LOBED INJECTOR FOR A GAS TURBINE COMBUSTOR

Abstract

 A lobed injector finger for a burner or a mixer of a gas turbine comprises a leading edge (23), a lobed trailing edge (24), a first and a second corrugated surfaces (40a, 40b) defining an airfoil cross section and converging in the trailing edge (24), a plurality of nozzles (27) located at the trailing edge (24) for injection of oil or fuel or carrier air in the burner or mixer, and a plurality of vortex generators (42) projecting from a suction side of lobes (28) defined by the first and second corrugated surfaces (40a, 40b) and positioned from the nozzles (27) to reduce flow separation and/or influence the location of a separation bubble.

Description

TECHNICAL FIELD

[0001] The present invention relates to the technology of gas turbines. It refers to a lobed injector for a gas turbine combustor according to the preamble of claim 1

BACKGROUND

[0002] In order to achieve a high efficiency, a high turbine inlet temperature is required in standard gas turbines. As a result, there arise high NOx emission levels and high life cycle costs. These problems can be mitigated with a sequential combustion cycle, wherein the compressor delivers nearly double the pressure ratio of a conventional one. The main flow passes the first combustion chamber (e.g. using a burner of the general type as disclosed in US 4,932,861, also called EV combustor, where the EV stands for environmental), wherein a part of the fuel is combusted. After expanding at the high-pressure turbine stage, the remaining fuel is added and combusted (e.g. using a burner of the type as disclosed in US 5,431,018 or US 5,626,017 or in US 2002/0187448 , also called SEV combustor or burner, where the S stands for sequential). Both combustors contain premixing burners, as low NOx emissions require high mixing quality of the fuel and the oxidizer.

[0003] An exemplary gas turbine of the applicant with sequential combustion is shown in Fig. 1.

[0004] Gas turbine 10 of Fig. 1 comprises a rotor 11 with a plurality of blades rotating about a machine axis 20 and being surrounded by a casing 12. Air is taken in at air inlet 13 and is compressed by compressor 14. The compressed air is used to burn a first fuel in a first (annular) combustor 15, thereby generating hot gas. The hot gas drives a first, high pressure (HP) turbine 16, is then reheated in a second (annular, sequential) combustor 17, drives a second, low pressure (LP) turbine 18 and exits gas turbine 10 through exhaust gas outlet 19.

[0005] Since the second combustor 17 is fed by expanded exhaust gas of the first combustor 15, the operating conditions allow self ignition (spontaneous ignition) of the fuel air mixture without additional energy being supplied to the mixture. To prevent ignition of the fuel air mixture in the mixing region, the residence time therein must not exceed the auto ignition delay time. This criterion ensures flame-free zones inside the burner. This criterion poses challenges in obtaining appropriate distribution of the fuel across the burner exit area. SEV-burners are currently designed for operation on natural gas and oil only. Therefore, the momentum flux of the fuel is adjusted relative to the momentum flux of the main flow so as to penetrate into the vortices. The subsequent mixing of the fuel and the oxidizer at the exit of the mixing zone is just sufficient to allow low NOx emissions (mixing quality) and avoid flashback (residence time), which may be caused by auto ignition of the fuel air mixture in the mixing zone.

[0006] It is known to provide lobed fingers having nozzles to inject oil/fuel and carrier air in the burner and/or mixer. One phenomenon that may occur at the outlet of nozzles of lobed fingers is the separation of the mixture of gases and oil/fuel from the fingers. Disadvantages related to flow separation are: a weak pressure gradient; a mixing vortex generates at a larger distance from the nozzle than where there is no separation; separation generates a bubble creating a pressure loss along the flow; and a separation region is a potential flame holder in the reheat combustion and this increases flashback risk.

[0007] Normally, conditions to avoid separation are such to cause relatively poor mixing. For example, it is known to reduce a penetration angle (see Fig. 6) of the lobed injector. This however causes a decrease in the pressure gradient and poor mixing.

