Counterflow flame experiments and chemical kinetic modeling of dimethyl ether/methane mixtures

As advanced engines become more controlled by the fuel reactivity, it is important to have a complete understanding of combustion chemistry of fuel blends at both high and low temperatures. While the high-temperature chemistry coupling with transport and heat release can be examined through the use of flame experiments, low-temperature chemistry has been traditionally limited to homogeneous reactor experiments at fixed temperatures, which leaves the heat release rate unconstrained. In this study, the kinetic coupling between dimethyl ether and methane is examined by studying hot flames, cool flames, and ozone-assisted cool flames in a counterflow burner. At fixed fuel mass fraction, it is found that methane addition to dimethyl ether raises the hot flame extinction limit but lowers the cool flame extinction limit. Ozone addition to cool flames is seen to lead to a substantial increase in the extinction limit, but it also produces a decrease in sensitivity of the extinction limit to the fuel mass fraction. The cool flame extinction measurements are then used to examine the uncertainties of reactions contributing significantly to the low-temperature heat release. The measurements indicate that the original kinetic model significantly overpredicts the cool flame extinction limits. However, by targeting the H-abstraction reaction of dimethyl ether by OH, among other reactions, an updated chemical kinetic model for dimethyl ether/methane mixtures is developed and validated. This study shows the value of the ozone-assisted counterflow cool flame platform in examining the key low-temperature reactions contributing to the heat release rate in cool flames.