An axial-flow compressor with variable stationary blades

Abstract

The axial-flow compressor comprises a casing and a rotor arranged in the casing for rotation about a rotation axis. The compressor further includes at least one annular row of stationary blades that radially extend inside a flow path of the axial-flow compressor towards the rotation axis. At least some of the stationary blades have a variable geometry and include: a forward section extending from a leading edge to an intermediate position of the stationary blade; and an aft section extending from the intermediate position to a trailing edge of the stationary blade. Both the aft section and the forward section are rotatable about respective radial axes with respect to the casing.

Description

AN AXIAL-FLOW COMPRESSOR WITH VARIABLE STATIONARY BLADESDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure concerns axial-flow compressors, such as axial-flow compressors for gas turbine engines, for instance. Embodiments disclosed herein specifically refer to axial-flow compressors having stationary blades with a variable geometry.BACKGROUND ART

[0002] Gas turbine engines are widely used to power aircrafts, as well as in industrial applications, for example to generate mechanical power to drive a load or to drive electric generators and generate electric power.

[0003] Typically, a gas turbine engine includes a compressor section, a combustor section and a turbine section. The compressor section can include one or more compressors in sequence, which can be axial-flow compressors or centrifugal compressors, or a combination thereof.

[0004] Compressed air delivered by the compressor section is mixed with a fuel, typically in liquid or gaseous form, in the combustor section and the air-fuel mixture is combusted to generate hot and compressed combustion gas, which is expanded in the turbine section, to convert heat and pressure of the combustion gas into mechanical power. A portion of the power generated by the turbine is used to drive the compressor section to ensure a continuous flow of combustion gas from the combustor section.

[0005] In industrial applications the remaining power generated by the turbine engine, i.e., the power which is not required to drive the compressor section, is available on an output shaft and used to drive a load, such as an electric generator, a compressor or any other load.

[0006] In aircraft applications, the additional power of the combustion gas, which is not used to drive the compressor, generates thrust that propels the aircraft on which the gas turbine engine is mounted.

[0007] Multistage axial -flow compressors with wide operating flow ranges and high efficiency are desirable in both aircraft and industrial applications. Variable inlet guide vanes and variable stator blades have been developed to improve the efficiency of the axial-flow compressors at part speed operating conditions. The variable inlet guide vanes are stationary blades located at the inlet of the axial-flow compressor. Each variable inlet guide vane is adapted to rotate about a respective radial axis. A radial axis, as understood herein, is an axis which is oriented generally perpendicular to a rotation axis of the compressor.

[0008] The stator blades are located along the airflow path and direct the incoming airflow in the correct direction towards the downstream rotor blades. Similar to the variable inlet guide vanes, the variable stator blades are pivotally adjustable about respective radial axes.

[0009] The inclination of the variable stator blades and of the variable inlet guide vanes can be adjusted depending upon the operating condition of the turbomachine, and specifically depending on the rotational speed thereof.

[0010] Flexible variable inlet guide vanes are also known, which are comprised of a forward section and an aft section. The forward section is stationary with respect to the casing of the axial-flow compressor and has a fixed angular orientation, while the aft section is pivotally adjustable about a respective radial axis to adjust the inclination of the aft section depending on the operating condition of the turbomachine. Flexible variable inlet guide vanes may be useful to increase the performance of the axial-flow compressor.

[0011] An object of the present disclosure is to further improve the performance of axial-flow compressors.SUMMARY

[0012] According to an aspect, disclosed herein is axial-flow compressor (aka axial compressor) comprising: a casing; a rotor arranged in the casing for rotation about a rotation axis; and at least one annular row of stationary blades that radially extend inside a flow path of the axial-flow compressor towards the rotation axis.

[0013] Moreover, in embodiments disclosed herein at least some of the stationary blades have a variable geometry. Each stationary blade having a variable geometry comprises: a forward section extending from a leading edge to an intermediate position of the stationary blade; and an aft section extending from the intermediate position to a trailing edge of the stationary blade. Each aft section of the stationary blades having a variable geometry is rotatable, about a radial aft axis, with respect to the forward section and to the casing; and each forward section of the stationary blades having a variable geometry is rotatable about a radial forward axis with respect to the aft section and to the casing.

