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Laser Light Scattering and Fluorescence Diagnostics of Rich Flames Produced by Gaseous and Liquid Fuels

  • A. D’Alessio

Abstract

Rich combustion systems present a much more complex chemistry than stoichiometric or lean ones: compounds from unsaturated C2 hydrocarbons to polynuclear aromatic hydrocarbons (PAH) are formed and destroyed, soot particles nucleate, grow and agglomerate and eventually burn out at different stages of the combustion. Furthermore, the fuel is introduced in the form of liquid droplets in many practical systems.

The present paper presents initially a short summary of the scattering and extinction properties of spherical particles, from the Rayleigh regime to the ray optics limit (geometric regime), referring particularly to the simultaneous presence of soot particles and fuel droplets.

Problems connected with polydispersion of spheres and the presence of particles of different shapes (spheroids, clusters, chains, etc.) are also briefly discussed.

The paper reviews subsequently the u.v./visible absorption and fluorescence spectra of some PAH compounds and discusses their interferences with the extinction due to soot particles and/or droplets and the spontaneous Raman effects due to low molecular mass compounds. The experimental section presents initially some laser light scattering (LLS) and extinction measurements carried out on large diesel oil spray flames. A procedure for distinguishing between fuel droplets and soot particles is illustrated, and average size and number concentration of soot particles are evaluated. The effects of swirl on the spatial distribution of soot particles and fuel droplets for these types of flames are also shown. Optical results obtained on premixed and diffusion CH4O2 flames are compared in the next section: LLS measurements provide evidence for zones of early formation of soot particles while u.v. absorption and fluorescence spectra are related to the build-up of PAH compounds.

The final part of the paper compares the experimental results with the theoretical models and discusses agreement and limits; suggestions are advanced both for more refined theoretical approaches and for more sophisticated and systematic experiments.

Keywords

Absorption Cross Section Soot Particle Diffusion Flame Premix Flame Complex Refractive Index 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

List of Symbols

C

cross section [cm2 sr−1]

Cλ

monochromatic cross section [cm sr−1]

D

particle diameter [Å], [nm], [m]

E

efficiency: 4C/πD2

f (α)

normalized size density distribution of soot

f (D)

size density distribution of droplets

I (υ,α, m)

scattering function: 4πQ/λ 2

Io

incident light intensity [W cm−2]

k

imaginary part of refractive index

m

complex refractive index

n

real part of refractive index

next

dispersion coefficient, d lnEext/dlnλ

p (υ, m)

phase function: 4πC(υ, m)/Cscatt

Q

scattering or fluorescence coefficient [cm−2 sr−1]

Qλ

monochromatic scattering or fluorescence coefficient [cm−2 sr−1]

α

size parameter. πD/λ

αm

mean size parameter

δ

polarization ratio: QHH/QVV

Δ

asymmetry ratio: QVV1)/QVV2)

ΔV

scattering volume [cm3]

ΔΩ

detection solid angle [sr]

λ

light wavelength [nm]

Λ

wavelength ratio: QVV11)/QVV2)

σ

standard deviation

abs

absorption

ext

extinction

f

fluorescence

H

horizontally polarized light

scatt

cattering

V

vertically polarized light

el

elastic

fl

fluorescence

d

droplets

g

gas

s

soot

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Copyright information

© Springer Science+Business Media New York 1981

Authors and Affiliations

  • A. D’Alessio
    • 1
  1. 1.University of NaplesNaplesItaly

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