Experimental and Numerical Investigations of Soot Formation in the Laminar to Turbulent Transition of an Acetylene Diffusion Flame

Soot formation from incomplete hydrocarbon combustion poses significant challenges for emission control in propulsion systems. This study employed experimental and numerical methods to investigate the transition process of the acetylene diffusion flame from laminar to turbulent flows, with particular focus on the evolutions of the soot formation rate (ṁ_soot), flame temperature (T), and sound pressure level (SPL). Results from different regimes indicate the following: (1) In the laminar state, ṁ_soot increased linearly with Re, with a growth rate positively correlated with the tube diameter. (2) After entering the transitional state, ṁ_soot decreased exponentially by over 95%; T gradually increased by 150 K; and both SPL and the standard deviation of T (σ_T) initially rose and then declined. (3) After entering the fully turbulent state, SPL increased again whereas σ_T stabilized at 14. When Re was decreased from the critical value at the occurrence of lift-off, the lifted flame could be maintained within the fully turbulent region (named as the reverse lifted flame) and ended upon entering the transitional region. ṁ_soot of the reverse lifted flame was about 2 to 3 times that of the attached flame, which was due to the enhanced O₂ entrainment from the bottom of the flame.