Exhaust diffuser for a gas turbine

1. An exhaust diffuser for a gas turbine, the exhaust diffuser comprising:

an annular duct having an inlet end and an outlet end, the annular duct being delimited by an inner wall and by an outer wall radially surrounding the inner wall;

a row of first struts arranged at a first axial position in the annular duct, the first struts connecting the inner wall and the outer wall and being distributed over the circumference of the annular duct, each of the first struts having a leading edge facing the inlet end and a trailing edge facing the outlet end, wherein the annular duct has a cross-sectional area which decreases towards the outlet end in a region downstream of the trailing edges of the first struts to a local minimum and then increases again towards the outlet end; and

a row of second struts arranged at a second axial position in the annular duct, the second struts connecting the inner wall and the outer wall and being distributed over the circumference of the annular duct, each of the second struts having a leading edge facing the inlet end and a trailing edge facing the outlet end of the annular duct, the leading edges of the second struts being arranged downstream of the trailing edges of the first struts and downstream of the local minimum of the cross-sectional area, and wherein the leading edges of the second struts are arranged downstream of the local minimum of the cross-sectional area at a distance from said local minimum which is larger by a factor of at least 3 than the distance from the trailing edges of the first struts to said local minimum, and

wherein the outer wall has a circumference, which decreases in a region downstream of the trailing edges of the first struts to the local minimum and continuously increases in a region downstream of the local minimum to the leading edge of the second struts.

2. The exhaust diffuser of claim 1 , wherein each of the first struts has a maximum thickness in a circumferential direction, and wherein the local minimum of the cross-sectional area is located at an axial distance downstream from the trailing edges of the first struts, the axial distance amounting to between 2 and 10 times the maximum thickness of the first struts.

3. The exhaust diffuser of claim 1 , wherein the cross-sectional area at its local minimum amounts to 82% to 97% of the cross-sectional area at the trailing edges of the first struts.

4. The exhaust diffuser of claim 1 , wherein the inner wall and the outer wall are separated by a radial distance which decreases in a region downstream of the trailing edges of the first struts to a local minimum of the radial distance and then increases again towards the outlet end.

5. The exhaust diffuser of claim 1 , wherein at least one of the inner wall and the outer wall, in a region around the local minimum of the cross-sectional area, has an axial profile which is convex away from the annular duct.

6. The exhaust diffuser of claim 5 , wherein both the inner wall and the outer wall, in a region around the local minimum of the cross-sectional area, have an axial profile which is convex away from the annular duct.

7. The exhaust diffuser of claim 1 , wherein the inner wall has a circumference which increases in a region downstream of the trailing edges of the first struts.

8. The exhaust diffuser of claim 1 , wherein the first struts have a strut length between the leading edge and the trailing edge, and wherein the outer wall and the inner wall are separated by a radial distance which decreases towards the outlet end between the leading edge and the trailing edge of the first struts at least along a last quarter of the strut length.

9. The exhaust diffuser of claim 1 , further comprising a section which acts as a Carnot diffuser, said section being located axially downstream of the outlet end of the annular duct.

10. The exhaust diffuser of claim 9 , wherein the inner wall is formed by a hub structure and the outer wall is formed by a casing, and wherein the section which acts as a Carnot diffuser is located axially downstream of the hub structure and is radially delimited by the casing.

11. A gas turbine comprising the exhaust diffuser of claim 1 .

12. The exhaust diffuser of claim 1 , wherein the cross-sectional area at the local minimum amounts to 85% to 95% of the cross-sectional area at the trailing edges of the first struts.

13. The exhaust diffuser of claim 1 , wherein the cross-sectional area at the local minimum amounts to 88% to 92% of the cross-sectional area at the trailing edges of the first struts.

14. The exhaust diffuser of claim 1 , wherein a number of second struts is less than a number of first struts.

15. The exhaust diffuser of claim 14 , wherein the number of second struts is 3 to 5 and the number of first struts is 5 to 20.

16. The exhaust diffuser of claim 15 , wherein the number of second struts is 3 and the number of first struts is 10.

17. The exhaust diffuser of claim 1 , wherein the inner wall has a circumference, which increases in a region downstream of the trailing edges of the first struts to the local minimum and decreases in a region downstream of the local minimum to the leading edge of the second struts.

18. The exhaust diffuser of claim 1 , wherein the inner wall has a circumference, which increases in a region downstream of the trailing edges of the first struts to the local minimum and continuously decreases in a region downstream of the local minimum to the leading edge of the second struts.