Broadband Entropy Noise Phenomena in a Gas Turbine Combustor

Broadband sound emission in the region of 1 kHz to 3 kHz probably related to entropy noise was found in a model gas turbine combustor. Entropy noise is caused by the acceleration of fluctuating hot spots in the outlet nozzle of aero-engine combustors. It was predicted in literature in the late 70’s. However, the entire relevance of entropy noise concerning the total noise emission of a combustor is not confirmed until now. Disagreements still exist in the dominating propagating frequencies. This paper presents a broadband noise phenomenon appearing from 1 kHz to 3 kHz in a model aero-engine combustor. In this frequency range the sound pressure level depends on the Mach number as the entropy noise theory expected. Experiments are performed for different types of combustion chambers varying in length and material at various power consumptions and equivalent ratios. A scale factor had been chosen to double the length of the combustion chamber, to allow the investigation of different states of temperature mixing in the exhaust flow following the primary flame zone. Due to the decomposition of the acoustic field into forward and backward propagating waves only the emitted sound power can be considered in the analysis. It can be shown that the broadband noise seems to be independent of changes in geometry or material. A similar cold test flow condition was investigated with respect to the broadband noise phenomena and compared to the reactive case.