  The effects of nanosecond pulsed non-equilibrium plasma discharges on
ignition characteristics and low-temperature chemistry or LTC of DME/O2/Ar
mixtures are numerically investigated in a plane-to-plane geometry at a reduced
pressure of 76 Torr. Detailed numerical investigations of the two-stage
ignition process in different temperature regimes of DME have been created
through a high-fidelity one-dimensional, multi-timescale plasma-discharge
model. Two different initial temperatures are set for comparison: 550K, the
negative temperature coefficient regime, where LTC plays a dominate role; 800K,
the intermediate temperature regime, where LTC normally does not play an
important role. The results have shown that, in NTC regime, non-equilibrium
plasma discharge improves the overall ignition delay time by 15 times with a
significant improvement of approximately 250 times for the first-stage
ignition. In Intermediate temperature regime, the two-stage ignition property
is reactivated by the addition of plasma, and the overall ignition delay is
shortened by approximately 75 times. This observation suggests that, in the
intermediate temperature regime, the plasma discharges can not only enhance DME
ignition characteristics but also alter the reaction pathways to a reactive LTC
reaction pathway; therefore, the two-stage ignition behavior occurred again.
Further investigation suggests that the plasma enhancement on ignition and LTC
in the intermediate temperature regime is non-linear. As the accumulative
energy input to the system increases, the enhancement from the non-equilibrium
plasma discharge reaches a limit. Considering the total amount of heat release,
the optimized voltage pulse number appears to be in the range of 35-40 to
provide the most efficient enhancement for the overall ignition delay time.
