SUMMARYNumerical simulation with detailed chemistry has been carried out to clearly discriminate the thermal and chemical contributions of added diluents (H 2 O and CO 2 ) to major flame structures and NO emission characteristics in H 2 /N 2 counterflow diffusion flame. The pertinence of GRI, Miller-Bowman, and their recent modified mechanisms are estimated for the combined fuel of H 2 , CO 2 , and N 2 . A virtual species X ; which displaces the individual CO 2 and H 2 O in the fuel sides, is introduced to separate chemical effects from thermal effects. In the case of H 2 O addition the chain branching reaction, H+O 2 ! O+OH is considerably augmented in comparison with that in the case of CO 2 addition. It is also seen that there exists a chemically super-adiabatic effect in flame temperature due to the breakdown of H 2 O. The reaction path of CH 2 O ! CH 2 OH ! CH 3 and the C1-branch reactions become predominant due to the breakdown of CO 2 . In NO emission behaviour super-equilibrium effects caused by the surplus chain carrier radicals due to the breakdown of added H 2 O are more superior to the enhanced effects of prompt NO with the breakdown of added CO 2 . Especially, it is noted that thermal NO emission is directly influenced by the chemical super-equilibrium effects of chain carrier radicals in the case of H 2 O addition. As a result the overall NO emission in the case of the addition of H 2 O is higher than that in the case of CO 2 addition.
SUMMARYNumerical simulation of CO 2 addition effects to fuel and oxidizer streams on flame structure has been conducted with detailed chemistry in H 2 -O 2 diffusion flames of a counterflow configuration. An artificial species, which displaces added CO 2 in the fuel-and oxidizer-sides and has the same thermochemical, transport, and radiation properties to that of added CO 2 , is introduced to extract pure chemical effects in flame structure. Chemical effects due to thermal dissociation of added CO 2 causes the reduction flame temperature in addition to some thermal effects. The reason why flame temperature due to chemical effects is larger in cases of CO 2 addition to oxidizer stream is well explained though a defined characteristic strain rate. The produced CO is responsible for the reaction, CO 2 +H=CO+OH and takes its origin from chemical effects due to thermal dissociation. It is also found that the behavior of produced CO mole fraction is closely related to added CO 2 mole fraction, maximum H mole fraction and its position, and maximum flame temperature and its position.
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.
SUMMARYA numerical study has been conducted to clearly grasp the impact of chemical effects caused by added CO 2 and of flame location on flame structure and NO emission behaviour. Flame location affects the major source reaction of CO formation, CO 2 +H ! CO+OH and the H-removal reaction, CH 4 +H ! CH 3 +H 2 . It is, as a result, seen that the reduction of maximum flame temperature due to chemical effects for fuel-side dilution is mainly caused by the competition of the principal chain branching reaction with the reaction, CH 4 +H ! CH 3 +H 2 , while that for fuel-side dilution is attributed to the competition of the principal chain branching reaction with the reaction, CO 2 +H ! CO+OH.The importance of the NNH mechanism for NO production, where the reaction pathway is NNH ! NH ! HNO, is recognized. In C-related reactions most of NO is the direct outcome of (R171) and the contribution of (R171) becomes more and more important with increasing amount of added CO 2 as much as the reaction step (R171) competes with the key reaction of thermal mechanism, (R237), for N atom. This indicates a possibility that NO emission in hydrogen flames diluted with CO 2 shows less dependent behaviour upon flame temperature.
SUMMARYA numerical study with momentum-balanced boundary conditions has been conducted to grasp the chemical effects of added CO 2 to fuel-and oxidizer-sides on flame structure and NO emission behaviour in H 2 -O 2 diffusion flames with varying flame location. A reaction mechanism is proposed to show better agreements with experimental results in CO 2 -added hydrogen flames.Oxidizer-side dilution results in significantly higher flame temperatures and NO emission. Flame location is dramatically changed due to high diffusivity of hydrogen according to variation of the composition of fuel-and oxidizer-sides. This affects flame structure and NO emission considerably especially the chemical effects of added CO 2 . The present work also displays separately thermal contribution and prompt NO emission due to the chemical effects caused by thermal dissociation of added CO 2 in NO emission behaviour. It is found that flame temperature and the flame location affect the contribution of thermal and prompt NO due to chemical effects considerably in NO emission behaviour.
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