Silencer with skirt for compressed gas systems

The present invention relates to a silencer for systems dealing with compressed gas, e.g. systems working with compressed air, in particular for the use in commercial vehicles. For instance, pneumatic brakes require compressed air.

Systems working with compressed gas, for example compressed air systems, can release large amounts of compressed gas to the environment within short time. This depressurization of the gas can result to remarkable noises. However, noise emissions have to be kept below an admissible level, and hence, silencers for systems of compressed gas are usually necessary.

In commercial electric vehicles, the largest part of the noise is generated by the mixing of a high velocity air flow with ambient air.

In prior art, document WO 2019/063350 A1 is known, disclosing a noise damper for compressed air systems with an inlet channel and a chamber for receiving sounddamping material. In this case, the inlet channel and the chamber are separated perpendicular to the compressed air flow by a separation plate, which in an outer peripheral region of the compressed air flow has a plurality of openings in order to introduce compressed air into the chamber. Such noise damper has a dampening function for noises created by a flow of compressed air, e.g. in a valve releasing compressed air.

For electric commercial vehicles with a pneumatic brake system, a very low noise is of extreme importance. Presently, a valve assembly through which compressed air flows should fulfill the requirement of at most 72 dB(A) in 7 m distance, according to ISO R362 and EEC92/97 and ECE R51. However, for an electric commercial vehicle with pneumatic brakes (for which the electric motor is very silent compared to other engines), the noise value of at most 72 dB(A) in 7 m distance is still too high.

Hence, there is a need for novel silencers, which are able to reduce a noise emission to below 68 dB(A), or more preferably below 60 dB(A) within 7 m distance from the vehicle. In commercial electric vehicles, the largest part of the noise is generated by the mixing of a high velocity flow with ambient air. Such noise (and noise created by other circumstances) regularly propagates in all directions.

However, noise dampers according to prior art do not provide a solution to deal with noise if noise is already generated.

Hence, the technical problem of the present application can be seen in providing an improved silencer, which is able to reduce propagation of the noise leaving the silencer.

The technical problem of the present application is solved by a silencer according to claim 1 and the use of a silencer according to claim 16. Further advantageous embodiments of the present application are subject-matters of the dependent claims.

A silencer according to the present application for compressed air system comprises: A housing with an inlet opening for compressed air flow, a chamber, sound-damping means provided in the chamber, and a grid, comprising at least two passages through which the compressed air flow is adapted to leave the chamber.

Furthermore, the silencer comprises a skirt, which is arranged downstream of the grid, and the skirt is adapted to guide compressed gas leaving the passages of the grid. The diameter of the skirt is more preferably at least as big as the diameter of the grid, optimally the skirt and the grid have approximately the same or even the same diameter.

The skirt serves that if noise is already created, the propagation of noise is terminated in the radial direction, as noise waves are reflected back to the center of the flow, but cannot be heard by a person standing away from the silencer to a great extent. In other words, the skirt drives the generated noise to the direction of the flow instead to the direction of an observer and hence works as a barrier. Hence, the propagation of the noise in a radial direction (of the silencer / grid) is terminated, as the skirt reflects noise waves to the center of the flow of air leaving the grid, i.e. streamline the noise propagation.

If the silencer and the skirt are directed to the ground direction (i.e. direction directed to the ground), noise might be directed mainly to the ground, and not to an ear of an observer (e.g. in a distance of 7 m from the silencer).

Furthermore, the skirt has an effect on the mixing, as there is less creation of mixing energy. The skirt reduces the surface for mixing (of compressed air and ambient air, which usually has a cone shape around the compressed air, i.e. high velocity flow), and hence reduces the surface for turbulence creation. A skirt is hence effective in particular for cases where a noise already has been created.

Furthermore, the skirt is adapted to terminate the mixing between the high velocity flow and the ambient air in the zone where the jet is not completely developed, as many individual jets are created by the grid.

Preferably, the skirt has a cylindrical or polygonal, preferably hexagonal, cross-section, or a combination of different cross-sections. Such cross-section is most effective for achieving the effects of the skirt.

Preferably, the skirt has a constant cross-sectional area over its length, or a cross- sectional area increasing or decreasing over the length of the skirt in the downstream direction.

The cross-section again is the area through which an air stream flows when flowing through the skirt.

In the case of an increasing cross-sectional area over the length of the skirt, the skirt works as a diffuser. In case of a decreasing cross-sectional area over the length of the skirt, the skirt acts as a confuser. The shape can be adapted to the flow cones which form in the mixing zone of compressed and ambient air. Preferably, the skirt has a length between 2 and 200 mm. Preferred minimal lenghts are at least 5 mm, even more preferably at least 10 mm, and even more preferably at least 20 mm, most preferably at least 50 mm. Such lengths have been proven to be most effective for achieving the effects of the skirt.

