Ignition properties of lean DME/H₂ mixtures at low temperatures and elevated pressures

Ignition delays of lean dimethyl ether (DME)/hydrogen mixtures with hydrogen mole fraction ranging from 0 to 85% were measured using a rapid compression machine with equivalence ratios from 0.3 to 0.5, temperatures from 621 to 852 K and pressures from 7 to 22 bar. Numerical analysis was performed based on the NUI Aramco Mech 2.0 and, in general, good agreement between measurements and predictions was demonstrated. Both measurements and predictions show that H₂ addition increases the ignition delay of DME/H₂ mixtures non-linearly. Moreover, the effect of H₂ addition on ignition delay becomes more pronounced at H₂ mole fraction above 70% and lower pressure. The dilution and chemical effects of H₂ addition were quantified, and the dilution effect decreases whereas the chemical effect increases with the increase of H₂ mole fraction. It is also found that DME/H₂ mixtures show a three-stage heat release behavior at very lean condition (φ = 0.3), namely, a low temperature heat release (LTHR) stage followed by two high temperature heat release (HTHR) stages. The first- and second-stage HTHR are caused by the conversion of HCHO to CO and the production of final reaction products CO₂ and H₂O respectively. Overall, DME is mainly consumed during the LTHR and first-stage HTHR whereas H₂ is mainly consumed during the second-stage HTHR. The two-stage HTHR behavior of DME/H₂ mixtures occurs when H₂ and HCHO produced during DME's LTHR stage are present in a proper proportion prior to high-temperature ignition and the temperature after LTHR has reached a proper level.