US20130149109A1

US20130149109A1 - Method of damping aerofoil structure vibrations

Info

Publication number: US20130149109A1
Application number: US13/817,621
Authority: US
Inventors: John R. Webster
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2010-08-23
Filing date: 2011-08-04
Publication date: 2013-06-13
Also published as: WO2012025359A1; EP2609297A1; GB2483061A; GB201014025D0

Abstract

The method described herein includes providing an aerofoil structure having a root portion and an aerofoil portion, wherein the aerofoil portion has a tip remote from the root portion. In certain aspects, the tip of the aerofoil structure can be substantially surrounded by a casing with a clearance gap between the tip and the casing. In certain aspects, the method described herein includes disposing the aerofoil structure relative to the casing such that an oscillation of the aerofoil structure from an unperturbed position in a first direction reduces the clearance gap and that an oscillation from the unperturbed position in a second direction increases the clearance gap. In certain aspects, the method further includes disposing the aerofoil structure such that the frequency of the clearance gap variation is equivalent to the frequency of the aerofoil structure vibration and permits a flow through the clearance gap in which the flow tends to dampen the aerofoil structure vibrations.

Description

The present invention relates to a casing for a blade, for example a fan blade as may be used in a turbofan gas turbine engine.
Fan flutter and other vibration continues to be a significant issue. The traditional route to reduce this is to avoid problematic engine running ranges or blade/fan set vibration modes, but this is particularly difficult at take off. Alternative methods include re-camber and increased blade chord.
Turbofan clapperless fan blades may suffer from vibration where aerodynamic forces lead to excitation of a fan blade's natural modes of vibration, e.g. second flap mode, away from coincidence with the harmonics of a fan blades rotational speed, i.e. a non integral vibration.
Flutter has continued to cause difficulties for many years, there is no fundamental solution which can be applied without a major performance penalty. As a result engines are designed as close to the limit as possible. Partial solutions which are used when flutter cannot be designed out include rolling take off and keep out zones, both of which are unattractive from an operational stand point.
Furthermore, re-camber and increased blade chord, reduce efficiency and increase weight respectively.
Accordingly the present invention seeks to address these issues.
According to a first aspect of the present invention there is provided a method of damping aerofoil structure vibrations, the method comprising: providing an aerofoil structure having a root portion and an aerofoil portion, wherein the aerofoil portion has a tip remote from the root portion, the tip of the aerofoil structure being substantially surrounded by a casing with a clearance gap between the tip and the casing; disposing the aerofoil structure relative to the casing such that an oscillation of the aerofoil structure from an unperturbed position in a first direction reduces the clearance gap and that an oscillation from the unperturbed position in a second direction increases the clearance gap; further disposing the aerofoil structure such that the frequency of the clearance gap variation is equivalent to the frequency of the aerofoil structure vibration; and permitting a flow through the clearance gap, the flow tending to damp the aerofoil structure vibrations.
The method may further comprise limiting the aerofoil structure from vibrating beyond a minimum in the clearance gap when moving in the first direction.
The method may further comprise providing a change in the curvature of the aerofoil portion of the aerofoil structure. As a result, the tip of the aerofoil structure may be displaced from the point of minimum clearance gap when in the unperturbed position.
The method may further comprise angling the root portion of the aerofoil structure. As a result, the tip of the aerofoil structure may be displaced from the point of minimum clearance gap when in the unperturbed position.
The method may further comprise angling the aerofoil portion with respect to the root portion. As a result, the tip of the aerofoil structure may be displaced from the point of minimum clearance gap when in the unperturbed position.
According to a second aspect of the present invention there is provided an aerofoil structure configured to damp vibrations in the aerofoil structure, the aerofoil structure comprising a root portion and an aerofoil portion, wherein the aerofoil portion has a tip remote from the root portion, the tip of the aerofoil structure being substantially surrounded by a casing with a clearance gap between the tip and the casing; wherein the aerofoil structure is disposed relative to the casing such that an oscillation of the aerofoil structure from an unperturbed position in a first direction reduces the clearance gap and that an oscillation from the unperturbed position in a second direction increases the clearance gap; the aerofoil structure being further disposed such that the frequency of the clearance gap variation is equivalent to the frequency of the aerofoil structure vibration; and wherein a flow through the clearance gap tends to damp the aerofoil structure vibrations.
The aerofoil structure may be further disposed such that the aerofoil structure may be limited from vibrating beyond a minimum in the clearance gap when moving in the first direction.
The aerofoil portion of the aerofoil structure may comprises a change in the curvature. As a result, the tip of the aerofoil structure may be displaced from the point of minimum clearance gap when in the unperturbed position.
The root portion of the aerofoil structure may be angled, for example, with respect to a hub or a radial line. As a result, the tip of the aerofoil structure may be displaced from the point of minimum clearance gap when in the unperturbed position.
The aerofoil portion may be angled with respect to the root portion. As a result, the tip of the aerofoil structure may be displaced from the point of minimum clearance gap when in the unperturbed position.
The tip of the aerofoil structure may be angled with respect to a line normal to the casing. A bend may be provided in the aerofoil portion of the aerofoil structure. A bend may be provided substantially at the root portion of the aerofoil structure.
The aerofoil structure may comprise a suction surface and a pressure surface disposed either side of the aerofoil portion. The suction and pressure surfaces may be arranged such that the flow through the clearance gap may tend to move the aerofoil structure in the first direction. Alternatively, the suction and pressure surfaces may be arranged such that the flow through the clearance gap may tend to move the aerofoil structure in the second direction.
The aerofoil structure may rotate with respect to the casing. The aerofoil structure may be a fan blade. A turbomachine may comprise the turbomachine aerofoil structure described above. A gas turbine engine may comprise the aerofoil structure described above.
In summary, embodiments of the present invention may provide for blade vibration damping by utilising passive modulation of blade tip clearance. Embodiments of the present invention may provide for extended blade life due to reduction in high cycle fatigue, reduced blade generated noise due to blade damping, reduced blade tip generated noise due to disrupted over tip vortex. With embodiments of the present invention problems of reduced fan efficiency and/or increased weight may be at least mitigated. Tip clearance modulation in accordance with embodiments of the present inventions may have a significant effect on blade vibration, for example in fans and/or compressors.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

