Four-wave mixing techniques, such as coherent anti-Stokes Raman spectroscopy (CARS), laser-induced grating spectroscopy (LIGS), and degenerate four-wave mixing (DFWM), have been widely used in combustion diagnostics due to their advantages of high signal-to-noise ratio (S/N), coherent signal, and spatial resolution. In this work, a nano-second pulsed laser is utilized to generate mid-infrared (near 3 µm) pump beams, exciting the rovibrational transitions of nascent water in flames. Combined LIGS and DFWM measurements are demonstrated in premixed laminar CH4/O2/N2 flames with equivalence ratios from 0.6 to 1.5, to achieve precise thermometry in a wide range of flame conditions. The flame temperatures were also measured by thermocouple as a reference, and the results from LIGS and DFWM align well with the trends shown in the thermocouple measurements. In fuel-lean flames, where the mass-to-specific-heat ratio variation is minimal, LIGS provides temperature data with a precision better than 16 K (0.8%). In fuel-rich flames, where the increased H2 concentration in the flame introduces uncertainty in gas constants thus affecting the accuracy of LIGS thermometry, DFWM is instead employed for temperature measurement since it is less sensitive to the gas composition within the measured volume. The high-precision LIGS temperatures in lean flames serve as temperature reference during the DFWM calibration of the degree of saturation, and a precision better than 90 K (4.5%) is achieved for DFWM thermometry. In addition to temperature, a theoretical model is employed to fit LIGS signal time waveforms, extracting the local speed of sound and thermal diffusivity with precisions better than 0.5% and 1.3%, respectively. These high-precision measurements contribute additional data for flame research and simulation calculations. This way, the combined use of DFWM and LIGS proves the potential for accurate thermometry and diagnostics of other thermodynamic parameters across a wide range of flame conditions.

中文翻译：

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使用红外四波混合技术对层流甲烷火焰进行温度和热扩散率诊断

四波混频技术，如相干反斯托克斯拉曼光谱（CARS）、激光诱导光栅光谱（LIGS）和简并四波混频（DFWM）等，由于其高分辨能力等优势，已广泛应用于燃烧诊断。信噪比 (S/N)、相干信号和空间分辨率。在这项工作中，利用纳秒脉冲激光产生中红外（近 3 µm）泵浦光束，激发火焰中初生水的振动跃迁。预混层流 CH 中演示了 LIGS 和 DFWM 组合测量4/O2/N2当量比为 0.6 至 1.5 的火焰，可在各种火焰条件下实现精确的测温。火焰温度也通过热电偶测量作为参考，LIGS 和 DFWM 的结果与热电偶测量中显示的趋势非常吻合。在贫燃料火焰中，质量比热比变化最小，LIGS 提供的温度数据精度优于 16 K (0.8%)。在富燃料火焰中，H 增加2火焰中的浓度引入了气体常数的不确定性，从而影响了 LIGS 测温的准确性，而采用 DFWM 进行温度测量，因为它对测量体积内的气体成分不太敏感。DFWM饱和度校准过程中，采用高精度稀火焰LIGS温度作为温度参考，DFWM测温精度优于90 K（4.5%）。除了温度之外，还采用理论模型来拟合 LIGS 信号时间波形，提取局部声速和热扩散率，精度分别优于 0.5% 和 1.3%。这些高精度测量为火焰研究和模拟计算提供了额外的数据。这样，DFWM 和 LIGS 的结合使用证明了在各种火焰条件下精确测温和诊断其他热力学参数的潜力。