Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 193

both fluid flow and the chemical kinetics that govern fuel combustion in aircraft components.
To properly model the chemical kinetics requires an understanding of the composition of the
blended fuel components, as well as appropriate chemical kinetic models that describe the cas-
cade of chemical reactions governing the combustion process. Once the constituents of the fuel are
determined, blending rules are applied to calculate the thermophysical properties of the hydrocar-
bonmixture, such as density, viscosity, specific energy, and others as needed. During a simulation,
these properties can evolve over time as a result of chemical reaction, which changes the com-
position of the fuel, or through changes to the local temperature and pressure. Considering the
variability shown in Figures 11.3, 11.5 and 11.7, it would be a disheartening task to attempt to
comprehensively model the entire range of possible fuel mixtures. This leads to the desire for
identifying representative fuel blends, i.e., jet fuel surrogates (Anand et al. , 2011; Colket et al. ,
2007; Dooley et al. , 2010; Herbinet et al. , 2010; Huber et al. , 2010; LeClercq and Aigner, 2009;
Mensch et al. , 2010; Pitz andMueller, 2011; Singh et al. , 2011; Westbrook et al. , 2009). Surrogate
fuels provide a standardized reference composition for comparison of different designs and oper-
ating conditions. Both experimental and numerical experiments have employed surrogate fuels to
simplify the chemistry while maintaining the key elements of the combustion process to keep all
of the essential physics in place. C16 ( n -hexadecane) is a primary reference fuel for experimental
work on diesel engines. The Lawrence Livermore National Laboratory (LLNL) developed a suite
of chemical kinetic models for alkanes from n -octane to n -hexadecane for low and high temper-
atures (Westbrook et al. , 2009). Their work showed that all n -paraffins in the range of C8-C16
exhibit nearly the same ignition behavior. If the right behavior can be appropriately captured in a
numerical study by substituting a lower carbon number fuel for n -hexadecane, this vastly reduces
the size of chemical kinetic model. As a reference point, LLNL's model for the combustion of
n -hexadecane has 8000 reactions and 2100 species, in contrast to n -decane (C10), which has
3900 reactions and 950 species (Westbrook et al. , 2009).
The identification of the right blend of fuel components for surrogate fuels is currently an active
area of research. Databases for petroleum fuel components have been laboriously built up over
decades. In contrast, the characterization of the more complex chemistry of biofuels is relatively
recent (Kohse-Hoinghaus et al. , 2010). Methyl decoanate is sometimes used as a surrogate for
biodiesel fuels. It includes reduced chemistry for the oxidation of n -heptane that is accurate for
both low and high temperatures (Herbinet et al. , 2008) More recent work has incorporated two
large, unsaturated esters to show the influence of the double bond (Herbinet et al. , 2010). Another
study used two of the five major components of biodiesel to examine toxic emissions (Kohse-
Hoinghaus et al. , 2010). They studied methyl stearate and methyl esters using LLNL's pre-existing
classes and rules for the chemical reactions, but added additional mechanisms (3500 chemical
species and >17,000 reactions) to represent some of the unique characteristics of methyl esters.
For example, the monounsaturated compound, methyl oleate (also called oleic acid methyl ester,
with a lipid number of C18:1 and a chemical formula of C 19 H 34 O 2 ) is slightly less reactive than
methyl stearate (stearic acid, C18:0, C 19 H 36 O 2 ). The double bond in methyl oleate inhibits some
of the reaction pathways that produce chain branching at low temperatures (Naik et al. , 2011).
It is also useful to develop a framework to compute fuel properties as a function of composition
in order to find the most efficient geometries and operating conditions. Reliable models for
properties such as density and viscosity will permit computational analysis of other aircraft
systems, such as calculating the power required to pump a given volume of fuel from its storage
reservoirs to the combustor. Figure 11.8 shows the effect of composition and temperature on
properties for certain fatty acid esters that are typical of biodiesel. At this point, we move into
lipid notation for the fuel's fatty acid subunit, C x : y , in which x represents the carbon number
and y represents the number of unsaturated carbon-to-carbon bonds. Figure 11.8(a) presents
experimental data representing the temperature dependence of density for saturated methyl esters
(C x :0) in the range of C16 to C22, as well as unsaturated methyl esters with one or three double
bonds (C x :1, C x :3) (Pratas et al. , 2011a). As with the pure hydrocarbons, density decreases
linearly as the temperature increases. The density also increases with increasing chain length and
increasing levels of unsaturation (Refaat, 2009). The data in Figure 11.8(b) show the temperature