Method for detuning a rotor-blade cascade

Abstract

A method for detuning a rotor-blade cascade of a turbomachine having a plurality of rotor blades includes: a) establishing at least one target natural frequency for at least one vibration mode; b) setting up a value table having discrete mass values and radial center-of-gravity positions, and determining respective natural frequency; c) measuring the mass and radial center-of-gravity position of one of the rotor blades; d) determining an actual natural frequency by interpolating the measured mass and radial center-of-gravity position in the value table; e) if actual natural frequency is outside a tolerance around target natural frequency, selecting a value pair that at least approximates target natural frequency, and removing material from the rotor blade in such a way that mass and radial center-of-gravity position correspond to the value pair; f) repeating steps c) to e) until actual natural frequency is within the tolerance around target natural frequency.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2014/051322 filed Jan. 23, 2014, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP13153956 filed Feb. 5, 2013. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for detuning a rotor-blade cascade.

BACKGROUND OF INVENTION

A turbomachine has rotor blades which are arranged in rotor wheels, which may be regarded as firmly clamped at their blade roots and can oscillate during operation of the turbomachine. Depending on the operating state of the turbomachine, oscillation processes may occur in which oscillating states with high and critical stresses in the rotor blade occur. In the event of long-term loading of the blade by critical stress states, material fatigue takes place which can ultimately lead to a lifetime reduction of the blade, necessitating replacement of the rotor blade.

Because of the centrifugal forces acting on the rotor blade during operation of the turbomachine, a prestress is generated in the rotor blade. Owing to this and the high temperature of the rotor blade during operation, the natural frequencies of the rotor blade during operation differ from the natural frequencies of the cold rotor blade at rest. As a quality-assurance measure during manufacture, only the natural frequencies when the turbomachine is at rest can be measured, although for the configuration of the rotor blade it is necessary to know the natural frequencies under the centrifugal force, so that the oscillation processes in which the oscillation states with high and critical stresses in the rotor blade occur can be avoided.

EP 1 589 191 discloses a method for detuning a rotor-blade cascade.

SUMMARY OF INVENTION

It is an object of the invention to provide a method for detuning a rotor-blade cascade of a turbomachine, the rotor blades having a long lifetime during operation of the turbomachine.

The method according to aspects of the invention for detuning, in particular rotor-dynamically detuning, a rotor-blade cascade, comprising a multiplicity of rotor blades, of a turbomachine, has the steps: a) establishing for each of the rotor blades of the rotor-blade cascade at least one setpoint natural frequency ν_F,S which the rotor blade has for at least one predetermined oscillation mode during normal operation of the turbomachine under the effect of centrifugal force, such that the oscillation load of the rotor-blade cascade under the centrifugal force lies below a tolerance limit; b) compiling a value table ν_F(m, r_S) with selected discrete mass values m and radial center-of-mass positions r_S, which result from variations of the nominal geometry of the rotor blade, and determining the respective natural frequency ν_F of the predetermined oscillation mode under the centrifugal force for each selected value pair m and r_S; c) measuring the mass m_I and the radial center-of-mass position r_S,I of one of the rotor blades; d) determining an actual natural frequency ν_F,I of the rotor blade under the centrifugal force by interpolation of the measured mass m_I and the measured radial center-of-mass position r_S,I in the value table ν_F(m, r_S); e) in the event that ν_F,I lies outside a tolerance around ν_F,S, selecting from the value table ν_F(m, r_S) a value pair m_S and r_S,S such that ν_F,S at least approximates ν_F,S, and removing material of the rotor blade in such a way that m_I and r_S,I correspond to the value pair m_S and r_S,S; f) repeating steps c) to e) until ν_F,I lies within the tolerance around ν_F,S.

