Prediction and measurement of thermoacoustic improvements in gas turbines with annular combustion systems

Environmental compatibility requires low emission burners for gas turbine p ower. plants. In the past, significant progress has been made developing lo w 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 c onventional burner designs. The oscillations may be excited to such nit ext ent that the risk of engine failure occurs. For this reason, the prediction of these thermoacoustic instabilities in the design phase of an engine bec omes more and more important. A method based on linear acoustic four-pole e lements has been developed to predict instabilities of the the ring combust or of the 3A-series gas turbines. The complex network includes the whole co mbustion system starting from both compressor outlet and fuel supply system and ending at the turbine inlet. The flame frequency response was determin ed by a transient numerical simulation (step-function approach). Based on t his method, possible improvements for the gas turbine are evaluated in this paper First, the burner impedence is predicted theoretically and compared with results from measurements oil a test rig for validation of the predict ion 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. Se cond, the interaction of the acoustic parts of the gas turbine system has b een detuned systematically in circumferential direction of the annular comb ustion chamber in order to find a more stable configuration. Stability anal yses show the positive effect of this measure as well. The results predicte d 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.