Sound absorbing structure

A sound absorbing structure includes multiple sound absorbers. The sound absorbers are assembled together into a sound absorbing member. The sound absorbing member absorbs sound emitted from a sound source. Each of the sound absorbers is shaped like a hollow tube. Each of the sound absorbers has a hole extending in its axis direction.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2011-150104 filed on Jul. 6, 2011, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a sound absorbing structure having multiple narrow tube-shaped sound absorbers assembled together into a sound absorbing member or having a porous sound absorber with multiple holes.

BACKGROUND

In a sound absorbing structure disclosed in US2005/0067220 corresponding to JP-2005-84693A, multiple Helmholtz resonators (i.e., resonant silencers) having different resonant frequencies are arranged to obtain a sound absorption characteristic over a wide frequency range.

Specifically, as shown inFIG. 6, the sound absorbing structure includes an inner member101and an outer member102that surrounds the inner member101.

The inner member101defines an inner wall104having openings103. The outer member102defines an outer wall105that extends along the inner wall104of the inner member101.

The outer wall105forms bellows106to improve the sound absorption characteristic and flexibility of the sound absorbing structure. The bellows106and the inner wall104define cavities107. The openings103of the inner wall104are open to the cavities107.

A resonator, which is used in the conventional sound absorbing structure, is a device designed to absorb sound of a specific frequency. Therefore, to obtain sound absorption characteristics over a wide frequency range, a lot of resonators having different resonant frequencies are needed. As a result, the size of the sound absorbing structure is increased.

The present inventors have conducted an experiment to evaluate the effect of the conventional sound absorbing structure to reduce intake noise. In the experiment, a speaker was placed at one end of the inner member101, and a microphone was placed at the other end of the inner member101. Then, white noise was generated from the speaker, and sound caught by the microphone was analyzed by using a fast Fourier transform (FFT) analyzer.FIG. 7shows a result of the experiment. As can be seen fromFIG. 7, the sound absorption characteristic of the conventional sound absorbing structure has an excessive frequency region where the noise is excessively (i.e., unnecessarily) absorbed. Further, the sound absorption characteristic has a worse frequency region near the excessive frequency region. In the worse region, the noise is increased rather than reduced.

SUMMARY

In view of the above, it is an object of the present disclosure to provide a relatively small sound absorbing structure having good sound absorption characteristics over a wide frequency range.

According to an aspect of the present disclosure, a sound absorbing structure includes multiple sound absorbers. The sound absorbers are assembled together into a sound absorbing member for absorbing sound emitted from a sound source. Each sound absorber has a hole extending in its axis direction.

According to another aspect of the present disclosure, a sound absorbing structure includes a porous sound absorber for absorbing sound emitted from a sound source. The porous sound absorber has multiple holes extending in its axis direction.

DETAILED DESCRIPTION

A first embodiment of the present disclosure is described below with reference toFIGS. 1-3.FIG. 1shows an engine control system.FIG. 2shows part of an intake duct (i.e., sound absorbing structure).

The engine control system includes a turbocharger and an intake air silencer. The turbocharger compresses intake air by using pressure of exhaust gas of an internal combustion engine E. The intake air silencer absorbs intake air noise generated in an intake pipe of the engine E. The engine control system controls intake air supplied to a combustion chamber of each cylinder of the engine E.

The intake air silencer has an intake duct1serving as a sound absorbing structure. The intake duct1defines an intake passage R (refer toFIG. 4) where intake air flowing out of a compressor of the turbocharger flows.

As shown inFIGS. 1 and 2, the intake duct1has a rectangular tube-shaped duct body2and multiple hollow sound absorbers4. Noise emitted from the compressor (i.e., sound source) is transmitted through the duct body2. The sound absorbers4are made of synthetic resin and assembled together into a sound absorbing member. The sound absorbing member is attached to an inner surface of a duct wall3of the duct body2and exposed to the intake passage R so that the noise emitted from the compressor can be absorbed by the sound absorbing member. The intake passage R is defined by the inner surface of the duct wall3of the duct body2. The sound absorbing member is directly attached to a predetermined portion of the inner surface of the duct wall3by an adhesive or the like. Alternatively, the sound absorbing member can be attached to a plate member, and the plate member can be attached to the predetermined portion of the inner surface of the duct wall3.

