EP1878009A1

EP1878009A1 - A device for damping of sound in a pipe

Info

Publication number: EP1878009A1
Application number: EP06733456A
Authority: EP
Inventors: Ragnar Glav; Victor-Hugo MÖLLER
Original assignee: Scania CV AB
Current assignee: Scania CV AB
Priority date: 2005-04-27
Filing date: 2006-04-26
Publication date: 2008-01-16
Also published as: SE0500958L; AU2006240549B2; KR20080005983A; EP1878009A4; WO2006115461A1; SE528857C2; KR101280597B1; AU2006240549A1; BRPI0609953A2

Abstract

The present invention relates to a device for damping sound in a pipe (7) which is adapted to having a gaseous medium flowing through it. The device (10) comprises at least one guide element (1 la-c) arranged inside the pipe (7) so that it divides an internal space in a portion (7') of the pipe into at least two flow channels (12a-d) for the gaseous medium. The guide element (1 la-c) comprises a plurality of holes running through it (11 ') which are so narrow that sound can pass through said holes (H') between mutually adjacent flow channels (12a-d) but only with a certain resistance.

Description

Device for damping of sound in a pipe

BACKGROUND TO THE INVENTION, AND STATE OF THE ART

The present invention relates to a device for damping of sound in a pipe according to the preamble of claim 1.

To reduce the amount of emissions in exhaust gases and increase the engine power of supercharged diesel engines in heavy vehicles, the charging pressure with which air is supplied to supercharged diesel engines has increased. The strength of the sound which occurs when air is drawn into the compressor of the turbo unit, i.e. the so-called turbo howl, has therefore also increased. This sound is of high frequency typically between 5 and 15 kHz. As the sound insulation of the driving cabs of vehicles has at the same time become more effective, the driver's perception of other disturbing sounds, which are usually at significantly lower frequencies, has been reduced. The so-called turbo howl has therefore become increasingly obvious to a driver of the vehicle.

GB 1 339 526 refers to a sound damper in an exhaust line of a gas turbine. The sound damper comprises a plurality of guide elements arranged in parallel which divide the exhaust line into parallel flow channels close to a curved portion of the exhaust line. The guide elements comprise sound-absorbing material. The sound-absorbing material results in the guide elements being relatively wide and space-occupying. To prevent the exhaust gases undergoing too great a pressure drop as they pass between the guide elements, the exhaust line portion has been widened in this region.

US 5,709,529 refers to a sound damper arranged in a curved pipe portion for supply of air to a turbo machine. The sound damper comprises a plurality of thin guide elements which lead the air into alternative flow channels in the curved pipe portion. The guide elements are of different lengths so that the sound emanating from the various flow channels has a definite mutual phase shift so that the sounds at least partly cancel one another out. SUMMARY OF THE INVENTION

The object of the present invention is to provide a device which results in simple but effective damping of sound in a pipe through which a gaseous medium flows, without the flow of the gaseous medium being substantially affected.

This object is achieved with the device of the kind mentioned in the introduction which is characterised by the features indicated in the characterising part of claim 1. The sound which occurs in the pipe may be defined as pressure fluctuations of the gaseous medium inside the pipe. The sound propagates substantially uniformly in all directions inside the pipe. The sound which reaches the surface of the guide element will thus be partly reflected and undergo a certain absorption. Some of the sound, however, will pass through the holes in the guide element together with the gaseous medium. As the holes are relatively narrow, the sonic energy is effectively reduced as the sound passes through the holes, resulting in damping of the sound.

According to the present invention, the guide element is arranged in a curved pipe portion. Dividing a curved pipe portion into flow channels considerably hinders sound propagation through the curved pipe portion. The sound will be reflected by the guide element and the wall surface of the curved pipe portion. The result is absorption of the sound and relatively good damping. With advantage, the guide element and the curved pipe portion have curvature of such magnitude that sound cannot pass through said flow channels without encountering at least one wall surface of the guide element or the curved pipe portion. All of the sound will thus be reflected by at least one surface and/or be pushed through one of said holes in the guide element. Thus none of the sound can pass totally undamped through the curved pipe portion. The guide element is preferably of a curved shape with a centre of curvature substantially corresponding to that of the curved pipe portion. The guide element can therefore divide the curved pipe portion into flow channels of substantially the same size with a substantially constant cross-sectional area along the whole of their extent. The result is a more uniform flow through the curved pipe portion. According to another preferred embodiment of the present invention, the thickness of the guide element is within the range 0.5 to 5 mm. A guide element made of, for example, sheetmetal can be made so thin that it substantially does not reduce the cross- sectional area for the gaseous medium passing through said pipe portion. In such cases the thickness of the guide element may be in the lower part of the range indicated above, i.e. about 1 mm. It is nevertheless necessary that the thickness of the guide element be sufficient to enable it to be fitted inside the curved pipe portion without risk of being deformed by the flowing medium. With advantage, the guide element is made of a metal material. The guide element may be made from a sheet of metal provided with holes running through it. Such a sheet may be of aluminium. If instead the guide element takes for example the form of a wing section, it will be thicker and may then be up to 5 mm thick.

