Effect of additional diluents on flame propagation speed and markstein length in outwardly propagating premixed methane/ethylene–air flames

An experimental study was conducted to understand the effects of additional diluents (CO2 and He) on unstretched flame speed and Markstein length in outwardly propagating spherical premixed CH4/C2H4–air flames at normal temperatures and elevated pressures of up to 0.3 MPa. Laminar burning velocities were measured and compared with predicted ones using reliable reaction mechanisms. The data were first validated by testing linear and nonlinear extrapolation models for premixed methane–air flames. Unstretched laminar burning velocities were presented for premixed methane/ethylene–air flames diluted with CO2 and He on the basis of the optimized range of flame radius and extrapolation model. Three kinetic mechanisms were evaluated and compared with the measured data, and findings showed that Sung Mech was best fitted to the current unstretched flame speeds. Experimentally determined Markstein lengths were compared with theoretically predicted ones by considering the definitions of Lewis (heat-release-weighted, diffusion-based, and volumeweighted) and Zel’dovich numbers (based on a temperature-dependent one). The theoretical Markstein lengths based on a temperaturedependent Zel’dovich number and a heat-release-weighted effective Lewis number agreed best with the experimental data. The capability in predicting theoretical Markstein length with the model of Matalon was better than that with the model of Chen.