STRUCTURAL ASSEMBLY FOR A COMPRESSOR OF A FLUID FLOW MACHINE

Abstract

A structural subassembly for a compressor of a turbomachine, which has: a stator with a multiplicity of guide blades which extend in a flow path of the turbomachine, wherein the guide blades have an axis of rotation and are designed to be adjustable in terms of their stagger angle; an inner flow path boundary, which delimits the flow path through the turbomachine radially at the inside; and an outer flow path boundary, which delimits the flow path through the turbomachine radially at the outside. Here, the guide blades have first partial gaps with respect to the outer flow path boundary and/or second partial gaps with respect to the inner flow path boundary. Provision is made whereby the guide blades are arranged and formed such that the axes of rotation of the guide blades have a combined inclination both with respect to the axial direction and in a circumferential direction.

Claims

1. A structural subassembly for a compressor of a turbomachine, which has: a stator with a multiplicity of guide blades which extend in a flow path of the turbomachine, wherein the guide blades have an axis of rotation and are designed to be adjustable in terms of their stagger angle, an inner flow path boundary, which delimits the flow path through the turbomachine radially at the inside, and an outer flow path boundary, which delimits the flow path through the turbomachine radially at the outside, wherein the guide blades have first partial gaps with respect to the outer flow path boundary and/or second partial gaps with respect to the inner flow path boundary, wherein in that the guide blades are arranged and formed such that the axes of rotation of the guide blades have a combined inclination both with respect to the axial direction and in a circumferential direction.

2. The structural subassembly according to claim 1, wherein the axes of rotation of the guide blades of the stator are inclined in the circumferential direction such that the respective radially inwardly directed elongations thereof do not intersect at a point of the stator axis.

3. The structural subassembly according to claim 2, wherein the axes of rotation of the guide blades of the stator are inclined in the circumferential direction such that the respective radially inwardly directed elongations thereof lie tangentially on a circle which extends around the stator axis in a section plane perpendicular to the stator axis.

4. The structural subassembly according to claim 1, wherein the inclination of the axes of rotation of the guide blades both with respect to the axial direction and in the circumferential direction is optimized such that a predefined minimum gap is not undershot in the first partial gap and/or in the second partial gap in the case of all settable stagger angles.

5. The structural subassembly according to claim 4, wherein the inclination of the axes of rotation of the guide blades both with respect to the axial direction and in the circumferential direction is optimized such that the first partial gap and/or the second partial gap maintains a minimum spacing to the adjacent flow path boundary in the case of all settable stagger angles.

6. The structural subassembly according to claim 1, wherein the axes of rotation are inclined in a positive direction in the circumferential direction.

7. The structural subassembly according to claim 1, wherein the axes of rotation are inclined in a negative direction in the circumferential direction.

8. The structural subassembly according to claim 1, wherein the axes of rotation are tilted in the circumferential direction by a tilt angle in the range between 0 and 10.

9. The structural subassembly according to claim 1, wherein the axes of rotation are inclined upstream with respect to the axial direction.

10. The structural subassembly according to claim 1, wherein the axes of rotation are inclined downstream with respect to the axial direction.

11. The structural subassembly according to claim 1, wherein the axes of rotation are tilted relative to the axial direction by a tilt angle in the range between 0 and 10.

12. The structural subassembly according to claim 1, wherein the partial gaps are formed in the region of the leading edge and/or in the region of the trailing edge of the guide blades, adjacent to the respective flow path boundary.

13. The structural subassembly according to claim 1, wherein the guide blades have a cut-back in the region of the trailing edge and adjacent to the radially outer flow path boundary and/or adjacent to the radially inner flow path boundary, such that said guide blades form, in the region of the trailing edge, a partial gap with respect to the adjacent flow path boundary.

14. The structural subassembly according to claim 1, wherein the guide blades have a cut-back in the region of the leading edge and adjacent to the radially outer flow path boundary and/or adjacent to the radially inner flow path boundary, such that said guide blades form, in the region of the leading edge, a partial gap with respect to the adjacent flow path boundary.

