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.
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.
SUMMARYNumerical analysis is conducted to clarify chemical effects of added steam to either fuel-or oxidizer-side on flame structure and NO emission behaviour with detailed chemistry in hydrogen-oxygen-nitrogen diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties to added H 2 O, is introduced to feasibly isolate chemical effects of added H 2 O. It is found that the reaction step (-R23) is the starting point to induce chemical effects of added steam. Special concern is, thus, focused on the impact of OH radical on flame structure and NO emission behaviour. A strong dependency of the amount of steam addition on OH radical behaviour is clearly displayed, and this modifies flame structure sufficiently to produce higher flame temperature at more than a certain mole fraction of added steam in comparison to that diluted with artificial species. It is also shown that the reaction step (-R23) is closely related to flame temperature and thereby the location of maximum flame temperature. The behaviour of NO emission index is shown to be greatly influenced by the competition between the reaction steps of (R63) and (R65) in addition to Zeldovich NO. It is, consequently, seen that the intermediate active species, HNO, affects NO emission behaviour remarkably. These results may be helpful to understand the role of recirculated steam in the combustion systems with flue gas recirculation to either fuel-or oxidizer-side.
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