FLUID EJECTOR

Abstract

A fluid ejector having: a diffuser duct for mixing of a first gas stream with a second gas stream having faster velocity than the first gas stream, wherein the diffuser duct extends along an axial direction; and wherein, in cross-section perpendicular to the axial direction, the diffuser duct is defined between a first wall extending around the axial direction, and a second wall extending around the axial direction, radially within the first wall.

Claims

1. A fluid ejector having: a diffuser duct for mixing of a first gas stream with a second gas stream having faster velocity than the first gas stream, wherein the diffuser duct extends along an axial direction; and wherein, in cross-section perpendicular to the axial direction, the diffuser duct is defined between a first wall extending around the axial direction, and a second wall extending around the axial direction, radially within the first wall.

2. The fluid ejector of claim 1, wherein the diffuser duct is a first diffuser duct, and wherein the fluid ejector includes: a second diffuser duct extending along the axial direction and extending within the second wall or outside the first wall, wherein the second diffuser duct is for mixing a third gas stream and a fourth gas stream, the third gas stream from the same source as the first gas stream and the fourth gas stream from the same source as the second gas stream.

3. The fluid ejector of claim 2, wherein in cross-section perpendicular to the axial direction, the second diffuser duct comprises a continuous space bounded the second wall.

4. The fluid ejector of claim 2, wherein in cross-section perpendicular to the axial direction, the second diffuser duct is defined between a perimeter wall extending around the axial direction and the first wall, wherein the first wall is radially within the perimeter wall.

5. The fluid ejector of claim 2, wherein the first and second diffuser ducts are contiguous in the radial direction, such that first wall forms an inner surface or outer surface of the second diffuser duct.

6. The fluid ejector of claim 2, including one or more further diffuser ducts, each further diffuser duct extending along the axial direction and arranged radially inside or outside the first and second diffuser ducts, each further diffuser duct for mixing a gas stream from the same source as the first gas stream and a gas stream from the same source as the second gas stream.

7. The fluid ejector of claim 2, wherein the ejector comprises a passage divided into two or more diffuser ducts in a radial direction, and wherein a ratio between the length of the passage along the axial direction, and a diameter of the passage in the radial direction is no more than 6:1.

8. The fluid ejector of claim 7, wherein the ratio between the length of the passage, and the diameter of the passage is no more than 4.5:1.

9. The fluid ejector of claim 1, wherein the diffuser duct extends from a first end to a second end, along the axial direction; wherein the first and second gas streams are provided at or near the first end, and travel along the axial direction such that a mixture of the first and second gas streams is provided at the second end of the or each duct; and wherein a radial width of the diffuser duct decreases along the axial length from the first end to a constriction, and widens along the axial length from the constriction to the second end, such that the diffuser duct forms a Venturi tube.

10. The fluid ejector of claim 1, wherein the diffuser duct includes one or more inlets for the first gas stream, and one or more inlets for the second gas stream.

11. The fluid ejector of claim 10, wherein the one or more inlets for the second gas stream comprises one or more separate nozzles arranged around the or each diffuser duct, at or near the first end.

12. The fluid ejector of claim 10, wherein the one or more inlets for the second gas stream comprises an annular nozzle provided at or near the first end of the diffuser duct.

13. The fluid ejector of claim 1, wherein the fluid ejector comprises a cabin bleed ejector for providing air from a compressor stage of a gas turbine engine of an aircraft to a passenger cabin of the aircraft.

14. The fluid ejector of claim 13, wherein the fluid ejector is arranged to take a first gas stream of air from a first compressor stage of a gas turbine engine, and the second stream of air from a second compressor stage, the second compressor stage operating at higher pressure than the first compressor stage.

15. A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and a fluid ejector according to claim 13, arranged to provide air from the compressor to a cabin of an aircraft on which the engine is mounted.

