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

The flammability of a mixture of a known (or assumed to be known) composition
can also be determined using Le Chatelier’s flammability rule [27], as described earlier in
equation 2.1. The LFL of a multicomponent mixture can then be estimated by the
relationship:
xi
LC = ( . 02 − 0.
000721∗T
)∑, i = 1 → N
LFL
1 ( 2.1 )
where xi is the mole fraction of species i in the mixture and LFLi is the lower
flammability limit (25°C) of species i. The mixture is considered flammable if LC>1. It
is more explicitly stated here than for the FAR flammability rule that the flammability of
a mixture is dependent upon not only the amount of fuel in the mixture but also the
composition of the fuel vapor, as LC is calculated considering both the fraction and the
LFL of each individual species in the mixture. Figure 6.6 shows the calculated Le
Chatelier’s ratio for the two fuel compositions with flashpoints of 115°F and 120°F. Le
Chatelier’s rule indicates that even the more volatile 115°F flashpoint fuel did not
become flammable throughout the length of the test. From a safety standpoint, the FAR
rule appears to be more conservative and indicates that mixtures may become flammable
earlier than the Le Chatelier’s rule does. Application to the present results obtained using
equivalent fuel species characterizations requires additional consideration, including
experimental verification, but for comparison purposes the computed fuel species mole
fractions in the ullage, represented in terms of C5 to C20 normal alkanes only, were used
with equation (1) to calculate the LC ratio as a function of time.
I
Figures 6.7 and 6.8 show calculated fuel to air mass ratios and Le Chatelier’s
ratio, respectively, using the input data from the first example (a heated tank at sea level),
as well as two other profiles with liquid temperatures 5°F and 10°F higher than the
i
53