Triple flame propagation and stabilization in a laminar axisymmetric jet

The propagation of a methane–air triple flame in a partially premixed jet is investigated experimentally and numerically. The flame is ignited with a Nd:YAG laser in a nonuniform jet-mixing layer downstream of the burner. The ignition and flame propagation processes are recorded using a high-speed video camera. The flamefront propagation velocity in laboratory coordinates is inferred from the video images. A comprehensive, time-dependent computational model is used to simulate the transient ignition and flame propagation phenomena. The model employs a detailed description of methane–air chemistry and transport properties. Following ignition, a well-defined triple flame is formed that propagates upstream towards the burner along the stoichiometric mixture fraction line. As the flame propagates upstream, the flame propagation speed, which is defined as the normal flamefront velocity with respect to the local gas velocity, decreases linearly. Near the burner wall, the flame curvature increases to two times the value of its downstream freely propagating counterpart. During the flame propagation process, the curvature-induced stretch dominates over the hydrodynamic stretch and the flame speed decreases with increasing stretch rate in accord with previous measurements. We also examine the dominant reaction rates to follow the transition from a triple flame to a double flame structure.