Characterization of combustion generated particulate matter produced in an inverted gravity flame reactor

Abstract

Many unknowns remain in the pursuit to describe the formation of combustion generated particles from first principles. Of most importance in the following thesis is the transition from young to mature soot that is evident by the formation of a graphitic shell on the periphery of particles. An inverted gravity flame reactor (IGFR) allows for flexibility in studying the effects of residence time, temperature, and flow properties. Combustion generated particles are characterized to probe the underlying mechanisms involved in the evolution of these particles. Soot particles are characterized using experimental and computational techniques that reveal particle nanostructure and its evolution. The structure is quantified using transmission electron microscopy (TEM) followed by image processing to obtain information on the crystal properties and molecular compounds. In the third chapter, the repeated exposure of soot in the fuel-rich and the fuel-lean region is explored to find how exposure to complex flow field influences final soot properties. The recirculating flame produces soot with primary particles 2-3 times the size of a non-recirculating flame. Due to the maturity of recirculated particles, computational results, and fringe profiles, we propose a recurring surface growth by the condensation of aromatics on the surface of the particles as the reason for the increased primary particle diameter. The fourth chapter explores the role temperature and dilution play on particle nanostructure. The prevalence of a shell on the periphery of mature soot particles is confirmed for high-temperature combustion (>1700 K); at low temperatures, the isotropic nature of young soot particles is observed. The young particles contain both shorter fringes and more curved fringes, pointing to the role of curved aromatics in the formation of soot particles. Finally, several other analysis methods to characterize soot produced in the diluted flames are presented in the fifth chapter. The particles are characterized using UV-Vis absorption, Raman spectroscopy, and ROS activity. These techniques require more analysis, but all show distinct differences between soot produced in the high dilution and low dilution flames.