Each sound absorber4has a circular through hole5extending straightly in its axis direction. The sound absorbers4are assembled together into the sound absorbing member in such a manner that a clearance (i.e., hole)6is formed between adjacent sound absorbers4. The details of the intake air silencer, in particular, the intake duct1, are described later.

The engine E includes, but is not limited to, a multicylinder gasoline with multiple cylinders. For example, the engine E can be multicylinder diesel engine with multiple cylinders.

The engine E is installed in an engine room of a vehicle together with the turbocharger. Each cylinder of the engine E is provided with an injector for injecting fuel into a corresponding intake port at an optimum time.

The engine E has an intake pipe and an exhaust pipe. The intake pipe defines an intake passage where intake air drawn into the combustion chamber of each cylinder flows. The exhaust pipe defines an exhaust passage through which exhaust gas flowing out of the combustion chamber of each cylinder is discharged to the outside.

The intake pipe is provided with the intake duct1, an air cleaner7, the compressor of the turbocharger, an intercooler8, an electronic throttle unit, and an intake manifold. The intake manifold is connected to the intake port of each cylinder of the engine E.

The exhaust pipe is provided with an exhaust manifold, a turbine of the turbocharger, an exhaust gas cleaner (i.e., catalyst), and a muffler9serving as an exhaust silencer. The exhaust manifold is connected to an exhaust port of each cylinder of the engine E.

The air cleaner7has a filter for filtering the outside air drawn into an air introduction passage from an outside air inlet located at an upstream end of an inlet duct (i.e., outside air introduction duct).

An outlet of the air cleaner7is connected to a compressor housing of the turbocharger through an intake duct defining an intake passage where the intake air passing through the air cleaner7flows.

An outlet of the compressor housing is connected to the intercooler8through the intake duct1, which defines the intake passage R where the intake air flowing out of the compressor of the turbocharger flows.

The intercooler8includes a cooler housing and a cooler core accommodated in the cooler housing. The cooler core cools the hot intake air compressed by the compressor of the turbocharger by exchanging heat between the intake air and a coolant such as cooling liquid or gas.

An outlet of the intercooler8is connected to a throttle body11of the electronic throttle unit through an intake duct (i.e., air connector, air hose) defining an intake duct where the intake air passing through the cooler core flows.

The electronic throttle unit includes the throttle body11, a throttle valve12, and an electric actuator13. The throttle valve12is opened and closed adjust the flow rate of the intake air.

The throttle valve12is supported by a shaft14. The electric actuator13rotates the shaft14. The electric actuator13has a motor for opening and closing the throttle valve12and a deceleration mechanism for decelerating rotation of the motor and for transferring the decelerated rotation to the shaft14. The motor is electrically connected to a battery of the vehicle through a motor drive circuit of an engine electronic control unit (ECU).

The intake manifold includes a surge tank for reducing a change in pressure of the intake air flowing from an outlet of the throttle body11and intake blanch pipes, each of which is connected to a corresponding intake outlet of the surge tank.

The surge tank has a surge tank room for temporarily storing the intake air and for distributing the intake air between the intake blanch pipes, each of which is connected to a corresponding combustion chamber and intake port of each cylinder of the engine E. InFIG. 1, the surge tank is not shown.

Each intake pipe branches from the surge tank room and is connected to the corresponding combustion chamber and intake port of each cylinder of the engine E. Each intake blanch pipe has an intake blanch passage for introducing the intake air to the intake port of each cylinder of the engine E.

The turbocharger has the compressor and a turbine. The compressor is located at a point in the intake pipe of the engine E. The turbine is located at a point in the exhaust pipe of the engine E. In the turbocharger, the intake air flowing through the intake pipe is compressed by the compressor, and the compressed air is supplied to the combustion chamber of each cylinder of the engine E.

When a turbine wheel15of the turbine is rotated by exhaust gas, a turbine shaft16and a compressor impeller17that are connected to the turbine wheel15rotates so that the intake air can be compressed by the compressor impeller17.

The turbine includes the turbine wheel15and the turbine housing. The turbine wheel15has turbine blades arranged in a circumferential direction of the turbine wheel15and is rotated by pressure of the exhaust gas of the engine E. The turbine wheel15is connected through the turbine shaft16to the compressor impeller17to directly rotate the compressor impeller17. A wheel accommodation space is formed in the center of the turbine housing. The turbine wheel15is rotatably held in the wheel accommodation space of the turbine housing.