16. The gas turbine engine of claim 15, wherein: the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft; the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft; and the first gas stream of the fluid ejector is taken from the first compressor or the second compressor, and the second gas stream is taken from the second compressor.

17. The gas turbine engine of claim 15, including: a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft.

18. A fluid ejector having: a passage extending along an axial direction from a first end to a second end, opposite the first, the diffuser duct arranged to mix a first gas stream with a second gas stream having faster velocity than the first gas stream, and provide the mixture at the second end; one or more inlets for the first gas stream at the first end of the diffuser duct; and a nozzle arrangement for providing the second gas stream at the second end of the diffuser duct, wherein the nozzle arrangement includes a plurality of nozzles arranged concentrically around the axial direction.

19. The fluid ejector of claim 18, wherein the nozzle arrangement includes: a cylindrical nozzle arranged along the axial direction; and an annular nozzle or arrangement of nozzles arranged around the cylindrical nozzle.

20. The fluid ejector of claim 18, wherein: the passage includes an inner wall extending within the passage; the inner wall is annular around the axial direction such that the passage is split into a cylindrical duct, and an annular duct around the cylindrical duct; and the nozzle arrangement is arranged to provide the second gas stream into the cylindrical duct and the annular duct.

Description

DESCRIPTION OF THE DRAWINGS

[0064] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0065] FIG. 1 is a sectional side view of a gas turbine engine;

[0066] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0067] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0068] FIG. 4 is a schematic view of a cabin air bleed system;

[0069] FIG. 5A is a schematic, side on sectional view of the ejector of the cabin air bleed system of FIG. 4;

[0070] FIG. 5B is a schematic, front on sectional view of the ejector of FIG. 5A;

[0071] FIG. 6 is a schematic sectional perspective view of a diffuser of an alternative ejector;

[0072] FIG. 7A is a schematic, side on sectional view of an alternative ejector for the cabin air bleed system of FIG. 4; and

[0073] FIG. 7B is a schematic, side on sectional view of the alternative ejector of FIG. 7A.

DETAILED DESCRIPTION

[0074] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0075] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into 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 is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0076] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to process around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0077] Note that the terms low pressure turbine and low pressure compressor as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the low pressure turbine and low pressure compressor referred to herein may alternatively be known as the intermediate pressure turbine and intermediate pressure compressor. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0078] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the present disclosure. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0079] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0080] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0081] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0082] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0083] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0084] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0085] FIG. 4 schematically illustrates a system 50 for providing pressurised air into a passenger cabin 52 of an aircraft 54. A gas turbine engine 10, as discussed above, is mounted on the aircraft 54, from the wing or fuselage.

[0086] The gas turbine engine 10 includes a cabin bleed ejector 56, which takes a low-pressure air supply 58 from the low pressure compressor 14 of the engine 10, and a high pressure air supply 60 from the high pressure compressor 15. As will be described in more detail below, the ejector 56 uses the high pressure supply 60 to increase the pressure of the low pressure supply 58. Air is then provided from the ejector 56, into the cabin 54, through cabin air ducting 62.

[0087] FIGS. 5A and 5B illustrate a first example embodiment of the ejector 56 in more detail. The ejector 56 extends along an ejector axis 64. FIG. 5A illustrates the ejector 56 in cross section along the axial direction 64, whilst FIG. 5B illustrates the ejector 56 in cross section perpendicular to the axis 64.

[0088] The ejector 56 includes an outer wall 66. The outer wall 66 is annular and extends along and around the ejector axis 64, defining a passage 68 extending from a first end 70 of the ejector 56 to a second end 72. The first end 70 of the passage 68 is closed by an end wall (not shown), or the like, whilst the second end 72 is open at an opening 108.

