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

Abstract 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.