The radical index and the effect of oxygen concentration on non-premixed cool flame extinction of large n-alkanes

The effects of fuel and oxygen concentrations on the extinction limits of non-premixed cool flames for large n-alkanes, ranging from n-heptane to n-dodecane, are measured in an atmospheric counterflow burner. The measurements are used (a) to derive a scaling law between the cool flame reactivity and...

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Bibliographic Details
Published in:Combustion and flame 2021-09, Vol.231 (C), p.111471, Article 111471
Main Authors: Zhou, Mengni, Yehia, Omar R., Xu, Wenbin, Reuter, Christopher B., Wang, Ziyu, Yan, Chao, Jiang, Bo, Ju, Yiguang
Format: Article
Language:English
Subjects:
Alkanes
Atmospheric models
Cool flame
Counterflow
Counterflow non-premixed flame
Dodecane
Enthalpy
Extinction limit
Extinguishing
Fuels
Heptanes
Low temperature
Mass transport
N-alkane
Oxygen
Oxygen effect
Radical index
Reactivity
Scaling laws
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Summary:The effects of fuel and oxygen concentrations on the extinction limits of non-premixed cool flames for large n-alkanes, ranging from n-heptane to n-dodecane, are measured in an atmospheric counterflow burner. The measurements are used (a) to derive a scaling law between the cool flame reactivity and the oxygen concentration and (b) to develop a radical index to rank the cool flame reactivity of n-alkanes, with the assistance of kinetic modeling. First, due to the significant role of multiple oxygen addition reactions in the low-temperature branching sequence, the relationship between cool flame extinction (and hence the chemical time scale) and oxygen concentration are examined. The results suggest that all measured n-alkane cool flame extinction limits exhibit similar nonlinear dependence on oxygen concentration. The results further show that, due to the combined effects of multi-oxygen addition processes and the negative temperature coefficient effect, the cool flame low temperature reactivity of n-alkanes is proportional to nth power of oxygen mole fraction, [O2]n, with n = 1.5–2. Furthermore, by accounting for thermal and mass transport effects using a mass-weighted enthalpy, a cool flame radical index is proposed to evaluate the role of cool flame reactivity on the extinction limits of large n-alkanes. We find that the radical index of large n-alkanes increases with the increase of carbon length of the fuel molecules, which is consistent with stronger low-temperature reactivity.
ISSN:0010-2180
1556-2921
DOI:10.1016/j.combustflame.2021.111471