Rieker Gregory B, Jeffries Jay B, Hanson Ronald K
High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering,Stanford University, Stanford, California 94305, USA.
Appl Opt. 2009 Oct 10;48(29):5546-60. doi: 10.1364/AO.48.005546.
We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.
我们展示了一种用于在恶劣环境中测量气体温度和浓度的无校准二次谐波检测波长调制光谱技术（WMS-2f）的实际应用。该方法适用于使用具有同步波长和强度调制的激光器（如注入电流调谐二极管激光器）进行的测量。实现无需通常与WMS相关的现场校准即可进行测量的关键因素包括：（1）用基波（1f）信号对WMS-2f信号进行归一化，以考虑激光强度；（2）在用于与测量的1f归一化WMS-2f信号进行比较以推断气体特性的光谱吸收模型中纳入特定于激光器的调谐特性。讨论了与无校准WMS方法相关的不确定性，特别强调了压力和光学深度对WMS信号的影响。这些不确定性中的许多也适用于校准后的WMS测量。展示了一个示例实验装置，该装置将六个波长在1.3至2.0μm之间的可调谐二极管激光源组合成一个探测光束，用于温度、H₂O和CO₂的测量。采用波长和频率解复用的混合组合来区分激光信号，并给出了最佳的激光调制波形集。该系统在地面测试超燃冲压发动机燃烧室的恶劣环境中得到了验证。对于超声速流中CO₂的测量，直接吸收与1f归一化WMS-2f的比较表明，WMS技术的信噪比提高了4倍。使用一种考虑视线（LOS）不均匀性对吸收测量影响的简单方法，将多维计算流体动力学（CFD）计算结果与温度和H₂O的测量结果进行了比较。比较结果表明，LOS无校准技术能够获取有关多维CFD模型的有用信息。
