Estimation of Flammability Limits of Selected Fluorocarbons with F(sub 2) and CIF(sub3) Page: 45 of 78
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have in predicted flammability boundaries. Hydrofluorocarbons, for example, might have
somewhat different combustion chemistry, and thus one should not use these models for such
species without some confirmation or recalibration with experimental data.
Some technical limitations exist for the present models. The detonation pressure model, in
particular, is set up to operate with a single fuel and oxidizer. Inclusion of another oxidizer or
fuel in the starting mix for any of the versions of these models results in the new reactant being
treated as an inert diluent. It would not be at all straightforward to attempt to modify the
spreadsheets to include mixed fuels and/or oxidizers. The equilibrium-based model, as
currently constituted, also assumes a single fuel and oxidizer. This limitation manifests itself
only in the "stoichiometry parameter" provided as part of the list of data sets to run. This
number is the ratio of oxidizer to fuel needed to completely react the fuel without excess of fuel
or oxidizer. For any particular composition of mixed fuels and oxidizers, such a value can be
readily defined. To do so for a large number of points in which the ratios of the various fuels or
oxidizers change could be cumbersome, and a more automated process could be developed.
Reaction-chamber size effects are not captured by any of the data used in calibration of the
flammability limit models or in the functional form of the model. Consequently, phenomena
such as quench distance and lower pressure limit of flammability (a related factor) will not be
predicted. These phenomena are probably not of great relevance to the intended use of these
The detonation pressure model is based on a well-developed and accepted theory of shock
propagation and combustion and rests on that theory rather than on directly applicable
experimental data for fluorocarbon-fluorine explosions. For the results to have a degree of
validity, however, it is necessary to choose an appropriate set of chemical reactions. It is not
necessary that the reaction chemistry be completely exact, but the energetics of the reactions
chosen should be approximately the same as the reactions that actually occur in combustion. The
results, however, will only be as accurate as the accuracy of the chemistry assumed. The models
as presently constituted do not have a provision for dissociating reaction products and thus may
slightly overestimate pressures. At the temperatures and times appropriate to the presumed
detonation, considerable dissociation of fluorocarbons can occur, major radical species being CF2
and CF3. Even at lower temperatures, ClF3, if in excess, will likely dissociate to CF and F2. Any
dissociation of this sort will lower the temperature of the gas mix but increase the number of
moles of gas, compensating factors that, however, would not cancel. To properly treat
dissociation would require a much more detailed treatment of the kinetics of combustion than is
contained in either of these models. An approximate treatment including dissociation could be
carried out by calculating the appropriate thermodynamic equilibrium at shock temperatures,
rather than, as at present, assuming that a specific set of reactions go to completion.
The detonation pressure model in general predicts what will happens assuming that one or two
reactions have initiated, propagated, and accelerated to fully developed shock conditions. It does
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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/m1/45/ocr/: accessed June 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.