[0089] An inner wall 74 is provided within the passage 68. The inner wall 74 is also is annular and extends along and around the ejector axis 64. The inner wall 74 splits the passage 68 into a first diffuser duct 76, and a second diffuser duct 78. The first diffuser duct 76 is annular in cross-section perpendicular to the ejector axis 64, whilst the second diffuser duct 78 is cylindrical, extending through the centre of the first diffused duct 76. Thus, the diffuser ducts 76, 78, separated by the inner wall 74, are concentric and contiguous.

[0090] As best shown in FIG. 5A, along the length of the ejector 56 from the first end 70, the outer wall 66 tapers inwards towards the ejector axis 64 until it reaches a narrow point or waist 80. The outer wall 66 then tapers outwards from the axis 64 after the waist 80. As such, the diameter of the passage 68, measured from the inner surface 92 of the outer wall 66, reduces towards the waist 80 from either the first end 70 or the second end 72 of the ejector 56.

[0091] The inner wall 74 includes an inner surface 82 facing radially inwards, and an outer surface 84 facing radially outwards. As also best shown in FIG. 5A, along the axial length of the ejector 56 from the first end 70, the thickness of inner wall varies, so that a radial spacing between the inner and outer surfaces 82, 84 varies. Along the axial direction 64, the spacing between the inner and outer surfaces 82, 84 increases from the first end 70 to a maximum spacing 86, and then reduces towards the second end 72 of the ejector 56.

[0092] The annular diffuser duct 76 has a radial width 88 defined between the outer surface 84 of the inner wall 74, and an inner surface 92 of the outer wall 66. Along the axial direction 64, the waist 80 in the outer wall 66 aligns with the maximum spacing 86 between the inner and outer surface 82, 84 of the inner wall 74. Therefore, along the axial direction 64 from the first end 70, the radial width 88 of the annular duct 76 narrows to a constriction 90a, and then widens to the second end 72. As such, the annular duct 76 forms a Venturi tube for gas passing through it.

[0093] Similarly, the cylindrical duct 78 has a radial width 94 defined by the spacing of the inner surface 82 of the inner wall 74 from the axis 64. Therefore, the radial width 94 of the cylindrical duct 78 also narrows, from the first end 72 to a constriction 90b along the axial length of the ejector 56, before widening towards the second end 72. Thus the cylindrical duct 78 also forms a Venturi tube for gas passing through it.

[0094] As best shown in FIG. 5A, at the first end 70 of the ejector 56, a first end 110 of the outer wall 66 is flush with a first end 112 of the inner wall 74. However, at the second end 72 of the ejector 56, a second end 114 of the outer wall 66 extends beyond a second end 116 of the inner wall 74. As such, the cylindrical diffuser duct 78 has an opening 118 that opens into the passage 68, before the second end 72 of the passage 68.

[0095] Each diffuser duct 76, 78 includes respective inlets 96a, 96b for a low pressure stream of gas 98a, 98b, such as air from a low pressure compressor stage 14 of the engine 10. In the example shown, the inlets 96a, 96b are in the form of openings provided in the wall at the first end 70 of the ejector 56.

[0096] Each diffuser duct 76, 78 also includes an inlet 100a, 100b for a high pressure stream of gas 102a, 102b, such as air from a high pressure compressor stage 15 of the gas turbine engine 10.

[0097] The inlets 100a, 100b for the high pressure stream 102a, 102b are in the form of nozzles that include a narrow (relative to the conduit feeding the stream) opening at the point of discharge. In the annular diffuser duct 76, the inlet is formed by an annular nozzle 104 extending around the first end 70 of the ejector 56. In the cylindrical diffuser duct 78, the inlet is in the form of a simple cylindrical nozzle 106 at the first end 70 of the ejector 56.

[0098] In use, air from the high pressure compressor 15 is provided in to the diffuser ducts 76, 78 through the nozzles 104, 106. The nozzles 104, 106 accelerate the air from the high pressure compressor 15 and so eject a high velocity stream of air 102a, 102b into the diffuser ducts 76, 78.

