Vaporization of JP-8 Jet Fuel in a Simulated Aircraft Fuel Tank ...

6.2 Flammability Assessment
6.2.1 Fuel Tank at Sea Level
Determination of the fuel to air mass ratio is important in fire hazard assessment because
it can be compared with a lower flammability limit (LFL) of a combustible mixture. For
multicomponent fuels, such as Jet A, it is not in general possible to identify a single LFL
since the ullage vapor composition may vary with loading ratio, with the liquid
temperature, and the time from initiation of heating. An approximate criterion used for
estimating the FAR at the LFL is that at the LFL the FAR for dry air volume of most
saturated hydrocarbons on a mass ratio basis is 0.035±0.004 at 0°C (32°F) [26]. This can
then be used as an estimate of the FAR at the LFL since Jet A consists mainly of
paraffinic saturated (75%-85%) hydrocarbons, and the results can be compared with
FAR’s computed in the present work since they are based on a straight chain alkane
characterization of the fuel.
Figure 6.5 shows the calculated temporal change in FAR of the heated tank at sea
level and the range of the FAR at the LFL using the two different fuel compositions.
According to the estimate of the lower flammability limit [26], the mixture became
flammable at about 1800 seconds, and from simultaneous inspection of figure 6.1, the
average liquid temperature at that point was about 116°F, which was very close to the
measured flashpoint of 117°F. By comparing the initial fuel temperature to the
temperature at which the mixture was flammable, a fuel temperature rise of about 43°F
was necessary to cause an initially non-flammable fuel tank to possibly become
flammable.
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