| Summary: | The structure of methane/air tubular diffusion flames with <inline-formula>
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<mn>65</mn>
<mo>%</mo>
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</inline-formula> fuel dilution by either CO<sub>2</sub> or N<sub>2</sub> is numerically investigated as a function of pressure. As pressure is increased, the reaction zone thickness reduces due to decrease in diffusivities with pressure. The flame with CO<sub>2</sub>-diluted fuel exhibits much lower nitrogen radicals (N, NH, HCN, NCO) and lower temperature than its N<sub>2</sub>-diluted counterpart. In addition to flame structure, NO emission characteristics are studied using analysis of reaction rates and quantitative reaction pathway diagrams (QRPDs). Four different routes, namely the thermal route, Fenimore prompt route, N<sub>2</sub>O route, and NNH route, are examined and it is observed that the Fenimore prompt route is the most dominant for both CO<sub>2</sub>- and N<sub>2</sub>-diuted cases at all values of pressure followed by NNH route, thermal route, and N<sub>2</sub>O route. This is due to low temperatures (below 1900 K) found in these highly diluted, stretched, and curved flames. Further, due to lower availability of N<sub>2</sub> and nitrogen bearing radicals for the CO<sub>2</sub>-diluted cases, the reaction rates are orders of magnitude lower than their N<sub>2</sub>-diluted counterparts. This results in lower NO production for the CO<sub>2</sub>-diluted flame cases.
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