Numerical Investigation of the Counterflow Diffusion Flame of CO/H₂ in Oxyfuel Combustion: Effects of H₂O as Diluent at High Strain Rates

The maximum water as a diluent in the nonpremixed flame is quantified, mimicking combustion cooling by evaporation. Simulations with Cantera, using established chemical mechanisms, were conducted with various mass flux rates of fuel, equivalence ratios, fuel compositions, pressures, and inlet temperatures of the fuel. More water and lower flame temperature promote the forward reaction H + O-2 + (M) <-> HO2 + (M) but demote H + O-2 <-> O + OH. Eventually, the heat generated by the first of these reactions could not compensate for the reduced contribution from H + OH + M <-> H2O + M, making the flame unable to sustain itself. A linear dependency was found between the flammability limit and the strain rate of the flame. The fuel composition and inlet temperature presented an insignificant impact on the flammability limit. Higher pressure prevented the flame from extinction as more water was added to the oxidizer. Under the reference configuration, the flammability limit was found at an H2O mole fraction of 0.75 in the oxidizer.