EP2630337A1

EP2630337A1 - Gas turbine provided with a cooling circuit for tip sections of rotor blades

Info

Publication number: EP2630337A1
Application number: EP11804793.5A
Authority: EP
Inventors: Massimiliano Maritano
Original assignee: Ansaldo Energia SpA
Current assignee: Ansaldo Energia SpA
Priority date: 2010-10-20
Filing date: 2011-10-20
Publication date: 2013-08-28
Also published as: WO2012052961A1; WO2012052961A8; ITMI20101919A1

Abstract

A gas turbine comprising an array of rotor blades (15) arranged around a rotation axis (A), a guide ring (23) arranged around the rotor blades (15), and a cooling circuit (25), configured to receive an air flowrate and to direct the air flowrate to the rotor blades (15). The guide ring (23) has through holes (27) coupled to the cooling circuit (25) and configured so that air coming from the cooling circuit (25) and passing through the holes (27) deviates from an inner surface (23a) of the guide ring (23) and forms jets (G) directed towards tip sections (20) of the rotor blades (15) and having an output speed from the holes (27) so as to reach the tip sections (20) of the rotor blades (IS).

Description

"GAS TURBINE PROVIDED WITH A COOLING CIRCUIT FOR TIP SECTIONS OF ROTOR BLADES"

TECHNICAL FIELD

The present invention relates to a gas turbine provided with a cooling circuit for tip sections of rotor blades.

BACKGROUND ART

As known, the gas turbine blades must be cooled during use, since they would otherwise be destroyed in a short time by the very high temperatures during use. Cooling is necessary both for the rotor blades, and for the stator blades. As for the rotor blades, however, the realization of effective cooling systems poses greater problems, due to rotation speed. Typically, a rotor blade of a gas turbine comprises a root or anchor, a platform and an elongated portion to form an airfoil, which extends radially outwards from the platform. The airfoil has a leading edge, a trailing edge, and a tip section, opposite to the root.

For cooling, fresh air drawn from the compressor and fed through channels built into the blade and forming a cooling circuit is normally used. The cooling air then exits through holes on the surface of the blades and is allowed to enter into the main evolving flow.

One of the most common problems in the gas turbines is the damage to the tip section of the rotor blades due to the leakage flow due to the difference in pressure between the front and back of the blade. The leakage flow crosses the interspace (usually called "tip clearance") defined between the tip section and the guide ring of the blades, which usually is placed around each rotor stage. The interspace is needed to avoid the dragging of the blades on the guide ring itself. Due to the high temperatures of the evolving fluid in the turbine and the rotation speed, the leakage flow exerts an intense thermal and mechanical action on the tip section of the blades, causing serious deterioration. The deterioration in turn causes an increase in the interspace and tends to prematurely damage the structure of the blades. On one hand, therefore, the performance of the machine is degraded (in particular for the increase in interspace) and, on the other, it requires more frequent and costly maintenance.

Currently, the tip section of the blade is cooled by means of holes made near the top edge of the blade itself and connected to the internal cooling circuit. The heat, in this way, is removed both due to the well effect, and by creating a film of cooler air in the area of intervention.

According to another solution, holes are made in the upper platform of stator blades upstream, so as to direct cooling air towards the tip sections of the blade immediately downstream.

The known solutions however have limits. First, the cooling air is fed into a highly disturbed flow and, both in the case of the holes on the rotor blade, and in the case of the holes on the platform of the stator blades, most of the time it is not possible to obtain an adequate coating of surface to cool. Because of turbulence, in practice, the cooling air is not able to form a layer around the entire tip section of the rotor blades, but is dragged elsewhere by the main gas flow. In addition, the holes may not always be made where it would be required. The arrangement of holes is therefore not bound to the geometry of the blade and this may limit the effectiveness of cooling.

DISCLOSURE OF INVENTION

The aim of the present invention is therefore to provide a gas turbine that is free of the restrictions above described and, in particular, allow to effectively cool the tip sections of the rotor blades.

