Transient flame spread during convective ignition of solid fuel in a sudden-expansion combustor

The convective ignition of solid fuel (PMMA) in a sudden-expansion combustor (Reh = 6200, u0 = 22 m/s, [O2] ∼ 11.7%, T0 = 810 °C) is investigated from the perspective of flame–vortex interactions. Three phases of the transient flame spread are identified via the diagnostics of flow visualization and particle image velocimetry (PIV). The dominance of small/large vortices is revealed, respectively, in the pre-/post-ignition regimes, which demonstrates the small-to-large vortex transformation due to heat release. Attributed to the decreased characteristic reaction time and enhanced mixing, the first ignition is observed at the downstream end of the fuel, after which a primitive flame is formed and initiates the opposed flame spread. During the spread, the rolling behavior of flame kernels are considered to be dominated by the small eddies. The combined effects of broken vortices and continuing pyrolysis introduce the periodical extinction–reignition around the reattachment region. At the final phase, the entrainment of flame kernels into the shear layer is facilitated by the large shedding vortices, and a sustained diffusion flame is established. The study not only provides novel insights into the convective ignition of solid fuel in the separation–reattachment flow, but also serves as a basis for the advancement of ignition control.