Influence of boundary conditions on the noise emission of turbulent premixed swirl flames

The main focus of subproject 6 was the development of time resolved experimental techniques for the analysis of the noise formation of turbulent premixed swirling flames. With these techniques the connection between the spatially and temporally fluctuating heat release in the reaction zone and the emitted noise spectrum could be investigated. In particular previous theoretical and experimental work could be extended such that a comprehensive picture of the noise-generating parameters in the flame was obtained. This understanding was cast into a new method [12, 36], which allows the prediction of the fluctuating heat release spectrum from local mean values of turbulence and heat release. With this method it is possible to estimate the acoustic power spectral density of premixed swirl flames with good accuracy. Furthermore, the basic influences of turbulence intensity and length scale, fuel and mixture composition on the generated acoustical frequency spectrum appear naturally in this analysis, which allows to specifically design and optimize combustion systems with respect to noise emission. The model was validated globally and in parts [36, 44] by using experimental mean data of turbulence and mean heat release as inputs and comparing the predicted model result with the corresponding measured quantity. It was shown successfully that the model could be used to calculate combustion noise on the basis of CFD results [11]. Further pursuing the title objective of this project, it was found that the model responded correctly to significant changes of boundary conditions. The basic theoretical analysis of a noise source in a confined acoustical system showed how to insert a CFD-based noise source spectrum into a thermo acoustical network code in order to calculate noise emission from a combustor. With this approach measured acoustic pressure spectra were reproduced with very good comparison.