Prediction of acoustic pressure spectra in combustion systems using swirl stabilized Gas Turbine Burners

A method to predict pressure spectra of gas turbine combustion chambers with premixed, turbulent, swirl-stabilized flames is presented. The combustion system is represented as a network of acoustic elements, where each element is characterized by its transfer matrix. Analytic equations can be derived for most of the elements in such a network (e.g. ducts with variable cross-sectional area, sudden area changes etc.). However, the description of the acoustic properties of the burner and flame still require experimental input because of the complex interaction between the turbulent swirling flow, fuel supply and unsteady heat release. Acoustic excitation can be applied up-, and downstream of the burner in order to determine the transfer matrix of burner and flame. Due to the turbulent flow and combustion, the flame itself may also act as an independent source of sound. Thus, the burner does not only transmit and reflect incoming signals, but generates its own signal, independent of the acoustic state upstream and downstream. This quantity, the source-Term, has been determined experimentally as well. Having determined the acoustic properties of all the elements (either analytically or by experiment) the thermoacoustic network can be built up. The frequency spectrum of the acoustic oscillations can then be investigated by solving the non-homogeneous system of equations, where the inhomogeneties are due to the source term of the flame. Because of the network approach, the influence of different acoustic boundary conditions on the frequency response has been determined. Application of the method to an atmospheric combustion test-rig with a gas turbine burner showed that the predicted frequency response and stability were in good agreement with experimental data.