GB2474683A

GB2474683A - Pipe unit with flow modifying formations

Info

Publication number: GB2474683A
Application number: GB0918598A
Authority: GB
Inventors: Ian David Hillary; Andrew James Grimshaw; David S Heaton; Ian James Barton
Original assignee: Leyland Trucks Ltd
Current assignee: Leyland Trucks Ltd
Priority date: 2009-10-23
Filing date: 2009-10-23
Publication date: 2011-04-27
Anticipated expiration: 2029-10-23
Also published as: GB2474683B; GB0918598D0

Abstract

A pipe unit 1 having a channelled section 4, such as a bellows de-coupler, presenting to the interior of the pipe unit axially spaced channels 12 that are open 5 to the interior of the pipe unit. Located at the upstream entrance of the channelled section and projecting inwardly from the inner wall of the pipe unit is a flow modifying element 20 formed around its peripheral extent adjacent to the inner wall with a plurality of peripherally spaced flow modifying edge formations 21 22, which serve to reduce noise level otherwise created by the presence of the channelled section. Preferably the walls of gas flow channels are defined by the modifying edge formations and the inner wall of the pipe unit, where the flow modifying element may be annular and the modifying edge formations may be provided as recesses at the outer peripheral edge of the annulus.

Description

Pipe Unit The present invention relates to pipe units and more particularly to such units having along their length a channelled section presenting to the interior of the pipe unit axially spaced channels that extend around the pipe and are open to the interior thereof. The invention relates more particularly (but not necessary exclusively) to pipe units in the form of exhaust pipe assemblies for use in commercial vehicles (e.g. trucks, lorries etc) incorporating a bellows type de-coupler.

The mass of certain exhaust components, such as the catalyst unit employed in emission control devices for commercial vehicles (lorries, trucks etc) is such that they cannot be suspended from the vehicle using conventional rubber mountings.

Therefore exhaust components are often rigidly mounted to the vehicle. However this requires that part of the exhaust pipe work be designed to be flexible such that it de-couples any transmission of vibration or displacement from the engine of the vehicle into the exhaust system. It is well known that a bellows type of de-coupler may be used as the flexible part of the exhaust system. Such a de-coupler is comprised of a thin walled metal tube which has been axially compressed under controlled conditions to form corrugations in the wall thereof, these corrugations presenting to the interior of the bellows de-coupler axially spaced channels or cavities that extend around the de-coupler and open to the interior thereof. Generally the de-coupler is of circular section and is formed with axially spaced, circumferential corrugations.

Bellows de-couplers of the type described above are employed not only in the exhaust systems of large commercial vehicles but in other automotive and non-automotive fluid flow applications.

Bellows type de-couplers for large commercial vehicles function perfectly adequately for the purpose for which they were designed, i.e. de-coupling transmission of vibration or displacement from the engine into the exhaust system. They have however been found to give rise to a noise problem in large commercial vehicles, e.g. those designed for compliance with the Euro 5 emission standard. More particularly, gas flow conditions in commercial vehicles designed for compliance with the Euro 5 standard have been found (under certain conditions particularly high rpm conditions) to generate an undesirable "whistling" noise with a discrete frequency. This "whistling" noise is also referred to as "cavity flow noise" and is generated by acoustic resonance of the gas flow over the cavity of the corrugation. The "whistle" is generally audible within the cab of the vehicle and also externally thereof. Emission of the noise externally of the vehicle may mean that the latter does not comply with noise emission standards. Within the cab, the noise is an irritant to drivers and passengers. Moreover the noise may lead to misdiagnosis or masking of any genuine vehicle issues. For example, the "whistling" could be misinterpreted as an engine problem when there may be another fault that should be correctly diagnosed. A further problem associated with the "whistling" is that the resultant oscillations in the bellows de-coupling can induce structural fatigue and lead to premature failure of the bellows de-coupler.

