WO2019158451A1

WO2019158451A1 - Acoustic damper for a gas duct system

Info

Publication number: WO2019158451A1
Application number: PCT/EP2019/053227
Authority: WO
Inventors: Lin Zhou
Original assignee: Creo Dynamics Ab
Priority date: 2018-02-13
Filing date: 2019-02-11
Publication date: 2019-08-22
Also published as: SE544189C2; SE1850154A1; EP3753009A1

Abstract

An acoustic damper (100) for a gas duct system (20), comprises a housing (1) comprising a first wall (2) and a second wall (3) opposite to and spaced from the first wall (2) so as to at least partly define a housing volume there between. The acoustic damper (100) comprises a slot (4) in the first wall (2), the slot (4) providing an elongate opening in the first wall (2) and being connected with a channel (4') extending from the slot (4) towards the second wall (3). At least one Helmholtz resonator (5, 5', 5") is arranged in the housing, comprising an enclosed resonator volume (6, 6', 6"), and at least one neck (7, 7', 7") having a first end (7a, 7'a, 7"a) and a second end (7b, 7'b, 7"b). The first end (7a, 7'a, 7"a) opens into the Helmholtz resonator volume (6, 6', 6") and the second end (7b, 7'b, 7"b) opens into the channel (4').

Description

ACOUSTIC DAMPER FOR A GAS DUCT SYSTEM

Technical Field

The present disclosure relates to an acoustic damper for a gas duct system, a gas duct damping system comprising the acoustic damper and to various systems comprising the gas duct damping system.

Background of the Invention

In a typical duct or pipe system, such as an intake/exhaust system in a vehicle, a vehicle air conditioning system, a pipe line in a heating, ventilation and air conditioning (HVAC) system, transportation and exchange of gas is performed with a power unit. Examples of power units are engines, turbochargers, turbines, and compressors. Such power units inherently generate transient pulsating gas flows and noise in the duct/pipe system. Noise from the power unit could propagate with the gas on the downstream side (exhaust side) or against the gas on the upstream side (intake side) of the system.

To decrease the noise level various kinds of mufflers have been designed and implemented in different duct/pipe systems. Mufflers may be applied in the intake and exhaust system of a vehicle engine, as silencers in HVAC systems, as acoustic liners in aero engines, etc.

Expansion chambers and perforated tubes have been widely used in muffler systems in vehicles. Such mufflers cause a large unwanted pressure drop in the duct/pipe. In HVAC systems porous material dampers are commonly used. A large muffler material volume is then needed in order to decrease the noise level, especially for decreasing noise of lower

frequencies. Acoustic liners with honeycomb structure are often used for aero engine noise control. For such liners the thickness of the liner should be one quarter of the wave length of the frequency to be dampened, and, hence, larger liner thicknesses are needed to dampen lower frequencies.

There is, hence, a need for an acoustic damper with good low frequency damping performance, and which damper is compact in volume and causes a low pressure drop of a gas transported in the duct/pipe system at which the damper is arranged.

Summary of the Invention

It is an object of the present disclosure to provide an acoustic damper for a gas duct system with improved low frequency damping performance, a gas duct damping system comprising a duct portion and the acoustic damper and various systems comprising the gas duct damping system.

The invention is defined by the appended independent claims.

Embodiments are set forth in the dependent claims, in the attached drawings and in the following description.

According to a first aspect there is provided an acoustic damper for a gas duct system, comprising a housing enclosing a housing volume, the housing comprising a first wall and a second wall opposite to and spaced from the first wall so as to at least partly define the housing volume there between. The acoustic damper comprises at least one slot in the first wall, the slot providing an elongate opening in the first wall and being connected with a channel which extends from the slot into the enclosed housing volume towards the second wall. The damper also comprises at least one Helmholtz resonator arranged in the housing, which Helmholtz resonator comprises an enclosed resonator volume, and at least one neck having a first end and a second end, wherein the first end opens into the Helmholtz resonator volume and the second end opens into the channel.

The acoustic damper may be used for a gas duct system which, typically, is designed to transport and exchange gas. Examples of such gas duct systems are intake and exhaust systems in vehicles, aero engines, vehicle air conditioning systems, pipe lines in heating, ventilation and air conditioning (HVAC) systems, or other similar duct systems for gas transportation. The gas transportation is performed with a power unit, e.g. an engine, a turbocharger, a turbine, or a compressor. These power units inherently generate transient pulsating flows and noise in the gas duct system.

The gas may be air, or any other gas transported in the gas duct system.

The first and second walls of the damper housing could completely define the housing volume there between. Alternatively, additional walls such as one or more side walls connected to the first and second walls are needed to completely define the housing volume.

The slot in the first wall is connected with a channel which extends from the slot into the enclosed housing volume towards the second wall.

There is, hence, no slot in the second wall and the channel does not extend through the second wall of the damper.

The shape of the slot opening in the first wall could e.g. be rectangular, elliptical or rectangular with rounded corners.

The number of slots in the first wall is strongly dependent on the size of the acoustic damper and may for example be 1 -20.

When there is more than one slot, each corresponding channel has its own Helmholtz resonator(s) connected thereto, i.e. a Helmholtz resonator may not be shared by adjacent channels.

A cross section of the housing could be of any shape, such as squared, rectangular, triangular, tube-shaped having an inner cross section being squared, rectangular, elliptical, circular, be of semi-tube shape, be a circular segment or an elliptical segment. The shape of the damper should be adapted to the position in the gas duct system at which the acoustic damper is to be arranged. A duct portion of the duct system at which the acoustic damper may be arranged could e.g. be squared and a squared acoustic damper could be arranged on one or more sides of the duct. The acoustic damper could be arranged as a tube around the duct portion, having an inner cross-section matching the outer dimensions of the duct portion, or be arranged as a semi-tube around the duct portion.

