PLENUM RESONANCE PREVENTION FOR GAS TURBINE ENGINE

Abstract

A gas turbine engine comprising at least one radially extending bleed passage in fluid communication with at least one generally circumferentially extending plenum. A plenum has an upstream end in fluid communication with a bleed passage and an outlet for releasing air from the plenum. A plenum further comprises a downstream surface defining a downstream closed end of the plenum and the downstream surface of one or more plenum is/are provided with an outwardly extending projection extending into the plenum.

Claims

1.-22. (canceled)

23. A gas turbine engine, comprising: at least one radially extending bleed passage in fluid communication with at least one generally circumferentially extending plenum, the at least one plenum having an upstream end in fluid communication with an bleed passage and an outlet for releasing air from the plenum, and further comprising a downstream surface defining a downstream closed end of the plenum, wherein the downstream surface of one or more plenum is/are provided with an outwardly extending projection extending into the plenum.

24. The engine of claim 23, wherein all or part of the outwardly extending projections extend from the downstream surface in an upstream direction towards the upstream end of the plenum.

25. The engine of claim 23, wherein an outwardly extending projection reduces the volume of a respective plenum or is arranged in use to cause positive interactions of pressure waves

26. The engine of claim 23, wherein a plenum comprises at least one outwardly extending projection.

27. The engine of claim 23, wherein a plenum comprises multiple outwardly extending projections.

28. The engine of claim 23 wherein the plenum is in the form of a circumferentially extending chamber surrounding a central portion of the engine.

29. The engine of claim 28, wherein the plenum is subdivided into a plurality of individual plenums.

30. The engine of claim 29, wherein each of the plurality of plenums comprises one or more outwardly extending projections.

31. The engine of claim 23, wherein an outwardly extending projection is in the form of a convex surface, the convex surface extending into the plenum.

32. The engine of claim 31, wherein the convex surface is formed by a hollow dome having an extending base portion connected to the downstream surface of a plenum.

33. The engine of claim 23 wherein an outwardly extending projection defines a volume all or partially filled with a sound absorbing material

34. The engine of claim 23, wherein an outer surface of an outwardly extending projection is all or partially covered or coated in a sound absorbing layer or includes a acoustically absorbent material or structure.

35. The engine of claim 23 wherein the closed end of a plenum is provided with coupling portions to receive an outwardly extending projection and to fix an outwardly extending projection thereto.

36. The engine of claim 23, wherein a closed end of one or more plenums is provided with at least one opening to receive an outwardly extending projection therethrough.

37. The engine of claim 23, wherein a radial wall portion of a plenum is provided with at least one opening to receive an outwardly extending projection therethrough.

38. The engine of claim 36, wherein a plenum wall is provided with a coupling arrangement to secure an inwardly extending projection thereto.

39. The engine of claim 23 wherein at least 50% of the surface of the rear end wall of the plenum is provided with an inwardly extending projection.

40. An intermediate compressor structure for a gas turbine engine, the intermediate compressor structure comprising at least one radially extending bleed passage in fluid communication with at least one generally circumferentially extending plenum, the at least one plenum having an upstream end in fluid communication with an bleed passage and an outlet for releasing air from the plenum, each plenum further comprising a downstream surface defining a downstream closed end of the plenum, wherein the downstream surface of one or more plenum is/are provided with an outwardly extending projection extending into the plenum.

41. An intermediate compressor structure for a gas turbine engine comprising a plenum arranged to receive bleed-off air from an up-stream portion of an engine, the plenum comprising one or more inwardly extending projections arranged within the space defining the plenum and arranged in use to cause a change to the acoustic resonant frequency of the plenum.

