NUMERICAL STUDY OF RADIATION REABSORPTION EFFECT ON NO_x FORMATION IN CH_4/AIR COUNTERFLOW PREMIXED FLAMES

摘要

The radiation reabsorption effects on NO_x (mainly on NO) formation in CH_4/air counterflow premked flame were numerically investigated by using a statistical narrow-band model (SNB) and a detailed chemistry (GRI-MECH 2.11). It was found that the reabsorption of emitting radiation leads to substantially wider flame thickness and higher flame temperature than those calculated by using the optically thin model. The results show that the level of NO_x is predicted to be highest in the adiabatic flames, that is, flames without radiation heat loss, and that the level of NO_x is predicted to be lowest in the flames by the optically thin model. In the flames by the SNB model, the predicted amount of NO_x lies between these two levels. In lean premked flames, N_2O is an important nitrogen oxide emission in addition to NO. As a result, the radiation reabsorption has a more significant influence on NO_x formation, not only on NO but also on N_2O production for lean premked flames. The calculated results also show that the radiation reabsorption effect on NO formation grows stronger as the stretch rate decreases and the equivalence ratio of flame approaches 1, particularly when CO_2, a strong absorber, is added to the unburned gas mixture. In this study, the effectiveness and validity of the optically thin radiation model for calculating NO_x formation in counterflow premixed flames was also investigated in comparison with the SNB model.