FIG. 1 shows a turbofan gas turbine engine having a fan blade to which the present invention can be applied;

FIG. 2 shows a fan blade to which the present invention can be applied;

FIG. 3 schematically illustrates a simplified tip modulation scenario, for assistance in understanding the present invention;

FIG. 4 schematically illustrates tip opening on a twisted fan blade for assistance in understanding the present invention;

FIG. 5 illustrates an aerofoil structure according to an example of the present invention (for simplicity s flat casing has been shown);

FIG. 6 illustrates an aerofoil structure according to a further example of the present invention (for simplicity s flat casing has been shown); and

FIG. 7 illustrates an aerofoil structure according to a yet further example of the present invention (for simplicity s flat casing has been shown).

A turbofan gas turbine engine 10, as shown in FIG. 1, comprises in flow series an inlet 12, a fan section 14, a compressor section 16, a combustion section 18, a turbine section 20 and an exhaust 22. The fan section 14 comprises a fan rotor 24 carrying a plurality of circumferentially spaced radially outwardly extending fan blades 26. The fan blades 26 are arranged in a bypass duct 28 defined by a fan casing 30, which surrounds the fan rotor 24 and fan blades 26. The fan casing 30 is secured to a core engine casing 34 by a plurality of circumferentially spaced radially extending fan outlet guide vanes 32. The fan rotor 24 and fan blades 26 are arranged to be driven in a conventional manner by a turbine (not shown) in the turbine section 20 via a shaft (not shown). The compressor section 16 comprises one or more compressors (not shown) arranged to be driven by one or more turbines (not shown) in the turbine section 20 via respective shafts (not shown).
An exemplary fan blade 26 to which the present invention may relate is shown more clearly in FIG. 2. The fan blade 26 comprises a root portion 36 and an aerofoil portion 38. The root portion 36 is arranged to locate in a slot 40 in the rim 42 of the fan rotor 24, and for example the root portion 36 may be dovetail shape, or fir-tree shape, in cross-section and hence the corresponding slot 40 in the rim 42 of the fan rotor 24 is the same shape. The aerofoil portion 38 has a leading edge 44, a trailing edge 46 and a tip 48 remote from the root portion 36 and the fan rotor 24. A concave pressure surface 50 extends from the leading edge 44 to the trailing edge 46 on one face of the fan blade 26 and a convex suction surface 52 extends from the leading edge 44 to the trailing edge 46 on an opposite face of the fan blade 26.
Aerodynamic disturbances caused by vibration of the blades 26 could excite appropriate modes in the casing 30 that would in turn modulate the tip clearance. It is suspected that changes in tip clearance cause a modulation in the energy loss due to tip leakage and hence a modulation in the aerodynamic loading, particularly around the tip 48. This loading modulation can provide a vibration excitation. Dependent on modal coincidences, mode strengths and exact phasing, the mechanism can provide strong excitation or damping.
Small changes in tip clearance may cause major performance penalties i.e. energy loss. This energy loss may be manifested as a reduction of the blade loading around the tip. A modulation in this energy loss can provide vibration forcing/damping.
A simplified illustration is shown in FIG. 3, which schematically illustrates tip modulation. In FIG. 3, a fan blade 26 is modeled as a flat plate, which operates close to a further flat plate (which represents a casing 30). As shown, a flap mode will provide a tip clearance modulation. This modulation opens the gap at the maximum displacement on each half-vibration cycle, so that the modulation occurs at twice the vibration frequency.
Since this is frequency doubled, it can have no effect on the blade vibration in the flap mode. However, in accordance with the present invention it has been appreciated that it is desirable to modulate once per cycle. Such a modulation has the potential to provide an aerodynamic forcing which is at the same frequency as the blade vibration and the phase of this forcing may be changed by 180° to provide damping.