15. The structural subassembly according to claim 13, wherein the axes of rotation of the guide blades of the stator in the circumferential direction are inclined in combined fashion both with respect to the axial direction and in the circumferential direction such that the upper corner point, at which the leading edge and the cut-back at the blade tip or the trailing edge and the cut-back at the blade tip converge, and/or the lower corner point, at which the leading edge and the cut-back at the blade root or the trailing edge and the cut-back at the blade root converge, describe, during an adjustment of the stagger angle, a circular trajectory which is oriented locally perpendicularly with respect to the adjacent flow path boundary.

16. The structural subassembly according to claim 15, wherein the spacing of a corner point to the adjacent flow path boundary is substantially constant in the case of every set stagger angle.

17. The structural subassembly according to claim 1, wherein the guide blades are, in order to provide rotatability for the adjustment of the stagger angle, connected rotationally conjointly to, or formed as a single piece with, a spindle.

18. The structural subassembly according to claim 1, wherein the guide blades are connected at their radially outer end in each case to an outer circular platform which is arranged in the radially outer flow path boundary.

19. A gas turbine engine having a structural subassembly according to claim 1.

20. A gas turbine engine according to claim 19, said gas turbine engine having: an engine core which comprises a turbine, a compressor having a structural subassembly, and a turbine shaft which is configured as a hollow shaft and connects the turbine to the compressor; a fan, which is positioned upstream of the engine core, wherein the fan comprises a plurality of fan blades; and a gearbox that receives an input from the turbine shaft and outputs drive for the fan so as to drive the fan at a lower rotational speed than the turbine shaft.

Description

[0060] The invention will be explained in more detail below on the basis of a plurality of exemplary embodiments with reference to the figures of the drawing. In the drawing:

[0061] FIG. 1 shows a sectional lateral view of a gas turbine engine;

[0062] FIG. 2 shows a close-up sectional lateral view of an upstream portion of a gas turbine engine;

[0063] FIG. 3 shows a partially cut-away view of a gearbox for a gas turbine engine;

[0064] FIG. 4 shows a guide blade cascade, with the stagger angle of the guide blades being illustrated;

[0065] FIG. 5 schematically shows a structural subassembly which has an inlet stator with adjustable stagger angle and partial gaps to the adjacent flow path boundaries;

[0066] FIGS. 6a-6c show, in a view from the front, in meridional section and in three-dimensional view, a structural subassembly corresponding to FIG. 5, with the trajectory of the trailing-edge corner points during a change of the stagger angle being illustrated;

[0067] FIG. 7 shows, in a view from the front, an exemplary embodiment of a structural subassembly in which the axis of rotation of the guide blades is arranged so as to be inclined both in an axial direction and in a circumferential direction;

[0068] FIG. 8a shows, in a schematic illustration perpendicular to the longitudinal axis of the structural subassembly, the inwardly directed elongations of the axes of rotation of the guide blades of the stator in the case of an exactly radial orientation of the axes of rotation;

[0069] FIG. 8b shows, in a schematic illustration perpendicular to the longitudinal axis of the structural subassembly, the inwardly directed elongations of the axes of rotation of the guide blades of the stator in the case of an inclination of the axes of rotation in the circumferential direction, wherein the elongations of the axes of rotation lie tangentially on an imaginary circle; and

[0070] FIG. 9 shows the partial gap in a manner dependent on the stagger angle for a stator with exactly radially oriented guide blades and a stator with guide blades whose axis of rotation is inclined in combined fashion with respect to the axial direction and in the circumferential direction.

[0071] FIG. 1 illustrates a gas turbine engine 10 having a main axis of rotation 9. The engine 10 comprises an air intake 12 and a thrust fan 23 that generates two air flows: a core air flow A and a bypass air flow B. The gas turbine engine 10 comprises a core 11 which receives the core air flow A. In the sequence of axial flow, the engine core 11 comprises a low-pressure compressor 14, a high-pressure compressor 15, a combustion device 16, a high-pressure turbine 17, a low-pressure turbine 19, and a core thrust nozzle 20. An engine nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass thrust nozzle 18. The bypass air flow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low-pressure turbine 19 by way of a shaft 26 and an epicyclic gearbox 30.

