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

Abstract

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 (Tc ) and species concentration (Xc ) 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; Tc = 1400–2200 K; and Xc = 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 Tc . In comparison, the variation of Xc 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.