According to another preferred embodiment, the device comprises at least one flow channel defined by a surface of the guide element and an opposite surface of the guide element, whereby said surfaces are situated at a predetermined distance from one another which corresponds to one-quarter of the wavelength of the sound primarily intended to be damped in the pipe. In many cases the frequency and wavelength of the sound which occurs in a pipe with a flowing gaseous medium are known. By dimensioning at least one of the flow channels in the manner mentioned it is possible to achieve very effective damping of sound at this wavelength. With advantage, the device comprises at least two guide elements, whereby said guide elements comprise mutually opposite surfaces which define an intermediate flow channel and whereby said mutually opposite surfaces are situated at a predetermined distance from one another which corresponds to half the wavelength of the sound primarily intended to be damped in the pipe. A flow channel with such dimensioning provides further damping of sound at that wavelength.

According to another preferred embodiment of the present invention, the magnitude of said holes is such that at least two substantially mutually opposite side edges of the holes are situated at a distance from one another of not more than 5 mm. The resistance to flow of air and sound through a hole is closely related to the narrowness of the hole. The narrower the hole, the greater the resistance to air and sound passing through it. With advantage, such a hole is made relatively elongate. The hole thus presents a relatively large cross-sectional area so that an ample amount of air can pass through the hole despite its relative narrowness.

According to another preferred embodiment, the guide element is arranged in a pipe portion situated substantially immediately upstream of a compressor in the direction of flow of the gaseous medium. When a gaseous medium is drawn into a compressor, a disturbing sound usually occurs. The construction of the compressor is usually optimised to meet certain capacity requirements. In many cases it is therefore not advantageous to subject a compressor to internal modifications in order to reduce the occurrence of such disturbing sound. The occurrence of the disturbing sound is also affected by the flow pattern of the gaseous medium before the medium reaches the compressor. Against this background, it seems most advantageous to arrange the guide element in a curved pipe portion substantially immediately upstream of the compressor. The compressor may form part of a turbo unit for a combustion engine. In many cases, turbo units give rise to a strong high-frequency whistling sound which may be perceived as disturbing. In such cases, a suitable number of guide elements can with advantage be fitted in a curved pipe portion upstream of the compressor in order to damp this sound.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below by way of examples with reference to the attached drawings, in which:

Fig. 1 depicts an arrangement with a sound-damping device and a turbo unit for supplying compressed air to a supercharged diesel engine, Fig. 2 depicts the sound-damping device in Fig. 1 in more detail,

Fig. 3 depicts a workpiece for making a guide element of the sound-damping device and Fig. 4 depicts an alternative workpiece for making a guide element of the sound- damping device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 depicts an arrangement with a turbo unit for supplying compressed air to a supercharged combustion engine. The combustion engine is here exemplified as a diesel engine 1. The diesel engine 1 may be intended for powering a heavy vehicle. The exhaust gases from the cylinders of the diesel engine 1 are led via an exhaust manifold 2 to at least one exhaust line 3. The exhaust gases in the exhaust line 3, which are at above atmospheric pressure, are led to a turbine 4 of a turbo unit. The turbine 4 is thus provided with driving power which is transmitted, via a connection, to a compressor 5 of the turbo unit. The compressor 5 thus compresses air which is led via an air filter 6 into an inlet line 7 to the diesel engine 1. A charge air cooler 8 is arranged in the inlet line 7 to cool the compressed air before it is led to the respective cylinders of the diesel engine 1 via a manifold 9. During operation of the turbo unit, a high-frequency whistling sound usually called turbo howl occurs. The sound may be at a frequency of about 10 kHz. To reduce this sound to an acceptable level, a schematically depicted sound-damping device 10 is fitted in a curved portion T of the inlet line. The curved pipe portion T is situated substantially immediately upstream of the compressor 5 with respect to the direction of air flow in the inlet line 7.

