GB2381832A

GB2381832A - Resonance Suppression Device

Info

Publication number: GB2381832A
Application number: GB0126864A
Authority: GB
Inventors: John Fenton Gamble; David Geoffrey Henshaw
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2001-11-08
Filing date: 2001-11-08
Publication date: 2003-05-14
Also published as: GB0126864D0

Abstract

A resonance suppression device for a first fluid containing duct (32) in which the fluid is capable of resonance. The device comprises means defining a second fluid containing duct (40) arranged in fluid communication with the first duct (32). The second duct (40) is formed such that fluid therein can resonate on resonance of the fluid in the first duct (32), thereby suppressing resonance in the first duct (32).

Description

Resonance Suppression Device This invention relates to resonance suppression devices. More particularly, but not exclusively, the invention relates to devices for suppressing resonance in gas containing ducts in a gas turbine engine.

In gas turbine engines, air is taken from the main flow of air through the compressor and turbine arrangements to be used for other purposes, for example internal engine and accessory unit cooling, bearing chamber sealing, prevention of hot gas ingestion into the turbine disc cavities, control of bearing axial loads, control of turbine blade clearances and engine anti-icing. Air is removed from the main flow via conduits which are provided with selectively openable valves. When the valves are closed, the air in the duct can be excited by the main flow of air across the opening to the duct and, thereby, caused to resonate. Attempts to suppress such resonance usually involved the provision of deflectors or splitters in the duct, but the use of these can detract from the designed purpose or performance of the duct.

According to one aspect of this invention, there is provided a resonance suppression device for a first fluid containing duct in which the fluid is capable of resonating, the device comprising a second fluid containing duct arrangeable in fluid communication with the first duct, wherein the second duct is formed such that, on resonance of the fluid in the first duct, the fluid in the second duct can resonate, thereby suppressing said resonance in the first duct.

According to another aspect of this invention, there is provided a resonance suppression arrangement comprising a first fluid containing duct in which the fluid is capable of resonating, and a resonance suppression device

comprising a second fluid containing duct arranged in fluid communication with the first duct, wherein the second duct is formed such that, on resonance of the fluid in the first duct, the fluid in the second duct can resonate, thereby suppressing said resonance in the first duct.

Damping means is advantageously provided, to remove acoustic energy from the resonating fluid. The damping means may comprise energy conversion means to convert the acoustic energy to another form of energy, for example, heat. The damping means is preferably disposed between the first and second ducts, conveniently adjacent the open end of the second duct.

The energy conversion means may comprise means for creating turbulence in the resonating air. Thus, in the preferred embodiments, the energy conversion means converts the laminar flowing air in the resonating fluid to turbulent flow, thereby converting the acoustic energy to heat.

Preferably the damping means is also capable of allowing air to pass through it. In one embodiment, the damping means comprises a gauze, for example in the form of a wire mesh. In another embodiment, the damping means comprises a restriction in the second duct. The restriction in the second duct may be provided by a restriction member, conveniently an apertured member. The member may be in the form of a plate and may have a single aperture therein, which is conveniently centrally defined in the member. Alternatively, the member may have a plurality of apertures therein. The or each aperture may be a perforation.

Preferably, the first duct has an open first end arrangeable to allow fluid communication with a flow of said fluid, and a closure, spaced from said first end which may be openable. The closure may be a valve member,

openable to allow fluid to flow through the first duct.

The second duct may have an open first end at said first duct to allow said fluid communication therewith, and preferably includes a closure at a second opposite end which is conveniently a closed end of the second duct.

The first duct has an open end, and the second duct is desirably arrangeable in said fluid communication with the first duct at a region of the first duct spaced from said open end. Advantageously, the second duct is arranged in said fluid communication with the first duct adjacent the closure in the first duct.

