On the Quantification of Boundary Layer Effects on Flame Temperature Measurements Using Line-of-sight Absorption Spectroscopy

ABSTRACT

We studied the boundary layer effects on flame temperature measurements using line-of-sight (LOS) absorption spectroscopy. The primary objective is to quantify the measurement uncertainty of the LOS-determined temperature, which is influenced by the thermal and species boundary layers existing in standard laminar premixed flames. The boundary layer thickness (δ), central flame temperature (T_c) and species concentration (X_c) are the major factors investigated in this work. Typical absorption lines of H₂O in the wavelength range of 1.4–2.9 μm were examined under different boundary layer conditions (δ/L = 0–50%, L is the optical path length above the flame; T_c = 1400–2200 K; and X_c = 14–20%). As a result, the thermal boundary layer is observed to contribute mainly to the temperature deviation of the LOS measurement, which increases with δ and T_c. In comparison, the variation of X_c between 14% and 20% has a negligible effect on the temperature measurement in the presence of both thermal and species boundary layers. A systematical investigation of all the selected line pairs reveals that a properly selected line pair reduces the temperature deviation by a maximum of 16.8% under typical laminar flame conditions. The particular line pair centered at 4029.52 cm⁻¹ and 4030.73 cm⁻¹, which could be detected by a single tunable semiconductor laser, is recommended for the LOS temperature measurement over a temperature range of 1400–2200 K. Finally, we performed a case study of five representative temperature measurements in laminar flames and successfully corrected the LOS-determined temperature by taking into account the boundary layer effects.

KEYWORDS:

• Flame temperature
• Boundary layer effects
• Laser absorption spectroscopy
• Line-of-sight

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Funding

This research is supported by National Natural Science Foundation of China (NSFC) (51776179), Natural Science Foundation of Guangdong Province (2019A1515011372), Science Foundation of Sichuan Province (No. 2020JDRC0034), and Open Funding from State Key Laboratory of High-temperature Gas Dynamics . We thank Professor Xing Chao and Mr. Zhenhai Wang from Tsinghua University for providing the set of flame temperature data.