A Computational Study on the Transient Ignition and NTC Behavior of Non-Premixed Dimethyl Ether/Air Counterflow under Elevated Pressure

For large hydrocarbon fuels with low-temperature chemistry, the non-premixed ignition and negative temperature coefficient (NTC) are affected by the coupling effects of chemical kinetics and transport. In this study, the transient non-premixed ignition process in dimethyl ether (DME)/air counterflow at low temperature and elevated pressure is studied numerically with detailed chemistry and transport. The emphasis is on the effects of strain rate, fuel/air temperature, and pressure on the nonpremixed ignition and the corresponding NTC characteristics. Similar to homogeneous ignition, the non-premixed ignition process consists of three stages, which are dominated by low-, intermediate-, and high-temperature chemistries, respectively. Multiple reaction zones are identified in these ignition stages. An increase in strain rate and a decrease in fuel temperature are both found to greatly inhibit the intermediate-temperature ignition stage while only marginally affecting the low- and high-temperature ignition stages. As a result, the non-premixed NTC behavior manifested by the global ignition delay time is reduced by either increasing the strain rate or decreasing the fuel temperature. However, while an increase of the strain rate greatly enhances the inhibitive effect of transport on the intermediate-temperature ignition stage, change of the fuel temperature has a minor influence on the reaction- transport interaction during the ignition process. Besides, by significantly promoting the kinetic effect over the transport effect, elevation of pressure greatly accelerates the intermediate-temperature ignition stage and weakens the inhibitive effect of increasing strain rate on the non-premixed ignition and the corresponding NTC behavior.