Quasi-steady combustion of normal-alkane droplets supported by Cool-Flame chemistry near diffusive extinction

Abstract

Two steady-state chemical-kinetic approximations are introduced into the apparently most relevant five cool-flame steps of the latest San Diego mechanism for n-heptane, supplemented by a sixth step chosen to capture the influence of hot-flame chemistry on the cool flame in that mechanism, in order to obtain an effectively four-step chemical-kinetic description for addressing quasi-steady combustion of normal-alkane droplets in the lower-temperature part of the negative-temperature-coefficient (NTC) range, where cool-flame extinction is observed to occur. A development paralleling the classical activation-energy-asymptotic (AEA) analysis of the partial-burning regime, accompanied by an approximate description of a distributed reaction, is then pursued to make predictions of the combustion process, accounting for the large Lewis numbers of the fuel and intermediate species for the first time. The predictions are compared with results of droplet-combustion experiments performed in the International Space Station (ISS), showing reasonable agreement between theory and experiment and pointing to some needed future improvements in values of rate parameters. In addition, the theory predicts, for the first time, a limiting oxygen index (LOI) for droplet combustion of normal alkanes, giving, for example, for heptane burning in oxygen-nitrogen mixtures at normal room temperature, a corresponding oxygen mole fraction on the order of 0.10 in the ambient atmosphere, below which cool-flame-supported combustion cannot occur.

Keywords:

• Asymptotic analysis
• partial burning regime
• cool flames
• droplet combustion
• extinction

Acknowledgments

The Flammability-Limit-Experiment (FLEX) project on ISS produced the experimental and computational results with which we compared our predictions, and we are indebted to the FLEX engineering and science teams for their invaluable contributions. We also especially wish to thank D. L. Dietrich, not only for his expert data acquisition and reduction, but also for his helpful ideas and observations throughout this study.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was supported by the NASA Space Life and Physical Sciences Research and Applications Program. Some of the most recent effort of the first author was supported by the National Science Foundation through Grant CBET-1740499.