Diffuser, in particular for an axial flow machine

Abstract

 The invention concerns a diffuser (20), in particular for an axial flow machine, preferably a stationary gas turbine, which diffuser (20) transforms from a ring channel (17) with a first cross sectional area into an outlet space (21) with a second, larger cross sectional area along a machine axis (31).
Improved efficiency is achieved in that the transition is effected in several steps (22a-c) and that an annular passage (29) is provided through which a gas flow is able to escape and to flow along the guiding surface (28) in the form of a Coanda flow (30).

Description

TECHNICAL SCOPE

[0001] The present invention relates to the field of axial flow machines. It concerns a diffuser as claimed in the preamble of claim 1.

PRIOR ART

[0002] Diffusers which are arranged at the outlet of stationary gas turbines and which are to reduce the speed of flow of the gases coming out of the turbine and to bring about a build-up of pressure in order to improve the efficiency of the gas turbine, have been known for a long time in the prior art (see, for example, document EP 0 491 966 A1 or document US 2011/058939 A1 along with the attached figure 1).

[0003] In the past, various proposals have been made in order to improve the action of the diffuser at the outlet of a gas turbine and consequently the overall efficiency of the machine. Thus, among other things, document EP 0 265 633 B1 proposes dividing the diffuser into several part diffusers in the radial direction by means of flow-conducting baffle plates.

[0004] In US 2011/058939 A1, already mentioned, to improve the flow conditions in the diffuser the inner tapering part of the diffuser is provided with a controllable Coanda flow by way of which the flow in the diffuser can be influenced in a favorable manner. The inner part of the diffuser, the hub, tapers downstream without forming a step. From an external source, a gas is guided toward a ring chamber in the hub and from there is injected by means of a number of slotted nozzles in the direction of flow of the hot exhaust gases parallel to the surface of the hub. As a result of the known Coanda effect, said additional gas flow sucks in hot exhaust gas and deflects it in the direction of the hub. The exhaust gas flow is accelerated there and adapts to the surface of the hub which tapers downstream. In order to achieve a desired influencing of the exhaust gas flow in the diffuser, up to 4% of the exhaust gas mass flow in additional gas has to be injected, which is equal to not insignificant expenditure.
Contrary to this, EP 0 265 633 B1 provides a sudden transition in the cross sectional area at the outlet of the diffuser which is designated as a Carnot diffuser.

[0005] Although said measures provide certain improvements in efficiency, the possibilities for exerting influence in the region of the diffuser have not been exhausted by a long way.

REPRESENTATION OF THE INVENTION

[0006] Consequently, it is an object of the invention to create a diffuser, in particular for an industrial gas turbine, which results, in a simple manner, in a further improvement in the overall efficiency of the gas turbine.

[0007] These and other objects are achieved by the totality of the features of claim 1.

[0008] The invention proceeds from a diffuser, in particular for an axial flow machine, preferably a stationary gas turbine, which diffuser transforms from a ring channel with a first cross sectional area into an outlet space with a second, larger cross sectional area along a machine axis. It is distinguished in that the transition is effected in several steps.

[0009] A first development of the invention is characterized in that the cross sectional area inside the diffuser is increased in two steps. Said diffuser is designed in a particularly simple manner.

[0010] As claimed in another development of the invention, the diffuser is realized as a Carnot diffuser.

[0011] A further development of the invention is distinguished in that the diffuser includes an outer casing and an inner casing, between which the medium flows through the diffuser, and that the steps are generated in the cross sectional area by diameter steps on the inner casing.

[0012] Another development of the invention is characterized in that a ring-shaped, convexly curved guiding surface which tapers in diameter is arranged between two adjacent steps, and that on the upstream step of the two steps there is provided an annular passage, through which a gas flow is able to escape and to flow along the guiding surface in the form of a Coanda flow. As a result, the flow in the diffuser is able to be influenced in a favorable manner.

[0013] The guiding surface is preferably arranged between the penultimate and the last step of the diffuser.

[0014] Yet another development of the invention is characterized in that the diffuser is arranged at the outlet of an industrial gas turbine.

BRIEF EXPLANATION OF THE FIGURES

[0015] The invention is to be explained below by way of exemplary embodiments in conjunction with the drawing, in which, in more detail:

fig. 1

shows the schematic design of a gas turbine with an exhaust gas diffuser, as is known per se;

fig. 2

shows the inside design of a conventional Carnot diffuser;

fig. 3

shows, in comparison to figure 2, the inside design of a multi-step diffuser according to one exemplary embodiment of the invention;

fig. 4

shows a perspective side view of a 2-step diffuser according to another exemplary embodiment of the invention; and

fig. 5

shows the inside design of a 2-step diffuser with Coanda control according to a further exemplary embodiment of the invention.

WAYS TO CARRY OUT THE INVENTION

[0016] Fig. 1 shows the schematic design of a gas turbine with an exhaust gas diffuser, as is known in the prior art. The gas turbine 10 shown in figure 1 includes a compressor 12, which sucks in air by means of an air inlet 11 and compresses it. The compressed air is supplied to a combustion chamber 13 and there is used for the combustion of a fuel 14. The resultant hot gas is expanded in a turbine 15 downstream under operating conditions and then flows through a diffuser 16 in order to slow down the speed of flow and to bring about a build-up of pressure.

