Influence of C2HF5 and C3HF7 on CH4 and C3H8 combustion: Flame speed and ignition delay time measurements
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C2HF5 (HFC-125) and C3HF7 (HFC-227) represent two of the best options as fire suppressant substitutes. Understanding the combustion properties of such alternatives is critical to optimizing their applications and to prevent undesirable events. Several studies have been conducted using chemical kinetics mechanism to predict the suppressants' behavior at different conditions. However, there are scarce experimental data of HFCs that can be used for model validation, especially in terms of un-stretched laminar flame speed and ignition delay times. The present work remedies this deficiency and focuses on providing accurate measurements for C3HF7 and C2HF5 systems that can be compared directly with modeled data. A HFC chemical kinetic mechanism together with a recently updated C0-C5 hydrocarbon and OH chemistry were used for numerical modeling. Experimental methodology included determination of ignition delay times using OH emission from shock-tube measurements and determination of un-stretched laminar flame speeds employing the spherical, freely propagating flame technique. Most of the experimental data provided in this work are the first measurements of their kind. Current data were also compared with CF3Br performance, which is used as the point of reference. Results show that fire suppressants can be ranked as CF3Br>C3HF7>C2HF5 based on their effectiveness at reducing CH4-air and C3H8-air flame speeds. Shock-tube measurements show that methane ignition was promoted, in all the cases, by the action of the tested fire suppressants. While in propane mixtures, only CF3Br presented an ignitioninhibiting effect. The present results suggest that the tested agents may not be good alternatives when used as ignition preventers; however, they still show inhibition properties as, in all the cases, they decreased the laminar flame speed. The most significant reactions that contribute to the promotion or suppression of the ignition process were identified through sensitivity analysis. It was surprising to see the number of fluorinated species that participate in ignition-promoting reactions, especially in the CH4 system. These findings provide fundamental insight and can be used as the baseline for further research.