Attempts made to overcome this problem have included providing a pipe within the bellows de-coupler. However flexing of the bellows de-coupler was found to cause contact between the de-coupler and the inserted pipe so that durability of the de-coupler was effected.

It is therefore an object of the present invention to obviate or mitigate the above mentioned disadvantages.

According to the present invention there is provided a pipe unit having a channelled section presenting to the interior of the pipe unit axially spaced channels that extend around the pipe unit and are open to the interior thereof wherein located at the upstream entrance of the channelled section and projecting inwardly from the inner wall of the pipe unit is a flow modifying element formed around its peripheral extent adjacent said inner wall with a plurality of peripherally spaced flow modifying edge formations.

In accordance with the invention, we have found that undesirable noise in a pipe unit caused by the presence in such a unit of a channelled section may be reduced by providing a flow modifying element at the upstream entrance region to the channelled section, the flow modifying element being one which has flow modifying edge formations adjacent the inner wall of the pipe unit. Whilst we do not wish to be bound by theory, we believe that the flow modifying edge formations result in the formation of vortices in the near wall region of the pipe which are effected to eliminate the eddies responsible for the noise. The invention has particular application to pipe units in which the channelled section is formed by a bellows de-coupler.

Pipe units in accordance with the invention may be constructed as exhaust pipe assemblies for use in vehicles having an internal combustion engine (petrol or diesel), particularly large commercial vehicles such as trucks and lorries. Experimental tests we have conducted on such vehicles incorporating an exhaust gas assembly in accordance with the invention have demonstrated that the unwanted noise was no longer audible in the cab of the vehicle and considerably reduced externally of the vehicle. Further experimental data in this regard is presented in, and discussed with reference to, Fig 7 (see below).

Generally, the flow modifying edge formations are at a distance from the inner wall of the pipe unit which is less than 25% of the distance between that inner wall and the centre of the pipe unit. Thus in the usual case of a circular section pipe unit, the flow modifying edge formations lie within a notional annular region for which the outer edge is the circular inner wall of the pipe unit and the inner edge of the notional annulus is a concentric circle with a diameter 75% of that of the inner wall of the pipe unit. However the closer the flow modifying edges are to the inner wall of the pipe unit then the better the results.

In preferred embodiments of the invention, the flow modifying edge formations of the flow modifying element define at least parts of the walls of the boundary walls of gas flow channels at or adjacent the inner wall of the pipe unit. Particularly good results are achieved with flow modifying elements in which such gas flow channels are defined partly by the flow modifying edge formation (of the flow modifying element) and partly by the inner wall of the pipe unit. Such edge formations (in the flow modifying unit) may for example be part circular and open at the periphery of the flow modifying element. The edge formations may provide the majority of the length of the periphery of the gas flow channel.

Whilst we have found that particularly good results have been achieved with flow modifying elements having flow modifying edge formations that cooperate with the inner wall of the pipe unit to define gas flow channels, it is also possible for the gas flow channels to be defined wholly within the gas flow element itself. Such gas flow channels may, for example, be circular.

The flow modifying element may be in the form of a ring comprised of a planar wall and a central open area. A preferred form of ring is a circular annulus although other geometric configurations are not precluded. In the case of a flow modifying element in the form of a ring, the flow modifying edge formations may be provided around the outer peripheral edge (to define gas flow channels with the inner wall of the pipe unit), around the inner peripheral edge and/or within the "body" of the ring. Flow modifying edge formations providing the outer periphery of the annulus may be part-circular or of any other geometric configuration, e.g. rectangular, triangular etc as may any flow modifying edge formations provided around the inner periphery. For example, flow modifying edge formations provided around the inner edge of the ring may, for example, be part circular or in the form of rectangular cut-outs (these cut-outs actually providing three sides of a rectangle) which gives the inner edge a castellated appearance. Flow modifying edge formations provided within the "body" of the annulus may, for example define circular gas flow channels.

