Transitional region for a secondary combustion chamber of a gas turbine

1. A gas turbine having a secondary combustion chamber and a first guide vane row of a low-pressure turbine, said row being arranged directly downstream of said chamber,

wherein a radially outer boundary of the secondary combustion chamber is formed by at least one outer wall segment, which is secured on at least one support element arranged radially outwardly,

wherein a hot gases flow path is bounded radially outwardly, in a region of the first guide vane row, by an outer platform which is secured at least indirectly on at least one guide vane support,

wherein there is a substantially radially extending gap-shaped cavity having a width in the range of 1-25 mm in an axial direction in an inlet region, between the at least one outer wall segment and the outer platform, an encircling projection, which locally narrows the inlet gap, and which is formed at the inlet gap leading to the cavity, on the wall of the outer wall segment, at least one step element, which reduces said width by between 10% and 70% in at least one step, extending substantially perpendicularly to the direction of the hot gas flow in the cavity, is arranged in the inlet region and spaced downstream from the encircling projection, and

wherein the at least one outer wall segment and the at least one support element form a boundary wall of the cavity, which extends substantially perpendicular to the axis of the turbine;

wherein the at least one step element is situated opposite of the at least one outer wall segment.

2. The gas turbine as claimed in claim 1 , wherein the at least one step element is configured to encircle the axis of the turbine.

3. The gas turbine as claimed in claim 1 , wherein the at least one step element is configured as encircling segments, and one such segment is assigned radially outwardly to each guide vane, wherein all the segments have a length in a circumferential direction, based on a circumferential spacing of the guide vanes, of 30-50% of the circumferential spacing.

4. The gas turbine as claimed in claim 1 , wherein the at least one step element is in the form of a rib which is mounted or formed on the wall region of the outer platform that adjoins the cavity, is substantially rectangular in axial cross section and has a length in the radial direction in the range of 10-100 mm.

5. The gas turbine as claimed in claim 1 , wherein the at least one step element is in the form of a rib which is mounted or formed on a wall region of the outer platform that adjoins the cavity, is substantially rectangular in axial cross section and has a length in the radial direction in the range of 20-50 mm, and is formed in combination with a recess of equal or greater length, which is arranged radially outwardly, which is formed in said wall region and the radially outer end of which is formed by a further step.

6. The gas turbine as claimed in claim 5 , wherein the outer platform is secured on the guide vane support by an intermediate ring, and wherein a further wall region of the cavity, said further wall region radially adjoining the wall region of the outer platform, is formed by said intermediate ring, and wherein a further step element is formed at the transition between the wall region of the platform and the further wall region of the intermediate ring.

7. The gas turbine as claimed in claim 1 , wherein the cavity also extends between the guide vane support and the support element.

8. The gas turbine as claimed in claim 1 , wherein the at least one step element reduces said width by between 20% and 70%.

9. The gas turbine as claimed in claim 1 , wherein the at least one step element reduces said width by between 30% and 70%.

10. The gas turbine as claimed in claim 1 , wherein the radially outwardly width of the at least one step element increases substantially to the original width in the inlet region, via a step extending substantially perpendicularly to the direction of flow of the hot gas in the cavity.

11. The gas turbine as claimed in claim 1 , wherein the width in the inlet region is in a range of 2-15 mm in the axial direction.

12. The gas turbine as claimed in claim 1 , comprising: the encircling projection, which locally narrows the inlet gap, is formed directly at the inlet gap leading to the cavity.

13. The gas turbine as claimed in claim 1 , comprising:

at least one second step element spaced radially apart from the at least one step element, the at least one second step element arranged on an intermediate ring, which is secured to an outer platform segment.

14. The gas turbine as claimed in claim 13 , wherein a wall region on the at least one outer platform segment between the at least one step element and the at least one second step element is recessed.

15. A gas turbine having a secondary combustion chamber and a first guide vane row of a low-pressure turbine, said row being arranged directly downstream of said chamber,

wherein a radially outer boundary of the secondary combustion chamber is formed by at least one outer wall segment, which is secured on at least one support element arranged radially outwardly,

wherein a hot gases flow path is bounded radially outwardly, in a region of the first guide vane row, by an outer platform which is secured at least indirectly on at least one guide vane support,

wherein there is a substantially radially extending gap-shaped cavity having a width in the range of 1-25 mm in an axial direction in an inlet region, between the at least one outer wall segment and the outer platform, at least one step element, which reduces said width by between 10% and 70% in at least one step, extending substantially perpendicularly to the direction of the hot gas flow in the cavity, is arranged in the inlet region, and

wherein the at least one step element is configured as encircling segments, and one such segment is assigned radially outwardly to each guide vane, wherein all the segments have a length in a circumferential direction, based on a circumferential spacing of the guide vanes, of 30-50% of the circumferential spacing.

16. A gas turbine having a secondary combustion chamber and a first guide vane row of a low-pressure turbine, said row being arranged directly downstream of said chamber,

wherein a radially outer boundary of the secondary combustion chamber is formed by at least one outer wall segment, which is secured on at least one support element arranged radially outwardly,

wherein a hot gases flow path is bounded radially outwardly, in a region of the first guide vane row, by an outer platform which is secured at least indirectly on at least one guide vane support,

wherein there is a substantially radially extending gap-shaped cavity having a width in the range of 1-25 mm in an axial direction in an inlet region, between the at least one outer wall segment and the outer platform, an encircling projection, which locally narrows the inlet gap, and which is formed at the inlet gap leading to the cavity, on the wall of the outer wall segment,

at least one step element, which reduces said width by between 10% and 70% in at least one step, extending substantially perpendicularly to the direction of the hot gas flow in the cavity, is arranged in the inlet region and spaced downstream from the encircling projection, and

wherein the at least one step element is situated opposite of the at least one outer wall segment.