Acetylene and ethylene mole fractions in methane/air partially premixed flames

Mole fractions of acetylene in flames play a major role in soot formation. An experimental and computational study of the structure of non-premixed and partially premixed methane/air flames was condducted, using a counterflow flame, with emphasis on investigating C2H2 and C2H4 mole fractions. The operating conditions were selected to maintain a fixed stretch rate on the oxidizer side for different levels of partial premixing. Moderately high stretch rates were used to prevent the buoyancy effects. The high stretch rates and selection of methane as the fuel resulted in flames with negligible soot formation based on their blue color. However, measurable mole fractions of C2H2 and C2H4 existed at all levels of partial premixing. Local measurements of species mole fractions were performed using sampling and gas chromatography. Numerical simulations involved 30 species, variable diffusivities, and C2 chemistry consisting of 278 steps. Analysis of formation and consumption of C2H2 and C2H4 was completed using quantitative reaction path diagrams (QRPD). The results show that the predictions of C2H2 and C2H4 mole fractions are significantly higher than measurements. Unavoidable errors associated with probe sampling are responsible for only a part of the discrepancies. A preliminary investigation of C3-C6 chemistry shows that only a small portion of the error results from the neglect of conversion to higher hydrocarbons. Therefore, critical studies of the rate constants of the relevant reactions, which are identified using the QRPD, are warranted.