Generally there will be a total of at least 5 and more preferably at least 10 of the flow modifying edge formations provided around the peripheral extent of the flow modifying element. Thus in the case where the flow modifying edge formations define gas flow channels with the inner wall of the pipe unit there are preferably at least 10 such flow channels defined. There may be additional flow modifying edge formations provided in the flow modifying element. The flow modifying edge formations in the flow modifying element may be regularly or irregularly spaced around the peripheral extent of the flow modifying element. Thus, in one preferred embodiment, the flow modifying element is annular and has 16 equiangularly spaced, part-circular flow modifying edge formations around its outer peripheral edge and 11 part-circular flow modifying edge formations around its inner periphery, again equiangularly spaced.

The flow modifying element should have sufficient open area to allow flow of gas along the pipe unit without generation of undesirable back pressure (e.g. which would adversely affect the operation of an engine in the case where the pipe unit is an exhaust gas assembly) under all operating conditions to which the pipe unit may be subjected. For this reason, the open area of the flow modifying element is preferably at least 50%, more preferably at least 60% and typically at least 75% of the cross-sectional area of the pipe unit. This percentage open area (of the flow modifying element) is calculated to include the open areas defined by the flow modifying edge formations. Thus, for example, in the case of an annulus having "cut-outs" defining flow modifying edge formations around both its inner and outer surfaces then the open area of the annulus is calculated as the central area of the annulus (i.e. the "hole") plus the areas of the individual cut-outs.

The invention will now be further described by way of example only with reference to the accompanying drawings, in which: Fig 1 is a perspective view of an exhaust pipe assembly for a commercial vehicle to which the present invention is applicable; Fig 2 is an axial section (to an enlarged scale) of the bellows de-coupler incorporated in the exhaust pipe assembly shown in Fig 1; Figs 3(a)-(c) schematically illustrate generation of cavity noise in the bellows de-coupler illustrated in Fig 2; Fig 4 illustrates a first embodiment of flow modifying device in accordance with the invention for incorporation in an exhaust gas assembly as shown in Fig 1 for reducing cavity noise; Fig 5 illustrates location of the device of Fig 4 in the exhaust pipe assembly of Fig 1; Fig 6 is a part-sectional view of the line X-X in Fig 5; Fig 7 illustrates Noise Attenuation Results for an exhaust pipe assembly of the type illustrated in Fig 1 both with an without the device of Fig 4; Fig 8 illustrates a plane annular ring; and Figs 9 and 10 respectively illustrate second and third embodiments of flow modifying device for use in accordance with the invention.

Fig 1 illustrates an exhaust pipe assembly 1 of a type found in commercial vehicles. In more detail, the exhaust pipe assembly 1 comprises an upstream exhaust pipe section 2 connected to a downstream exhaust pipe section 3 by means of a conventional bellows de-coupler 4. Provided at the upstream end of exhaust pipe section 2 is a mounting 5 by means of which the exhaust pipe assembly 1 is mounted on the vehicle engine (not shown). Provided at the downstream end of exhaust pipe section 3 is an exhaust muffler and catalyst unit 6. Both this unit 6 and downstream exhaust pipe section 3 are rigidly mounted on the vehicle, the former by means of studs 7 and the latter by means of a bracket 8. Bellows de-coupler 4 (for which an enlarged axial sectional view is shown in Fig 2), is of conventional construction and comprises upstream and downstream end regions each formed with circumferential corrugations 9 that are axially spaced relatively close to each other and an intermediate region with corrugations 10 of lesser height, and greater axial spacing, as compared to the corrugations 9. It will be appreciated that the internal configuration of the bellows de-coupler 4 is such that annular channels or cavities which are open to the interior of the bellows de-coupler are defined by the configuration of the corrugations 9 and 10.

