Initiation and formation of the corrugated structure leading to the self-turbulization of downward propagating flames in a combustion tube with external laser absorption

Combustion tube (length 45 cm, inner diameter 5 cm) experiments with flames of premixed gas of C2H4/CO2-O2 (Le < 1) were conducted. The flame fronts propagated downward to the closed bottom of an open-ended tube. An initially steadily propagating flat flame was deformed by an external laser irradiation method to investigate its evolution under the interaction with acoustic vibration. Results showed that the locally deformed flame evolved into a corrugated structure at the flame front followed by self-turbulization. The process to form this corrugated structure was investigated in detail based on the images captured using high-speed cameras. From the observations, a possible mechanism for the initiation of the corrugated structure, explained mainly by periodic acoustic acceleration, was proposed. Then, according to the mechanism an alternative definition for the inverse Froude number is proposed in this work and used as criterion for the initiation of the corrugated flame structure. To prove the validity of the criterion two mixtures having different flame speed were tested and it was confirmed that the criterion provided transition condition very well for both tested mixtures.