Acoustic Instability in Premixed Flames

Onset mechanism of primary acoustic instability in downward-propagating flames

“…Then, the increasing velocity gradients induce change in the flame surface area, which is in phase with pressure fluctuation, and the primary acoustic sound is generated. Finally, the variation in the flame surface area caused by the acoustic velocity field forces the amplification of the primary acoustic sound and leads to the stabilization of D-L instability [1,2,11]. Their calculation indicates that the maximum growth rate takes place in the lower half of the tube, which is in good agreement with Searby's experimental observations [1].…”

“…Ethylene gas plays an important role as the main absorption medium of CO 2 laser light according to the NIST chemical database [20]. The CO 2 Searby's work [1], he observed that the primary acoustic sound occurs when the flame has reached the lower half of the tube. Therefore, half of the tube is an appropriate field to observe the generation process of the primary acoustic instability, and this captured region is called as ""view area" in the present work.…”

“…An analytical analysis of this mechanism was further performed by Pelcé and Rochwerger [11] in connection with the experiments conducted by Searby [1]. They studied the coupling with variation in the flame surface area produced by the presence of an initial curved structure due to D-L instability and demonstrated that the growth rate of the primary acoustic instability is proportional to (ak) 2 , where a is an amplitude and k is a wave number of the curved flame.…”

“…The experimental observations by Searby [1] constitute a typical example of research in a combustion tube. He reported four distinct regimes of downward-propagating flames in a tube: (1) a curved flame with no acoustic sound just after ignition; (2) a primary acoustic instability with a flat flame surface when the flame has reached the lower half of the tube; (3) a violent secondary acoustic instability with a corrugated flame; and (4) turbulent flame.…”

“…Finally, the variation in the flame surface area caused by the acoustic velocity field forces the amplification of the primary acoustic sound and leads to the stabilization of D-L instability [1,2,11]. Their calculation indicates that the maximum growth rate takes place in the lower half of the tube, which is in good agreement with Searby's experimental observations [1]. However, it should be noted that the amplitude of a curved flame in their research is too small to apply to an actual flame, since this analysis assumes that the maximum value is ak = 1.…”