Shock-tube study of dimethyl ether ignition by high-voltage nanosecond discharge

A shock tube with a discharge cell was used to experimentally analyze the kinetics of dimethil ether (DME) ignition in DME:O2:Ar and DME:O2:Ar:He mixtures at temperatures from 1075 to 1860 K. We measured ignition delay time in lean and stoichiometric mixtures after a reflected shock wave. Measurements were made in the mixtures activated by a high-voltage nanosecond discharge and without discharge plasma. The dominant mechanisms of DME ignition were studied on the basis of a numerical simulation of the discharge and ignition stages. We calculated the densities of chemically active species generated in the discharge and used these data to model ignition. The calculated ignition delay times agreed reasonably with the measured data for autoignition and plasma-assisted ignition. The limiting processes during the discharge and ignition were shown using sensitivity analysis. It was demonstrated that the partial replacement of Ar with He in the mixtures studied allowed the same values of plasma-assisted ignition delay times at much lower gas temperatures being opposite to ignition without plasma where the delay time was not sensitive to the replacement of Ar with He. Calculations showed that this is explained by an increased specific discharge energy spent on the excitation of DME, O2 and Ar, whereas the excitation of He was negligible. It followed from the calculations that electron-impact ionization of DME molecules is of great importance for the production of active species in the discharge phase and consequently for ignition with plasma. The electron-impact ionization cross section of DME is poorly known and its variation influences the calculated ignition delay time. In addition, at reduced gas temperatures, the effect of plasma nonuniformity is important for shock-tube studies.