Provided according to the present invention, is a gas turbine as defined in claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the annexed drawings, which illustrate a non limitative example of implementation, in which:

- Figure 1 is a side view partially longitudinally sectioned of a turboassembly of a plant for the production of electric energy incorporating a gas turbine according to an embodiment of the present invention;

- Figure 2 is a front perspective view of a rotor blade of the gas turbine of Figure 1;

- Figure 3 is a rear perspective view of a rotor blade and of a portion of a guide ring of the gas turbine of Figure 1;

- Figure 4 is a schematic side view of a portion of the gas turbine of Figure 1;

- Figure 5 is a plan view from above of a guide ring developed on a plane of the gas turbine of figure 1;

- Figure 6 is a side view, sectioned along the track plane A-A of Figure 5, of an enlarged detail of the guide ring of Figure 5 in an embodiment;

- Figure 7 is a side view, sectioned along the track plane A-A of figure 5, of an enlarged detail of the guide ring of Figure 5 in a different embodiment;

- Figure 8 is a plan view from above of a guide ring developed on a plane of a gas turbine according to another embodiment of the present invention;

- Figure 9 is a plan view from above of a guide ring developed on a plane of a gas turbine according to another embodiment of the present invention, and

Figure 10 is a plan view from above of a guide ring developed on a plane of a gas turbine according to another embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Figure 1 shows a portion of a plant for the production of electric energy, indicated as a whole with reference number 1. The plant 1 comprises a shaft 2 which extends around a rotation axis and along which are arranged a compressor 3 and a gas turbine, hereinafter simply referred to as turbine 5. In addition, a combustion chamber 6 of an annular type is arranged around the shaft 2, between the compressor 3 and the turbine 5. The compressor 3, the combustion chamber 6 and the turbine 5 form a turbogas unit, which is housed in an external case 4.

The shaft 2 comprises a plurality of rotor discs 7, 8 arranged in a succession between a front hollow shaft 10 and a rear hollow shaft 11 and clamped by a central tie rod 12. A central hollow shaft 13 separates the rotor discs 7 in the region of the compressor 3 from the discs 8 in the region of the turbine 5 and extends through the combustion chamber 6. The rotor discs 7 and the rotor discs 8 present respective annular expansions 7a, 8a, which extend axially in opposite directions and are in contact with annular expansions 7a, 8a, of the adjacent rotor discs 7, 8.

Each of the discs 7 carries a respective array of rotor blades 14 of the compressor, while the rotor discs 8 carry respective arrays of rotor blades 15 of the turbine.

As shown in Figure 2, each rotor blade 15 comprises a root (or anchor) 17, a platform 18 and an elongated portion to form an airfoil 19, which extends from platform 18 towards the outside. The airfoil 19 has a tip section 20 opposite to the root 16, a leading edge 21 and a trailing edge 22.

As shown in more detail in Figure 3, each array of rotor blades 15 of the turbine is surrounded by a respective outer guide ring 23, fixed to the blade-holder case 24 (for simplicity, Figure 3 shows a single rotor blade 15 and a portion of the corresponding guide ring 23; see also Figure 1) . The guide ring 23 is designed to shield the blade-holder case 24 from high-temperature gas flowing into the turbine 5.

The plant 1 comprises a cooling circuit 25, of which only part is schematically illustrated in Figure 1. The cooling circuit 25 draws a fresh airflow rate from the compressor 3 and conveys it to the rotor blades 15 and the guide ring 23. Part of the cooling circuit 25 is defined within the airfoil 19 of each rotor blade 15 and is not shown here for simplicity. A branch 25a of the cooling circuit 25 serves to cool the enclosure blade-holder case 24 (in the vicinity of the turbine 5), of the guide rings 23 and of the tip sections 20 of the rotor blades 15.

