Combustion characteristics and primary particle size of soot in ethylene/propylene-air coflow flames under dynamic pressure rise environment

Abstract

The effect of dynamic pressure rise rate on the burning characteristics and soot particle size in laminar coflow flames of ethylene and propylene is studied. Flame characteristics are observed at constant fuel flow rates under five pressure rise rates. Soot particles are collected using a thermophoretic sampling method at a pressure of 65 kPa during the pressure increase, and the primary soot particle diameters in ethylene flames are measured using a transmission electron microscope (TEM). The results show that the flame height increases with chamber pressure until 65 kPa, then slightly decreases. The flame becomes shorter with a higher pressure rise rate, influenced by both diffusion and buoyancy effects. As pressure increases, the transverse diffusion of fuel molecules diminishes, causing the flame to become slender. Simultaneously, the buoyancy effect enhances air entrainment, contributing to a reduction in flame height. A relationship between the flame height, pressure and dynamic pressure rise rate is derived based on the Burke-Schumann theory by assuming the pressure as a function of time. The proposed model can successfully predict the experimental data. The length of the soot-free main reaction zone (exhibiting a blue color) decreases with increasing pressure and is longer at smaller pressure rise rates. The relationship between the blue flame zone length and the dynamic pressure rise rate, which is characterized by soot formation time, correlates well with experimental results. The measured soot particle sizes range from 25 to 35 nm. The soot particle sizes are larger at lower pressure rise rates. The fractal dimension decreases with increasing pressure rise rates, while the pre-factor increases as the pressure rise rates become higher.