Investigation on flame behaviors in the mesoscale channel of one sudden-expansion step during the preliminary stage of ignition

The dynamic flame behaviors at the preliminary stage of ignition in the mesoscale sudden-expansion channel are numerically investigated in this study under the isothermal condition (300 K) at walls by using a two-dimensional model without symmetric assumption at the centerline. It is found that the flashback velocity is dominated by the upstream channel height; nevertheless, the blowoff velocity is determined by not only the downstream channel height but also the flow recirculation behind the sudden-expansion steps. As the expansion ratio is sufficiently large, the flame could exist within a substantially wider range of inlet flow velocity. In addition, four types of flame behaviors are found at the expansion ratio of 2: (I) steady convex flame, (II) steady concave flame, (III) simple oscillating flame, and (IV) complex oscillating flame. Above the flashback velocity, the convex flame exists steadily. With the increase in flow velocity, the flame becomes concave to the upstream and is stabilized by the wall-quenching effect of sudden-expansion steps. If the flow velocity is further increased, the flame becomes unstable and oscillates periodically (simple oscillating flame) due to the interaction of flame and the symmetric flow field in the sudden-expansion channel, while the occurrence of complex oscillating flame at high flow velocities is attributed to the asymmetric flow pattern. The frequency of oscillating flames decreases with the increase in flow velocity.