Analysis of Structure, Extinction and Broadening in Oxygen-Enhanced Non-Premixed Flames

In the present numerical study laminar oxygen-enhanced and oxy-fuel CH4/N-2-O-2/N-2 non-premixed flames were systematically investigated in counter-flow configuration. The compositions of the fuel and the oxidizer stream were chosen in a way, which allows a study of the flame structure at different flame temperatures in a range of approximately 1800-3000 K at constant stoichiometric mixture fraction of Z(st) = 0.2. The calculations were performed with the GRI 3.0 as a base kinetic mechanism, while selected cases were also investigated with the Appel et al. mechanism and the GDFkin (R) 3.0. The flame structure, extinction and flame broadening effects were investigated for various strain rates by analysing the heat release rate and temperature profiles. Four different heat release rate zones were identified, namely the radical chain branching, the main water and carbon dioxide formation, the fuel oxidation and the fuel pyrolysis zone. While those zones overlap in case of air-combustion, a separation accompanied by a flame broadening is found in oxygen-enhanced flames as a result of the higher flame temperatures. The four heat release zones get affected dissimilarly by increased oxygen content and flame temperature, while the flame broadening is dominated by the shift of the radical branching zone towards the oxidizer side. A comparison between air combustion and pure oxygen combustion shows that the flame is broadened by a factor of approximately 10 as long as the flame conditions are not close to the extinction limits. With increasing strain rate flame extinction starts from the oxidizer side. An increased radical pool and temperature level in the chain branching zone extents the extinction limits towards higher strain rates in oxygen-enhanced flames. On the fuel side, the increased oxygen-content with the associated higher temperature level leads to a strong endothermic zone and an intense activation of the C-2-pathway.