Abstract:
A noise reduction device, for example for use in a bleed assembly of a gas turbine engine, comprises partitions having apertures which provide contractions and sudden expansions of flow passing through the device, turbulators being provided between the partitions to break up jets issuing from the Apertures. Breaking up the jets before impact with the partitions enables the partitions to be disposed closer together, so enabling adequate noise reduction to be achieved in a device of relatively small thickness.

Description:
TECHNICAL FIELD 
     This invention relates to a noise reduction device, and is particularly, although not exclusively, concerned with such a device for use with a bleed valve in a gas turbine engine to release compressed air from a compressor stage to a bypass duct of the engine. 
     BACKGROUND 
     When a gas turbine engine is operating under transient conditions, for example when decelerating, it may be necessary to bleed air at high pressure from the core gas flow through the engine. Such air may be transferred to a bypass flow within the engine. Bleed valves are provided to control this transfer of air. The flow of bleed air from the core gas flow into the bypass flow takes place over a substantial pressure drop, and can generate significant noise. It is therefore usual to provide a noise reduction device in, or at the exit of, the flow passage between the core gas flow and the bypass duct. A typical measure is to discharge the bleed air into the bypass duct through a perforated plate, sometimes referred to as a “pepper pot” as disclosed, for example, in US2001/0042368. The pepper pot serves to break the single body of air flowing towards the bypass duct into a large number of smaller jets which promote small-scale turbulence and hence quicker mixing with the main flow through the bypass duct. 
     SUMMARY OF THE INVENTION 
     In order to avoid a single large pressure drop and sudden expansion from the high pressure core flow to the bypass flow, two or more pepper pots have been used in series, in order to break the single large pressure drop into a series of smaller pressure drops. Pepper pots are typically made from thin metallic sheets in which holes are formed, for example by laser cutting, and tend to be expensive. If a series of pepper pots are used downstream of a single bleed valve, the cost is multiplied. Also, pepper pots are subjected to high transient pressure drops, and the shock loadings can cause them to deform or disintegrate. 
     According to the present invention there is provided a noise reduction device for a flow of gas, the device comprising a flow passage and a partition extending across the flow passage, the partition being provided with apertures which cause contraction followed by sudden expansion of flow passing through the flow passage, the partition bounding a turbulence zone of the flow passage, the turbulence zone being provided with turbulators for enhancing turbulence in gas flowing through the turbulence zone towards the partition. 
     The turbulators may be distributed along the flow direction in the turbulence zone, and may comprise elongate elements which extend transversely of the general flow direction in the flow passage. The turbulators may be disposed obliquely with respect to the partition. 
     The partition may be situated at an exit from the noise reduction device, but in an alternative embodiment, further turbulators may be provided between the partition and the exit. 
     In one embodiment, the turbulators may comprise diagonal struts extending between the partition, comprising a first partition, and a second partition spaced from the first partition. At least some of the turbulators may be arranged to form truss structures, each truss structure comprising a row of struts lying in a common plane extending between the first partition and the second partition, adjacent struts extending diagonally between the partitions at opposite angles of inclination. Adjacent truss structures may be arranged out of phase with each other, so that the struts of one truss structure cross a respective strut of the other truss structure. 
     The apertures in the partitions may be disposed in rows which lie in the common planes of the respective truss structures. 
     The partition, or the first partition, and the turbulators disposed in the turbulence zone bounded by the partition, or by the first partition, may constitute a noise reduction stage, the device comprising at least one further noise reduction stage comprising a further partition bounding a further turbulence zone provided with further turbulators. The stages may comprise a pre-formed unit or cassette installed in the flow passage. 
     The total area of the apertures in one of the partitions may be greater than the total area of the apertures in another, upstream, partition. The variation in total area may be achieved by providing different numbers of apertures in the respective partitions, and/or by varying the cross-sectional areas of the apertures. 
     At least part of the pre-formed unit may be manufactured by a stereolithographic process, such as a selective laser sintering process or a laser direct metal deposition process. Such processes enable the manufacture of complex structures, including the or each partition and the turbulators. 
