Estimation of Flammability Limits of Selected Fluorocarbons with F(sub 2) and CIF(sub3) Page: 17 of 78
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Nothing in the explosion theory spreadsheets directly speaks to the question of whether a given
mixture will burn if ignited, though intermediate values calculated in those spreadsheets were
used (outside the context of the spreadsheet calculations) to assist in such a prediction. The
specific criteria used by Barber in devising the flammability boundaries reported in K/ETO-111
were not explicitly stated in that document. A search of his files did indicate the probable course
of evolution of the method used. Early experimental work (1960s and 1970s) on CFC-114
systems suggested that the thermal ignition temperature for coolant plus CIF3 or F2 mixtures was
above -750*F (450*C) and below 1300*F (750 C) . The thermal ignition temperature is that at
which the mixture will autoignite, essentially a flash point for that mixture. In the late 1980s,
several references in letters and memos suggested that this ignition temperature might be used as
a guide to estimate flammability. The reasoning was that when a mixture burns, it should briefly
reach the vicinity of its adiabatic temperature. If the calculated adiabatic temperature reached
upon reaction lies below the thermal ignition temperature, insufficient energy is released to heat
and ignite adjacent unburned gas.
This reasoning worked for a limited range of compositions, but when applied to a wider range of
compositions, it overestimated the extent of the flammability envelope (i.e., mixtures are
predicted to burn that experimentally will not do so). Closer examination of mixtures whose
compositions lie on the experimental flammability boundary reveals that the predicted adiabatic
temperatures vary with composition of the mixture. This became evident during the preparation
of K/ETO-111, and a modified empirical method was used to predict the boundary. Based on
notes from a private communication , the Barber flame limit estimate used in K/ETO-111
was derived as follows:
1. Select experimental points on the flammability boundary systems of interest and record the
mole fractions of fuel and oxidizer on such boundary points.
2. Calculate the adiabatic temperature for these mixtures.
3. Plot the following for all these points: T,&,6 vs. In (Xid./Xfuel).
4. Draw or fit a straight line that lies below essentially all plotted points, deriving
fit parameters A and B for the line.
5. To predict flammability limits of other (similar) systems or compositions, compute the
adiabatic temperature necessary for burning at a given mole ratio. Run the (appropriately
modified) model to derive the adiabatic temperature of that composition. The mixture is
predicted to sustain a flame if
Tdi.e > A In [Xoxjdp./Xfu] + B
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Trowbridge, L.D. Estimation of Flammability Limits of Selected Fluorocarbons with F(sub 2) and CIF(sub3), report, September 1, 1999; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc623234/m1/17/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.