The acoustic damper may be arranged to be in contact with a wall of the duct portion of the gas duct system where sound damping is wanted, such that there is a fluid connection between the gas flow in the duct portion and the Helmholtz resonator(s) connected to the slot of the acoustic damper. Alternatively, the first wall of the damper may constitute at least a portion of a wall of a duct portion of the gas duct system, i.e. be integrated with the wall of the duct portion.

A Helmholtz resonator is an acoustic filter element and may be seen as a mass on a spring. The volume of gas in the resonator volume is the spring and the gas in the neck, i.e. a hollow tube, is the oscillating mass. When installed with its first wall in contact with an outer wall of the duct portion or with its first wall as an integrated part of an outer wall of the duct portion pulsating flows and noise in the duct system causes gas to be pumped in and out of the Helmholtz resonator volume(s) via the slot(s) and corresponding channels and via the at least one resonator neck, which gas movement causes friction between the gas and the walls of the neck(s) and the walls of the channel. In the end, flow pulsation is damped by transferring its dynamic energy into heat through the friction.

The frequency range over which Helmholtz resonators are effective is relatively narrow and they need to be tuned to the noise source to achieve significant attenuation. Parameters of the acoustic damper, i.e. slot/channel width and the resonator frequency/frequencies of the Helmholtz resonator(s), may be tuned and optimized for different damping applications.

A resonance frequency of a Helmholtz resonator may be described by the formula f₀ * where c is the sound speed in the gas, A is the

opening area of the second end of the neck, L is the length of the neck, and V is the volume of the Helmholtz resonator volume.

The present acoustic damper may be arranged to have a compact geometry, i.e. have a small thickness (distance between first and second walls of the damper), even when the damper comprises more than one Helmholtz resonator, as one channel may be arranged to be connected with one or more Helmholtz resonators arranged in the housing volume. In comparison, traditional porous dampers and liners with honeycomb structures need large volumes/thicknesses to decrease the noise level, especially for dampening of lower frequencies. The damper may be arranged to give pulsation damping in multi- frequencies or a broad band frequency, i.e. at least one Helmholtz resonator may be arranged per frequency to be dampened.

The material of the walls of the damper, the walls of the resonator volume and the walls of the neck may e.g. be plastics (such as polyvinyl chloride (PVC)), a metal (such as aluminum), a metal alloy, or combinations thereof. The choice of material(s) depends on the intended application of the acoustic damper.

The volume of the Helmholtz resonator is empty.

The acoustic damper may be produced by using 3D printing. Other production methods, such as different cutting methods, may be used. The production method is dependent on the material(s) in which the damper is to be produced and the intended application of the acoustic damper.

A cross-sectional area and/or shape of the channel in each cross- section taken along the extent of the channel may be substantially equal to the slot opening in the first wall.

A cross section is understood as a cross section taken in a plane that is perpendicular to a flow direction of the channel. The term“substantially equal to” should be understood as allowing some variation, e.g. due to manufacturing tolerances. Such variation may amount to less than 5 % of any dimension of the cross section, preferably less than 1 % of said dimension.

At least one cross-sectional area and/or shape of the channel in a set of cross-sections taken along the extent of the channel may differ from the other cross-sectional areas.

The width of a channel may be thinner or wider at some portions. The channel may be straight or curved.

Alternatively, all cross-sectional areas of the channel in a set of cross- sections taken along the extent of the channel may be substantially the same.

As above, the term“substantially the same” should be understood as allowing some variation, e.g. due to manufacturing tolerances. The damper may comprise at least two slots with associated channels arranged in the damper, and the channels may extend from the first wall into the enclosed volume in directions substantially parallel with each other.

The damper may comprise at least two slots with associated channels arranged in the damper, wherein at least one of the slots has a direction of extension in the first wall being non-parallel with the direction of extension of another slot in the first wall.

Non-parallel here meaning having a direction of extension which forms an acute angle or an obtuse angle to the direction of extension of another slot in the first wall, or which is perpendicular to the direction of extension of another slot in the first wall.

The channel may extend from the slot towards the second wall a distance of 40%-100%, of 50%-100%, 60-100%, 70-100%, 80-100% or 90- 100% of a distance between the first and second wall.

The channel may extend perpendicularly or radially from the first wall into the enclosed housing volume towards the second wall.

For example, if the damper is tube shaped having a circular inner cross section the channel may extend radially from the slot in the first, inner, wall into the enclosed housing volume. If the first wall of the damper is planar, e.g. for a damper having a square cross section, the channel may extend perpendicularly from the slot in the first wall into the housing volume.

The channel may be connected with 1 -10, 1 -5 or 1-3 Helmholtz resonators.

The acoustic damper may comprise two or more Helmholtz resonators, wherein at least two of the resonators may provide the same resonance frequency.

With same resonance frequency is here meant that the resonance frequency of different Helmholtz resonators used in an acoustic damper differ at most with 5%, or at most with 2%.

Alternatively, when the acoustic damper comprises two or more

Helmholtz resonators, at least one of the resonators may provide a resonance frequency which is different from the other resonator(s). E.g. one resonator may have a resonance frequency of 400 Hz and another may have a resonance frequency of 800 Hz.

At least one channel may be connected with two or more Helmholtz resonators, wherein the second end of the neck of a Helmholtz resonator having the lowest resonance frequency may be arranged to open into the channel at a distance further away, as seen in a direction along the channel, from the first wall than the second end of the neck of a Helmholtz resonator having a higher resonance frequency.

By arranging a Helmholtz resonator with lower resonance frequency to open into the channel further away than a Helmholtz resonator with a higher resonance frequency, an acoustic damper for damping low frequency noise can be made more compact. The channel may be seen as an extension of the neck of a Helmholtz resonator. By arranging the Helmholtz resonator with lower resonance frequency to open into the channel further away from the first wall it is possible to utilize the channel as an extension of the neck, thereby achieving a lower resonance frequency for the Helmholtz resonator.

