Experimental study on soot formation and primary particle size in oxy-combustion ethylene diffusion flames under CO2 substitution for N2

This paper reports an experimental study on the effects of O2 concentration and CO2 concen-tration on soot volume fraction and primary particle size of flame centre in ethylene laminar co -diffusion flame. The incandescent light was induced by a two-color band method and laser -excited by wavelength 1064 nm. Previous studies have shown that the oxygen concentration affects the soot volume fraction in the flame. However, the effects of O2 concentration on the primary particle size under CO2 atmosphere are unclear and need to be further discussed. Seven flames were optically detected, in which the oxygen concentration of the accompanying gas varied from 21% to 50% by volume. One primary flame was burned in co-flowing ethylene/air. Six ethylene flames were burned in an oxygen-rich O2/N2 or O2/CO2 atmosphere with an oxygen mole fraction from 30% to 50%. The C2H4 flow rate remained constant in all operating condi-tions. The results show that the SVF observed by the LII method and spectroscopy in the oxygen -enriched flame are in good agreement. Furthermore, the soot's primary particle diameter measured by the time-resolved LII method is in good agreement with the transmission electron microscope image analysis. With the increase of oxygen concentration, the flame height becomes shorter, the luminosity becomes brighter, and the soot yield increases. Compared with an N2 atmosphere with the same oxygen concentration, the soot yield in a CO2 atmosphere was significantly inhibited. In all the flames studied, the SVF along the flame axis showed a similar distribution: it first increased to the maximum value and then decreased rapidly until the flame tip. The maximum SVF on the axis appears between 0.64 and 0.78 based on the normalized height of the visible flame. In all ethylene/(O2/N2) flames, the maximum primary particle diameter and average primary particle diameter increase with oxygen content. For ethylene/(O2/CO2) flame, the maximum diameter of primary particles decreases with the increased oxygen content. The uncertainty of primary diameter is mainly caused by the thermal adaptation coefficient, the lower LII signal of smaller particles and the uncertainty of local flame temperature.