Divot for outer case shroud

Abstract

The invention concerns a turbine exhaust casing (TEC) for a gas turbine engine in which portions of the inner surface of the casing against which exhaust gas flows are provided with recesses extending into the surfaces. The recesses are positioned proximate to the leading edges of struts which extend between an outer shroud and inner hub of the casing.

Claims

1. A turbine exhaust casing for a gas turbine engine, comprising: an outer cylindrical shroud; an inner cylindrical hub; and a plurality of struts extending radially between the shroud and the hub, each strut intersecting with the shroud at a first end and with the hub at an opposing end; wherein an outer surface of the hub and an opposing inner surface of the shroud define a channel through which gas may pass, whereby a surface of the channel includes the outer surface of the hub and the inner surface of the shroud; wherein, for at least one of the struts, a recess is formed in a portion of the surface of the channel at an intersection of the surface of the channel and the at least one of the struts; wherein the strut intersects with the shroud or hub at a first intersection, where a smooth fillet extends between an outer surface of the strut and the inner surface of the shroud or the outer surface of the hub, wherein the smooth fillet has a radius of curvature; wherein the strut intersects with the recess at a second intersection, where the strut intersects with a continuous and smooth surface of the recess; and wherein: the radius of curvature has a radius of R.sub.F, the strut has a maximum width measured perpendicular to its length of S.sub.W, the strut has a length between its leading edge and a point of maximum thickness (S.sub.W) of the strut of S.sub.A; and wherein the outer periphery of each recess is located within a region defined by: (a) 2×S.sub.A from the leading edge of the strut measured away from the strut in an upstream or downstream direction; and (b) (S.sub.W+R.sub.F)/2 measured perpendicularly from a center line running through the strut between a leading and trailing edge of a strut.

2. The turbine exhaust casing of claim 1, wherein the recess extends radially outwardly at the intersection of strut and shroud and radially inwardly at the intersection of strut and hub.

3. The turbine exhaust casing of claim 1, wherein the recess is concave and extends into the surface of channel.

4. The turbine exhaust casing of claim 1, wherein the periphery of the recess intersects with the surface of the channel as a continuous and smooth surface.

5. The turbine exhaust casing of claim 1, wherein the recess extends along an upstream portion of the surface of the channel away from the leading edge of a strut measured in a gas flow direction through the casing.

6. The turbine exhaust casing of claim 1, wherein the recess extends along a downstream portion of the surface of the channel away from the trailing edge of a strut measured in a gas flow direction through the casing.

7. The turbine exhaust casing of claim 1, wherein at least a plurality of the struts are provided with respective recesses proximate to a leading or trailing edge of each respective strut in the plurality of the struts.

8. The turbine exhaust casing of claim 1, wherein a portion of an outer surface of the shroud aligning with the position of the recess on the inner shroud surface is provided with an increased shroud wall thickness over an area corresponding to the area of the recess.

9. The turbine exhaust casing of claim 8, wherein the increased shroud wall thickness is in the form of a convex portion corresponding in shape to the concave recess on the inner surface of the shroud.

10. The turbine exhaust casing of claim 9, wherein the recess and increased shroud wall thickness are complimentary such that the wall thickness of the shroud at the recess is at least as thick as the wall thickness of the remaining parts of the shroud wall.

11. The turbine exhaust casing of claim 10, in which t=shroud case thickness without a modified recess; d=the thickness of the shroud measured from the deepest part of the recess to the most protruding part of the reinforcement portion or pad; e=the depth of the recess measured from the inner surface; and r=the radius of the fillet of curvature at which the recess intersects with the leading edge of the strut; and wherein: d>=t, t>e>0, r>=the normal fillet radius between the strut and the shroud/hub.

12. A turbine exhaust casing comprising: an inner hub portion, an outer shroud portion, and a plurality of struts extending radially between the hub and shroud portions, each strut having a leading edge arranged to face an upstream direction of gas flow and a trailing edge arranged to face a downstream direction of gas flow; wherein an inner surface of the outer shroud portion immediately adjacent to the leading edge of at least one of the struts is provided with a recess extending into the inner surface of the outer shroud portion; an outer surface of the outer shroud portion is provided with a reinforcement at a position on the outer surface of the outer shroud portion corresponding to the recess on the inner surface of the outer shroud portion; the strut intersects with the hub at a first intersection, where a smooth fillet extends between an outer surface of the strut and the outer surface of the hub, wherein the smooth fillet has a radius of curvature; the strut intersects with the recess at a second intersection, where the strut intersects with a continuous and smooth surface of the recess; and wherein: the radius of curvature has a radius of R.sub.F, the strut has a maximum width measured perpendicular to its length of S.sub.W, the strut has a length between its leading edge and a point of maximum thickness (S.sub.W) of the strut of S.sub.A; and wherein the outer periphery of each recess is located within a region defined by: (a) 2×S.sub.A from the leading edge of the strut measured away from the strut in an upstream or downstream direction; and (b) (S.sub.W+R.sub.F)/2 measured perpendicularly from a center line running through the strut between a leading and trailing edge of a strut.

