What is it about?
Thermoacoustic combustion instabilities are a severe challenge for the development of reliable, low-emission and flexible combustion technology for propulsion and power generation. These self-excited instabilities result from feedback between flow, flame and acoustics. Unless effectively controlled, they can lead to limited operating range, increased emissions, or even rapid destruction of a combustion systems
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Why is it important?
Intrinsic thermoacoustic (ITA) modes results from a flow/flame/acoustic feedback mechanism that has escaped attention until recently. The ITA feedback loop may be described as follows: An upstream velocity disturbance induces a modulation of the heat release rate, which in turn generates an acoustic wave traveling in the upstream direction, where it influences the acoustic velocity and thus closes a feedback loop. This feedback has important consequences for the dynamics and stability of combustion processes. This paper reports for the first time on ITA modes observed in high-resolution simulations of a premixed laminar flame in an anechoic environment. This gives convincing evidence that such "intrinsic modes" - first described and discussed by the group at TU/e - are not merely an artefact of simplistic low-order models. Our results were corroborated by Courtine et al (CNF 162, 11, 2015).
Perspectives
This study started with Camilo Silva wanting to determine the FTF of the "Kornilov flame" as a warm-up exercise in CFD/SI. Following established wisdom, he imposed non-reflecting boundary conditions at in- and outflow boundaries -- and observed a self-excited instability !? This was not understood at all, and first it was assumed that he'd encountered some kind of numerical instability. Strange enough, with non-zero reflection coefficients the simulations did not "blow up".... Sebastian Bomberg had identified ITA feedback as the underlying cause of strong, narrow-band peaks in instability potentiality (see Emmert et al, CNF, 2015), and eventually we realized that the instability observed in the simulations resulted from ITA feedback and was indeed a manifestation of the "Intrinsic Instability of Flame-Acoustic Coupling" that the TU/e group (Hoeijmakers et al, CNF, 2014) had observed previously in low-order models with n-tau heat source. Incidentally, the analysis presented in this paper also explains in a very satisfactory manner puzzling observations made by Kaess et al. (CTR Summer Programm 2008). In that study, which was also concerned with numerical analysis of laminar flame dynamics, we observed pronounced peaks in the frequency response (see section 4.3 and Fig. 10 in the report) which were inexplicable back then, but we now understand to result from ITA feedback
Professor Wolfgang Polifke
Technische Universitat Munchen
Read the Original
This page is a summary of: Numerical study on intrinsic thermoacoustic instability of a laminar premixed flame, Combustion and Flame, September 2015, Elsevier,
DOI: 10.1016/j.combustflame.2015.06.003.
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