Investigation of High-Frequency Thermoacoustic Instabilities

The motivation of the present work was to develop a simple laboratory-scale atmospheric test case that can be used for the analysis and modeling of thermoacoustic oscillations.

To gain a better understanding of the interaction of the transverse acoustic waves and the flames, forced-response large-eddy simulations (LES) were performed for two types of acoustic flame interaction: a) velocity forcing: the flame was placed at the acoustic pressure node and b) pressure forcing: the flame was placed at the acoustic pressure anti-node. The comparison of the results of the two acoustic/flame interactions showed that the flame at the pressure anti-node is generally more susceptible to thermoacoustic instabilities. Density oscillations were identified as the main driving mechanism in the case of pressure forcing and the flame displacement mechanism in the case of velocity forcing.

Based on the lessons learned from the forced response studies, a generic combustor was constructed using LES, which can exhibit self-excited thermoacoustic instabilities in a frequency range often observed in real gas turbines. Additional investigations allowed a gradual increase in the complexity of the combustor in terms of realistic boundary conditions and geometry until a feasible configuration for the experiment was obtained. This configuration was subsequently investigated experimentally. For a given air mass flow, thermoacoustic instabilities of the first radial mode were identified when a critical equivalence ratio was exceeded. The largest amplitude was observed at an equivalence ratio of a slightly lean mixture. The ability of the numerical method with respect to high-frequency thermoacoustics and stable combustion prediction were taken into account. It could be demonstrated that CFD could reliably predict the results with a reasonable agreement for both states.

The developed experiment proves the capability of LES as a design tool for thermoacoustic applications and opens the door for more sophisticated experiments to gain a better understanding of these kinds of instabilities.


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