The second duct desirably has a geometry selected to allow the fluid in the second duct to resonate at or near the natural frequency of the fluid in the first duct. The natural frequency of the fluid in the second duct may be between substantially 0.5 times to substantially 2 times the natural frequency of the fluid in the first duct, preferably between substantially 0.8 times to substantially 1.25 times the natural frequency of the fluid in the first duct, and more preferably, between substantially 0. 98 times to substantially 1.02 times the natural frequency of the fluid in the first duct.

The second duct may comprise an elongate enclosed channel, which may be in the form of a tube. The channel preferably has a substantially constant cross-sectional area. Preferably the channel also has a substantially constant cross-sectional shape. Preferably the crosssectional shape of the channel is substantially circular, although it may be any other suitable shape.

The cross-sectional area of the second duct may be at least substantially 0. 2% of the cross-sectional area of the first duct, preferably at least substantially 2% of the cross-sectional area of the first duct and, more preferably, at least substantially 10% of the cross-

sectional area of the first duct. In the preferred embodiments, the effectiveness of the second duct has been found to increase with increasing cross-sectional area. For example, in one embodiment where the cross-sectional area of the second duct is substantially 0. 2% of the crosssectional area of the first duct, the resonance suppression effect becomes noticeable, where the cross-sectional area of the second duct is substantially 2% of the crosssectional area of the first duct, the resonance suppression effect becomes significant, and the optimum resonance suppression effect is achieved where the cross-sectional area of the second duct is substantially 10% or above that of the cross-sectional area of the first duct.

In one embodiment, the most preferred length of the second duct, preferably from the damping means to the closure thereof, is substantially one quarter of the wavelength of the natural frequency of the fluid in the first duct. However, the length of the second duct may be substantially 0.5 times, to substantially 2 times one quarter of the wavelength of the natural frequency of the fluid in the first duct, preferably substantially 0.8 times to substantially 1.25 times one quarter of the wavelength of the natural frequency of the fluid in the first duct, more preferably substantially 0.98 times to substantially 1.02 times one quarter of the wavelength of the natural frequency of the fluid in the first duct.

In an embodiment where the length of the second duct is within the said more preferred range described in the paragraph above, the resonance suppression effect may be substantially twice the resonance suppression effect of an embodiment where the length of the second duct is either substantially 0.8 times or substantially 1.25 times one quarter of the wavelength of the natural frequency of the fluid in the first duct, and may be substantially four

times the resonance suppression effect of an embodiment where the length of the second duct is either substantially 0.5 times or substantially 2 times one quarter of the wavelength of the natural frequency of the fluid in the first duct.

The second duct may comprise a first portion extending from the first duct and a second portion which extends transverse to the first portion, conveniently generally orthogonal thereto. Conveniently, the second duct is generally J shaped, but it will be appreciated by those skilled in the art that the second duct could be any suitable shape, for example, straight, coiled, spiral.

In another embodiment, the second duct includes a first portion, which may be generally straight, extending from the first duct and a second portion which may comprise a chamber portion extending from the first portion, the chamber portion being wider than the first portion.

Preferably, the length and cross-sectional areas of the first portion, and the volume of the chamber portion are selected so that the fluid in the second duct resonates, at or near the natural frequency of the fluid in the first duct. The natural frequency of the fluid in the second duct may be between substantially 0.5 times to substantially 2 times the natural frequency of the fluid in the first duct, preferably between substantially 0.8 times to substantially 1.25 times the natural frequency of the fluid in the first duct, and more preferably, between substantially 0.98 times to substantially 1.02 times the natural frequency of the fluid in the first duct.

In embodiments where the natural frequency of the fluid in the second duct is within said more preferred range described eleventh and seventeenth paragraphs above, the resonance suppression effect may be substantially twice the effect of the embodiment where the natural frequency of

the fluid in the second duct is either substantially 0.8 times or substantially 1.25 times the natural frequency of the fluid in the first duct, and may be substantially four times the resonance suppression effect of the embodiment where the natural frequency of the fluid in the second duct is either substantially 0. 5 times or substantially 2 times the natural frequency of the fluid in the first duct.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of the top half of a ducted fan gas turbine engine; Fig. 2 is a close up of part of the gas turbine engine showing a resonance suppression arrangement; Fig. 3 is a close up of a part of a gas turbine engine showing an alternative construction of a resonance suppression arrangement.