Preferably, the cross-sectional area of the skirt at the inlet of the skirt is larger than the sum of the cross-sectional areas of all passages of the grid at their respective outlets. In other words: The overall cross-sectional area covered by skirt is larger than the overall cross-sectional area of all passages. In this case, the skirt can most effectively drive the generated noise in the direction of the flow. Further, this ensures that there is no increase of flow velocity in the skirt, which might lead to additional noise.

Preferably, the at least two passages have a length between 3 mm and 200 mm. Preferred minimal lengths are at least 4 mm, even more preferably at least 5 mm, even more preferably at least 7 mm, most preferably at least 10 mm. The passages with a length of at least 3 mm ensure a homogeneous flow of gas, and hence, the turbulence is reduced in the mixing region, where highly pressurized gas is mixed with ambient air. Hence, passages of at least 3 mm can effectively reduce the noise generated by the mixing of air, which flows out of the chamber and the sound-damping means. Passages of at least 3 mm hence contribute to avoid the noise creation by turbulent flows. The longer the passages are, the more the effect of the flow homogenization is.

Preferably, wherein the cross-sectional area of the at least two passages respectively decreases from the inlet of the passage until a portion with lowest diameter, and increases from the portion with lowest diameter towards the outlet of the passage.

The specific shape of the passages (cross-sectional area of the at least two passages decreasing from the inlet of the passage until a portion with the lowest diameter within the passage, and then again increasing from the portion with lowest diameter towards the outlet of the passage) mimics a nozzle, which usually has optimized flow characteristics. Such nozzle shape is able to decrease the velocity of the flow. The specific structure of the cross-sectional area of the at least two passages, which is a streamlined shape, reduces the separation vortices and the drag vortices, which might form after the grid. Such streamlined passages reduce the drag caused by the grid, and also reduce the flow resistance which affects the operation speed of e.g. a pneumatic valve. Furthermore, such streamlined passages can reduce the size and the intensity of the vortices behind the grid (which form as compressed air is mixed with ambient air), and hence reduce the noise energy created by the turbulent flow.

Therefore, an effective reduction of turbulence wakes (vortexes) is possible, in particular by reducing the turbulence intensity in the grid, the turbulence being formed by wakes created by the air separation through the grid.

Such passage length and shape contributes to direct the internally generated noise in the direction of the flow, instead of the direction of an observer (e.g. in a distance of 7 m).

Preferably, the effective surface ratio is between 60% and 90%, more preferably above 80%. The effective surface ratio is defined by the cross-sectional area covered by the passages (i.e. where flow through the grid can occur) divided by the overall cross- sectional area of the grid. A higher number of channels with smaller size has a higher rigidity. The optimal effective surface ratio depends on the available surface, the length of the passages, and the rigidity requirements

Preferably, the sound-damping means comprises a fibrous material, more preferably a polyethylene fiber mesh. Such fibrous material is adapted to damp the noise which is already generated, e.g. originating from a valve.

Preferably, the cross-section of the at least two passages is polygonal, more preferably hexagonal (i.e. forming a honeycomb structure). Hexagons can be divided by other shapes as well. For such hexagonal shape of the passages, the effective flow surface is best, as the residual area covered by the solid grid structure, where dead areas can emerge after leaving the passages, is minimal. Hence, there is an increased ratio from passage area to area covered by solid (i.e. the grid). Hence, such structure allows the highest ratio of the effective flow area over the total grid area. Therefore, the best efflux area can be used for the flow of compressed air over the grid area.

Alternatively, the cross-section of the at least two passages is round. Also with such shape, a very homogenous flow can be ensured.

Preferably, the portion of the lowest diameter (for example approximately in the middle of the length of the passages) has a cross-sectional area which is between 80% and 95% of the cross-sectional area of the inlet of the passage and/or the outlet of the passage, preferably above 85%.

Such geometrical constraints mimic the shape of a nozzle. By such shape of the channels, there is a decrease of the velocity of the flow of compressed gas. Such geometrical constraints lead to a low drag at the inlet, and a low wake at the outlet.

Furthermore, due to the nozzle shape, the mixing point I region (of pressurized and ambient gas) is towards from the grid, and the separation wakes decrease regarding their size. Further, the flow cores (velocity profile of flow leaving each channel of grid) get broader, leading to very homogenous flow with less turbulence, hence resulting in less noise.

