Improved laser absorption spectroscopy measurements of flame temperature via a collisional line-mixing model for CO2 spectra near 4.17 µm

An experimental and theoretical study of CO2 absorption spectroscopy near 4.172 μm was conducted to acquire measurements of gas temperature and CO2 concentration in flames at near-atmospheric pressure. Scanned-wavelength laser absorption spectroscopy measurements were obtained in laminar non-premixed flames produced using either a Hencken burner or inside a fixed-volume combustion vessel. For the Hencken burner flame, the non-uniform distribution of gas conditions along the line-of-sight was estimated from computational fluid dynamics (CFD) simulations and accounted for in simulations of path-integrated absorbance spectra. Gas properties were inferred from measured absorbance spectra using two models: (1) an isolated line model employing the Voigt Profile and (2) a collisional line-mixing model. Regarding the latter, the relaxation matrix for CO2 in the line-mixing model was modified and scaled based on an empirically derived collisional-relaxation matrix for CO2–Ar. The collisional line-mixing model reduced the residuals of the best-fit spectra by approximately a factor of 4. In addition, the flame temperature obtained from the line-mixing model agreed well with that predicted by adiabatic equilibrium calculations and was typically 10% more accurate than those produced by the isolated line model, thereby illustrating the importance of accounting for collisional line mixing at the conditions and wavelengths studied despite the modest gas pressure and high temperature.