UNSTEADY MOTION OF POOL FIRE ON SMALL-SCALE BURNER

Abstract

The flame motion of a pool fire on a small-scale burner has been experimentally investigated from the viewpoint of nonlinear dynamics, focusing on the relationship between the flame motion behavior and increasing burner diameter d. Hexane ( C 6 H 14) was used as a liquid fuel in the present study. For small burner diameters of up to d ≈ 10 mm , a stable conical flame was observed. The flame tip of the stable flame began to oscillate at low frequency (approximately 10 Hz) due to the buoyancy-driven hydrodynamic instability when d exceeded 13 mm. With further increase in d, the flame tip oscillation began to exhibit an interesting oscillation mode. These flame motions can be shown quantitatively by drawing an attractor, and evaluated by estimating the correlation dimension Dc. For d ≤ 10 mm , the attractor was a fixed point and Dc was approximately zero. When d reached 13 mm, the attractor became a limit cycle and Dc was estimated to be approximately unity, indicating a periodic motion. With larger burner diameters of up to d ≈ 50 mm , the trajectories of the attractor seemed to be rolled up and Dc approached approximately 2, indicating a quasi-periodic motion. These results indicate that the flame motion of the small-scale pool fire switches from stable to quasi-periodic, throughout periodic with increasing the burner diameter. The present results also show that a nonlinear analysis based on deterministic chaos theory, such as that using the attractor and the correlation dimension, would be a valid method by which to discuss the flame instability issue of the pool fire.