By measuring the mass m_I and the radial center-of-mass position r_S,I and by interpolating these values in the value table ν_F(m, r_S), the natural frequency ν_F,I under the centrifugal force can advantageously be determined with a high accuracy. With the method according to the invention, it is likewise advantageously possible to adjust this natural frequency ν_F,I with a high accuracy and approximate it to the established setpoint natural frequency ν_F,S. The oscillation load of the rotor blade during operation of the turbomachine can therefore be reduced, so that the lifetime of the rotor blade is extended. Furthermore, the method can be carried out straightforwardly because, for accurate determination of the actual natural frequency ν_F,I, it is surprisingly sufficient to measure m_I and r_S,I of the rotor blade without its full geometry. Furthermore, m_I and r_S,I are quantities which are simple to measure; for example, m_I can be measured with a balance.

The predetermined oscillation modes are particularly selected in such a way that the natural frequencies ν_F,S associated with the oscillation modes are equal to or of lower frequency than a multiple harmonic of the rotor rotation frequency, in particular the eighth harmonic, a value table ν_F(m, r_S) respectively being compiled for a multiplicity of or all of the oscillation modes, the actual natural frequency ν_F,I being determined for each value table and the value pair m_S and r_S,S being selected in such a way that the determined ν_F,I are at least approximated to the established ν_F,S.

The method according to the invention for detuning, in particular rotor-dynamically detuning, a rotor-blade cascade, comprising a multiplicity of rotor blades, of a turbomachine, has the steps: a) establishing for each of the rotor blades of the rotor-blade cascade at least one setpoint natural frequency ν_F,S which the rotor blade has for at least one predetermined oscillation mode during normal operation of the turbomachine under the effect of centrifugal force, such that the oscillation load of the rotor-blade cascade under the centrifugal force lies below a tolerance limit; b) compiling a value table ν_F(m, r_S) and a value table ν_S(m, r_S) with selected discrete mass values m and radial center-of-mass positions r_S, which result from variations of the nominal geometry of the rotor blade, and determining the respective natural frequency ν_F of the predetermined oscillation mode under the centrifugal force and the respective natural frequency ν_S with the rotor blade at rest for each selected value pair m and r_S; c) measuring the mass m₁ and the radial center-of-mass position r_S,I of one of the rotor blades; d) determining an actual natural frequency ν_F,I of the rotor blade under the centrifugal force by interpolation of the measured mass m_I and the measured radial center-of-mass position r_S,I in the value table ν_F(m, r_S); e) in the event that ν_F,I lies outside a tolerance around ν_F,S, selecting from the value table ν_F(m, r_S) a value pair m_S, r_S,S such that ν_F,I at least approximates ν_F,S, and removing material of the rotor blade in such a way that m_I and r_S,I correspond to the value pair m_S, r_S,S; f) in the event that material has been removed, measuring an natural frequency ν_S,I of the rotor blade at rest; g) repeating steps e) to f or c) to f) until ν_F,I lies within the tolerance around ν_F,S and ν_S,I lies within a tolerance around ν_S,S corresponding to the tolerance.

By the additional measurement of the natural frequency ν_{S j}, the actual natural frequency ν_F,I under the centrifugal force can advantageously be determined with an even higher accuracy. It is also possible to use the measurement of the natural frequency ν_S,I at rest in order to monitor the removal, without repeating the measurement of m₁ and r_S,I.

The predetermined oscillation modes are particularly selected in such a way that the natural frequencies ν_F,S associated with the oscillation modes are equal to or of lower frequency than a multiple harmonic of the rotor rotation frequency, in particular the eighth harmonic, respectively a value table ν_F(m, r_S) and respectively a value table ν_S(m, r_S) being compiled for a multiplicity of or all of the oscillation modes, the actual natural frequency ν_F,I and the actual natural frequency ν_S,I being determined for each value table and the value pair m_S and r_S,S being selected in such a way that the determined ν_F,I are at least approximated to the established ν_F,S and the natural frequencies ν_S,I being measured for the predetermined oscillation modes.

The variations of the nominal geometry may comprise thickening and/or thinning of the rotor blade in each radial section or in radial sections. It is advantageous for the variations of the nominal geometry to comprise a linear variation of the thickness of the rotor blade over the radius. It is advantageously possible to combine the value table using the thickening and thinning of the nominal geometry with an accuracy sufficient for determining the natural frequencies ν_F and ν_S.