Further, a turbine inlet passage and a turbine outlet passage are formed in the turbine housing. The turbine inlet passage leads the exhaust gas flowing from a confluence of the exhaust manifold to the wheel accommodation space. The turbine outlet passage leads the exhaust gas flowing from the wheel accommodation space to the exhaust gas cleaner.

The compressor includes the compressor impeller17and the compressor housing. The compressor impeller17has impeller blades arranged in a circumferential direction of the compressor impeller17. The compressor impeller17is connected through the turbine shaft16to the turbine wheel15and directly rotated by the turbine wheel15.

An impeller accommodation space is formed in the center of the compressor housing. The compressor impeller17is rotatably held in the impeller accommodation space of the compressor housing.

Further, a compressor inlet passage and a compressor outlet passage are formed in the compressor housing. The compressor inlet passage leads the intake gas flowing from the air cleaner7to the impeller accommodation space. The compressor outlet passage leads the intake gas flowing from the impeller accommodation space to the, intercooler8through the intake duct1.

An engine body (i.e., cylinder block and cylinder head) of the engine E has the intake ports opened and closed by an intake valve21and the exhaust ports opened and closed by an exhaust valve22.

When the engine E is a gasoline engine, the cylinder head of the engine E is provided with a spark plug23having a tip portion exposed to the chamber of each cylinder.

When the engine E is a four-cylinder engine, four combustion chambers (i.e., cylinder bores) are arranged in the cylinder block of the engine E in a direction in which the cylinders are arranged. A piston24connected through a connecting bar to a crank shaft is supported in each cylinder bore so that the piston24can slidably reciprocate in the cylinder bore.

The intake air silencer, in particular, the intake duct1according to the first embodiment is described in detail below with reference toFIGS. 1-3.

The intake duct1defines the intake passage R where the intake air flowing out of the compressor of the turbocharger flows.

Since the intake air is compressed by the compressor impeller17, the intake air flowing out of the compressor is at high pressure. Therefore, it is preferable that the intake duct1should absorb sound over a wide range of high frequencies (e.g., from 1 kHz to 4 kHz).

As mentioned previously, the intake duct1is configured as a sound absorbing structure having a sound absorbing member for absorbing noise emitted from the compressor of the turbocharger (i.e., sound on the downstream side of the compressor). The intake duct1includes the rectangular tube-shaped duct body2and multiple hollow sound absorbers4. The sound absorbers4are assembled together into the sound absorbing member.

The duct body2has a rectangular cross-section perpendicular to its axis. For example, the duct body2can be a single piece of synthetic resin (i.e., heat-resistant resin) such as polyamide (PA), polyphenylene sulfide (PPS), polypropylene (PP), or polybutylene terephthalate (PBT).

An upstream end of the duct body2in the intake air flow direction (i.e., inlet of the intake duct1) is connected to the outlet of the compressor housing of the turbocharger. A downstream end of the duct body2in the intake air flow direction (i.e., outlet of the intake duct1) is connected to an inlet of the intercooler8.

The duct wall3of the duct body2extends parallel to a traveling direction of the noise emitted from the compressor.

Each sound absorber4is a circular tube having a circular cross-section perpendicular to its axis. The sound absorbers4are made of synthetic resin (i.e., heat resistant resin) and integrated as a single piece. Alternatively, the sound absorbers4can be separate pieces. For example, each sound absorber4can be made of polyamide (PA), polyphenylene sulfide (PPS), polypropylene (PP), or polybutylene terephthalate (PBT).

For example, the sound absorber4can be a narrow straight tube having the hole5with a diameter of 1 mm or less. The sound absorbers4are attached to the inner surface of the duct wall3of the duct body2and arranged in parallel to each other in the traveling direction of the noise emitted from the compressor so that the axis direction of each sound absorber4can be perpendicular to the traveling direction of the noise.

Each sound absorber4has at least one open end in the axis direction so that the open end can be perpendicular to the traveling direction of the noise emitted from the compressor.

As described in detail later, a sound absorbing characteristic of the sound absorber4depends on a ratio L/φd of a length L (mm) of the sound absorber4in the axis direction to a diameter φd (mm) of the hole5of the sound absorber4. For example, as the ratio L/φd is increased, an average amount of sound absorbed by the sound absorber4is increased.