[0072] During use, the core air flow A is accelerated and compressed by the low-pressure compressor 14 and directed into the high-pressure compressor 15, where further compression takes place. The compressed air expelled from the high-pressure compressor 15 is directed into the combustion device 16, where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high-pressure and low-pressure turbines 17, 19 before being expelled through the nozzle 20 to provide some thrust force. The high-pressure turbine 17 drives the high-pressure compressor 15 by means of a suitable connecting shaft 27. The fan 23 generally provides the major part of the thrust force. The epicyclic gearbox 30 is a reduction gearbox.

[0073] An exemplary assembly for a gearbox fan gas turbine engine 10 is shown in FIG. 2. The low-pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun gear 28 of the epicyclic gearbox assembly 30. Radially to the outside of the sun gear 28 and meshing therewith are a plurality of planet gears 32 that are coupled to one another by a planet carrier 34. The planet carrier 34 limits the planet gears 32 to orbiting around the sun gear 28 in a synchronous manner while enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled by way of linkages 36 to the fan 23 so as to drive the rotation of the latter about the engine axis 9. Radially to the outside of the planet gears 32 and meshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0074] It is noted that the terms low-pressure turbine and low-pressure compressor as used herein can be taken to mean the lowest-pressure turbine stage and the lowest-pressure compressor stage (that is to say not including the fan 23) respectively and/or the turbine and compressor stages that are connected to one another by the connecting shaft 26 with the lowest rotational speed in the engine (that is to say not including the gearbox output shaft that drives the fan 23). In some literature, the low-pressure turbine and low-pressure compressor referred to herein may alternatively be known as the intermediate pressure turbine and intermediate-pressure compressor. Where such alternative nomenclature is used, the fan 23 can be referred to as a first compression stage or lowest-pressure compression stage.

[0075] The epicyclic gearbox 30 is shown in an exemplary manner in greater detail in FIG. 3. Each of the sun gear 28, the planet gears 32 and the ring gear 38 comprise teeth about their periphery to mesh with the other gears. However, for clarity, only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the person skilled in the art that more or fewer planet gears 32 may be provided within the scope of protection of the claimed invention. Practical applications of an epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0076] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, wherein the ring gear 38 is fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of a further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held so as to be fixed, wherein the ring gear (or annulus) 38 is allowed to rotate. In the case of such an arrangement, the fan 23 is driven by the ring gear 38. By way of a further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0077] It is self-evident that the arrangement shown in FIGS. 2 and 3 is merely an example, and various alternatives fall within the scope of protection of the present disclosure. Purely by way of example, any suitable arrangement may be used for positioning the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of a further example, the connections (such as the linkages 36, 40 in the example of FIG. 2) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have a certain degree of stiffness or flexibility. By way of a further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts of the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the person skilled in the art would readily understand that the arrangement of output and support linkages and bearing positions would typically be different to that shown by way of example in FIG. 2.

[0078] Accordingly, the present disclosure extends to a gas turbine engine having an arbitrary arrangement of gearbox types (for example star-shaped or planetary), support structures, input and output shaft arrangement, and bearing positions.

[0079] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate-pressure compressor and/or a booster compressor).

[0080] Other gas turbine engines to which the present disclosure can be applied may have alternative configurations. For example, engines of this type may have an alternative number of compressors and/or turbines and/or an alternative number of connecting shafts. By way of a further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed-flow or split flow) may have a fixed or variable area. While the example described relates to a turbofan engine, the disclosure may be applied, for example, to any type of gas turbine engine, such as an open-rotor engine (in which the fan stage is not surrounded by an engine nacelle) or a turboprop engine. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0081] The geometry of the gas turbine engine 10, and components thereof, is/are defined by a conventional axis system, comprising an axial direction (which is aligned with the axis of rotation 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the view in FIG. 1). The axial, radial and circumferential directions are mutually perpendicular.

