Abstract:
At least one Helmholtz damper is arranged at a combustion chamber for a gas turbine in order to damp thermoacoustic oscillations; the damping volume of this Helmholtz damper is in communication with the combustion chamber via a connecting passage. Optimum damping is achieved in a simple way by virtue of the Helmholtz damper being designed in such a manner that its damping frequency is adjustable.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of the U.S. National Stage designation of co-pending International Patent Application PCT/CH02/00696 filed Dec. 16, 2002, the entire content of which is expressly incorporated herein by reference thereto. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention deals with the field of gas turbine engineering. It relates to a combustion chamber for a gas turbine.  
       BACKGROUND OF THE INVENTION  
       [0003]     A combustion chamber is known, for example, from EP A1 0 597 138 and U.S. Pat. No. 5,373,695.  
         [0004]     As is explained in the introduction to the above documents, the problem of thermoacoustic oscillations is becoming increasingly significant in modern low-NOx combustion chambers of gas turbines. Therefore, the prior art has given various proposals for arranging what are known as Helmholtz dampers at the combustion chamber of a gas turbine; the configuration of these dampers, in which a damping volume is in communication with the combustion chamber via a thin connecting passage, means that they are able to effectively damp certain oscillation frequencies in the combustion chamber.  
         [0005]     Since the frequency and amplitude of the thermoacoustic oscillations that occur in a combustion chamber are influenced by a very wide range of geometric and operational parameters of the combustion chamber, the likely oscillations in a new combustion chamber cannot be predicted with anything like a sufficient degree of accuracy. It may therefore be the case that the Helmholtz dampers used at the combustion chamber are not optimally matched to the oscillations that actually occur in the combustion chamber.  
         [0006]     It has therefore been proposed in the documents mentioned in the introduction for the Helmholtz dampers to be completely or partially exchangeable, in order to allow retrospective changes to be made to the resonant frequency. For this purpose, a manhole is provided in the turbine casing, through which the Helmholtz dampers can be exchanged.  
         [0007]     Drawbacks in this context are firstly that matching to a resonant frequency can only take place in stages, that it is very difficult to exchange parts of dampers or entire dampers, and that a considerable design outlay is required at the turbine casing and the combustion chamber for this exchange to be performed.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, the invention relates to providing a combustion chamber for a gas turbine with a Helmholtz damper that avoids the drawbacks of known combustion chambers and in particular is distinguished by greatly simplified adaptation to the frequencies that are to be damped.  
         [0009]     The Helmholtz damper is to be designed in such a manner that its damping frequency is adjustable, in particular continuously adjustable. This makes it easy to match the damping to the thermoacoustic characteristics of the combustion chamber, so that it can be optimized accordingly. There is no need to replace parts or entire dampers, and consequently there is no need for correspondingly large access features. At the same time, the adjustability of the Helmholtz dampers eliminates the need to produce and keep available damper parts or dampers of different configuration for different resonant frequencies.  
         [0010]     One preferred configuration of the invention is distinguished by the fact that the damping volume of the Helmholtz damper is continuously variable. This type of adjustability for the damping frequency can be realized in a particularly simple and effective way.  
         [0011]     In this context, it is particularly expedient for the damping volume to be divided into a fixed damping volume and a variable damping volume, and for the damping volume to be altered by changing the variable damping volume.  
         [0012]     It is preferable for the variability of the volume to be achieved by virtue of the variable damping volume being delimited on one side by a displaceable piston. This configuration is in mechanical terms very simple to realize and is functionally reliable and simple to actuate in operation.  
         [0013]     A tried-and-tested form of actuation is characterized in that an adjustment element, in particular in the form of a threaded rod, by means of which the piston can be displaced, is arranged at the Helmholtz damper.  
         [0014]     Since the combustion chamber is arranged inside a turbine casing, it is particularly advantageous for actuation of the Helmholtz damper if the adjustment element can be actuated through a closeable access opening in the turbine casing. The adjustment element may in this case easily be designed in such a way that only a small opening, which requires only insignificant changes to the turbine casing, is required for its actuation.  
