Developing Surrogates for Liquid Transportation Fuels: The Role of Spherically Symmetric Droplet Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 387

Fig. 14 Comparison of the
evolution of droplet diameter
of gasoline with toluene and
mixtures of toluene with
heptane (Liu et al. 2012 ). The
closest match with gasoline is
for 5 % heptane in toluene

simplest blend for a surrogate. Varying the composition is a relatively simple task
for a binary system, and optimization techniques are not needed to identify the
composition that produces the best match of combustion properties between the
surrogate and real fuel.
Heptane and iso-octane were found to have essentially the same droplet burning
characteristics for the con guration of Fig. 4 . Figure 14 shows that neither heptane

(nor iso-octane since heptane and iso-octane burn in an almost identical manner
(Liu et al. 2012 ) can replicate the evolution of droplet diameter, which is consistent
with the less luminous

fl flames of heptane and iso-octane compared to gasoline.

flames are, however, more luminous than gasoline (Fig. 12 ). As
such, from a mixing perspective, blending toluene with heptane (or iso-octane, for
that matter) can result in sooting propensities that are closer to gasoline. Further-
more, because toluene burns slower than gasoline while heptane burns faster as
shown in Fig. 14 , it will be possible to identify a heptane mixture fraction of
heptane and toluene that closely matches the evolution of droplet diameter of
gasoline. Figure 14 shows that a toluene/heptane mixture containing 5 % heptane
and 95 % toluene accomplishes this quite well.
It is important to note that a surrogate may not be able to replicate all combustion
properties of a real fuel. Considering the data in Fig. 15 which show the relative
position of the

Toluene droplet fl

fl flame to the droplet, gasoline droplet fl flames are much closer to the

droplet surface than are toluene or heptane fl flames. A binary mixture evidently does

not have enough flexibility to match more than a few combustion properties (one in

this case). Including more constituents in a surrogate provides more

fl

flexibility to
match combustion properties of a surrogate with a real fuel. Jet fuel surrogates
(Dooley et al. 2010 , 2012 ) are a case in point.
Two surrogates were formulated for jet-A: one containing three components
(3CS) and the other four components (4CS). The surrogates were developed around
four targets (average molecular weight (MW), DCN, TSI, and hydrocarbon-to-carbon
ratio (H/C)) and the mixture fraction of the three or four components determined by

fl