Structure and Behavior of Water-laden Methane/Air Flames
An experimental and chemical kinetic study has been conducted to determine the role of water when it is added as a diluent to the fuel side of methane-air counterflow diffusion flames. This work is relevant to combustion processes where water is incorporated in the fuel; e.g., methane hydrates and applications for emission reduction such as in flares and H2O /fuel emulsions. The flame temperature profile and extinction limits were measured, and the flame structure was computed using detailed kinetic mechanisms. Predictions and experiments demonstrate that water mainly acts thermally to lower the flame temperature until extinction. Very close to extinction, however, the calculations show that water has a small but distinct chemical effect as it scavenges important chain branching radicals. Experiments and computations show that extinction limits (in terms of the H2O mass fraction) decrease with increasing strain rates; that is, flames can sustain more H2O vapor at low strain rates. Maximum flame temperature at extinction increases with increasing strain rates because there is less H2O to act as a thermal sink. Temperature and hydroxyl radical PLIF measurements confirm qualitative features found in the calculations, showing a reduction of peak values, a decrease in the reaction zone thickness, and a movement of the flame location with water addition. The two representative well-known chemical kinetic mechanisms examined overpredict the water carrying capacity of these flames in comparison to the experiments. A similar disagreement was not found when using carbon dioxide as the fuel diluent, which suggests that water-specific reactions may be responsible for the final stages of flame extinction in water-laden non-premixed flames. Computationally, chemical effects were distinguished from thermal effects by introducing of a nonreactive diluent molecule with the same thermal and transport properties as water. The study indicates that water's chemical effects are to change the production and depletion of OH, H and O radicals, especially near extinction. Furthermore, a reaction path analysis shows that atomic hydrogen plays a critical role in extinction through chain branching and chain terminating reactions. This chemical kinetic interpretation of water's role confirms some findings described in the literature, in particular, the influence from the chain terminating reaction and the considerable uncertainty in the value of water's chaperon efficiency. It is difficult to ascribe detailed behaviors to individual reactions in complex chemical kinetic mechanisms but the data presented in this dissertation provide a robust set of outcomes that can be used to refine key reaction parameters in water-laden flame chemistry.