Shock Tube and Kinetic Study on the Effects of CO₂ on Dimethyl Ether Autoignition at High Pressures

First-stage and overall ignition delay times of dimethyl ether (DME) were measured in a high-pressure shock tube for various mixtures containing high amounts of CO₂. The data are available for DME/air mixtures diluted with 40% CO₂ as well as for the mixture of DME and oxidizer containing 20.5% O₂ and 79.5% CO₂. As a reference, data were also collected for mixtures containing the corresponding amounts of N₂. Measurements were conducted for pressures of 15, 35, and 50 bar and a range of temperatures (744–1316 K). For the DME/air mixture diluted with CO₂, the equivalence ratio was varied in the range of 0.5–2.0. The results demonstrate that CO₂ dilution has a strong effect on ignition delay times in the NTC (negative temperature coefficient) region. The kinetic study that was conducted showed that this phenomenon can be attributed to a thermal effect resulting from the high heat capacity of CO₂. In low and high temperature ranges, the effect of CO₂ is less pronounced. Additionally, a chemical effect was identified. At high temperatures this effect can be attributed mostly to the influence of CO₂ on third-body reactions and leads to slight acceleration of ignition. Various chemical-kinetic models available were evaluated with respect to their accuracy in prediction of ignition delay times for mixtures containing DME and large amounts of CO₂.