Description

BRIEF SUMMARY OF THE DRAWINGS

[0032] Examples will now be described, by way of example only, with reference to the accompanying figures in which:

[0033] FIG. 1 shows a cross-section of a gas turbine engine incorporating a plenum;

[0034] FIGS. 2A and 2B show cross-section through an aero-engine plenum;

[0035] FIG. 3 shows a schematic of a conventional plenum volume;

[0036] FIG. 4 shows a schematic of a modified plenum volume according to an example described herein;

[0037] FIG. 5 shows a modified plenum incorporating a plenum insert;

[0038] FIG. 6 shows a plenum insert incorporating a sound abatement layer;

[0039] FIG. 7 shows a segment arrangement of a plenum as described herein;

[0040] FIG. 8 shows a concave plenum insert and installation method; and

[0041] FIGS. 9A, 9B and 9C show an alternative installation method for a modified plenum.

[0042] While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood however that the drawings and detailed description attached hereto are not intended to limit the invention to the particular form disclosed but rather the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention

[0043] It will be recognised that the features of the aspects of the invention(s) described herein can conveniently and interchangeably be used in any suitable combination

DETAILED DESCRIPTION

[0044] FIG. 1 shows a cross-section of a gas turbine engine 1 incorporating an annular plenum 13.

[0045] The skilled person will understand the principal components of a gas turbine engine and their operation. In summary the engine 1 comprises an air intake 2 which permits air to flow into the engine to the fan 3 located at the upstream end of the engine. All of the components are housed within the engine nacelle 4.

[0046] The engine comprises a bypass channel downstream of the fan and a central engine core which contains the compressors, combustors and turbines. The core of the engine is formed of a first low pressure compressor 5 and a second high pressure compressor 6. This multi-stage compressor arrangement takes air from ambient pressure and temperature to high temperature and pressure. Compressed air is then communicated to the combustion chamber 7 where fuel is injected and combustion occurs.

[0047] The combustion gases are expelled from the rear of the combustions chamber 7 and impinge first on a high pressure turbine 9 and then on a second low pressure turbine 10 before leaving the rear of the engine through the core nozzle 11. Thrust from the engine is created by two gas flows: a first from the fan nozzle 8 (receiving thrust from the fan) and secondly from the exhaust gases from the core nozzle 11.

[0048] A transition duct 14 is arranged to receive air from the low pressure compressor 5 and communicate it radially inwards to be supplied to the high pressure compressor 6.

[0049] As shown both compressors are coaxial with the central axis of the turbine. The low pressure compressor 5 has a larger outer radius (measured from the central axis of the compressor) than the outer radius of the high pressure compressor 6 because of the efficiency reasons (examples discussed above).

[0050] This requires that the duct or channel communicating air between the two compressors is a generally S shaped to communicate the compressed air towards the central axis of the turbine and into the high pressure turbine 6.

[0051] As described herein, it is desirable to be able to release or bleed some air within the transition duct out of the engine. The bleed can be also positioned between the last LPC rotor and its OGV, although in such designs the OGV are usually at the very inlet of the transition duct and can be considered to be in it/part of it. This may be used to control the volume of air being passed to the high pressure compressor and prevent a compressor stall, for example.

[0052] As shown in FIG. 1 an outlet 15 is provided which provides an openable passage allowing air to selectively flow from the transition duct 14 to an annular chamber 13, often referred to as a plenum.

[0053] The plenum 13 may be arranged downstream of the low pressure compressor. Specifically the plenum may be arranged radially outside of the core and the bleed passage is usually located downstream of the LPC.

[0054] The plenum is an annular chamber extending all or part of the way around the engine and arranged to receive air that is released from the main flow path. In effect the plenum acts as a collecting chamber or reservoir for air released from the main flow path.

[0055] FIG. 2A illustrates an enlarged cross-section view of the plenum and its position with respect to the core flow or main flow path and the transition duct. The air passing into the plenum in a conventional engine by means of the bleed passage connected to the main flow path at the location A. Air then leaves the plenum through a port or valve B proximate to the bypass channel C of the engine.