In one embodiment, the inlet of the passage and the outlet of the passage have the same cross-sectional area, which leads to a homogenous flow. The cross-sectional area is the area through which the air flows in the inlet, the outlet or somewhere between them.

Preferably, the cross-sectional area of the passage is larger at the inlet of the passage than at the portion with the lowest diameter and the outside of the passage, the cross- sectional area of the passage is larger at the outlet of the passage than at the portion with the lowest diameter.

By such nozzle design, a very homogenous flow can be guaranteed.

In this case, the passages act as a confusor, focusing the flow at the outlet of the passage. Alternatively, the cross-sectional area of the passage is larger at the outlet of the passage than at the portion with a lowest diameter and at the inlet of the passage, wherein the cross-sectional area of the passages is larger at the inlet of the passage than at the portion with the lowest diameter.

Such design of the nozzle is a diffusor, which also leads to a very rapid pressure reduction, however without generating noise, in particular by moving the mixing zone (of pressurized air and ambient air) away from the grid.

An inventive use of a silencer is a use for a brake system, preferably a braking system of a commercial vehicle, more preferably a braking system of an electrical commercial vehicle.

In the following, advantageous embodiments of the present application are described more in detail with the attached figures.

Fig. 1 shows the silencers with skirts - in Fig. 1 a), the skirt acts as a confuser, and in Fig. 1 b), the skirt acts as a diffuser.

Fig. 2 shows flow velocity profiles when a skirt is used downstream of passages of a grid. Fig. 2 a) shows such flow profiles directly downstream of the grid, and Fig. 2 b) shows the development of the flow velocity profile within the whole skirt.

Fig. 3 shows a general structure of a silencer according to the present invention. Fig. 3 a) to 3 d) show different embodiments, in particular with regard to the shape of the passages.

Fig. 4 shows a structure of the grid with passages more in detail - Fig. 4 a) to 4 d) and Fig. 4 f) to 4 i) show different shapes of the passages, and in Fig. 4 e), an isometric view of a structure according to Fig. 4 d) is shown. Fig. 5 shows the physical background of flow resistance of objects with different shapes.

Fig. 6 shows flow velocity profiles, wakes and vortices, Fig. 6 a) herein shows a silencer according to prior art, and Fig. 6 b) and 6 c) show the influence of the passage length and shape on the flows.

Fig. 1 shows a silencer 1 comprising a skirt 8 downstream of a grid 6. The silencer 1 comprises a housing 2 with an inlet opening 3, through which compressed air can flow in. Within the silencer 1 , there is a chamber 4, which is filled with a sound-damping means 5, in this case a fibrous material. In the end of the silencer 1 in a flow direction, there is a grid 6, which has several passages 7. In Fig. 1 a), the skirt has a decreasing cross-sectional area (and diameter) in the flow direction and works as a condenser, and in Fig. 1 b), the skirt has an increasing cross-sectional area (and diameter) in the flow direction, hence working as a diffuser.

Fig. 2 shows flow velocity profiles when a skirt 8 is used downstream of passages 7 of a grid 6.

Fig. 2 a) shows such flow profiles directly downstream of the grid 6. Herein, it can be seen that individual, cone-shaped flow cores form at the outlet of the passages 7 of the grid. Between these flow cores, wake separation occurs. The skirt 8 acts as a barrier, and reflects noise to the middle of the flow. Little noise can reach and observer or receiver (e.g. a microphone) spaced apart radially form the skirt 8.

Fig. 2 b) shows the development of the flow velocity profile within the whole skirt 8. Herein, it can be seen that the individual flow cores form a developed flow core downstream of the skirt 8. In this region, there are no turbulences any more.

Little noise can reach and observer or receiver (e.g. a microphone) spaced apart radially form the skirt 8.

In Fig. 3, the general structure of a silencer 1 according to the present invention is described. The silencer 1 comprises a housing 2 with an inlet opening 3, through which compressed air can flow in. Within the silencer 1 , there is a chamber 4, which is filled with a sound-damping means 5, in this case a fibrous material. In the end of the silencer 1 in a flow direction, there is a grid 6, which has several passages 7. Each passage 7 has an inlet 7a, a portion with the lowest diameter 7b, and an outlet 7c. The cross- sectional area decreases from the inlet 7a until the portion with the lowest diameter 7b, and then increases again until the outlet 7c.

In Fig. 3 a), the passages 7 have a polygonal shape. The cross-section of walls 7d between the passages 7 has a rhomboid shape.

In Fig. 3 b), the passages 7 have a round shape, wherein the cross-sectional area (and the diameter) of the inlet 7a and the outlet 7c of the passage 7 are about the same. The cross-section of the walls 7d between the passages 7 has a quasi-oval shape.