The neck of a Helmholtz resonator may be straight, straight comprising at least one bend, curved, coiled or any combination thereof.

A neck does not have to be straight. It could comprise at least one bend or it could be curved or coiled in order to obtain the required resonance frequency. A bend may be perpendicular to the major extension of the neck or be of any other angle. The number of bends may be 1 -10, or 1 -5.

A total length of the neck may be 1 -100 mm, 10-100 mm, 20-100 mm, 30-100 mm, 40-100 mm, 50-100 mm, 60-100 mm, 70-100 mm, 80-100 mm, 90-100 mm, 1-10 mm, 1 -20 mm, 1 -30 mm, 1 -40 mm, 1 -50 mm, 1 -60 mm, 1 - 70 mm, 1-80 mm, or 1 -90 mm.

The neck may extend into the resonator volume a distance of 50-99%, 60-99%, 70-99%, 80-99%, 90-99%, 50-90%, 50-80%, 50-70%, or 50-60% of a total length of the neck.

Each Helmholtz resonator may have 1-5 necks, preferably 1-2 necks.

The necks of one Helmholtz resonator could be the same or could be different from each other in length, width, shape etc. A cross section of an opening area of the second end of the neck may be square, rectangular, triangular, circular or elliptical.

The Helmholtz resonator may be provided with a hole in the second wall, thereby connecting the housing volume with the outside.

In some applications it may be of importance to avoid water or oil accumulation in the housing volume. With the use of the holes in the second wall, problems related to such accumulation may be avoided.

A largest area of the hole may be smaller than (an) opening area(s) of the second end(s) of the neck(s) of the Helmholtz resonator(s).

Preferably, the hole opening area is less than 10 % of the cross section of the second end of the neck, preferably less than 5 % of the neck cross section or less than 1 % of the neck cross section.

A cross-section of the enclosed resonator volume taken where the first end opens into the Helmholtz resonator volume and coinciding with the direction of extent of the channel may be square, rectangular, triangular, circular, elliptical, semi-circular, semi-elliptical, a circular segment, or an elliptical segment.

According to a second aspect there is provided a gas duct damping system comprising a duct portion having a duct wall extending between ends of the duct portion and defining a gas flow path, and an acoustic damper as described above, wherein a duct wall slot is provided in the duct wall and connected to the channel, such that the Helmholtz resonator(s) is (are) in fluid connection with the gas flow path.

There is no restriction of the flow area of the gas flow path caused by the acoustic damper, as compared to if traditional expansion chambers or perforated tubes are used as dampers. Thereby, there is a near zero mean flow pressure drop in the duct portion and the impact of the present acoustic damper on the performance of a system comprising the duct portion and acoustic damper is low.

The duct portion could e.g. be squared and a squared acoustic damper could be arranged on one or more sides of the duct. The acoustic damper could be arranged as a tube around the duct, having an inner cross-section matching the outer dimensions of the duct, or be arranged as a semi-tube around the duct. The first wall of the damper could constitute a portion the duct wall, i.e. be integrated with the duct wall, or the duct portion and the acoustic damper could be two separate units.

The duct wall slot may extend perpendicularly to the gas flow path in the duct portion.

The duct wall slot may extend parallel to the gas flow path in the duct portion.

Ducts having two or more duct wall slots could be provided with at least one slot extending parallel to the gas flow path in the duct portion and at least one slot extending perpendicularly to the gas flow path in the duct portion, or the slots could all be arranged to extend in the same direction.

The first wall of the acoustic damper may constitute at least a portion of the duct wall of the duct portion.

That is the first wall of the damper may be an integrated part of the duct wall of the duct portion and the duct wall slot and the slot in the first wall of the damper may be the very same slot.

The slot in the first wall of the acoustic damper may alternatively be arranged in fluid connection with the duct wall slot of the duct portion.

That is the first wall of the damper and the duct wall of the duct portion are separate walls arranged in contact with each other such that the slots thereof are in fluid connection.

According to a third aspect there is provided an air conditioning system comprising a compressor and at least one gas duct damping system described above, wherein the duct portion is connected upstream and/or downstream of the compressor.

Such an air conditioning system may for example be used to cool or heat rooms or other spaces like a vehicle cabin.

According to a fourth aspect there is provided an engine system comprising an engine and at least one gas duct damping system described above, wherein the duct portion is connected upstream and/or downstream of the engine.

The engine system may be an internal engine combustion system. According to a fifth aspect there is provided a fan system comprising a fan and at least one gas duct damping system described above, wherein the duct portion is connected to a gas intake duct and/or gas exhaust duct.

According to a sixth aspect there is provided a turbine engine system comprising a turbine engine and the acoustic damper described above, wherein the acoustic damper is arranged at a wall of a bypass duct and/or at a wall of a bypass nozzle and/or at a wall of an exhaust nozzle.

Brief Description of the Drawings

Figs 1 a and 1 b show the installation of acoustic dampers at gas duct portions having circular cross section and rectangular cross section, respectively.

Fig. 2 shows a longitudinal cross section of the duct portion and acoustic damper of Fig. 1 a or Fig. 1 b.

Fig. 3 shows a cross-section of an example of an acoustic damper in 2D. The damper being provided with a slot and associated channel, three Flelmholtz volumes and thereto connected necks.

Figs 4, 5 and 6 show acoustic dampers with different structures in 3D.

Fig. 7 shows the transmission loss for an acoustic damper arranged at a small and medium sized duct portion.

Fig. 8 shows the absorption coefficient for an acoustic damper arranged at a medium and large sized duct portion.

