The most common and reliable technique used for flame stabilization of industrial combustors with high thermal loads is the application of strongly swirling flows. In addition to stabilization, swirl flames offer the possibility to influence emission characteristics by simply changing the swirl intensity or the type of swirl generation. Despite of these major advantages, swirling flows tend to evolve flow instabilities, that considerably constitute a significant source of noise. In general, noise generation is substantially enhanced, when such a swirling flow is employed for flames. Thus, the minimization of the resulting noise emissions under conservation of the benefit of high ignition stability is one major design challenge for the development of modern swirl stabilized combustion devices. The present investigation makes an attempt to determine mechanisms and processes to influence the noise generation of flames with underlying swirling flows. Therefore, a new burner has been designed, that offers the possibility to vary geometrical parameters as well as the type of swirl generation, typically applied in industrial devices. Experimental data has been acquired for the isothermal flow as well as swirl flames by means of 3-D-LDV-diagnostics comprising the components of long-time averaged mean and rmsvelocities as well as spectrally resolved velocity fluctuations for all components. The noise emission data was acquired with microphone probes resulting in sound pressure levels outside the zone of the perceptible fluid flow. Along to the experiments, numerical simulations using RANS and LES have been carried out for isothermal cases with different burner outlet geometries. The results of the measurements show a distinct rise of the sound pressure level, obtained by changing both the test setup from the isothermal into the flame configuration as well as the geometrical parameters. This is also resembled by the LES simulation results. Furthermore, a physical model has been developed from experiments and verified by the LES simulation, that
The development of modern jet engine and stationary gas turbine applications is focused on the reduction of pollutants and, increasingly, on the reduction of noise emissions including combustion noise. This requires the minimization of noise sources, namely noise from the turbulent flow, combustion noise and noise caused by periodic flow and combustion instabilities or fluctuations of the ignition zone. This has to be achieved under conservation of the benefits of swirl flames, i.e. high ignition stability and broad operation ranges. The presented experimental work enables to relate the total noise generation of swirl flames to its physical sources:turbulent flow noise, combustion noise, noise caused by a lifted, unstable flame stabilization and the combustion of coherent flow structures, the latter being caused by the burners exit geometry [1, 2]. Different positions of the flame stabilization can cause an increase of combustion noise up to 3 dB. With the results it is possible to prevent additional noise to the unavoidable minimal combustion noise, evoked by stochastic, turbulent fluctuations of the mixture density, i.e. optimization of the ignition stability with a perfectly premixed pilot flame. Further insight was gained about the influence of the gas injection direction on the combustion noise, where some configurations lead to an increase up to 6 dB in comparison to the results of premixed flames [2], which in general are louder than all investigated non-premixed flames.
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