Experimental Investigations On Soot Nucleation And Growth In Laminar Premixed Burner-stabilized Stagnation Flames

Soot,the byproducts of incomplete combustion of hydrocarbon fuels,has severe negative impacts on human health and environment.To reduce soot emissions,it is essential to understand the detailed mechanism of soot formation for building soot models to be used in the design of combustion devices.With complex flow and boundary conditions,practical combustors are often not suitable for the fundamental studies of soot formation.Instead,a laminar premixed burner-stabilized stagnation flame,with a simple flame structure and a well-defined boundary condition,has been chosen in this work to study the soot formation mechanism.Systematical studies have been carried out to explore the evolution of the dynamics,morphology,and structure of nascent soot particles by means of various experimental methods.In the meanwhile,combustion kinetic modeling of the flame chemistry was performed to understand the experimental observations by analyzing the flame structure and gas-phase species profiles.Firstly,to study the characteristics of soot nucleation,a diethylene glycol condensation particle counter was adopted to measure soot particle size distributions down to 1 nm.The soot nucleation peak,the smallest soot particle size,as well as the relationships between the peak diameter and the absolute number density of particles were studied.The results indicated the minimum diameter of the incipient soot particles appeared at¹.54 nm,and the absolute number density of particles no bigger than the first peak diameter was found to be positively related to the first peak diameter and the geometric mean diameter of these particles.Furthermore,sooting limits of different fuel structures were studied quantitatively by combining visual observations with direct measurements of soot particle size distributions.The?_c-T_maxcurves of the sooting limits present a U-shape for all studied flames,indicating the flame temperature has a strong effect on sooting limits.At a fixed temperature,sooting propensity generally increases with increasing fuel carbon/hydrogen atom ratio.Secondly,to elucidate the effects of fuel structure and flame temperature on soot nucleation,growth and structural characteristics of soot aggregates,typical straight-chain alkanes and aromatics were selected to study their soot formation characteristics.The effects of fuel structure on gas-phase chemistry,dynamics and morphological evolutionof nascent soot particles were investigated at the same residence time and under similar flame temperatures,respectively.The results show that in straight-chain alkane flames,soot nucleation rate is slower,the structure of aggregates is less compact,and the primary particles are bigger due to faster surface growth as indicated by the higher concentration of acetylene.By contrast,in the flames with addition of aromatic fuels,soot nucleation is faster and occurs at lower flame heights because of the higer concentrations of aromatics such as benzene and pyrene,but the primary particle size of soot aggregates was significantly reduced due to slower soot surface growth.Finally,for a better understanding of the exhaust gas recirculation on soot production,the effect of CO₂ addition on the dynamic of soot formation was investigated.To explain the experimental observations,numerical simulations were carried out to analyze the thermal and chemical effects of CO₂ addition on the precursor chemistry.In short,the nucleation and growth of soot were deeply explored experimentally.The findings of these investigations have deepened our understanding of soot formation mechanisms.In addition,the experimental data are useful and suitable for developing and validating soot models.