Dissertation/Thesis Abstract

Experimental investigation of soot nucleation and growth in premixed flat flames
by Abid, Aamir Dawood, Ph.D., University of Southern California, 2009, 209; 3355374
Abstract (Summary)

Soot formed from incomplete combustion has significant health effects and contributes to atmospheric pollution. The details of soot nucleation and growth have been studied experimentally and numerically and recent advances in experimental techniques have enabled the investigation of soot particles in the size range of 1 to 10 nm. This range is particularly interesting as soot nucleation occurs at the lower end of this range and mass growth is fast within this range.

This work utilizes a variety of complementary experimental techniques to probe soot nucleation and growth from premixed lightly sooting flat flames. A scanning mobility particle sizer (SMPS) in conjunction with probe sampling is used to study detailed particle size distributions, soot volume fraction and number density. Soot volume fraction is also measured using a thermocouple particle densitometry technique. Soot morphology is investigated using transmission electron microscopy and atomic force microscopy. For the ethylene-oxygen-argon flames studied, the particle size distributions are bimodal. The trough of this bimodal distribution shifts gradually to smaller sizes as the flame temperature is increased resulting in an apparent unimodal size distribution. Morphology studies show that nascent particles are spherical and not fully carbonized.

A standard SMPS system is modified to extend the lower size detection limit from 2.5 nm to 1.6 nm. Using this lower detection limit the particle distributions at high flame temperatures are observed to be persistently bimodal and the apparent unimodality previously seen is concluded to be due to instrumentation limitation. In addition, the ethylene flames are doped with benzene to examine the effect of nucleation rate variation on nascent soot size distribution. The size distributions for benzene doped flames are similar to that of pure ethylene flames yet further study needs to be done to decouple the influence of flame temperature resulting from benzene doping on the evolution of the size distributions.

An inherent challenge of probe sampling is that it is intrusive and the effect of probe perturbation needs to be quantified. To address this issue, a burner stabilized stagnation flow sampling technique is developed and fully characterized. The key advantage of this technique is that the flame can be modeled as the flow field and boundary conditions are known. The size distributions measured by this technique are similar to that observed with tubular probe sampling with the onset of the second size mode occurring at somewhat short residence times.

Using the new sampling technique developed, n-dodecane sooting flames are investigated. This liquid fuel is of particular interest as it is an ideal surrogate component for real jet fuels. The size distributions for these flames show a bimodal characteristic and the nucleation mode for n-dodecane flames is stronger compared to similar ethylene flames. The measured and simulated temperature profiles show good agreement. The modified sampling probe and development of a liquid fuel system makes this configuration ideal for studying soot formation for a broad range of conditions and fuels.

The nature of nano-organic carbon (transparent soot) in non-sooting rich ethylene-air flames is studied. Experiments are performed to allow nanoparticles with particle diameter around 2 nm to coagulate and deposit on larger substrate particles that can easily be detected by a standard SMPS. Coagulation of nanoparticles from sooting flames showed growth of the substrate particles yet no growth was seen for substrate particles introduced to flame gases in a non-sooting fuel-rich flame. These results cast further doubt on the existence of nanoparticles in non-sooting flames.

Indexing (document details)
Advisor: Wang, Hai
Commitee: Campbell, Charles S., Egolfopoulos, Fokion N., Phares, Denis J., Sioutas, Costas
School: University of Southern California
Department: Mechanical Engineering
School Location: United States -- California
Source: DAI-B 70/05, Dissertation Abstracts International
Subjects: Mechanical engineering, Environmental science
Keywords: Flat flames, Nucleation, Premixed flames, Probe sampling, Soot
Publication Number: 3355374
ISBN: 978-1-109-13810-8
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