Autoignition of DME/H2 mixtures in a rapid compression machine under low-to-medium temperature ranges

Ignition delays of dimethyl ether and hydrogen (DME/H2) mixtures (hydrogen blending ratio ranging from 0% to 60%) were measured in a rapid compression machine at compressed temperatures ranging from 655 to 810 K and compressed pressures from 16.9 to 24.5 bar with an equivalence ratio range of 0.8-1.6. Experimental measurements are compared with results from numerical simulations using the NUIG Mech_56.54 mechanism and, in general, a good agreement is obtained. The first-stage ignition delay of DME/H2 mixtures proves to be insensitive to compressed pressure and equivalence ratio, whereas the total ignition delay decreases distinctly with the increase of compressed pressure and equivalence ratio, especially in the negative temperature coefficient (NTC) region. Moreover, the simulation results show that the NTC region shifts to a higher temperature when compressed pressure is increased. Increasing the equivalence ratio enhances the overall system reactivity but does not significantly change the temperature range of the NTC region. Both first-stage and total ignition delay increase with the increase of hydrogen blending ratio. The total ignition delay as a function of hydrogen blending ratio increases nonlinearly, and the effect of hydrogen addition becomes more prominent in the NTC region and at 60% hydrogen blending ratio. Kinetic analysis indicates that hydrogen addition consumes OH radical and inhibits DME low-temperature oxidation. More fuel molecules undergo chain propagation reaction subsequently, leading to the decrease of heat release and radicals generated during the first-stage ignition and longer total ignition delay.