[0014] Advantageously, a plurality of actuators is provided, adapted to adjust a stagger angle of at least the aft section of the stationary blades having a variable geometry. Each actuator of said plurality of actuators is mechanically coupled to a respective subset of stationary blades, including at least one stationary blade, to adjust the stagger angle of the aft sections thereof. The stagger angle of each subset of stationary blades having a variable geometry can thus be adjusted independently of the remaining subsets, such that different stagger angles can be set for different subsets of stationary blades.

[0015] It shall be understood that in addition to the variable geometry blades defined above, the compressor may include stationary vanes which have a simplified variable geometry, i.e. with only one of the forward section or aft section capable of pivoting about a respective forward or aft axis. Different kinds of variable geometry blades and / or of stationary blades having a fixed geometry, can be provided in different annular rows of stationary blades, or can even be interspersed with one another.

[0016] Thus, in some embodiments, in addition to blades having both pivotable forward sections and pivotable aft sections, stationary blades with a variable geometry can be foreseen, which have a fixed (non-pivotable) aft section and a pivotable forward section, or vice versa.

[0017] In some embodiments, an annular row of stationary blades including blades with a variable geometry with either one or both the forward and aft sections being pivotable, may also include blades having a non-variable geometry. The various kinds of blades (non-pivotable, with both aft and forward section pivotable, or only one ofthe forward and aft section pivotable) can be interspersed with one another in the same annular row.

[0018] When both the forward and the aft sections of a variable geometry blade are pivotable to adjust the inclination thereof, the radial axes about which these sections are pivotable can be parallel and spaced from each other, or can be coincident.

[0019] In general, the pivotable aft and forward section of a blade are adapted to rotate simultaneously in the same direction, but usually by different angles, the aft section usually rotating about greater angles than the forward section.

[0020] The rotation of the forward section of a stationary blade having a variable geometry reduces losses due to separation of the air flow caused by the angle of incidence with the aft section of the same blade.

[0021] In general, the variable geometry stationary blade can be a stationary blade between two annular rows of rotor blades, i.e. blades inside a compression airflow path, or variable inlet guide vanes. In embodiments disclosed herein, an axial-flow compressor can include both variable inlet guide vanes as well as variable stator vanes inside the compression airflow path between rows of rotor blades. In other embodiments, it may be envisaged that only the inlet guide vanes include variable geometry blades with pivoting forward and aft sections. Conversely, in yet further embodiments, it may be envisaged that the inlet guide vanes have a fixed geometry, or a state-of-the art variable geometry, with only aft sections of the vanes having a variable inclination, followed by one or more annular rows of stationary blades, wherein one, some or all the stationary vanes have both a forward as well as an aft section with a variable inclination, i.e. variable geometry.

[0022] Further embodiments and features according to the present disclosure are outlined in the annexed claims and will be described with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Reference is now made briefly to the accompanying drawings, in which:Fig.1 illustrates a schematic sectional view of an axial-flow compressor alonga plane containing the rotation axis;Fig.2 illustrates a sectional view of a variable inlet guide vane of the axial- flow compressor of Fig.1 in one embodiment;Fig.3 illustrates a sectional view of a variable stator vane of the axial-flow compressor of Fig.1;Fig.4 illustrates a sectional view of a variable inlet guide vane of the axial- flow compressor of Fig.1 in another embodiment;Fig.5 illustrates an axonometric view of a variable stator vane and relevant actuator means in one embodiment;Fig.6 illustrates a partial sectional view of the variable stator vane of Fig.5;Fig.7 illustrates a sectional view according to line VII- VII of Fig.6;Fig.8 illustrates a schematic sectional view of an actuation arrangement for variable inlet vanes or variable stator vanes, in another embodiment;Fig.9 illustrates a schematic sectional view of an actuation arrangement of variable inlet vanes or variable stator vanes, in another embodiment;Fig.10 illustrates an axonometric view of the actuation arrangement of Fig.9;Fig.11 illustrates a schematic sectional view an actuation arrangement of variable inlet vanes or variable stator vanes, in another embodiment; andFig.12 illustrates an axonometric view of the actuation arrangement of Fig.11.DETAILED DESCRIPTION

