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Combustion Noise and Dynamics of partially premixed flames

Subject Area Technical Thermodynamics
Energy Process Engineering
Fluid Mechanics
Term from 2021 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 442581657
 
Final Report Year 2024

Final Report Abstract

Thermoacoustic combustion instabilities remain a significant challenge for the development of reliable, low-emission gas turbines. Predicting these self-excited instabilities requires thorough understanding of the interactions between unsteady heat release rate, acoustics, flow and mixture perturbations as well as entropy waves. The aim of the transfer project was to enhance the comprehension of flame dynamics and thermoacoustic interactions in technically relevant configurations, particularly partially premixed combustors. To this end, experimental postprocessing techniques based on acoustic measurements and low-order network models were developed to enable a consistent prediction of flame dynamics, high-fidelity simulations with subsequent system identification were performed to predict the flame and entropy response, and an analytical framework was proposed to explain the underlying physics of entropy wave generation. NoiSI project work demonstrated that the state-of-the-art post-processing approach for determining the flame dynamics from microphone measurement data is invalid when applied to test rigs with complex features, such as acoustic cross-communication bypassing the flame or area contraction at the combustor outlet. To address this limitation, a novel method has been developed that integrates reactive transfer matrix measurements of the entire combustor with a low-order network model of the rest rig. The Rolls-Royce scaled acoustic rig for low-emission technology (SCARLET), operated under realistic engine conditions, was used to demonstrate the capabilities of this physics-based method. Furthermore, it was demonstrated that acoustic wave propagation in large eddy simulation (LES) is not relevant for the system identification (SI) of the dynamics of velocity sensitive, acoustically compact, premixed flames. Specifically, the flame response, described in terms of the flame transfer function (FTF), can be accurately predicted without explicitly modeling the acoustic-flame interaction, i.e., by utilizing incompressible LES. This approach exhibits several advantages over its compressible counterpart, particularly in terms of the identification procedure and computational requirements. This paper settled a long-standing dispute in the research community! In addition, the evaluation of the entropy transfer function (ETF) at various positions within the combustion chamber sheds light on the convective dispersion of entropy waves, revealing significant amplitudes at the combustor exit. By separating the entropy responses to equivalence ratio and velocity fluctuations utilizing multiple-input, single-output (MISO) identification, the dominance of equivalence ratio fluctuations in this process was found, which aligns with the existing literature. It is demonstrated that entropy waves can even be generated in fully premixed combustors, specifically through wall heat losses resulting from unsteady heat transfer, but surprisingly also through the differential diffusion of hydrogen. To further investigate the generation of entropy waves, a consistent derivation of its analytical terms based on an arbitrary Lagrangian-Eulerian (ALE) framework was proposed.

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