On the hydro-acoustic coupling responsible for the flashback limit-cycle of a premixed flame at a backward-facing step

Abstract

This paper examines the combustion dynamics of a methane/air premixed flame establishing past a backward-facing step placed in a ducted combustor. Spectral analysis performed on high-speed recordings of the flame allows to show that the flame dynamics is governed by two main components. The first one is an acoustic longitudinal resonance, which imposes a flapping motion of the flame around the step. The second one is a hydrodynamic instability, which corresponds to the roll-up of the flame by the recirculation region in the trail of the step. When the frequencies associated to each mechanism are far apart, the flame is relatively stable. When the two frequencies get closer, the roll-up process is amplified and leads to a more pronounced dilatation of the recirculation bubble, as well as an enhanced vertical flame flapping. When the two frequencies coincide, a fully-coupled hydro-acoustic instability is established and prompts the flame to periodically flashback in that it is favored by the fact that the fluctuations of the hydro-acoustically modulated flow velocity are so large that the flow velocity drops below the flame speed. Besides, an acoustic analysis of the ducted combustor reveals the presence of an acoustic velocity anti-node at the position of the step, which is exacerbating the amplitude of the fluctuations. Time series analyses of the flame position, reduced to a 1D description of its rotational motion, reveals that the flashback dynamics is locked to a limit-cycle attractor. The flashback motion is intermittently accompanied with the stochastic generation of an autoignition front developing from a hot spot located on the top wall.