As outlined above, the incorporation of a bellows de-coupler such as that indicated by reference numeral 4 in Fig 1 in commercial vehicles designed to comply with the Euro 5 standard has resulted in unwanted, additional cavity flow noise which is generated by acoustic resonance of the gas flow over the cavity of the corrugation.

More particularly, under certain load and driving conditions, exhaust gas flow over corrugations in the bellows generates a "whistling" noise with a discrete frequency.

This can make it difficult for the vehicle to comply with external regulations.

Additionally, the additional noise is a discomfort to the driver and may lead to a misdiagnosis or masking of any genuine vehicle issues. Additionally, oscillations generated by the cavity flow noise can also induce structural fatigue to possible premature failure of components.

The manner in which the noise is generated is schematically illustrated in Figs 3(a)-(c) to which reference is now made. In these Figures, a portion of the interior surface of a bellows de-coupler 4 is schematically illustrated to a much enlarged scale and is referenced generally by numeral 10, the outer surface being referenced as 11.

Formed within the inner surface 10 are cavities or channels 12 schematically representing the circumferentially extending channels defined by the corrugations 9 and 10 of the bellows de-coupler as illustrated in Fig 2.

In Fig 3, exhaust gas flow is represented by reference numeral 13 and (as seen in Fig 3a) results in the generation of vortices 14 at the upstream edge 15 of a cavity 12. The vortices 14 in the shear layer impinge on the downstream edge 16 of cavity 12 and initiate a pressure wave depicted in Fig 3b by reference numeral 17. This pressure wave propagates from downstream edge 16 back to the upstream edge 15, as depicted in Fig 3(c). It will be appreciated that pressure wave 17 is then reflected back from upstream cavity edge 15 to downstream cavity edge 16 and that reflection between these edges is self-sustaining, thus leading to the generation of the unwanted cavity flow noise.

Reference is now made to Fig 4 which illustrates a first embodiment of flow modifying device 20 that may be incorporated in an exhaust system as illustrated in Fig 1 to reduce cavity flow noise generation. Device 20 is in the form of an annular ring and is formed with arcuate recesses on both its exterior and interior peripheral surfaces. More specifically, the outer peripheral surface of device 20 is formed with sixteen equiangularly spaced 16 recesses 21, each of which comprises about three-quarters of the circumference of a circle (the circle being open at the outer periphery of the device 20). The centres of recesses 21 lie on a pitch circle centred at the same point as the annulus that forms device 20. Provided around the inner peripheral edge of the device 20 is a further set of part-circular recesses 22, in this case generally semicircular. In this case, there are eleven recesses 22 which open into the central region of the annulus and have centres lying on a pitch circle centred at the same point as the annular device 20.

Flow modifying device 20 may be of metal or any other material withstanding the temperature and thermal cycling conditions to which the exhaust gas assembly 1 is subjected in use.

Flow modifying device 20 is intended to be located in the exhaust pipe system 1 (see Fig 1) immediately upstream of the bellows de-coupler 4 (in other words at the downstream end of exhaust pipe section 2). Reference is now made to Fig 5 which shows exhaust pipe section 2 separate from the remainder of the exhaust pipe assembly 1. More particularly, Fig 5 shows a "face on" view of the downstream end of pipe 2 with the flow modifying device 20 located in position by a collar 30 (see also the part-sectional view of Fig 6 which is taken on the line X-X of Fig 5. With the flow modifying device 20 located in position as described, the recesses 21 (formed in the outer peripheral surface of the device 20) form flow channels 31 with the interior surface of collar 30.

By way of example for an exhaust pipe section 2 having an internal diameter of mm the flow modifying device 20 may have an external diameter of 89.50 mm and an internal diameter of 52 mm. The part-circular recesses 21 may have a diameter of 12 mm and be located on an 82 mm Pitch Circle Diameter. The part-circular recesses 22 may have a diameter of 12 mm and be located on an 52 mm Pitch Circle Diameter.

Device 20 has been found to be effective in reducing cavity flow noise otherwise generated in the exhaust pipe assembly 1 (i.e. without the provision of the device 20).

