Abstract:

The oscillatory extinction mechanism of micro-gravitational dimethyl ether (DME) spherical diffusion flame in hot- and cool-flame conditions was studied by numerical modeling with detailed chemistry and transport model. The results show that a stable self-sustaining cold flame can be established under microgravity conditions, and the cold flame reaction can significantly expand the flammability limit of flameout. Oscillation existed prior to the steady-state extinction turning point of either hot or cool flame. The oscillatory extinction of DME hot flame was governed by a single oscillatory mode, and its frequency (1 Hz or so) was independent of the ambient oxygen mole fraction. By contrast, the cool-flame oscillatory extinction was governed by dual mode of oscillation with distinct frequencies, and the oscillation period of the high-frequency mode significantly increased when approaching the extinction limit. Moreover, the dual oscillatory modes showed strong coupling interaction, so the cool-flame extinction was more complicated than hot flames. The sensitivity analysis indicated that the hot flame extinction was controlled by the competition reactions of high-temperature exothermicity/endothermicity and chain-branching/termination involving small molecules, and the cool flame extinction was controlled by competition of low-temperature branching and termination reactions in the negative temperature coefficient regime.

Key words: computational fluid dynamics, chemical reaction, reaction kinetics, fuel, fluid mechanics

摘要：

采用详细的化学反应机理和组分输运模型，对二甲醚(DME)微重力球形扩散火焰在热焰与冷焰条件下的振荡熄火机理开展数值研究。结果表明，在微重力条件下可以建立稳定自持的冷焰，而且冷焰反应可以显著拓展熄火的可燃极限。在热焰与冷焰的稳态熄火点之前均观察到了振荡熄火过程。DME热焰的振荡熄火受单个振荡模式所控制，且振荡频率(约1 Hz)与环境氧含量无关。而冷焰的振荡熄灭受两个具有不同频率的双振荡模式所控制，在靠近熄火极限时高频振荡模式的振荡周期急剧增加。此外，高频与低频振荡模式之间存在着强烈耦合作用，导致冷焰的振荡熄火过程更加复杂。基于敏感性分析的结果表明，热焰的振荡熄火受小分子所参与的高温吸热/放热以及链分支/断裂反应之间的竞争关系所控制，而冷焰的振荡熄火受负温度系数机制下低温链分支与断裂反应之间的竞争关系所控制。

关键词: 计算流体力学, 化学反应, 反应动力学, 燃料, 流体力学