Influence of steam dilution on ignition delay time of dimethyl ether/air mixtures at high pressure measured behind reflected shock waves

This work provides the first experimental data on ignition delay times of steam-diluted dimethyl ether/air mixtures at high pressure obtained behind reflected shock waves. Mixtures are prepared with 20 and 40 vol% steam dilution and at equivalence ratios of 0.5, 1.0 and 2.0. The measurements comprise temperatures between 731 and 1314 K and are recorded at nominal pressures of 15 and 35 bar. The obtained experimental data is used for comparison and evaluation of predictions of common chemical-kinetic models. The effect of dilution on ignition delay time is furthermore studied numerically utilizing zero-dimensional reactor simulations with the chemical-kinetic model AramcoMech 3.0. Thermal and chemical effects of steam-dilution are analyzed in comparison to CO2 and N2 dilution. Steam dilution features a smaller heat capacity than CO2 at the investigated temperatures, which leads to a lower thermal effect and therefore shorter ignition delay time. The chemical effect of steam decreases ignition delay time at intermediate and high temperatures below the ignition delay time of a mixture with the same amount of N2 dilution. The chemical effect is mainly due to the high collision efficiency of H2O in the third body reactions H2O2 + H2O 4-* 2OH + H2O, H + O2 + H2O 4-* HO2 + H2O and CH3OCH3 + H2O 4-* CH3 + CH3O + H2O. At high temperatures the direct participation of H2O in reactions and its chemical conversion additionally exerts a further decreasing effect on ignition delay time.