Fig. 9 illustrates a process for optimizing parameters of an acoustic damper.

Fig. 10 is a graph showing the transmission loss for three different acoustic dampers.

Fig. 11 is a graph showing the absorption coefficient for three different acoustic dampers.

Fig. 12 shows an air conditioning system with a gas duct damping system arranged before and after a compressor.

Fig. 13 shows an engine system with a gas duct damping system arranged at the intake and exhaust ducts. Fig. 14 shows a cross sectional view of a fan system with a gas duct damping system arranged after the intake duct and before the exhaust duct.

Fig. 15 shows a turbine engine comprising an acoustic damper arranged at a wall of a bypass duct, and/or at the wall of a bypass nozzle and/or at the wall of an exhaust nozzle.

Detailed Description of the Drawings

Figs 1 a and 1 b show gas duct damping systems 300, wherein acoustic dampers 100 are installed at duct portions 200 of a gas duct system, the gas duct portions 200 having different cross section, circular and rectangular, respectively. In Fig. 2 is a cross section of any of the systems in Fig. 1 a or Fig. 1 b. The duct portion 200 has a first and second end 201 a, 201 b and define a gas flow path in the duct portion 200. The acoustic damper 100 may be used for a gas duct system such as intake and exhaust systems in vehicles, aero engines, vehicle air conditioning systems, pipe lines in heating, ventilation and air conditioning (FIVAC) systems, or other similar duct systems for gas transportation. The gas transportation is performed with a power unit, e.g. an engine, a turbocharger, a turbine, or a compressor. These power units inherently generate transient pulsating flows and noise in the duct portion of a gas duct system. Noise from the power unit could propagate with the gas on the downstream side (exhaust side) or against the gas on the upstream side (intake side) of the system. The acoustic damper 100 may decrease the noise level.

The acoustic damper 100 shown in the figures comprises a housing 1 enclosing a housing volume. The housing comprises a first wall 2 and a second wall 3 opposite to and spaced from the first wall 2 so as to at least partly define the housing volume there between. The acoustic damper 100 may be provided with one slot 4 on the first wall 2, as shown in Fig. 3, or with several slots, as shown in the 3D cross sections in Figs 4-6. A slot 4 provides an elongate opening in the first wall 2 and is connected with a channel 4^' which extends from the slot 4 into the enclosed volume of the housing towards the second wall 3. A width of the slot 4 in the first wall 2 may be 0.01 - 2 mm and a ratio between a total slot 4 opening area in the first wall 2 and a total area of the first wall 2 may be 0.1 -10%. The number of slots 4 in the first wall 2 and thereto associated channels 4^' largely depend on the intended application and the size of the damper and the area of the first wall 2.

The channels 4^' connected with the slots 4 may extend from the first wall 2 into the enclosed volume in a parallel fashion, as shown in Figs 4-6. In Fig. 6 is further shown one slot 4 which is arranged to extend in a direction in the first wall 2 which is substantially vertical to the direction of extension of the other slots 4 in the first wall 2. Other angels may also be possible, i.e. acute or obtuse angels.

Channels 4^' may extend into the volume of the acoustic damper in a direction which is substantially vertical to the first wall 2, as shown in Figs 2-6. The channel 4^' may extend from the first wall 2 towards the second wall 3 a distance of 40%-100% of a distance between the first 2 and second wall 3. In the figures the channel 4^' extends at least 90% of such a distance.

In the figures a cross-sectional area and/or shape of the channel 4^' in each cross-section taken along the extent of the channel is substantially equal to the slot 4 opening in the first wall 2. Alternatively, not illustrated, at least one cross-sectional area and/or shape of the channel 4^' in a set of cross-sections taken along the extent of the channel 4^' differs from the other cross-sectional areas.

The channels 4^' in e.g. Figs 4 and 5 extend perpendicularly from the first wall 2 into the enclosed housing volume towards the second wall 3. The channels in Fig. 6 extend radially from the first wall 2 into the enclosed housing volume towards the second wall 3.

The acoustic damper 100 also comprises at least one Flelmholtz resonator 5, 5^', 5^", 5^"'. The Flelmholtz resonators are arranged in the housing of the damper 100. A Flelmholtz resonator comprises an enclosed resonator volume 6, 6^', 6^", 6^"' and at least one neck 7, 7^', 7^", 7^"', i.e. a hollow tube. The neck has a first end 7a, 7a^', 7a^", 7a^{' "} and a second end 7b, 7b^', 7b^", 7b^{' "}. The first neck end 7a, 7a^', 7a^", 7a^'" opens into the Flelmholtz resonator volume 6, 6^', 6^", 6^{' "} and the second neck end 7b, 7b^', 7b^", 7b^{' "} opens into the channel 4^'. Each channel 4^' may be associated with 1 -10, 1 -5 or 1-3 Flelmholtz resonators. In Fig. 3 a channel 4^' is connected with three different Helmholtz resonators 5, 5^', 5^”. In Fig. 5 one channel 4^' is connected with four different Helmholtz resonators. As seen in the figures, a Helmholtz resonator 5, 5^', 5^", 5^{" '} may not be shared by adjacent channels 4^'.

The acoustic damper may be provided with inner walls 50, Figs 4 and 5, to support the damper structure. Such inner walls 50 do not, however, effect the resonance frequencies or the damping properties of the acoustic damper 100.

The acoustic damper 100 may be arranged such that the slot 4 and channel 4^' in the first wall 2 of the damper 100 is in fluid contact with a duct wall slot 204 of the duct portion 200. Alternatively, the first wall 2 of the acoustic damper 100 may constitute at least a portion of the duct wall 202 of the duct portion 200 and the slot 4 in the damper is the same slot as the slot in the duct wall 202. In both embodiments, the Helmholtz resonator(s) 5, 5^', 5^”, 5^'" are in fluid connection with the gas flow path in the duct portion 200.

