Characterization of the chemical kinetics of dimethyl ether (DME) in a controlled trajectory - rapid compression and expansion machine (CT-RCEM)

A numerical study of the chemical kinetics of dimethyl ether (DME)-air mixture is presented when compressed in a controlled trajectory - rapid compression and expansion machine (CT-RCEM). CT-RCEM accurately steers the piston trajectory, allowing a broad operating condition range and an easy operation without needing hardware intervention. This feature enables us to modify the thermodynamic route of compression/expansion by choosing the appropriate piston trajectory. Based on the computational fluid dynamics (CFD) data, DME-air's chemical kinetics is examined via reaction pathway analysis to understand the effect of the thermodynamic path of compression on the ignition delay. This work first shows how the creviced piston head effectively suppresses roll-up vortices, resulting in spatial uniformity of temperature, species, and, ultimately, a single dominant global reaction pathway of C, H, and O elements of DME. Next, this study shows that for a given compression ratio, the longer compression time, i.e., the longer residence time for more chemical reactions to progress by the end of compression (EOC), does not necessarily have higher reaction progress at the EOC, and in turn, lengthens the ignition delay. This counter-intuitive effect is due to the higher average core temperature of the shorter compression time piston trajectory right before EOC for sufficient time, which triggers the reactions much faster than the longer compression time piston trajectory and shortens the ignition delay. Hence, CT-RCEM's novel feature, i.e., the ability to attain the same EOC thermodynamic state with different thermodynamic paths of compression, leading to different species build-up by EOC and thereby affecting ignition delay, provides essential information for a deeper understanding of chemical kinetics.