The Combination of Detailed Kinetics and CFD in Automotive Applications J. M. Deur and S. Jonnavithula Analysis and Design Application Co., Ltd. Melville, New York E. Meeks Reaction Design San Diego, California There are many automotive applications where complex chemistry plays an important role. For example, more stringent regulatory requirements for automobiles and trucks have led to a need to fine-tune combustors in IC engines to reduce emissions of pollutants such as NO x and introduce catalytic converter systems for aftertreatment of exhaust gases to further reduce pollutant emissions. However, most computational fluid mechanics (CFD) simulations of chemically reacting flows utilize very simplified chemistry. STAR-CD, a general purpose commercial CFD code, has been combined with complex chemistry solver technology of CHEMKIN and SURFACE CHEMKIN to provide an easy-to-use package capable of applying detailed chemistry in complex multi-dimensional geometries. CHEMKIN or more accurately the CHEMKIN Collection is a set of software programs and object libraries that work together to facilitate the formation, solution, and interpretation of problems involving gas-phase and heterogeneous (gas-surface) chemical kinetics. The Collection provides flexible and powerful tools for simulating complex chemical kinetics in a reacting flow. Examples of areas where CHEMKIN tools have been successfully applied include combustion, catalysis, chemical vapor deposition, and plasma etching.(1) Applications within the CHEMKIN Collection address idealized geometry and flow conditions, including perfectly stirred reactors, plug-flow reactors, stagnation flows, and channel flows where boundary-layer effects are important (Fig. 1). Kinetics mechanisms can be described as a set of the most elementary reactions, including a variety of pressure-dependence formulations, or as a set of empirically derived or global reactions. A rich surface kinetic formalism allows detailed descriptions of gas-surface kinetics, including the effects of surface site coverage and competition between adsorbing species for surface sites.(2) Although extremely useful, one limitation of the CHEMKIN Collection has been the inability to extend analyses beyond these idealized zero- and one-dimensional problems. One solution to this limitation would be to adapt the Collection's acknowledged chemistry solution methodology to a CFD platform where full use could be made of that platform's complex geometry handling. Ideally, the CFD code would also provide a wide suite of complementary physical models for turbulence, sprays, etc. STAR-CD, a general purpose finite volume CFD code which uses the PISO algorithm (a multi-corrector variant of SIMPLE), provides a particularly well-suited platform for combination with the complex chemistry solver technology of the CHEMKIN Collection.(Fig. 2) STAR-CD incorporates a variety of physical models for sprays and turbulence and already has a variety of models for combustion chemistry, albeit no model for handling detailed chemistry. Sprays are treated via a Lagrangian approach with sub-models for break-up, turbulent dispersion, and collision. Initial spray distributions can be readily implemented to match available experimental data, although STAR-CD also incorporates semi-empirical models to create distributions from available injector geometry information as well. Turbulence can be treated with a variety of models ranging from the standard k-e approach up to Large Eddy Simulation (LES). STAR-CD supports completely unstructured meshes, key to handling complex geometries, and has a very versatile mesh motion capability (for describing piston and valve motion in IC engines for example). To simplify the mesh generation process for STAR-CD, a variety of mesh generation tools are available, e.g., ProICE which provides an automated IC engine mesh and mesh motion description methodology.(3,4,5,6,7) While the combination of the chemistry solver technology from the CHEMKIN Collection and the flow solver technology of STAR-CD produces a very capable tool within itself, the combined STAR-CD + CHEMKIN code