The setpoint natural frequencies ν_F,S are particularly established in such a way that rotor blades arranged next to one another in the rotor-blade cascade have unequal setpoint natural frequencies ν_F,S, and that the setpoint natural frequencies ν_F,S are different to the rotor rotation frequency during normal operation of the turbomachine up to and including a multiple harmonic of the rotor rotation frequency, in particular the eighth harmonic of the rotor rotation frequency. This prevents an oscillating rotor blade being able to excite a rotor blade next to it in an oscillation, and coupling of the rotation of the rotor-blade cascade with the oscillations of the rotor blades taking place. The oscillation loads of the rotor blades are therefore low and their lifetime is long.

It is advantageous for the measurement of the mass m_I and of the center-of-mass position r_S,I to be carried out relatively in a different difference measurement with respect to a reference blade which has been three-dimensionally measured, in particular by a coordinate measuring device and/or by an optical method. The accuracy of a measurement depends on the size of the measurement range, a larger measurement range resulting in a lower accuracy. By carrying out the measurement of m_I and r_S,I relative to a reference blade, a small measurement range with a high accuracy can be used. It is therefore necessary only to take a single rotor blade as thereference blade and to characterize it once by a cost-intensive three-dimensional method, so that m_I and r_S,I of all the other rotor blades can also be measured with the high accuracy.

It is advantageous for the value pairs m_S and r_S,S to be selected in such a way that the unbalance of the rotor is reduced and/or that the outlay for the removal is minimal. Knowledge of the value pair m_S and r_S,S is sufficient for an unbalance of the rotor, so that detuning and balancing of the rotor-blade cascade can be carried out in a common method step by the removal of the material. The removal of the material may also be carried out in such a way that the amount of material to be removed is minimized.

The predetermined oscillation mode is particularly selected in such a way that the natural frequency ν_F,S of the predetermined oscillation mode is equal to or of lower frequency than a multiple harmonic of the rotor rotation frequency, in particular the eighth harmonic. The natural frequencies ν_F and/or ν_I are particluarly determined computationally, in particular by a finite element method.

It is advantageous that, during the measurement of the frequency ν_S,I, the rotor blade is clamped at its blade root, and the oscillation of the rotor blade is excited and measured. The oscillation is particularly measured by oscillation transducers, acceleration sensors, strain gages, piezoelectric sensors and/or optical methods. This constitutes a simple method for determining the natural frequency.

Adaptation of the model for determining the natural frequencies ν_F and ν_S is particularly carried out by a comparison of the measured natural frequency ν_S,I with an actual natural frequency determined by interpolation of m_I and r_S,I in the value table ν_S(m, r_S). In this way, influences of the material on the natural frequencies can advantageously be taken into account as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below with the aid of the appended schematic drawings, in which:

FIG. 1 shows longitudinal sections of three rotor blades with a nominal geometry of the rotor blade and variations of the nominal geometry,

FIG. 2 shows a two-dimensional graph of natural frequencies ν_S of the rotor blade at rest and a two-dimensional graph of the natural frequencies ν_F of the rotor blade under centrifugal force, as a function of the mass m and the radial center-of-mass position r_S of the rotor blade, and

FIG. 3 shows a flowchart of the method according to the invention.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows three rotor blades 1 of a turbomachine, the first rotor blade being represented in its nominal geometry 5, the second rotor blade both in its nominal geometry 5 and in a first variation 6 and a second variation 7, and the third rotor blade both in its nominal geometry 5 and in a third variation 8 and a fourth variation 9. The rotor blades 1 have a blade root 2, which is firmly fitted on a rotor 4 of the turbomachine, and a blade tip 3 facing away from the blade root 2. In the event of an oscillation of the rotor blade 1 during operation of the turbomachine, an oscillation node is arranged at the blade root 2. The radius r of the rotor blade 1 is directed from the blade root 2 to the blade tip 3.

The second rotor blade shows variations 6, 7 of the nominal geometry 5, in which, starting from the nominal geometry 5 the mass m is varied but the radial center-of-mass position r_S of the rotor blade is not. In the first variation 6, the mass m is increased by uniformly thickening the second rotor blade at each radial distance r from the rotation axis, and in the second variation 7 the mass m is reduced by radially thinning the second rotor blade at each radial distance r.

