Experimental and numerical study of curvature effects and NO formation in ammonia Bunsen flames

Analysis of ammonia Bunsen flame is meaningful for understanding ammonia combustion details and solving the two major problems, the poor combustion characteristics and the severe NO emission. In the present work, ammonia Bunsen flames with equivalence ratio from 0.9 to 1.2 were successfully achieved at an elevated preheat temperature, 873 K, using an elevated-preheat-temperature jet flame facility. Detailed simulations were con- ducted using the CFD method combined with a chemical kinetic model. A good agreement was achieved between the calculated flames and the experimental flame images. The strong effects of curvature at the flame tip were revealed both experimentally and numerically, which were consistent with the literature data. In the lean Bunsen flame, curvature inhibits combustion, while, in the stoichiometric or rich flames, curvature enhances combus- tion. Based on the calculated flames, comparison of the detailed flame structures between the tip and the planar portion was conducted and the mechanism of the curvature effects was uncovered, which were closely related to the H2 transport characteristics. Besides, NO formation was investigated and a strong dependence on the local combustion intensity and atmosphere was observed. In stoichiometric and rich flames, two specific NO peaks were uncovered at the flame tip and on the base, respectively.