More in detail, the airflow rate drawn from the compressor 3 through the branch 25a of the cooling system 25 is injected into chambers (not shown) defined between the guide rings 23 and the blade-holder case 24 and then drawn in the flow channel of the turbine 5 by through hole systems 27 made in the guide rings 23 (Figures 4 and 5) . As explained below, in each guide ring 23 the holes 27 are defined and distributed so that the air coming from the cooling circuit 25 form air jets G directed towards the tip section 20 of the rotor blades 15, rather than a layer adhering to the surface of the guide ring 23 which defines the flow channel of the turbine 5 (hereinafter briefly referred to as inner surface 23a) . In one embodiment, the holes 27 are configured so that, during operation of the turbine 5, the speed of the air jets G exiting the holes 27 is sufficient to cause separation of the refrigerant flow from the inner surface 23a of the guide ring 23, particularly in the absence of a canopy or "shroud" of the rotor blades 15. In addition, holes 27 are configured to allow the air jets G to reach the tip sections 20 of the rotor blades 15 of the corresponding array. The holes 27 are also shaped so that the outlet speed of the air jets G has a component parallel and a component substantially radial with respect to the axis A. For this purpose, it is possible to select, according to the specified operative conditions in the turbine, the number of holes 27, their distribution, the pressure drop associated with each hole 27 and the inner profile. In one embodiment, the distribution of the holes 27 is such that the air jets G reach the entire surface of the tip sections 20 of the rotor blades 15.

Figure 5 shows on a plane the development of one of the guide rings 23 and, schematically, the tip section 20 of one of the rotor blades 15 with the respective leading edges 21 and trailing edges 22. The arrow indicates the direction of motion V of the rotor blades 15 during the rotation, which lies in a plane perpendicular to the axis A of the shaft 2.

The holes 27 are arranged on the guide ring 23 along a path PI transverse to the direction V, and are distributed, with respect to the direction of the axis A, between the leading edge 21 and the trailing edge 22.

In the embodiment of figure 5, in particular, the path PI developed in a plane is rectilinear, extending between a front margin 23b and a rear margin 23c of the guide ring 23 and forms an angle a in the range between 45° and 75° with the direction of axis A. In practice, the path PI is helical and has a pitch which basically corresponds to the distance in the axial direction between the front margin 23b and the rear margin 23c of the guide ring 23, so that the entire tip section 20 of the rotor blades 15 is invested with fresh air jets once for each revolution of the shaft 2.

Moreover, denoting by D the diameter of holes 27 at the exit, the distance S between consecutive holes 27 is uniform and is preferably between 2D and 10D. The diameter D is preferably between about 0.5 mm and 2 mm.

In one embodiment, referred to by figure 6, the holes 27 have a conical profile, with a half vertex angle a in the range between about 5° and 20° and an internal axis F forming an angle γ in the range between about 20° and 60°, preferably in the range between 45° and 60°, with respect to the inner surface 23a of the guide ring 23. In addition, the holes 27 converge towards the inside of the guide ring 23. The conical shape determines an increase in the speed of the cooling air and promotes release of the jet from the inner surface of the case .

In a different embodiment, shown in Figure 7, the holes 27 are cylindrical with F inclined at an angle γ in the range between 50° and 60° with respect to the inner surface 23a of the guide ring 23.

In the embodiment shown in Figure 8, the holes 27 are arranged on the guide ring 23 along a path P2 that, developed on a plane, comprises two rectilinear stretches P2¹ , P2". The two stretches P2¹ , P2" extend consecutively each on substantially half the length of the development on a plane of the inner surface 23a of the guide ring 23. In the direction of the axis A, moreover, the stretches P2 ' , P2" extend both between the front margin 23b and the rear margin 23c of the guide ring 23, so that the entire tip section 20 of the rotor blades 15 is invested with fresh air jets once for each revolution of the shaft 2. In practice, the stretches P2 ' , P2" extend along respective portions of a helical line with a pitch equal to twice the distance between the front margin 23b and the rear margin 23c of the guide ring 23.

The spacing between the stretches P2 ' , P2" is greater than L/8, being L the length of the development of the guide ring 23. In the embodiment of figure 9, the holes 27 are distributed on the guide ring 23 along a path P3 comprising the rectilinear stretches P3', P3", P3¹", arranged transversely to the movement direction V of the rotor blades 15 and extending between the front margin 23b and the rear margin 23c of the guide ring 23. The spacing between the stretches P3 ' , P3" P3 ' " is greater than L/8, being L the length of the development of the guide ring 23.