13. A method of forming an exhaust gas casing for a gas turbine engine, the casing comprising an outer cylindrical shroud; an inner cylindrical hub; and a plurality of struts extending radially between the shroud and the hub, each strut intersecting with the shroud at a first end and with the hub at an opposing end; wherein an outer surface of the hub and an opposing inner surface of the shroud define a channel through which gas may pass, whereby a surface of the channel includes the outer surface of the hub and the inner surface of the shroud, the method comprising forming a recess in a portion of the surface of the channel at an intersection of the surface of the channel and the at least one of the struts, wherein the recess extends radially inwardly or radially outwardly into the surface of the channel with respect to the surface of the channel surrounding the intersection; wherein the strut is formed to intersect with the shroud or hub at a first intersection, where a smooth fillet is formed to extend between an outer surface of the strut and the inner surface of the shroud or the outer surface of the hub, wherein the smooth fillet has a radius of curvature; wherein the strut is formed to intersect with the recess at a second intersection, where the strut intersects with a continuous and smooth surface of the recess; and wherein: the radius of curvature has a radius of R.sub.F, the strut has a maximum width measured perpendicular to its length of S.sub.W, the strut has a length between its leading edge and a point of maximum thickness (S.sub.W) of the strut of S.sub.A; and wherein the outer periphery of each recess is located within a region defined by: (a) 2×S.sub.A from the leading edge of the strut measured away from the strut in an upstream or downstream direction; and (b) (S.sub.W+R.sub.F)/2 measured perpendicularly from a center line running through the strut between a leading and trailing edge of a strut.

14. The method of claim 13, wherein the recess is concave and extends into the surface of channel.

Description

BRIEF SUMMARY OF THE DRAWINGS

(1) Aspects of the disclosure will now be described, by way of example only, with reference to the accompanying figures in which:

(2) FIG. 1 shows an aft looking forward view of an exhaust gas casing;

(3) FIG. 2 shows a side elevation of one of the struts 7;

(4) FIG. 3 is a cross-section through a shroud in which the leading edge and trailing edge are shown;

(5) FIG. 4 is a plan view of the outer surface of the shroud viewed radially inwardly;

(6) FIG. 5 is a cross-section through section A-A′ in FIG. 4;

(7) FIG. 6 is an additional cross-sectional view of the leading edge of the strut;

(8) FIG. 7 illustrates the respective positions of the recesses on the leading edge (R1 and R2) and on the trailing edge (R3 and R4);

(9) FIG. 8 illustrates the boundary of the zone into which a recess is formed; and

(10) FIGS. 9 and 10 illustrate views of the shroud and hub respectively incorporating the reinforcements and recesses.

(11) 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 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. 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. It will also be recognised that the invention covers not only individual embodiments but also combinations of the embodiments that have been discussed herein.

DETAILED DESCRIPTION

(12) FIG. 1 shows an aft looking forward view of an exhaust gas casing 1 suitable for use in a gas turbine engine. Such a component will be well known to the person skilled in the art of gas turbine engines.

(13) The casing 1 is formed of an outer ring or shroud 2 and a radially inwardly located hub 3 concentric with the shroud 2. Both shroud and hub are concentric with a centrally located and rotatable shaft (not shown) that runs along the length of the engine and is located in the central space 4 within the hub 3. The hub 3 locates bearings and other equipment which support the shaft and allow it to rotate.

(14) The shroud 2 is provided with a plurality of lugs 5 which provide structural couplings to connect the engine to the body of the aircraft.

(15) The radial space between the shroud and the hub defines an annular channel 6 through which gas (exhaust gas in the case of an exhaust gas casing) may pass. Extending between the shroud and hub are a plurality of struts or vanes 7 (described in more detail below).

(16) The struts or vanes 7 serve a number of purposes.

(17) First, the struts provide a mechanical coupling connecting the shroud and hub together and rigidly spacing the shroud from the hub (which itself supports the rotating shaft of the engine). This provides an aft end support and coupling for the engine.

(18) Secondly, the struts or vanes 7 each have an aerodynamic profile to turn the swirling air leaving the combustors towards an axial outlet direction, i.e., directing exhaust gas out of the back of the engine. This is described in more detail below.

(19) Each of the struts 7 has a leading edge, that is an up-stream edge against which the exhaust gas first impinges and a trailing edge, that is the edge downstream of the leading edge.

(20) Described herein is a modification to the casing at the portions of the casing at which the strut meets (or intersects) with the shroud at one end and hub the hub at the other. More specifically the casing is modified to incorporate recesses or depressions proximate to the points at which the struts meet the hub or shroud.

(21) FIG. 2 shows a side elevation of one of the struts 7. Each strut 7 has a leading edge 9 in a forward direction of the engine and a trailing edge 10 in an aft direction of the engine. A radially inward end 11 of the strut 7 is coupled to the hub 3 and at an opposing end of the strut 7 a radially outward end 12 of the strut is coupled to the shroud 2 as shown in FIG. 2. The present disclosure is concerned with portions of the inner surface of the casing (illustrated by reference 13 in FIG. 2) where the leading or trailing edges of the strut meet the inner surface 14 of the channel.

