Thermoacoustic instabilities limit the operation of a gas turbine combustor under very lean conditions, which are required to achieve the strict regulations of emissions of NO_x and CO_2. An effective means to extend the operation window is a Helmholtz resonator, which provides a narrow-band damping of acoustic pressure amplitudes. While the prediction of the damping characteristic of a single volume Helmholtz resonator is well known under isothermal conditions, uncertainties still exist for non-isothermal conditions as they prevail in gas tubine combustors where the resonator volume and its neck are purged with colder air from the compressor that is released eventually through the neck mouth into the hot grazing flow in the combustion chamber. Based on the multi-microphone method, the reflection coefficent of a purged Helmholtz resonator mounted to a combustion test rig has been determined experimentally for different grazing flow temperatures. On basis of the results the change of the length correction of the resonator neck is modeled, being a function of the temperature difference and the excitation amplitude. Latter is provided by loudspeakers plugged to the combustor rig downstream of the Helmholtz resonator. It is shown, that the decay of the length correction for higher grazing flow temperatures basically scales with the density ratio between the hot combustion gases and the colder gas in the resonator neck, as suggested in recent publications. Small deviations are detected which can be explained by physical phenomena that have been observed by optical measurement methods. If these phenomena are incorporated into the model, the accuracy of estimation of the resonance frequency improves considerably.
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