With reference to Fig. 1 a ducted fan gas turbine engine generally indicated at 10 comprises, in axial flow series, an air intake 12, a propulsive fan 14, an intermediate pressure compressor 16, a high pressure compressor 18, combustion equipment 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28.

The gas turbine engine 10 works in the conventional manner so that air entering the intake 12 is accelerated by the fan 14 to produce two air flows, a first air flow into the intermediate pressure compressor 16 and a second airflow which provides propulsive thrust. The intermediate pressure compressor 16 compresses the air flow directed into it before delivering the air to the high pressure compressor 18 where further compression takes place.

The compressed air exhausted from the high pressure compressor 18 is directed into the combustion equipment 20

where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through and thereby drive the high, intermediate and low pressure turbines 22,24 and 26 respectively before being exhausted through the nozzle 28 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 22, 24 and 26 respectively drive the high and intermediate pressure compressors 16 and 18 and the fan 14 by suitable interconnecting shafts (not shown). The flow of air through the engine, as described above is referred to herein as"the main flow of air".

At certain regions in the engine, for example indicated as X and Y in Fig 1, air is taken from the main flow of air to perform other functions, for example internal engine and accessory unit cooling, bearing chamber sealing, prevention of hot gas ingestion into the turbine disk cavities, control of bearing axial loads, control of turbine blade tip clearances, and engine anti-icing.

Referring to Figs. 2, there is shown a region of the gas turbine engine 10 in which a wall 30 defines a passage 31 in which the main flow of air through the engine passes, as represented by the arrow A. The region shown in Fig. 2 may be for example one of the regions X or Y in Fig. 1.

Air is taken from the main flow of air via a conduit 132 to perform other functions, for example as described above. The conduit 132 extends from the wall 30 and is in fluid communication with the channel 31 via an opening 34 in the wall 30.

The conduit 132 is provided with a selectively openable valve member 36, which is opened when air is required to be delivered along the conduit 132. The conduit 132 defines a first duct 32 between the opening 34 and the valve member 36. When a flow of air is not required through the conduit 132, the valve member 36 is closed. On

closure of the valve member, the air in the first duct 32 is then static and has a resonant or natural frequency. The air flowing across the opening 34 as represented by the arrow A can excite the air in the first duct 32 causing it to resonate at its natural frequency.

A resonance suppression device 38 is provided on the first duct 32. In the embodiment shown, the resonance suppression device 38 comprises a second duct 40 in fluid communication with the first duct 32 via an opening 42.

The second duct 40 is closed, having a closed end 44 opposite the opening 42. The opening of the second duct 40 is defined adjacent to the valve member 36 in the first duct 32. In the arrangement shown in Fig. 2, the second duct 40 is generally J shaped, although it could be any other suitable shape.

The geometry of the second duct 40 and the position of the opening 42 are selected such that the resonance of air in the first duct 32 excites the air in the second duct 40 causing it to resonate at or near the same frequency as the resonance of the air in the first duct 32. For example, the natural frequency of air in the second duct 40 is in the range of substantially 0.98 times to substantially 1.02 times the natural frequency of the air in the first duct 32.

Damping means 50 is provided in the connecting passage 46 and is provided to remove acoustic energy from the air resonating in the second duct 40. The damping means 50 can be a restrictor plate having a single aperture therein and thereby reducing the diameter of the second duct 40.

Alternatively, the damping means 50 may be a gauze capable of absorbing sound waves. The gauze may be in the form of a wire mesh. The damping means 50 is provided adjacent the opening 42. The resonant or natural frequency of the air in the second duct 40 is dependent upon the length of the

second duct 40, from the damping means 50 to the closed end 44. This length is preferably selected to be substantially one quarter of the wavelength of the resonating air in the first duct 32. In the embodiment shown in fig. 2 the cross-sectional area of the second duct 40 is substantially the same as the cross-sectional area as the first duct 32.

