Ring diffuser for an axial turbomachine

1. An arrangement for an axial turbomachine, comprising:

a ring-shaped flow duct which is delimited by a first wall surface and a second wall surface of an outer wall and of an inner wall, respectively, and between which is provided a guide vane cascade through which a medium is capable of flowing;

the guide vane cascade, comprising,

a plurality of aerodynamically curved vane leaves, the free-standing vane leaf tips of which lie, radially on the inside, opposite the inner wall of the flow duct or, radially on the outside, opposite the outer wall of the flow duct, in each case so as to form a gap; and

a ring diffuser arranged downstream of the guide vane cascade, with a ring diffuser outer wall and with a ring diffuser inner wall coaxial with the ring diffuser outer wall, between which a diffuser duct extends in ring form along an axial extent so as to diverge from an inflow-side end to an outflow-side end,

wherein the ring diffuser inner wall and the ring diffuser outer wall in each case comprising a second ring diffuser wall surface and a first ring diffuser wall surface, respectively, delimiting the diffuser duct,

wherein the ring diffuser outer wall is arranged downstream of the outer wall of the flow duct and/or the ring diffuser inner wall is arranged downstream of the inner wall of the flow duct,

wherein the second ring diffuser wall surface of the ring diffuser inner wall and/or the first ring diffuser wall surface of the ring diffuser outer wall is rotationally asymmetrical at least in a first portion of the ring diffuser,

wherein the second ring diffuser wall surface at which the gaps are arranged further upstream is a rotationally asymmetrical design, and

wherein the first portion is arranged at the inflow-side end of the ring diffuser and merges into a rotationally symmetrical second portion which is arranged downstream of the rotationally asymmetrical first portion in relation to the throughflow direction of the ring diffuser.

2. The arrangement as claimed in claim 1 , wherein the first and second ring diffuser rotationally asymmetrical wall surfaces have elevations and depressions along the circumference.

3. The arrangement as claimed in claim 2 ,

wherein between the second ring diffuser wall surface of the inner wall and the first ring diffuser wall surface of the outer wall, there is an inflow-side duct height to which a maximum height of each elevation and a maximum depth of each depression are related, and

wherein the maximum height or maximum depth amounting at most to 25% of the duct height.

4. The arrangement as claimed in claim 2 , wherein the elevations have different heights and/or the depressions have different depths in one axial position on the first portion.

5. The arrangement as claimed in claim 2 , wherein the elevations have different heights and/or the depressions different depths in different axial positions on the first portion.

6. The arrangement as claimed in claim 1 ,

wherein the first portion includes a first portion wall surface which is rotationally asymmetrical, and

wherein the first portion merges steplessly into the rotationally symmetrical second portion.

7. The arrangement as claimed in claim 2 , wherein the rotational asymmetry has a critical axial contour which includes the elevations and the depressions, which form with the axial extent an angle, the size of which amounts to between 0° and 40°.

8. The arrangement as claimed in claim 7 , wherein the angle is of different size in different axial positions on the first portion.

9. The arrangement as claimed in claim 1 , wherein at least the first portion is designed as an axial diffuser.

10. The arrangement as claimed in claim 1 , wherein at least the first portion is designed as a diagonal diffuser.

11. The arrangement as claimed in claim 1 , wherein the guide vane cascade is designed as a guide vane cascade of a vane stage.

12. The arrangement as claimed in claim 1 , wherein the guide vane cascade is designed as a guide vane cascade of a follow-up guide wheel.

13. The arrangement as claimed in claim 1 , wherein the second ring diffuser wall surface of the outer wall merges into the first wall surface of the outer wall of the flow duct and/or the first ring diffuser wall surface of the inner wall merges into the second wall surface of the inner wall of the flow duct.

14. The arrangement as claimed in claim 1 , wherein the first wall surface of the outer wall of the flow duct and/or the second wall surface of the inner wall of the flow duct have in each case a portion in which one wall surface or both wall surfaces is or are rotationally asymmetrical.