[0056] FIG. 2B shows a cross-section through A-A′ in FIG. 2A. FIG. 2B shows an arrangement in which the continuous plenum is sub-divided into 4—13.1, 13.2, 13.3 and 13.4. The spaces between each plenum may be used as part of the structure of the engine and also to allow services/control signals to pass radially inwards and outwards from the engine core. Different arrangements with more sub-divisions or even with no sub-division are possible.

[0057] The plenum may be formed of a discrete component connected to the intermediate compressor structure or alternatively formed by walls themselves forming part of the intermediate compressor structure (ICS). Other terms are “compressor casing” or “compressor frame”. It will be recognised that the walls of the plenum may belong to different components. For example, the plenum may be the space between the intermediate compressor structure, a firewall, a compressor outer casing and the core cowl

[0058] Although the plenum described herein is an annular arrangement, the plenum may be any suitable chamber arranged to receive the released air from the primary flow path.

[0059] FIG. 3 shows a cross-section through one portion of a plenum. It will be recognised that the plenum may be in the form of a torus extending around the engine and coaxial with the axis running along the engine core. The plenum may be a continuous chamber or may be a number of discrete chambers each performing the same function of collecting released air.

[0060] As shown schematically in FIG. 3, air passing through the main flow path 14 can induce flow phenomena at the inlet of passage 15, or transport flow phenomena from the upstream compressor to the inlet of passage 15, which then interacts with the plenum 13. The present disclosure is concerned with the aero-acoustic effects of this arrangement and how airflow in the main flow path can create unwanted acoustic and vibrational effects within the plenum and engine.

[0061] The plenum 13 defines a volume with characteristic resonant frequencies F.sub.r which depend on various factors of the plenum design and volume. The air in the main gas path passing over the inlet of the bleed passage 15 can cause excitation of air within the plenum at, for example, a frequency F.sub.e.

[0062] In situations where the excitation frequency is close to or equal to a resonant frequency (i.e. when F.sub.r=F.sub.e) acoustic resonance may occur within the plenum creating highly undesirable aero-acoustic effects and potentially damaging vibrations within the engine.

[0063] The frequencies at which this resonance may occur will depend on operating conditions of the engine and the acoustic characteristics of the plenum.

[0064] FIG. 4 shows a schematic of a modified plenum volume described herein including the way the acoustic performance may be modified by consuming a portion of the plenum with an inwardly extending projection 16.

[0065] In FIGS. 3 and 4 the outlet from the plenum (which would allow air to be released out of the engine in a generally radial direction) is not illustrated but will be understood by someone skilled in the art of aero-engines.

[0066] In FIG. 4, a modified plenum volume or chamber is provided by means of the projection 16. The cross-section is one example of such a or plurality of plenums.

[0067] More specifically, air (as shown by the dotted arrow) passes over the leading edge L1 and becomes an unstable shear layer of airflow as it passed over the opening to the plenum. The airflow then interacts with the trailing edge surface T1 where the air collides with the trailing edge surface. The unstable shear flow over the opening causes pressure waves W to propagate into the plenum and to resonate within the plenum causing the unwanted aero-acoustic effects.

[0068] According to the present document, in the modified state the plenum has a modified acoustic performance by virtue of the modified geometry of the plenum as shown in FIG. 4. The specific geometry and extent to which the projection extends into the plenum volume is discussed in more detail below.

[0069] FIG. 5, illustrates the inwardly extending projection in more detail.

[0070] The inwardly extending projection 16 is arranged to modify the plenum characteristics in a predetermined way. For a given engine and plenum configuration, the acoustic resonant frequencies of the plenum can be disrupted i.e. modified so as to prevent the unwanted plenum resonance occurring and therefore its damaging acoustic effects.

[0071] As shown in FIG. 5, the plenum comprises an end closed wall or surface 17 which forms the end of the plenum (and extends around all or part of the circumference of the engine). The plenum is arranged so as to extend from the wall 17 in an upstream direction towards the inlet 15. The precise shape and geometry of the plenum and the consequential volume change of the plenum may be calculated using acoustic analysis of the engine and the operating conditions which cause the unwanted resonance. For example the length/and width w of the plenum may be determined based on calculations or trials to establish a geometry that disrupts the resonant frequency. Similarly the curvature of the plenum insert 16 may further be selected according to those determinations.

