CORE COWL AFTER-BODY

Abstract

A gas turbine engine includes a core engine having a working gas annulus which ends in a core exhaust nozzle for exit of hot gas from the core engine. The engine further includes a bypass duct which ends in a bypass exhaust nozzle for exit of bypass air from the bypass duct, the bypass exhaust nozzle being forward of the core exhaust nozzle, and the bypass duct having an inner wall which forms a surrounding cowl of the core engine and ends at the bypass exhaust nozzle. The engine further includes a core cowl after-body which provides a frustoconical air-washed surface continuing unbroken, on a longitudinal section through the engine, the line of the bypass duct inner wall downstream of the bypass exhaust nozzle. The air-washed surface of the core cowl after-body has plural axially-spaced circumferentially extending corrugations which locally disturb the flow of air over the surface.

Claims

1. A gas turbine engine comprising: a core engine having a working gas annulus which ends in a core exhaust nozzle for exit of hot gas from the core engine; a bypass duct which ends in a bypass exhaust nozzle for exit of a flow of bypass air from the bypass duct, the bypass exhaust nozzle being forward of the core exhaust nozzle, and the bypass duct having an inner wall which forms a surrounding cowl of the core engine and ends at the bypass exhaust nozzle; and a core cowl after-body which provides a frustoconical air-washed surface continuing unbroken, on a longitudinal section through the engine, the line of the bypass duct inner wall downstream of the bypass exhaust nozzle; wherein the air-washed surface of the core cowl after-body has plural axially-spaced circumferentially extending corrugations which locally disturb the flow of air over the surface.

2. The gas turbine engine of claim 1, wherein the frustoconical air-washed surface continues unbroken the line of the bypass duct inner wall downstream of the bypass exhaust nozzle to terminate at a lip of a circumferentially extending vent forward of the core exhaust nozzle.

3. The gas turbine engine of claim 1, wherein the height of the corrugations is from 0.1 to 3 mm.

4. The gas turbine engine of claim 3, wherein the height of the corrugations is from 0.2 to 1 mm.

5. The gas turbine engine of claim 1, wherein one or at least some of the corrugations are configured such that, on a longitudinal section through the engine, they form discrete steps in the profile of the air-washed surface on the longitudinal section.

6. The gas turbine engine of claim 5, wherein the height of the corrugations is from 0.1 to 3 mm.

7. The gas turbine engine of claim 6, wherein the height of the corrugations is from 0.2 to 1 mm.

8. The gas turbine engine of claim 5, wherein one or at least some of the corrugations are configured such that, on a longitudinal section through the engine, they form smooth waves in the profile of the air-washed surface on the longitudinal section.

9. The gas turbine engine of claim 8, wherein the height of the corrugations is from 0.1 to 3 mm.

10. The gas turbine engine of claim 9, wherein the height of the corrugations is from 0.2 to 1 mm.

11. The gas turbine engine of claim 1, wherein one or at least some of the corrugations are configured such that, on a longitudinal section through the engine, they form smooth waves in the profile of the air-washed surface on the longitudinal section.

12. The gas turbine engine of claim 11, wherein the height of the corrugations is from 0.1 to 3 mm.

13. The gas turbine engine of claim 12, wherein the height of the corrugations is from 0.2 to 1 mm.

14. A core cowl after-body of a gas turbine engine comprising: a core engine having a working gas annulus which ends in a core exhaust nozzle for exit of hot gas from the core engine; a bypass duct which ends in a bypass exhaust nozzle for exit of a flow of bypass air from the bypass duct, the bypass exhaust nozzle being forward of the core exhaust nozzle, and the bypass duct having an inner wall which forms a surrounding cowl of the core engine and ends at the bypass exhaust nozzle; and a core cowl after-body which provides a frustoconical air-washed surface continuing unbroken, on a longitudinal section through the engine, the line of the bypass duct inner wall downstream of the bypass exhaust nozzle; wherein the air-washed surface of the core cowl after-body has plural axially-spaced circumferentially extending corrugations which locally disturb the flow of air over the surface, wherein one or at least some of the corrugations are configured such that, on a longitudinal section through the engine, they form smooth waves in the profile of the air-washed surface on the longitudinal section.

