Dynamical Behaviors of Small-scale Buoyant Diffusion Flame Oscillators in Externally Swirling Flows

TL;DR

This study computationally investigates small-scale buoyant diffusion flames in swirling flows, identifying six distinct dynamical modes and analyzing their vortex dynamics and frequency responses to swirling intensity.

Contribution

It introduces a detailed classification of flame behaviors under varying swirl conditions and links these modes to vortex dynamics and flow parameters.

Findings

Six flame dynamical modes identified and characterized.

Flame flicker frequency nonlinearly increases with swirl intensity.

High swirl can extinguish or stabilize the flame, inducing different modes.

Abstract

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.