  Small-scale flickering buoyant diffusion flames in externally swirling flows
were computationally investigated with a particular interest in identifying and
characterizing various distinct dynamical behaviors of the flame oscillators
under different swirling flow conditions. By varying the external swirl, six
distinct flame dynamical modes, such as the flickering flame, the oscillating
flame, the steady flame, the lifted flame, the spiral flame, and the flame with
a vortex bubble, were computationally identified in both physical and phase
spaces and analyzed from the perspective of vortex dynamics. Specifically, the
frequency of buoyancy-induced flame flicker nonlinearly increases with the
swirling intensity in the weak swirl regime. Further increasing the swirling
intensity causes the vortex shedding to occur either around the flame tip or
downstream of the flame, and the flame stops flickering but oscillates with
small amplitude or stays in a steady state. A sufficiently high swirling
intensity locally extinguishes the flame at its base, leading to a lifted
flame. In addition, the spiral mode and the vortex-bubble mode were found for
the flame at large swirl angles. Through establishing the phase portrait for
featuring the flow in flames, the dynamical behaviors are presented and
compared in phase space.