The real situation is more complex than is illustrated in FIG. 3. For example, the casing may be curved and the fan blade may comprise high levels of blade twist, which gives significant modification to the tip motion. The effect will increase towards the leading and trailing edges. In the case of a twisted blade, the motion is not perpendicular to the tip of the aerofoil with modulation once per cycle. FIG. 4 schematically illustrates tip opening on a twisted fan blade.
With a simple model, the effect from the leading and trailing edges would however be equal and opposite so would cancel each other out. Asymmetry in geometry or local aerodynamic loading could lead to an out of balance effect that will result in blade forcing. This may be likely to occur in existing designs and may be the root of some vibration problems.
With reference to FIGS. 5 to 7, a turbomachine casing assembly according to the present invention comprises an aerofoil structure, for example a fan blade 26, configured to damp vibrations in the aerofoil structure. The fan blade 26 comprises a root portion 36 and an aerofoil portion 38. The aerofoil portion 38 has a tip 48 remote from the root portion 36. The tip 48 of the fan blade is substantially surrounded by a casing 30 with a clearance gap 60 between the tip 48 and the casing 30. The fan blade 26 is disposed relative to the casing 30 such that an oscillation of the fan blade from an unperturbed position in a first direction 62 reduces the clearance gap 60 and that an oscillation from the unperturbed position in a second direction 64 increases the clearance gap 60. The fan blade 26 is further disposed such that the frequency of the clearance gap 60 variation is equivalent to the frequency of the fan blade vibration.
A flow through the clearance gap 60 tends to damp the fan blade vibrations. The fan blade 26 may comprise a pressure surface 50 and a suction surface 52 disposed either side of the aerofoil portion 38. The pressure and suction surfaces 50, 52 may be arranged such that the flow through the clearance gap 60 may tend to move the fan blade 26 in the first direction 62. Alternatively, the pressure and suction surfaces 50, 52 may be arranged such that the flow through the clearance gap 60 may tend to move the fan blade 26 in the second direction 64.
As shown in FIGS. 6 and 7, the fan blade 26 may move between first and second positions 26′ and 26″. (NB, The unperturbed position is denoted 26 in FIG. 7.) The first position 26′ may be the furthest displacement of the fan blade in the first direction 62, whilst the second position 26″ may be the furthest displacement of the fan blade 26 in the second direction 64. The fan blade 26 may be constructed such that it is limited from vibrating beyond a minimum in the clearance gap 60′ when moving in the first direction 62. (By contrast, the maximum in the clearance gap is denoted 60″ and is reached when moving in the second direction 64.) For example, the unperturbed position of the fan blade 26 and/or the rigidity of the fan blade may be configured such that under the maximum loading experienced in normal operation the furthest displacement in the first direction does not go beyond the minimum clearance gap 60′. In other words, as the fan blade vibrates in the first direction, the clearance gap 60 may decrease only. Movement of the fan blade 26 in the first direction 52 may also be restricted by the tip 48 of the fan blade contacting the casing 30.
As shown in FIG. 5, the root portion 36 of the fan blade 26 may be angled in a spanwise direction, for example, with respect to a radial line emanating from a rim 42. As a result, the tip 48 of the fan blade 26 may be displaced from the point of minimum clearance gap 60′ when in the unperturbed position. Alternatively, the aerofoil portion 38 may be angled with respect to the root portion 36. As a result, the tip 48 of the fan blade 26 may be displaced from the point of minimum clearance gap 60′ when in the unperturbed position.
The example shown in FIG. 5 may be difficult to design in terms of stresses in the fan blade 26. However, the same effect may be achieved by applying a bent over tip 48. For example, as shown in FIG. 6, the aerofoil portion 38 of the fan blade 26 may comprises a change in the spanwise curvature 66 of the fan blade. The change in the curvature 66 may be localised such that it resembles a bend in the fan blade 26. As a result, the tip 48 of the fan blade 26 may be displaced from the point of minimum clearance gap 60′ when in the unperturbed position.