Next, an operation of the engine control system is described below with reference toFIGS. 1-3.

The exhaust gas, discharged from the combustion chamber of each cylinder of the engine E to the exhaust manifold, flows into the wheel accommodation space through the turbine inlet passage as an inlet of the turbine housing of the turbocharger so that the turbine wheel15can rotate. Then, the exhaust gas flows out of the turbine outlet passage as an outlet of the turbine housing.

The rotation of the turbine wheel15is transferred through the turbine shaft16to the compressor impeller17of the compressor so that the compressor impeller17can rotate.

The intake air, drawn into the intake passage from the air cleaner7, flows into the impeller accommodation space through the compressor inlet passage as an inlet of the compressor housing. The intake air flowing into the impeller accommodation space is compressed by the compressor impeller17rotating with the turbine wheel15so that pressure of the intake air can be increased. The compressed intake air flows from the compressor outlet passage of the compressor housing to the intercooler8through the intake passage R of the duct body2of the intake duct1and is cooled by the intercooler8. The cooled intake air flows out of the cooler core of the intercooler8and is drawn into the intake port and the combustion chamber of each cylinder of the engine E through a throttle bore of the throttle body11and the intake blanch passage of the intake manifold.

As described above, according to the first embodiment, the intake duct1includes multiple hollow sound absorbers4. The sound absorbers4are assembled together into a sound absorbing member for absorbing sound emitted from a sound source such as the compressor. Each sound absorber4is tube-shaped and has the hole5with a predetermined diameter, for example, 1 mm or less. The intake duct1is connected to the outlet of the compressor housing of the turbocharger. The sound absorbing member is installed inside the duct body2and exposed to the intake passage R where the intake air flowing out of the compressor flows. Specifically, the sound absorbing member is attached to the inner surface of the duct wall3that defines the intake passage R.

The sound absorber4is made of synthetic resin. Sound (i.e., noise) entering the hole5of the sound absorber4hits against an inner wall of the sound absorber4so that energy of the sound can be damped. Further, the energy of the sound can be damped by the vibration of the air in the hole5of the sound absorber4.

A desired sound absorption characteristic of the sound absorber4can be obtained by adjusting the specifications of the sound absorber4. For example, the desired sound absorption characteristic can be obtained by adjusting the ratio L/φd of the length L of the sound absorber4in the axis direction to the diameter φd of the sound absorber4.

According to the first embodiment, each sound absorber has the same ratio L/φd. Alternatively, for example, the sound absorbers4can be divided into a first group and a second group. In this case, each of the sound absorbers4of the first group can have a first ratio L/φd, and each of the sound absorbers4of the second group can have a second ratio L/φd different from the first ratio L/φd. In such an approach, the sound absorbing member constructed with the sound absorbers4having different ratios L/φd can absorb noise over a wide frequency range.

As mentioned above, it is preferable that the intake duct1should absorb sound over a high frequency range from 1 kHz to 4 kHz to absorb the noise emitted from the compressor of the turbocharger.FIG. 3is a graph illustrating a relationship between the ratio L/φd and the average amount of sound absorbed by the sound absorber4. As can be seen fromFIG. 3, the average sound absorption amount of 15 dB over the frequency range from 1 kHz to 4 kHz can be obtained by adjusting the ratio L/φd to 33.8 or more. For example, when the length L is 10 mm, the ratio L/φd of 33.8 or more can be obtained by adjusting the diameter φd to a value less than 0.3 mm.

Thus, the sound absorbing structure according to the first embodiment intake can exhibit high sound absorption performance over a wide frequency range, although the intake duct1has a relatively small size and does not require a large mounting space. In particular, the sound absorbing structure can effectively absorb noise over a wide range of high frequencies. Further, unlike the conventional sound absorbing structure, the sound absorbing structure can have a sound absorption characteristic without a worse frequency region where noise is increased rather than reduced.