The duct wall slot 204 in the duct wall 202 may extend perpendicularly to the gas flow path in the duct portion 200, see Fig. 5, or may extend parallel with the gas flow path in the duct portion 200. In Fig. 6 is shown two duct wall slots 204 arranged perpendicular to the gas flow path and one duct wall slot 204 arranged perpendicular to the gas flow path in the duct portion 200.

When the first wall 2 of the damper is not a part of the duct wall 204, the dimensions of the duct wall slot 204 and the slot 4 in the first wall 2 of the damper 100 should preferably match when the damper is installed at the duct portion.

Pulsating flows and noise in the duct system 200 causes gas to be pumped in and out of the Helmholtz resonators 5, 5^', 5^", 5^"' through the slots 204, 4 and the channels 4^' connected thereto into the resonator neck 7, 7^', 7^", 7^"' and into the resonator volume 6, 6, 6^", 6^"'. The gas movement causes friction between the gas and the walls of the neck 7, 7^', 7^", 7^{" '} and the walls of the channel 4^', and flow pulsation is damped by transferring its dynamic energy into heat through the friction. A resonance frequency of a Helmholtz resonator 5, 5^', 5 ^", 5^{" '} may be described by the formula f₀ where c is sound speed in the gas, A is

the opening area of the second end 7b, 7b^', 7b^", 7b^'" of the neck, L is the length of the neck 7, 7^', 7^", 7^'", and V is the volume of the Helmholtz resonator volume 6, 6^', 6^", 6^{' "}. The frequency range over which Helmholtz resonators are effective is relatively narrow and need to be tuned to the noise source to achieve significant attenuation.

Typical resonance frequencies for the Helmholtz resonators 5, 5^', 5^", 5^'" of the present acoustic damper may range between 400 Hz and 800 Hz. The acoustic damper 100 may be tuned and optimized for certain frequencies by varying the slot 4/channel 4^' width and the resonance

frequency/frequencies of the Helmholtz resonator(s) 5, 5^', 5^", 5^{' "}.

When the acoustic damper 100 comprises two or more Helmholtz resonators 5, 5^', 5^", 5^'" at least two or all of the resonators may be arranged to have the same resonance frequency. Alternatively, the Helmholtz resonators 5, 5^', 5^", 5^{' "} may have different resonance frequencies

attenuating different noise frequencies.

When one channel 4^' is connected with two or more Helmholtz resonators 5, 5^', 5^", 5^'" as shown in Figs 4-6, the second end 7b^', 7b^', 7b^", 7b^'" of the neck 7, 7^', 7^", 7^{' "} of a Helmholtz resonator 5, 5^', 5^", 5^{' "} having the lowest resonance frequency may be arranged to open into the channel 4^' at a distance further away, as seen in a direction along the channel 4^' from the first wall 2, than the second end of the neck 7b, 7b^', 7b^", 7b^'" of a Helmholtz resonator 5, 5^', 5^", 5^{' "} having a higher resonance frequency.

As seen in Figs 4-6 the second end 7b, 7b^', 7b^", 7b^{' "} of the neck of a Helmholtz resonator 5, 5^', 5^", 5^{' "} may be arranged to extend substantially orthogonally to the direction of extension of the channel 4^'.

As shown for example in Figs 3-5 the neck 7, 7^', 7^" of a Helmholtz resonator 5, 5^', 5^" may be straight or straight comprising a plurality of straight bends, see e.g. Fig. 3. Alternatively, or additionally, the neck 7^{' "} may be curved, Fig. 6, or coiled. The shape of the neck could be adapted to obtain a Helmholtz resonator with a resonance frequency of interest. Each Helmholtz resonator 5, 5^', 5^”, 5^'" may have 1 -5 necks. As shown in e.g. Fig. 3, one Helmholtz resonator 5^" may have two separate necks. The necks 7, 7^', 7^", 7^"' of one Helmholtz resonator 5, 5^', 5^", 5^{" '} could be the same or could be different from each other in length, width, shape etc.

A length of the neck may be 1-100 mm and could extend into the resonator volume 6, 6^', 6^", 6^"' 50-99% of a total length of the neck. In Fig. 3 the neck 7 of the resonator 5 to the right in the figure is longer than the necks 7^', 7^" of the resonators 5^', 5^" seen to the left in the figure.

A cross-section of the enclosed resonator volume 6, 6^', 6^", 6^{" '} taken where the first end 7a, 7^'a, 7^"a, 7a^'" opens into the Helmholtz resonator volume 6, 6^', 6^", 6^{" '} and coinciding with the direction of extent of the channel 4^' may be square, rectangular, triangular, circular, elliptical, semi- circular, semi-elliptical, a circular segment, or an elliptical segment.

As seen in Figs 1a, 1 b, 2 and 4-6 the shape of the acoustic damper 100 should be adapted to the shape of the duct portion 200. A cross-section of the duct portion 200 could e.g. be square or rectangular as shown in Fig.

1 b, 4 and 5 or elliptical or circular as shown in Figs 1 a and 6.

Acoustic dampers 100 could be arranged at one side of the duct portion 200 as a separate unit or as an integrated part, Figs 4 and 5, or be arranged at two or more sides of the duct portion 200. The acoustic damper 100 could be arranged as a tube or semi-tube around the duct portion as a separate unit or as an integrated part, Figs 1 a, 1 b and Fig. 6.

As the first wall 2 of the damper 100 is provided in contact with the duct wall 202 or is arranged as a portion of the duct wall 202 there is no restriction of the flow area of the gas flow path caused by the acoustic damper 100. Thereby, there is a near zero mean flow pressure drop in the duct portion 200 and the impact of the acoustic damper 100 on the performance of a system comprising the duct portion 200 and damper 100 is low.