     The present invention also provides a bleed valve assembly for a gas turbine engine, the assembly comprising a noise reduction device as defined above, having a bleed valve at an inlet end of the flow passage. The present invention also provides a gas turbine engine having a compressor, a bypass duct, and a bleed valve assembly as defined above, the flow passage extending between the compressor and the bypass duct. 
     For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of a gas turbine engine; 
         FIG. 2  is a sectional view of a bleed assembly; 
         FIGS. 3 and 4  correspond to  FIG. 2  but show alternative embodiments of the bleed assembly; 
         FIG. 5  is a diagrammatic view of a noise reduction device of the bleed assemblies of  FIGS. 2 to 4 ; 
         FIG. 6  is an alternative view of part of the structure of  FIG. 5 ; 
         FIG. 7  is a view in the direction of the arrow VII in  FIG. 6 ; 
         FIG. 8  is a diagrammatic representation of a variant of the structure shown in  FIG. 6 ; and 
         FIG. 9  shows a further alternative structure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , an intermediate pressure turbine  18 , a low-pressure turbine  19  and a core exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines the intake  12 , a bypass duct  22  and an exhaust nozzle  23 . 
     The gas turbine engine  10  works in the conventional manner so that air entering the intake  11  is accelerated by the fan  13  to produce two air flows: a first airflow A into the intermediate pressure compressor  14  and a second airflow B which passes through the bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the airflow A directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  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  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  17 ,  18 ,  19  respectively drive the high and intermediate pressure compressors  15 ,  14  and the fan  13  by suitable interconnecting shafts. 
     The fan  13  is circumferentially surrounded by a structural member in the form of a fan casing  24 , which is supported by an annular array of outlet guide vanes  28 . The fan casing  24  comprises a rigid containment casing  25  and attached inwardly thereto is a rear fan casing  26 . 
     During engine operations and particularly when changing rotational speed at low power it is important to ensure that the pressure ratio across each compressor  14 ,  15  remains below a critical working point, otherwise the engine  10  can surge and flow through the engine  10  breaks down. This can cause damage to engine&#39;s components as well as aircraft handling problems. 
     To maintain a preferred pressure difference across a compressor  14 ,  15 , or even just one stage of a compressor  14 ,  15 , bleed assemblies  30  are provided to release pressure from an upstream part of a compressor  14 ,  15 . Operation of a bleed assembly  30  and engine operability are described in “The Jet Engine” 6th Edition, 2005, Rolls-Royce plc, pages 79-80, and details of such operation will therefore only be briefly mentioned herein. 
       FIG. 2  shows one of the bleed assemblies  30 . Each bleed assembly  30  comprises an inlet  32 , a bleed valve  34 , a duct  36  and an outlet  38 . A noise reduction device  40  is situated at the junction between the duct  36  and the outlet  38 . The duct  36  and the outlet  38  together define a flow passage  39 . Parts of core engine airflow A may be diverted through the bleed assembly  30  by opening the bleed valve  34 , such that the bleed airflow enters the inlet  32 , passes through the bleed valve  34  and is channelled the flow passage  39  defined by the duct  36  and the outlet  38  into the bypass flow B in the bypass duct  22 . There is usually an annular array of bleed valves around the core engine&#39;s casing  27 . 
     When the bleed valve  34  is open, one or more high velocity jets of air from the compressor  14  or  15  travel through the bleed duct  36 . If these jets of air are discharged directly into the bypass duct  22 , the resulting energy transfer between the bleed air and bypass flow B and the turbulence that is created in the bypass flow B, generates substantial noise. In order to avoid this, the noise reducing device  40  is provided. 
     In the embodiment shown in  FIG. 2 , a baffle  42  is provided at the exit from the bleed valve  34 . In an alternative embodiment a baffle plate  42  is not provided. The noise reducing device  40  is in the form of a monolithic component, or cassette, which is supported at the junction between the bleed duct  36  and outlet  38  on a partition  44 . 