[0024] Fig. l illustrates a schematic axial-flow compressor 1 in one embodiment. Specifically, Fig. l illustrates half sectional view of the axial-flow compressor according to a plane containing a rotation axis A-A of the compressor. The axial-flow compressor is substantially axially symmetric and therefore the lower part of the sectional view is omitted in Fig.l. The compressor 1 comprises a casing 3 and a rotor 5 housed in the casing 3 for rotation about the rotation axis A-A. Bearings (not shown) rotatably support the rotor 5 in the casing 3.

[0025] The rotor comprises a plurality of annular rows of rotor blades 7 arranged sequentially from a forward end F to an aft end A of the axial-flow compressor 1. Each rotor blade 7 is an integral part of the rotor 5 and rotates therewith about the rotation axis A-A. Annular rows of stator blades 9 are stationarily mounted in the casing 3 andinterspersed with the annular rows of rotor blades 7. Each stator blade 9 extends radially from the casing 3 within a flow path FP of the axial-flow compressor towards the rotation axis A-A.

[0026] Upstream of the first (i.e. most forward) annular row of rotor blades 7 an annular row of inlet guide vanes 11 defines the inlet of a compression flow path FP through which the process gas, for instance air, flows and is gradually compressed by mechanical energy supplied by the rotor blades 7 and which is converted into pressure of the process gas.

[0027] In the embodiment of Fig.1 the inlet guide vanes 11, or at least some of them, are variable inlet guide vanes, i.e., the inlet guide vanes 11 have a variable geometry as will be described in more detail below. In the embodiment of Fig.1 the first four annular rows of stator blades 9 also have a variable geometry as described below. In other embodiments, not shown, only the inlet guide vanes 11, or some of them, may have a variable geometry, while the stator blades 9 may be conventional stator blades with a fixed geometry. In yet further embodiments, not shown, the inlet guide vanes 11 may have a fixed geometry and the stator blades 9 may have a variable geometry.

[0028] While the exemplary axial-compressor 1 of Fig.1 has four annular rows of variable geometry stator blades 9, a different number (greater or less than four) of annular rows of variable geometry rotor blades 9 can be envisioned.

[0029] A schematic sectional view of a variable inlet guide vane 11 and of a variable geometry stator blade 9 are shown in Fig.2 and Fig.3, respectively. As shown in Fig.2, each inlet guide vane 11 having a variable geometry according to the present disclosure comprises a forward section 11.1 and an aft section 11.2. The forward section 11.1 extends from a leading edge 11.3 to an intermediate position 11.4 of the inlet guide vane 11. The aft section 11.2 extends from the intermediate position 11.4 to a trailing edge 11.5 of the inlet guide vane 11.

[0030] In some embodiments the forward section 11.1 has a shorter chord and the aft section 11.2 has a longer chord, as shown in Fig.2.

[0031] Each forward section 11.1 is adapted to rotate according to double arrow fF about a radial forward axis Fx with respect to the casing 3. Similarly, each aft section11.2 is adapted to rotate according to double arrow fA about a radial aft axis Ax. In the embodiment of Fig. 2 the radial forward axis Fx and the radial aft axis Ax of the same inlet guide vane 11 coincide.

[0032] The rotation fA of the aft sections 11.2 of the inlet guide vanes 11 can be used to provide the required flow swirl to the rotor blades 7 downstream thereof. A separate rotation fF of the forward section 11.1, which can be different from the rotation of the aft section 11.2, is provided to minimize the flow incidence of the incoming flow of the process gas, pictorially represented by arrows fpg. As will be described, in some embodiments the forward section 11.1 and the aft section 11.2 of the inlet guide vane 11 can rotate through two angles which can be selected independently of each other according to the operating conditions of the axial-flow compressor 1. In other embodiments, the two angles by which the forward section 11.1 and the aft section 11.2 rotate can be different from each other but not independent of each other. Rather, the two angles may be related to each other by a given ratio.

