An observation of near-planar spinning premixed flames in a sudden expansion tube

Unusual structures of premixed flames are observed experimentally in a sudden expansion tube. A unique flame having a near-planar surface spinning inside a sudden expansion tube exists in both propane/air and methane/air flames which resembles an X-shape in bare eyes. Stable near-planar spinning flames are observed in rich propane and lean methane flames and lifted spinning flames which migrate slowly toward the downstream through the expanded tube and having the speed in the order of 1 cm/min are found in lean propane and rich methane flames. Variations of near-planar spinning flames are also observed which include the top part of the spinning flame surface is bent toward the upstream with axisymmetric and asymmetric spinning and both the top and the bottom parts bent horizontally. The rotational speed is sensitive to the equivalence ratio and linearly decreases with the step height of the expansion tube. The importance of preferential diffusion, heat loss through the recirculation zone to the outer expanded tube, and hydrodynamic effect are demonstrated on the existence and spinning of the near-planar flame surface.