激光诱导击穿光谱技术(LIBS)作为一种新型光谱分析方法, 近年来被逐渐应用于燃烧诊断领域。 在甲烷/空气混合气中建立了一套LIBS测试系统, 利用H, O, N不同特征谱线强度比实现了对甲烷、 空气燃空当量比的定量测量。 在“门控”模式下, 比较分析了H656/O777与H656/N746这两种定标曲线对燃空当量比的测量效果, 发现H656/O777可以实现更好的预测精度与更高的灵敏度;“非门控”模式可以利用更多的谱线强度比(H656/O777, H656/N+500, H656/N+567和H656/N746)进行标定测量, 其中H656/O777的测试效果最好。 比较分析了“门控”与“非门控”两种检测模式对标定测量的影响: “门控”检测模式在拟合优度与预测精度方面都要略优于“非门控”模式。 此外, 在保持聚焦点能量密度相同的情况下, 研究了1 064, 532和355 nm三种不同激光波长对H, O和N特征谱线及H656/O777标定曲线斜率的影响: H, O和N特征谱线强度、 信噪比以及H656/O777标定曲线的斜率均随激光波长的增加而增加。 在三种波长中, 1 064 nm的激光最适合作为LIBS技术的光源, 定量测量甲烷/空气混合气当量比。 最后, 分别从等离子体电子密度、 温度对谱线强度与分布函数的影响方面, 对以上实验结果给出理论上分析与说明。

Laser-induced breakdown spectroscopy (LIBS) has been increasingly used in combustion diagnostics as a novel spectral analysis method in recent years. The quantitative local equivalence ratio of methane/air mixture is determined by LIBS using different emission intensity ratios of H/O and H/N. The comparison between calibration curves of H656/O777 and H656/N746 is performed in gated mode, which shows that H656/O777 can achieve better prediction accuracy and higher sensitivity. More spectral intensity ratios (H656/O777, H656/N+500, H656/N+567 and H656/N746) can be used to make calibration measurements in ungated mode and H656/O777 is also tested best among them. The comparison between gated and ungated detection modes shows that gated mode offers better accuracy and precision. In addition, the effects of different laser wavelengths (1 064, 532 and 355 nm) on LIBS spectra and calibration curves are investigated with laser focal point size and laser fluence kept constant. The results show that with longer laser wavelength, the peak intensity and SNR of H, O and N lines increase, as well as the slope of calibration curve of H656/O777. Among these three wavelengths, 1 064 nm laser is best suited to measure the equivalence ratio of CH4/air mixture by LIBS. The experimental results are explained in terms of plasma electron density and temperature, which have a significant impact on the emission intensity and the partition function of hydrogen and oxygen, respectively.