Schadstoffarme Verbrennung durch elektrische Kontrolle
Zusammenfassung der Projektergebnisse
Stable operation is an essential requirement for technical combustion systems. The subject of thermoacoustic investigations is the interaction between the sound generated by a flame and the acoustic behavior of the surrounding system, because this interaction can lead to combustion instabilities as a result of feedback mechanisms. In the well-studied field of low-frequency (LF) thermoacoustics, the relevant flame length scale is small compared to the acoustic wavelength, which allows the simplistic consideration of the flame as a compact source in the acoustic field. In the high-frequency (HF) regime, however, the smaller acoustic wavelengths can lead to more complex interactions of the flame with the acoustic field that preclude treating the flame as a compact source. Non-planar acoustic modes can become relevant, exhibiting variations in transverse directions. In cylindrical combustors, the simplest transverse mode is the first-order azimuthal mode, also designated as the T1 mode. Searching for methods to affect and suppress combustion instabilities, previous research has rendered the generation of Nanosecond-Repetitively Pulsed (NRP) plasma discharges as an attractive means to address thermoacoustic oscillations in the LF regime. In the present project, we have installed a new experimental setup for azimuthal excitation of a swirl flame in a cylindrical combustion chamber with acoustic and multi-electrode NRP forcing systems. Using this setup to investigate the flame response up to the T1 mode frequency, the knowledge on HF thermoacoustic oscillations and the application of NRP plasma as a combustion actuation tool could be extended in two ways: (1) Azimuthal acoustic excitation of HF thermoacoustic oscillations with transverse variations: Due to high damping of HF oscillations at short wavelengths, lab-scale burners usually display no or only self-excited non-compact flame phenomena. Without the presence of selfexcited HF instabilities, it could be shown that the newly developed azimuthal acoustic forcing system is able to excite modes with azimuthal variations in a cylindrical combustion chamber. Investigation of the flame response to the azimuthal acoustic forcing system with phase-averaged images identifies the excitation of hydrodynamic structures in the low- and medium-frequency range, whereas in the high-frequency range a direct flame response to the acoustic field in the combustion chamber can be observed. (2) Azimuthally distributed generation of plasma discharges with a multi-electrode NRP forcing system: The application range of NRP plasma could be extended to address HF combustion phenomena with spatial variations: To this purpose, a novel multi-electrode NRP plasma forcing system was developed and implemented that allows for the generation of plasma discharges at six azimuthal positions in the burner outlet. Six separate pulse generators provide nanosecond-short high voltage pulses for one of six anodes positioned at the lip of the burner outlet. Phase-locked images of the intensity fluctuation resulting from excitation with the azimuthal plasma forcing system confirm the controlled generation of plasma discharges at the flame foot. Flame response images of the flame excited with combined plasma and acoustic forcing show the presence of successive positive intensity fluctuation spots in the wake over an azimuthal electrode position. This observation indicates an interaction of the gas kernels with the acoustic field and can be explained with a three-dimensional kinematic kernel evolution model proposed to describe the advection and expansion of the gas kernels produced by the plasma discharges.
Projektbezogene Publikationen (Auswahl)
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Flame Response of a Lean Premixed Swirl Flame to High Frequency Azimuthal Forcing. Volume 3B: Combustion, Fuels, and Emissions. American Society of Mechanical Engineers.
Mangold, Tobias O.; Orchini, Alessandro; Paschereit, Christian Oliver; Moeck, Jonas P. & Bohon, Myles D.