In the embodiment of Figure 10, the holes 27 are distributed on the guide ring 23 along a helical path P4 at a variable pitch, in particular gradually decreasing. Developed on a plane, the path P4 is curvilinear with decreasing curvature radius between the front margin 23b and the rear margin 23c of the guide ring 23.

The invention advantageously allows to effectively cool the tip section of the rotor blades. First, it is possible to direct jets of air at high speed towards the tip of the blades, so as to overcome the resistance due to the strongly perturbed motion in the flow channel of the gas turbines. It is therefore assured a coolant flow that laps the tip of the blades and causes a great cooling effect. In addition, the holes are made on the guide ring surrounding the rotor blades, which offers great freedom to create different patterns of cooling holes, according to the needs of the turbine.

It is also clear that to the described method and plant modifications and variations can be made, without going beyond the scope of the present invention, as defined in the appended claims .

Claims

1. Gas turbine comprising:

at least an array of rotor blades (15) arranged around a rotation axis (A) ;

a guide ring (23) arranged around the rotor blades (15); and a cooling circuit (25) , configured to receive an airflow rate and to convey the airflow rate toward the rotor blades (15) ; characterized in that the guide ring (23) has through holes (27) fluidly coupled to the cooling circuit (25) and configured so that the air deriving from the cooling circuit (25) and passing throughout the holes (27) deviates from an inner surface (23a) of the guide ring (23) and forms jets (G) directed toward tip sections (20) of the rotor blades (15) and having outlet speed from the holes (27) so as to reach the tip sections (20) of the rotor blades (15) .

2. Turbine according to claim 1, wherein the holes (27) have an outlet diameter (D) and adjacent holes (27) are separated by a distance (S) in the range between about twice the outlet diameter (D) and about ten times the outlet diameter (D) .

3. Turbine according to claim 2, wherein the outlet diameter (D) of the holes (27) is in the range between about 0,5 mm and about 2 mm.

4. Turbine according to any one of the foregoing claims, wherein the holes (27) extend around respective internal axes (F) , which form angles (γ) in the range between 20° and 60° with an internal surface (23a) of the guide ring (23) .

5. Turbine according to any one of the foregoing claims, wherein the holes (27) are conical.

6. Turbine according to claim 5, wherein the holes converge towards the inside of the guide ring (23) and preferably have a half vertex angle ( β ) in the range between about 5° and about 20 ° .

7. Turbine according to any one of claims 1 to 4, wherein the holes (27) are cylindrical.

8. Turbine according to any one of the foregoing claims, wherein the holes (27) are configured so as the outlet speed of the air jets (G) has a parallel component and a radial component with respect to the rotation axis (A) .

9. Turbine according to any one of the foregoing claims, wherein, the rotor blades (15) are rotatable around the rotation axis (A) and the holes (27) are aligned along a path (P2; P2; P3; P4) transversal to a motion direction (V) of the rotor blades (15) around the rotation axis (A).

10. Turbine according to claim 9, wherein, in a direction parallel to the rotation axis (A), the holes (27) are distributed in a band comprised between leading edges (21) and trailing edges (22) of the rotor blades (15) .

11. Turbine according to claim 9 or 10, wherein the path (Pi) developed on a plane is rectilinear and extends between a front margin (23b) and a rear margin (23c) of the guide ring (23).

12. Turbine according to claim 11, wherein the path (Pi) forms an angle (a) in the range between 45° and 75° with the rotation axis (A) .

13. Turbine according to any one of claims 9 or 10, wherein the path (P4) developed on a plane is curvilinear with decreasing curvature radius between a front margin (23b) and a rear margin (23c) of the guide ring (23) .

14. Plant for the production of electric energy, comprising a gas turbine (5) according to any one of the foregoing claims.

15. Plant according to claim 14, comprising a compressor (3) mechanically coupled to the gas turbine (5), wherein the cooling circuit (21) is fluidly coupled to the compressor (3) for drawing the airflow rate from the compressor (3) .