An Experimental Study on Premixed Methane-Air Flame Propagation with Intrinsic Instabilities in a Hele-Shaw Chamber at Various Inclined Angles

Flame propagation in narrow gaps can manifest intrinsic instabilities with appearance of wrinkles/cells on the reaction front, which is attributed to various physical mechanisms. For instance, the buoyancy effect is believed to be able to either stabilize or destabilize the reaction front depending on the flame propagation direction. In this paper, an experimental study of propagation of premixed methane-air flames at a variety of mixture equivalence ratios (from fuel-lean to fuel-rich) in a narrow gap (12 mm) formed by two parallel plates (termed Hele-Shaw chamber) is conducted. The magnitude of the gravitational component along the direction of flame propagation is manipulated via altering the inclined angle of the rotatable combustion chamber. Evolution features of flame-surface shapes during the propagation process under various conditions are recorded using high-speed imaging. Results show that generations of the flame wrinkles/cells are mainly driven by the Darrieus-Landau (DL) and Rayleigh-Taylor (RT) instability. For slowly propagating flames at an extremely fuel-lean condition, the RT is a pronounced mechanism, showing a stabilizing effect for downward propagating flames and a destabilizing effect for upward propagating ones. For the fast burning system at higher equivalence ratios, the DL plays a predominant role in generating instabilities among all the chamber inclined angles while the RT's effect is noticeably weakened.