EXPERIMENTAL AND THEORETICAL-STUDY OF A PREMIXED VIBRATING FLAME

An experimental study of vibrating flames above a cylindrical burner has been conducted in order to examine some of the fundamental characteristics of the flow perturbations-combustion interaction. Here the perturbation chosen is a space-time sinusoidal flow velocity. The vibrational nature of the flow has been studied with and without the flame, using a LDV setup and a tomography system. The prevalent kinematic phenomenon is an effect of flow pumping-indeed the velocity perturbation propagates along the vertical axis nearly at the mean flow velocity. A laminar flame in such a flow modulation (< 200 Hz) responds with a periodic distortion around its steady, almost conical. configuration. The oscillation is the same frequency as that produced by the flow pumping. The deformation and the kinematics of the flame, studied by the means of a tomography system and a photographic record, are characterized by the height, the length, the area, and at the top by the flapping. the absolute speed, the thickness of the thermodiffusive zone, and the normal burning speed. In particular, the burning speed evolves globally between 0.5 and 2 times the laminar burning speed measured at the top of the reference steady flame. The area variations evolve from 11% to 25%, for the RMS intensity of the velocity fluctuations well below 10%. Nevertheless over a cycle the mean area is approximately equal to that of the laminar steady reference flame. On average the instantaneous stretching effects are negligible on the front structure, in spite of the presence of large instantaneous effects like at the top. An analytical model, based on small disturbances theory and a constant burning speed, shows that the space-time front deformation results from waves originating at the flame base towards its upper limit. A comparison between theoretical and experimental results is in good agreement.