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ISBN 9783843927505

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978-3-8439-2750-5, Reihe Strömungsmechanik

Stephan Schlimpert
Numerical Analysis of Hydrodynamic and Acoustic Flame Response Mechanisms

174 Seiten, Dissertation Rheinisch-Westfälische Technische Hochschule Aachen (2016), Softcover, A5

Zusammenfassung / Abstract

Thermoacoustic instabilities can cause serious problems, e.g., structural damages which limit the operating envelope of low-emission lean premixed combustion systems. Predicting the onset of combustion instabilities requires a description of the unsteady heat release driving the instability. Motivated by the understanding of this phenomenon, this thesis describes how a disturbed flow field interacts with laminar and turbulent flames and the acoustic sound field. First, the nonlinear flame response characteristics are studied by periodically exciting the incoming flow field of a laminar flame. By comparing the findings to a reduced-order model (ROM), it is found that the convective Darrieus-Landau instability and the vortical structures induced by the shear layer lead to flame response discrepancies generated by both effects at high and low gas expansion ratios. Furthermore, it is demonstrated that the flame foot response is mainly controlled by the presence of the shear layer inducing vortical structures at the burner exit and that the combustion solver correctly captures the flame foot kinematics. Thereafter, the turbulent flame response is studied for a three-dimensional rectangular slot flame. It is found that the heat release rate spectrum can be divided into three typical regimes, i.e., a low (energy containing), medium (energy transferring), and high frequency (energy releasing) range. The low frequency range is controlled by the modes excited by the geometry and flame height oscillations, where the magnitude of these modes depends on the hydrodynamic instability and shear layer effect. The spectral roll-off behavior in the medium frequency range depends on the gas expansion ratio. However, in the higher frequency range it is a function of the Strouhal number and independent on the investigated parameter range. Finally, the sound emission of the turbulent slot flames is studied by the acoustic perturbation equations (APE). The acoustic flame response function and the APE source terms are used to determine how the flame responds acoustically to heat release fluctuations. The location where sound is emitted is determined by the spatial and spectral acoustic source term distributions and a spectral flame front response study. The acoustic flame response of the slot jet flames is similar to the response determined experimentally in the round jet study, indicating that the acoustic response is similar in turbulent premixed flames.