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SUMMARYNumerical study on flame structure and NO emission behaviour has been conducted to grasp chemical effects of added H 2 O 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 2 O, 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 2 O 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 2 O 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 2 O. The contribution of the reaction steps related to HNO intermediate species to the production of prompt NO is also stressed.

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SUMMARYNumerical study on flame structure and NO emission is conducted covering a wide range of atmospheric temperature, high temperature, and mild combustion regimes in H 2 -Air laminar flames diluted with steam. Special concern is focused on the difference of flame structure and NO emission behaviour between hightemperature combustion and mild combustion modes. The important role of chemical effects of added steam in flame structure and NO emission behaviour is also discussed. It is seen that there exists an oxidizer-side temperature limit which the combustion mode changes from high temperature combustion to mild combustion. In high temperature combustion modes the OH production via the reaction step, (-R23) is suppressed while in mild combustion modes is enhanced by the increase of oxidizer-side temperature. It is also found that chemical effects of added steam are influenced by the competition between both the reaction steps, (R21) and (-R23).NO emission index increases with increasing oxidizer-side temperature and decreases with mole fraction of added steam. The remarkably produced OH due to chemical effects of added steam does not contribute to the increase of NO but plays a role of holdback on NO in thermal mechanism. It is also seen that in both the high temperature combustion and mild combustion modes NO emission indicates a consistently similar tendency, and is consequently recognized that in the whole ranges steam addition suppresses NO emission.