METHOD FOR OPTIMIZING A NATURAL FREQUENCY OF A ROTOR BLADE, AND ROTOR BLADE

Abstract

A method for optimizing a design of a rotor blade which has a blade root and a blade airfoil, of a turbomachine in which an actual natural frequency of the rotor blade is detected and compared with a reference value or reference range and, if a deviation or match between the actual natural frequency and the reference value or reference range, which will impair the proper operation of the rotor blade, is identified, a structural change to the rotor blade is undertaken in order to change the natural frequency thereof, wherein as the structural change at least one cutout is formed at a predetermined position on at least one side face of the rotor blade root. A rotor blade for a turbomachine, which blade has a blade root and a blade airfoil, wherein at least one cutout is formed on at least one side face of the rotor blade root.

Claims

1. A method for optimizing a design of a rotor blade, which has a blade root and an airfoil, of a turbomachine, the method comprising: detecting an actual natural frequency of the rotor blade, comparing the detected actual natural frequency with a reference value or reference range and, for a detection of a deviation or coincidence between the actual natural frequency and the reference value or reference range, which impairs the designed used of the rotor blade, undertaking a structural modification on the rotor blade for changing its natural frequency, forming at least one cutout at a predetermined position on at least one side face of the blade root as the structural modification, wherein for detecting the actual natural frequency, the rotor blade is inserted by its blade root into a blade root socket of an excitation device of vibration test bench so that surfaces of the blade root and of the blade root socket are in contact, a vibration is excited in the rotor blade and the excited vibration of the rotor blade is measured, wherein the predetermined position of the at least one cutout is determined by a plate with at least one hole being arranged between a side face of the blade root and the blade root socket of the excitation device of the vibration test bench, and the natural frequency of the rotor blade is detected again and compared with the reference value or reference range.

2. The method as claimed in claim 1, wherein the plate is a thin plate which is releasably fastened on the side face of the blade root.

3. The method as claimed in claim 2, wherein the thin plate is arranged at different positions between the side face of the blade root and the blade root socket, or exchanged with other thin plates with different hole arrangements, until no deviation or coincidence between the last detected natural frequency and the reference value or reference range, which impairs a designed use of the rotor blade, is detected any longer, and then the at least one cutout is formed at the position of the at least one hole.

4. The method as claimed in claim 3, wherein the at least one cutout is formed by removing a small amount of blade material in such a way that a contact between the blade root and a blade root socket is directly prevented during a designed use of the rotor blade.

5. The method as claimed in claim 1, wherein the plate is inserted into a recess which is provided on the side face of the blade root, in such a way that the plate terminates with the side face of the blade root in a flush manner.

6. The method as claimed in claim 5, wherein the recess is formed during production of the rotor blade, wherein after production of the rotor blade, its actual natural frequency is initially detected using a hole-free reference plate which is inserted into the recess, or the recess is introduced into the side face of the blade root only when the deviation or coincidence between the actual natural frequency and the reference value or reference range is detected.

7. The method as claimed in claim 5, wherein plates with different hole arrangements are inserted into the recess of the blade root until no deviation or coincidence between the last detected natural frequency and the reference value or reference range is detected any longer, and the plate then remains in an unmodified state and permanently as a component part of the rotor blade in its recess so that the at least one hole of the plate forms the at least one cutout.

8. A rotor blade for a turbomachine, comprising: a blade root having side faces, a blade airfoil, at least one cutout which is formed on at least one of the side faces of the blade root against which the rotor blade butts during a designed use in the turbomachine during operation by centrifugal force on bearing flanks of a blade root socket of the rotor, and a recess which is formed on a surface of the blade root, in which recess a corresponding plate is fastened, wherein the plate terminates with the surface of the blade root in a basically flush manner and the at least one cutout is formed in the plate.

9. The rotor blade as claimed in claim 8, wherein the at least one cutout comprises a drilled hole with a circular cross section.

10. The rotor blade as claimed in claim 8, wherein the recess extends over approximately 90% of a side face of the blade root or over an entire length of the side face and into two end faces of the blade root.

11. The method as claimed in claim 1, wherein the predetermined position of the at least one cutout is determined by a plate with at least one circular hole.

12. The method as claimed in claim 1, wherein the predetermined position of the at least one cutout is determined by a plate with a hole pattern.

