Experimental study on effects of CO₂ addition on the H₂/CO₂/air turbulent expanding flames

This study presents a comprehensive investigation of spherical flame morphology, propagation, and turbulent burning velocity of H₂/CO₂/air mixtures at wide equivalence ratios (φ = 0.8–2.0), with diluted with volumetric CO₂ additions ranging from 0 % to 30 %. The effects of CO₂ addition from wrinkled flamelets to thin reaction zones were studied by employing fans to provide homogeneous turbulence intensity (u’ = 0–2.72 m/s) within the combustion chamber. Shadowgraph imaging reveals that a non-monotonic variation in the wrinkling degree of laminar flame fronts with increasing CO₂ content, characterized by an initial increase followed by a decrease in the critical Peclet number of the transition acceleration stage, which is attributed to the synergistic effect of enhanced differential diffusion and weakened hydrodynamic instability. External turbulence leads to finer flame wrinkles and larger-scale flame curvature, and rapid attainment of a self-similar propagation stage for the flame. Normalized turbulent flame propagation speeds ((d<r>/dt)/σS_L) are scaled as the power-law fitting of flame Reynolds number (Re_T,flame) to describe the turbulent self-similar propagation behavior, as d<r>/dt)/σS_L = aRe_{T, flame}^b. The pre-factor a increases with higher CO₂ content and lower equivalence ratios, which is attributed to the larger diffusional-thermal instability effects manifested as lower Le. The scaling exponent b exhibits different values in the wrinkled flamelets, corrugated flamelets, and thin reaction zone, indicating a transition from instability-dominated to turbulence-dominated flame propagation. While the preferential diffusion of hydrogen enhances flame propagation and further improves the scaling exponent in each turbulent regime. The correlations from literature based on Le and Ka (or Da) have been demonstrated to effectively describe the effects of differential diffusion and turbulent stretch on turbulent burning velocity in H₂/CO₂/air mixtures. Furthermore, a concise correlation was proposed to better predict the present data, which could play a crucial role in elucidating the behavior of H₂/CO₂/air turbulent flames.