The cool-flame oxidation of n-heptane. Part I. The kinetic features of the reaction

Abstract

The oxidation of n-heptane has been investigated at subatmospheric pressures and temperatures from 220° to 360°C, and detailed kinetic data have been collected in the cool-flame region in an attempt to understand the phenomenon of cool-flame formation. A detailed ignition diagram has been constructed for a 1:1 fuel-oxygen mixture. At high pressures at temperatures above 355°C, ignition is a single stage process while at lower temperatures it is always initiated by a cool-flame, and appears in a two-stage phenomenon. Although there is a large temperature rise during this first stage it is insufficient to cause thermal ignition of the fuel, suggesting that initiation of the second stage is both chemical and thermal. During a cool-flame the temperature rise is sufficient to take the reaction mixture into the high temperature slow combustion region, when, presumably, the intermediate responsible for cool-flame formation is no longer formed. The flame is quenched, the temperature falls, and if sufficient intermediate is formed again, another flame propagates. Most of the work reported in this paper supports the “thermokinetic” theory of cool-flame formation involving one chain-branching intermediate. It is shown that in the cool-flame region there are complex relationships between cool-flame parameters (induction periods, pressure rise and maximum rate of pressure rise) and initial temperatures and pressures. These give rise to the observed complex shapes of the cool-flame and ignition limits. In turn all these phenomena reflect the complex temperature dependence of the propagation and branching reactions involved in the oxidation.