Temperature and CO₂ concentration measurements in the exhaust stream of a liquid-fueled combustor using dual-pump coherent anti-Stokes Raman scattering (CARS) spectroscopy

Single-shot, dual-pump coherent anti-Stokes Raman scattering (CARS) measurements of N₂ and CO₂ were performed in the exhaust stream of a swirl-stabilized JP-8-fueled combustor under sooting conditions. The combustor is designed to study particulate formation and particle-size distributions for different flame conditions and therefore is operated at near-stoichiometric overall fuel-air ratios. Various jet fuels and additive concentrations were studied. These conditions pose a significant challenge for temperature measurements using standard N₂ CARS due to strong flame emission and absorption of the CARS signal by the C₂ Swan band. With the dual-pump CARS technique employed in this study, the N₂ CARS signal is generated at a wavelength (496 nm) that is not absorbed by C ₂, and concentration measurements of CO₂ can be performed. The standard deviations of the single-shot temperature measurements were approximately 3-4% of the mean values for equivalence ratios ranging from 0.4 to 1.1, whereas those of the single-shot CO₂ concentration measurements were between 9 and 20% of the mean values. Previous single-shot temperature and CO₂ concentration measurements using dual-pump CARS in this liquid-fueled combustor were limited to an equivalence ratio of 0.45, with standard deviations in temperature of about 5-6% of the mean value of 1143 K (Lucht et al., AIAA J. 41 (4) (2003) 679-686). The current study demonstrates a significant improvement in the applicability of single-shot CARS temperature and CO₂ concentration measurements to practical, swirl-stabilized combustors under sooting conditions.