[0072] The inwardly (that is into the plenum volume) extending projection 16 may be a solid component connected to the wall 17. Alternatively the surfaces of the extending projection may be covered with acoustic liners conveniently selected to provide sound absorbing properties to the projection or comprise a semi permeable wall allowing a weak acoustic communication between the inner and outer volumes of the modified plenum. The modification may be perforated in a similar to the way an acoustic liner face sheet operates.

[0073] In an arrangement where a perforated surface or projection is used a foam layer may be optional.

[0074] The foam may be conveniently selected to further enhance the acoustic properties of the projection. As stated above the surface of the projection extending into the plenum may be perforated to allow sound waves to penetrate into the projection where the foam (or other sound absorbing material) may further enhance the acoustic properties of the projection.

[0075] In such as arrangement the projection functions in multiple ways to control the pressure fluctuations and sound caused in the plenum. First, the geometry of the projection is selected to modify the resonant frequencies of the plenum in order to prevent pressure fluctuations due to resonance. Secondly the projection itself is provided with sound absorbing or sound abatement properties to further reduce the pressure fluctuations and vibrations that may occur as a result of those.

[0076] Alternatively, or additionally, as shown in FIG. 6 the projection may itself be provided with an outer layer 19 of sound absorbing material such as an acoustic liner conventionally used in other parts of a jet engine, e.g. the intake.

[0077] The inwardly extending projection may be any suitable shape according to the engine design and operation. For example the projection could be in the form of one or more projections distributed across the surface 17. The projections may be triangular prisms, convex portions or other suitable shapes which change the resonant frequencies of the plenum by consuming a portion of the internal volume and by promoting a “cancelling-out” interaction of the pressure waves in the plenum.

[0078] FIG. 7 illustrates how the plenum inserts or projections may be sub-divided into a plurality of discrete portions, each portion extending around a portion of the plenum. Advantageously dividing the projection or insert allows inserts to be positioned in a non-continuous plenum i.e. a plenum that does not extend continuously around the engine circumference but is divided into individual sections or segments.

[0079] The plenum projections or inserts may be located into a plenum in a variety of ways.

[0080] Referring to FIG. 8, one such installation arrangement is illustrated. Here the surface 17 is provided with a plurality of apertures 20 through the surface and into the plenum. In the example shown a conical projection insert 21 is formed and located into each of the apertures. The ring 22 of the projections may then be coupled to the wall to hold the projections in position. The coupling may be a permanent coupling such as by welding or may alternatively be a selectively removable coupling allowing the insert to be removed and re-installed.

[0081] An arrangement shown in FIG. 8 may be designed as part of the initial engine design or may advantageously be retro-fitted to an existing engine or engine design by modification of the end wall of the plenum.

[0082] For a retrofit application, an aero-acoustic and also a structural assessment of the design would be needed. Then other changes could be required (reinforcement of that area, for instance).

[0083] FIGS. 9A, 9B and 9C show an alternative installation arrangement for a plenum insert as described herein. In such an arrangement the plenum insert 22 is positioned into the plenum through an aperture 23 located on an outer surface of the plenum body. The aperture may be a removable panel for example allowing access to the plenum.

[0084] According to the installation arrangement the insert 22 passes through the aperture and is located against the rear wall 17 of the plenum. The insert may then be coupled to the wall structure, for example using bolts, rivets or other suitable connections to secure the insert to the inner wall of the plenum.

[0085] Furthermore, the plenum inserts may be non-uniformly spaced around the circumference of the engine, for example some plenums may not include an insert at all. Advantageously at least 50% of the circumference of the plenum may be provided with a suitable plenum insert i.e. at least 50% of the plenum circumference may be modified.