Two-stage ignition of DME/air mixture at low-temperature (<500 K) under atmospheric pressure

Low-temperature ignition characteristics of dimethyl ether (DME)/air mixture were studied in an external heated, straight-shaped, plug-flow reactor under atmospheric pressure. Auto-ignition of the mixture was attained under a specific narrow range of temperature and equivalence ratio with relatively longer exposure time. Three kinds of ignition behaviors were identified accordingly as the equivalence ratio increased, such as (1) periodic hot flames, followed by (2) periodic two-stage ignitions (weak flame(s) and subsequent hot flame), lastly (3) chaotic weak flames. Time-sequential gas analyses were conducted for the case showing a typical periodic two-stage ignition in order to investigate the mechanism of the observed low-temperature auto-ignition under 500 K. The results revealed that the formation of CH4 and C2H2 was promoted prior to the hot flame ignition, suggesting that the chain-branching reaction pathway might exist. The transition from weak flame to hot flame was clearly observed as the mixture equivalence ratio decreased, implying the oxygen could be responsible to trigger this transition. It is suspected that the chain-branching precursor CH2OCH2OOH radicals should be relatively stable in the examined temperature range here, thus can gradually accumulate through a longer exposure time, and eventually give rise to the chain-branching explosion.