Diffusion flames are ubiquitous in domestic and industrial applications that have been shaping human civilization. The development of flame instabilities could impair combustion performance, cause ignition failure or flame extinction, damage combustion devices, and trigger uncontrollable fire hazard. A prominent phenomenon related to the stability of a buoyant diffusion flame is flame flickering, or puffing, which describes the vibratory motion of the luminous flame. Previous experimental investigations have confirmed a famous scaling relation between the flickering frequency and the diameter of the fuel inlet. However, the fundamental mechanism for this relation has not been clearly understood.
To unveil the secret of the flickering of buoyant diffusion flames, Dr. Xi Xia, research fellow, and Dr. Peng Zhang, associate professor of Department of Mechanical Engineering, proposed a vortex-dynamical theory that connects the periodicity of flame flickering to the periodic formation and detachment of the toroidal vortices, that result from the buoyancy-induced shearing at the flame sheet (as illustrated in the figure). By incorporating the theory on vortex pinch-off, this work successfully establishes a theoretical scaling theory for the flickering frequency, which has been validated by the existing experimental data of pool flames and jet diffusion flames. This work has been included in the latest issue of the Journal of Fluid Mechanics. [X. Xia and P. Zhang, J. Fluid Mech. (2018), vol. 855, pp. 1156-1169]