[0099] The high velocity stream 102a, 102b in each diffuser duct 76, 78 then draws in a low velocity stream of air 98a, 98b from the low pressure compressor 14 through the respective inlets 96a, 96b.

[0100] The incoming streams 98, 102 are mixed along the length of the diffuser ducts 76, 78. Furthermore, the constrictions 90a, 90b and subsequent widening of the diffuser ducts 76, 78 towards the second end 72, causes a reduction in the velocity and an increase in pressure of the mixture due to the Venturi effect. The mixture of air streams 98, 102, is provided through the opening 108 at the second end 72 of the passage 68.

[0101] The air at the opening 108 is at lower pressure than the air from the high pressure compressor 15. However, the effect of the ejector 56 means that the air pressure is above the pressure of the air from the low pressure compressor 14. Thus, a small amount of air from a high pressure source can be used to provide pressurised air into the cabin 52.

[0102] FIG. 6 illustrates an alternative example of an ejector 56. In the example shown in FIG. 6, the ejector 56 has three diffuser ducts 76, 78, 120. As in the example discussed in relation to FIGS. 5A and 5B, a cylindrical diffuser duct 78 is arranged within an annular diffuser duct 78. However, in this example, a further annular diffuser duct 120 is provided concentrically outside the first annular duct 76.

[0103] In the example shown in FIG. 6, the outer wall 66 defines a passage 68, as discussed above. The passage 68 is split into three diffuser ducts 76, 78, 120 by Also, as discussed above, an inner wall 74 defines a first and second inner walls 74, 124. As in the example discussed above, the inner walls 74, 124 extend annularly around the ejector axis 64, and along the ejector axis 64, in order to split the passage 68 into the respective diffuser ducts 76, 78, 120.

[0104] A first inner wall 74 defines the cylindrical diffuser duct 78 along the centre of the passage 68. The first annular passage 76 is defined radially outside the first inner wall 74, between the first inner wall 74 and the second inner wall 124. The second annular passage 120 is defined radially outside the second inner wall 124, between the second inner wall 124 and the outer wall 66.

[0105] As such, the passages 76, 78, 120 are arranged concentrically and are contiguous.

[0106] At the first end 70 of the ejector 56, the walls 66, 74, 124 are flush. However, at the second end 72 of the ejector 56, the ends of the walls 66, 74, 124 are staggered, such that the innermost wall 74 (the first inner wall) extends the shortest distance along the axial direction 64, and the outer wall 66 extends the longest. As such, the end of the first inner wall 74 is within the second inner wall 124, and the end of the second inner wall 124, is within the outer wall 66.

[0107] The inner surface 92 of the outer wall 66 of the ejector 56 shown in FIG. 6 tapers in from the first end 70 of the ejector 56 to a waist 80, and then outwards towards the second end 72, in a similar manner as described in relation to FIG. 5A. In this example, the radial thickness of the wall 66 increases and reduces to form the waist 80, rather than the wall 66. It will be appreciated that the waist 80 can be formed in either manner.

[0108] Similarly, the radial spacing between the inner surface 82 and outer surface 84 of each inner wall 74,124 increases and then decreases along the axial direction.

[0109] Therefore, each diffuser duct 76, 78, 120 includes a constriction 90a, 90b, 90c as discussed above. Furthermore, each diffuser duct 76, 78, 120 includes inlets in a similar manner to the ejector 56 discussed in relation to FIGS. 5A and 5B. Therefore, in use, the ejector 56 of FIG. 6 works in a similar manner to the ejector 56 of FIGS. 5A and 5B.

[0110] FIGS. 7A and 7B illustrate an example of an alternative embodiment of an ejector 56.

[0111] As in the example discussed above, the ejector 56 is formed by an outer wall 66 that is annular in cross section perpendicular to the ejector axis 64. The outer wall 66 extends along and around the ejector axis 64, defining a passage 68 extending from a first end 70 of the ejector 56 to a second end 72. The first end 70 of the passage 68 is closed by an end wall (not shown), including inlets for air, discussed below in more detail. The end wall may have any suitable structure or shape. The second end 72 of the passage 68 is open, forming an opening 108.

