10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Particle Formation in Combustion Environments: Importance of Charge Distributions on Evolution of Aerosol Size Distributions
YANG WANG, Girish Sharma, Michel Attoui, Pratim Biswas, Washington University in St Louis
Abstract Number: 428 Working Group: Combustion-Generated Aerosols: the Desirable and Undesirable
Abstract Combustion systems result in the production of particles both inadvertently and intentionally. The evolution of the size distribution is an important factor in both cases - either for the control of emissions of particles or for the production of novel materials. Charging in flames significantly affects the properties of the particles produced because of its influence in almost all stages of particle formation. In situ measurements with enhanced particle detectors in a premixed flat flame showed that up to 95% of the flame-generated sub-3 nm particles were charged at a sampling height of 5 mm above the burner, indicating the existence of a strong ionization process. The flame-generated ions play an important role during particle inception, as shown by the attachment of ionic species on the particles. Hence, the charging process and the resultant aerosol charge distributions should be considered in aerosol dynamics modeling to accurately predict the behavior of the high-temperature aerosol system. First, this is contrary to reports that most particles below a certain size (~ 5 nm) are not charged. Second, existing models do not consider particle charging, which may lead to a bias in the calculated size distributions.
In this study, Fuchs’ charging theory was coupled with aerosol dynamics models to investigate the influence of charging on the evolution of aerosol size distributions in combustion environments. A monodisperse particle growth model and a model using the method of moments were developed and compared in simulating the simultaneous particle charging and coagulation at high temperatures. In order to quantify the charging characteristics of nanoparticles, a high-resolution DMA was used to measure the mobilities of ions generated from a premixed flat flame operated at various conditions. The effect of temperature on ion-particle and particle-particle combination coefficients was further examined. The proposed models showed that the influence of charging on particle growth dynamics was more prominent when the ion concentration was comparable to or higher than the particle concentrations, a condition which may be encountered in flame synthesis and solid fuel-burning. Simulated results also showed that unipolar ion environments strongly suppressed the coagulation of particles. In the end, a simplified analysis of the relative importance of particle charging and coagulation was proposed by comparing the characteristic time scales of these two mechanisms.