Dimethyl ether autoignition in a rapid compression machine: Experiments and chemical kinetic modeling

Dimethyl ether (DME) autoignition at elevated pressures and relatively low temperatures is experimentally investigated using a rapid compression machine (RCM). DME/O2/N2 homogeneous mixtures are studied over an equivalence ratio range of 0.43–1.5 and at compressed pressures ranging from 10 to 20 bar and compressed temperatures from 615 to 735 K. At these conditions RCM results show the well-known two-stage ignition characteristics of DME and the negative temperature coefficient (NTC) region is noted to become more prominent at lower pressures and for oxygen lean mixtures. Furthermore, the first-stage ignition delay is found to be insensitive to changes in pressure and equivalence ratio. To help interpret the experimental results, chemical kinetic simulations of the ignition process are carried out using available detailed kinetic models and, in general, good agreement is obtained when using the model of Zhao et al. [Int. J. Chem. Kinet. 40, 2008, 1–18]. Sensitivity analyses are carried out to help identify important reactions. Lastly, while it is implicitly assumed in many rapid compression studies that chemical changes from the initial charge conditions that might occur during compression are negligible, it is herein shown with the help of Computational Singular Perturbation (CSP) analyses that chemical species formed during compression with little evolved exothermicity can considerably affect autoignition observations. Therefore, it is essential to simulate both compression and post-compression processes occurring in the RCM experiment, in order to properly interpret RCM ignition delay results.