Effects of stretch and preferential diffusion on tip opening of laminar premixed Bunsen flames of syngas/air mixtures

• Tip structure of laminar premixed Bunsen flames was obtained using OH-PLIF.
• Tip structure was investigated combined with experiment and simulation.
• Local stretch rates due to flame curvature and flow strain were separated and calculated.
• Effects of stretch and preferential diffusion on tip structure were studied in quantitatively.

Effects of stretch and preferential diffusion on tip opening of laminar premixed Bunsen flames of syngas/air mixtures were investigated with experiments and numerical simulation. The reaction zone of the laminar flames at different hydrogen fractions (XH2 = 0.6, 0.8), equivalence ratios (φ = 0.6–1.0) and Reynolds numbers (Re = 1400, 1800) was obtained by OH-PLIF technique, while the species profile, velocity field, etc. were simulated by STAR-CD. The stretch rates due to the flame curvature and flow strain was calculated separately combined with experimental image and simulation. The results show that the flame tip of laminar premixed Bunsen flame is strongly negative stretched with a strong negative stretch due to flame curvature combined with slight positive stretch due to flow strain. At lean condition, the local preferential diffusion at the flame tip is intensified due to the maximum negative stretch, which promotes the mass diffusion of the fuel at the flame tip and makes the fuel mixture much leaner. This results in the reduction of local combustion intensity and is the main reason for the flame tip opening. The tip opening is promoted with the decrease of equivalence ratio, the increase of hydrogen fraction and the increase of Reynolds number. The strong negative stretch due to flame curvature at flame tip combined with preferential diffusion is the dominant effect of the flame tip opening. While, the slight positive stretch due to flow strain has weak effect.