Fig. 2 depicts the sound damping device 10 in more detail. In this case the curvature of the curved pipe portion T is about 90 degrees. Three guide elements 1 Ia, b, c are fitted successively peripherally externally to one another in the curved pipe portion T. The guide elements 1 la-c each likewise have a curvature of about 90 degrees between an upstream end and a downstream end. The curved pipe portion T may with advantage have a substantially rectangular cross-sectional area to make it easier to shape and fit the guide elements 1 la-c. The curved pipe portion T may nevertheless have a cross-sectional area of substantially any desired shape. The guide elements 1 la-c are made of relatively thin sheetmetal which may be sheet aluminium. The thickness of the guide elements 1 la-c is within the range 0.5 to 1.5 mm. The guide elements 1 la-c are thus so thin as to reduce only negligibly the cross-sectional area for air flowing in the curved pipe portion 7'. The guide elements 1 la-c are nevertheless rigid enough to guide the air flowing in the pipe 7 without becoming deformed. In this case, the guide elements 1 la-c divide the internal space of the curved pipe portion T into four flow channels 12a-d which are successively situated radially externally to one another. Dividing the curved pipe portion T into such flow channels 12a-d reduces the variation in the velocity of the air flowing between different regions of the curved pipe portion T. The presence of the guide elements 1 la-c results in a more uniform flow of air to the compressor 5. The respective guide elements 11 a-c and the curved pipe portion T have a curved shape with a substantially common centre of curvature 13. Mutually adjacent guide elements 1 la-c can therefore be arranged at a substantially constant distance d₂, d₃ from one another along their whole extent. The radially innermost and outermost guide elements 1 Ia, c are situated at substantially the same distance dj, d₄ from the adjacent surface of the curved pipe portion T along the whole of their respective extent. The radially innermost and outermost flow channels di, d₄ have in this case substantially half the width of the middle flow channels d₂, d₃ in a radial direction throughout the curved pipe portion T.

The guide elements 1 la-c are provided with a large number of holes 11 ' running through them. It is thus possible for the air flowing through the flow channels 12a-d to be led through the holes 11 ' into adjacent flow channels 12a-d. The air is led through the holes 11 ' when there are pressure differences on opposite sides of the holes 11'. As sound takes the form of pressure fluctuations which propagate like waves within the pipes 7, the air on opposite sides of the holes 11 ' is subject to varying pressure, which causes a rather continuous occurrence of pressure difference between said sides. The result is that air is led through the holes 11 ' towards the side which at the time has the lower pressure. The holes 11 ' are nevertheless narrow enough to create an appreciable resistance to the air being led through the holes 11'. The sound which propagates through the holes 11 ' is therefore likewise subject to an appreciable resistance resulting in a damping of its sonic energy. Fig. 3 depicts a front view of a substantially rectangular metal sheet 14 before it is shaped to form a curved guide element 1 la-c. The metal sheet 14 is microslit so that it comprises a large number of holes 11 ' running through it. The holes 11 ' have an elongate shape which in this case is substantially rectangular. The shorter the short sides of the rectangular holes 1 V, the narrower the holes 11 ' and the greater the resistance which air has to overcome in order to flow through the holes 11'. The short sides may be of the order of magnitude of 0.1 mm. However, the elongate shape of the holes 11 ' results in the holes 11 ' providing a sufficient cross-sectional area for air to pass between mutually adjacent flow channels 12a-d in the curved pipe portion T . Fig. 4 depicts a front view of a rectangular metal net 15 before it is shaped to form a curved guide element 11. The metal net 15 comprises a large number of holes 11 ' running through it which in this case are substantially square in shape. The size of the holes 11 ' may be about 1 mm x 1 mm. The holes 11 ' are therefore so narrow that air is subject to an appreciable resistance as it is led through the holes 11'.

