  An experimental study of methane, propane and dimethyl ether (DME) pre-mixed
flames propagating in a quasi-two-dimensional Hele-Shaw cell placed
horizontally is presented in this paper. The flames are ignited at the open end
of the combustion chamber and propagate towards the closed end. Our experiments
revealed two distinct propagation regimes depending on the equivalence ratio of
the mixture as a consequence of the coupling between the heat-release rate and
the acoustic waves. The primary acoustic instability induces a small-amplitude,
of around 8 mm, oscillatory motion across the chamber that is observed for lean
propane, lean DME, and rich methane flames. Eventually, a secondary acoustic
instability emerges for sufficiently rich (lean) propane and DME (methane)
flames, inducing large-amplitude oscillations in the direction of propagation
of the flame. The amplitude of these oscillations can be as large as 30 mm and
drastically changes the outline of the flame. The front then forms pulsating
finger-shaped structures that characterize the flame propagation under the
secondary acoustic instability. The experimental setup allows the recording of
the flame propagation from two different points of view. The top view is used
to obtain accurate quantitative information about the flame propagation, while
the lateral view offered a novel three dimensional perspective of the flame
that gives relevant information on the transition between the two oscillatory
regimes. The influence of the geometry of the Hele-Shaw cell and of the
equivalence ratio on the transition between the two acoustic-instability
regimes is analyzed.
