The autoignition of cyclopentane and cyclohexane in a shock tube

TL;DR

This study measures ignition delay times of cyclopentane and cyclohexane in a shock tube, revealing cyclopentane's lower reactivity, and develops detailed kinetic models using an improved software tool and ab initio calculations.

Contribution

It provides new experimental data on cyclopentane and cyclohexane ignition and introduces enhanced kinetic models including new reaction pathways for cyclanes.

Findings

Cyclopentane has ignition delay times about ten times longer than cyclohexane.

Detailed kinetic models successfully reproduce experimental ignition delay times.

Ab initio calculations help estimate key reaction rate constants.

Abstract

Ignition delay times of cyclohexane-oxygen-argon and cyclopentane-oxygen-argon mixtures have been measured in a shock tube, the onset of ignition being detected by OH radical emission. Mixtures contained 0.5 or 1 % of hydrocarbon for equivalence ratios ranging from 0.5 to 2. Reflected shock waves allowed temperatures from 1230 to 1800 K and pressures from 7.3 to 9.5 atm to be obtained. These measurements have shown that cyclopentane is much less reactive than cyclohexane, as for a given temperature the observed autoignition delay times were about ten times higher for the C5 compound compared to the C6. Detailed mechanisms for the combustion of cyclohexane and cyclopentane have been proposed to reproduce these results. The elementary steps included in the kinetic models of the oxidation of cyclanes are close to those proposed to describe the oxidation of acyclic alkanes and alkenes. Consequently, it has been possible to obtain these models by using an improved version of software EXGAS, a computer package developed to perform the automatic generation of detailed kinetic models for the gas-phase oxidation and combustion of linear and branched alkanes and alkenes. Nevertheless, the modelling of the oxidation of cyclanes requires to consider new types of generic reactions, and especially to define new correlations for the estimation of the rate constants. Ab initio calculations have been used to better know some of the rate constants used in the case of cyclopentane. The main reaction pathways have been derived from flow rate and sensitivity analyses.