Propagation of unsteady tribrachial flames in laminar non-premixed jets

The characteristics of propagating tribrachial flames in non-premixed lamin ar jets have been investigated experimentally. Free jets of methane are ign ited downstream with a Nd:YAG laser to eliminate possible flow disturbances during ignition. The flame propagation is visualized and its speed measure d by a high-speed shadowgraphy and schlieren technique. Results show that t he edge of the propagating flame in a laminar jet regime has a tribrachial flame structure: a lean premixed flame, a rich premixed flame, and a diffus ion flame, all extending from a single location; The flame displacement spe ed maintains a constant value for a specified flow rate and decreases with jet velocity. Flame propagation speeds of tribrachial flames have been dete rmined from the velocity fields measured by laser Doppler velocimetry and f rom jet velocity modeling. This flame speed correlates well with flame curv ature and mixture fraction gradient. It is found that the unsteady curvatur e term dominates over the steady curvature term in flame stretch and that t he flame propagation speed correlates well and decreases with flame stretch . (C) 1999 by The Combustion Institute.