15. The core cowl after-body of the gas turbine engine of claim 14 wherein at least one of the corrugations are configured such that, on a longitudinal section through the engine, they form discrete steps in the profile of the air-washed surface on the longitudinal section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0026] FIG. 1 shows schematically a longitudinal section through a rear portion of a conventional gas turbine engine;

[0027] FIG. 2 shows schematically a typical natural shock/pressure wave pattern resulting from air flows over a conical core cowl after-body of FIG. 1;

[0028] FIG. 3 shows a longitudinal cross-section through a gas turbine engine;

[0029] FIG. 4A shows the air-washed surfaces of a core cowl after-body and a vent after-body;

[0030] FIG. 4B shows a close-up view of a portion of the after-body air-washed surface;

[0031] FIG. 5A shows the air-washed surface of a variant core cowl after-body;

[0032] FIG. 5B shows a close-up view of a portion of the after-body air-washed surface;

[0033] FIG. 6 shows schematically, a controlled shock/pressure wave pattern resulting from air flows over a core cowl after-body having circumferentially extending corrugations; and

[0034] FIG. 7 shows a rear perspective view of a gas turbine engine having a variant core cowl after-body and vent topology.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0035] With reference to FIG. 3, a gas turbine engine incorporating the invention is generally indicated at 10 and has a principal and rotational axis X-X. Although the engine shown is a three-shaft engine, the invention is equally applicable to two-shaft engines and single shaft engines. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, combustion equipment 15, a high pressure turbine 16, an intermediate pressure turbine 17, a low pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 encases a bypass duct 22 which surrounds the engine 10, and extends from an air intake highlight to a lip of a bypass exhaust nozzle 23.

[0036] During operation, air entering the intake 11 is accelerated by the fan 12. Aft of the fan the air stream is split into two separate air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.

[0037] The compressed air exhausted from the high pressure compressor 14 is directed into the combustion equipment 15 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 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

[0038] The bypass duct 22 has an inner wall 24 which forms a surrounding cowl of the core engine and ends at the bypass exhaust nozzle 23. A core cowl after-body 25 then provides a frustoconical air-washed surface which continues the line of the bypass duct inner wall downstream of the bypass exhaust nozzle to terminate at a lip of a circumferentially extending vent (not shown in FIG. 3) located axially between the bypass exhaust nozzle and the core exhaust nozzle 19. The vent is used to exhaust air syphoned from the second air flow B into a core rear fire zone 28 of the engine. A separate vent after-body 27 provides the air washed surface between the vent and the core exhaust nozzle 19.

[0039] FIG. 4A shows the air-washed surface of the core cowl after-body 25, the circumferentially extending vent 26 for exhausting air from the core rear fire zone 28, and the air-washed surface of the vent after-body 27, while FIG. 4B shows a close-up view of a portion of the air-washed surface of the core cowl after-body 25. The air-washed surface has axially-spaced circumferentially extending corrugations which form discrete steps in the profile of the surface. These steps locally disturb the flow of air over the surface to enabling manipulation of the location and strength of shocks and pressure waves within the airflow exiting the bypass duct exhaust nozzle 23. As shown best in the close-up view of FIG. 4B, these steps can be configured such that the overall slope of the air-washed surface gives the surface a 15 conic half angle, as typically required by air frame manufacturers. However, the surface can be configured to give a steeper or gentler slope as desired, for example by changing the angle of the surface sections between the steps.

[0040] The location, the height C, the circumferential extent, the number and/or the spacing D of the steps may be varied as necessary in order to better manipulate the location and strength of shocks and pressure waves in the airflow.