[0082] In the context of the present invention, the design of stators with a variable stagger angle in the compressor of the gas turbine engine is of importance.

[0083] Here, firstly, on the basis of FIG. 4, the basic construction of a guide blade cascade of a stator will be described, and the stagger angle will be defined. The guide blade cascade is illustrated in a conventional illustration in meridional section and in a developed view. Said guide blade cascade comprises a multiplicity of guide blades S, which each have a leading edge S.sub.VK and a trailing edge S.sub.HK. The leading edges S.sub.VK lie on an imaginary line L.sub.1, and the trailing edges S.sub.HK lie on an imaginary line L.sub.2. The lines L.sub.1 and L.sub.2 run parallel. The guide blades S furthermore each comprise a suction side SS and a pressure side DS. Their maximum profile thickness is denoted by d.

[0084] The guide blade cascade has a cascade pitch t and a profile chord s with a profile chord length s.sub.k. The profile chord s is the connecting line between the leading edge S.sub.VK and the trailing edge S.sub.HK of the profile. The blade stagger angle (hereinafter referred to as stagger angle) .sub.s is formed between the profile chord s and the perpendicular to the line L.sub.1 (wherein the perpendicular at least approximately corresponds to the direction defined by the machine axis). The stagger angle .sub.s indicates the inclination of the blades S.

[0085] The invention may be realized on each stator with variable stagger angle. The invention will be discussed below on the basis of an exemplary embodiment, in which said invention is realized on a stator with adjustable guide blades, which is arranged upstream of the first rotor of a compressor. Such a stator is referred to as an inlet stator or pre-stator (IGVInlet Guide Vane). Inlet stators with variable stagger angle improve the working range of a compressor. The invention may however additionally or alternatively also be realized on any other stator of the compressor which has a variable stagger angle of the guide blades.

[0086] Before the invention itself is discussed, the basic construction of a structural subassembly under consideration will firstly be discussed on the basis of FIG. 5.

[0087] FIG. 5 shows, in a sectional view, a structural subassembly, which defines a flow path 25 and which comprises an inlet stator 5, a rotor 6 of a compressor stage of a compressor and flow path boundaries. The flow path 25 guides the core air flow A as per FIG. 1 through the core engine.

[0088] Radially on the inside, the flow path 25 is delimited by a hub 95, which forms an inner flow path boundary 950. Radially on the outside, the flow path 25 is delimited by a compressor casing 4, which forms a radially outer flow path boundary 410. The flow duct 25 is formed as an annular space. The inlet stator 5 has stator blades or guide blades 50 which are adjustable in terms of stagger angle and which are arranged in the flow duct 25 so as to be distributed in the circumferential direction. The guide blades 50 each have a leading edge 51 and a trailing edge 52.

[0089] The swirl in the flow is increased by the inlet stator 5 and, as a result, the downstream rotor 6 is driven more effectively. The rotor 6 comprises a row of rotor blades 60, which extend radially in the flow path 25.

[0090] For adjustability of the stagger angle, the guide blades 50 are mounted so as to be rotatable. For this purpose, said guide blades are each connected rotationally conjointly to, or formed integrally with, a spindle 7. The spindle 7 has an axis of rotation 70, which is identical to the axis of rotation of the guide blades 50. Here, the spindle 7 is accessible and adjustable from outside the flow duct 25.

[0091] Specifically, provision is made for the guide blade 50 to be connected at its radially outer end to an outer circular platform 75, which forms a further rotary plate and which is connected to a radially outer spindle portion 71 of the spindle 7. The platform 75 and the spindle portion 71 are in this case mounted in a shroud 61, which is part of the compressor casing 4. Correspondingly, the guide blade 50 is connected at its radially inner end to an inner circular platform 76, which forms a further rotary plate and which is connected to a radially inner spindle portion 72 of the spindle 7. The platform 76 and the spindle portion 72 are in this case mounted in an inner shroud 62, which locally forms the inner flow path boundary 950.

