Premixed flame response to axisymmetric axial instabilities has been studied in great detail. Recently, the authors extended this analysis to consider the direct excitation of premixed flames by transverse acoustic modes in a can combustor configuration. The modes are high frequency in nature leading to acoustically non-compact flames as opposed to the compact flames in the axial instability case. This non-compact flame stability is assessed using the Rayleigh criterion and thus, prior modeling efforts by the authors has presented a modeling framework using the G-equation to capture the local flame dynamics. Along with the spatially distributed pressure mode shape, the Rayleigh integral was evaluated over a range of control parameters for purely transverse modes showing that radial modes were always unstable whereas mixed radial-azimuthal modes showed regions of stability. However, the transverse acoustic motions induce axial acoustic fluctuations at the nozzle-combustor intersection, thereby leading to an axial excitation of the flame at the transverse mode frequency. Thus, the overall thermoacoustic stability of the flame is determined by both the response to the induced axial and inherent transverse velocity fluctuations. This paper focuses on the relative importance of the purely transverse mode response with respect to the response to the induced axial fluctuations. A key parameter in this study is the transfer function between the induced axial fluctuations and the source transverse fluctuations at the nozzle, which is mode dependent. The study will present comparisons of the Rayleigh index with and without these axial fluctuations to illustrate this relative importance.


Significance of the Direct Excitation Mechanism for High-Frequency Response of Premixed Flames to Flow Oscillations

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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