Influence of the Propagation Direction for an Acoustic Wave Interacting with a Turbulent Premixed Flame

We investigate in this work the conditions leading to amplification or damping of a planar, Gaussian acoustic wave interacting with a CO/H₂/Air turbulent premixed flame. We examine in particular the influence of the direction of propagation. In a first case the acoustic wave is coming from the right direction (starting in burnt gas zone), while it propagates from the left direction (starting in fresh gas zone) in the second case. This investigation is based on fully compressible Direct Numerical Simulation (DNS) results. Chemical processes are computed using a complete reaction scheme and accurate transport properties are taken into account. A local version of the classical Rayleigh's criterion is used to analyze the results, based on the single assumption that the celerity of sound is independent of time and only function of the spatial coordinates. We observe that, after the interaction, the wave is wrinkled leading to a non-planar geometry, similar to that of the initial flame front. For both cases the heat release fluctuation is concentrated in very small zones, associated either with positive or negative values. Rayleigh's criterion is then used to localize large positive and negative values (amplification resp. damping conditions). This criterion shows that wave amplification occurs only inside well-located small zones in the two cases. The propagation direction has no influence on these results, confirming that amplification or damping is mainly controlled by a coupling process between pressure and heat release fluctuations through the chemical reactions. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)