Lean flammability limit as a fundamental refrigerant property: Phase 2. Interim technical report, 1 April 1995--30 March 1996 Page: 4 of 47
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ABSTRACT
The flammability of alternative, non-ozone depleting refrigerants is an issue of
growing importance to the air-conditioning and refrigeration industry. Test methods
developed decades ago are being stretched to their limits when measuring the combustion
behavior of weakly flammable refrigerants. This work is Phase II of a three part project to
determine the feasibility, accuracy, and applicability of a premixed opposed-flow burner as
an alternative means of measuring lean flammability limits.
In this work, the Phase II burner demonstrates the precision available to the
opposed-flow technique for evaluating the lean flammability limit of weak fuels. Using
opposed, converging nozzles, two jets support a premixed twin flame at different global
strain rates and permit evaluation of the corresponding fuel concentration at the extinction
point. Comparisons with published data support that the LFLo, a lean flammability limit
value defined by the extrapolation of the extinction conditions to zero global strain, yields a
consistent value. Using a computer simulation to analyze the uncertainty, the lean
flammability limit of R-32 (CH2F2) in dry air is found to be 14.0 % 0.8 % by volume; with
a 43 % relative humidity in air the LFLo of R-32 is 14.1 % 0.6 % by volume. Tests with
R-134a (C2H2F4) demonstrated that no flame could be sustained for ambient test conditions
in dry air without adding a significant amount (2 % by volume) of methane (CH4) to the
mixture.
Concurrent computational modeling of the combustion of R-32 in air, individually
and in mixtures with CH4, R-125 (C2HF5), and R-134a, has been performed with the
chemical kinetics code CHEMKIN. Estimates of the impact of the initial conditions
(equivalence ratio, fuel composition, temperature, and relative humidity) on the magnitude of
the laminar flame speed of a zero strain flame are made. This knowledge has been used to
help interpret the counter-flow flame experiments and to predict the influence of other
parameters that have not yet been investigated.ii
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Womeldorf, C. & Grosshandler, W. Lean flammability limit as a fundamental refrigerant property: Phase 2. Interim technical report, 1 April 1995--30 March 1996, report, April 30, 1996; Gaithersburg, Maryland. (https://digital.library.unt.edu/ark:/67531/metadc688489/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.