Blue emission in premixed hydrogen flames

The origin of the blue emission in premixed hydrogen flames has been studied. Experiments examining the blue flame were performed using premixed hydrogen/air mixtures in an axisymmetric burner at a jet velocity of 12 m/s and varied equivalence ratios. Nitrogen was used to prevent the influence of a diffusion flame between the ambient air and the hydrogen flame. A spectrometer with a CCD camera measured emission spectra between 385 and 495 nm to identify species that affect the blue emission. Reaction rates, mole and mass fractions were calculated using Chemkin with the GRI 3.0 mechanisms. The blue flame is identified in visible pictures by its distinct emissions, which can be categorized into dark blue and light blue. Clear peaks have been identified in the wavelength range of 385–495 nm. The most pronounced peaks occur between 385 and 420 nm under fuel-lean and stoichiometric combustion conditions, while the peaks between 420 and 495 nm exhibit lower intensities. Emission intensities associated with hydroxyl radicals (OH) and atomic oxygen (O) demonstrate a strong correlation with the reaction rates of chemical reactions and their mole and mass fractions. Emission intensities linked to molecular hydrogen (H₂) and molecular oxygen (O₂) reveal a pronounced correlation with the reaction rates. Conversely, emissions related to atomic hydrogen (H) do not reveal a consistent relationship with their mole and mass fractions. The blue emission observed is likely influenced by a combination of OH, H₂, O₂, and O. The distinguishable peaks between 390 and 420 nm appear to result from the combined contributions of these species. The comparatively low peaks in the range of 440–495 nm are primarily associated with H₂. It is important to recognize that the blue emission in hydrogen flames could be affected by OH, H₂, O₂, and O.