A fiber assembly, such as a felt, a fiberglass, or glass wool, or a porous absorber such as a thin film, a non-woven fabric, or a urethane foam can be used as a sound absorbing member for absorbing noise emitted from a sound source. For example, the fiber assembly or the porous absorber is used as an insulator mounted on a floor of a vehicle. Generally, the fiber assembly or the porous absorber has flexibility. Therefore, assuming that such a fiber assembly or a porous absorber is used as a sound absorbing member mounted in an intake duct of an engine in order to absorb intake noise emitted from the engine or a turbocharger, the sound absorbing member is largely deformed by intake pulsation due to a change in pressure in the intake duct. As a result, a clearance or hole in the sound absorbing member may be widened or narrowed, and sound absorption characteristic of the sound absorbing member may be deteriorated.

For example, according to the first embodiment, narrow tube-shaped sound absorbers4, each having the hole5with the diameter of 1 mm or less, are assembled together into the sound absorbing member. In such an approach, it is less likely that the sound absorbers4are deformed by the pressure change. Therefore, the sound absorbing structure can maintain its sound absorption characteristic.

Since the sound absorbing member is attached to the inner surface of the duct wall3of the intake duct1by an adhesive or the like, the intake duct1can be reinforced by the sound absorbing member. Accordingly, the intake duct1, which is connected to the outlet of the compressor housing of the turbocharger, can withstand the pressure compressed by the compressor of the turbocharger even when the thickness of the duct wall3of the duct body2is reduced.

Since the sound absorbing member is located inside the duct body2, the intake duct1can have a small size and does not require a large mounting space. Therefore, the intake duct1can be easily mounted in a vehicle, for example, in an engine room without increasing the size of the engine room.

According to the first embodiment, the clearance6is formed between adjacent sound absorbers4. Since the clearance6can absorb noise emitted from a sound source such as the compressor, the sound absorbing characteristic of the sound absorbing structure can be improved. Alternatively, the clearance6can be filled with, for example, synthetic resin.

According to the first embodiment, the sound absorbing member is directly attached to the inner surface of the duct wall3. Alternatively, the sound absorbing member can be accommodated in a tube-shaped housing, and then the housing can be attached to the inner surface of the duct wall3.

An intake duct1according to a second embodiment of the present disclosure is described below with reference toFIGS. 4 and 5. A difference of the second embodiment from the first embodiment is as follows.

Like the first embodiment, the intake duct1as a sound absorbing structure includes multiple hollow sound absorbers4that are made of synthetic resin and assembled together into a sound absorbing member. The sound absorbing member is attached to the inner surface of the duct wall3, which defines the intake passage R of the duct body2, by using an adhesive or the like.

Each sound absorber4is a circular tube having a circular cross-section perpendicular to its axis. The sound absorbers4are made of synthetic resin and integrated together. For example, the sound absorber4can be a narrow straight tube having an inner hole5with a diameter of 1 mm or less.

The sound absorbing member is attached to the inner surface of the duct wall3of the duct body2in such a manner that the axes of the sound absorbers4are parallel to each other.

The sound absorber4has at least one open end in its axis direction.

The axes of the sound absorbers4are inclined in a direction toward (or away from) the compressor of the turbocharger at a predetermined angle θ with respect to the inner surface of the duct wall3of the duct body2.

That is, the axes of the sound absorbers4are inclined in the direction toward (or away from) the compressor of the turbocharger at the predetermined angle θ with respect to the traveling direction of noise emitted from the compressor.

As described above, according to the second embodiment, the axes of the sound absorbers4of the sound absorbing member are inclined in the direction toward (or away from) the compressor of the turbocharger at the predetermined angle θ with respect to the inner surface of the duct wall3of the duct body2. In such an approach, the sound absorbing member can effectively absorb a progressive wave of the noise emitted from the compressor. Thus, noise leaking out of the duct body2can be reduced rapidly.

In this case, the sound absorbing member may not effectively absorb a reflection wave of the noise emitted from the compressor. However, the refection wave is reduced with a reduction in the progressive wave. Therefore, the noise leaking out of the duct body2can be reduced by reducing the progressive wave of the noise.

Specifically, as shown inFIG. 5, when the axes of the sound absorbers4of the sound absorbing member are inclined in the direction toward a sound source, from which a sound D1is emitted, at the angle with respect to the inner surface of the duct wall3of the duct body2, it is likely that the sound absorbing member absorbs the sound D1. In contrast, when the axes of the sound absorbers4of the sound absorbing member are inclined in the direction away from a sound source, from which a sound D2is emitted, at the angle θ with respect to the inner surface of the duct wall3of the duct body2, it is less likely that the sound absorbing member absorb the sound D2.

