Investigation of the thermoacoustic characteristics of a lean premixed gas turbine burner

To model thermoacoustic oscillations, a combustion system can be described as a network of acoustic elements, representing for example fuel and air supply, burner and flame, combustor, cooling channels, suitable terminations, etc. For most of these elements, simple analytical models provide an adequate description of their thermoacoustic properties. However, the complex response of burner and flame to acoustic perturbations has - at least in a first step - to be determined by experiment. In our approach, we describe the burner as an active acoustical two-port, where the state variables pressure and velocity at the inlet and the outlet are coupled via a four element transfer matrix. This approach is similar to the `black box' theory in communication engineering. To determine all four coefficients, two independent test states have to be created. This is achieved by using acoustic sources upstream and downstream of the burner, respectively. In application to a full size gas turbine burner, the method's accuracy was tested in a first step without combustion and the results were compared to an analytical model for the burner's acoustic properties. Then the method was used to determine the burner transfer matrix with combustion and to investigate the influence of various parameters such as acoustic amplitude and equivalence ratio. An experimental swirl stabilized burner was used for this purpose. The treatment of burners as acoustic two-ports with feedback used to model the thermoacoustic behavior of combustion chambers and the experimental determination of the burner transfer matrix is novel.