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

6.2.2 Fuel Tank Under Varying Ambient Conditions
The FAR rule and Le Chatelier’s ratio rule were again used to determine the level of
flammability for the second example, an initially heated fuel tank exposed to simulated
flight conditions. Figures 6.11 and 6.12 show, respectively, the calculated FAR and the
calculated Le Chatelier’s ratio, both calculated using the fuel compositions with
flashpoints of 115°F and 120°F [21]. From figure 6.11, the mixture was not in the
flammable region until the ambient pressure was decreased during ascent. This was due
to the fact that the component vapor pressures are functions of temperature only, and
although the ambient pressure outside of the fuel tank was decreasing, the component
vapor pressures are fixed for the liquid fuel temperature. So in order for the fuel vapors
to exert the same pressure on the enclosure at a reduced ambient pressure, more fuel
molecules were required to vaporize into the ullage space. Le Chatelier’s rule was again
seen to be more conservative than the FAR rule by comparing figures 6.11 and 6.12, so
the FAR rule will again be used to assess the effects of fuel temperature and mass loading
on flammability for this flight profile test.
The liquid temperature effects on flammability are shown in figures 6.13 and
6.14. Three different liquid temperature profiles were used in the model to calculate the
FAR and LCR using the fuel composition of the 115°F flashpoint fuel. The original
average measured liquid temperature profile is referred to as TLIQ and the other two
profiles were obtained by adding 5°F and 10°F to TLIQ and are referred to as TLIQ+5
and TLIQ+10, respectively. Figure 6.13 includes the predicted LFL range using the FAR
criterion from reference [26]. It shows that for the conditions tested, the tank ullage was
within the LFL range for part of the level flight at 30,000’ altitude. However, increasing
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