Study On The Morphological And Nanostructural Characterization And Agglomeration Forces Of Soot Particles Formed In Premixed Flames

Soot formation mechanism is one of the important research directions in the areaof combustion science. Laminar premixed flames are the preferred laboratory systemsto study the kinetics of soot formation. The focus of this dissertation is on theformation history and properties of soot in premixed CH4/O2/N2flames. The majorresearch work and results of this dissertation are listed as follows:1. Combustion analysis system has been developed to investigate flame-formedsoot in laminar premixed flames, which consists of a commercial McKenna planepremixed flame burner, flame temperature measurement system, mixture supplysystem, and a thermophoretic sampling particle diagnostic(TSPD) system. Thissystem can produce stable laminar premixed flames with precisely controled fuel/airequivalence ratio and flame temperatures. Moreover, soot particles in different heightabove the burner also can be precisely sampled by the TSPD system for the study ofsoot formation history.2. Transmission electron microscopy(TEM) and atomic force microscopy(AFM)are used for imaging the particles in flames sampled by the TSPD system. Resultsshow that three types of soot morphology are found, condensed soot with a typicalsize of～17nm, nascent soot with a typical size of～8nm, mature soot with atypical size of10～30nm.3. The effects of flame temperature and fuel/air equivalence ratio on morphologyand nanostructure of soot are studied. The diameter of primary particles measuredfrom TEM graphs is in a range of5.7～31.4nm and show a unimodal distributionwith peaking at15～20nm. The volume-equivalent-diameter (ED) of primaryparticles measured by AFM in a range of1.6～32.8nm, and the aspect ratio ofprimary particles ranges from0.006to0.48. The ED distribution changes fromunimodal shape to bimodal shape as flame temperature or fuel/air equivalence ratioincreases.4. The fringe length, fringe separation distance and tortuosity for most carbonlattice in soot are0.41～5.85nm,0.32～0.44nm and1～2，respectively. The fractaldimension of soot aggregates ranges from1.57to2.01. An increase in flametemperature results in a decrease in the diameter, mean tortuosity, separation distance of primary particles and the fractal dimension of soot agglomerates but an increase inthe mean fringe length of primary particles. An increase in fuel/air equivalence ratioresults in a decrease in the mean tortuosity, separation distance of primary particlesand the fractal dimension of soot agglomerates but an increase in the diameter, meanfringe length of primary particles. The results mentioned above suggest that the sootevolves toward a more orientational ordering and organization, more graphiticstructure and lower oxidative activity with increasing the flame temperature or fuel/airequivalence ratio.5. The agglomeration forces and coagulation model of soot particles are discussed.The force between soot and AFM tip are measured by AFM. The pull–off force is3.03±0.24nN, which is consistent with the calculated van der Waals force. This indicatesthat van der Waals force is one of the predominant contributors to the agglomerationof soot particles.