SUMMARYThe dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically simulated with detailed chemistry in CH 4 /air counterflow diffusion flame. The volume percentage of diluents (H 2 O, CO 2 , and N 2 ) in air stream is systematically changed from 0 to 10. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour especially at low strain rates. The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO 2 >H 2 O>N 2 , even if heat capacity of H 2 O is the highest. It is also found that the addition of CO 2 shows the tendency towards the reduction of flame temperature in both the thermal and chemical sides, while the addition of H 2 O enhances the reaction chemically and restrains it thermally due to a super-equilibrium effect of the chain carrier radicals caused by the breakdown of H 2 O in high-temperature region. The comparison of the nitrogen chemical reaction pathway between the cases of the addition of CO 2 and H 2 O clearly displays that the addition of CO 2 is much more effective to reduce NO emission.

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.

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The NO x emission characteristics of dimethyl ether (DME) in laminar co-axial jet and counterflow nonpremixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NO x emissions of DME were compared to those of C 2 H 6 , which has equivalent methyl structures but lacks an oxygen atom. Experimental results showed that, in the co-axial jet case, the combustion of DME had the characteristics of a partial premixed flame. Additionally, it had a shorter flame and lower NO x emissions compared to the C 2 H 6 flame. It is thus concluded that the major cause of low NO x emissions from DME co-axial jet flames may be the short flame length because of the lower stoichiometric air/fuel ratio. The activation of reburning NO chemistry because of the characteristics of the partially premixed flame may also play a role. The computational results of the DME counterflow non-premixed flame revealed that the EI NO decreased by approximately 50% relative to that of the C 2 H 6 flame. Although the overall NO x reaction path of the DME flame is similar to that of the C 2 H 6 flame, it is concluded that the DME non-premixed flame has a distinct NO reduction mechanism. This is associated with reburning NO chemistry in the fuel-rich region because of fast pyrolysis and oxidation reactions in comparison to that of the C 2 H 6 flame.