grilles, which, for example, have a truncated cone-like shape to enable multiple protective grilles to be stacked one inside the other, have particularly unfavorable noise behavior.

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

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 strongly excessive force fluctuations occur, particularly in the range of approximately 20° and particularly at a ratio d/D of the distance d and the wire diameter D greater than or equal to 3.

The angle at which the cylinders are relative to one another is the angle at which a downstream cylinder or wire is arranged behind an upstream cylinder or wire relative to the flow direction or the air flow. The flow direction or the air flow can be parallel to the axis of rotation, particularly in the free-blowing operating point. Depending on the fan and the operating point under consideration, the flow direction or the air flow can also have a radial component, so that the air flow is oblique to the axis of rotation and forms an angle α with it.

A further advantageous variant 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.

Since the angle at which the wires or cylinders are arranged to one another also depends on a radial speed component and therefore on the operating point of the fan, the ratio d/D, in contrast to a specific angle, has an influence on a large number of operating points the noise behavior.

Given the suitability for protection against contact and the stability of the wires, a ratio d/D <3 is particularly advantageous in the predetermined range.

As stated, 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, in relation to the entire protective device, care must be taken to ensure that the pressure loss at the protective device is not too high and that the suitability of contact protection is not lost. However, since a small distance d and a large diameter D increase the obstruction by the protective grille and thus the pressure loss, the wire diameter D of each grille ring or each area of the protective device should not be "too large" and the distances between the wires should not be "too large." "wide" or "too narrow".

Correspondingly, the ratio d/D of the distance d to the wire diameter D of two immediately adjacent wires of the and in particular all wires in a second area adjacent to the predetermined (first) area along the common curve is, in a likewise advantageous embodiment, of the ratio in the predetermined one Area different and in particular larger than 2.6, especially larger than 3 and smaller as 4. For example, the ratio in such a second range can be 3.3.

Seen from the predetermined area, such a second area can be adjacent both radially on the outside and radially on the inside, whereby the ratios d/D in the two second areas can also differ.

Through such second areas with a ratio d/D that deviates from the predetermined area, both the use of material and the influence on the aerodynamic characteristics can be reduced or minimized.

The proposed protective device can also be designed to be stackable. For this purpose, the common curve goes from radially outside to radially inside from a steep first area into a flat second area, so that the protective device is designed in particular in the shape of a cup or truncated cone with a diameter that widens radially outward and several protective devices can be stacked one inside the other.

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

In addition to the proposed advantageous ratio d/D, further measures to improve noise behavior can also be provided.

For example, a surface roughness of the wires in the predetermined range may be increased compared to the wires outside the predetermined range.

This increased surface roughness can be provided, for example, by a varnish covering the wires. By using textured paint, for example, a boundary layer becomes turbulent as the flow flows around the cylinder or wire. This allows the vortex area downstream of the wire to be reduced and the flow resistance decreases.

Accordingly, in an advantageous development it is provided that the wires in the predetermined range have a relative surface roughness k/D in a range of 1% to 10%. Where k is the average depth of the roughness and where D is the diameter of the respective wire.

Furthermore, one or the surface of the wires can be structured in the predetermined area and, in particular, have projections which extend in the longitudinal direction of the wires and which further spiral in particular around the longitudinal direction.

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

Through an upstream pre-grid made of a plurality of second wires or through upstream meshes made of a second wire, the flow can be preconditioned, in particular adjacent to the predetermined area, with an additional modification of the rings by at least the proposed ratio d/D.

The meshes 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, which influence the boundary layer flow at the grid rings in a similar way to increased surface roughness.

If the wire mesh diameter is small, the additional pressure loss is small and no significant additional noise is to be expected.

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

The features disclosed above can be combined in any way, as long as this is technically possible and they do not contradict each other.

Fig. 1

ein Längsschnitt durch eine herkömmliche Schutzvorrichtung;

Fig. 2

ein Längsschnitt durch eine erste Variante einer Schutzvorrichtung;

Fig. 3

ein Längsschnitt durch eine zweite Variante einer Schutzvorrichtung;

Fig. 4

eine vergrößerte Darstellung des vorbestimmten Bereichs der Schutzvorrichtung gemäß Figur 3

Fig. 5

ein Längsschnitt durch eine dritte Variante einer Schutzvorrichtung.

Other advantageous developments of the invention are characterized in the subclaims or are shown in more detail below together with the description of the preferred embodiment of the invention with reference to the figures. Show it:

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 Figure 3

Fig. 5

a longitudinal section through a third variant of a protective device.

