Impact of damper parameters on the stability margin of an annular combustor test rig

An experimental investigation of the influence of dampers on the stability margin of an atmospheric lab-scale annular combustor is presented. The rig allows the comparison of different damper configurations regarding the number of dampers, their spatial distribution and the amount of purge air used for temperature control. The porosity of the perforated plates at the damper exits as well as the influence of the purge air are systematically investigated. Eigenfrequencies and damping rates are computed from the acoustic pressures measured in the combustor with three different methods: The first method is based on the analysis of the decay of the pulsating pressures after sudden shut-down of sirens providing single frequency acoustic excitation. The second method employs Lorentzian fitting to the pressure spectra resulting from turbulent combustion noise and the third method consists of the analysis of the autocorrelation of the acoustic pressures. Since with all three methods very similar eigenfrequencies and damping rates are obtained for the reference configuration without dampers the more complicated first method with forcing does not exhibit advantages over the simpler two other methods, which analyze the acoustic response of the combustor to turbulent combustion noise. As expected, adding dampers leads to a significant increase of the damping rate, but surprisingly the magnitude of this increase depends on the evaluation method employed. Because of the deterioration of the quality of Lorentzian fitting it cannot be applied in cases with strong damper influence. Furthermore, the forcing needed for the decay method induces hot gas ingestion in the dampers leading to detuning. This means that this method modifies the system to be diagnosed and that the results obtained characterize the case with over-Temperature in the dampers, which is not representative for normal combustor operation. In conclusion, best results are obtained with the simple autocorrelation method. Pressure data analysis reveals that combustor damping increases with porosity and purge air mass flow rate.