Effects of C-2-chemistry on the structure of partially premixed methane-air flames

Partially-premixed flames (PPF) can contain multiple reaction zones, e.g., one or two with a premixed-like structure and one being a nonpremixed react ion zone. An intrinsic feature of partially premixed flames pertains to the synergistic interactions between these two types of reaction zones that ar e characterized by heat and mass transfer between them. Since these interac tions 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 C-1-chemistry in determining the structure of partially premixed methan e-air flames is investigated herein by employing two relatively derailed ch emical mechanisms. The first involves only C-1-containing species and consi sts of 52 reactions involving 17 species, while the second mechanism repres ents both C-1- and C-2-chemistry and consists of 81 reactions that involve 24 species. A planar two-dimensional partially premixed flame established o n a rectangular slot burner is simulated. The simulation is based on the nu merical solution of the time-dependent conservation equations for mass cont inuity, momentum, species, and energy. The computations are validated by co mparison with the experimentally-obtained chemiluminescent emission from ex cited-C-2* free radical species, as well as with velocity measurements usin g particle image velocimetry. A numerical study is then conducted to examin e the applicability of C-1 and C-2 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 stoichio metry. Since the C-1 mechanism is known to be inadequate for fuel-rich prem ixed flames, its relatively good performance can be attributed to the inter actions between the two reaction zones that characterize the PPF structure. There are, however, important quantitative differences between the predict ions of the two mechanisms. The C-2 mechanism is overall superior compared to the C-1 mechanism in that its predictions are in closer agreement with o ur experimental results. The rich premixed reaction zone height obtained wi th the C-2 mechanism is more sensitive to variations in the equivalence rat io as compared with predictions that are obtained using the C-1-mechanism. In addition, for high levels of partial premixing, the methane consumption in the inner reaction zone is significantly increased when the C-2-mechanis m is employed. compared to when the C-1-mechanism is used. Consequently, th e amount of methane that leaks from the rich premixed to nonpremixed reacti on zone is significantly lower when the C-2-mechanism is used. The interact ions between the inner and outer reaction zones are stronger when the C-2-m echanism is employed. Finally, the maximum temperature predicted by the C-2 -mechanism is slightly lower as compared to that obtained using the C-1-che mistry alone. These differences are attributed to the presence of the C-2-c hain in the 81-step mechanism, which strongly affects the inner premixed re action zone.