Self-excited instabilities in aero- and thermoacoustics
Acoustics
Technical Thermodynamics
Final Report Abstract
During the Selfixs project a method derived from phasor diagrams has been developed, which allows an in-situ categorization of acoustic vs. ITA modes in an ideal resonator with a velocity-sensitive heat source, such as a premixed flame. This categorization is especially important in studies that investigate the inherent and sometimes paradoxical properties of ITA modes, which are known to differ in an essential manner from conventional thermoacoustic modes. The nature of a thermoacoustic mode, either acoustic or ITA, was shown to not only correlate to the mode of oscillation in the vicinity of the flame, but also to the magnitude and phase of the flame transfer function. Besides, there is also a relationship between the abundance of ITA modes with the location of the flame with respect to the boundaries. It is important to take into account these findings during the layout of passive control strategies for thermoacoustic instabilities in combustion systems. Furthermore, the working principles of the standing wave and traveling wave engines were investigated in collaboration with projcet partners at LAUM with the help of phasor diagrams, which provide an insight towards the general phase relationship between pressure and velocity fluctuations in a traveling wave engine. This investigation elucidates functional differences between the two types of TA engines and helps explain why traveling wave engines are superior. The surprising finding of this study is the discovery of the indirect contribution of the thermal viscous resistance to acoustic gain in a traveling wave engine by establishing traveling wave phasing, which was previously thought to only contribute to acoustic losses. This study may help the development of more targeted optimization strategies in thermoacoustic engine designs. The simulation of thermoacoustic engines is significantly simplified by including the thermoacoustic stack in the state-space based ‘taX’ toolbox. A complete set of eigenvalues can be computed at a fraction of the time needed by conventional numerical solver such as DeltaEC. This toolbox will see use in designing new thermoacoustic engines or optimizing existing engines in the future. Unfortunately the COVID pandemic had a severe impact on the research progress. The lock-downs and travel restrictions have dramatically impeded the communications with cooperation partner and colleagues. Frequent and extended visits among the project partners were not possible in particular in the initial project phase. As a consequence, joint research on the non-Hermitian aspects of aero- and thermo-acoustic could not develop as originally envisaged. The pandemic also hindered the planned outreach of the research topic to students and a wider audience through workshops or science fair. Publication of results achieved at TUM has concentrated on journal articles.
Publications
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A categorization of marginally stable thermoacoustic modes based on phasor diagrams. Combustion and Flame, 228, 236-249.
Yong, Kah Joon; Silva, Camilo F. & Polifke, Wolfgang
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Categorization of thermoacoustic modes in an ideal resonator with phasor diagrams. Combustion and Flame, 249, 112605.
Yong, Kah J.; Silva, Camilo F.; Fournier, Guillaume J.J. & Polifke, Wolfgang