[0092] To permit rotatability of the guide blades 50 or adjustability of the stagger angle, it is necessary for the guide blades to form, in the region of their trailing edge 52 and radially adjacent to the outer flow path boundary 410 and radially adjacent to the inner flow path boundary 950, cut-backs 53, 54 which ensure that the guide blades 50, in their axially rear region, form in each case one partial gap 81 to the radially outer flow path boundary 410 and one partial gap 82 to the radially inner flow path boundary 950. This prevents, during an adjustment of the guide blade 50 by rotation about the axis of rotation 70, said guide blade colliding with the outer flow path boundary 410 and/or with the inner flow path boundary 950.

[0093] The gaps 81, 82 are referred to here as partial gaps because they do not extend over the entire axial length of the guide blades 50.

[0094] Provision may alternatively be made whereby the guide blades 50 are formed without a shroud at their radially inner end, for which case they end in freely floating fashion, forming a continuous gap, in a manner radially spaced apart from the inner flow path boundary 950. Provision may also alternatively be made for partial gaps to be formed in the region of the leading edge 51 or both in the region of the leading edge 51 and in the region of the trailing edge 52.

[0095] Referring again to FIG. 5, it is furthermore the case that the guide blade 50 forms an upper corner point 55 of the trailing edge 52 and a lower corner point 56 of the trailing edge 52. The upper corner point 55 is defined as the point at which the trailing edge 52 and the cut-back 53 at the blade tip converge. The lower corner point 56 is defined as the point at which the trailing edge 52 and the cut-back 54 at the blade root converge.

[0096] FIG. 6a, in a view from the front, FIG. 6b, in meridional section, and FIG. 6c, in a perspective view, show the course of the trajectory of the trailing-edge corner points 55, 56 during an adjustment of the stagger angle. Here, correspondingly to the prior art, the guide blade 50 is oriented in exactly radial orientation in the flow path 25, that is to say the axis of rotation 70 runs in the radial direction.

[0097] The upper corner point 55 defines a first trajectory T1 during variation of the stagger angle. The lower corner point 56 defines a second trajectory T2 during variation of the stagger angle. As can be seen from the perspective illustration of FIG. 6c, the trajectories T1, T2 are circular. This follows from the fact that a rotation of the corner points 55, 56 about the axis of rotation 70 occurs.

[0098] FIG. 7 shows an exemplary embodiment of the invention in a view from the front, that is to say in a section plane perpendicular to the axial direction or machine axis of the structural subassembly. Provision is made whereby the guide blades of the stator are arranged and formed such that their axes of rotation 70 have a combined inclination both with respect to the axial direction and in the circumferential direction cp. Owing to the illustration from the front, the inclination in the axial direction cannot be seen in FIG. 7. To illustrate this, FIG. 5 illustrateswithout the corresponding guide bladean axis of rotation 70 inclined with respect to the axial direction. The spindle 7, the circular platforms 75, 76 and the shrouds 61, 62 are of correspondingly adapted design. The axis of rotation 70 is, in the exemplary embodiment illustrated in FIG. 5, tilted by the angle toward the axial direction, that is to say the axis of rotation 70 assumes the angle relative to the radial direction r, wherein the angle is defined as being positive clockwise.

[0099] For better comparison with the prior art, FIG. 7 shows both the trajectories T1, T2 that arise in the case of an axis of rotation 70 inclined correspondingly to the present invention if the upper corner point 55 and the lower corner point 56 are rotated about the axis of rotation 70, and the trajectories T1, T2 that arise in the case of an axis of rotation 70 running in a radial direction correspondingly to the prior art if the upper corner point 55 and the lower corner point 56 are rotated about the axis of rotation 70.

