The kinetics and warm flame chemistry associated with radiative extinction of spherical diffusion flames

Abstract

Several studies have found that microgravity diffusion flames with sufficient heat loss cool until they extinguish at a critical temperature near 1130 K. In this work, the associated chemical kinetics are explored for spherical diffusion flames burning ethylene. The flames are simulated with a transient numerical model with detailed chemistry, transport, and radiation. This incorporates the UCSD mechanism with 57 species and 270 reactions. Species concentrations, reaction rates, and heat release rates are examined. Upon ignition, the peak temperature is above 2000 K, but this decreases until extinction due to radiative losses. This allows the chemistry to be studied over a wide range of peak temperatures for the same fuel and oxidiser. When the peak temperature is high, the dominant chemistry is similar to that for typical normal-gravity ethylene diffusion flames. There are two distinct zones: an ethylene pyrolysis zone and an oxidation zone, and negligible reactant leakage. The ethylene mainly reacts with H and OH. The concentration of OH in the ethylene pyrolysis zone is high due to the long residence times and reactions of CO₂ and H₂O with H. As the flame cools and the peak temperature approaches the critical extinction temperature, there is increased reactant leakage leading to higher O, OH, and HO₂ concentrations on the fuel side. Most reactions shift towards the oxidiser side and there is large overlap between the two zones. Reactions involving HO₂ become more significant and the large consumption rates of H by HO₂ increase the concentrations of OH and O on the fuel side. The appearance of warm flame chemistry delays extinction but is not sufficiently exothermic to prevent it.

Keywords:

• ethylene
• international space station
• microgravity combustion
• reaction kinetics

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Correction

Acknowledgments

The authors thank Dennis Stocker for many helpful discussions.

Disclosure statement

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

Supplemental data

Supplemental data for this article can be accessed at https://umd.box.com/s/tcttrl8verrt50opa9kl86vy7k67c69n

Correction Statement

This article was originally published with errors, which have now been corrected in the online version. Please see Correction (http://dx.doi.org/10.1080/13647830.2024.2354112).

Additional information

Funding

The authors gratefully acknowledge the support of the National Aeronautics and Space Administration [80NSSC20M0072 and 80NSSC20M0073] and the National Science Foundation [CBET1740490].