Experimental and numerical analysis of the influence of oxygen on soot formation in laminar counterflow flames of acetylene

Axisymmetric counterflow flames at atmospheric pressure are used to investigate the influence of oxygen on soot formation. An acetylene/nitrogen mixture is blown against the hot postflame gases of a lean premixed flame stabilized on a sinter metal plate, thereby preheating the oxidizer. Oxygen is then added separately to the fuel and oxidizer side. Measured temperature, species concentration and soot volume fraction profiles are compared with calculations. The latter are performed using a detailed kinetic reaction mechanism with 452 reactions and a detailed soot model describing particle nucleation, coagulation, polycyclic aromatic hydrocarbon (PAH) condensation, surface growth, and surface oxidation. It is found that oxygen addition increased the maximum soot concentration in both cases. The model predicts the same trends, and satisfactory agreement between measured and calculated values for all investigated parameters is observed. To distinguish between chemical influences of oxygen addition and thermal and dilution effects, further numerical studies are carried out following a procedure similar to the one proposed by Axelbaum et al. [1]. For fuel side addition of oxygen, the increased soot volume fraction is attributed to a catalytic effect triggering the production of benzene via C3 and C4 species. For the case of O2 addition to the oxidizer stream, it is found that the higher flame temperature was the main cause for the increased soot concentration.