The reduction in noise is achieved since the flow modifying device 20 acts to induce eddies, particularly in the near wall region, and disrupt the vortices created across the cavity, which lead to the production of sound due to acoustic resonance, and thus reduce the level of noise generated. The eddies which are believed primarily to be effective in reducing the cavity flow noise are those generated around the edges of the flow channels 31 defined partly by the part-circular recesses 21 and partly by the interior surface of the (30). Part-circular recesses 22 (provided on the inner periphery of the device 20) also have an effect in generated eddies effective for reducing cavity noise but their effect is believed to be less significant than the eddies generated by the edges of flow channels 31.

The effectiveness of flow modifying device 20 in reducing cavity flow noise is shown in Fig 7 which shows noise attenuation tests measured on a DAF LF55 truck fitted with a Cummins ISB 6.7 litre 6 cylinder engine, and measured using a MPI SO Analyser with SmartOffice file translator software, 2 off 130D20 lOP Microphones with integral preamplifier and Nl-DAQ USB4 9234 Dynamic Signal Acquisition Module.

The upper two graphs in Fig 7 (referenced as (a) and (b)) show noise measurements taken both inside the cab (upper left hand graph (a)) and outside the cab approximately 0.5m from the exhaust tailpipe (upper right hand graph (b)) for a vehicle not fitted with the flow modifying device 20. As circled in the graph (a), there was a noise "peak" of around 40 decibels at just below 8 kHz in the cab of the vehicle (see dotted circle) which was also manifested as a noise "peak" of around 100 decibels as measured outside the vehicle.

The corresponding measurements for the vehicle fitted with the flow modifying device 20 are shown in graphs (c) and (d) of Fig 7, the former representing the noise level within the cab and the latter representing the noise level externally of the vehicle. It will be seen from the lower left hand graph (c) of Fig 7 that the noise "peak" present in the upper left hand graph (a) (vehicle not fitted with flow modifying device) has been totally removed. Thus that noise is no longer an irritant to a driver or passenger in the cab. It will also be seen from the lower right hand graph (d) that, as measured externally of the vehicle, the noise "peak" has been reduced by about 14 dB as compared to the value for the vehicle not fitted with device 20 (upper right hand graph (b) of Fig 7).

Reference is now made to Fig 8 which shows a plain annular ring 40, i.e. without any part-circular recesses in either its inner or outer peripheral surfaces. Such an annular ring 40 has been found to be ineffective in reducing cavity flow noise when incorporated (in the same manner as device 20) in exhaust gas assembly 1. The ineffectiveness of annular ring 40 due to the fact that it does not generate eddies in the near wall region which are capable of disrupting the vortices which cause the cavity flow noise.

Further embodiments of flow modifying device that may be employed to provide for cavity flow noise reduction are illustrated in Figs 9 and 10 and all function by inducing eddies in the gas flow (particularly in the near wall region) to disrupt vortices created across the cavity.

The device 50 shown in Fig 9 is generally annular and has an "unmodified" outer peripheral surface which is therefore truly circular. However the inner peripheral surface has a number of castellations 51 that project generally radially inwardly of the device. The device functions by generating eddies at the edge regions 52 between adjacent castellations 51. The castellations 51 in the device 50 are of somewhat irregular configuration for introducing a degree of asymmetry to the flow in the bellows de-coupler 4 although castellations that are both uniform in size and circumferential spacing would also serve to induce eddies and reduce cavity flow noise.

The flow modifying device 60 illustrated in Fig 10 is also in the form of an annulus but, in this case, with a central opening which is somewhat lesser than that in the device 20 of Fig 4 or the device 50 of Fig 9. As shown in Fig 10, the device 60 has an outer row of circular apertures 61 and a concentric inner row of circular apertures 62. Purely by way of example, device 60 may have an outer diameter of 103 mm, an inner diameter of 35 mm, there may be sixteen circular apertures 61 having a diameter of 12 mm (on an 82 mm PCD) and eleven circular apertures 62 each with a diameter of also 12mm (on a 52 mm PCD).

