Computed flammability limits of opposed-jet H₂/CO syngas diffusion flames

Extensive computations were made to determine the flammability limits of opposed-jet H₂/CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate that a minimum hydrogen percentage is required for H₂/CO syngas mixture to be combustible in air. The minimum hydrogen concentration becomes larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s^-1 where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7 % for the equal-molar syngas fuels (H ₂/CO=1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and ambient pressures or CO₂ dilution percentages as the coordinates. By adding dilution gases such as CO₂, H₂O and N₂ to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H₂O contributes to higher flame temperature, while the radiation effect of H ₂O and CO₂ plays an important role in the flame extinction at low strain rates.