Extinction limits and structure of counterflow nonpremixed water-laden methane / air flames

In order to better understand nonpremixed combustion processes when large amounts (water/fuel molar ratios on the order of unity) of water naturally incorporate into the fuel stream, the extinction limits and structure of counterflow nonpremixed flames of mixtures of water vapor, methane and air were identified experimentally and computationally. Such conditions arise, for example, in the combustion of methane hydrates and water/fuel emulsions. With water vapor addition, the extinction limits and flame temperature and location of methane/air flames were experimentally determined, while the extinction limits and the detailed flame structure were computed using a detailed kinetic mechanism, including a statistical narrow-band radiation model. Results generally show narrowing of the extinction limits (in terms of the water to methane molar ratio) with increasing strain rates, implying that flames can sustain more water vapor at low strain rates. The maximum flame temperature at the extinction limit increases with increasing strain rate because there is less water present to act as a thermal sink. This result shows that the extinction is not due simply to water as a diluent but also involves reactant leakage. For a fixed strain rate, the maximum flame temperature decreases with water addition. With water addition flame location shifts towards the air stream due to the increased momentum of the water vapor-laden jet. Comparative predictions assuming added non-reactive water vapor indicate that the chemical effects of water addition on flame structure are relatively small but may be critical near extinction. Predicted and measured tendencies of extinction limits and temperature for various conditions exhibit encouraging agreement, but quantitative discrepancies among the predictions and measurements indicate a need in additional consideration for heat loss modeling.