On micro-buoyancy spherical diffusion flames and a double luminous zone structure of the hydrogen/methane flame

Almost spherically symmetry diffusion flames of substantial dimension (diameter ∼ several centimeters) were established in earth gravity by ejecting an O₂/N₂ mixture from a porous sphere into a subatmospheric pressure environment consisting of low-molecular-weight fuels such as methane and hydrogen. In the present investigation, a double-luminous zone structure, consisting of a green luminous zone near the fuel side, a blue or violet luminous zone near the oxidizer side, and a dark space between the two luminous zones, was observed for subatmospheric diffusion flames of either air or an O₂/N₂ mixture against a mixture of hydrogen with a small quantity of methane. The two luminous zones eventually merge with increasing methane addition, characterized by the green luminous zone shifting toward the blue luminous zone. Computational simulation using the experimental boundary conditions shows that the blue and green luminous zones respectively correspond to the main consumption layers of H₂-O₂ and CH₄; that the breakdown of CH₄ is primarily due to its attack by H, leading to the formation of CO, additional H₂, and eventually H₂O and CO₂; and that the electronically excited CO₂ and C₂ are respectively responsible for the blue and green luminescence. This double-luminous zone structure also exhibits a secondary peak in the heat release rate profile; important reactions accounting for the two heat release peaks were identified.