Estimation of Flammability Limits of Selected Fluorocarbons with F(sub 2) and CIF(sub3) Page: 21 of 78
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Adiabatic (isentropic) model
The adiabatic model calculates the temperature rise in product and diluent gases in a constant
volume system, assuming no energy gain or loss from the system: that is, the heat of reaction will
all go into heating the gases present, with none lost as heat or work to the surroundings. This is a
static model, unlike the original 1992 version, which was a dynamic model of (subsonic) flame
From the ratio of fuel (coolant) to oxidizer (fluorinating agent) the model will deduce which
reactions will occur and calculate final products and energy release from tables of enthalpy of
formation (AHf). Since this is a constant volume process, we need to determine the reaction's
internal energy change, AE, rather than its enthalpy change, AH. The two are related per:
AE= AH - A(PV)
Since AE is a state function, it does not depend on the route chosen from initial to final
conditions, but only on the physical parameters at the end points. We imagine first that the
reaction takes place (at constant volume and temperature), with a possible change in the number
of moles of gas. Treating all gases as ideal,
A(PV)= An RT .
Consequently, the internal energy change for this segment is
AE(Ti) = AH(T.) - An RT.
Here, AE, AH, and T are evaluated at the standard thermodynamic reference temperature, T, , of
298.15K, and at standard pressure. The term An is obtained from the reaction stoichiometry
(representing the gain or loss in numbers of moles of gas). At standard temperature and pressure,
AH is obtained from appropriately weighted sums of tabulated enthalpy of formation data for
reactants and products. To correct AE from the tabulated value at the reference temperature to
the initial temperature Ti, the following adjustment must be applied:
Since for an ideal gas, (dE/dV)-r is zero (i.e., the internal energy is independent of volume at
constant temperature), no correction is needed if the initial pressure is other than the standard
thermodynamic reference pressure of 1 bar. The term AE represents the energy released at
constant volume upon completion of the reactions postulated. The second part of the problem is
to calculate the temperature to which the product mixture will rise as it absorbs this energy. The
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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/m1/21/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.