| The formation process of soot particles emitted by conventional fuel-powered engines is quite complicated and many problems have not yet been scientifically explained.The evolution of soot morphology and nanostructure during the combustion of single and multiple hydrocarbon fuels has not been studied in detail.In this paper,based on the basic combustion means,the formation characteristics of soot particles in laminar flame were studied in order to further understand the soot evolution process.First of all,the morphology and nanostructure of soot particles in laminar diffusion flame has been investigated in this work.The fuels considered were diesel surrogate fuels,including n-heptane,n-butanol and the volumetric mixture 50% n-heptane/50% n-butanol(H50B50).Soot particles were sampled by thermophoresis probe method and observed by transmission electron microscope(TEM).The TEM images were processed by image processing software and self-developed software SootFringe to obtain the morphology and nanostructure parameters of soot.The primary particle diameter shows a trend of increasing first and then decreasing due to soot growth and oxidation respectively.The fractal dimension of aggregates almost keeps increasing along the flame centerline.In addition,n-butanol addition can effectively reduce the number and size of soot particles,having significant effect on reducing soot emissions.In the process of soot growth,the fringes become shorter and less curved,meanwhile inter-fringe spacing increases due to electron repulsion between the adjacent carbon layers.Subsequently,the degree of graphitization would increase,concretely,fringe length increases,while tortuosity and inter-fringe spacing decrease.At the flame tip,most microcrystalline carbon layers in the outer shell are oxidized,which causes a sharp increase in the tortuosity.In addition,for mature soot near the flame tips,particles in n-butanol flame has the maximum fringe length and tortuosity while the minimum inter-fringe spacing.Secondly,based on the same experimental and analytical methods,the evolution and differences of the soot morphology and nanostructures in diesel and the volumetric mixture 50%diesel/50% n-butanol(D50B50)were also studied.The results showed that the evolution trends of primary particle size and aggregates fractal dimension are almost the same as those of the n-heptane flame.In terms of reducing the generation of soot,the effect of n-butanol addition on diesel flame is more remarkable than on n-heptane flame.In the aspect of soot nanostructure,statistics shows that with the increase of flame height in diesel flame,the fringe length gradually decreases and the tortuosity and inter-fringe spacing.Moreover,the nanostructure parameters in different radius ranges of single primary soot particle were statistically analyzed.With the distance from the core increasing,the fringe length increases monotonically.The tortuosity firstly increases and then decreases.And the inter-fringe spacing decreases monotonically.Finally,based on Laser Induced Incandescent(LII)system,the soot concentration field of n-heptane/n-butanol blends and diesel/n-butanol blends flames were measured and the distribution characteristics of the soot volume fraction in axial direction and radial direction of the flames were quantitatively analyzed.In axial direction of the flame,the soot volume fraction firstly increases and then decreases,and the space span of soot produced in the axial direction decreases with the addition of n-butanol.The curve of the volume fraction of soot in the flame radial direction gradually changes from bimodal distribution to unimodal distribution as the height increases,and the n-butanol addition can reduce the height of the turning point. |
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