CONTAINING GAS

Background of the Invention The present invention relates to the technology of gas turbines. It refers to a mixing arrangement for mixing a fuel with a stream of oxygen containing gas according to the preamble of claim 1 .

Prior Art

Gas turbines with sequential combustion are well known in the art. Fig. 4 shows the general scheme of a gas turbine 25 with sequential combustion. The gas turbine of Fig. 4 comprises a compressor 26, a first combustor 27, a first high- pressure turbine 28, a second combustor 29 and the low-pressure turbine 30. The second combustor 29 reheats the exhaust gas of the first turbine 28 by injecting fuel into the hot gas stream. This is for example done by a mixing arrangement according to Fig. 5, where an actually extending fuel lance 33 is placed in a hot gas channel 31 to inject one or more fuels into the hot gas 32 streaming along an axis 16.

Such a gas turbine reheat system (sequential combustion) proved to excel in achieving very low emissions at high firing temperature in big industrial gas turbines of the type GT24/GT26.

Such combustion system fulfils also a number of stringent requirements such as: low pressure drop, low cooling air consumption, long lifetime, fuel flexibility. One of the key elements the reheat combustion systems relies on, is the mixing levels achieved between hot gas and fuel before the flame.

The concepts developed up to now provide very good mixing levels and

combustion performances.

However, they rely either on high momentum flux ratios, complicated geometries or complex sealing systems between the fuel injector and the hot gas path (see for example documents US 5,645,410, WO 201 1/037646 A1 , US 5,351 ,474 A, US 6, 192, 688 B1 , WO 201 1 /054757, WO 201 1 /054766 or EP 2 21 1 109 A1 ).

The multipoint injection systems described in document WO 201 1 /054757 and in document WO 201 1 /054766 provide an effective way to mix the fuel and hot gas with minimal pressure drop, by using a multipoint injection scheme, supported by fluid-dynamic structure generated by dedicated vortex generators or lobes.

Summary of the Invention It is an object of the present invention to provide a mixing arrangement for sequential combustion applications, which results in an improved mixing and a shorter mixing length.

This and other objects is obtained by a mixing arrangement according to claim 1 .

The mixing arrangement for mixing a fuel with a stream of oxygen containing gas flowing along an axis in an axial channel, especially in the second combustor of a gas turbine with sequential combustion is characterised in that said mixing arrangement comprises an injector with at least one injector ring, which is passed by said stream of gas inside and outside.

According to a first embodiment of the invention said at least one injector ring is circular with respect to said axis. According to a second embodiment of the invention said at least one injector ring has an aerodynamical profile in the axial direction. Especially, said at least one injector ring has the aerodynamical profile of a symmetric airfoil with the symmetry plane being aligned with said axis.

According to another embodiment of the invention said at least one injector ring is designed to inject a plurality of streams of different fluids.

Especially, said at least one injector ring is designed to inject a stream of liquid fuel, especially oil, a stream of air and a stream of fuel gas.

According to a further embodiment of the invention said at least one injector ring has a plurality of inline injection nozzles being distributed circumferentially, and with their injection direction being aligned with said axis.

According to another embodiment of the invention said at least one injector ring has a plurality of injection nozzles being distributed circumferentially, and with their injection direction being in crossflow orientation with respect to said stream of gas.

Preferably, vortex generators are provided on said at least one injector ring.

Especially, said vortex generators are circumferentially distributed on said at least one injector ring such that each of said vortex generators corresponds to one of said injection nozzles.

More specifically, said vortex generators are arranged alternating on the inside and on the outside of said at least one injector ring.

Alternatively, lobes are provided between neighbouring inline injection nozzles. Especially, said lobes are extending alternating to the inside and outside of the at least one injector ring.

According to a further embodiment of the invention a plurality of injector rings is provided in a multi-ring configuration.

Specifically, the injector rings are of different size and are arranged in the same plane. Alternatively, the injector rings are of different size and are arranged at different axial locations.

Brief Description of the Drawings

The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

Fig. 1 shows in an axial view a circular injector, which is designed to inject a liquid fuel, shielding air and a fuel gas, according to an embodiment of the invention;

Fig. 2 shows the cross section of the injector ring of Fig. 1 , which is similar to a symmetrical aerodynamic profile;

Fig. 3 shows in one figure 3 different embodiments (a), (b) and (c) of the invention regarding in-line and crossflow arrangements and related vortex generators and lobes; Fig. 4 shows a simplified scheme of a gas turbine with sequential

combustion; and Fig. 5 shows a mixing arrangement for a reheat system according to the prior art.

