Estimation of Flammability Limits of Selected Fluorocarbons with F(sub 2) and CIF(sub3) Page: 28 of 78
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original inverse power series format. For perfluorocarbon-related species (c-C4F8, C4F0, and
products thereof), such formulae were not present in the original spreadsheets, and the Cp
formulae for CFC-114 and CFC-115 exhibited unphysical behavior at high temperatures. For
these compounds, estimates were made, with the formulae pegged to the limited number of
known low temperature data points, and forced to approach a high temperature limiting value
plausible on statistical mechanical grounds, that is,
Cp = (3n -3) R ,
when the temperature is high enough to saturate vibrational modes, n being the number of atoms
in the molecule). For all species Cp formulae were checked for reasonable behavior from 300 to
4000 K and several were refit to improve agreement with published tabular data from ref.13.
Values used in the model for the thermodynamic parameters needed are listed in Appendix C,
2.2 EQUILIBRIUM-BASED FLAME LIMIT MODEL
2.2.1 1992 Equilibrium-Based Flame Limit Model
In ref. 1, the second model used to predict flame limits for fluorocarbon/fluorinating agent gas
mixtures was based on an empirical correlation observed between the adiabatic thermodynamic
temperature of gas mixtures and the flammability of those mixtures. The adiabatic temperature is
that attained by the mixture when it reaches thermodynamic equilibrium at constant pressure
without gain or loss of thermal energy outside the system (though, at constant P, expansion does
transfer work to the outside). A typical chemical system allowed gaseous components including
reactants, inert diluents, and potential reaction products. For flammable gas mixtures, the
equilibrium condition will strongly favor the formation of reaction products (e.g., CF4) at the
expense of reactants (e.g. F2 and c-C4F8). The user needs only to consider possible products,
however, and is not required to designate exactly which will form or in what proportions. The
calculation of the equilibrium state (done with the aid of thermodynamics software) will
determine the product mix.
Prediction of flame limits was based on purely empirical correlations observed between the
adiabatic temperature of "reacted" gas mixtures known experimentally to lie on the composition
limits of flammability and some simple functions of mixture composition.
Several correlations were examined, and none were perfectly satisfactory at matching all
available experimental boundary data points. The correlation finally used was one that bounded
known flammability composition boundary points for the specific systems of interest (or for
closely related systems). Specifically, two boundaries were established: one for rich mixtures,
Trch = AIch / Xfuel + BCh ,
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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/m1/28/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.