| Summary: | It is important to monitor the temperature and H<sub>2</sub>O concentration in a large combustion environment in order to improve combustion (and thermal) efficiency and reduce harmful combustion emissions. However, it is difficult to simultaneously measure both internal temperature and gas concentration in a large combustion system because of the harsh environment with rapid flow. In regard, tunable diode laser absorption spectroscopy, which has the advantages of non-intrusive, high-speed response, and in situ measurement, is highly attractive for measuring the concentration of a specific gas species in the combustion environment. In this study, two partially overlapped H<sub>2</sub>O absorption signals were used in the tunable diode laser absorption spectroscopy (TDLAS) to measure the temperature and H<sub>2</sub>O concentration in a premixed CH<sub>4</sub>/air flame due to the wide selection of wavelengths with high temperature sensitivity and advantages where high frequency modulation can be applied. The wavelength regions of the two partially overlapped H<sub>2</sub>O absorptions were 1.3492 and 1.34927 μm. The measured signals separated the multi-peak Voigt fitting. As a result, the temperature measured by TDLAS based on multi-peak Voigt fitting in the premixed CH<sub>4</sub>/air flame was the highest at 1385.80 K for an equivalence ratio of 1.00. It also showed a similarity to those tendencies to the temperature measured by the corrected R-type T/C. In addition, the H<sub>2</sub>O concentrations measured by TDLAS based on the total integrated absorbance area for various equivalent ratios were consistent with those calculated by the chemical equilibrium simulation. Additionally, the H<sub>2</sub>O concentration measured at an equivalence ratio of 1.15 was the highest at 18.92%.
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