[0112] In this example, the ejector 56 has a single diffuser duct 78 that is cylindrical in cross section perpendicular to the axial direction 64. Thus, in this example, the passage 68 is not divided into separate diffuser ducts.

[0113] The diffuser duct 78 includes inlets 96a for a low pressure stream of gas 98a such as air from a low pressure compressor stage 14 of the engine 10. As discussed in the examples above, the inlets 96a are in the form of openings provided in the wall at the first end 70 of the ejector 56.

[0114] The diffuser duct 76 also includes a plurality of inlets 100a, 100b for a high pressure stream of gas 102a, 102b, such as air from a high pressure compressor stage 15 of the gas turbine engine 10.

[0115] A first inlet 100a is in the form of cylindrical nozzle 106 arranged at the centre of the end wall (i.e. on the axis). A second inlet 100b is formed by an annular nozzle 104 extending around the first end 70 of the ejector 56, and thus around the cylindrical nozzle 106. Thus, the ejector 56 of FIGS. 7A and 7B is similar to the ejector 56 of FIGS. 4A and 4B, with the inner wall 74 removed, so that the passage 68 forms a single duct 78.

[0116] As discussed in relation to FIGS. 5A and 5B, the diffuser duct 78 also includes a constriction 90 along its axial length, such that the diffuser duct 78 forms a Venturi tube. As discussed above, the constriction is formed by narrowing the passage 68, and then subsequently widening it, along the axial length from the first end 70 to the second end 72.

[0117] In use, the ejector 56 discussed in relation to FIGS. 7A and 7B works in a similar manner to the ejector 56 discussed in relation to FIGS. 5 and 6, using the high pressure gas to accelerate the low pressure gas.

[0118] From the example discussed above, it will be appreciated that the ejector may include any number of diffuser ducts 76, 78, 120. In one example, there may be a single annular diffuser duct 76, a single cylindrical diffuser duct 78, with a concentric arrangement of nozzles 104, 106.

[0119] In alternative examples, there may be two or more concentric, contiguous ducts 76, 78, 120. In some cases, a cylindrical duct 78 may be provided at the centre of the arrangement. However, in other cases, the ducts may all be annular, and the central space may be left as a void for other components.

[0120] The structure of the diffuser ducts 76, 78, 120 given above is by way of example only. Any suitable structure may be adopted.

[0121] In one example, the walls 66, 74, 124 may be supported by struts (not shown) extending radially through the passages 76, 78, 120, and may be mounted from the inner most wall 74, 124 to an axially extending support structure (not shown) or from the outer wall 66 to an external support structure. In other examples, the struts may be provided at one or both ends of the ejector 56.

[0122] Where ducts 76, 120 have been described as annular, it will be appreciated that this may include any suitable annular shape. For example, as shown in the Figures, the annular ducts 76, 120 may have a constant radial height around their circumference. However, in other examples, the annular ducts 76, 120 may have varying radial height around the circumference.

[0123] Although the ducts 76, 78, 120 shown are formed by walls that are circular in cross section, the walls may have any shape. For example the walls may be elliptical, square, rectangular, or any suitable shape.

[0124] In any of the above examples, the length of the passage 68, measured from the point along the axial direction 64 at which the high pressure stream (120a,b) is introduced into the low pressure stream (98a,b), to the second end 72 is at most six times the diameter of the passage 68 formed by the outer wall 66. Sufficient mixing of the gas streams can be achieved in this length. In some cases, the length may be at most four times the diameter of the passage 86.

[0125] For an annular diffuser duct 76, 120, the ratio of the length of the duct 76, 120 to the radial height may be 7:1. It will, however, be appreciated that the radial height of an annular duct 76, 120 is significantly less than the radius from the axis 64 to the outer wall of the annular duct, and so the length is significantly less than a cylindrical duct having the same outer diameter.

