Study On The Morphology And Force Analysis Of Soot Particles In A CH₄/Air Diffusion Flame

Because the soot formation mechanism can provide theoretical and technical support for reducing particulate emissions,it has presently received more attention.In order to study the evolution of soot in the diffusion flame and improve the cognition of soot formation mechanism,a methane / air diffusion flame combustion system was established in this study.Soot was collected from the flame at different heights above the burner（HAB）using thermophoretic sampling particle diagnostic（TSPD）system.Besides,the morphology and force analysis of soot in different HABs were studied using atomic force microscopy（AFM）.The main research contents and conclusions of this thesis are listed as follows:Through analyzing AFM photographs,the soot particles sampled in the used HAB can represent the soot formation process: growth,coagulation,aggregation and oxidation.The 3D morphology for the soot particles formed in the methane/air flame was investigated by AFM.The population-averaged equivalent volume diameter of single soot particles increases from 8.72 to 11.36 nm with an increase in the HAB from 30 to 45 mm,and then decreases to 9.41 nm in the subsequent combustion.Single soot particles spread out on the mica substrate and possess a low sphericity ratio（SR）in the range of 0.01-0.03,indicating that these soot particles are liquid-like and far from being fully carbonized.The SR increases along with the increase in the equivalent volume diameter,especially at higher HAB.This phenomenon suggests that the graphitic degree of particles is related to the particle size and the HAB.The roundness and compactness of the soot particles ranges from 0.464-0.484 and 0.671-0.694,respectively.The roundness increases firstly and then decreases,while the compactness shows an increasing trend with the increase in HAB.The average attractive force(AF_at)between soot particles and probe tip is in the range of 2.22-2.65 nN,and shows a decreasing trend with the increase in HAB.Van der Waals forces between soot particles and probe tip ranges from 1.89 to 2.18 nN.The attractive force is dominated by the Van der Waals force.The ratios of the Van der Waals force calculated to the average attractive force at different HABs are 0.71-0.90.The average adhesive force(AF_ad)increases from 12.1 to 17.3 nN with the increase in the HAB,and is approximately 4-6 times larger than the AF_at.Similar to the AF_ad,the average adhesion energy also shows an increase as the HAB increases,with a minimum of 1.11×10^-16 J observed at HAB = 30 mm,and a maximum of 2.11×10^-16 J observed at HAB =60 mm.