Line-of-sight temperature and species profiles determined from spectral transmittances

Line-of-sight (LOS) variations of temperature and concentrations of CO and H₂O within a simulated high-pressure flame are deduced using an inverse analysis. In this work, synthetic spectral transmittances, acquired by a Fourier transform infrared spectrometer along a single LOS, represent the experimental data. The theoretical basis of the analysis is that spectral variations in the absorption coefficient contain information about spatial variations in temperature and species concentrations. The Marquardt-Levenberg method is used to solve for the temperature and species concentrations. Accurate spatial variations of temperature and species concentrations can be recovered when changes in the spectral transmittances caused by noise are smaller than those changes caused by spatial variations in temperature and species concentrations. The recovered centerline temperatures and species concentrations are, respectively, within 5 and 20% of the synthetic values, when the variations in spectral transmittance caused by noise are about the same as that caused by spatial variations in temperature and species concentrations. As temperature increases, a redistribution in the molecular states decreases the spectral absorptances, thereby causing the analysis to become more sensitive to the effects of noise.