In situ plasma activated low temperature chemistry and subsequent S-curve transition in DME/oxygen/helium mixture

The effect of plasma activated low temperature chemistry on the ignition and extinction of a Dimethyl Ether (DME)/O₂/He diffusion flame was investigated in a counterflow burner with in situ nanosecond pulsed discharge at 72 Torr. A uniform discharge was generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of DME/O₂/He were studied by employing CH₂O Planar Laser Induced Fluorescence (PLIF) at constant strain rates and O₂ mole fraction on the oxidizer side while changing the DME mole fraction. Contrary to the conventional understanding, strong low temperature reactivity was observed for DME with non-equilibrium plasma activation even at 72 Torr. At lower O₂ mole fraction, it was found that with the increase of DME mole fraction on the fuel side, the CH₂O PLIF intensity increased significantly before ignition and decreased rapidly after ignition. Moreover, at higher O₂ mole fraction and discharge repetition frequency, it was found that the in situ discharge could significantly modify the characteristics of ignition and extinction and create a new monotonic and fully stretched ignition S-curve with a smooth transition between low temperature chemistry and high temperature chemistry. The transition from the conventional S-curves to a new stretched ignition curve at high plasma repetition rate demonstrated that the plasma could dramatically change the chemical kinetic pathways of DME oxidation by activating the low temperature chemistry. The chemical kinetic model for the plasmaflame interaction was developed based on the assumption of constant electric field strength in the bulk plasma region. Both experiments and modeling revealed that the plasma activated low temperature chemistry had a much shorter time scale comparing with that of thermally activated low temperature chemistry. The reaction pathways analysis showed that atomic O generated by the discharge was critical to controlling the radical production. The radical production from the plasma at low temperatures significantly accelerated both the low temperature and high temperature kinetics.