[0033] According to the embodiment of Fig.3, each stator blade 9 having a variable geometry comprises a forward section 9.1 and an aft section 9.2. The forward section 9.1 extends from a leading edge 9.3 to an intermediate position 9.4 of the stator blade 9. The aft section 9.2 extends from the intermediate position 9.4 to a trailing edge 9.5 of the stator blade 9.

[0034] In some embodiments, the forward section 9.1 has a shorter chord and the aft section 9.2 has a longer chord, as shown in Fig.3.

[0035] Each forward section 9.1 of the stator blade 9 is adapted to rotate about a radial axis Fy according to the double arrow labeled again with fF with respect to the casing 3. Similarly, each aft section 9.2 of the stator blade 9 is adapted to rotate about a radial axis Ay according to double arrow labeled again fA. In the embodiment of Fig. 4 the radial axis Fy and the radial axis Ay of the stator vane 9 coincide.

[0036] The rotation fA of the aft sections 9.2 of the stator blade 9 can be used to provide the required flow swirl to the rotor blades 7 downstream thereof. A separate rotation fF of the forward section 9.1, which can be different from the rotation of the aft section 9.2, can be provided to minimize the flow incidence of the incoming flowof the process gas. As mentioned with above with respect to the variable inlet guide vanes 11, the forward section 9.1 and the aft section 9.2 of the stator blade 9 can rotate through two angles which can be selected independently of each other according to the operating conditions of the axial -flow compressor 1, or the two angles through which the forward section 9.1 and the aft section 9.2 rotate can be different from each other but related to each other by a given ratio.

[0037] In general, the forward section 11.1 and the aft section 11.2 of the inlet guide vanes 11 rotate in the same direction, about different angles. In the same way, the forward section 9.1 and the aft section 9.2 of the stator blades 9 rotate in the same direction about different angles.

[0038] In other embodiments, each variable inlet guide vane 11 and / or stator vane 9 can have respective forward and aft sections which rotate about forward and aft axes Fx, Ax and Fy, Ay that are not coincident as shown in Figs. 2 and 3, but rather spaced apart and approximately parallel to each other. An embodiment relating to a variable inlet guide vane 11 having parallel and spaced apart axes Fx, Ax is shown in Fig.4. The same arrangement can be provided for the stator blades 9.

[0039] A gap 11.6 is provided in the intermediate position 11.4 of the variable inlet guide vane 11. The gap forms a calibrated flow passage between a pressure side and a suction side of the respective inlet guide vane 11. A controlled flow of process gas can stream through the flow passage. The controlled process gas flow reduces the flow separation region on the suction side of the respective inlet guide vane 11. In other words, if a flow separation arises on suction side of the inlet guide vane due to change in inclination between forward section 11.1 and aft section 11.2 of vane 11, such flow separation can be reduced or suppressed with the momentum of flow that enters through the gap 11.6.

[0040] A similar arrangement with spaced apart axes Ay and Fy can be foreseen for the stator vanes 9 for the same purpose.

[0041] It shall be understood that in some embodiments, one or more annular rows of stator blades 9 can be configured with coincident axes Ay, Fy and one or more different annular rows of stator blades 9 can be configured with parallel, non-coincident axes Ay, Fy. Moreover, the variable inlet guide vanes 11 configured as shown in Fig.4 or in Fig.2 can be combined with annular rows of stator blades 9 which have all coincident axes Ay, Fy, or all parallel and non-coincident axes Ay, Fy, or partly coincident and partly non-coincident axes Ay, Fy.

[0042] In some embodiments in a same annular row of inlet guide vanes 9 the two configurations of Figs. 2 and 4 can be mixed. Similarly, in a same annular row of stator blades 9, some of said stator blades can have coincident axes Ay, Fy, and some can have non-coincident axes Ay, Fy.

[0043] Some constraints on the choice of the arrangement of the aft and forward axes Ax, Fx and Ay, Fy may result from the structure of the actuation arrangement used to control the rotation of the stator blades 9 and inlet guide vanes 11, as will become apparent from the following detailed description of exemplary embodiments of actuation arrangements.

