Study On The Morphology And Mechanical Properties Of Combustion-generated Soot Particles

Because soot formation and evolution consist of many complex and concurrent physical and chemical processes that all finish within a few milliseconds,the mechanism of soot formation is not yet understood completely.In this dissertation,the soot particles generated from diesel engine,CH₄/Air premixed flame and CH₄/Air diffusion flame are sampled by total cylinder sampling system and thermophoresis sampling system.The 3D morphology and mechanical properties of these particles during the combustion processes are studied by atomic force microscope.The present work is helpful to deeply understand the mechanism of formation and evolution of soot particles,and provide the theoretical guidance to reduce the soot emission.In diesel engine and CH₄/Air premixed and diffusion flames,the peak for the distributions of equivalent diameters?ED?of the particles are found to vary in the range of 2.5-4.5 nm.After an initial increase,the averaged equivalent diameter?AED?of isolated particles decreases as the combustion proceeds.For the isolated particles formed in the CH₄/Air premixed and diffusion flames,the sphericity ratio?SR?increases as an increase of ED,indicating that the carbonization degree of soot particles is related to the ED in this case.By contrast,this phenomenon is not discovered for the diesel particles,which is probabaly attributed to the fact that diesel particles at various stages of formation and growth coexist in each combustion phase due to the heterogeneous nature of diesel combustion.With the aid of AFM,which has the detected limitation of less than 1nm in size,the nanoparticles with the ED < 7 nm and the SR < 0.02 are found to exist throughout the combustion process for the three combustion systems as mentioned above.The 3D images are significantly different from the corresponding amplitude and phase images in the initial combustion phase,while this differentiation reduces as the combustion goes on.For the CH₄/Air premixed flame,the particles at low height above the burner?HAB?exhibits as single ones,and the aggregates appear at HAB ? 8 mm,suggesting the agglomeration occurrence after the formation of spherical primary particles as suggested by Lahaye.For the CH₄/Air diffusion flame and the diesel engine,however,the aggregation takes place prior to the complete carbonization of particles because aggregates are observed at the start of combustion,which follows the mechanism proposed by Reilly.Throughout the combustion process,diesel in-cylinder particles show three types of force curves,corresponding to the carbonized soot particles,nascent soot particles and unburned fuel respectively.By contrast,two types of force curves are observed for the soot particles generated from the CH₄/Air premixed and diffusion flames,which are assigned to the carbonized soot particles and nascent soot particles respectively.Due to the presence of plastic deformation for the nascent soot particles,the force study only addresses the carbonized primary soot particles.As the combustion proceeds,the averaged attractive force(AF_at)of particles formed in diesel engine and the CH₄/Air diffusion flame gradually decreases,while the AF_at of particles in the CH₄/Air premixed flame shows an increasing trend.The calculated Van der Waals force?Fvdw?accounts for more than 60 % of the AF_at in the diesel engine and CH₄/Air diffusion flame,and more than 75 % in the CH₄/Air premixed flame,indicating that the Fvdw is the largest contributor to the F_at.For these three combustion systems,the averaged adhesive force?AFad?of particles decreases as the combustion goes on.Because the averaged adhesion energy?AWad?is three orders of magnitude larger than the thermal kinetic energy,the particles are likely to stick together once they collide.Although the F_at,Fad and Wad measured are those between the soot particles and the AFM probe tip rather than between soot particles,they can be used to qualitatively characterize the interaction between soot particles to some extent because the same tip is used for all measurements and no significant differences in the cantilever elastic constant during the measurement process.The Young's modulus?EY?of soot particles obtained is much lower than that of graphene,and the AEY has a negative correlation with the separation distance of graphene layers in the soot structure.These findings show that EY of soot particles may be derived from the bonding strength between the graphene layers rather than the covalent bonding of graphene layer.