Environmental Engineering Reference
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Fig. 11 Selected photographs from the burning history of a D o = 2.78 mm n-octane droplet
showing backlighted images (a) and development of the sooting dynamics, and
fl
flame-illuminated
images (b) showing development of the
fl
flame (Liu et al. 2014b )
of backlighting. Figure 11 displays backlighted images of an unsupported large
n-octane droplet (D o = 2.78 mm Liu et al. 2014a ) from ISS experimentation, as well
as a
flame-illuminated sequence (Fig. 11 b) of the same droplet (i.e., no backlighting
is used). The soot shell is again clearly visible as are individual aggregates that
formed.
When developing a surrogate, qualitative comparisons of the flame luminosity
can provide a clue about the ef
fl
ame
luminosity for several small hydrocarbon droplets considered as surrogate con-
stituents for an octane 87 gasoline (heptane, octane, toluene, and iso-octane).
The images are arranged according to their
cacy of the surrogate. Figure 12 compares the
fl
(i.e., the sensitivity of
the eye to wavelengths in the visible region of the electromagnetic spectrum due to
oxidation and incandescence of soot aggregates that are transported to the
'
brightness
'
ame).
The image brightness qualitatively correlates with the amount of soot formed, being
greater with increasing soot formation.
Based on a qualitative assessment of the image brightness in Fig. 12 , the sooting
propensities (highest to lowest) consistent with Fig. 6 would be toluene > heptane/
toluene > gasoline > iso-octane > heptane/iso-octane > heptane. The backlighted
images show the intensity of the soot shell and are consistent with this approximate
ordering. It could be concluded that if matching sooting propensity were an
important consideration in developing a surrogate for gasoline, neither heptane, iso-
octane, or their mixtures would work. Toluene, on the other hand, has a noticeably
brighter
fl
flame than gasoline. As such, one could envision that blending toluene with
heptane or iso-octane would produce a feasible surrogate for gasoline, at least in
terms of sooting propensities.
fl
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