SUMMARY

[0008] The invention is accordingly based on the object of providing a lobed injector able to avoid the above-mentioned separation and, at the same time, provide enhanced mixing.

[0009] This is achieved by a lobed injector finger for a burner or a mixer of a gas turbine comprising a leading edge, a lobed trailing edge, a fist and a second corrugated surfaces defining an airfoil cross section and converging in the trailing edge, a plurality of nozzles located at the trailing edge for injection of oil or fuel or carrier air in the burner or mixer, and a plurality of vortex generators projecting from a suction side of lobes defined by the first and second corrugated surfaces and positioned from the nozzles to reduce flow separation and/or influence the location of a separation bubble.

[0010] These and other objects are obtained by the injector finger according to the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a better understanding of the present invention, the latter will further be disclosed with reference to the accompanying figures in which is shown:

Fig. 1 is a a perspective view of an exemplary gas turbine with sequential combustion;

Fig. 2 shows a lobed injection unit for a secondary combustor of rectangular design;

Fig. 3 shows an enlarged view of a trailing edge section of an injector finger according to a first embodiment of the present invention;

Fig. 4 shows an alternative embodiment of the present invention; and

Fig. 5a, 5b, 5c show respective enlarged sketches of vortex generators applied according to the indication of figures 3 and 4; and

Fig. 6 shows a sketch of an airfoiled cross of an injector finger to define a penetration angle.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0012] According to the invention grouped or alternating neighbouring lobed fingers are the cause to have local combined vortices (grouped) or not (alternating); thus it defines the level of large scale mixing of fuel, cooling air and hot gas. The arrangement is defined based on the burner size and possible number of lobed fingers. With current rectangular sequential burner, a four finger arrangement is proper. However, the arrangements will not be limited to four finger arrangement.

[0013] Within a reheat burner an arrangement of three lobed fingers behaves differently than an arrangement with four fingers or more. For such an arrangement grouped lobes allow the vortices to combine with each other (two or more vortices can combine into a single vortex) and thereby create large scale structures, which enhance mixing and are thus beneficial for NOx, CO and overall temperature distribution factor (OTDF).

[0014] The lobed fingers according to the present invention can also be used in mixers and/or burners of a gas turbine. For example, the burner may be either annular or rectangular and the relative combustor may be either an annular combustor or a can combustor.

[0015] A lobed injector unit according to an example of the invention is shown in Fig. 2. Lobe lance 21 of Fig. 2, which is preferably to be used with a rectangular burner, comprises four separate fingers 22a-d extending in parallel between an upper plate 25 and a lower plate 26. Each finger 22 is configured as a streamlined body which has a streamlined cross-sectional profile (like an airfoil). The body has two lateral surfaces essentially parallel to an axial hot gas flow with inflow direction 32, which passes through the lance between upper and lower plates 25, 26. The lateral surfaces are joined at their upstream side by a leading edge 23 and joined at their downstream side forming a trailing edge 24.

[0016] A plurality of nozzles 27 for injecting a gaseous and/or liquid fuel mixed with air is distributed along the trailing edge 24. Each of said fingers 22 has an air plenum 30 for air supply, a gas plenum 31 for gaseous fuel supply, and a liquid fuel supply 29. Means for improving the mixing quality and reducing pressure loss in said secondary combustor are provided in the trailing edge region of said body in form of lobes 28 running between the nozzles 27 at the trailing edge 24.

[0017] Lobes 28 of the various fingers 22 generate vortices in the downstream flow of the fuel/air mixture, whereby the vortex flow of the different fingers 22 interact with each other. This interaction, which is able to enhance the mixing effect, depends on the orientation of lobes 28 in each finger.

