二甲醚球形扩散火焰的振荡熄火动力学机理研究

doi:10.11949/0438-1157.20190808

摘要/Abstract

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

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

中图分类号:

TK 4

亢银虎, 张弋, 张朋远, 卢啸风. 二甲醚球形扩散火焰的振荡熄火动力学机理研究[J]. 化工学报, 2020, 71(4): 1469-1481.

Yinhu KANG, Pengyuan ZHANG, Xiaofeng LU. Study on oscillatory extinction dynamics mechanism of dimethyl ether spherical diffusion flame[J]. CIESC Journal, 2020, 71(4): 1469-1481.

图/表 11

图1 经典的“S曲线”以及H2在熄火极限附近的燃烧产热速率变化特性(TE是熄火点E的温度)

Fig.1 Canonical S-curve and near-extinction heat release behavior of H2 (TE is temperature at extinction point E)

图2 球形扩散火焰示意图以及GRAD=CURVE=0.1(网格个数486)、GRAD=CURVE=0.2时(网格个数303)XO2*=42.1%条件下的火焰仿真结果对比（GRAD和CURV分别代表流场中标量的梯度与曲率）

Fig.2 Diagram of spherical diffusion flame, and comparison of predicted results using GRAD=CURVE=0.1 (486 grids) and GRAD=CURVE=0.2 (303 grids) for flame at XO2*=42.1%

图3 稳态DME球形扩散火焰的最高温度随XO2*的响应

Fig.3 Response of maximum temperature with respect to XO2* for steady-state spherical diffusion flame

图4 环境成分分别为42.1%O2/57.9%He与18.9%O2/81.1%He时的热焰和冷焰的火焰结构

Fig.4 Flame structures of hot flame at ambient composition 42.1%O2/57.9%He and cool flame at 18.9%O2/81.1%He, respectively

图5 不同环境氧气摩尔分数条件下的热焰温度场经摄动之后最高温度的瞬态响应过程(δT是温度场所受到的扰动比例)

Fig.5 Response of maximum temperature upon a temperature perturbation for hot flames at different ambient oxygen mole fractions (δT is temperature perturbation ratio)

图6 不同环境氧气摩尔分数条件下的冷焰振荡熄火过程以及相应的频谱分析图

Fig.6 Oscillatory extinction process of cool flames at different ambient oxygen mole fractions as well as spectral analyses

图7 对热焰与冷焰的振荡熄火过程起控制作用的关键反应及其SIr值

Fig.7 Key reactions that govern oscillatory extinction process with their SIr values for hot and cool flames

图8 关键反应的速率常数与关键标量的输运参数乘以某倍数之后所得的热焰分支图

Fig.8 Response of hot flame branch after key reaction rate constants and transport parameters are multiplied by a factor

图9 关键反应的速率常数与关键标量的输运参数乘以某倍数之后所得的冷焰分支图

Fig.9 Response of cool flame branch after key reaction rate constants and transport parameters are multiplied by a factor

图10 XO2*=24.02%、δT=+0.3%条件下的热焰振荡熄火过程中关键反应R1、R29、R53、R56、R30、R48的最大反应速率与最高温度之间的相位关系(圆圈表示经摄动之后振荡的起始点，正交虚线的交点表示摄动前的稳态点)

Fig.10 Phase functions of peak temperature and maximum reaction rate for key reactions R1, R29, R53, R56, R30 and R48 during oscillation extinction process of hot flame at XO2*=24.02% and δT=+0.3%

图11 XO2*=6.1%、δT=+1.5%条件下的冷焰振荡熄火过程中，200~250 s期间内，关键反应R240、R264、R273、R274、R272、R44的最大反应速率与最高温度之间的相位关系

Fig.11 Phase functions of peak temperature and maximum reaction rate for key reactions R240, R264, R273, R274, R272, and R44 within 200—250 s of oscillation extinction process for cool flame at XO2*=6.1% and δT=+1.5%

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