Referring to FIG. 7, rather than having a localised bend 66, the fan blade 26 may comprise a gradual change in the curvature so that the tip 48 of the fan blade 26 may be displaced from the point of minimum clearance gap 60′ when in the unperturbed position. As a result, the mode shape of the fan blade 26 may be modified such that it allows more radial movement at the tip 48.
In addition to the above, the present invention provides a method of damping aerofoil structure vibrations. The method comprises providing an aerofoil structure, for example a fan blade 26, having a root portion 36 and an aerofoil portion 38, wherein the aerofoil portion has a tip 48 remote from the root portion, the tip of the fan blade being substantially surrounded by a casing 30 with a clearance gap 60 between the tip 48 and the casing 30. The method further comprises disposing the fan blade 26 relative to the casing 30 such that an oscillation of the fan blade 26 from an unperturbed position in a first direction 62 reduces the clearance gap 60 and that an oscillation from the unperturbed position in a second direction 64 increases the clearance gap 60. The method further comprises disposing the fan blade 26 such that the frequency of the clearance gap variation is equivalent to the frequency of the fan blade vibration. A flow tending to damp the fan blade vibrations is permitted through the clearance gap 60.
The method may further comprise limiting the fan blade 26 from vibrating beyond a minimum in the clearance gap 60′ when moving in the first direction 62.
The method may further comprise providing a change in the curvature of the aerofoil portion 38 of the fan blade 26. As a result, the tip 48 of the fan blade 26 may be displaced from the point of minimum clearance gap 60′ when in the unperturbed position.
The method may further comprise angling the root portion 36 of the fan blade 26. As a result, the tip 48 of the fan blade 26 may be displaced from the point of minimum clearance gap 60′ when in the unperturbed position.
The method may further comprise angling the aerofoil portion 38 with respect to the root portion 36. As a result, the tip 48 of the fan blade 26 may be displaced from the point of minimum clearance gap 60′ when in the unperturbed position.
The above description uses the first flap vibration mode as an example. Similar effects can be achieved with other modes if they are problematic. In any event, it is desirable to achieve a significant radial component in the mode shape similar to that described above.
The present invention alleviates or reduces blade flutter by a purely passive means. In other words, the present invention damps blade vibration by utilising passive modulation of the blade tip clearance. As a result, the blade life may be extended due to a reduction in high cycle fatigue. Likewise, noise levels may be reduced due to the blade damping and the disrupted over tip vortex. The present invention may achieve these advantages without reducing the fan efficiency and/or increasing the weight, which may be the case for current solutions to the aforementioned problem.
The present invention is for example applicable to clapperless fan blades which lead to excitation of other natural modes of vibration, e.g. first flap mode, third flap mode, first torsion mode, second torsion mode or combinations thereof or any of the first ten fundamental vibration modes. The present invention is applicable to any aerofoil structure such as metal fan blades and fan blades having a hybrid structure, e.g. composite fan blades. In the case of some designs of hybrid structured fan blades there may be other natural modes of vibration that are not easy to describe using first flap mode, second flap mode, third flap mode, first torsion mode or second torsion mode because the complex structure of these hybrid structured fan blades may distort such mode shapes out of recognition.
The present invention is however also applicable to other fan or turbine applications or turbomachinery blades, including e.g. fans in ventilation subsystems or automotive applications, centrifugal compressors etc.

