TY - JOUR
T1 - The radical index and the effect of oxygen concentration on non-premixed cool flame extinction of large n-alkanes
AU - Zhou, Mengni
AU - Yehia, Omar R.
AU - Xu, Wenbin
AU - Reuter, Christopher B.
AU - Wang, Ziyu
AU - Yan, Chao
AU - Jiang, Bo
AU - Ju, Yiguang
N1 - Funding Information:
The authors acknowledge financial support from NASA microgravity grant NNX16AK07G, ARO grant W911NF-16-1-0076, and DOE BES research grant DE-SC0021135.
Publisher Copyright:
© 2021
PY - 2021/9
Y1 - 2021/9
N2 - 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.
AB - 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.
KW - Cool flame
KW - Counterflow non-premixed flame
KW - Extinction limit
KW - N-alkane
KW - Oxygen effect
KW - Radical index
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U2 - 10.1016/j.combustflame.2021.111471
DO - 10.1016/j.combustflame.2021.111471
M3 - Article
AN - SCOPUS:85105354798
SN - 0010-2180
VL - 231
JO - Combustion and Flame
JF - Combustion and Flame
M1 - 111471
ER -