Effects of C2-Chernistry on the Structure of Partially Premixed Methane-Air Flames

Abstract

Partially-premixed flames (PPF) can contain multiple reaction zones, e.g., one or two with a premixed-Iike structure and one being a nonpremixed reaction zone. An intrinsic feature of partially premixed flames pertains to the synergistic interactions between these two types of reaction zones that are characterized by heat and mass transfer between them. Since these interactions are strongly dependent on the distribution of the radical and stable species concentrations, an accurate representation of the flame chemistry involving these species is critical for simulating their behavior. The role of C2-chemistry in determining the structure of partially premixed methane-air flames is investigated herein by employing two relatively detailed chemical mechanisms. The first involves only C1-con-taining species and consists of 52 reactions involving 17 species, while the second mechanism represents both C1- and C2-chemistry and consists of 81 reactions that involve 24 species. A planar two-dimensional partially premixed flame established on a rectangular slot burner is simulated. The simulation is based on the numerical solution of the time-dependent conservation equations for mass continuity, momentum, species, and energy. The computations are validated by comparison with the experimentally-obtained chemiluminescent emission from excited-C2 free radical species, as well as with velocity measurements using particle image velocimetry. A numerical study is then conducted to examine the applicability of C1 and C2 mechanisms for predicting the structure of partially premixed flames for different levels of partial premixing and reactant velocity. Results indicate that both the mechanisms reproduce the global structure of PPF over a wide range of reactant velocity and stoichiometry. Since the C1 mechanism is known to be inadequate for fuel-rich premixed flames, its relatively good performance can be attributed to the interactions between the two reaction rones that characterize the PPF structure. There are, however, important quantitative differences between the predictions of the two mechanisms. The C2 mechanism is overall superior compared to the C, mechanism in that its predictions are in closer agreement with our experimental results. The rich premixed reaction zone height obtained with the C2 mechanism is more sensitive to variations in the equivalence ratio as compared with predictions that are obtained using the C1-mechanism. In addition, for high levels of partial premixing, the methane consumption in the inner reaction zone is significantly increased when the C2-mechanism is employed, compared to when the C1-mechanism is used. Consequently, the amount of methane that leaks from the rich premixed to nonpremixed reaction zone is significantly lower when the C2 mechanism is used. The interactions between the inner and outer reaction zones arestronger when the C2 mechanism is employed. Finally, the maximum temperature predicted by the C2-mechanism is slightly lower as compared to that obtained using the C1-chemistry alone. These differences are attributed to the presence of the C2-chain in the 81-step mechanism, which strongly affects the inner premixed reaction zone.