Low-temperature ignition of dimethyl ether: transition from cool flame to hot flame promoted by decomposition of HPMF (HO2CH2OCHO)

Abstract

Low-temperature (< 600 K) ignition of dimethyl ether (DME) is investigated both experimentally and numerically with particular attention given to the transition from a cool flame to a hot flame. The premixed DME/air mixture is fed into a laminar flow reactor with prescribed temperature profiles at atmospheric pressure. Periodic behaviors involving ignition and flame propagation are captured. A limit in reactor temperature (TR) is observed, above which cool flame behaviors dominate, while below which the cool flame transits to a hot flame at fuel rich conditions (equivalence ratio (ϕ) > 1). In order to better understand the experiments, a transient two-dimensional axisymmetric numerical computation with a 36-step kinetic model and detailed transport properties is conducted. Accordingly, ignition and flame propagation observed in the experiments are successfully reproduced. The transition limit is found and depicted in the TR–ϕ plane. It suggests that the intermediate species HPMF (hydroperoxy-methylformate, HO2CH2OCHO) plays a critical role in the transition from a cool flame to a hot flame. During the ignition process, the reactivity in the negative temperature coefficient (NTC) regime can be enhanced by numerous of OH radicals that are produced via the decomposition reaction of HPMF, thus promoting the transition. Not only is more HPMF formed in the first-stage ignition as the reactor temperature decreases, but the fuel rich conditions also support the dominant reaction pathways in the NTC regime. Accordingly, transition limits exist in both reactor temperature and equivalence ratio. Finally, diffusion effects and potential reasons for the discrepancy in predicting the transition limit are discussed.