Indirect combustion noise, as an important source of noise in gas turbines, was traditionally attributed solely to entropy waves. In recent years, compositional waves were introduced as another contributor to indirect combustion noise. Nonetheless, unlike that of entropy waves, the annihilation of compositional waves by the mean flow has remained largely unexplored. Hence, the current numerical study analyses the spatiotemporal evolution of different components of compositional waves and compares them with the decay of entropy waves. A convecting wave, including a mixture of combustion products at elevated temperature, is introduced at the inlet of a simple channel. This allows simultaneous analysis of entropy and compositional waves. The passage of these along the channel is modeled using a large eddy simulation and the annihilation of the waves' components is examined in the frequency domain. It is shown that the turbulence level of the mean flow and convective heat transfer on the walls can both result in a considerable wave deterioration.However, the effects of heat losses from the channel walls are found to be stronger than that of turbulence intensity. Importantly, as the wave is convected, the chemical potential function remains coherent for most of the channel length and deterioration of the compositional wave majorly ensues from the mixture fraction gradient. The results indicate that, overall, the compositional sources feature 10% to 20% more dissipation in comparison with the entropic sources. Therefore, compositional waves are less likely to survive the flow and generate noise.
Entropy wave, as the convecting hot spot, is one of the source of combustion instabilities, which is less explored through the literature. Convecting in a highly turbulent flow of a combustor, entropy waves may experience some levels of dissipation and deformation. In spite of some earlier investigations in the zero acceleration flow, the extent of the wave decay has not been clear yet. Further, there exist no results upon the wave decay in non-zero accelerated flows. This is of crucial importance, as the wave passes through the end nozzle of the combustor or gas turbine stages. The current experiment, therefore, compares the wave decay in both flow of constant and variable bulk velocity, meaning respectively a uniform pipe and a convergent nozzle. The comparison will aid the theoretical models to reduce complexity by simplifying the relations of non-zero acceleration flow to those of no acceleration, as followed by the earlier effective-length method. Reynolds number and inlet turbulence intensity are considered as the governing hydrodynamic parameters for both investigated flows. The entropy wave is generated by an electrical heater module and detected using fast-response thermocouples. The results show that the entropy wave variation is point-wise and frequency-dependent. The accelerated flow of the nozzle is generally found to be more dissipative in comparison with the zero acceleration flow.
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