Leading edge stabilisation of vertical boundary layer diffusion flames

Abstract

The leading edge stability of vertical boundary layer diffusion flames established over a 3D-printed porous gas burner is revealed through Planar Laser-Induced Fluorescence imaging of the OH radical (OH-PLIF). Flame stability is studied by premixing methane and ethylene with increasing volume fractions of bromotrifluoromethane (CF3Br/Halon-1301) to increase the characteristic chemical timescale. Blow-off occurs at 15.7% and 37.3% CF3Br addition for methane and ethylene respectively, which are remarkably large limits compared to other flame configurations. As CF3Br is added, the flame stand-off distance increases and the reaction zone broadens, thereby increasing the flame length. This accelerates the buoyancy-induced flow ahead of the leading edge, promoting O2 entrainment into the flame anchor. Consequently, the radical scavenging effects on the flame anchor reactivity are dampened, resulting in the flame re-anchoring slightly downstream along the plate. Towards extinction, the flame shortens dramatically due to efficient catalytic cycling and radiative quenching at the flame tip resulting from the brominated species and excessive soot formation. This reduces the O2 mass flux into the kinetically dampened flame anchor resulting in blow-off extinction. Therefore, blow-off is controlled by both the leading edge reactivity and trailing edge length. Methane is shown to be considerably more sensitive compared to ethylene to CF3Br owing to its larger flame speed. These results demonstrate that fire-induced buoyancy greatly increases blow-off limits when using chemically active or inert agents in vertical wall fire configurations.