Experimental and numerical study of soot formation and evolution in co-flow laminar partially premixed flames

In this work, a set of four partially premixed ethylene-air laminar flames, characterized by different equivalent ratios, was studied. The aim was to investigate the effect of partial premixing on soot formation, which is a relevant parameter for real combustion devices. Quantitative and qualitative information on soot particles were obtained using a modified version of the thermophoretic particle densitometry (TPD) method. Particle thermal emissivity values were measured from the transient thermocouple temperature response. Results show that the emissivity of flame-formed carbon nanoparticles ranges from about 0.5 for soot precursor particles up to 0.95, i.e., the typical emissivity value attributed to mature soot particles. Particle volume fraction was then accurately evaluated by means of the TPD method, using the values of particle emissivity previously measured. The changing of particle features throughout the flame, evidenced by the variation in thermal emissivity, was further investigated using Raman spectroscopy. The analysis of Raman spectra showed that the increase in thermal emissivity follows a reduction in the hydrogen-to-carbon ratio of the investigated soot nanoparticles. Measurements performed with the TPD method also evidence that both particle emissivity and volume fraction are strongly affected by oxidation, which prevents to accurately measure soot concentration in oxidative regions of the flames. An attempt to include soot oxidation in the TPD method for a correct evaluation of volume fraction procedure is also illustrated. OH concentration profiles, obtained from a modeling analysis and validated by radical chemiluminescence measurements, were used in the TPD method to correct particle concentration due to oxidation.