Nonlinear behavior of the thermoacoustic instabilities in the limousine combustor

The topic of this dissertation are the large amplitude pressure perturbations which are sometimes observed in gas turbine burners and boilers. The pressure oscillations are the result of unstable feedback between the combustion process and pressure waves. The amplitude of these oscillations reach such large amplitudes that the oscillation is limited by non-linear phenomena. This phenomenon is known as Limit Cycle Oscillation (LCO) of pressure and has a negative impact on the technical lifetime of the gas turbine engine. This research is part of the LIMOUSINE project, funded by the European Commission under the Marie Curie FP7 Initial Training Network, grant agreement number 214905. The flame dynamics have been studied empirically with laboratory scale, atmospheric combustion setups specifically designed for this project. These combustion chambers have rectangular cross-section and they use a wedge-shaped "bluff body" to stabilize the flame. The burner uses methane as fuel and the thermal power ranges between 30 and 70 kW. The burner can sustain different combustion regimes, depending on the fuel and air flows. The combustion process may be stable without any remarkable dynamics or may show pressure Limit Cycle Oscillations. The frequency of the LCO is related to the acoustic resonance frequency of the combustion chamber, as can be seen from the sensitivity of the Helmholtz and Strouhal dimensionless numbers. One of the features of the limit cycle is the non-linearity of the signal. The pressure spectrum shows regularly spaced secondary peaks which do not match any of the acoustic modes. The origin of these dynamics lies on the temporal waveform. The characteristics of the limit cycle were investigated with non-linear methods, such as the dimension of the attractor of the measured time series for the pressure and the heat release rate in the phase space. In general, stable combustion have chaotic motion while LCO cases present an associated dimension of 3. Nevertheless, the motion of the structural elements is deterministic for both combustion regimes. Last but not least, the interaction between wall vibration and combustion induced oscillations was examined. The coupling of the two physical systems is visible to some extent.