Effects of hydrogen addition on the Markstein length and flammability limit of stretched methane/air premixed flames

A computational study is performed to investigate the effects of hydrogen addition on the fundamental characteristics of steady and unsteady stretched methane/air premixed flame in an opposed flow configuration. The problem is of interest as a potential application to gas turbines and sparkignition engines, where the addition of a small amount of hydrogen allows combustion at leaner conditions to achieve lower NOx emission. The flame response is first studied under steady conditions with different levels of hydrogen addition. The effective Markstein length is found to exhibit a nonmonotonic function of the level of blending due to the competing effects between the Zeldovich and Lewis number variations. The results also show that the lean flammability limit is significantly extended due to the presence of hydrogen in the mixture, consistent with previous studies. On the other hand, the consumption speed and time scale of the flame at the extinction condition are found to be rather insensitive to the extent of blending. Unsteady flame response is subsequently studied by imposing oscillatory equivalence ratio at the boundary, as a means to characterize the effects of mixture stratification at various time scales. Consistent with the steady results, the attenuation of the dynamic flammability limits in a normalized scale collapse very well for various levels of hydrogen blending, implying that the unsteady flame response depends strongly on the characteristic chemical time scale irrespective of the amount of fuel blending. A simple analytical solution is derived to predict consistent qualitative behavior. The net effects of unsteady composition fluctuation on the NOx formation are also discussed.