The Helmholtz resonator 5^{" '} may be provided with one or more holes 9 in the second wall 3, Fig. 6, connecting the housing volume with the outside. In some applications it may be of importance to avoid water or oil accumulation in the housing volume. With the use of the hole(s) 9 in the second wall 3, problems related to such accumulation may be avoided.

The hole 9 area may be smaller than (an) opening area(s) of the second end(s) 7b^{' "} of the neck(s) of the Helmholtz resonator(s) 5^"'.

In Fig. 7 is illustrated the transmission loss, TL, for an acoustic damper 100 installed at a small or medium sized duct portion 200. Small and medium sized duct portions here meaning e.g. duct portions in air conditioning systems or intake or exhaust ducts of engines in vehicles. The transmission loss defines how much energy can be stopped by arranging a certain acoustic damper at a duct portion. An acoustic wave (noise) Pi from a noise source, e.g. from an engine, compressor, fan or the like, is partly reflected P_R because of the application of the acoustic damper 100, and only a portion of the acoustic wave P_T is transmitted downstream of the acoustic damper. The transmission loss may be expressed as

P_R

TL = log_{j 0}(— )

In Fig. 8 is illustrated the absorption coefficient, a, for an acoustic damper 100 installed at a medium and large sized duct portion 200. Medium and large sized duct portions here meaning e.g. a bypass duct in an aero engine, or a duct system in a HVAC system. The absorption coefficient defines how much energy can be absorbed by arranging a certain acoustic damper at a duct.

An acoustic wave (noise) Pi from a noise source is partly reflected P_R from the surface of the acoustic damper 100. The absorption coefficient may be expressed as o _ 11 ^PR

Pi

The parameters of an acoustic damper 100, i.e. slot 4/channel 4^' width, volume and neck dimensions of the Helmholtz resonator(s) can be optimized based on the transmission loss and/or the absorption coefficient.

When optimizing the parameters for an acoustic damper 100, a process as illustrated in Fig. 9 may be used. In a first step 1001 , a frequency or frequencies to be dampened by the acoustic damper 100 is/are

determined. In a second step 1002, a set of damper parameters: slot/channel width, resonator volume, and neck dimensions (length, shape, neck opening area etc.), are determined. In a third step 1003, the transmission loss, TL, or the absorption coefficient, a, is determined through simulations for a damper with the set of damper parameters from the second step 1002. The

simulations may be based on the acoustic transfer matrix method or the finite element method by solving the Helmholtz equation. Each part of the structure, the channels, necks and volumes of the Helmholtz resonators are modeled based on its effective gas density and sound speed for acoustic wave propagation in channels, necks and in the volumes. Viscosity (or the friction between the gas and walls) and its thermal effects are considered based on the theory of acoustic wave propagation in a narrow tube/channel for modeling the parts of the channel and the necks of the Helmholtz resonators.

In a fourth step 1004, the transmission loss or absorption coefficient from the third step 1003, is compared with a target value, i.e. does the damper 100 attenuate the frequency/frequencies to be dampened to (a) predetermined target value(s). If the predetermined target values(s) have been reached, the fifth step 1005 is to finalize the optimization process and produce the damper 100 with the set of damper parameters. If in the fourth step 1004 the predetermined target value(s) are not reached the set of damper parameters are adjusted in a sixth step 1006. The adjusted damper parameters are entered into the second process step 1002 and thereafter the process steps are repeated until the predetermined target values are obtained in the fourth step 1004.

In Fig. 10 is a graph showing the transmission loss for acoustic dampers described above having a channel 4^' connected with one Helmholtz resonator (dashed line marked with squares), two Helmholtz resonators (dashed line marked with stars) and three Helmholtz resonators (solid line), respectively. The dashed line shows the transmission loss for an empty chamber, the chamber having the same size and outer shape as the acoustic dampers.. From this graph it can be seen that for the damper having one Helmholtz resonator the damping of a frequency of 500 Hz results in a transmission loss of about 40 dB, while the empty chamber shows a transmission loss for the same frequency of about 7 dB. For the damper having two Helmholtz resonators the damping of frequencies of 350 Hz and 500 Hz, respectively, results in transmission losses of about 35 dB and 38 dB, respectively. The empty chamber shows transmission losses for the same frequencies of about 5 dB and 7 dB, respectively. For the damper having three Helmholtz resonators the damping of frequencies of 250 Hz, 425 and 550 Hz, respectively, results in transmission losses of about 30 dB, 28 dB and 38 dB, respectively. The empty chamber shows transmission losses for the same frequencies of about 2 dB, 5 dB and 7 dB, respectively.

Fig. 11 is a graph showing the absorption coefficient for acoustic dampers described above having a channel 4^' connected with one Helmholtz resonator (dashed line marked with stars), two Helmholtz resonators (dashed line marked with circles) and three Helmholtz resonators (solid line), respectively. From this graph it can be seen that for the damper having one Helmholtz resonator the damping of a frequency of 450 Hz results in an absorption coefficient of about 0.9. For the damper having two Helmholtz resonators the damping of frequencies of 450 Hz and 800 Hz, respectively, results in absorption coefficients of about 1 and 0.45, respectively. For the damper having three Helmholtz resonators the damping of frequencies of 350 Hz, 800 Hz and 1250 Hz, respectively, results in absorption coefficients of about 1 , 0.7 and 0.85, respectively.

For the acoustic damper having three Helmholtz resonators, the thickness of the damper, i.e. the distance from an outer surface of the first wall 2 to an outer surface of the second wall 3, was 35 mm. Such a damper is much thinner and more compact compared to traditional liners used in e.g. aero engines or HVAC ducts. For such traditional liners the thicknesses need to be one quarter of the wavelength of the frequency to be attenuated. For a frequency of 400Hz a thickness of 214 mm is, hence, needed.

