Counterflow water-laden flames to simulate fuel hydrate combustion

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 statistical narrow-band radiation, conductive, and convective heat loss models. 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. Thus, the maximum flame temperature at the extinction limits increases with increasing strain rates because there is less water to act as a thermal sink. For a fixed strain rate, the maximum flame temperature decreases with water addition. The observed flammable range of the water to methane molar ratio is comparable to that found in self-sustained combustion of methane hydrates. 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 insignificant. Predicted and measured extinction limits, temperature and flame position exhibit encouraging agreement when incorporating appropriate reaction mechanisms and heat losses.