A further embodiment is shown in Fig. 3 which comprises many of the same features as shown in Fig. 2 and these have been designated with the same reference numeral.

Fig. 3 differs from Fig. 2 in that the second duct, designated 140 in Fig. 3, comprises a connecting passage 146 and a main portion 148. In this embodiment, the main portion 148 is in the form of a chamber or bulb. The main portion 148 is in fluid communication with the passage 146 via an opening 149. The cross-sectional area of the main portion 148 is much greater than the cross-sectional area of the first duct 32 and of the connecting passage 146.

The natural frequency of the air in the second duct 140 of the embodiment shown in Fig. 3 is dependent upon the volume of the main portion 148, the length of the connecting passage 146 from the damping means 50 to the opening 149 and the cross-sectional a are of the connecting passage 146. These three parameters can, thus, be varied in a manner which would be appreciated by a person skilled in the art to design a second duct 140 in which the air has a desired natural frequency, and in which the main portion 148, and a connecting passage 146 have appropriate dimensions to fit into the region adjacent the first duct 32, taking into account the other components in the vicinity of the first duct 32. For example, a second duct 140 having a longer connecting passage 146 would require a main portion 148 having a lesser volume than a second duct 140 having a shorter connecting passage so that the air in both would resonate at the same frequency.

While not wishing to be limited to any particular theory it is believed that the above embodiments are able to suppress resonance in the first duct 32 due to the resonating air in the first duct 32 establishing resonance of the air in the second duct 40,140. When resonance in the second duct 40,140 occurs, it is believed that the air in the first duct 32 interacts dynamically with the air in the second duct 40,140. An oscillating flow is thought to occur between the two ducts 32,40 or 140. The provision of the damping means 50 removes acoustic energy from the resonating air in the second duct 40,140, converting it to heat to provide a significant reduction in the overall level of oscillation.

Thus, there is described in relation to the above embodiments, a device for suppressing resonance in a closed first duct which has the advantage of reducing or preventing excess noise from the resonating air in the first duct and reducing or preventing damage caused by such resonance. The first duct acts like a first resonator with the air therein being excited by the main flow of air. The second duct acts like a second resonator (Helmholtz or otherwise), with the air in the second duct being excited by the resonance of air in the first duct. Thus, the provision of said resonance suppression devices means that there is no need for changes in the form of the first duct, which means that the first duct can be designed primarily to meet its main performance or function objectives.

Various modifications can be made without departing from the scope of the invention. For example, the second duct 40,140 could be of other shapes, and the damping means 50 could be of other forms.

Although the preferred embodiment has been described with reference to a gas turbine engine, it will be appreciated that embodiments falling within the scope of

the invention can be used in other situations, for example in air scoops on the fuselage of an aeroplane, in motor vehicles or in buildings.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

CLAIMS 1. A resonance suppression device for a first fluid containing duct in which the fluid is capable of resonating, the device comprising a second fluid containing duct arrangeable in fluid communication with the first duct, wherein the second duct is formed such that, on resonance of the fluid in the first duct, the fluid in the second duct can resonate, thereby suppressing said resonance in the first duct.
2. A resonance suppression device according claim 1 wherein damping means is provided in the second duct to remove acoustic energy from fluid resonating in the first duct.
3. A resonance suppression device according to claim 2 wherein the second duct comprises an elongate region extending from the damping means, the elongate region having a length of substantially one quarter of the wavelength of the natural frequency of the fluid in the first duct.
4. A resonance suppression device according to claim 2 wherein the second duct comprises an elongate region extending from the damping means, the elongate region having a length of between substantially 0.5 times to substantially 2 times one quarter of the wavelength of the resonating fluid in the first duct when resonating at its natural frequency.
5. A resonance suppression device according to claim 4 wherein the elongate region has a length of between substantially 0.8 times and substantially 1.25 times one quarter of the wavelength of the resonating fluid in the first duct when resonating at its natural frequency.
6. A resonance suppression device according to claim 4 or 5 wherein the elongate region has a length of between