During operation of the turbo unit, air is drawn into the pipe portion 7 via the air filter 6 to the compressor 5. When the air reaches the curved pipe portion 7', the air flow is distributed substantially equally over the curved flow channels 12a-d. As the guide elements 1 la-c are so thin, they substantially do not reduce the flow area for air flowing through the curved pipe portion T . The flow of air close to the turbocompressor 5 nevertheless normally gives rise to a relatively strong whistling sound which on a large truck diesel is at a frequency of about 10 kHz. The division of the air flow in the curved flow channels 12a-d leads to a more uniform flow of air into the compressor 5. The result is that the whistling sound is greatly reduced. The geometry of each of the flow channels 12a-d is such that the sound from the compressor 5 cannot propagate through the curved pipe portion T without encountering the guide elements 1 la-c or the internal surfaces of the curved pipe portion 7'. Part of the sound is reflected by said surfaces, resulting in successive damping of the sound. Part of the sound which encounters the guide elements 1 la-c propagates through the holes 11 ' with a certain resistance. This sound undergoes a loss of energy and is therefore damped. Since the frequency range within which the disturbing sound occurs is known, it is also possible to adapt the radial width di-d₄ of the flow channels 12a-d so as to achieve substantially optimum damping of a specific sound. Effective damping of a specific sound is achieved when the radially innermost and outermost flow channels 12a, d have an extent dj, d_t in a radial direction in the curved pipe portion 7' which corresponds to one-quarter of the wavelength of the sound primarily intended to be damped in the pipe 7. As the frequency and velocity of the sound are known, the radial extent d_l5 Cl₄ of the flow channels 12 can be determined. When the sound is at a frequency of 10 kHz, the radial extent di, d_i of the flow channels 12a, d will be 7 to 8 mm for maximum damping of sound at that frequency. The result is very effective damping of sound at that frequency. For even more effective damping of this specific sound, the middle flow channels 12b, c between mutually adjacent guide elements 1 Ia- c may be given a radial extent d₂, d₃ corresponding to half the wavelength of said specific sound. When the sound is at a frequency of 10 kHz, the radial extent d₂, d₃ of the flow channels 12b, c will be about 15 mm for maximum damping of sound at that frequency. The present device thus provides very effective damping of the sound from a turbo unit in a relatively simple manner and substantially without disturbing the pattern of air flow and without auxiliary means which would require more space.

The invention is in no way limited to the embodiments described with reference to the drawings but may be varied freely within the scopes of the claims. The device is not limited to being used in an inlet line for air to a combustion engine but may be used in substantially any desired pipe through which a gaseous medium flows. The gaseous medium need not be air. The guide elements may be of substantially any desired but functional shape and substantially any desired number of them may be fitted inside the pipe and form flow channels of any desired width.

Claims

1. A device for damping sound in a pipe (7) which is adapted to having a gaseous medium flowing through it, whereby the device (10) comprises at least one guide element (11 a-c) arranged inside the pipe (7) so that it divides an internal space in a portion (7') of the pipe into at least two flow channels (12a-d), characterised in that the guide element (1 la-c) comprises a multiplicity of holes running through it (11 ') which are so narrow that sound can pass through said holes (H') between mutually adjacent flow channels (12a-d) but only with a certain resistance.

2. A device according to claim 1, characterised in that the guide element (1 la-c) is arranged in a curved pipe portion (7').

3. A device according to claim 2, characterised in that the guide element (1 la-c) and the curved pipe portion (7') have curvature of such a magnitude that sound cannot pass through said flow channels (12a-d) without encountering at least one wall surface of the guide element (1 la-c) or an internal wall surface of the curved pipe portion (7').

4. A device according to claim 3, characterised in that the guide element (1 la-c) has a curved shape with a centre of curvature substantially corresponding to that of the curved pipe portion (7').

5. A device according to any one of the foregoing claims, characterised in that the guide element (1 la-c) has a thickness within the range 0.5 to 5 mm.

6. A device according to any one of the foregoing claims, characterised in that the device comprises at least one flow channel (12a-d) defined by a surface of a guide element (1 la-c) and an opposite surface of the curved pipe portion (7'), whereby said surfaces are situated at a predetermined distance (di, d_i) from one another which corresponds to one-quarter of the wavelength of the sound primarily intended to be damped in the pipe (J).

7. A device according to any one of the foregoing claims, characterised in that the device comprises at least two guide elements (1 la-c) whereby said guide elements (1 la-c) comprise mutually opposite surfaces which define an intermediate flow channel (12b, c) and whereby said mutually opposite surfaces are situated at a predetermined distance (d₂, d₃) from one another which corresponds to half the wavelength of the sound primarily intended to be damped in the pipe (7).

8. A device according to any one of the foregoing claims, characterised in that the size of said holes (H') is such that at least two mutually substantially opposite side edges of the respective holes (11 ') are situated at a distance of not more than 5 mm from one another.

9. A device according to any one of the foregoing claims, characterised in that the guide element (11) is arranged in a pipe portion (7') which is situated substantially immediately upstream of a compressor (5) with respect to the direction of flow of the gaseous medium.

10. A device according to claim 9, characterised in that said compressor (5) comprises a turbo unit (4, 5) for a combustion engine (1).