Simultaneous Co and Oh Imaging in Flames Using a Single Broadband Femtosecond Laser Pulse

Hydrocarbon combustion is the primary source of energy for aviation and power-generation gas turbine applications.  In such conditions, quantitative studies of spatially resolved CO formation in the flame zone is critical because of its involvement in the reaction, CO+OH  CO₂+H, which accounts for a large fraction of heat release during hydrocarbon combustion.  In this work, we report a novel experimental approach for simultaneous excitation/detection of carbon monoxide (CO) and hydroxyl (OH) in flames by using a single femtosecond (fs)-duration laser source. Two-photon transition of CO A¹←←X¹Ʃ⁺ (4, 0) and single-photon transition of OH A²Ʃ⁺←X¹ (1, 0) are simultaneously excited using a single broad laser pulse near 283 nm. Subsequently, crosstalk-free fluorescence bands are detected in the 203–225 nm region for CO and 310–325 nm region for OH. The present study focusses on simultaneous imaging CO and OH in a McKenna flat-flame burner with C₂H₄-air as a pilot flame and non-premixed turbulent C₂H₄ jet at the center. The location of CO with respect to the reaction zone marked by OH is revealed. The measured CO and OH concentration profiles as a function of the equivalence ratio are well predicted by the equilibrium flame calculations. Such simultaneous measurements of CO and OH  enables a better understanding of the combustion processes which is vital for future gas turbine applications. The present experimental challenges and further improvement strategies are also discussed.