Mode transition of interacting buoyant non-premixed flames

The dynamic behavior, especially in the transition to oscillation mode (in-phase and anti-phase), of two interacting non-premixed methane-air jet flames was investigated experimentally. A well-controllable experimental system for the present purpose was constructed and key parameters; such as fuel flowrate ( Q), burner diameter (d), and burner separation distance (L), were varied systematically. A well-known periodic motion of the flame was observed and the frequency monitored by thermocouples mounted adjacent to the burner exit. Time-variation of flame shape was recorded by a high speed camera associated with the optical imaging visualization. It was found that the flickering frequency was insensitive to the fuel flowrate, Q, implying that inertia played secondary role in the transition. Instead, the burner critical separation distance for the transition (L-crt) varied when various burner diameters were used, confirming that the difference in distance played an important role in the transition. It was found that the critical condition could be summarized by an updated correlation as d x L-crt(3) similar to const. This is slightly different from the one recently proposed by Yang et al. (2019), which was given under a narrower range of the fire scale. Accordingly, the critical condition can also be described by the critical value of the updated global parameter, such as alpha(3)Gr(4/3), where alpha and Gr denote the length ratio (L-crt/d) and Grashof number based on the burner diameter, respectively.