Burning velocity and flamelet structure of turbulent premixed CH₄/H₂/air flames with CO₂ dilution

Turbulent burning velocity and statistical flame front structure parameters of turbulent premixed CH₄/H₂/air flames diluted with CO₂ were measured with OH-PLIF technique. Hydrogen fractions are up to 0.2 and CO₂ dilution ratios are up to 0.1. The instantaneous flame front structure was detected using the OH-PLIF technique on a turbulent premixed Bunsen burner. Through image processing and statistic analysis, turbulent burning velocity S_T and statistic quantitative parameters, i.e. flame surface density Σ and flame brush thickness δ_T were obtained. Results show that turbulent burning velocity S_T normalized by laminar burning velocity S_L decreases with CO₂ dilution. Increased Markstein number L_M and flame intrinsic instability l_i with CO₂ dilution leads to the passive response of flame to turbulence wrinkling which resulting in less wrinkled flame front, smaller Σ, lower local burning velocity and S_T/S_L. Convex structure is more frequent than that of concave ones at normal pressure and the bias distribution is much weaker comparing to that of high pressure. The positive structure decreases while the negative structure increases slightly with CO₂ dilution. This would be one of the mechanisms leading to the decrease of the effective contact surface between reaction zone and unburned mixture which resulting in the decrease of S_T/S_L with CO₂ dilution.