[0044] In general, irrespective of the arrangement of the aft axes Ax, Ay and forward axes Fx, Fy, the arrangement is such that the inclination of both the forward section 11.1, 9.1 and the aft section 11.2, 9.2 of the stationary blades (either inlet guide vanes 11 or stator blades 9) can be adjusted, i.e., the stagger angle thereof can be modified as a function of the operating conditions of the axial -flow compressor 1. As understood herein, and according to the standard meaning in the art, the stagger angle is the angle between the chord of the relevant airfoil section (forward or aft section of the blade) and the axial direction of the turbomachine, i.e. the direction of the rotation axis A-A.

[0045] The following description of embodiments of the actuation arrangements for controlling the angular displacement of the forward section and of the aft section of the variable geometry stationary blades can be used for both the variable geometry inlet guide vanes 11 and the variable geometry stator blades 9. Therefore, when describing Figs. 5 to 12 the controlled member will be referred as “stationary blade” having a variable geometry, it being understood that the stationary blade can be either an inlet guide vane or a stator blade. The stationary blade is labeled “10” in Figs.5 to 12, it being understood that the stationary blade 10 can be either a variable inlet guide vane 11 or a stator blade 9.

[0046] A first embodiment of an actuation arrangement to rotate the stationary blades is shown in Figs 5, 6 and 7. The actuation arrangement is globally labeled 21 and is arranged outside of the casing 3. In this embodiment, the actuation arrangement 21 includes, for each stationary blade 10 having a variable geometry, a first actuator 23 and a second actuator 25, for instance a first electric motor and a second electric motor. The first actuator 23 is drivingly coupled to a first shaft 27, which connects the first actuator 23 to the forward section 10.1 of the stationary blade 10. The second actuator 25 is drivingly coupled to a second shaft 29, which connects the second actuator 25 to the aft section 10.2 of the stationary blade 10. The shafts 27 and 29 are coaxial, i.e. they are concentric to one another, as shown in Fig.7 and their common axis features the two coincident forward axis Fx and aft axis Ax. A pair of meshing gears 28 can be provided to drivingly couple the second actuator 25 with the second shaft 29. In this embodiment the first actuator 23 is coaxial with the first shaft 27. In other embodiments, a pair of gears can be provided to transmit the rotary motion of the first actuator 23 to the first shaft 27, when the first actuator 23 and the first shaft 27 are not coaxial.

[0047] The two actuators 23, 25 can control the angular displacement of the forward section 10.1 and of the aft section 10.2 of the stationary blade 10 one independently from the other, such that the stagger angle of the forward section 10.1 can be adjusted independently of the stagger angle of the aft section 10.2. As a pair of actuators 23, 25 is provided for each stationary blade 10, each blade can be adjusted according to needs and each stationary blade can be adjusted with own stagger angles for the respective forward and aft sections 10.1 and 10.2 thereof.

[0048] A further embodiment of the actuation arrangement for controlling each individual stationary blade 10 is shown in FIG. 8. The actuation arrangement is again labeled 21. In this embodiment, a single actuator 31 is provided for each stationary blade 10. The actuator 31 controls the rotation of a single motor shaft 32 on which a first gear 33 and a second gear 35 are keyed. The first gear 33 meshes with a corresponding gear 37 keyed to the first shaft 27. The first shaft 27 is integrally coupled to the forward section 10.1 of the stationary blade 10. The second gear 35 meshes with a corresponding gear 39 keyed to the second shaft 29. The second shaft 29 is integrally coupled to the aft section 10.2 of the stationary blade 10. The two pairs of gears 33, 37 and 35, 39 provide different transmission ratios between the motor shaft 32 and the first shaft 27and the second shaft 29, such that rotation of the actuator 31 causes the forward section 10.1 and the aft section 10.2 to perform different angular movements in the same direction. For example, the transmission ratio is such that the aft section 10.2 performs a greater angular displacement than the forward section 10.1.

