Abstract Autoignited lifted flames of dimethyl ether (DME) in laminar nonpremixed jets with high-temperature coflow air have been studied experimentally. When the initial temperature was elevated to over 860 K, an autoignition occurred without requiring an external ignition source. A planar laser-induced fluorescence (PLIF) technique for formaldehyde (CH2O) visualized qualitatively the zone of low temperature kinetics in a premixed flame. Two flame configurations were investigated; (1) autoignited lifted flames with tribrachial edge having three distinct branches of a lean and a rich premixed flame wings with a trailing diffusion flame and (2) autoignited lifted flames with mild combustion when the fuel was highly diluted. For the autoignited tribrachial edge flames at critical autoignition conditions, exhibiting repetitive extinction and re-ignition phenomena near a blowout condition, the characteristic flow time (liftoff height scaled with jet velocity) was correlated with the square of the ignition delay time of the stoichiometric mixture. The liftoff heights were also correlated as a function of jet velocity times the square of ignition delay time. Formaldehydes were observed between the fuel nozzle and the lifted flame edge, emphasizing a low-temperature kinetics for autoignited lifted flames, while for a non-autoignited lifted flame, formaldehydes were observed near a thin luminous flame zone. For the autoignited lifted flames with mild combustion, especially at a high temperature, a unique non-monotonic liftoff height behavior was observed; decreasing and then increasing liftoff height with jet velocity. This behavior was similar to the binary mixture fuels of CH4/H2 and CO/H2 observed previously. A transient homogeneous autoignition analysis suggested that such decreasing behavior with jet velocity can be attributed to partial oxidation characteristics of DME in producing appreciable amounts of CH4/CO/H2 ahead of the edge flame region.

中文翻译：

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加热的coflow空气中二甲醚的自燃火焰

摘要 实验研究了二甲醚（DME）在高温协流空气层流非预混射流中的自燃离火火焰。当初始温度升高到 860 K 以上时，无需外部点火源即可发生自燃。一种用于甲醛 (CH2O) 的平面激光诱导荧光 (PLIF) 技术定性地可视化了预混火焰中的低温动力学区域。研究了两种火焰配置；(1) 具有三臂边缘的自燃离火火焰，具有三个不同的分支，即带有尾随扩散火焰的贫和富预混火焰翼；(2) 当燃料高度稀释时，自燃离火火焰温和燃烧。对于临界自燃条件下的自燃三臂边缘火焰，由于在爆燃条件附近表现出重复的熄灭和重新点火现象，特征流动时间（升空高度与射流速度成比例）与化学计量混合物的点火延迟时间的平方相关。升空高度也作为喷射速度乘以点火延迟时间的平方的函数相关联。在燃料喷嘴和离火边缘之间观察到甲醛，强调自燃离火的低温动力学，而对于非自燃离火，在薄的发光火焰区域附近观察到甲醛。对于温和燃烧的自燃升起火焰，尤其是在高温下，观察到独特的非单调升起高度行为；随着射流速度降低然后增加升空高度。这种行为类似于之前观察到的 CH4/H2 和 CO/H2 的二元混合燃料。瞬态均匀自燃分析表明，这种随射流速度下降的行为可归因于二甲醚的部分氧化特性，在边缘火焰区域之前产生可观数量的 CH4/CO/H2。