In the variations 8, 9 of the third rotor blade, starting from the nominal geometry 5 the thickness of the rotor blade is varied linearly over the radius r in the circumferential direction and/or the axial direction. According to the third variation 8, starting from the nominal geometry 5 the rotor blade is thickened at its blade root 2 and thinned at its blade tip 3, and according to the fourth variation 9, starting from the nominal geometry 5 the rotor blade is thinned at its blade root 2 and thickened at its blade tip 3. Because of this, in the third variation 8, the radial center-of-mass position r_S is displaced radially inward and in the fourth variation 9 it is displaced radially outward, although the mass m does not change. The variations 8, 9 may, however, be carried out in such a way that both the mass m and the radial center-of-mass position r_S are varied. Furthermore, it is possible to carry out the mass m and the radial center-of-mass position r_S by thickening and/or thinning the rotor blade 1 in selected radial sections.

A multiplicity of variations of the nominal geometry 5 are carried out, and for each variation the natural frequency ν_S of the lowest-frequency bending oscillation of the rotor blade 1 clamped at its blade root 2 and at rest is calculated by a finite element method. Furthermore, for each variation the natural frequency ν_F of the same bending oscillation is calculated, the centrifugal force acting on the rotor blade 1 during operation of the turbomachine being taken into account. Optionally, an elevated temperature and material properties therefore varying may be taken into account in the calculation of ν_F. For a given rotor-blade cascade, it is advantageously possible only to carry out the variations of the nominal geometry once.

Subsequently, for each variation of the nominal geometry 5, the mass m and the radial center-of-mass position r_S of the rotor blade 1 are determined and a value table ν_S(m, r_S) with value triplets ν_S, m, r_S and a value table ν_F(m, r_S) with value triplets ν_F, m, r_S are compiled. The value table ν_S(m, r_S) is represented in the left-hand graph of FIG. 2 and the value table ν_F(m, r_S) is represented in the right-hand graph of FIG. 2, by plotting the respective natural frequency ν_S 10 and ν_F 11 against the mass m 12 and the radial center-of-mass position r_S 13. The natural frequencies ν_S 10 and ν_F 11 are plotted in arbitrary units and the nominal geometry 5 is respectively plotted for m=0 and r_S=0. It can be seen from FIG. 2 that a reduction of the mass m and a displacement of the center-of-mass position r_S inward are associated with an increase of the natural frequencies ν_S 10 and ν_F 11.

FIG. 3 represents the method according to the invention in a flowchart. For each of the rotor blades 1 of the rotor-blade cascade, a setpoint natural frequency ν_F,S, which the rotor blade 1 has for the lowest-frequency bending oscillation of the rotor blade 1 firmly clamped at its blade root 2 during normal operation of the turbomachine under a centrifugal force, is established 14 such that the oscillation load of the rotor-blade cascade under the centrifugal force lies below a tolerance limit. This is achieved in that rotor blades arranged next to one another in the rotor-blade cascade have unequal setpoint natural frequencies ν_F,S, and that the setpoint natural frequencies ν_F,S are different to the rotor rotation frequency of the turbomachine up to and including the eighth harmonic of the rotor rotation frequency.

Subsequently, for each setpoint natural frequency ν_F,S, a corresponding setpoint natural frequency ν_S,S, which the rotor blade 1 has for the lowest-frequency bending oscillation of the rotor blade 1 firmly clamped at its blade root 2 at rest, is determined 15. Following this, as described above, the value table ν_S(m, r_S) and the value table ν_F(m, r_S) are compiled 16 using the variations of the nominal geometry 5.

After manufacture 18 of the rotor blade 1, its mass m and radial center-of-mass position r_S are measured 19. Subsequently, the actual natural frequency ν_F,I of the rotor blade 1 under the centrifugal force is determined 17 by interpolation of the measured mass m_I and the measured radial center-of-mass position r_S,I in the value table ν_F(m, r_S).

