Estimation of Flammability Limits of Selected Fluorocarbons with F(sub 2) and CIF(sub3) Page: 69 of 78
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Two boundaries are drawn which divide the region of the graph containing boundary points from
a region not containing such points. The one labeled "best fit" is the line drawn closest to the
lowest temperature boundary points. It is evident from inspection of the graph, however, that
there is considerable system-to-system and inter-system variability in the points (when plotted
using these functions). It is likely that were data available for additional systems, the variability
would persist, and some data points would fall below the "best fit" line. Since the prediction
scheme used claims that any composition whose isentropic temperature falls below the line is not
flammable, that would mean that the correlation would predict that some mixtures are non-
flammable which, in fact, might prove to be flammable. In an attempt to allow for this variation,
a second correlation line is drawn which attempts to capture and allow for a certain degree of
variability in this (imperfect) correlation. That line is labeled "conservative fit" in Figure D.1.
The parameters for these two lines are as follows::
"Best fit": A = 383 , B = 1617
"Conservative fit": A = 250, B = 1251
D.3 EQUILIBRIUM BASED CORRELATION
Among various correlation functions examined for use in the equilibrium-based flammability
limit model, the function chosen was a two-part function, with a threshold temperature function
fit for lean and rich mixtures. The function for lean mixtures is:
T,. = A,. / X.,e + Blea
and for rich mixtures,
Tch = Arich / (S Xuel) + Brich
The two threshold temperatures are combined smoothly by the function
Thhld= (T Iean4a + Tdich)_"4~
As before, experimental compositions known to burn are plotted in the form appropriate to the
correlation equations used. In this case, the data points are separated into "lean" (excess
oxidizer) and "rich" (excess fuel) sets and plotted separately. Fig. D.2 plots the adiabatic
temperature for lean mixtures vs 1/Xojd and Fig. D.3 plots adiabatic temperatures for rich
mixtures vs 1/ S Xf,,, , where S is the ideal stoichiometry ratio, the number of moles of oxidizer
needed to completely react the fuel to CF4 (and C12, if applicable). As before, two correlation
lines are drawn in each plot, one the closest line to the available data that could be drawn and
still encompass all flammable compositions, and the other ("conservative") correlation which
moves the line away from the flammable region to account for data variability. The resulting
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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/m1/69/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.