         [0015]     The damping action of the Helmholtz damper is particularly great if, in a combustion chamber that has a plurality of burners opening out into the combustion chamber at its entry side, the at least one Helmholtz damper is arranged on the entry side, in the immediate vicinity of the burners. If the combustion chamber is annular and the burners are arranged in concentric rings, the at least one Helmholtz damper is preferably arranged between the rings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The invention is to be explained in more detail below on the basis of exemplary embodiments in conjunction with the drawings, in which:  
         [0017]      FIG. 1  shows an excerpt from a cross-section through the entry side of a gas turbine combustion chamber with two rings of double-cone burners and adjustable Helmholtz dampers arranged therebetween, in accordance with a preferred exemplary embodiment of the invention; and  
         [0018]      FIG. 2  shows an enlarged sectional illustration of the Helmholtz damper from  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]      FIG. 1  shows an excerpt from a cross-section through the entry side of the combustion chamber of a gas turbine with two rings of double-cone burners and adjustable Helmholtz dampers arranged therebetween, in accordance with a preferred exemplary embodiment of the invention. The gas turbine  10  is surrounded by a gas turbine casing  11 , inside which there is a plenum  12  filled with compressed air. The plenum  12  surrounds the combustion chamber  16 , which is separated from the plenum  12  by a combustion-chamber casing  13 . The arrangement of the combustion chamber  16  within the gas turbine  10  is substantially the same as that described in EP A1 0 597 138, which was cited in the introduction. On the entry side, the combustion chamber  16  is delimited within the combustion-chamber casing  13  by a front cover  26 . The combustion chamber  16  is annular in design and is fitted with burners  14 ,  15  that are configured in a known way as double-cone burners and are arranged in rings around the axis of the gas turbine, as disclosed by EP A1 0 597 138.  
         [0020]     The burners  14 ,  15  are arranged in corresponding openings in the front cover  26  and open out into the combustion chamber  16 . Helmholtz dampers  17  are provided between the rings comprising the burners  14 ,  15  in order to damp the thermoacoustic oscillations excited in the combustion chamber  16  during the combustion operation. As shown in  FIG. 2 , the Helmholtz dampers  17  each have a damping volume  20 ,  21 , that is composed of a fixed cylindrical damping volume  20  and a variable cylindrical damping volume  21 . The damping volume  20 ,  21  is connected to the combustion chamber  16  via a relatively narrow connecting passage  18 . The arrangement comprising connecting passage  18  and damping volume  20 ,  21  forms a damping resonator, the resonant frequency of which is determined, inter alia, by the size of the damping volume  20 ,  21 .  
         [0021]     The fixed damping volume  20  is selected in such a way that the damping frequency that can thereby be attained is in the vicinity of the frequency of one of the thermoacoustic oscillations to be expected in the combustion chamber  16 , and that the possible range of variations in this frequency is covered when the variable damping volume  21  is added. It is in this way possible for the Helmholtz dampers  17  in a gas turbine that is to be newly commissioned to be accurately matched to the oscillation frequencies that occur and were not accurately known in advance, so that optimum damping is obtained by the easiest possible route. It will be readily understood that differently dimensioned Helmholtz dampers  17  can also be used in combination to damp different oscillation frequencies.  
         [0022]     The change in the variable damping volume  21  may in principle be brought about in various ways. For example, it is conceivable for the variable damping volume to be composed of a plurality of partial volumes that can be connected up in succession. However, the configuration shown in  FIGS. 1 and 2 , in which the variable damping volume can be altered continuously by means of a piston  22  arranged displaceably in the volume, is particularly favorable for the adjustability. The piston  22  is displaced in a particularly simple and reliable way by means of an adjustment element  23  in the form of a threaded rod that is mounted rotatably in a threaded hole  25  in the cover  24  and closes off the variable volume  21  with respect to the outside. Alternatively, the piston  22  also may be fixedly connected to the adjustment element  23 . In this case, the adjustment is effected by a screw thread in the cover  24 , in which the adjustment element  23  is guided. By way of example, a slot in which the blade of a screwdriver can engage may be provided on the outer end side of the adjustment element  23 . If the adjustment element (the threaded rod)  23  is rotated, the piston  22  moves along the cylinder axis of the damping volume  20 ,  21  and can adopt various positions, as indicated in  FIG. 1 . The frequency at which the damping occurs or reaches its maximum also changes correspondingly with the damping volume  20 ,  21 .  
         [0023]     The design of the adjustment element  23  creates the option of simple actuation of the adjustment element  23  from outside the turbine casing  11  without extensive features having to be added to the turbine casing. According to  FIG. 1 , a relatively small access opening  19  which comprises a screwed-in, closeable connection piece is provided on the turbine casing  11 , aligned with the axis of rotation, for actuation of the adjustment element  23 . It is in this way possible without great difficulty to optimally match the damping properties of the individual Helmholtz dampers  17  to the thermoacoustic oscillations that actually occur when the combustion chamber  16  is operating.  
         [0000]     List of Designations  
         [0024]      10  gas turbine  
         [0025]      11  turbine casing  
         [0026]      12  plenum  
         [0027]      13  combustion chamber casing  
         [0028]      14 ,  15  burners  
         [0029]      16  combustion chamber  
         [0030]      17  helmholtz damper  
         [0031]      18  connecting passage  
         [0032]      19  access opening  
         [0033]      20  damping volume (fixed)  
         [0034]      21  damping volume (variable)  
         [0035]      22  piston  
         [0036]      23  adjustment element (e.g. threaded rod)  
         [0037]      24  cover  
         [0038]      25  threaded hole  
         [0039]      26  front cover