Propagation of Unsteady Tribrachial Flames in Laminar Non-premixed Jets

The characteristics of propagating tribrachial flames in non-premixed laminar jets have been investigated experimentally. Free jets of methane are ignited downstream with a Nd:YAG laser to eliminate possible flow disturbances during ignition. The flame propagation is visualized and its speed measured by a high-speed shadowgraphy and schlieren technique. Results show that the 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 diffusion flame, all extending from a single location. The flame displacement speed maintains a constant value for a specified flow rate and decreases with jet velocity. Flame propagation speeds of tribrachial flames have been determined from the velocity fields measured by laser Doppler velocimetry and from jet velocity modeling. This flame speed correlates well with flame curvature and mixture fraction gradient. It is found that the unsteady curvature term dominates over the steady curvature term in flame stretch and that the flame propagation speed correlates well and decreases with flame stretch. r 1998 by The Combustion Institute