13. The method as claimed in claim 2, wherein the thin plate consists of metal.

14. The method as claimed in claim 2, wherein the thin plate has a thickness of between 0.1 and 2 mm.

15. The method as claimed in claim 2, wherein the thin plate is releasably fastened by adhesive fixing.

16. The method as claimed in claim 4, wherein the at least one cutout is formed by removing a small amount of blade material by eroding, drilling, milling, grinding, and/or smooth blending.

17. The rotor blade as claimed in claim 8, wherein the recess is an elongate groove.

18. The rotor blade as claimed in claim 8, wherein the at least one cutout comprises a blind hole or a through-hole formed in the plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Further features and advantages of the present invention become clear based on the following description of two exemplary embodiments of a method for optimizing a design of a rotor blade having a rotor blade according to the present invention with reference to the attached drawing. In the drawing

[0025] FIG. 1 shows a schematic front view of a rotor blade during the conducting of a first step of a method according to an embodiment of the present invention;

[0026] FIG. 2 shows a perspective schematic view of a blade root of the rotor blade shown in FIG. 1 during the conducting of a second method step;

[0027] FIG. 3 shows a perspective schematic view of a blade root of the rotor blade shown in FIG. 1 during the conducting of a third method step;

[0028] FIG. 4 shows a perspective schematic view of a blade root of the rotor blade shown in FIG. 1 during the conducting of a fourth method step;

[0029] FIG. 5 shows a perspective schematic view of a blade root of the rotor blade shown in FIG. 1 after the conducting of a fifth method step;

[0030] FIG. 6 shows a perspective schematic view of a rotor blade during the conducting of a second step of a method according to a second embodiment of the present invention;

[0031] FIG. 7 shows a perspective schematic view of a blade root of the rotor blade shown in FIG. 6 during the conducting of a third method step; and

[0032] FIG. 8 shows a perspective schematic view of a blade root of the rotor blade shown in FIG. 6 after the conducting of a fourth method step.

DETAILED DESCRIPTION OF INVENTION

[0033] FIGS. 1 to 5 show a rotor blade 1 with a blade root 4, which defines side faces 2 and end faces 3, and a blade airfoil 5 during five consecutive steps of a method according to a first embodiment of the present invention, which method is implemented for optimizing the design of the rotor blade 1.

[0034] The blade root 4 is usually of firtree or dovetail design so that its end face 3 is flat. The rotor blade 1 shown in the figures is provided for example for being inserted in a rotor with axial grooves. If in the following text mention is made of the side face 2 of the blade root, then it is understood as those regions which interconnect the two oppositely disposed, flat end faces 3 and which butt against walls of a blade retention groove of the rotor by centrifugal force during the designed use of the rotor blade in a turbine or in a compressor during operation.

[0035] According to FIG. 1, an actual natural frequency of the rotor blade 1 is initially detected in a first step. To this end, the rotor blade is inserted by its blade root 4 into a blade root socket 6 of an excitation device 7 of a vibration test bench 8 in such a way that the side faces 2 of the blade root 4 and the surfaces of the blade root socket 6 are in contact with each other. After this, the rotor blade 1 is excited into vibrations, whereupon an actual natural frequency of the rotor blade 1 is detected. During this, the blade root socket 6 of the excitation device 7 simulates the blade root socket of a rotor of that turbomachine in which the rotor blade 1 is to be subsequently used. The detected actual natural frequency is then compared with an acceptable, previously determined reference range. If a deviation between the actual natural frequency and the acceptable reference range, which impairs the designed use of the rotor blade 1, is detected, then a structural modification is undertaken on the rotor blade 1 according to the invention in such a way that a cutout 9 is formed in one of the side faces 2 of the blade root 4 at at least one predetermined position in order to change the actual natural frequency to the desired extent.

