Study On The Fragmentation Behaviors Of Diesel Soot Particles In A Lean Flame

The fragmentation of soot particles consistently takes place throughout the combustion and emission processes of diesel engines.Therefore,the related researches are of important theoretical and practical significance.Due to the complexity of combustion process in diesel engines,it is difficult to study the impact of a single factor on the soot fragmentation behavior.This paper employed the McKenna premixed flame as the test bench due to its flexible control on the experimental conditions.The flame temperature was modified by adding the inert gases with different thermal diffusion rates(i.e.,He and Ar)into methane and oxygen gases.Diesel soot particles was homogeneously dispersed into the lean premixed flame without soot generation through an aerosol generator,and thereby the formation and oxidation of soot were separated out.In this way,the behaviors of the induced-oxidation fragmentation of soot particles can be studied.The soot was collected using thermo-sampling and capillary sampling systems,and the obtained soot samples were characterized with field emission transmission electron microscope,particle size spectrometer,Raman spectrometer and thermogravimetric analyzer.The main research contents and results are as follows:In the lean flames with He and Ar gases,the soot particle number concentrations as a function of particle size show the trend of “unimodal distribution→distribution bimodal→bimodal disappearance”,as the height above burner(HAB)increases.The peak particle sizes of the nuclear particle number concentration lie between 6.98 and16.5 nm.The nuclear particle number concentrations increase at HAB = 4 mm for the He flame and at HAB = 6 mm for the Ar flame,indicating that soot particles break up at these flame heights.As the HAB further increases,the nuclear particle number concentrations show a decrease after an initial increase,and reach the peak value at HAB = 14 mm.The decrease in flame temperature results in the soot fragmentation occurring earlier,and prolongs the time when the fragmentation takes place.As a consequence,the nuclear particles produced from fragmentation increase the number concentration.This phenomenon indicates that low flame temperature can promote soot fragmentation.Based on the image analysis technology,a calculation method for the diffusion path of oxygen into the interior of soot particles was developed in order to evaluate the effect of internal oxidation on soot fragmentation.With the increase in HAB,the effective factors for evaluating the possibility of soot internal oxidation show an increase,suggesting that oxygen more readily diffuses into the interior of the primary particles and thus improves the possibility of soot fragmentation.After fragmentation,soot particles show decreases in the fringe length and the degree of graphitization and increases in the fringe separation and tortuosity.As a result,soot structure become more disordered.In addition,the apparent activation energy of soot is reduced and the oxidation activity is enhanced,which are beneficial to the soot oxidation elimination.