Claims

1. A method of damping aerofoil structure vibrations, the method comprising:

providing an aerofoil structure having a root portion and an aerofoil portion, wherein the aerofoil portion has a tip remote from the root portion, the tip of the aerofoil structure being substantially surrounded by a casing with a clearance gap between the tip and the casing;

disposing the aerofoil structure relative to the casing such that an oscillation of the aerofoil structure from an unperturbed position in a first direction reduces the clearance gap and that an oscillation from the unperturbed position in a second direction increases the clearance gap;

further disposing the aerofoil structure such that the frequency of the clearance gap variation is equivalent to the frequency of the aerofoil structure vibration; and

permitting a flow through the clearance gap, the flow tending to damp the aerofoil structure vibrations.

2. A method of damping aerofoil structure vibrations as claimed in claim 1, the method further comprising:

limiting the aerofoil structure from vibrating beyond a minimum in the clearance gap when moving in the first direction.

3. A method of damping aerofoil structure vibrations as claimed in claim 1, the method further comprising:

providing a change in the curvature of the aerofoil portion of the aerofoil structure such that the tip of the aerofoil structure is displaced from the point of minimum clearance gap when in the unperturbed position.

4. A method of damping aerofoil structure vibrations as claimed in claim 1, the method further comprising:

angling the root portion of the aerofoil structure such that the tip of the aerofoil structure is displaced from the point of minimum clearance gap when in the unperturbed position.

5. A method of damping aerofoil structure vibrations as claimed in claim 1, the method further comprising:

angling the aerofoil portion with respect to the root portion such that the tip of the aerofoil structure is displaced from the point of minimum clearance gap when in the unperturbed position.

6. An aerofoil structure configured to damp vibrations in the aerofoil structure, the aerofoil structure comprising a root portion and an aerofoil portion, wherein the aerofoil portion has a tip remote from the root portion, the tip of the aerofoil structure being substantially surrounded by a casing with a clearance gap between the tip and the casing;

wherein the aerofoil structure is disposed relative to the casing such that an oscillation of the aerofoil structure from an unperturbed position in a first direction reduces the clearance gap and that an oscillation from the unperturbed position in a second direction increases the clearance gap; the aerofoil structure being further disposed such that the frequency of the clearance gap variation is equivalent to the frequency of the aerofoil structure vibration; and

wherein a flow through the clearance gap tends to damp the aerofoil structure vibrations.

7. An aerofoil structure as claimed in claim 6, wherein the aerofoil structure is further disposed such that the aerofoil structure is limited from vibrating beyond a minimum in the clearance gap when moving in the first direction.

8. An aerofoil structure as claimed in claim 6, wherein the aerofoil portion of the aerofoil structure comprises a change in the curvature such that the tip of the aerofoil structure is displaced from the point of minimum clearance gap when in the unperturbed position.

9. An aerofoil structure as claimed in claim 6, wherein the root portion of the aerofoil structure is angled such that the tip of the aerofoil structure is displaced from the point of minimum clearance gap when in the unperturbed position.

10. An aerofoil structure as claimed in claim 6, wherein the aerofoil portion is angled with respect to the root portion such that the tip of the aerofoil structure is displaced from the point of minimum clearance gap when in the unperturbed position.

11. An aerofoil structure as claimed in claim 6, wherein the aerofoil structure comprises a suction surface and a pressure surface disposed either side of the aerofoil portion, the suction and pressure surfaces being arranged such that the flow through the clearance gap tends to move the aerofoil structure in the first direction.

12. An aerofoil structure as claimed in claim 6, wherein the aerofoil structure is a fan blade.

13. A turbomachine comprising the aerofoil structure of claim 6.

14. A gas turbine comprising the aerofoil structure of claim 6.

15. (canceled)

16. (canceled)