(22) FIG. 3 is a cross-section through a strut in which the leading edge 9 and trailing edge 10 are shown. The recess or depression 8 formed into the channel surface 14 is also shown. In the example shown in FIG. 3 the depression is only shown at the leading edge. However, the depression may be positioned at either the leading edge, trailing edge or both

(23) FIG. 4 is a plan view of the outer surface of the shroud viewed radially inwardly. FIG. 4 illustrates the reinforcement portion 16 which will be described with reference to FIG. 5.

(24) FIG. 5 is a cross-section through section A-A′ in FIG. 4. FIG. 5 illustrates the point at which the strut 7 intersects with the shroud 2 and the way the recess or depression 8 is integrated into the surface geometry of the channel surface 14.

(25) As shown in FIG. 5 the normal geometry of the shroud is shown by dotted line 8′ on the inner surface of the shroud and by 16′ on the outer surface.

(26) Reference 8 shows the recess extending radially outwardly and into the channel surface. The recess 8 defines a smooth and uninterrupted surface extending from the upstream channel wall at reference A to the leading edge 9 of the strut at reference B.

(27) A smooth and continuous profile is provided which can be sub-divided into three regions L1, L2 and L3. Region L1 represents a straight portion of the leading edge of the strut; Region L2 represents the smooth curvature of the recess or divot at the intersection of strut and recess; and Region L3 represents the upstream portion of the recess which intersects with the upstream channel inner surface at reference A.

(28) Each of the three regions intersects smoothly with the next providing a continuous smooth surface that leaves the channel inner surface and extends, in a curve, radially outwardly into the thickness of the shroud immediately before the intersection of strut and inner shroud surface. In effect material is removed from the root of the strut at a point at which it meets the shroud.

(29) As shown in FIGS. 4 and 5 the shroud may be optionally provided with an outer reinforcement portion or pad 16 which effectively thickens the shroud at a portion corresponding to the recess located on the opposing inner surface of the shroud.

(30) FIG. 6 is an additional cross-sectional view of the leading edge of the strut incorporating the recess or depression. The enlarged portion of FIG. 6 illustrates the relationship within the thickness of the shroud according to the modified shroud arrangement described herein.

(31) The shroud thickness at the intersection of the strut and the shroud is arranged according to the following parameters where t=shroud case thickness without a modified recess) d=the thickness of the shroud measured from the deepest part of the recess to the most protruding part of the reinforcement portion or pad e=the depth of the recess measured from the inner surface; and r=the radius of the fillet of curvature at which the recess intersects with the leading edge of the strut

(32) The following Applies: d>=t t>e>0 r>=the normal fillet radius between the strut and the shroud/hub

(33) FIG. 7 illustrates the respective positions of the recesses on the leading edge (R1 and R2) and on the trailing edge (R3 and R4). The reinforcement portions of pads may be optionally introduced on the outer surface of the shroud at positions corresponding to R1 and R4.

(34) The position of the recess or depression will now be described with reference to FIG. 8 which is a cross-section through a portion of a strut proximate to the intersection of the strut and shroud or hub.

(35) As described herein the recess or depression takes the form of a change in the surface profile of the channel in the area immediately upstream of the leading edge or immediately downstream of the trailing edge.

(36) In the case of a recess at a leading edge of a strut the recess is formed of two portions (a) the smooth fillet or radius that intersects with the radially extending leading edge of the strut and (b) the smooth and continuous surface that extends from the fillet or radius and joins the inner surface of the channel upstream of the radius.

(37) In the case of a recess at a trailing edge the recess is inverted, i.e., there is (a) an upstream smooth fillet or radius that intersects with the radially extending trailing edge of the strut and (b) a smooth and continuous surface that extends from the fillet or radius and joins the inner surface of the channel downstream of the radius.

(38) In both cases the recess must be contained within a zone either upstream of the leading edge or downstream of the trailing edge. This zone or region is described with reference to FIG. 8.

(39) Specifically, as illustrated in FIG. 8, the zone is defined as follows: the radius has a radius of R.sub.F the strut has a maximum width measured perpendicular to its length of S.sub.W the strut has a length between its leading edge and a point of maximum thickness (S.sub.W) of the strut of S.sub.A; and
wherein the outer periphery of each recess is located within a region defined by: (a) 2×S.sub.A from the leading edge of the strut measured away from the strut in an upstream or downstream direction; and (b) (S.sub.W+R.sub.F)/2 measured perpendicularly from a centre line running through the strut between a leading and trailing edge of a strut.

(40) FIGS. 9 and 10 illustrate views of the shroud and hub respectively incorporating the reinforcements and recesses.

(41) It has been established that locating the recesses within the zone shown in FIG. 8 and also illustrated in FIG. 10 provides a surprising technical effect in that the strength and fatigue life of the strut can be improved. Surprisingly, introducing a recess in the specific location described herein not only provides the advantages discussed herein but is not detrimental to the airflow within the casing. Convention dictates that the inner surfaces of the cases must not be interrupted owing to the disruption to the air flow. However, the inventors has established that applying the criteria set out herein allows the advantages of improved strength to be realised whilst not encountering airflow problems.