In this study, a strategy was developed for in situ, non-intrusive, and quantitative measurement of the oxides of nitrogen (NO and NO₂) to describe emission characteristics in gas turbines. The linear calibration-free wavelength modulation spectroscopy (LCF-WMS) approach combined with the temperature profile-fitting strategy was utilized for trace NO and NO₂ concentration detection with broad spectral interference from gaseous water (H₂O). Transition lines near 1308 nm, 5238 nm, and 6250 nm were selected to investigate the H₂O, NO, and NO₂ generated from combustion. Experiments were performed under different equivalence ratios in a combustion exhaust tube, which was heated at 450–700 K, with an effective optical length of 1.57 m. Ultra-low NO_x emissions were captured by optical measurements under different equivalence ratios. The mole fractions of H₂O were in agreement with the theoretical values calculated using Chemkin. Herein, the uncertainty of the TDLAS measurements and the limitation of improving the relative precision are discussed in detail. The proposed strategy proved to be a promising combustion diagnostic technique for the quantitative measurement of low-absorbance trace NO and NO₂ with strong H₂O interference in real combustion gases.

This paper performed a comprehensive study of the thermal nonequilibrium effects of CO/Ar mixtures with various degrees of N2 additions and probed the N2 relaxation behaviors via the CO rovibrational thermometry. The rovibrational temperature time histories of shock-heated CO/N2/Ar mixtures were measured via a laser-absorption technique, and the corresponding vibrational relaxation data were summarized at 1890–3490 K. The measured results were compared with predictions from the Schwartz–Slawsky–Herzfeld (SSH) formula and the state-to-state (StS) approach (treating CO and N2 as pseudo-species). The vibrational state-specific inelastic rate coefficients for N2–N2 collisions were supplemented using the mixed quantum–classical calculations. The StS predictions, informed by experimentally measured pressures, showed good agreement with experimental data. Additionally, the impact of coupling between flow dynamics and StS kinetics behind reflected shock waves was evaluated using two different one-dimensional approaches, which provide limiting bounds (accounting for unsteady flow and end wall effects) in post-reflected shock flow conditions. Moreover, the vibrational relaxation data of the N2–N2 system were modified via sensitivity analysis to improve the performance of the SSH formula. Further analysis highlighted that the vibration–vibration–translation path provides an efficient way for vibrational energy transfer between CO and N2, resulting in almost the same vibrational temperature time histories for CO and N2. Therefore, the N2 relaxation behaviors can be characterized by the CO rovibrational thermometry, considering N2 is infrared inactive. Finally, the heat sink effects and the reflected-shock-bifurcation phenomena were highlighted.

As an effective optical diagnosis method, tunable diode-laser absorption spectroscopy (TDLAS) has increasingly moved to examine nonuniform flows, such as two-dimensional combustion diagnosis. To investigate the effect of nonuniformity along the line of sight in a measurement using TDLAS, the integrated absorbance (IA, the key intermediate quantity in TDLAS) error was quantified. The error distribution is obtained from the line-shape parameters through the comprehensive analysis of the line-shape function and the fitting method. The effects of the fitting function and the absorption line overlap are also considered. A general method for estimating the error is given. The work illustrates the applicability of TDLAS technology in nonuniform flow fields and provides input parameters for the evaluation of tunable diode laser absorption tomography error.