Laminar burning velocities of cyclopropane flames

Abstract

Cyclopropane, c-C3H6, the simplest cycloalkane, is seldom included in detailed kinetic mechanisms for hydrocarbons, though it may exhibit unusual kinetic features yet to be analysed due to a lack of studies of its combustion characteristics. In this work, laminar burning velocities of cyclopropane flames have been determined using the heat flux method at atmospheric pressure and an initial gas mixture temperature of 298 K. The fuel consisting of 50% c-C3H6 + 50% N2 was burnt with air covering the range of equivalence ratios 0.6 – 1.5. The detailed kinetic model of the authors was extended by the reactions of cyclopropane and cyclopropyl radical, c-C3H5, with the rate constants selected from the literature. This mechanism, as well as the most recent mechanisms for c-C3H6 of Wang et al. (2022) and of Lei et al. (2022), have been compared with the burning velocities of propylene + air flames and with the new experimental results for cyclopropane flames. The model of Lei et al. (2022) significantly underpredicts the burning velocities for both fuels, on the other hand, good agreement with predictions of the present model and of Wang et al. (2022) was observed at 1 atm. However, further sensitivity and rate-of-production analyses revealed important differences in the pathways of c-C3H6 oxidation predicted by the two mechanisms. The present kinetic model was also tested using all available measurements of cyclopropane ignition delays in shock tubes, which combined cover the range of equivalence ratios from 0.33 to 3, at pressures 1 - 10 atm, and temperatures 1100 – 2100 K. Overall good performance of the model was demonstrated across these ranges of conditions and compositions of the mixture. A direct comparison of the experimental data shows that ignition delays of propylene are slightly longer than those of cyclopropane, yet in most cases within the overlapping uncertainties or scattering between different experimental facilities. The laminar burning velocities of c-C3H6 + air are slightly higher than those of C3H6 + air at least according to the predictions of the present mechanism and the model of Wang et al. (2022).