Since the axes of the sound absorbers4of the sound absorbing member are inclined at the angle θ with respect to the inner surface of the duct wall3of the duct body2, the thickness of the sound absorbing member is reduced so that the size of the intake duct1can be reduced. Thus, the intake duct1can be easily mounded in an engine room of the vehicle.

Further, according to the second embodiment, as shown inFIG. 4, a clearance6is formed between adjacent sound absorbers4. Since the clearance6can absorb noise emitted from a sound source such as the compressor, the sound absorbing characteristic of the sound absorbing member can be improved. Alternatively, the clearance6can be filled with, for example, synthetic resin.

Further, according to the second embodiment, the sound absorbing member is directly attached to the inner surface of the duct wall3. Alternatively, the sound absorbing member can be accommodated in a tube-shaped housing, and then the housing can be attached to the inner surface of the duct wall3.

In the embodiments, as shown inFIG. 1, the intake duct1is located on the downstream side of the compressor impeller17. That is, the intake duct1is located between the compressor impeller17and the intercooler8. Alternatively, the intake duct1can be located on the upstream side of the compressor impeller17. That is, the intake duct1can be located between the compressor impeller17and the air cleaner7.

In the embodiments, the sound absorber4is a circular tube having a circular cross-section perpendicular to its axis. The sound absorber4is not limited to such a circular tube. For example, the sound absorber4can be a rectangular or hexagonal tube having rectangular or hexagonal cross-section perpendicular to its axis.

In the embodiments, both ends of the sound absorber4in its axis direction are opened. Alternatively, one end of the sound absorber4in its axis direction can be closed.

In the embodiments, the shape and size of one end of the sound absorber4is the same as the shape and size of the other end of the sound absorber4in its axis direction. That is, the hole5of the sound absorber4has a uniform diameter. Alternatively, at least one of the shape and size of one end of the sound absorber4can be different from the shape and size of the other end of the sound absorber4in its axis direction. That is, the hole5of the sound absorber4can have a non-uniform diameter.

The sound absorber4can have a curved or bent tube shape instead of a straight tube shape.

In the embodiments, the sound absorber4is made of synthetic resin. Alternatively, the sound absorber4can be made of a material other than synthetic resin.

For example, the sound absorber4can be formed by punching a hole in a solid base. Alternatively, the sound absorber4can be formed by melting a fiber or the like contained in a solid base.

In the embodiments, multiple hollow sound absorbers4are assembled together into a sound absorbing member. Alternatively, a porous sound absorber having multiple holes extending in its axis direction can be employed as a sound absorbing member.

For example, the porous sound absorber can be a thin film, a non-woven fabric, or a urethane foam. Alternatively, the porous sound absorber of the second embodiment can be formed by filling the clearance6of the sound absorbing member of the first embodiment with the same material as the sound absorber4.

A porous sound absorber made of resin and a porous sound absorber made of metal can be assembled together into a single porous sound absorber.

The holes of the porous sound absorber can extend parallel to each other in a traveling direction of sound emitted from a sound source.

Each hole of the porous sound absorber can have at least one open end in its axis direction.

The porous sound absorber can be attached to the inner surface of the duct wall3of the duct body2.

The axes of the holes of the porous sound absorber can be inclined in a direction toward (or away from) a sound source at a predetermined angle θ with respect to the duct wall3of the duct body2or a traveling direction of sound emitted from the sound source.

The sound absorbing structure according to the present disclosure can be applied to a vehicle part other than the intake duct1, such as an air cleaner housing, a surge tank, an intake manifold, a floor, a ceiling, a wall, a door, a panel, or a hood of the vehicle. Further, the sound absorbing structure can be applied to an object other than a vehicle, such as a floor, a ceiling, a wall, a door, a panel, or a hood of a an the intake duct1, such as air cleaner housing, a surge tank; or an intake manifold. Further, the sound absorbing structure can be applied to a floor, a ceiling, a wall, a door, a panel, a hood of a building with an acoustic chamber such as a concert hall.

In the embodiments, the duct body2has a straight tube shape. Alternatively, the duct body2can have a curved tube shape. Further, the duct wall3can have a concave or convex surface with a predetermined curvature radius instead of a flat surface.