Flame Describing Function measurements for limit cycle characterization

In a gas turbine combustor, limit cycles of pressure oscillations may occur due to the coupling of combustion dynamics, the acoustic field present inside the combustor chamber and the aerodynamics flow in the burner passages. The high amplitude of the dynamic pressure oscillations have many damaging effects on engine components. Since these components have design limits with respect mechanical vibration loads, the result will be poor reliability, reduced time between overhauls and increased risk of catastrophic failure. In order to explore the nonlinear processes involved in limit cycle combustion oscillations, experimental research is carried out in a laboratory combustor, operating in a lean, partially premixed methane/air flame, where the flame is stabilized over a triangular bluff body. Depending on the operating point, the flame showed a stable or unstable behavior. In this last case, amplitudes up to 160 dB (ref 20 μPa) were recorded. The behavior of the flame as a function of operating conditions is well established in the experiments. The relation between periodic oscillations of the velocity of air in the inlet and rate of heat release by the flame is called Flame Describing Function. This is measured for different operating points. The flow was excited by a siren unit in the air feeding line, and the rate of heat released by the flame was measured on basis of its chemiluminescence with a photo multiplier tube. A multi microphone technique was used to measure the amplitude and phase of the excitations. The results are related to the characteristics of the limit cycle such as resonant frequency, amplitude and phase with respect to the inlet passage. The range of frequencies of interest is 40 to 600 Hz and variations of excitation of the inlet velocity were set to different levels, from 2 to 10 %. The final FDF has non linear behavior with excitation level and a constant time delay for the considered operating conditions. It is shown that the frequency of the limit cycle is not related to purely aerodynamic phenomena such as Kelvin Helmholtz or Von Karman instabilities.