[0100] By means of a combined inclination of the axis of rotation 70 both with respect to the axial direction and in the circumferential direction, it is made possible for the partial gaps 81, 82 (see FIG. 5) to be made narrower. In particular, provision is made here for the axis of rotation 70 of the guide blades in an inclined arrangement to be oriented such that, during adjustment of the stagger angle, the circular trajectory T1, T2 is locally oriented perpendicular to the adjacent flow path boundary 410, 950. In this way, the spacing of the respective corner point 55, 56 to the adjacent flow path boundary 410, 950 is substantially constant in the case of every set stagger angle. Variations of the influencing of the flow by the partial gaps 81, 82 in a manner dependent on the set stagger angle are thus avoided.

[0101] It is pointed out that the inclination may exist in the circumferential direction (+) or counter to the circumferential direction (), wherein the circumferential direction is defined by the clockwise direction. The angle of inclination lies for example in the range between 0 and 10.

[0102] The inclination in the axial direction may be upstream () or downstream (+), see FIG. 5, wherein the angle relative to the exactly radial direction r is defined as being positive clockwise. In this case, too, the angle of inclination lies for example in the range between 0 and 10.

[0103] FIGS. 8a and 8b illustrate the different orientation of the axis of rotation 70, 70 of the guide blades in the case of an arrangement according to the prior art (FIG. 8a) and in the case of an arrangement according to the invention (FIG. 8b). In the case of an arrangement according to the prior art, when the axes of rotation 70 run in the exactly radial direction, the radially inwardly directed elongations of the axes of rotation 70 intersect at a point which lies on the stator axis, which is identical to the machine axis 9 of the aircraft engine in which the structural subassembly is formed (see FIGS. 1 and 2). In other words, the axes of rotation 70 are, in the radially inward direction, aligned toward one point.

[0104] By contrast, in the case of an inclination of the axes of rotation 70 in the circumferential direction, it is the case that, correspondingly to FIG. 8b, the radially inwardly directed elongations of the axis of rotation 70 are not aligned toward one point, but rather lie tangentially on an imaginary circle 96, which extends circularly around the machine axis 9 in a section plane perpendicular to the stator axis or machine axis 9.

[0105] This course, illustrated in FIG. 8b, of the elongations of the axis of rotation 70 is based on the inclination of the axis of rotation 70 in the circumferential direction . The inclination that is likewise present with respect to the axial direction does not play a role in this respect.

[0106] FIG. 9 illustrates the advantages associated with the structural subassembly according to the invention. FIG. 9 illustrates the radial width G of the partial gaps 81, 82 in a manner dependent on the stagger angle .sub.s. The curve 101 shows the thickness of the partial gap 82 at the radially inner flow path boundary for guide blades whose axis of rotation is formed so as to be inclined exclusively in the axial direction. The curve 102 shows the thickness of the partial gap at the radially inner flow path boundary for guide blades whose axis of rotation is formed so as to be inclined in combined fashion with respect to the axial direction and in the circumferential direction. The curve 103 shows the thickness of the partial gap 81 at the radially outer flow path boundary for guide blades whose axis of rotation is formed so as to be inclined exclusively in the axial direction. The curve 104 shows the thickness of the partial gap 81 at the radially outer flow path boundary for guide blades whose axis of rotation is formed so as to be inclined in combined fashion in the axial direction and in the circumferential direction.

[0107] It can be seen in each case that, in the case of an orientation of the axis of rotation of the guide blades with a combined inclination both with respect to the axial direction and in the circumferential direction, the partial gaps that arise are reduced. The associated reduced gap leakage reduces the flow losses, leading to an increase in efficiency. At the same time, the disadvantages of the stators are less pronounced, which results in reduced excitation of vibrations of the rotors arranged downstream.

[0108] It is self-evident that the invention is not restricted to the embodiments described above and that various modifications and improvements can be made without departing from the concepts described here. It is also pointed out that any of the features described may be used separately or in combination with any other features, unless they are mutually exclusive. The disclosure also extends to and comprises all combinations and sub-combinations of one or a plurality of features which are described here. If ranges are defined, said ranges thus comprise all of the values within said ranges as well as all of the partial ranges that lie in a range.