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Show FLAME MODELING STUDIES Overview A counterflow diffusion flame, shown in Fig. 1, offers a convenient geometry for modeling the detailed processes that occur in diffusion flames. In this stagnation flow, where the fuel and oxidizer streams are opposed, a flame is established on the oxidizer side of the stagnation point. The mathematical description of this flow along the stagnation streamline is one-dimensional, i.e., the species mass fractions, temperature, and velocity are functions only of the axial coordinate x [3-5]. As a result of this simplification to one-dimensionality, very extensive chemical kinetic mechanisms can be incorporated into numerical flame models, with relatively modest computer run-time requirements. The counterflow flame model used in the present study is that developed by Kee et al. [3] and extended by Lutz et al. [6]. The coupled governing conservation equations for mass, species, momentum, and energy form a steady-state, boundary-value problem, which is solved by discretizing the partial differential equations in space using finite differences. Details of the flame model and the solution technique are provided in Lutz et al. [6]. Thermodynamic and molecular transport properties are provided by Chemkin libraries and subroutines [7,8]. The detailed mechanisms for methane combustion and oxides of nitrogen chemistry from Miller and Bowman [9] were employed to evaluate the reaction rate terms in the species and energy conservation equations. This mechanism includes 52 species and 235 elementary reactions. The presence of the diluent, or recirculated gases, is taken into account by specifying the species mass fractions at both the fuel-side boundary (x = 0) and the oxidizer-side boundary (x = L) as part of the boundary conditions to the problem. In the present study, pure N2 was used as the diluent. Although the heat capacity of pure N2 is somewhat less than recirculated combustion product gases, the use of N2 does not alter the chemical phenomena in any substantial way since N2 is the primary constituent (>60%) in the products. Runs using a mixture of C02, H20, and N2 with typical product-gas composition as the diluent showed the same qualitative behavior as for pure N2 . 2 |