Flame base structures of micro-jet hydrogen/methane diffusion flames

In this study, a comparison of flame base structures of hydrogen/methane-air diffusion flames formed over a tiny-jet is made numerically for both isothermal and thermal conductive burner conditions, in order to clarify the fuel dependent flame stabilization mechanisms. It is found that, unlike a methane flame, the flame base of a hydrogen flame always attaches to the burner. The analyses indicate that the dominant intermediate-consumption steps have significantly lower activation energies for the hydrogen flame as compared to a methane flame. More importantly, one of the HO2 production reactions (R43f: H + O2 + M → HO2 + M), which has a dominant role in sustaining reactivity at the flame base, shows a negative temperature dependence, causing the heat release rate in the flame base kernel to increase as the burner wall temperature decreases. With a thermal conductive burner (thermal conductivity of 16 W/m-K) over a wide range of fuel jet velocities (0.5-4.0 m/s), it is found that the burner tip is heated to a significantly higher temperature by a hydrogen flame due to its unique stabilization mechanism. The mixing effects of hydrogen and methane are then considered. It is found that the burner tip temperature can be reduced by adding methane into the fuel flow. This is because, according to the investigation of the structures of the hydrogen/methane jet diffusion flames, the reaction rate of R43f is suppressed due to the included intermediates (e.g., CH3, CH2O) consumption steps of methane. It is expected that the flame attachment feature associated with the flame base structure can be easily controlled by mixing hydrogen and methane, making it possible to control the burner tip temperature in advance.