[0017] Figure 2 shows a highly simplified representation of the inside design of a conventional Carnot diffuser. In this case, the diffuser 16, which is realized in a concentric manner with respect to a machine axis 31, on the inlet side includes a ring channel 17, by means of which the exhaust gas 19 of the turbine flows into the diffuser 16. Connecting to the ring channel 17 with its comparatively small cross sectional area is an outlet space 21, the cross sectional area of which is substantially larger for the flow. The transition between the ring channel 17 and the outlet space 21 is effected, in this example, by means of a sudden step 22, which characterizes the diffuser 16 as a Carnot diffuser. Radial struts 18, which connect the inside part and the outside part of the diffuser 16 and at the same time serve for steering the flow, can be arranged in the ring channel 17.

[0018] In contrast, the invention now proposes, according to the exemplary embodiment shown in figure 3, to realize the transition between the ring channel 17 and the outlet space 21 in multiple steps in the case of a diffuser 20. In the example shown, two steps 22a and 22b are provided for this purpose. A further step 22c (shown by the broken line in figure 3) is optional. The number of steps, however, is not limited upward. The diameter jumps connected to the steps 22a-c are limited in the exemplary embodiment in figure 3 to the inside part of the diffuser 20. However, it is also just as conceivable to provide diameter jumps on the outside part of the diffuser.

[0019] Such a multiply stepped inside contour produces a gain in the build-up of pressure which can be 0.1 % of the turbine efficiency and in the case of a GT26 model gas turbine of the Applicant signifies a gain in capacity of almost half a megawatt.

[0020] In practice, a corresponding diffuser looks, for example, as shown in figure 4. The diffuser 20a of figure 4 includes a ring-shaped outer casing 23 which surrounds an inner casing 24 in a concentric manner and together with the inner casing 24 defines a flow channel. The inner casing 24 and the outer casing 23 are connected by means of radial struts 25. Two rings 26 and 27, which are stepped in diameter and by means of which the multiply stepped expansion of the diffuser 20a is brought about, are arranged one behind the other in the axial direction at the outlet of the diffuser 20a.

[0021] In addition to the multi-stepped expansion of the cross sectional flow, the flow conditions in the diffuser can be influenced by means of a Coanda flow, as has been proposed, in principle, in document US 2011/058939 A1 mentioned in the introduction. To this end, according to figure 5, a ring-shaped, convexly curved guiding surface 28 which tapers in diameter is arranged between two steps 22a and 22b in the case of a diffuser 20b. On the upstream step of the two steps 22a and 22b is provided an annular passage 29, through which a gas flow is able to escape and to flow along the guiding surface 28 in the form of a Coanda flow 30. In this case, the gas feed for the Coanda flow 30 can be effected in different ways. Contrary to what the aforementioned document teaches, however, as claimed in the invention an external reference source for an actively injected additional gas is to be omitted. By arranging the components in a proper manner, the pressure conditions prevailing in the region of the erratic cross sectional expansion of the diffuser are to be utilized in such a manner that, during operation, a wall flow 30 is automatically built up along the curved guiding surface 28 and said wall flow deflects the parallel exhaust gas flow 19. The static pressure p₂ behind the ring body 27 is higher than the inlet pressure p₁ at the annular passage on account of the deceleration of the flow through the cross sectional expansion. Accordingly, a flow 32 is formed from the higher pressure region into the lower one.
If more than two steps are present in the diffuser, the Coanda flow is preferably inserted between the penultimate and the last step.

LIST OF REFERENCES

[0022] 

10

Gas turbine

11

Air inlet

12

Compressor

13

Combustion chamber

14

Fuel

15

Turbine

16,20

Diffuser

17

Ring channel

18,25

Strut

19

Exhaust gas

20a,b

Diffuser

21

Outlet space

22a-c

Step (cross sectional area)

23

Outer casing

24

Inner casing

26,27

Ring

28

Guiding surface (curved convexly)

29

Annular passage

30

Coanda flow

31

Machine axis

32

Return flow

Claims

1. A diffuser (20, 20a, 20b), in particular for an axial flow machine, preferably a stationary gas turbine (10), which diffuser (20, 20a, 20b) transforms from a ring channel (17) with a first cross sectional area into an outlet space (21) with a second, larger cross sectional area along a machine axis (31), characterized in that the transition is effected in several steps (22a-c).

2. The diffuser as claimed in claim 1, characterized in that the cross sectional area inside the diffuser (20, 20a, 20b) is increased in two steps (22a, 22b).

3. The diffuser as claimed in claim 1 or 2, characterized in that the diffuser (20, 20a, 20b) is realized as a Carnot diffuser.

4. The diffuser as claimed in one of claims 1-3, characterized in that the diffuser (20a) includes an outer casing (23) and an inner casing (24), between which the medium flows through the diffuser (20a), and in that the steps (22a-c) are generated in the cross sectional area by diameter steps on the inner casing (24).

5. The diffuser as claimed in one of claims 1-4, characterized in that a ring-shaped, convexly curved guiding surface (28) which tapers in diameter is arranged between two adjacent steps (22a, 22b), and in that on the upstream step of the two steps (22a, 22b) there is provided an annular passage (29), through which a gas flow is able to emerge and flow along the guiding surface (28) in the form of a Coanda flow (30) .

6. The diffuser as claimed in claim 5, characterized in that the guiding surface (28) is arranged between the penultimate and the last step of the diffuser (20b).

7. The diffuser as claimed in one of claims 1-6, characterized in that the diffuser (20, 20a, 20b) is arranged at the outlet of a stationary gas turbine (10).

Drawing