Experimental Investigation Of Characteristics Of Soot Formation In Premixed Ethylene And Propene Flames

Soot models of engine combustion are important tools for the design and development of clean and high-efficiency engines.The researches of soot formation characteristics for the small molecule hydrocarbons(e.g.,ethylene and propene)are considered to be an important basises for the fundamental understanding of soot formation mechanisms of hydrocarbon fuels and for the development of soot models of engine combustion.The development and validation of soot models requires accurate basic experimental data of soot formation processes.At present,the BSS(Burner Stabilized Stagnation)method is an internationally recognized standard method for the measurements of soot formation basic experimental data such as soot particle size distribution,number density and volume fraction in premixed flames.In this study,based on the establishment of standard test specification for BSS method,soot nucleation and grow processes in premixed ethylene and propene flames were investigated.The important characteristic parameters such as soot particle size distribution,number density and volume fraction were systematically measured.The main research work includes:(1)The standard test specification for BSS method was established.The effects of various factors,such as the size and material of burner and porous plug,the stagnation surface temperature and the velocity of shroud gas,on the experimental measurements of soot formation characteristics(e.g.,soot particle size distribution,number density and volume fraction)in premixed flames were systematically analyzed with BSS method.Based on the standard test specification for BSS method,the experimental results of soot particle size distribution,number density and volume fraction measured by different researchers at different experimental apparatuses are in good agreement with each other.(2)Based on the standard test specification,the effects of flame conditions(e.g.,flame temperature and equivalence ratio)on the soot formation characteristics were systematically investigated in premixed ethylene flames with BSS method.The result shows that soot particle size distribution is observed to be bimodal in premixed ethylene flames.The bimodality is composed of nucleation mode particles at small sizes and coagulation mode particles at larger sizes.The concentrations of nucleation mode particles are observed to be high,indicating that persistent nucleation processes exsit at the later stage of soot growth even in the heavily sooting flames(?= 2.5).In addition,soot particle size distribution,number density and volume fraction are highly sensitive to the equivalence ratio and flame temperature.Soot formation processes are limited by lower soot nucleation rate at lower flame temperatures and by the reversibility of soot nucleation processes(thermal decomposition of soot precursors)at higher flame temperatures.This limitation is evident in the lightly sooting flames(?= 1.8)but gets weaker in the heavily sooting flames(?= 2.5).(3)The impacts of flame conditions(e.g.,flame temperature and equivalence ratio)on the soot formation characteristics were systematically investigated in premixed propene flames with BSS method.The bimodality of soot particle size distribution and the sensitivity of soot particle size distribution,number density and volume fraction to equivalence ratio and flame temperature are observed in premixed propene flames.The effect of the reversibility of soot nucleation processes on soot nucleation and growth processes is highly significant in the lightly sooting propene flames(?= 1.8)but becomes almost negligible in the heavily sooting propene flames(?= 2.07).(4)The comparison of soot formation characteristics between premixed ethylene and propene flames is made.The result shows that soot nucleates and grows in premixed propene flames faster than in premixed ethylene flames,and that the maximum soot volume fraction in premixed propene flames is significantly higher than in premixed ethylene flames.Soot formation processes in both ethylene and propene flames are impacted by the reversibility of soot nucleation processes.However,the degree of impact varies a lot.Especially at the equivalence ratio of 2.07,the maximum soot volume fraction significantly decreases in ethylene flames but varies a little in propene flames as the flame temperature increases.The results mentioned above indicate that bonding way between carbon atoms in parent fuels has significant effect on sooting processes in premixed flames.The experimental measurements of soot formation characteristics in premixed ethylene and propene flames made in this study offer systematic canonical datas useful for the validation of soot formation mechanism/models.