Vortex-dynamical interpretation of anti-phase and in-phase flickering of dual buoyant diffusion flames

Abstract

Anti-phase and in-phase flickering modes of dual buoyant diffusion flames were numerically investigated and theoretically analyzed in this study. Inspired by the flickering mechanism of a single buoyant diffusion flame, for which the deformation, stretching, or even pinch-off of the flame surface result from the formation and evolution of the toroidal vortices, we attempted to understand the antiphase and in-phase flickering of dual buoyant diffusion flames from the perspective of vortex dynamics.The interaction between the inner-side shear layers of the two flames was identified to be responsible for the different flickering modes. Specifically, the transition between anti-phase and in-phase flickering modes can be predicted by a unified regime nomogram of the normalized flickering frequency versus a characteristic Reynolds number, which accounts for the viscous effect on vorticity diffusion between the two inner-side shear layers. Physically, the transition of the vortical structures from symmetric (in-phase) to staggered (anti-phase) in a dual-flame system can be interpreted as being similar to the mechanism causing flow transition in the wake of a bluff body and forming the Karman vortex street.

Mode transition of interacting buoyant non-premixed flames

“…). The predicted values (three plots) by Yang et al (2019) are also embedded in the figure, showing their plots are well-fitted to the trend obtained by the present experiment. With this respect, our experimental work for wider a range of fire scale (d) may reveal a "universal" trend for transition behavior.…”

“…Based on the dimensional analysis, we can successfully propose the updated global parameter to describe the transition as 3 4/3 , where and stand for the ratio of two length scales (L/d) and Grashof number, respectively. This is somewhat different from the one proposed by Yang et al (2019) because their prediction is based on the limited range of the fire scale.…”

“…Recall that in Yang's work (Yang et al, 2019), a modified Grashof number ( 1/2 ) was proposed with a characteristic global parameter, to summarize the transition under the range considered, which is the limited range of the fire scale with limited data (see Fig. 8).…”

“…Together with optical imaging technique (Bunkwang et al, 2019), it was confirmed that the fluid dynamics induced by buoyancy played a key role in the appearance of two modes and their transition. Yang et al (2019) performed 3-D numerical simulation for pool fires and proposed that the transition can be summarized by a modified Grashof number. Although their model successfully revealed the simple relation between the fire scale (d: pool/burner (inner) diameter) and separation distance (L) as × 2~.…”

“…Relation between and d based on the measured data in this study. Predicted values byYang et al (2019) are also embedded in the figure.…”