Interferometric Detection of Thermoacoustic Oscillations in Flames

In order to meet the stringent pollutant regulations set by the governments, low-emission concepts of combustion systems in turbomachinery have been developed. Unfortunately, combustors operating near the lean flammability limit have a strong tendency towards combustion instabilities. Unsteady heat release will cause sound radiation and might amplify these combustion instabilities as well. This unsteady heat release can be related to the density fluctuations in the flame, or to be more precise, with the time derivative of these density fluctuations. In a previous project funded by the Austrian Science Fund FWF it was shown that so-called laser-vibrometers - interferometers used in engineering for surface vibration detection - can directly record the time derivatives of density fluctuations in flames. Using three of them simultaneously enables quantitative and local recording of these important density fluctuations, as well as, average velocities within the flow field. The underlying hypothesis of this project proposed, claims that from the local laser-vibrometer recordings within the combustion zone the acoustic field in near-and far-field of a flame can be obtained, since laser-vibrometers can detect the first time derivative of density fluctuations quantitatively. With the simultaneously and experimentally recorded acoustic pressure distribution in the far field of the flame, this assumption can be tested. Such a mode shape analysis of the acoustic field around the flame by interferometric detection of thermoacoustic oscillation within the flame provides an innovative aspect for experimental thermoacoustic research. On the one side laser-vibrometers are well established in engineering, with easy access and handling, so their application is also cost-effective. On the other side western society strongly depends on fast transportation and permanent availability of energy, thus research in the field of turbomachinery always is energy related research, with a strong impact on society.

In order to meet the stringent pollutant regulations set by the governments, low-emission concepts of combustion systems for turbomachinery in power plants and aero-engines have been developed. Unfortunately, combustors operating near the lean flammability limit (low fuel, high air) have a strong tendency towards combustion instabilities. Unsteady heat release will cause sound radiation and might again amplify combustion fluctuations as well. This unsteady heat release can be related to the density fluctuations in the flame.In this and a previous project both funded by the Austrian Science Fund FWF - it was shown that so-called laser-vibrometers interferometers used in engineering for surface vibration detection can directly record density fluctuations in flames. Using two or three such systems simultaneously, allows quantitative and local recording of these important density fluctuations, as well as, average velocities within the flow field. In this project it also was shown that from local laser-vibrometer recordings within the combustion zone the acoustic field in near- and far-field of a flame can be obtained. In order test this new method the experiments were also performed at TU Munich with a test-rig relevant to industry. Also, a cooperation with TU Dresden started to combine this novel measurement technique with other modern laser-optical diagnostic tools.On the one side laser-vibrometers are well established in engineering, with easy access and handling, making their application also cost-effective. On the other side western society strongly depends on fast transportation and permanent availability of energy. Since turbines are widely used in airplanes and power plants, thus research in the field of turbomachinery always has a strong impact on society.