An actual/setpoint match 21 is carried out by comparing ν_F,I with ν_F,S. In the event that ν_F,I lies outside a tolerance around ν_F,S, a value pair m_S and r_S,S is selected from the value table ν_F(m, r_S) such that ν_F,I at least approximates ν_F,S, and material is removed 24 from the rotor blade 1 in such a way that m_I and r_S,I correspond to the value pair m_S and r_S,S. As can be seen from the right-hand graph of FIG. 2, a multiplicity of value pairs m_S and r_S,S are generally available for achieving a certain natural frequency ν_F,S. From the multiplicity of value pairs, it is possible to select a value pair m_S and r_S,S such that the rotor of the turbomachine is unbalanced and/or the outlay for the removal is minimal. The removal 24 may, for example be carried out by grinding.

In order to monitor the removal 24, the natural frequency ν_S,I of the rotor blade 1 at rest may be measured 20. To this end, the rotor blade 1 is clamped at its blade root 2, the oscillation of the rotor blade 1 is excited, for example by impact, and the sound emitted by the rotor blade 1 is measured. As an alternative, in order to monitor the removal 24, the mass m and the radial center-of-mass position r_S of the rotor blade 1 may be measured 19. The monitoring can be carried out with a particularly high accuracy by measuring both the natural frequency ν_S,I 20 and the mass m and the radial center-of-mass position r_S 19.

It is also possible to measure both the mass m and the radial center-of-mass position r_S 19 and the natural frequency ν_S,I 20 already before the removal 24 of the material, so as to measure the actual natural frequency ν_F,I with a particularly high accuracy. By a comparison of the measured natural frequency ν_S,I with an actual natural frequency determined by interpolation of m_I and r_S,I in the value table ν_S(m, r_S), adaptation of the model for determining the natural frequencies ν_F and ν_S can be carried out.

In the event that ν_F,I lies inside a tolerance around ν_F,S, method steps 22 may optionally be carried out on the rotor blade 1, for example removal of a coating. The rotor blade 1 is subsequently installed in the rotor-blade cascade 23.

Although the invention has been illustrated and described in detail with reference to the preferred exemplary embodiments, the invention is not restricted by the examples disclosed and other variants may be derived therefrom by the person skilled in the art without departing from the protective scope of the invention.

Claims

1. A method for detuning a rotor-blade cascade, comprising a multiplicity of rotor blades, of a turbomachine, the method comprising: a) establishing for each of the rotor blades of the rotor-blade cascade at least one setpoint natural frequency νF,S which the rotor blade has for at least one predetermined oscillation mode during normal operation of the turbomachine under the effect of centrifugal force, such that the oscillation load of the rotor-blade cascade under the centrifugal force lies below a tolerance limit;b) compiling a value table νF(m, rS) with selected value pairs of discrete mass values m and radial center-of-mass positions rS, which result from variations of the nominal geometry of the rotor blade, and determining the respective natural frequency νF of the predetermined oscillation mode under the centrifugal force for each selected value pair m and rS;c) measuring the mass mI and the radial center-of-mass position rS,I of one of the rotor blades;d) determining actual natural frequency νF,I of the rotor blade under the centrifugal force by interpolation of the measured mass mI and the measured radial center-of-mass position rS,I in the value table νF(m, rS);e) in the event that the actual natural frequency νF,I lies outside a tolerance around the setpoint natural frequency νF,S, selecting from the value table νF(m, rS) a value pair mS and rS,S such that the actual natural frequency νF,I at least approximates the setpoint natural frequency νF,S, and removing material of the rotor blade such that mI and rS,I correspond to the value pair mS and rS,S;f) repeating steps c) to e) until the actual natural frequency νF,I lies within the tolerance around the setpoint natural frequency νF,S.

2. The method as claimed in claim 1, wherein in addition to step b), further comprising:b1) compiling a value table νS(m, rS) with selected value pairs of discrete mass values m and radial center-of-mass positions rS, which result from variations of the nominal geometry of the rotor blade,and determining the respective natural frequency νS of the predetermined oscillation mode with the rotor blade at rest for each selected value pair m and rS;f) in the event that material has been removed, measuring a natural frequency νS,I of the rotor blade at rest;g) repeating steps e) to f or c) to f) until the actual natural frequency νF,I lies within the tolerance around the setpoint natural frequency νF,S and the natural frequency νS,I at rest lies within a tolerance around a setpoint natural frequency νS,S at rest corresponding to the tolerance.