The figures are schematic examples and each show the cut surfaces or a cross section of the wires of a protective device cut in the longitudinal direction, ie in a longitudinal section of the protective device. The same reference numbers in the figures indicate the same functional and/or structural features. Figure 4 corresponds to an enlarged representation of the predetermined area 32 Figure 3 , whereby the reference numbers shown also refer to an embodiment according to Figure 2 or 5 are transferable.

In Figure 1 a protection device 3 is shown according to a known configuration. The protective device 3 is shown upstream of an impeller 2, which generates the air flow L through rotation. The air flow L flows through the protective device parallel to the axis of rotation R with a flow velocity u varying in the radial direction r, which, as shown as an example for a free-blowing working point, increases from the radial inside, ie from the axis of rotation R towards the radial outside, to a maximum at the point P of maximum flow in the area 32 and falls further radially outside or to a housing of the impeller 2, not shown, which surrounds it in the circumferential direction. In a different operating point, the air flow L can also run obliquely with respect to the axis of rotation R and form a common flow angle with it, as is the case, for example, in Figure 5 is shown.

This in Figure 1 The impeller shown has a large number, here 31, grid rings, each of which is formed by an "endless" wire 10 which completely encircles the rotation factor R in the circumferential direction, the cross section of which is shown in each case.

The wires 10 or their cross sections in the longitudinal section of the protective device lie on a common curve 20 which is from the radial outside radially inwards from a steep first area 21, ie an area with a positive gradient, to a flat second area 22, ie an area with a lower gradient, to a third area 23 inclined towards the first area 21, ie an area with a negative gradient .

In particular, due to the first area 21 and the second area 22, the protective device 3 is designed in the shape of a truncated cone with a diameter that widens radially outward, so that a plurality of similarly designed protective devices 3 can be stacked one inside the other.

For the operating point shown here, which can be the free-blowing operating point of the fan or the impeller 2, the maximum of the flow velocity u, which is primarily responsible for noise development, is approximately 90% of the impeller diameter X. It is therefore suggested, especially for this operating point in the predetermined range 32 ± 5% around the point P of maximum flow to take measures to reduce noise. Since the point P of maximum flow velocity in the present case is approximately 90% of the impeller diameter X, the area 32 here extends from approximately 85% to 95% of the impeller diameter. The exact range of maximum air flow can be different depending on the impeller 2 used and can also lie outside the impeller diameter, so that the illustration is purely an example.

In the present case, the stackability means that two immediately adjacent wires 10 are arranged in an unfavorable flow-wise manner in this predetermined area 32. If two immediately adjacent wires 10 are considered in the predetermined area 32, the downstream of the two wires 10 along the air flow L is arranged within an angle β of less than 30° relative to the upstream of the two wires 10. See also this Figure 4 .

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

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

Such an acoustically favorable, i.e. low-noise, protective device can be provided by the in Figure 2 and Figure 3 shown variants can be achieved.

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

For the variant according to Figure 1 This results in an unfavorable ratio d/D of approximately 3.3 for all wires 10.

However, it is advantageous if 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 2 shown - the distance d between the wires is reduced or - as in Figure 3 shown - the wires in this area 32 are made thicker or with a larger diameter D.

If the number of wires or their diameter were increased not only in the predetermined area 32, but over all areas 31, 32, 33 of the protective grille 1, other properties of the protective grille 1, such as the pressure loss, would deteriorate significantly, so that the proposed Modification of the protective grilles 1 according to Figures 2 and 3 only in the predetermined area 32 and not in the areas 31, 33 adjacent to this. In these it will Leave the ratio d/D in a range between 3 and 4 or make no modifications compared to the variant according to Figure 1 carried out.

In the variant according to Figure 2 the acoustic optimization or noise reduction of the protective grille 1 is achieved in that in the predetermined area 32 in comparison to the embodiment according to Figure 1 more wires 11 can be provided. Because a wire 11 is added in this area 32, the distances d between the wires 11 are reduced.

The protective grille 1 in Figure 3 On the other hand, provides that the number of wires 12 in the predetermined area 32 compared to the variant according to Figure 1 remains unchanged, but the diameter D of the wires 12 in this area 32 compared to the original design Figure 1 are enlarged.

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

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

The grid rings or their wires 10, 12 of the total of n wires each have a distance a ₁ , a ₂ ... a _n from the axis of rotation R. Each of the wires 10, 12 has a diameter D, the diameters D of the wires 12 in the predetermined area 32 being larger than the diameters D of the wires 10 in the adjacent areas 30, 31. The distances d between two immediately adjacent wires 12 of the wires 12 in the predetermined area 32 and the distances d between two immediately adjacent wires 10 of the wires 10 in the same neighboring areas 31, 33 is essentially the same in the present case. The ratio d/D in the predetermined area is approximately 2.6 in the present case and approximately 3.3 in the areas adjacent to it.

