Abstract
The influence of pressure waves on the mean structure of a turbulent flame is described. Two cases are presented : (1) When the pressure wave is generated by the flame itself, the system is submitted to combustion instability. In some extreme circumstances, the coupling between combustion and acoustics can lead to a flame structure characterized by the shedding of large vortices behind the flame holders; for this regime, the mean flame structure definition is a result of a cyclic formation and destruction of the reactive pattern at the frequency of the pressure oscillation. The characteristics of this cycle are studied by a phase average imaging method of the local reaction rate. This low frequency combustion instability is described in the first part of this paper. (2) Pressure waves can also be externally imposed to the flame. Arrays of driver units (loudspeakers) can be used to excite the flame at a chosen frequency and with a predetermined acoustic structure. With this technique it is possible to excite the flame in the transverse sloshing mode. In the case of a multiple flame combustor, this excitation induces strong modifications of the flame pattern : small vortices generated by the sloshing motion interact and set the jets of fresh mixture into a flapping motion. The flame structure appears as the result of the nonlinear interaction between small vortices generated by high frequency excitation and a low frequency induced flapping motion of the jets. Phase average measurements cannot be used in such circumstances because the fluctuations are the result of the superposition of many different modes. A spectral imaging method is developed to analyse these nonlinear coupling effects and its application is described in the second part of the paper. KeywordsAcoustic ModeTurbulent FlameFlame StructureSmall Scale TurbulenceCombustion InstabilityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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