Interaction of the combustion front of methane-air mixture at low pressures with obstacles of cylindrical shape

• The interaction of flames with obstacles is valuable both for the development of reliable numerical models and for safety issues. • It was shown that the methane-oxygen flame does not form von Karman vortex street behind the cylindrical obstacle. • Von Karman instability occurs in the flow of products. In the perforated cylinder, ignition centers on its inner surface occur. • The model of compressible Navier–Stokes equations in a low Mach number approximation allows obtaining both instability modes. • The occurrence of local primary ignition centers on inner surface of the obstacle was qualitatively illustrated. It was experimentally observed that the front of a propagating flame of a well-mixed diluted methane-oxygen mixture at 298 K and 100–300 Torr does not form von Karman vortex shedding behind the obstacle of cylindrical shape of 30–50 mm in diameter, including a perforated cylinder; however, the instability under the same conditions occurs in the flow of hot products. In the perforated cylinder, the occurrence of local primary ignition centers on its inner surface is observed. In the mathematical modeling, the main observed features of the flame front propagation were taken into account: the chain branched mechanism of gaseous combustion and the absence of vortex shedding behind the obstacle at flame propagation. It was shown that a qualitative model of compressible dimensionless non-reactive/reactive Navier–Stokes equations in low Mach number approximation yields both the mode of the emergence of von Karman instability in chemically inert gas and the absence of the instability in the mode of flame propagation in a reacting flow. The model computations confirmed the occurrence of local primary ignition centers on the inner surface of the obstacle. (a), (b) High speed color filming of the flame propagation in 15.4% CH 4 + 30.8% O 2 + 46% CO 2 + 7.8% Ar mix against an obstacle in the form of a cylinder 40 mm in diameter, P = 175 Torr, 600 s −1 , 298 K. Numbers in each frame correspond to a consecutive number of the video image during the ignition starting from the moment of a discharge. (a) flame propagation from the left to the right, (b) propagation of the reflected flow of reaction products from the right to the left. (c), (d) numerical simulation of the gas density in flame propagation against an obstacle in the form of a cylinder, (c) – reactive flow propagates from the left to the right; (d) – inert pressure driven flow propagates from the right to the left (for ease of comparison with the experiment (a, b)).