Effect of Flow Unsteadiness on Soot Morphology

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Title: Effect of Flow Unsteadiness on Soot Morphology
Author: Xiao, Jidong
Advisors: Dr. william Roberts, Committee Chair
Abstract: The morphology of combustion-generated soot is of fundamental interest, but is difficult to measure. A new planar diagnostic technique was developed for morphology measurements in simple laminar flames. Additionally, Polycyclic Aromatic Hydrocarbons, (PAH) the precursors to soot formation, were measured non-intrusively in an unsteady laminar flame to determine the characteristic time scales of their formation. An existing point-wise laser scattering technique to determine soot morphology was extended to a planar technique and applied to a laminar ethylene air co-flow flame. An Nd:YAG laser (532 nm) was used to illuminate the soot particles and the resulting scattered light was collected over a wide range of angles. A low power laser was used to eliminate fluorescence and incandescence interferences. Simultaneous laser induced incandescence measurements were used to determine the soot volume fraction. Six morphological parameters, including primary spherule size and mass fractal dimension, were derived using this new planar measurement technique based on RDG/PFA theory. The results clearly show the soot formation processes, including inception, agglomeration, and oxidization. A two-angle approximate method was also demonstrated and shown to be applicable for instantaneous measurements in unsteady flames. The approximate method can provide limited soot morphology information including primary particle size and number of primary particles per aggregate, when the fractal dimension and distribution are assumed. The approximate method was not suitable over the entire region of this flame because the fractal dimension varies significantly from the assumed value of 1.8, particularly in the soot inception zone. Three different size classes of PAH were measured qualitatively via laser induced fluorescence by detecting fluorescence in different spectral regions. The relative concentrations of these PAH were measured in counterflow diffusion flames subjected to both steady and oscillating strain. With increasing strain rate, the PAH concentration was observed to decrease dramatically, though unequally, among the three size classes measured. With an unsteady strain rate, the PAH concentration oscillates in phase for frequencies below 50 Hz. At frequencies above 100 Hz, the PAH zone no longer responds to the imposed oscillations. Smaller PAH continue to respond to the instantaneous strain rate at a higher frequency than larger PAH.
Date: 2005-04-07
Degree: PhD
Discipline: Mechanical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/5040

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