[0036] According to a first embodiment of a method according to the invention, in this connection according to FIGS. 2 and 3 the predetermined positions at which cutouts 9 are to be formed in order to effect the desired change of the natural frequency of the rotor blade 1 are initially established. In this connection, a thin, approximately 0.5 mm thick plate 10 of elongate design and produced from metal, which is provided with a plurality of holes 11, is releasably fastened, for example by means of adhesive fixing, on a side face 2 of the blade root 4, after which the blade root 4 according to FIG. 4 is inserted into the blade root socket 6 of the excitation device 7 again and the new natural frequency of the rotor blade 1 with the plate 10 mounted thereupon is determined. The new natural frequency does not coincide with the original natural frequency since the holes 11 of the plate 10 in the contact region between the blade root 4 and the blade root socket 6 form imperfections or non-contact surfaces which influence the natural frequency. If the new natural frequency continues to lie outside the acceptable reference range, then the method steps shown in FIGS. 2 to 4 are repeated, shifting the plate 10 in the direction of the double arrow 12 until a desired natural frequency is established. If this should not be the case, then another plate 10 with a different hole pattern can be used. As soon as the natural frequency lies within the acceptable reference range, the predetermined positions at which the cutouts 9 are located are marked, using the plate 10 as a template, after which the plate 10 is removed from the blade root 4. After that, in a last step, advantageously flat cutouts 9 are formed on the side face 2 of the blade root 4 at the predetermined positions with minor material removal, for example by means of drilling, milling, smooth blending or the like, so that the arrangement shown in FIG. 5 is produced. During the designed use of the rotor blade 1 in a turbomachine these cutouts 9 form non-contact points between the side faces 2 of the blade root 4 and a blade root socket of the turbomachine, which lead to a corresponding natural frequency of the rotor blade 1 which lies outside the resonance range.

[0037] It should be obvious that the shape and dimensions of the plate 10, as well as the shape, the dimensions and the number of cutouts 9, can vary. Also, in the steps shown in FIGS. 2 to 4 a plurality of plates 10 can be arranged on the blade root 4, for example on both side faces 2 of the blade root 4.

[0038] FIGS. 6 to 8 show method steps of an alternative method according to an embodiment of the present invention. In the case of this method, in a first method step similar to FIG. 1 the actual natural frequency of the rotor blade 1 is determined. If this does not lie within the acceptable reference range, then in a second step, similar to the previously described method, predetermined positions at which cutouts 9 are to be provided on a side face 2 of the blade root 4 of the rotor blade 1 in order to shift the natural frequency of the rotor blade 1 into the acceptable reference range, are established.

[0039] To this end, as is shown in FIG. 5, a recess 13, in the form of a groove in the present case, is formed in a side face 2 of the blade root 4, for example by means of milling or the like, which recess extends in the present case in a straight line from one end face 3 to the opposite end face 3 of the blade root 4. Alternatively, the recess 13 can also extend over approximately 90% of the side face 2 and especially into neither of the two end faces 3. A plate 14, which is provided with a plurality of holes 15, is then inserted into the recess 13 in such a way that the holes 15 point outward. The dimensions of the plate 14 correspond in the main to those of the recess 13, wherein the upper side of the plate 14, in the state inserted into the recess 13, advantageously terminates flush with the blade root 4 or slightly projects from the surface of the blade root.

[0040] In a further step, according to FIG. 6, similar to FIG. 4, the new natural frequency of the rotor blade 1 with the plate 14 mounted thereupon is determined. If this does not lie within the acceptable reference range, then the steps shown in FIGS. 6 and 7 are repeated using plates 14 which have different hole patterns until a desired natural frequency is established. If this is the case, then that plate 14 by means of which the desired natural frequency was achieved is fastened inside the recess 13 on the blade root 4, for example by means of soldering or the like, so that the arrangement shown in FIG. 8 is produced.

[0041] The holes 15 now define cutouts similar to the cutouts 9 shown in FIG. 5 and, during the designed use of the rotor blade 1 in a turbomachine, form non-contact points between the side faces 2 of the blade root 4 and a blade root socket of the turbomachine, which lead to a corresponding natural frequency of the rotor blade 1 which lies outside the resonance range.

[0042] It should be obvious that the shape, the dimensions as well as the number of recesses 13 and plates 14 as well as the shape, the dimensions, the positions and the number of holes 15 provided in the plate 14 can vary. Furthermore, the recess 13 can also already be provided during production of the rotor blade 1. In this case, the first detecting of the actual natural frequency is conducted using a reference plate, without holes 15, inserted into the recess 13.

[0043] Although the invention has been fully illustrated and described in detail by means of the advantageous exemplary embodiment, the invention is not limited by the disclosed examples and other variations can be derived therefrom by the person skilled in the art without departing from the extent of protection of the invention.