A numerical study of the auto-ignited laminar lifted methane/hydrogen mixtures in heated co-flow air

摘要

In the present study, the liftoff characteristics of a laminar HCNG flame are investigated using 2-D numerical simulations by varying the fuel jet velocity. It is observed that the liftoff height decreases with increasing fuel jet velocity. Gradual transition from the tribrachial flame to the MILD combustion is also observed. From both Da analysis and transport budget analysis, it is verified that auto-ignition is the main stabilization mechanism of the lifted flame. Additional simulations are carried out with modified hydrogen diffusivity, showing that the decrease of liftoff height is mainly attributed to the high diffusivity of hydrogen molecule. Preferential diffusion effect of hydrogen is clarified by revising the ignition delay time term in the liftoff height relation. It is also shown that the revised ignition delay time increases as the fuel jet velocity decreases since hydrogen ratio R_H decreases at the upstream of flame front with decreasing fuel jet velocity. The high diffusive nature of hydrogen molecule increases the ignition delay time at lower fuel jet velocity, which, in turn, leads to the increase of the liftoff height.