[0126] Where two or more ducts 76, 78, 120 are provided, it will be appreciated that addition of further ducts enables further reduction in the overall length, whilst achieving the same mixing.

[0127] Although in examples with multiple diffuser ducts 76, 78, 120 the ejector 56 has been described as a single ejector having multiple diffuser ducts 76, 78, 120 each with corresponding inlets, it will be appreciated that it may also be considered as an ejector system having multiple ejectors, arranged concentrically and contiguously. Each diffuser duct and the associated inlets form one of the ejectors.

[0128] In the examples with multiple diffuser ducts 76, 78, 120 discussed above, the ends of the diffuser ducts 76, 78, 120 are staggered such that the outer ducts 78, 120 extend further along the axial length. This is by way of example only, and the ducts 76, 78, 120 may be flush at both ends. In yet further examples, the ducts 76, 78, 120 may be staggered at the first end 70 of the ejector 56 instead of or as well as at the second end 72. In yet further examples, the ducts 76, 78, 120 may have different lengths, but the arrangement is not necessarily staggered from inner to outer.

[0129] The arrangement of the diffuser ducts 76, 78, 120, as Venturi tubes, with constrictions 90, is optional, and may be omitted. In some examples having multiple diffuser ducts 76, 78, 120, some but not all of the ducts may be arranged as Venturi tubes.

[0130] Where ducts 76, 78, 120 are arranged with constrictions 90, this may be achieved in any suitable manner. For example, the walls may thicken or taper as discussed above, or inserts may be provided to form the constrictions 90.

[0131] Furthermore, although the constrictions 90 in the examples discussed above are all aligned at the same position along the axis 90, this need not necessarily be the case, and the constrictions 90 may be at different lengths.

[0132] In one example, the high pressure source may take air from the engine 10 at 75 psi (0.6 MPa) and the low pressure source may take air from the engine 10 at 25 psi (0.15 MPa). However, it will be appreciated that air may be taken at any suitable pressure.

[0133] It will also be appreciated that the inlets for air from the low and high pressure sources may be arranged in any suitable manner.

[0134] For example, any suitable nozzle may be used for the air from the high pressure source. Rather than using an annular nozzle in the annular diffuser ducts 76, 78, 120, a set of cylindrical nozzles may be used, or several nozzles formed in partial arcs. Furthermore, where a number of concentric diffuser ducts 76, 78, 120 are provided, including a central cylindrical duct 78, the nozzle in the central duct 78 may be an annular nozzle.

[0135] In examples with a single cylindrical diffuser duct 78, as discussed in relation to FIGS. 7A and 7B, it will be appreciated that further annular nozzles 104 may be formed at the first end 70 of the ejector 56. The further nozzles may be concentric with the cylindrical nozzle 106 and first annular nozzle 104. In some examples, the cylindrical nozzle 106 may be omitted, and multiple annular nozzles may be provided in a concentric pattern.

[0136] In further examples where a number of concentric diffuser ducts 76, 78, 120 are provided, one or more of the ducts 76, 78, 120 may have an annular arrangement of nozzles, as discussed in relation to FIGS. 7A and 7B. In one example, the central cylindrical diffuser duct 78 may be arranged in the same manner as the diffuser duct 78 in FIGS. 7A and 7B, with further annular ducts arranged outside it.

[0137] The inlets 96 for the air from the low pressure source may also be arranged in any suitable manner. For example, the inlets may be formed in the outer wall 66 or inner wall 74, 124, rather than an end wall.

[0138] In use, the cabin bleed ejector 56 may be provided within the engine core 11, or at any other suitable position in the engine 10.

[0139] The ejector 56 may also be used in any situations other than providing pressurised cabin air in an aircraft, and may be used for mixing any gasses or fluids.

[0140] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.