The effects of acoustic disturbances and mechanical vibration on laminar premixed flames: A comparative study

The energy conversion between the combustion process and acoustic waves is very complex in high-performance propulsion systems, which may trigger large-amplitude combustion instabilities, potentially leading to severe vibration or structural damage. Both acoustic disturbances and mechanical vibration are found to be able to cause the combustion instability, but their differences on the influence of laminar premixed flames are still unknown. In this paper, a premixed methane flame burner was disturbed by either a loudspeaker or an electromagnetic vibration exciter, and the effects of their forcing frequencies and amplitudes on the dynamic responses of the premixed methane flame were evaluated. Specifically, the unsteady heat release rate was measured by a photomultiplier tube, and the flame dynamic behaviors were captured by a high-speed camera with image intensifier. Furthermore, the acoustic disturbances upstream of the flame was measured by the two-microphone method, and the structure vibration was measured by an accelerometer. Then, the flame transfer functions were obtained at various equivalence ratios and mean flow velocities. The results demonstrated that the premixed flame exhibited obvious band-pass filtering characteristics when subjected to external forcing, whether through a loudspeaker or an electromagnetic vibration exciter. In addition, increasing either acoustic or structure excitation amplitude to a large extent can make the flame respond in a nonlinear manner. However, when the flame was subjected to mechanical vibration, the peak frequency corresponding to the maximum gain of the flame transfer function had minimal sensitivity to variations in equivalence ratios and mean flow velocities. In contrast, the acoustic disturbances were more likely to induce the nonlinear flame responses and flame front wrinkling.