When incorporated in the exhaust gas system 1, at least the edges of the apertures 61 serve to induce eddies in the gas flow in the near wall region so as to disrupt vortices created across the cavities and provides for reduction of cavity flow noise. In the manner described more fully above.

However the flow modifying device 60 shown in Fig 10 is less preferred than that shown in Figs 4 and 9 due to the lesser amount of open area (particularly at the central region) through which exhaust gas may flow. This reduced area can generate an undesirable increase in back pressure so that some of the power from the engine which would otherwise be used to propel the truck is taken to "pump" exhaust gas through the exhaust gas assembly 1.

Claims

CLAIMS1. A pipe unit having a channelled section presenting to the interior of the pipe unit axially spaced channels that extend around the pipe unit and are open to the interior thereof wherein located at the upstream entrance of the channelled section and projecting inwardly from the inner wall of the pipe unit is a flow modifying element formed around its peripheral extent adjacent said inner wall with a plurality of peripherally spaced flow modifying edge formations.
2. A pipe unit as claimed in claim 1 wherein said flow modifying edge formations of the flow modifying element define at least parts of the walls of gas flow channels at or adjacent the inner wall of the pipe unit.
3. A pipe unit as claimed in claim 2 wherein said gas flow channels are defined partly by said edge formations and party by the inner wall of the pipe unit.
4. A pipe unit as claimed in claim 2 or 3 wherein said edge formations are part circular.
5. A pipe unit as claimed in claim 2 wherein said gas flow channels defined by said flow modifying edge formations are defined wholly within the gas flow modifying element.
6. A pipe unit as claimed in claim 1 wherein the flow modifying element is annular.
7. A pipe unit as claimed in claim 6 wherein said flow modifying edge formations are provided as recesses at the outer peripheral edge of the annulus and define gas flow channels with the inner wall of the pipe unit.
8. A pipe unit as claimed in claim 7 wherein said edge formations are part circular.
9. A pipe unit as claimed in any one of claims 6 to 8 wherein flow modifying edge formations are provided around the inner peripheral edge of the annular gas flow modifying element.
10. A pipe unit as claimed in claim 9 wherein said flow modifying edge formations around the inner peripheral edge of the annular gas flow modifying element are part circular.
11. A pipe unit as claimed in claim 10 wherein said flow modifying edge formations around the inner peripheral edge of the annular gas flow modifying element are regular or irregular castellations.
12. A pipe unit as claimed in claim 6 wherein the flow modifying edge formations define gas flow channels between the inner and outer peripheries of the gas flow modifying element.
13. A pipe unit as claimed in claim 12 wherein said gas flow channels defined between the inner and outer peripheries of the annular gas flow modifying element are circular.
14. A pipe unit having a channelled section presenting to the interior of the pipe unit axially spaced channels that extend around the pipe unit and are open to the interior thereof wherein located at the upstream entrance to the channelled section is an annular flow modifying element around the inner wall of the pipe unit and being positioned with its plane transverse to the direction of gas flow, wherein the annular flow modifying element is formed around its outer peripheral edge with a plurality of peripherally spaced flow modifying edge formations that cooperate with the inner wall of the pipe unit to define gas flow channels at the wall of said unit.
15. A pipe unit as claimed in claim 14 wherein said annular flow modifying element formed with additional flow modifying edge formations around its inner peripheral surface.
16. A pipe unit as claimed in any one of claims 1 to 15 wherein said corrugated is provided by a bellows de-coupler.
17. A pipe unit as claimed in any one of claims ito 16 which is an exhaust gas assembly.
18. A vehicle with an internal combustion engine and having an exhaust gas assembly as claimed in claim 17 for the discharge of exhaust gas.