Numerical Simulation Of Thermoacoustic Combustion Instability In Gas Turbines

The main origin of combustion instability in modern gas turbine is considered to be related to the interaction between acoustic wavesand flame perturbations. An important role is played by the characteristics of combustion chamber and burners, because they influencethe operating conditions at which the instability may occur.Experimental test cases carried out on single burner arrangements fail to give adequate indications for the design of a full scalecombustion chamber, due to the interaction of the local flame fluctuations with the propagation of the pressure waves, that have awavelength of the same order of magnitude of the main dimensions of the chamber. Therefore there is a large interest on developingtechniques able to make use of the data gathered from tests carried out on a single burner for predictingthe thermoacoustic behavior of the combustion chamber at full scale with its actual geometry.A method, which uses a three dimensional finite element code and one dimensional approaches, has been developed for predictingacoustically driven combustion instabilities in combustion systems with complex geometries. The code allows one to identify the fre-quencies at which thermoacoustic instabilities are expected and the growth rate of the pressure oscillations, at the onset of instability,under the hypothesis of linear behaviour of the acoustic waves. The code permits to represent heat release fluctuations through an n − τ Flame Transfer Function (FTF) model and to adopt the transfer matrix method for modelling the burners. The FTF and the burnertransfer matrix, as well as the temperature field and the flame location, needed for the simulation, can be obtained from experimentaltests. Moreover, the code is able to make use of the local distribution of n and τ that can be evaluated from computational fluid dynamicstudies on the single burner