     The internal structure of the device  40  is illustrated in  FIGS. 5 to 7 . As shown in  FIG. 5 , inward flow from the bleed valve  34  (or baffle  42 ) is indicated by arrows  46 , and outlet flow into the bypass chamber  22  is indicated by arrows  48 . As the flow progresses through the device  40 , it encounters, in sequence, a first turbulence zone  50 , a first apertured partition  52 , a second turbulence zone  54 , a second apertured partition  56 , a third turbulence zone  58  and a third apertured partition  60 . The successive sets of partitions  52 ,  56 ,  60  and the associated turbulence zones  50 ,  54 ,  58  can be regarded as individual noise reduction stages of the noise reduction device, the device shown in  FIG. 5  thus having three stages. 
     Each of the turbulence zones  50 ,  54 ,  58  is provided with a respective array of turbulators  62 , one of which is shown, for illustrative purposes, in  FIG. 6 . The turbulators  62  of the first turbulence zone  50  serve to break up the jet or jets of bleed air issuing from the bleed valve  34 . Thus, the kinetic energy of the flow is converted into heat. The flow then passes through apertures  64  in the partition  52 , from which it issues as a further series of jets which are, in turn, broken up by the turbulators  62  of the second turbulence zone  54  before reaching the second partition  56 . The process is then repeated through the partition  56 , the turbulence zone  58  and the partition  60 . 
     At each partition  52 ,  56  and  60 , the respective apertures  64 ,  66 ,  68  act as contractions in the flow, followed by sudden expansions, causing successive pressure drops across the partitions  52 ,  56 ,  60 . As the pressure reduces through the device, the volume of the bleed gas expands, and consequently the successive partitions  52 ,  56 ,  60  have an increased total flow cross-section which is achieved either by increasing the size of each aperture  66 ,  68  or by increasing the number of apertures  66 ,  68 , or both, by comparison with the upstream apertures  64 . 
       FIG. 6  represents the structure between the partitions  56  and  60  of  FIG. 5 .  FIG. 5  can be regarded as a sectional view in the direction of the arrow V in  FIG. 6 . 
     The internal structure comprises an array of walls  70 ,  72  which intersect one another at right angles. The walls  72  correspond to the partitions  52 ,  56 ,  60  in  FIG. 5 , and are provided with the apertures  64 ,  66 ,  68 , one of which is indicated in  FIG. 7 , as a slot extending over the full width of a wall  72  between adjacent intersections with walls  70 . 
     An arrow F indicates the direction of flow through the structure. It will be appreciated that each section of wall may have the aperture (slot)  64 ,  66 ,  68  at different positions, and consequently flow in the direction F through the aperture  64  in one wall  72  will need to deflect in order to reach the aperture  66  in the next wall  72 . Also, it will be appreciated that the turbulators  62  extend across each cell formed by the walls  70 ,  72 , as indicated in  FIG. 6  by a single turbulator pin  62 . It will be appreciated that each cell has a plurality of such pins, as indicated in  FIG. 5 . Consequently, as described above, the flow undergoes a series of contractions at the apertures (slots)  64 ,  66 ,  68 , followed by sudden expansions, the resulting jet being broken up by impact with the turbulators  62 . 
     The structure shown in  FIGS. 5 and 6  may be formed by means of a stereolithographic process, such as selective laser sintering or laser direct metal deposition, sometimes referred to as Powder Bed direct laser deposition (DLD). In such methods, the structure is built up from a base plate, for example a base plate positioned at the bottom of  FIG. 6 , using a metal powder which is melted by a laser at locations where the structure is to be formed, but which is removed at locations which are not melted. Such processes enable complex internal structures to be formed. 
     In order to reduce the thickness of the cassette  40  in the general flow direction, it is desirable to form the internal structure, including the apertured partitions  52 ,  56 ,  60 , in such a way that the flow is accelerated in the apertures  64 ,  66 ,  68  to as high a velocity as possible within acceptable noise limits. This usually means acceleration of the flow to high subsonic or low supersonic velocities. However, higher speeds result in greater noise generation, and consequently the number of stages (ie the number of partitions  52 ,  56 ,  60 ) is selected to achieve a relatively compact structure while minimising the generation of noise. By introducing the turbulators  62  between adjacent partitions  52 ,  56 ,  60 , the high speed jets issuing from the apertures  64 ,  66 ,  68  are baffled efficiently, so mixing the jets in a short space, before they encounter the succeeding partitions  56 ,  60 . 