3. The method as claimed in claim 1, wherein the predetermined oscillation modes are selected such that the setpoint natural frequencies νF,S associated with the oscillation modes are equal to or of lower frequency than a multiple harmonic of the rotor rotation frequency,wherein the value table νF(m, rS) is respectively compiled for a multiplicity of or all the oscillation modes, the actual natural frequency νF,I is determined for each value table and the value pair mS and rS,S is selected such that the determined actual natural frequencies νF,I are at least approximated to the established setpoint natural frequencies νF,S.

4. The method as claimed in claim 2, wherein the predetermined oscillation modes are selected in such a way that the setpoint natural frequencies νF,S associated with the oscillation modes are equal to or of lower frequency than a multiple harmonic of the rotor rotation frequency,wherein respectively the value table νF(m, rS) and respectively the value table νS(m, rS) are compiled for a multiplicity of or all the oscillation modes, the actual natural frequency νF,I under the effect of centrifugal force and the actual natural frequency νS,I at rest are determined for each value table and the value pair mS and rS,S are selected in such a way that the determined actual natural frequencies νF,I are at least approximated to the established setpoint natural frequencies νF,S and the actual natural frequencies νS,I at rest are measured for the predetermined oscillation modes.

5. The method as claimed in claim 1, wherein the variations of the nominal geometry comprise thickening and/or thinning of the rotor blade in each radial section or in radial sections.

6. The method as claimed in claim 1, wherein the variations of the nominal geometry comprise a linear variation of the thickness of the rotor blade over the radius.

7. The method as claimed in claim 1, wherein the setpoint natural frequencies νF,S are established in such a way that rotor blades arranged next to one another in the rotor-blade cascade have unequal setpoint natural frequencies νF,S, and that the setpoint natural frequency νF,S are different to the rotor rotation frequency of the turbomachine up to and including a multiple harmonic of the rotor rotation frequency.

8. The method as claimed in claim 1, wherein the measurement of the mass mI and of the center-of-mass position rS,I is carried out relatively in a difference measurement with respect to a reference blade which has been three-dimensionally measured.

9. The method as claimed in claim 1, wherein the value pairs mS and rS,S are selected such that the unbalance of the rotor is reduced and/or that the outlay for the removal is minimal.

10. The method as claimed in claim 1, wherein the predetermined oscillation mode is selected such that the setpoint natural frequency νF,S of the predetermined oscillation mode is equal to or of lower frequency than a multiple harmonic of the rotor rotation frequency.

11. The method as claimed in claim 1, wherein the natural frequencies νF and/or νI are determined computationally.

12. The method as claimed in claim 2, wherein, during the measurement of the actual natural frequency νS,I at rest, the rotor blade is clamped at its blade root, and the oscillation of the rotor blade is excited and measured.

13. The method as claimed in claim 2, wherein adaptation of the model for determining the natural frequencies νF and νI is carried out by a comparison of the measured actual natural frequency νS,I with an actual natural frequency determined by interpolation of m1 and rS,I in the value table νS(m, rS).

14. The method as claimed in 3, wherein the multiple harmonic of the rotor rotation frequency is the eighth harmonic.

15. The method as claimed in 4, wherein the multiple harmonic of the rotor rotation frequency is the eighth harmonic.

16. The method as claimed in 7, wherein the multiple harmonic of the rotor rotation frequency is the eighth harmonic.

17. The method as claimed in 10, wherein the multiple harmonic of the rotor rotation frequency is the eighth harmonic.

18. The method as claimed in 8, wherein the measurement of the mass mI and of the center-of-mass position rS,I is carried out by a coordinate measuring device and/or by an optical method.

19. The method as claimed in 11, wherein the natural frequencies νF and/or νI are determined computationally by a finite element method.