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substantially 0.98 times and substantially 1.02 times one quarter of the wavelength of the resonating fluid in the first duct when resonating at its natural frequency.
7. A resonance suppression device according to any of claims 2 to 6 wherein the damping means comprises energy conversion means to convert acoustic energy to another form of energy.
8. A resonance suppression device according to claim 7 wherein the energy conversion means comprises means for creating turbulence in the resonating air.
9. A resonance suppression device according to any of claims 2 to 8 wherein the damping means comprises a gauze.
10. A resonance suppression device according to any of claims 2 to 8 wherein the damping means comprises a restriction in the second duct to reduce the crosssectional area thereof.
11. A resonance suppression device according to claim 10 wherein the restriction in the second duct is provided by a restriction member, having at least one aperture defined therein.
12. A resonance suppression device according to any preceding claim wherein one end of the first duct is open to allow fluid to enter the first duct, and the second duct is arrangeable in said fluid communication with the first duct at a region of the first duct spaced from said open end.
13. A resonance suppression device according to claim 12 wherein the opposite end of the first duct has a closure to prevent the flow of fluid therethrough, and the second duct is arrangeable in said fluid communication with the first duct adjacent the closure in the first duct.
14. A resonance suppression device according to any preceding claim, wherein the second duct has an open first end allowing said communication between the first duct and

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the second duct, and a closed opposite second end.
15. A resonance suppression device according to claim 14 wherein the second duct includes a first portion which can extend from the first duct and a second portion comprising a chamber portion extending from the first portion, the chamber portion being wider than the first portion.
16. A resonance suppression device according to claim 15 wherein the length of the first portion, the crosssectional area of the first portion and the volume of the second portion are selected to provide the desired natural frequency of the air in the second portion.
17. A resonance suppression device according to any preceding claim wherein the second duct has a geometry selected to allow the fluid in the second duct to resonate at or near the natural frequency of the fluid in the first duct.
18. A resonance suppression device according to any of claims 1 to 16 wherein the natural frequency of the fluid in the second duct is between substantially 0.5 times to substantially 2 times the natural frequency of the fluid in the first duct.
19. A resonance suppression device according to claim 18 wherein the natural frequency of the fluid in the second duct is between substantially 0.8 times and substantially 1.25 times the natural frequency of the fluid in the first duct.
20. A resonance suppression device according to claim 18 or 19 wherein the natural frequency of the fluid in the second duct is between substantially 0.98 times and substantially 1.02 times the natural frequency of the fluid in the first duct.
21. A resonance suppression device according to any preceding claim wherein the cross-sectional area of the second duct is at least substantially 0. 2% of the cross-

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sectional area of the first duct.
22. A resonance suppression device according to claim 21 wherein the cross-sectional area of the second duct is at least substantially 2% of the cross-sectional area of the first duct.
23. A resonance suppression device according to claim 21 or 22 wherein the cross-sectional area of the second duct is at least substantially 10% of the cross-sectional area of the first duct.
24. A resonance suppression arrangement comprising a first fluid containing duct in which the fluid is capable of resonating, and a resonance suppression device comprising a second fluid containing duct arranged in fluid communication with the first duct, wherein the second duct is formed such that, on resonance of the fluid in the first duct at its natural frequency, the fluid in the second duct can resonate, thereby suppressing resonance in the first duct.
25. A resonance suppression arrangement according to claim 24 wherein the resonance suppression device is as claimed in any of claims 1 to 23.
26. A gas turbine engine incorporating at least one resonance suppression arrangement as claimed in claim 24 or 25.
27. A resonance suppression device substantially as herein described with reference to Figs. 2 and 3.
28. A resonance suppression arrangement substantially as herein described with reference to Figs. 2 and 3.
29. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.