Air conditioning systems 400, Fig. 12, may comprise a compressor 402 and the gas duct damping system 300 as described above. The duct portion 200 may be connected upstream and/or downstream of the compressor 402 to reduce the pulsation wave. Such an air conditioning system 400 may be used to cool or heat rooms or spaces like a vehicle cabin by compressing, condensing, expanding and then evaporating a refrigerant. Such an air conditioning system 400 may, apart from the compressor 402 comprise, a condenser 405, an accumulator/dryer 406, an expansion valve 407 and an evaporator 408. Reciprocating gas compressors are a type of compressor that quite often are used in vehicles. It compresses gas by using a piston in a cylinder and a back-and-forth motion. The periodic movement inherently generate transient pulsating flows and vibration, which can transfer through the duct and further induce some correlated noise in the vehicle cabin.

An engine system 500, Fig. 13, may comprise an engine 502 and the gas duct damping system 300 as described above. The duct portion 200 may be connected upstream and/or downstream of the engine 502. In most internal combustion engine systems, gas is sucked in at an intake duct and compressed and burned gas is blown out at an exhaust duct. Noise from engines could propagate in both directions, at upstream and downstream sides, of the engine. Alternatively, the engine of the system shown in Fig. 13 may be exchanged with a piston compressor.

A fan system 600, Fig. 14, may comprise a fan 601 and the gas duct damping system 300 as described above. The duct portion 200 may be connected to a gas intake duct 602 or a gas exhaust duct 603. The fan 601 generates high pressure flow for the gas transportation and generates noise at the same time. The noise could propagate upstream of the fan against the flow and downstream of the fan with the flow.

A turbine engine system 700, Fig. 15, may comprise a turbine engine 705 and the acoustic damper 100 described above. The acoustic damper 100 may be arranged at a wall of a bypass duct 701 and/or at a wall of a bypass nozzle 702 and/or at a wall of an exhaust nozzle 703 to dampen the noise level. Air entering the gas turbine engine 705 is accelerated by a propulsive fan to produce two air flows. The first airflow enters into the core part of the engine, is further compressed, and burned with fuel and is exhausted through the exhaust nozzle 703. The second airflow passes through the bypass duct 701 and provide propulsive thrust. The fan in the intake duct, the compressor and the combustor in the core generate high level noise, which propagates against and with the flow in the bypass duct 701 and exhaust nozzle 703.

Claims

1. An acoustic damper (100) for a gas duct system, comprising a housing (1 ) enclosing a housing volume, the housing comprising a first wall (2) and a second wall (3) opposite to and spaced from the first wall (2) so as to at least partly define the housing volume there between,

c h a r a c t e r i z e d in that

the acoustic damper (100) comprises:

at least one slot (4) in the first wall (2), the slot (4) providing an elongate opening in the first wall (2) and being connected with a channel (4^') which extends from the slot (4) into the enclosed housing volume towards the second wall (3),

at least one Helmholtz resonator (5, 5^', 5^", 5^"') arranged in the housing,

which Helmholtz resonator (5, 5^', 5^", 5^"') comprises an enclosed resonator volume (6, 6^', 6^", 6^{" '}), and at least one neck (7, 7^', 7^", 7^'") having a first end (7a, 7^'a, 7^"a, 7a^"') and a second end (7b, 7^'b, 7^"b, 7b^{' "}), wherein the first end (7a, 7^'a, 7^"a, 7a^'") opens into the Helmholtz resonator volume (6, 6^', 6^", 6^{' "}) and the second end (7b, 7^'b, 7^"b, 7b^{' "}) opens into the channel (4^').

2. The acoustic damper of claim 1 or 2, wherein a cross-sectional area and/or shape of the channel (4^') in each cross-section taken along the extent of the channel is substantially equal to the slot (4) opening in the first wall (2).

3. The acoustic damper of claim 1 or 2, wherein at least one cross- sectional area and/or shape of the channel (4^') in a set of cross-sections taken along the extent of the channel (4^') differs from the other cross- sectional areas.

4. The acoustic damper of claim 1 or 2, wherein all cross-sectional areas of the channel (4^') in a set of cross-sections taken along the extent of the channel (4^') are substantially the same.

5. The acoustic damper (100) of any of the preceding claims, wherein a width of the slot (4) in the first wall (2) is 0.01-2 mm, preferably 0.05-1.5 mm or 0.1 -1 mm.

6. The acoustic damper (100) of any of the preceding claims, wherein a ratio between a total slot (4) opening area in the first wall (2) and a total area of the first wall (2) is 0.1 -10%, 0.1 -8%, 0.1-6%, 0.1 -4%, 0.1 -2%, 0.5-10%, 2- 10%, 4-10%, 6-10% or 8-10%.

7. The acoustic damper (100) according to any of the preceding claims, wherein the damper comprises at least two slots (4) with associated channels (4’) arranged in the damper (100), and wherein the channels (4^') extend from the first wall (2) into the enclosed volume in directions substantially parallel with each other.

8. The acoustic damper (100) of any of the preceding claims, wherein the damper comprises at least two slots (4) with associated channels (4^') arranged in the damper (100), wherein at least one of the slots (4) has a direction of extension in the first wall (2) being non-parallel with the direction of extension of another slot (4) in the first wall (2).

9. The acoustic damper (100) of any of the preceding claims, wherein the channel (4^') extends from the slot (4) towards the second wall (3) a distance of 40%-100%, of 50% -100%, 60-100%, 70-100%, 80-100% or 90-100% of a distance between the first (2) and second wall (3).

10. The acoustic damper (100) of any of the preceding claims, wherein the channel (4^') extends perpendicularly or radially from the first wall (2) into the enclosed housing volume towards the second wall (3).