     As a result of the structure disclosed in  FIGS. 5 to 7 , it is possible to construct a noise reduction device  40  with compact dimensions (having a thickness of the order of 10 mm), in a single structure, all or most of which may comprise a monolithic component. The structure has sufficient thickness to have good mechanical properties, including sufficient strength to resist the bending forces created by the pressure differential across the device  40 . Similarly, the structure is able to withstand vibration. 
     The walls  70 ,  72  may have a maximum span between intersections of the order of 5 mm, which means they can be made relatively thin (for example around 0.3 mm) and light in weight. 
     The turbulators  62  are represented in  FIG. 5  as pins of substantially circular cross-section, but it will be appreciated that other cross-sections, such as oval, triangular or rectangular, may be used. The turbulators  62  may have a diameter, or equivalent dimension, of approximately 0.3 mm, and the passages defined by the walls  70 ,  72  may have dimensions of approximately 5 mm by 15 mm, with the width of the apertures (slots)  64 ,  66 ,  68  being approximately 2 mm. As mentioned above, the number of apertures in the partitions  52 ,  56 ,  60  may be increased in the flow direction to allow for the increasing volume flow as the pressure reduces across the device  40 . 
     A further embodiment is shown in  FIG. 9 , in which apertured partitions  84 ,  86  are supported with respect to each other by truss structures each comprising struts  88 ,  90  arranged as alternating diagonals, in the manner of a warren truss. In  FIG. 9 , struts  88 ,  90  lie in the plane of the Figure, while struts  88 ′  90 ′ extend in a plane parallel to that of the Figure and displaced out of that plane. It will be appreciated that further such truss structures  88 ,  90 , with adjacent structures being out of phase with one another, are situated in further parallel planes. The planes may, for example, contain rows of apertures  92 ,  94  in the partitions  84 ,  86 . Thus, for example, the struts  88 ,  90  may be aligned with one row of holes  92 ,  94 , while the struts  88 ′  90 ′ are aligned with an adjacent row of holes  92 ,  94 . 
     The struts  88 ,  90 ,  88 ′,  90 ′, as well as supporting the partitions  84 ,  86  with respect to one another, constitute turbulators having the same function as the turbulators  62  in  FIG. 5 . Thus, flow entering the structure through the apertures  92  is forced to deviate out of the plane of the Figure on encountering the strut  90  shown in  FIG. 3 , as indicated by arrow  98 . Subsequently, the flow must divert again, arrow  100 , to pass over the strut  88 ′ shown in the Figure before issuing from the apertures  94  in the partition  86 . The struts  88 ,  90 ,  88 ′,  90 ′ thus serve not only to break up the jets issuing from the apertures  92 , but also to force the flow to undergo a tortuous path, further causing mixing-in of the jet flows. 
     Although the apertures  92  in the partition  84  and the apertures  94  in the partition  86  are shown as having the same size and the same pitch as each other, this is for the sake of simplicity; as with the embodiment of  FIG. 5 , the downstream apertures will have a smaller pitch and/or a larger size to allow for the increasing volume flow rate in the downstream direction. 
       FIGS. 3 and 4  show alternative bleed assemblies  30 . In the embodiment of  FIG. 3 , the outlet  38  is closed by a pepper pot diffuser  96 , which may be of conventional form. In the embodiment of  FIG. 4 , the noise reducing device  40  and the pepper pot  96  are integrated into a single cassette, simplifying the manufacture of the bleed assembly, with a reduced number of components. In the embodiments of  FIGS. 3 and 4 , the pepper pot  96  could be replaced by a vaned outlet, or otherwise suitably configured to control the plume entering the bypass duct by directing the flow in a desired direction or pattern.