Evaluation of Reduced Kinetic Mechanisms in Numerical Simulations of Nonpremixed Flames

The goal of this study is to understand the limits of applicability of progressively more accurate reduced kinetic mechanisms for nonpremixed methane-air flames using one-dimensional steady and two-dimensional time dependent numerical simulations. One-, threeand four-step reduced kinetic mechanisms for methane-air flames are studied in a Tsuji burner configuration using simplified transport mechanism. Flame structure and flame extinction characteristics are studied in detail. The time dependent flame response to interaction between a pair of counter rotating vortices with an initially laminar unstrained flame is studied. Different combinations of sizes and strengths for the vortex pair are selected. Simulations in which continuous burning and local extinction of the flame by the vortex pair have been observed for all three reduced kinetic mechanisms. Extinction mechanism for the one-step flame is primarily due to fluid strain, while strain and chemistry effects contribute to the threeand four-step flame extinctions. INTRODUCTION AND OUTLINE Nonpremixed flames are those in which fuel and oxidizer are initially separated, and react upon mix* Supported by American Chemical Society/Petroleum Research Fund. t Copyright © 2000 The American Institute of Aeronautics and Astronautics Inc. All rights reserved. * Member AIAA ing. When the flow field is turbulent, the flame can be considered to be interacting with an ensemble of vortices of different length and time scales. Thus, the interaction of a flame with an individual pair of vortices is of significant theoretical interest. Several researchers have employed detailed chemistry in their investigation of laminar diffusion flame structure, they include Peters and Williams (1987), Peters and Kee (1987) and Bilger et al. (1990). With detailed chemistry, Smooke and Giovangigli (1991) have examined counterflow diffusion flame structure with simplified transport. Many studies, including Laverdant and Candel (1988), Rutland and Ferziger (1991), Thevenin and Candel (1995) and Takahashi and Katta (1995) have investigated the effects of vortex motion on premixed and nonpremixed combustion using finite and infinitly fast chemistry. Cuenot and Poinsot (1994) and Rehm et al. (1996) have used simplified chemistry in examining nonpremixed combustion and diffusion flame reaction zone structure in a vortex. With the recent advancement in parallel computing, the implementation of detailed chemistry in numerical studies has been realized. Thevenin et al. (1996, 1998) and Katta et al. (1998) have employed detailed chemistry to study quenching patterns of a vortex interacting with a hydrogen-air flame. In most of these investigations, either simply, irreversible, infinitely fast chemistry was employed in the computation, or detailed chemistry which describes the combustion processes using many species and reversible reac(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. tions. Since there exists several well documented reduced chemical kinetic schemes for hydrocarbon flames (Maas and Warnatz (1988) and Bilger et al (1990)), it is possible to carry out investigations which the level of chemistry involved can be analyzed under the same condition. The present study is driven by a desire to obtain a better understanding on combustion processes using flame-vortex interaction studies with implementation of different degrees of reduced kinetics, which is currently lacking. We specifically consider one-, threeand fourstep reduced kinetic mechanisms for methane-air combustion to model the flame. We begin by verifying and documenting the flame structure and extinction characteristics under a steady flow condition in a one-dimensional Tsuji burner configuration for the three kinetic mechanisms with the simplified transport model suggested by Smooke and Giovangigli (1991), against previous experimental and numerical studies (Tsuji and Yamaoka (1970) and Dixon-Lewis et al. (1984)). Then we utilize time dependent numerical simulations to examine two-dimensional, unsteady flamevortex interaction in viscous, compressible, reacting flows with the same reduced kinetic mechanisms and simplified transport model in the steady flame investigation. In this case, the flame interacts with a pair of counter-rotating vortices initiated on the fuel side. In both cases, the treatment of transport mechanisms and thermodynamic properties such as viscosity, thermal diffusivity and species diffusion, are the same. The time dependent flame structure, its response to increasing levels of unsteady strain are examined systematically, which is accomplished by varying the strength and size of the vortices. Different regimes of flame-vortex interaction including continuous burning and events leading to local extinction with various levels of reduced chemistry are studied and compared. REDUCED KINETIC MECHANISMS Reduced one-, three-, and four-step kinetic mechanisms for methane-air combustion are used in both the steady, one-dimensional laminar flame in a Tsuji configuration and the two-dimensional, unsteady flame-vortex interaction study. This choice is motivated by the fact that extensive work has been done on developing reduced kinetic schemes for methane-air flames (Peters and Williams (1987) and Peters and Kee (1987)). The global one-step reduced mechanism for methane-air combustion is as follows (Bui-Pham (1992)): CH4 + 2O2 -» C02 + 2H20 (I) The three-step reduced kinetic mechanism given in Peters and Williams (1987) is: CH4 + 02 -> CO + H2 + H20 (I} CO + H20 C02 + H2 (II)