OCR Text |
Show mechanism-for C2H2 is incomplete. The presence of C2H6 in fuel was due to a fuel impurity. The addition of NO to the oxidant had negligible effects on all the above mentioned profiles, for both model predictions and experimental results. Profiles of NOx and HCN are shown in Fig. 8 under both purely thermal NO (Case I) and doped NO (Case In conditions. For both cases the measured NOx profiles were closely followed by the model. The sharp decrease near the flame indicated a destruction mechanism taking place on the fuel rich side immediately next to the flame. With 3200 ppmv NO added to the oxidant (Case II), the HCN, formed at the same location due to conversion from both NO and N2 , was predicted well. With zero NO doping in the oxidant stream (Case I), the model over-predicted measured HeN values, by a factor of two. This is consistent with exhaust HCN results (35 ppmv predicted vs. 33 ppm measured for 2040 ppm NO inlet, and 5.3 ppmv predicted vs. 0.8 and 1.9 ppmv measured with zero NO inlet). NH) was less than 2 ppmv which, for these samples, was the measurement threshold. NO Destruction Pathways Mechanisms governing the destruction of NO in counter-flow diffusion flames were explored, using the modeling results discussed above. In this overall fuel lean diffusion flame, NO formation was dominated by the Prompt NO mechanism. The Zeldovich NO mechanism was relatively unimportant due to the low flame temperature. Destruction of NO, which occurred on fuel rich side of the flame, led to conversion to HCN through the intermediate species HCNO. Diffusing towards the flame front, HCN was then converted to NCO which in tum broke down into NH and N. Upon formation at flame zone, most of the N atoms reacted with OH to form NO, to complete a cycle, while the remainder reacted with NO to form molecular nitrogen, leading to NO reduction. When NO at the flame front ceases to be continuously replenished from the oxidant stream, as in Configuration B, it is continuously destroyed as it travels alongside the flame, until the reaction zone is extinguished. CONCLUSIONS Given the range of environments and temperatures present in a counter-flow flame, the Miller and Bowman mechanism[ll] predicts NO formation and destruction in these systems very well. Other simulations, using the mechanism of Glarborg, Miller and Kee[13], yielded poorer comparisons. The good agreement between model and experiment for in-flame profiles supports the validity of the measured data, and strengthens the conclusions drawn from exhaust measurements. In addition, detailed modeling such as this allowed a possible destruction pathway of NO to be identified, and the exhaust data to be interpreted. Taken as a whole, these results demonstrate that the formation and destruction of NO in a stretched, flat, counterflow diffusion flame can be satisfactorily theoretically simulated. However, great net destruction of NO cannot be expected from a flat stretched flame, although it may be achieved in other, overall fuel lean, diffusion flames. |