In Figure 5 is a protective device 1 with a protection against the protective devices Figures 1 to 4 different common curve 20 is shown, in which the areas 21, 22, 23 of the curve have different slopes. At the same time, the air flows through the protective device at an angle to the rotation axis R, so that the air flow L forms an angle α with the rotation axis R. As described, the flow velocity u varies orthogonally to the flow direction or to the air flow L and also flows through the protective device 1 at a point P with a maximum flow velocity. Orthogonal to the flow direction, the flow velocity increases from the radial inside to the radial outside to the maximum and then falls further towards the radial outside. Although the course of the flow velocity is defined orthogonally to the flow direction in the present case, it can in principle also be converted to a coordinate system in which the X-axis is parallel to the radial direction, as in Figure 1 and is particularly the case for the free-blowing operating point. In the present case, this point P of maximum flow velocity lies in the radial direction outside an impeller, as shown 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 are designed in the maximum flow velocity around the point P with the ratio d/D proposed according to the invention, as is the case the Figures 3 and 4 was described.

Also for such a non-stackable protective device 1 according to Figure 5 , this results for the predetermined operating point with the The air flow L described results in a significant reduction in noise and therefore an acoustic improvement.

The implementation of the invention is not limited to the preferred exemplary embodiments given above. Rather, a number of variants are conceivable, which make use of the solution presented even in fundamentally different designs.

Claims (10)

Protective device (1) for arranging downstream of an impeller (2) which can be rotated about an axis of rotation (R) and has an impeller diameter (X), which is designed to generate an air flow (L) at a predetermined operating point, which turns the protective device (1) into a with the rotation axis (R) flows through a flow direction enclosing a flow angle α and has a flow velocity (u) that varies orthogonally to the flow direction, so that the protective device (1), viewed in a longitudinal section through the protective device (1), is orthogonal to the flow direction at a predetermined point (P ) is flowed through at a maximum flow rate, wherein the protective device (1) has a plurality of grid rings arranged around the axis of rotation (R), each of which is formed by a wire (10, 11, 12) with a wire diameter D running around the axis of rotation (R) in the circumferential direction, 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 in longitudinal section are arranged on a common imaginary straight line (30) which forms an angle β with the air flow (L). includes and along which the two wires are spaced apart from each other by a distance d, characterized 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 predetermined area (32) along the common curve (20) around the point (P) of maximum flow velocity less than 3 or greater than 4. Protective device according to claim 1,
wherein the maximum flow rate (u) flows through the protective device (1) in the area (32) predetermined along the common curve (20) and in particular in the middle of the predetermined area (32). Protective device according to claim 1 or 2,
wherein the area (32) predetermined along the common curve (20) extends ±10%, in particular ±5% of the impeller diameter (X) around the point (P) of maximum flow velocity. Protective device according to one of the preceding claims,
wherein the angle β is greater than 20°, in particular greater than 30°, at least in the predetermined area (32). Protective device according to 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 a second region (31, 33) adjacent along the common curve (20) depends on the ratio d/D in the predetermined region ( 32) differs and in particular is greater than 2.6 and less than 4. Protective device according to one of the preceding claims,
wherein the common curve (20) transitions from radially outside to radially inside from a steep first region (21) into a flat second region (22), so that several protective devices (1) can be stacked one inside the other. Protective device according to the preceding claim,
wherein the common curve (20) transitions radially inwards from the second area (22) into a third area (23), which is inclined towards the first area (21). Protective device according to one of the preceding claims,
wherein the wires (11, 12) in the area (32) predetermined along the common curve (20) have a relative surface roughness k/D in a range of 1% to 10%. Protective device according to one of the preceding claims,
wherein a surface of the wires (11, 12) is structured in the area (32) predetermined along the common curve (20) and in particular has projections which extend in the longitudinal direction of the wires (11, 12) and which further spiral in particular around the longitudinal direction . Protective device according to one of the preceding claims,
wherein the protective device (1) has a pre-grid upstream of the grid rings, which adjoins in the flow direction at least the area (32) predetermined along the common curve (20) and is in particular formed from at least one second wire, the diameter of which is smaller than the diameter of the wires (10, 11, 12) is the grid rings, and which in particular forms meshes, so that turbulent vortices can be generated in the air flow (L) upstream of the grid rings.

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