Fig. 3 c) shows an embodiment where the cross-sectional area (and the diameter) of an inlet 7a of the passage 7 is larger than the cross-sectional area (and the diameter) of the portion 7b with the lowest diameter and the outlet 7c of the passage 7. Herein, the cross-sectional area (and the diameter) of the outlet 7b of the passage 7 is greater compared to the portion 7b with the lowest diameter of the passage 7. The crosssection of the walls between the passages 7 has a shape of an elongated drop.

In Fig. 3 d), an embodiment is shown where the cross-sectional area (and the diameter) of an outlet 7c of the passage 7 is larger than the cross-sectional area (and the diameter) of the portion 7b with the lowest diameter and the inlet 7a of the passage 7. Herein, the cross-sectional area (and the diameter) of the inlet 7a of the passage 7 is larger compared to the portion 7b with the lowest diameter of the passage 7. The crosssection of the walls between the passages 7 has a carrot shape.

Fig. 4 a) shows a top view on a grid 6 with several passages 7a, which have a hexagonal shape. In Fig. 4 b), the same structure is shown, however the hexagons are larger compared to Fig. 4 a). In Fig. 4 c), even larger passages 7a (still hexagonal, but higher cross-sectional area) are shown. Fig. 4 d) shows a structure where the walls of the passages are tapered, but still have a hexagonal shape. Fig. 2 e) shows an isometric view of a grid 6 according to Fig. 4 d), again with hexagonal passages 7. The walls 7d separating the passages 7 are very thin in this embodiment (e.g. between 1 and 4 mm). A thickness below 1 mm would cause insufficient structural strength, while a thickness above 4 mm would reduce the cross-sectional area available for the flow too much. Fig. 4 f) shows a grid 6 with rhomboid-shaped cross-sectional areas of the passages 7, also maximizing the cross-sectional area available for the flow. Fig. 4 g) and 4 h) show a grid 6 with triangle-shaped cross-sectional areas of the passages 7, also maximizing the cross-sectional area available for the flow. Fig. 4 i) shows a grid 6 with the arrangement of passages 7 as a star. In this case, pentagon-shaped cross-sectional areas of the passages 7 are used.

Fig. 5 shows the physical backgrounds of flow resistances objects 0 with a different shape. In Fig. 5 a), there is a thin wall, and it can be seen that there are dead volumes after the object 0, and also vortices are generated. In Fig. 5 b), the object 0 has the shape of a ball or a sphere, it can be seen that there are still dead volumes, but less vortices are produced compared to Fig. 5 a). In Fig. 5 c), the object 0 has a combined shape, it is a drop shape (the broader part facing the flow direction). It can be seen that there are much less dead volumes and vortices compared to Fig. 5 a) and 5 b). In Fig. 5 d), a very thin object is shown, which is elongated in the flow direction. It can be seen that there are almost no dead volumes, and no vortices are generated. Hence, the flow resistance is minimal in Fig. 5 d).

Fig. 6 a) shows a flow through passages 7 of a grid 6 of a silencer according to prior art. Fluid can flow through a sound-damping means 5, and can be seen that before reaching the grid 6 with the passages 7, drags are generated. As the grid 6 and hence the channel 7 are very thin, vortices are created within the flow in each channel 7, and the merging zone is directly downstream of the grid 6. In the merging zone, compressed and ambient air are mixed.

The velocity profiles of the flows having left the passages 7 have a cone shape. Between the cones, a wake separation occurs. This leads to vortices and hence a high turbulence. Noise is generated by the mixing of compressed and ambient air.

In Fig. 6 b), a thicker grid 7 (compared to Fig. 4 a)) is shown. It shows that there are less and smaller drags, and less vortices within the flows. Still, there is a wake separation in the merging zone, but the noise can be reduced. Fig. 6 c) shows an elongated grid 6 (similar to Fig. 6 b)), however with a shape where the cross-sectional area of the passages decreases from the inlet to the section with the lowest diameter, but then again increases - a structure like a nozzle. It can be seen that there are even less drags in this embodiment, there is a little wake separation in the merging zone, but it can be seen that the flow cores are very broad, and hence less turbulence is generated, leading to almost no noise.

LIST OF REFERENCE SIGNS

1 Silencer

2 Housing

3 Inlet opening

4 Chamber

5 Sound damping means

6 Grid

7 Passage

7a Inlet of passage

7b Portion with lowest diameter

7c Outlet of passage

7d Wall of passage

8 Skirt

0 Object

Braking system