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Abstract
Experimental studies were carried out on turbulences in open premixed jet flames by using a double coaxial type burner with propane-air mixtures. Shadow and schlieren graphs showed that low frequency oscillations of the flame occur in the vicinity of the boundary between the outer flame and the surrounding air stream. The phenomena are surely caused by the hydrodynamic instability and it was also asserted from the fact that the phenomena could never be seen in the case of the surrounding flow velocity being large enough in measurement by a counter-type laser doppler velocimeter (LDV). Moreover, the organized motion with periodic vortices was entirely common in premixed, rich-premixed, and diffusion flames in the open atmosphere. The normalized frequencies had a tendency similar to the results obtained by some workers
On the other hand, from the measurement of velocity fields by LDV, it was found that strong turbulences occurred on the flame front mainly by the velocity gradient, not |∂∪|∂r| but |∂V|∂r|. That is, the turbulence on the flame front is principally caused by the acceleration and especially, the outward bending of stream lines accompanied with local thermal expansion. Accordingly, the turbulence is not directly concerned with the oscillation phenomenon.