Environmental Engineering Reference
In-Depth Information
Freeze temperature (Fig. 11.3c) measured in units of K , here in C: Since jet fuels are a blend of
different hydrocarbon compounds, it freezes over a temperature range rather than at a single tem-
perature, as for a pure liquid. This is due to the fact that, as the temperature is decreased, the heaviest
hydrocarbons freeze into waxy crystals before the lighter components solidify. To create a system-
aticmeans of comparing the freezing properties of jet fuels, the term“freeze temperature” (not “the
freezing temperature”) is defined through the following procedure: the hydrocarbon fuel blend is
cooled until wax crystals form. As the fuel is gradually warmed back up, the lowest temperature
at which all of the wax crystals have melted is defined as the freeze temperature (also sometimes
denoted as the “freeze point”). Consequently, the freeze temperature is well above the temperature
at which the fuel completely solidifies. A related term is “cloud point”, which is the temperature at
which wax crystals first start to form as the temperature is lowered. Roughly 10 C below the cloud
point, the freezing fuel reaches the “pour point”, at which the wax in the fuel has built up sufficient
solid structure to prevent pouring. The combination of viscosity and freezing point define the
pumpability of a fuel, that is, the ease of pumping fluid through the fuel lines (Chevron, 2006).
Flash temperature or flash point (Fig. 11.3d) measured in units of K , here in C: The flash
point is the lowest temperature at which vaporized fuel above a flammable liquid will burn when
exposed to an ignition source. Vapor burns only when the air/vapor mixture is in a certain range.
Below the lower flammability limit, there is insufficient fuel in the mixture to combust. For
kerosene-type jet fuel, the range is 0.6-4.7 volume percent (%v/v) vapor, while for wide-cut
fuel, it is 1.3-8.0%v/v (Chevron, 2006). The upper flammability limit is a function of the local
temperature and pressure. The flash point of wide-cut fuels like Jet B is not specified, but is below
0 C (Chevron, 2006).
Specific energy (heat of combustion) (Fig. 11.3e) measured in units of L 2 T 2 , here in mega-
Joules per kilogram [MJ/kg]: The specific energy can be used to compare fuels by the relative
energy content that a kilogram of fuel could release through complete and perfect combustion.
The WFSP found that all but two samples reached the minimum required energy content.
Energy density (Fig. 11.3f) measured in units of ML 1 T 2 , here in megaJoules per liter [MJ/L]:
The fuel energy content per unit volume is measured by the energy density, which is the product
of the specific energy and density. If these properties are measured by the units used for this
work, that product must also be multiplied by the necessary factors to convert between liters and
cubic meters (1m 3
= 1000 L). When a system is limited by volume, as in a completely filled fuel
tank, a fuel with higher energy density can release more net energy, which can be used to travel
a longer distance.
Aromatics content measured as a nondimensional number, the percentage of aromatic hydro-
carbons in terms of volume per total fuel volume (%v/v): Aromatics are unsaturated hydrocarbon
molecules with one or more carbon rings. As the fuel combusts, carbonaceous particles form that
become incandescent at the high temperatures and pressures in the combustor. The hot particles
emit infrared radiation, which can heat up the surrounding walls and create hot spots. The result
can be a loss in combustion efficiency, or even worse, a loss of structural integrity. Carbon that
deposits on the wall can change the carefully designed flow pattern in the combustor by inhibiting
the entrance of diluting air through combustor walls. If the particles are not completely consumed
by the time they reach the turbine blades and the stator, they can damage these key engine com-
ponents. Since aromatics tend to produce more of these carbonaceous particles, their content is
limited to a maximum of 25%v/v in aviation fuels. On the other hand, the presence of aromatics
can be critical for maintaining aircraft seals. Aromatics can cause seals and sealants to swell,
developing a “set” to a particular swell level, which is a function of the aromatic content and
exposure time. If the seals are subsequently exposed to jet fuel with either very low aromatics
content or a sufficiently different mix of aromatics, the absorbed petroleum leaches out of the seal
material, resulting in geometric shrinkage and possibly a leak (Hadaller and Johnson, 2006). An
aging aircraft would be susceptible to this condition if it has used a wide-cut fuel like JP-4 for a
long time and it is switched over to a narrower cut like JP-8 or one of the synthetics. Synthetic fuels
tend to have fewer aromatics than petroleum fuels and may require supplementation to meet jet
fuel requirements (Corporan et al. , 2011). DEFSTAN 91-91 requires a minimum of 8% aromatics
Search WWH ::




Custom Search