Numerical study on flame structure and NO formation in CH_4-O_2-N_2 counterflow diffusion flame diluted with H_2O

摘要

Numerical study on flame structure and NO emission behaviour has been conducted to grasp chemical effects of added H_2O on either fuel- or oxidizer-side in CH_4-O_2-N_2 counterflow diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties of added H_2O, is introduced to feasibly isolate the chemical effects. Special concern is focused on the important role of remarkably produced OH radicals due to chemical effects of added H_2O on flame structure and NO emission. The reason why the difference of behaviours between the principal chain branching reaction rate and flame temperature appear is attributed to the drastic change of reaction step (R120) from the production to the consumption of OH. It is also, however, seen that the most important contribution of produced OH due to chemical effects of added H_2O is through reaction step (R127). The importantly contributing reaction steps to NO production are also examined. The production rates of thermal NO and prompt NO are suppressed by chemical effects of added H_2O. The contribution of the reaction steps related to HNO intermediate species to the production of prompt NO is also stressed.