[0018] As can be seen at the lobe lance 21 shown in Fig. 2, the lobes 28 of the different fingers 22a-d may have two different orientations. In this case, the lobes 28 of the left two fingers 22a and 22b have the same orientation, which is opposite to the orientation of the lobes 28 of the right two fingers 22c and 22d. The lobe orientation of fingers 22a and 22b is said to be R (for right), while the lobe orientation of fingers 22c and 22d is said to be L (for left).

[0019] In particular, Fig. 2 shows, in a bottom right corner, rotation direction of vortexes due to lobes at the outlet of two nozzle 27. Each vortex is induced by the pressure difference between two corrugated surfaces 40a and 40b of fingers 22, which surfaces converge and are joined along corrugated trailing edge 24 where nozzles 27 are located. In particular, surfaces 40 define the airfoil cross section of finger 22.

[0020] In order to prevent flow separation or limit flow separation effects, in particular the bubbles, in conditions of relatively high mixing, fingers 22 comprises a plurality of vortex generators, in particular micro vortex generators, projecting from corrugated surfaces 40a, 40b to control in a passive way the flow in the region of lobes 28.

[0021] Fig. 3-4 depict respective enlarged suction or concave sides of a lobe 28. In Fig. 3, vortex generators 42 are set in an array and are substantially parallel to one another and substantially parallel to a transversal direction D perpendicular to a straight line connecting the axes of two adjacent nozzles 27. By guiding the flow along direction L, separation is substantially prevented. In particular, direction D is parallel to direction 32. Furthermore, according to a preferred embodiment, location along the flow direction is such that vortex generators 42 intercept a plane containing the axes of two adjacent nozzles 27. A trace of such a plane is the straight vertical line of figure 3 connecting the axes of nozzles 27. Alternatively, vortex generators 42 are located between such plane and trailing edge 24, as shown in Fig. 3. Such a location provides an improved interaction with the flow in order to decrease separation.

[0022] In Fig. 4, vortex generators 42 are inclined with respect to direction D such that a trailing edge 43 of a vortex generator 42 is proximal to a respective nozzle 27 and a leading edge 44 of the vortex generator 42 is distal from the respective nozzle 27. Vortex generators 42 of fig. 4 are divergent along inflow direction 32. According to the layout of fig. 4, the separation is not prevented but the bubble is forced to be in a position with a low or null influence to the reheat combustion. In particular, vortex generators 42 of fig. 4 control the flow in the surroundings of the relative nozzle 27 in a region A1 at the convergence of lobe 28 with the relative nozzle 27. In such regions, boundary layer flow is accelerated to prevent separation. Furthermore, vortex generators 42 of figure 4 locate separation, if any, in a transversal tip region A2 of lobe 28 that is far away, in particular furthest away, from nozzles 27. In this way there is little if any impact of possible separation in the area of nozzles 27. Preferably, location of vortex generators of Fig. 4 is the same as that described in the previous paragraph. In particular, Fig. 4 shows an example of vortex generators 42 intercepting the plane containing the axes of two adjacent nozzles.

[0023] Depending on the flow conditions in the surroundings of lobed finger 22, vortex generators 42 may have additional positions on the suction side of the respective lobe 28.

[0024] Fig. 5a shows a preferred two dimensional embodiment of a vortex generator 42 having a fin-like or substantially triangular shape with a length L along surfaces 40a or 40b, a height H that is proximal to or coincides with trailing edge 43 and a width W, preferably a constant width. Figure 5b shows an alternative and three-dimensional embodiment of a vortex generator 42, having a tetrahedral shape. Figure 5c shows a further three-dimensional embodiment obtained by halving the tetrahedral shape of Fig. 5b with a symmetry plane. In the embodiments of Fig. 5, a trailing half portion TP of vortex generator 42 has a maximum height that is greater than a maximum height of a leading half portion LP. A leading portion and a trailing portion of vortex generator 42 are defined with respect to flow direction (shown by the arrow of Fig. 5b). In particular, embodiments of Fig. 5 show a maximum height of the vortex generator 42 defined by the relative trailing edge 43.
Numerical simulation and experiments have shown that preferred dimensional ranges for the shape of vortex generators 42 are: 5-10 mm length and less than 4 mm height. For two dimensional and three dimensional vortex generators 42 (e.g. Fig. 5a), maximum width is 1-2 mm. Incidence with inflow is equal or less than 5°: figure 4 is therefore not in scale. With reference to vortex generator of figure 5b, incidence is measured with reference to a mid-plane.

