Multispectral infrared absorption spectroscopy for quantitative temperature measurements in axisymmetric laminar premixed sooting flames

Multispectral infrared absorption spectroscopy was developed for in situ, non-intrusive and quantitative measurements of temperature distributions in laminar premixed ethylene/air sooting flames. Tunable distributed feedback (DFB) lasers near 1343 nm, 1392 nm and 2482 nm were used to exploit multiple H2O absorption lines with varied temperature sensitivities. Scanned-wavelength direct absorption spectroscopy combined with the multi-line profile-fitting strategy was conducted for temperature sensing along the line-of-sight. This optical method was first numerically investigated for representative combustion fields to evaluate the measurement accuracy and uncertainty under different noise levels (2-10%). In the experiment, optical measurements were performed at different heights above the burner (3-15 mm) under three representative flame conditions (equivalence ratio Phi = 1.9, 2.1 and 2.3) with different co-flow gases (N-2 and air). Our measurements successfully captured the temperature field and were in excellent agreement with the thermocouple data within the high-temperature region. A slight temperature rise in the central flame was quantitatively differentiated when co-flow gas was changed from N-2 to air. The proposed method proves to be a promising combustion diagnostic technique for quantitative temperature measurements with the line-of-sight information.