On the decay of entropic-compositional sources of indirect noise in combustors.

Combustion noise is a well-known, potentially harmful occurrence in real combustors. For years, indirect combustion noise was addressed as the entropy noise, which was made by an irreversibly thermal fluctuation. After introducing the chemical source of indirect combustion noise, some more questions have been raised about the fate of the compositional waves in the combustors. They have not been directly addressed, yet. The current numerical work aims to clarify the evolution of the whole parts of indirect noise sources, as compositional or thermal ones. A convecting wave of a mixture of combustion product gases is introduced at the inlet of a cold channel flow. The degeneration of the wave's component is evaluated in the frequency domain by a statistical function using the results of a large eddy simulation. It is shown that both turbulence level and cooling heat transfer from the walls can result in wave deterioration. However, the effects of the latter are found to be stricter than the former. The compositional sources can endure at a higher frequency range than the thermal source of the indirect noise. On the other hand, the evolution of the compositional sources is more dependent on the length of the combustor compared to the thermal source. It is discussed that the chemical potential function remains coherent through a long distance while the wave convects and the evolution of the compositional wave is majorly ensuing from the mixture fraction gradient. A magnitude comparison among the various sources is performed and it is concluded that compositional source including the heat capacity variation has the least importance. [ABSTRACT FROM AUTHOR]