11. The acoustic damper (100) of any of the preceding claims, wherein the channel (4^') is connected with 1 -10, 1 -5 or 1 -3 Helmholtz resonators (5, 5^',

5).

12. The acoustic damper (100) of any of the preceding claims, wherein the acoustic damper (100) comprises two or more Helmholtz resonators (5, 5^',

5^”, 5^'"), wherein at least two of the resonators are arranged to have the same resonance frequency.

13. The acoustic damper (100) of any of the preceding claims, wherein the acoustic damper (100) comprises two or more Helmholtz (5, 5^', 5^", 5^'") resonators, wherein at least one of the resonators is arranged to have a resonance frequency which is different from the other resonator(s).

14. The acoustic damper (100) of any of the preceding claims, wherein at least one channel (4^') is connected with two or more Helmholtz resonators (5, 5^', 5^", 5^"'), wherein the second end of the neck (7b, 7^'b, 7^"b, 7b^"') of a Helmholtz resonator having the lowest resonance frequency is arranged to open into the channel (4^') at a distance further away, as seen in a direction along the channel (4^') from the first wall (2) than the second end of the neck (7b, 7b^', 7b^", 7b^{' "}) of a Helmholtz resonator having a higher resonance frequency.

15. The acoustic damper (100) of any of the preceding claims, wherein the neck (7, 7^', 7^", 7^'") of a Helmholtz resonator (5, 5^', 5^", 5^'") is straight, straight comprising at least one bend, curved, coiled or any combination thereof.

16. The acoustic damper (100) of any of the preceding claims, wherein a total length of the neck (7, 7^', 7^", 7^{' "}) is 1 -100 mm, 10-100 mm, 20-100 mm 30-100 mm, 40-100 mm, 50-100 mm, 60-100 mm, 70-100 mm, 80-100 mm, 90-100 mm, 1-10 mm, 1-20 mm, 1-30 mm, 1 -40 mm, 1 -50 mm, 1 -60 mm, 1 - 70 mm, 1-80 mm, or 1 -90 mm.

17. The acoustic damper (100) of any of the preceding claims, wherein the neck (7, 7^', 7^", 7^"') extends into the resonator volume (6, 6^', 6^”, 6^'") a distance of 50-99%, 60-99%, 70-99%, 80-99%, 90-99%, 50-90%, 50-80%, 50-70%, or 50-60% of a total length of the neck.

18. The acoustic damper (100) of any of the preceding claims wherein each Helmholtz resonator (5, 5^', 5^", 5^'") has 1-5 necks (7, 7^', 7^”, 7^{" '}), preferably 1 -2 necks.

19. The acoustic damper (100) of any of the preceding claims, wherein a cross section of an opening area of the second end of the neck (7b, 7b^', 7b^”, 7b^'") is square, rectangular, triangular, circular or elliptical.

20. The acoustic damper (100) of any of the preceding claims, wherein the Helmholtz resonator (5^'") is provided with a hole (9) in the second wall (3), thereby connecting the housing volume with the outside.

21. The acoustic damper (100) of claim 20, wherein a largest area of the hole (9) is smaller than (an) opening area(s) of the second end(s) (7b^{' "}) of the neck(s) of the Helmholtz resonator(s) (5^{' "}).

22. The acoustic damper (100) of any of the preceding claims, wherein a cross-section of the enclosed resonator volume (6, 6^', 6^", 6^{' "}) taken where the first end (7a, 7^'a, 7^"a, 7a^{' "}) opens into the Helmholtz resonator volume (6, 6^', 6^", 6^{' "}) and coinciding with the direction of extent of the channel (4^') is square, rectangular, triangular, circular, elliptical, semi-circular, semi- elliptical, a circular segment, or an elliptical segment.

23. A gas duct damping system (300) comprising a duct portion (200) having a duct wall (202) extending between ends (201 a, 201 b) of the duct portion (200) and defining a gas flow path, and an acoustic damper (100) of any of claims 1 -22, wherein a duct wall slot (204) is provided in the duct wall (202) and connected to the channel 4^', such that the Helmholtz resonator(s) (5, 5^', 5^", 5^{" '}) is (are) in fluid connection with the gas flow path.

24. The gas duct damping system (300) of claim 23, wherein the duct wall slot (204) extends perpendicularly to the gas flow path in the duct portion (200).

25. The gas duct damping system (300) of any of claim 23 or 24, wherein the duct wall slot (204) extends parallel with the gas flow path in the duct portion (200).

26. The gas duct damping system (300) of any of claims 23-25, wherein the first wall (2) of the acoustic damper (100) constitutes at least a portion of the duct wall (202) of the duct portion (200).

27. The gas duct damping system of any of claims 23-25, wherein the slot (4) in the first wall (2) of the acoustic damper (100) is arranged in fluid connection with the duct wall slot (204) of the duct portion (200).

28. An air conditioning system (400) comprising a compressor (402) and at least one gas duct damping system (300) of any of claims 23-27, wherein the duct portion (200) is connected upstream and/or downstream of the

compressor (402)

29. An engine system (500) comprising an engine (502) and at least one gas duct damping system (300) of any of claims 23-27, wherein the duct portion (200) is connected upstream and/or downstream of the engine.

30. A fan system (600) comprising a fan (601 ) and at least one gas duct damping system (300) of any of claims 23-27, wherein the duct portion (200) is connected to a gas intake duct (602) and/or a gas exhaust duct (603).

31. A turbine engine system (700) comprising a turbine engine (705) and the acoustic damper (100) of any of claims 1 -22, wherein the acoustic damper (100) is arranged at a wall of a bypass duct (701 ) and/or at a wall of a bypass nozzle (702) and/or at a wall of an exhaust nozzle (703).