Optical diagnostics of nanosecond and nanometer ignition and quenching

为了获得分析点火机理的基础数据，对甲烷/空气和乙炔/空气层流预混火焰的火焰前锋和锚点进行了化学发光测量。使用化学发光光谱，锚点位置，局部当量比，和火焰温度进行了检查。局部当量比是用OH^* 和CH^* 的强度比测量的，温度是用OH^* 光谱的P和R分支的强度比测量的。在锚点附近，局部当量比和温度较低，当强脉冲激光聚焦到气体中的一个小光斑时，会发生击穿现象，产生高温高压的火花等离子体。如果激光束与可燃气体相互作用，等离子体可以点燃气体。等离子体的发射光谱由ICCD光谱仪获得，并使用分幅ICCD相机捕获。从这些结果中，得到以下结论。（1）等离子体的形状首先是圆柱形。之后，等离子体膨胀成球形。等离子体尺寸在2000 ns之前一直增大，之后迅速减小。（2）韧致辐射和复合辐射的连续谱出现在等离子体发展的早期。然后，连续谱减弱，观察到氢、氮、氧和碳的离子谱和原子谱。（3）等离子体尺寸与入射激光能量有关。发现H/O和H/N的强度比是混合物当量比的线性函数。实验结果表明LIBS技术可以测量甲烷或丙烷预混层流火焰的局部当量比。

In order to obtain basic data for analysis of ignition mechanism, chemiluminescence was measured at the flame front and the anchor point of laminar premixed CH_4/air and C_3H_8/air flames. Using the chemiluminescence spectra, anchor point location, local equivalence ratio, and flame temperature were examined. The local equivalence ratio was measured using the intensity ratio of OH^* and CH^* and the temperature were using measured the intensity ratio of the P and the R branches of the OH^* spectra. The local equivalence ratio and the temperature were lower near the anchor point.When a powerful pulsed laser is focused to a small spot in gas, breakdown phenomena occurs and a spark plasma of high temperature and high pressure is created. If the laser beam interacts with a combustible gas, the plasma can ignite the gas. The emission spectra of the plasma were obtained by ICCD spectrometer and were captured using Framing ICCD camera. From these results, the following conclusions are obtained. (1)The shape of plasma turns out to be cylindrical at first. After that, the plasma expands spherically. The plasma size increases until 2000ns, after that time it was decreases rapidly. (2)The continuum spectrum by the bremsstrahlug and the recombination radiation came up at the earliest stage of the plasma development. Then the continuum spectrum decreased, the ionic spectra and the atomic spectra of hydrogen, nitrogen, oxygen, and carbon observed. (3)The plasma size was a correlated with the incident laser energy. The intensity ratios of H/O and H/N were found to be a liner function of equivalence ratio of the mixture. These results demonstrated that LIBS application can measure the local equivalence ratio of methane or propane premixed laminar flame.