## LSU Historical Dissertations and Theses

1997

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Mechanical Engineering

Tryfon T. Charalampopoulos

#### Abstract

Accurate determination of the morphology and dynamics of aerosols consisting of agglomerated sub-micron sized particulates is needed for soot formation studies, process monitoring in flame-based materials synthesis applications, and pollution control studies. The objective of the current study is to investigate the application of dynamic light scattering (DLS) to flames as an extension of current measurement technologies. Unlike previous DLS flame studies, conventional polarized DLS is supplemented with depolarized DLS measurements to account for the agglomerates' anisotropy. Because unavailable relationships between the agglomerates' morphological parameters and the Brownian translational and rotational diffusion coefficients under non-continuum conditions are required for accurately determining agglomerate morphology from DLS flame measurements, the potential of combining DLS-determined diffusion coefficients with independently-determined morphology measurements to provide the necessary experimental relationships is investigated. For this study DLS measurements and thermophoretic agglomerate sampling were performed on chainlike iron oxide agglomerates occurring within a 1/2$\sp{\prime\prime}$ diameter Fe(CO)$\sb5$-seeded CO/O$\sb2$ diffusion flame. Systematic detection system errors in the DLS measurements, caused by dead time and afterpulsing, were virtually eliminated by cross-correlating the outputs of two separate photomultiplier tubes. The accompanying decrease in the signal-to-noise ratio was compensated by increasing the incident laser power and experiment duration time. This duration time increase was accomplished by combining the data from multiple five-minute experiments because of severe iron oxide deposition on the burner and flame stabilizer occurring with longtime, continuous flame operation. Accurate depolarized DLS data was thus generated from a flame environment for the first time. Inverse Laplace transform analyses of the DLS data using CONTIN produced unacceptable fits of the data. Cumulants fits were more accurate but systematically produced negative second cumulant coefficients, which prevent a complete linewidth distribution analysis and indicate the presence of invalid assumptions used in the conventional theoretical DLS interpretations. Additional experiments implicated incident beam nonuniformities as a partial cause of this effect. Average diffusion coefficients extracted from the first cumulants exhibited an order-of-magnitude agreement with theoretical estimates based on ex situ morphology and flame temperature measurements.

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