UV absorption of CO₂ for temperature diagnostics of hydrocarbon combustion applications

At room temperature, CO₂ is transparent in the ultraviolet (UV) at wavelengths longer than 205 nm; however, at temperatures above 1000 K the CO₂ absorption cross-section becomes significant in the region between 200 and 320 nm. Because CO₂ is a major product of hydrocarbon combustion and because both the magnitude of the absorption cross-section and the shape of the UV absorption spectrum vary strongly with temperature, measurements of UV optical absorption spectra offer the potential to infer gas temperature in combustion systems. In this paper, we demonstrate the first use of UV absorption measurements to determine temperature using five different experimental examples to illustrate the utility in hydrocarbon combustion applications of this new temperature diagnostic strategy. (1) Transmission measurements of cw laser light at 266 nm are used to determine time-resolved temperature in shock-heated CO₂. (2) Similar transmission measurements are used to infer time-resolved temperature behind a detonation wave in a pulse-detonation engine using absorption from equilibrium concentrations of the CO₂ combustion product. (3) The absorption of pulsed laser light near 226 nm is used to infer temperature in the burned gases of a premixed high-pressure methane flame. (4) Wavelength-resolved absorption of light from a broadband UV deuterium lamp is time-resolved with a kinetic spectrograph to acquire time-resolved absorption spectra illustrating the measurement of temperature in a system with changing temperature and CO₂ mole fraction. (5) Time-gated, spectrally resolved transmission of a deuterium lamp is used to derive temperature at specific crank angles in a piston engine. These examples demonstrate that temperature measurements based on UV optical absorption of CO₂ have good potential for use in a wide variety of hydrocarbon combustion applications.