[0025] Vortex generators 42 are applicable to any lobed finger 22. Preferably, in order to improve mixing, the shape of lobes 28 is such that the following relation is satisfied, with reference to figure 3:

[0026] Where:

H1 is the height between a bottom point of a lower lobe and a top point of an upper lobe (with reference to Fig. 6);

L1 is the length of lobes 28; and

T1 is the thickness of a leading body of fingers 22.

[0027] Furthermore, in some instances, it is not possible to provide lobes with holes for providing a cooling film, i.e. holes in the range of 0.5 mm approximately or lower.

[0028] It is understood that the features and embodiments disclosed above may be combined with each other. It will further be appreciated that further embodiments are conceivable within the scope of the present disclosure and the claimed subject matter which are obvious and apparent to the skilled person.

Claims

1. A lobed injector finger for a burner or a mixer of a gas turbine comprising a leading edge (23), a lobed trailing edge (24), a first and a second corrugated surfaces (40a, 40b) defining an airfoil cross section and converging in the trailing edge (24), and a plurality of nozzles (27) located at the trailing edge (24) for injection of oil or fuel or carrier air in the burner or mixer, characterized by comprising a plurality of vortex generators (42) projecting from a suction side of lobes (28) defined by the first and second corrugated surfaces (40a, 40b) and positioned from the nozzles (27) to reduce flow separation and/or influence the location of a separation bubble.

2. The lobed injection finger according to claim 1, characterized in that the vortex generators (42) are substantially parallel one to the other and to a direction (D) substantially perpendicular to a straight line connecting the axes of two adjacent nozzles (27).

3. The lobed injection finger according to claim 1, characterized in that the vortex generators (42) are inclined with respect to a direction (D) substantially perpendicular to a straight line connecting the axes of two adjacent nozzles (27).

4. The lobed injection finger according to claim 3, characterized in that the vortex generators (42) diverge towards the trailing edge (24) so as to locate a separation region (A2) adjacent to a transversal tip of the lobe (28) and accelerate boundary layer flow in a region (A1) adjacent to the nozzle (27).

5. The lobed injection finger according to any of claims 3 to 4, characterized in that the vortex generators (42) have an angle of incidence lower than or equal to 5° with respect to an inflow direction (32).

6. The lobed injection finger according to any of the preceding claims, characterized in that a trailing half portion (TP) of the vortex generator (42) has a first maximum height that is greater than a second maximum height of a leading half portion (LP) of the vortex generator (42).

7. The lobed injection finger according to any of the preceding claims, characterized in that the vortex generators (42) have a triangular or fin-like shape or tetrahedral shape or half-tetrahedral shape.

8. The lobed injector finger according to claim 7, characterized in that the vortex generators (42) have a height (H) less than or equal to 4 mm and/or a length (L) between 5 and 10 mm and/or a width (W) between 1 and 2 mm.

9. The lobed injector finger according to any of the preceding claims, characterized in that a penetration angle of the lobed trailing edge (24) is greater than 40°.

10. The lobed injector finger according to any of the preceding claims, characterized in that no holes are provided on surfaces (40a, 40b) in the range of 0.5 mm approximately or lower to provide a cooling film to the suction side.

11. A burner of a gas turbine comprising a plurality of lobed injection fingers (22) according to any of the preceding claims.

12. A mixer of a gas turbine comprising a plurality of lobed injection fingers (22) according to any of the preceding claims.

Drawing