High temperature ignition and combustion enhancement by dimethyl ether addition to methane-air mixtures

The effects of dimethyl ether (DME) addition on the high temperature ignition and burning properties of methane-air mixtures were studied experimentally and numerically. The results showed that for a homogeneous system, a small amount of DME addition to methane resulted in a significant reduction in the high temperature ignition delay. The ignition enhancement effect by DME addition was found to exceed that possible with equivalent amounts of hydrogen addition, and it was investigated by using radical pool growth and computational singular perturbation analysis. For a non-premixed methane-air system, it was found that two different ignition enhancement regimes exist: a kinetic limited regime and a transport limited regime. In contrast to the dramatic ignition enhancement in the kinetic limited regime, the ignition enhancement in the transport limited regime was significantly less effective. Furthermore, laminar flame speeds as well as Markstein lengths were experimentally measured for methane-air flames with DME addition. The results showed that the flame speed increases almost linearly with DME addition. However, the Markstein length and the Lewis number of the binary fuel change dramatically at small DME concentrations. Moreover, comparison between experiments and numerical simulations showed that only the most recent DME mechanism well reproduced the flame speeds of both DME-air and CH ₄-air flames.