10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Morphology and Mobility Diameter of Carbonaceous Aerosols during Agglomeration and Surface Growth
EIRINI GOUDELI, Georgios Kelesidis, Sotiris E. Pratsinis, University of Minnesota
Abstract Number: 1058 Working Group: Aerosol Physics
Abstract Combustion is essential to the manufacture of carbon black, fumed oxides, optical fibers and high-value products, like carbon nanotubes. Carbon black is the largest flame-made nanostructured material by volume and value. Similarly to soot, a major air pollutant, carbon black particles have irregular, fractal-like structure, consisting of chemically-bonded and polydisperse primary particles. The morphology of carbon black determines its end-use and performance in nanocomposites (e.g. tires) or battery electrodes. The mobility size estimation of such evolving fractal-like aggregates is not trivial, as it depends on their structure as well as the number and size of their constituent primary particles. Scaling laws describing the mobility size of agglomerates consisting of monodisperse primary particles in point contact have been developed in the free molecular, transition and continuum regimes. However, these models overestimate up to 37% the mobility diameter of mature soot aggregates as they neglect primary particle polydispersity.
Here nascent soot dynamics, after nucleation or inception, are investigated by Discrete Element Modeling (DEM) from free molecular to transition regime. During nascent soot formation, polydisperse spheres and aggregates are formed by agglomeration and surface growth, attaining an asymptotic geometric standard deviation of 1.2 ± 0.01, in agreement with experiments in premixed, diffusion flames and wood combustion. When surface growth stops, mature soot grows by agglomeration of these polydisperse spheres and aggregates before oxidation and condensation take over. The evolution of nascent soot structure from spheres to aggregates is quantified by the mass fractal dimension and mass–mobility exponent, in excellent agreement with microscopic and mass–mobility measurements in a standard burner-stabilized stagnation ethylene flame. Based on aggregate projected area, a scaling law is derived for determining the primary particle size of nascent soot aggregates from mass–mobility measurements rather than tedious image counting. Furthermore, easy-to-use relationships between the mobility diameter and the average number of primary particles per agglomerate as well as the relative effective density of soot are proposed.