Divot for outer case shroud

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 F ,

the strut has a maximum width measured perpendicular to its length of S W ,

the strut has a length between its leading edge and a point of maximum thickness (S W ) of the strut of S A ; and

wherein the outer periphery of each recess is located within a region defined by:

(a) 2×S A from the leading edge of the strut measured away from the strut in an upstream or downstream direction; and

(b) (S W +R 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 F ,

the strut has a maximum width measured perpendicular to its length of S W ,

the strut has a length between its leading edge and a point of maximum thickness (S W ) of the strut of S A ; and

wherein the outer periphery of each recess is located within a region defined by:

(a) 2×S A from the leading edge of the strut measured away from the strut in an upstream or downstream direction; and

(b) (S W +R 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 F ,

the strut has a maximum width measured perpendicular to its length of S W ,

the strut has a length between its leading edge and a point of maximum thickness (S W ) of the strut of S A ; and

wherein the outer periphery of each recess is located within a region defined by:

(a) 2×S A from the leading edge of the strut measured away from the strut in an upstream or downstream direction; and

(b) (S W +R 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.