Impact of UV absorption by CO2 and H2O on NO LIF in high-pressure combustion applications

The influence of UV light absorption by hot CO2 and H2O is evaluated for laser-induced fluorescence (LIF) measurements of NO in high-pressure combustors. UV lasers are ubiquitously used to measure LIF from species like NO, OH, HCO, and O-2, as well as Raman and Rayleigh scattering in combusting environments. However, attenuation of the laser probe and/or signal by optical absorption from major combustion species is seldom considered. In this paper, we show that neglecting UV attenuation by major product species like CO2 may lead to large errors in combustion measurements. Absorption cross sections between 190 and 320 mn are measured in shock-heated CO2 and H2O at temperatures ranging from 900 to 3050 K. The absorption cross section of CO2 has strong temperature dependence and increases by 4 orders of magnitude at 193 run between 300 and 2000 K. The measured temperature-dependent absorption spectra for CO2 and H2O are fit to an empirical function to provide a tool for facile assessment of potential errors and quantitative corrections for UV combustion diagnostics. LIF measurements of NO in a high-pressure burner and an internal combustion engine are adjusted for CO2 and H2O absorption to demonstrate the importance of these corrections.