[0049] Both embodiments of Figs. 5, 6, 7 and Fig.8 allow controlling each stationary blade 10 independently from the others. Thus, the stagger angle of the forward section 10.1 and aft section 10.2 of each stationary blade 10 can be controlled individually. This can be used for instance to address problems arising from rotating stalls which may occur in the turbomachine.

[0050] Rotating stalls occur in axial-flow compressors due to circumferential propagation of disturbances or pressure waves at a fraction of the rotational speed and causes local flow variations, i.e. stall cells in some blade channels that limit both performance and stability of axial-flow compressor. Moreover, the rotating stall cells induce both airfoil and shaft vibrations which may lead to catastrophic mechanical failures.

[0051] Rotating stalls typically occur at part-speed operation during the engine startup. They may occur also at part-load and at the edge of operating envelop.

[0052] Rotating stalls can be mitigated or suppressed by adjusting the stagger angle of the stationary blades of the annular row of stationary blades, in which the stall occurs, by selecting different stagger angles for different stationary blades belonging to the same annular row. Different combinations of stagger angles for different blades and for the forward sections and aft sections thereof is possible in this embodiment.

[0053] In some other embodiments, not shown, stationary blades 10 can be grouped together such that each group comprises an actuation arrangement with one or two actuators, to control the first shafts and the second shafts of all stationary blades of the group, either independently from one another (as in Figs. 5, 6, 7) or in combination (Fig.8)

[0054] A yet further embodiment of an actuation arrangement for controlling rotation of the forward sections 10.1 and aft sections 10.2 of the stationary blades is shown in Figs. 9 and 10. The actuation arrangement is labeled again 21.

[0055] In this embodiment, each forward section 10.1 of the stationary blades 10 is drivingly coupled with a first shaft 27 to a respective first gear 37 keyed on the first shaft 27. Each aft section 10.2 of the stationary blades 10 is drivingly coupled with a second shaft 29 to a respective second gear 39 keyed on the second shaft 29.

[0056] The first gears 37 mesh with a first annular rack 41 which extends through 360° about the rotation axis A-A. The second gears 39 mesh with a second annular rack 43 which extends through 360° about the rotation axis A-A. The first annular rack 41 and the second annular rack 43 are monolithic, i.e. are formed as a single ring 45. On a side opposite to the first annular rack 41 and the second annular rack 43 the ring 45 features a third annular rack 47, which meshes with an actuating gear 49 keyed on a motor shaft 51, which is driven by an actuator 53, for example an electric motor.

[0057] The transmission ratio between the first annular rack 41 and the first gear 37 differs from the transmission ratio between the second annular rack 43 and the second gear 39. In this way by rotating the motor shaft 51 by a given angle, two different angular displacements of the forward section 10.1 and of the aft section 10.2 are obtained, to provide different adjustments to the respective stagger angles of the forward section 10.1 and aft section 10.2 of the stationary blade 10.

[0058] Thus, in this embodiment all stationary blades 10 are controlled by one and the same actuator and are therefore adjusted by the same angle. Nevertheless, some stationary blade 10 of the same annular row can have a non-variable geometry and may be separate from the annular racks. For instance two, four, six or eight stationary blades can be configured as rigid struts.

[0059] A yet further embodiment of an actuation arrangement for controlling the rotation, i.e. the angular displacement of the forward sections 10.1 and aft sections 10.2 of the stationary blades is shown in Figs. 11 and 12. The actuation arrangement is labeled again 21. In this embodiment, each forward section 10.1 of the stationary blades 10 is drivingly coupled with a respective first shaft 27 to a respective first gear 37 keyed on the first shaft 27. Each aft section 10.2 of the stationary blades 10 is drivingly coupled with a respective second shaft 29 to a respective second gear 39 keyed on the second shaft 29. The first gears 37 mesh with a first annular rack 41 and the second gears 39 mesh with a second annular rack 43.

