Comparative Study of the Structure and Height of CH₄ Laminar Diffusion Flames: Effects of Fuel-Side versus Air-Side Dilution

Abstract

The strategy for introducing diluents is a critical practical concern in diluted combustion; however, a comprehensive understanding of the effects of fuel-side dilution versus air-side dilution is currently lacking. This numerical investigation systematically studied the effects of dilution strategies on methane coflow diffusion flames, with a focus on the flame structure and flame length. Common additives in practical combustion, specifically H2O and CO2, were introduced to either the fuel or the air streams, with dilution ratios (Z) ranging from 0 to 0.2, and the impacts of four dilution strategies were quantified and ranked. Detailed simulations were conducted using a well-validated two-dimensional (2D) flame code to gain a deep understanding of OH formation, flame attachment, temperature of the burner nozzle, and flame height. Systematic analyses in terms of heat transfer, molecular diffusion, and chemical kinetics were conducted. Results demonstrate that introducing diluents into the air stream exerts a more profound influence on suppressing OH formation compared with fuel-side dilution. Moreover, air-side dilution has a negligible influence on flame attachment, while increasing Z on fuel side significantly inhibits flame attachment, and the latter behavior is attributed to the diminished mass diffusion of CH4 toward the oxidizer side. As the flame attachment weakens, it causes a consequential reduction in heat transfer from the flame base to the burner. Accordingly, the nozzle temperature exhibits a more remarkable decrease with the fuel-side dilution ratio than with the air-side dilution ratio. Simultaneously, a more profound influence of Z on flame length was observed for fuel-side dilution than for air-side dilution, and the underlying mechanisms governing these two distinct dilution strategies were theoretically elucidated.