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978-3-8439-1811-4, Reihe Strömungsmechanik
Thermoacoustic Noise Sources in Premixed Combustion
148 Seiten, Dissertation Rheinisch-Westfälische Technische Hochschule Aachen (2014), Softcover, A5
The investigation of aero- and thermoacoustic noise source mechanisms is very complex due to the impact and non-linear interaction of all variables describing the physical state of the fluid. Here, an augmented acoustic analogy approach in terms of the acoustic perturbation equations (APE) is utilized that offers a wave operator accounting for time-invariant convection and refraction effects. Based on the governing equations of compressible reactive fluids the APE are reformulated to account for thermoacoustic noise sources in compressible fluids. Several sources that account for specific phenomena are identified using a differential pressure-density relation. Additionally, an ansatz to extend the concept of Riemann invariants to higher dimensional cases is given.
A generic discrete convolution filter framework is used to develop the numerical methods to simulate acoustic propagation processes at high accuracy. These methods are utilized in a newly developed generic structured multi-block framework that is characterized by a simple and compact program structure and a very high efficiency at massively parallel computations.
The numerical method is validated by the investigation of the noise emission of a hot coaxial jet. Here, clear spectral and directional impacts of certain noise sources are identified and found to be consistent with jet noise theory. The investigation of a perturbed laminar premixed flame identifies flame annihilation events as a potential noise source by chemical heat release fluctuations. A significant impact by combustion modeling on the acoustic emission is found. The analysis of two unconfined turbulent premixed flames shows a considerable effect of indirect convective and mixing sources on the noise emission. One flame is investigated at various operation conditions which allows to identify certain impacts on the emitted noise spectrum. However, unstable burner behavior and a restricted parameter space does not allow to deduce universal scaling laws yet.