[0060] In this embodiment, in contrast to the embodiment of Figs. 9 and 10, the first annular rack 41 and the second annular rack 43 are separate from each other. Each annular rack 41, 43 is drivingly coupled with a respective actuator, namely a first actuator 42 and a second actuator 44. The first actuator 42 can control the rotation of a first gear 46 and the second actuator 44 can control the rotation of a second gear 48. The first gear 46 meshes with a third annular rack 50 integral with the first annular rack 41 and the second gear 48 meshes with a fourth annular rack 52 integral with the second annular rack 41.

[0061] Thus, in this embodiment a first mechanical transmission comprising the racks 50 and 41 and the gear 46 connects the first actuator 42 to all forward sections 10.1 of all stationary blades 10. A second mechanical transmission comprising the racks 52 and 43 and the gear 48 connects the second actuator 44 to all aft sections 10.2 of all stationary blades 10.

[0062] As mentioned in connection with the embodiment of Figs. 9 and 10, also in the embodiment of Figs. 11 and 12 one or some stationary blades 10 may be devoid of a variable geometry and may be formed for instance as a monolithic airfoil in the form of a strut.

[0063] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. An axial-flow compressor comprising: a casing; a rotor arranged in the casing for rotation about a rotation axis; and at least one annular row of stationary blades that extend radially inside a flow path of the axial-flow compressor towards the rotation axis; wherein: at least some of the stationary blades have a variable geometry; each stationary blade having a variable geometry comprises: a forward section extending from a leading edge to an intermediate position of the stationary blade; and an aft section extending from the intermediate position to a trailing edge of the stationary blade; each aft section is rotatable, about a radial aft axis, with respect to the forward section and to the casing; each forward section is rotatable, about a radial forward axis, with respect to the aft section and to the casing; a plurality of actuators is provided, adapted to adjust a stagger angle of at least the aft section of the stationary blades having a variable geometry; and each actuator of said plurality of actuators is mechanically coupled to a respective subset of stationary blades, including at least one stationary blade, to adjust the stagger angle of the aft sections thereof.

2. The axial -flow compressor of claim 1, wherein the at least one annular row of stationary blades comprises a plurality of inlet guide vanes.

3. The axial-flow compressor of claim 1 or 2, wherein the at least one annular row of stationary blades is positioned between an upstream annular row of rotor blades and a downstream annular row of rotor blades.

4. The axial-flow compressor of any one of the preceding claims, wherein the forward axis and the aft axis of each stationary blade with a variable geometry are coaxial.

5. The axial-flow compressor of any one of claims 1 to 3, wherein the forward axis and the aft axis of each stationary blade with a variable geometry are parallel and spaced from one another.

6. The axial-flow compressor of claim 5, wherein each stationary blade with a variable geometry comprises a gap between the forward section and the aft section; and wherein the gap forms a flow passage between a pressure side and a suction side of the stationary blade.

7. The axial-flow compressor of any one of the preceding claims, wherein the forward section and the aft section of each stationary blade with a variable geometry are rotatable independent from each other, such that a stagger angle of the forward section is adjustable independently of the stagger angle of the aft section of the same stationary blade.

8. The axial-flow compressor of claim 7, further comprising an actuation arrangement adapted to adjust the stagger angle of the forward section of the the stationary blades with a variable geometry..

9. The axial-flow compressor of claim 8, wherein the actuation arrangement comprises a plurality of further actuators; and wherein each further actuator is mechanically coupled to a respective subset of stationary blades having a variable geometry, said subset comprising at least one stationary blade, to adjust the stagger angle of the forward sections of the stationary blades belonging to the respective subset of stationary blades.

10. The axial -flow compressor of claim 8, wherein the actuation arrangement comprises: a single further actuator adapted to adjust the stagger angle of the forward section of all stationary blades with variable geometry; and a first mechanical transmission mechanically connecting the single actuator to the forward sections of all stationary blades with variable geometry.

11. The axial-flow compressor of any one of claims 1 to 6, wherein the forward section and the aft section of each stationary blade with variable geometry are mechanically coupled to each other by a mechanical transmission, such that each rotation of the forward section corresponds to a predetermined rotation of the aft section of the same stationary blade.

12. The axial-flow compressor of claim 11, wherein the mechanical transmission is such that the aft section and the forward section rotate in the samedirection.

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