can be even more useful when combined with the CHEMKIN Collection in a hierarchical approach analyzing reacting flow problems. CHEMKIN offers the ability to test reaction mechanisms and perform screening calculations on some key physical parameters within the context of its idealized reactor modeling facilities. Further, reaction mechanism reduction can also be performed within its suite of models. From here, full multidimensional simulations with these same reaction mechanisms or with the reduced mechanisms developed with the aid of CHEMKIN can be performed with the combined STAR-CD + CHEMKIN code. In addition, both Reaction Design and adapco are members of a Department of Energy sponsored CRADA whose goal is to develop a chemistry workbench that will start from molecular modeling (to estimate rates for reactions for which experimental data is lacking), pass through the idealized reactor modeling level, and eventually reach the multidimensional reacting flow CFD level (Fig. 3).(8) Results will be presented to show the application of both CHEMKIN Collection and the combined STAR-CD + CHEMKIN program to IC applications. In particular, the Aurora perfectly stirred reactor simulator from the CHEMKIN Collection will be used to examine Heptane auto-ignition characteristics (Fig. 4), while STAR-CD + CHEMKIN incylinder calculations of a heavy -duty Diesel utilizing a reduced hydrocarbon mechanism (9) will be described (Figs. 5-7). Discussion of application of both programs to catalytic combustion and catalytic aftertreatment will also be provided as time permits. Finally, methods for combining chemistry with turbulence, the combustion-turbulence interaction, will be discussed in the context of the combination of CHEMKIN and STAR-CD. Although the combustion-turbulence interaction is a broad subject, only a limited preview of the options being considered for use in the present combined code will be offered.(10,11) 1. McRae, G. J., and Meeks, E., "Coupling Computational Fluid Mechanics with Detailed Chemical Kinetics," AIChE Annual Meeting, 2000. 2. Meeks, E., "Coupling Fluid Dynamics and Chemistry," CD-adapco Japan Annual Users' Conference, 2000. 3. Deur, J. M., et al., "An Analytical Study of Piston Bowl and Fuel Injector Geometry Effects on Diesel Engine Simulation," Eighth International Engine Combustion Multi-Dimensional Modeling Conference, 1998. 4. Deur, J. M., et al., "Application of the Weller Flame Area Model in SI Engine Simulation," Ninth Inter-national Engine Combustion Multi-Dimensional Modeling Conference, 1999. 5. Deur, J. M., et al., "Combustion and Spray Simulation of a GDI Engine Incorporating a Multi-Component Fuel," Tenth International Engine Combustion Multi-Dimensional Modeling Conference, 2000. 6. Deur, J. M., et al., "Comparison of NO x Prediction Methodologies for Gas Turbine Combustor Simulations," 35th Intersociety Energy Conversion Engineering Conference, 2000. 7. Deur, J. M., "Chemistry in Multi-Dimensional Fluid Flow," Second International Workshop on CHEMKIN in Combustion, 2000. 8. Meeks, E., "Near-Term and Future Development of CHEMKIN," Second International Workshop on CHEMKIN in Combustion, 2000. 9. Kundu, K. P., et al., "Simplified Jet-A/Air Combustion Mechanisms for Calculation of NO x Emissions," 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 1998. 10. Kong, S.-C., and Reitz, R. D., "Use of Detailed Chemical Kinetics to Study HCCI Engine Combustion with Consideration of Turbulent Mixing Effects," ASME ICE Division Fall 2000 Technical Meeting, 2000. 11. Goldberg, U., and Palaniswamy, S., "Turbulent Reacting Flow Modeling - An Engineering Perspective," 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 1999.
Fuel + Oxidizer Burne d Gas AURORA PREMIX PLUG CRESLAF Fue Oxidize OPPDIF SPIN Figure 1. Examples of CHEMKIN Collection Applications. Figure 2. STAR-CD + CHEMKIN Collection.
Figure 3. Hierarchical Approach to Analyzing Reacting Flow Problems. Figure 4. Example of CHEMKIN Collection Predictions of Heptane Auto-Ignition.
Figure 5. NASA Glenn Reduced Reaction Mechanism for Generic Hydrocarbon Combustion.(9) Figure 6. Simplified Sector Mesh for Heavy -Duty Diesel Simulation with STAR-CD + CHEMKIN.
1 7 2 8 3 9 4 10 5 11 6 12 Figure 7. Centerline Temperature (every 2 ca) during Spray and Ignition in Heavy -Duty Diesel Simulation.