Detailed Description of different Embodiments of Invention

In the present invention a system is proposed, in which a novel fuel injector is providing excellent fuel/air mixing without the need of high momentum flux ratio, and fitting a cylindrical burner. Such configuration could be well-suited for annular combustion systems (as in the GT24/GT26 gas turbines) as well as a can combustor configuration.

The present invention also relies on multipoint injection with dedicated vortex generators or lobes, which are structured in an axisymmetric manner, fitting a cylindrical burner and allowing more homogeneous fuel distribution.

The key advantage of such configuration compared with the system described in document EP 2 21 1 109 is its independence on the momentum flux ratio, thanks to the strong inline component of the fuel injectors as in WO 201 1 /054757 and in WO 201 1 /054766.

In order to improve the mixing of fuel and air the following mixer is proposed:

Fig. 1 shows in a first embodiment of the invention a general arrangement of a single "ring" mixer.

The basic principle of the "ring" system is that of a circular injector 10 comprising an injector ring 1 1 , which is able to inject three different fluid streams: liquid fuel 13 (e.g. oil), fuel gas (15) and shielding air 14 in between (similar to the well-known SEV lance of the GT24/GT26 gas turbines, as described for example in EP 0 620 362 A1 ). The three fluids are fed to the injector ring 1 1 by means of a concentric duct arrangement in a fluid supply arrangement 12. In the axial direction (axis 16) the injector ring 1 1 will have an aerodynamic profile, similar to a symmetric airfoil (Fig. 2), with a design similar to that described in documents WO 201 1 /054757 and WO 201 1 /054766, which allows low pressure drop and effective convective cooling inside the lance. Fig. 2 shows a schematic of such configuration, indicating the aerodynamic profile aligned with the main hot gas flow direction 17.

Three embodiments of such an injector ring 18 are considered and shown in a simplified scheme in Fig. 3 (in each of the three embodiments the respective injection nozzle and vortex generator configuration extends along the whole circumference of the ring, while in Fig. 3 only exemplary sections are shown for simplicity).

In the first embodiment (Fig. 3(a)) vortex generators (VGs) 20a,b at the inside and outside of the injector ring 18a are coupled with inline injection: The injector nozzles 19 are aligned with the main hot gas flow (axis 16). Mixing is enhanced by the dedicated VGs 20a and 20b creating vertical structures helping to distribute of the fuel. The advantage of inline injection is cheaper manufacturability, lower impact of momentum flux ratio on performances, higher flashback margin/fuel flexibility.

In the second embodiment (Fig. 3(b)) vortex generators 22a and 22b are coupled with inclined injection: The injector nozzles 21 a, 21 b are inclined in a crossflow arrangement so that the fuel can better penetrate into the vertical structures created by VGs 22a, 22b similar to the previous embodiment. This can provide better mixing for lower NOx emissions.

The third embodiment (Fig. 3(b)) is related to lobes 24a, 24b combined with inline injection by means of injection nozzles 23: Lobed structures create the velocity field necessary to optimize mixing of fuel and air. Lobes can achieve similar mixing levels with smaller pressure drop. They are derived from WO 201 1 /054757 and WO 201 1 /054766 and include all variants described in those patent applications. For large round contours one could as well imagine a multi-ring blender (e.g. several injector rings of different sizes (radius etc.) at the same plane

(perpendicular to axis 16) or several rings of different sizes at different axial locations along axis 16).

For large cylindrical burners or cans, a multi-ring configuration is envisaged, e.g. several rings of different sizes at the same axial plane or several rings of different sizes at different axial locations. The advantages of invention are:

• Better mixing => less emissions released;

• shorter mixing length => compact design;

• axial staging of fuel injection possible => prevention of combustion

dynamics due to different time lags;

· fitting a cylindrical burner or can;

• easily possible to be adapted to different burner or can sizes, by adding nozzles or rings.

List of Reference Numerals

10,18 circular injector

1 1 injector ring

12 fluid supply arrangement

13 liquid fuel

14 shielding air

15 fuel gas

16 axis

17 main flow direction (axial)

18a-c injector ring

19,23 inline injection nozzle

20a,b vortex generator

21 a,b cross flow injection nozzle

22a,b vortex generator

24a,b lobe

25 gas turbine (with sequential combustion)

26 compressor

27,29 combustor

28,30 turbine

31 hot gas channel

32 hot gas

33 fuel lance