The operating range of heavy duty gas turbines that feature lean premixed combustion to achieve low NO, emissions is limited by thermoacoustic oscillations. T o extend the operational envelope of the gas turbine, passive means have to be developed to suppress thermoacoustic instabilities. In order to develop passive means the complex interaction between acoustics and thermal heat release has to be taken into account. A new stability chart applicable to the qualification of industrial design has been developed that accounts for the acoustic properties of the combustion system including its boundary conditions and the flame response data. The method has been validated using detailed measurements of the eigenmodes in an operating gas turbine as well as experimental data from component test rigs. An explanation is given of the significant extension of the operation envelope of the gas turbine a s an effect of cylindrical extensions to the burner nozzle.

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Environmental compatibility requires low emission burners for gas turbine power plants.In the past, significant progress has been made developing low NO x and CO burners by introducing lean premixed techniques in combination with annular combustion chambers. Unfortunately, these burners often have a more pronounced tendency to produce combustion-driven oscillations than conventional burner designs. The oscillations may be excited to such an extent that the risk of engine failure occurs. For this reason, the prediction of these thermoacoustic instabilities in the design phase of an engine becomes more and more important. A method based on linear acoustic four-pole elements has been developed to predict instabilities of the ring combustor of the 3A-series gas turbines. The complex network includes the whole combustion system starting from both compressor outlet and fuel supply system and ending at the turbine inlet. The flame frequency response was determined by a transient numerical simulation (step-function approach). Based on this method, possible improvements for the gas turbine are evaluated in this paper. First, the burner impedance is predicted theoretically and compared with results from measurements on a test rig for validation of the prediction approach. Next, the burner impedance in a gas turbine combustion system is analyzed and improved thermoacoustically. Stability analyses for the gas turbine combustion system show the positive impact of this improvement. Second, the interaction of the acoustic parts of the gas turbine system has been detuned systematically in circumferential direction of the annular combustion chamber in order to find a more stable configuration. Stability analyses show the positive effect of this measure as well. The results predicted are compared with measurements from engine operation. The comparisons of prediction and measurements show the applicability of the prediction method in order to evaluate the thermoacoustic stability of the combustor as well as to define possible countermeasures. Downloaded From: http://gasturbinespower.asmedigitalcollection.asme.org/ on 05/24/2015 Terms of Use: http://asme.org/terms Fig. 16 Stability limits for different cylindrical burner outlet "CBO… configurations "Berendbrink and Hoffman †19 ‡… Journal of Engineering for Gas Turbines and Power JULY 2001, Vol. 123 Õ 565 Downloaded From: http://gasturbinespower.asmedigitalcollection.asme.org/ on 05/24/2015 Terms of Use: http://asme.org/terms

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Providing gas turbine combustion chambers with Helmholtz-resonators is a promising approach for extending the operating range of gas turbines towards higher thermal power input whilst minimizing the risk of thermoacoustic instabilities. The work currently being reported gives an overview of experimental and computational analyses carried out for a full annular combustor test-rig located at Gioia del Colle in Italy. The thermoacoustic stability characteristics of this test-rig were thoroughly analyzed both for a base configuration without Helmholtz-resonators and for an extended configuration with 14 Helmholtz-resonators. An increase of power input to the combustor by 8.5–20% can be realized when the test-rig is equipped with resonators. The experimental analyses are reproduced by a computational model.

For the development of modern Low-NOx gas turbine combustors featuring high power densities due to their compact design a detailed knowledge about thermoacoustically induced combustion oscillations is required. In order to design passive and active means to suppress thermoacoustic oscillations and to extend the stable operation range of the gas turbine an investigation of the acoustic eigenmodes of the combustor already in the design phase is necessary. In a combined experimental and computational project, tools to determine the mode shapes of a gas turbine combustor have been developed. The mode shapes of an annular combustor of the 3A series have been identified under operating conditions in the test bed of Berlin by cross correlating pressure signals mounted on twelve different azimuthal locations. These data have been used in order to validate the new numerical steady state response method. It has been found that taking into account appropriate acoustic boundary conditions at the burner outlet the numerical predictions are in good agreement with the measurements.

Environmental compatibility requires low emission burners for gas turbine power plants. In the past, significant progress has been made developing low NOx and CO burners by introducing lean premixed techniques in combination with annular combustion chambers. Unfortunately, these burners often have a more pronounced tendency to produce combustion-driven oscillations than conventional burner designs. The oscillations may be excited to such an extent that the risk of engine failure occurs. For this reason, the prediction of these thermoacoustic instabilities in the design phase of an engine becomes more and more important. A method based on linear acoustic four-pole elements has been developed to predict instabilities of the ring combustor of the 3A-series gas turbines (Krüger et al. (1999b)). The complex network includes the whole combustion system starting from both compressor outlet and fuel supply system and ending at the turbine inlet. The flame frequency response was determined by a transient numerical simulation (step-function approach). Based on this method, possible improvements for the gas turbine are evaluated in this paper. First, the burner impedance is predicted theoretically and compared with results from measurements on a test rig for validation of the prediction approach. Next, the burner impedance in a gas turbine combustion system is analyzed and improved thermoacoustically. Stability analyses for the gas turbine combustion system show the positive impact of this improvement. Second, the interaction of the acoustic parts of the gas turbine system has been detuned systematically in circumferential direction of the annular combustion chamber in order to find a more stable configuration. Stability analyses show the positive effect of this measure as well. The results predicted are compared with measurements from engine operation. The comparisons of prediction and measurements show the applicability of the prediction method in order to evaluate the thermoacoustic stability of the combustor as well as to define possible countermeasures.

Modern low-emission gas turbine combustion systems often experience thermo-acoustic instabilities at certain operating conditions, which adversely affect the performance of the engine. One way to mitigate the detrimental effect of such instabilities is to place passive damping devices along the wall of the combustion chamber. To achieve greatest overall damping requires good understanding of the acoustic properties of the damping devices at engine conditions and determination of the undesired acoustic mode shapes for optimal placement at the wall. This paper presents an experimental study which characterises the acoustic properties of bias flow liners operating at frequencies in the low kilohertz regime (> 1 kHz). The engine conditions are simulated in the experiment at ambient conditions by maintaining dynamic similarity, i.e. by matching a number of non-dimensional parameters in the experiment which characterise the engine conditions. The present experimental study contributes to the existing measurement database by taking into account the strong gradient in characteristic impedance between grazing and bias flow medium. The acoustic properties of the investigated damper configurations are assessed in terms of the surface impedance at the interface between grazing and bias flow. An impedance model is suggested which accounts for the strong gradient in characteristic impedance between grazing and bias flow medium. The impedance model may serve conveniently as input to an acoustic mode shape prediction in the combustion chamber to identify the optimal placement of the damping devices.