Double blue zones in inverse and normal laminar jet diffusion flames

Past experimental and computational works indicate that double blue zones are possible in hydrocarbon diffusion flames. This study examines these zones in inverse (IDF) and normal (NDF) laminar gas jet diffusion flames. Over 100 diffusion flames were observed with various fuels, diluents, dilution levels, and flow rates. Upon close examination, the IDFs and NDFs are seen to have double blue zones, separated at the flame tip by up to 1.6 and 0.9 mm, respectively, with a relatively dark region in between. By partially premixing the fuel and oxidizer streams, it was observed that, for both IDFs and NDFs, the zone toward the fuel side is rich, while that toward the oxidizer side is stoichiometric. Chemiluminescence was investigated using cameras and bandpass filters emphasizing emissions from OH* (310 nm), CH* (430 nm), CO₂* (455 nm), and C₂* (515 nm). The images were deconvolved using onion peeling. Broadband CO₂* emissions were subtracted from the images of OH*, CH*, and C₂* emissions to find profiles of spectral emissive power for these species. The thin, blue-green zone on the fuel side coincides with the peaks in CH* and C₂* emissions in the fuel-consumption zone, while the broader, blue stoichiometric zone coincides with the peaks in OH* and CO₂* emissions in the oxygen-consumption zone. The spectral emissive power peaks for C₂*, CO₂*, CH*, and OH* are typically 3 times as high in the IDF as in the NDF, which is attributed to the higher scalar dissipation rates.