| Particulates have a significant impact on global climate and human health.In the area of human activity,particulate matters come mainly from the burning of fossil fuels.Considering that the soot formation mechanism is still not mature,the experimental study of soot formation characteristics in flames of accurately controlled conditions is still the basis for soot model development and validation.The detailed particle size distribution is of great help in understanding the nucleation and growth process of soot particles(the particle size is typically at nano scale).In this paper,the particle size evolution of different fuels under certain operating conditions(flame temperature,equivalence ratio,etc.)was measured in the laminar premixed flames.The burner stabilized stagnation flame configuration,coupled with the small orifice sampling and the scanning mobility particle sizer,was applied.The objective of this paper is to investigate the effect of fuel composition and structure on soot formation characteristics.Main results and conclusions are as follows:(1)Different proportions of methane,ethane,propane and ethylene were doped into premixed propene flame with a carbon-to-oxygen ratio of 0.6 and a maximum flame temperature of 1829 K,to investigate the sooting characteristics of the small-molecular gas mixtures.A constant carbon to oxygen ratio and similar maximum flame temperatures as well as temperature-time histories were maintained.Qualitatively,the particle size distribution of the mixed fuel flame was similar to that of the propene flame,presenting a bimodal distribution,namely the coexistence of the nucleation mode and the coagulation mode.Quantitatively,it was observed that the size distribution of the propene flame was not sensitive to methane,ethane or propane addition,while the particle number density of small nucleated particles increased with the presence of ethylene.The particle growth in flame with a small amount of ethylene addition was stronger than that of either the pure propene or ethylene flame,indicating the synergistic effect between propene and ethylene on soot formation.The calculated results of gasphase products showed that this synergistic effect on soot formation could be attributed to the combination of odd-numbered carbon pathway and even-numbered carbon pathway towards aromatics formation.(2)The size distributions of soot particles from premixed n-heptane and methylcyclohexane flames were measured and compared at different particle residence times.For both flame series,the mixture’s carbon-to-oxygen mole ratio was 0.6,and the maximum flame temperature was controlled around 1835 K to investigate the fuel structure effect on soot propensity.It was observed that with the increase of particle residence time,the particle size distribution of both flames evolved from the unimodal(only the nucleation mode)to the bimodal(both nucleation and coagulation modes)distribution.The particle nucleation strength and the particle growth rate of the methylcyclohexane flame were quantitatively lower than those of the n-heptane flame.Considering the equivalence ratio difference(4.8%)between the two flames,the sooting propensity disparity between cycloalkanes and n-alkanes was not considerable under the test condition.The calculated results of the gas-phase products(acetylene,methyl,and benzene)were qualitatively consistent with the experimental results.(3)The evolution of soot particle size distribution in premixed flames of gasoline(34% aromatics by volume)and a n-heptane / toluene blend(66% n-heptane / 34% toluene by volume)was investigated.The aim was to explore the reliability of fuel blends in imitating soot formation of practical fuels.The mole ratio of carbon to oxygen in the unburned gas was kept constant at 0.6,and the maximum flame temperatures and temperature-time histories were kept similar between the two cases,so that we focused on the fuel composition effects on sooting propensity.It was observed that the particle size distribution of both gasoline and heptane / toluene flames evolved from the unimodal distribution(nucleation mode only)to the bimodal(both nucleation and coagulation modes)distribution.Compared to the heptane / toluene blend,soot formation in gasoline flame featured more persistent nucleation and much faster growth rate,which was likely due to the complex fuel composition of gasoline.It was suggested that in the aspect of soot formation characteristics,using a fuel blend to imitate the practical fuel may have some uncertainties.(4)Gasoline was distilled into the light distillate and the heavy distillate fraction.The particle size distribution in premixed flames of the two gasoline distillates was measured.The results were also compared with particles yielding from the undistilled gasoline to study the correlation and difference in soot formation tendency between the practical fuel and its distillates.It was observed that the particle size distribution from the light and heavy distillates was qualitatively similar with that of gasoline,showing typical unimodal and bimodal distribution.Quantitatively,the heavy distillate had much higher particle growth rate and soot volume fraction than the light distillate,and those of gasoline were between the two distillates.However,the particle nucleation strength in the premixed gasoline flame was higher than those in flames of either distillate and this also caused gasoline’s highest particle number emission.This demonstrated a synergistic effect on particle nucleation between different distillate fractions of the practical fuel.This synergistic effect was probably due to the diversity of fuel components after mixing,which increased the diversity of PAHs and soot formation pathways.The experimental results in this paper enriched the soot database from the combustion of different fuel components and fuel structures,which is the basis for further development and validation of soot models. |
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