惰化条件下甲烷燃爆火焰特征试验与数值模拟分析

Experimental and numerical simulated analysis on combustion and explosion flame characteristics of methane under inerting conditions

为分析甲烷惰化极限条件下燃爆火焰特征,采用高速纹影对20 L球形装置内CO_(2)惰化抑制CH4燃爆的火焰图像进行采集,并利用数值模拟方法对燃爆过程进行重现。通过观察和分析火焰形状的演化过程、火焰熄灭现象以及火焰传播速度,可掌握浮力主导下的燃爆火焰的特征,揭示惰化条件下燃爆火焰形态演化机理和各方向扩展速度演化,分析导致火焰熄灭的原因。结果显示:爆炸初期,火焰在各方向扩展速度相同,火焰形态呈“球”形。之后,添加较少CO_(2)的工况中,火焰各方向扩展速度基本相同,一直呈“球”形。在惰化极限的工况中,火焰的已燃区和未燃区内体积分数差显著增大,与惰化弱的工况相比,浮力变得显著,“球”形火焰将转变为“ω”形火焰,且火焰出现整体上浮或熄灭现象。火焰上浮过程中,向下扩展的速度出现负值,且明显大于两侧速度。此外,数值模拟可以补充试验中视窗以外的火焰部分,模拟结果显示火焰在上浮过程中底部向上速度逐渐增大,且明显大于两侧速度,导致火焰面底部中心出现凹陷,最终形成“ω”形火焰。“ω”形火焰在上浮过程中,会与周围未燃气体或容器壁面换热进而导致火焰熄灭,但整个过程没有产生超压。

To analyze the characteristic change of explosion flame under limited conditions of methane inerted,high-speed schlieren was used to capture the flame images of CO,inerting inhibiting CH4 explosion in the 20 L spherical device.Besides,a numerical simulation method was used to reproduce the explosion process of CH,inerted.Through observing and analyzingthe evolutionprocess ofthe flameshape,extinguishment phenomenon,and velocity of flame,the characteristics of explosion flame dominated by buoyancy were mastered.It is found that the velocity of flame in all directions is almost the same,so the flame shape is“spherical”.Then,in the cases of adding less CO_(2),the flame shape is always spherical since the spread velocity in all directions is almost the same.Under the cases of inerting limit,the density difference between the burned zone and unburned zone of the flame increases significantly,which causes the buoyancy becomes significant.Thus,the“spherical”flame changes into a “ω”flame,and then the flame floats or extinguishes.During flame floating up,the velocity of the bottom is negative and significantly greater than that of the top and both sides.In addition,the numerical simulation can supplement the flame part outside the visualization window in the experiment.The simulation results show that the bottom velocity that directions upward gradually increases during the process of rising,which is significantly higher than that of both sides,resulting in a depression at the bottom center of the flame surface,and finally a “ω”-flame is formed.During the process of flame floating,the flame is extinguished due to the heat exchange with the surrounding unburned gas or the vessel wall,but there is no overpressure.