[0041] Indeed, the shape of the corrugations can be varied, and differently shaped corrugations can be used on the same core cowl after-body. FIG. 5A shows the air-washed surface of a variant core cowl after-body 25, and FIG. 5B shows a close-up view of a portion of the air-washed surface. In the variant, instead of the corrugations forming discrete steps in the profile of the surface, they form smooth waves with peaks and troughs. These waves can be formed as e.g. constant radius peaks and troughs, peaks and troughs of differing size radii, a Bezier spline, or any combination thereof.

[0042] Whatever shape is selected for the corrugations, the heights C of the corrugations may be at least 0.1 mm, and preferably at least 0.2 mm and/or the heights C may be at most 3 mm, and preferably at most 1 mm. Such heights are generally sufficient to enable manipulation of the airflow, without being so large that the local disturbances create unacceptable losses. However the heights of individual corrugations on the same core cowl after-body 25 may differ as necessary to achieve the desired result.

[0043] By controlling the location and strength of pressure waves/shocks in the airflow exiting the bypass exhaust nozzle 23 it is possible to improve the exhaust thrust coefficient Cv and the engine specific fuel consumption (sfc), where Cv=Actual Gross Thrust/Ideal Gross Thrust, and defines the level of efficiency of the whole exhaust system. In addition, by reducing the strengths of the shocks, noise reduction in shock cells can be achieved.

[0044] More particularly, profiling the air-washed surface of the core cowl after-body 25 as described above can force the airflow to change direction at a specific location and in doing so initiate a pressure wave/shock of controlled magnitude. In this way it is possible to: disrupt the airflow to a relatively small extent upstream in order to reduce or eliminate stronger shocks downstream, move shocks to lower Mach number regions to reduce their strength, and/or increase the angle of a pressure wave relative to the air-washed surface normal to reduce the magnitude of shocks which propagate.

[0045] In particular, up to a point, a series of small shocks is considered more desirable than a lesser number of shocks of greater magnitude. This is because: Several smaller shocks equate to a reduced overall pressure loss. Pressure waves which initiate along the air-washed surface of the core cowl after-body 25 traverse the jet stream exiting the bypass exhaust nozzle 23 and reflect back off the shear layer, the shear layer being the interface between the jet stream and the free stream air. Having a degree of control over both the originating pressure wave and hence its reflection allows the path of either or both to be guided to coincide with an undesirable shock propagating along a separate pressure wave within the jet stream. This can cause a disruption within the shock path so as to reduce the strength of the shock.

[0046] FIG. 6 shows schematically an exemplary controlled shock/pressure wave pattern over a core cowl after-body 25 of the type described above having circumferentially extending corrugations which locally disturb the flow of air over the surface.

[0047] In the embodiment of FIGS. 3 to 6, the vent 26 extends the full circumferential extent of the core cowl after-body 25. However, other options are possible for venting the core rear fire zone 28 of the engine. These options are compatible with the provision of circumferentially extending corrugation(s) on the air-washed surface of the core cowl after-body 25. For example, FIG. 7 shows an embodiment in which, instead of a single circumferentially extending vent 26, plural (in this case six) circumferentially spaced, slot-like vents 26 are provided. To accommodate thermal expansion of the core engine, the core cowl after-body 25 may also include a relatively compliant annular member (not shown). As another example, the single circumferentially extending vent 26 can be replaced by a series of axially-spaced circumferentially extending vents 26. A multi-vent topology allows different vents to be dedicated to different purposes. Thus one vent can exhaust the core rear fire zone 28 at a required rate, while another vent can exhaust gear box heater matrix air (on a geared fan engine). The individual vents, being radially narrower than a single vent, can reduce losses in the jet stream by reducing the extent to which the jet stream has to change direction when it passes over a given vent. Also the cushion of relatively low velocity air flow exhausted out through the vents can be spread more evenly over the air-washed surfaces of the core cowl after-body 25 and subsequent after-bodies 27, helping to reduce the magnitude of the viscous forces between the jet stream and the boundary layer on the air-washed surfaces.

[0048] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the scope of the invention.