Simulation of Engine Exhaust Aftertreatment with CFD using Detailed Chemistry J. M. Deur, S. Jonnavithula, S. Dhanapalan, K. Schulz, B. Raghunathan, and H. Nakla Analysis and Design Application Co., Ltd. Melville, New York E. Meeks and C. P. Chou Reaction Design San Diego, California A variety aftertreatment systems have been or are being developed to reduce emissions of NO x and other pollutants from auto and truck exhausts. In addition to three-way catalytic converters for automobiles, several catalytic technologies are also being devised to reduce emissions from Diesel-powered vehicles. These include selective catalytic reduction (SCR), lean NO X traps (LNT), and catalyzed soot filters (CSF).(Fig. 1) In the past, computational fluid mechanics (CFD) simulations of such devices have simplified the problem by introducing porous media descriptions of the catalyst monoliths and utilizing very simple descriptions of the catalysts' surface chemistry.(1,2) To remedy this latter deficiency, STAR-CD, a general purpose commercial CFD code, has been combined with the complex chemistry solver technology of the CHEMKIN Collection to provide an easy-to-use package capable of applying detailed surface, as well as gas phase, chemistries in complex multi-dimensional geometries. The CHEMKIN Collection provides flexible and powerful tools for simulating complex chemical kinetics in a reacting flow. 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. STAR-CD, a 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. STAR-CD utilizes a fully unstructured solver with a wide variety of tools to handle complex geometries. STAR-CD also offers a while variety of physical models to complement the detailed chemistry capabilities offered by the CHEMKIN Collection-derived solver. For example,sprays are treated via a Lagrangian approach with sub-models for break-up, turbulent dispersion, and collision, while turbulence, when present, can be treated with several models ranging from the standard k-e approach up to Large Eddy Simulation (LES).(3) Of course, the ability to utilize detailed chemistry requires the availability of detailed reaction mechanisms. These are now being developed for aftertreatment devices by various research teams. For example, a detailed mechanism for Pt/Rh three-way catalytic converters is now available.(fig. 2) In order to test these new mechanisms and calibrate them for a given system, it has been found that a combination of CHEMKIN Collection and single-channel STAR-CD+CHEMKIN calculations are particularly effective.(figs. 3 and 4) With the capability to incorporate detailed surface chemistry within the channels of a catalyst monolith, a methodology is needed to describe the flow within the monolith in greater detail than possible with a porous media description. The chief problem becomes one of balancing the detail desired against computational cost. In the end, the decision has been made to opt for a representative channel approach, where only selected channels are modeled within the monolith. As this approach's name implies, these channels are postulated to be representative of their neighbors. Where the monolith mates with inlet and outlet ducts, some means is needed to couple the regions represented by these individual channels with the corresponding portions of the inlet and outlet faces of the ducts.(fig. 5) By utilizing the user-subroutine facilities of STAR-CD, this coupling can be readily accomplished. The resultant solution continuously couples pressure and other flow variables across the interface with a methodology that allows arbitrary associations between all domains. This, combined with STAR-CD's existing conjugate heat transfer methodology, allows the flow, chemistry, and heat transfer effects to all be considered in detail.(fig. 6) Diesel aftertreatment devices include additional complexities. For example, SCR systems add urea injection, as well as the thermolysis and hydrolysis of the urea to ammonia, upstream of the actual SCR catalyst. The spray
injection is readily treated with STAR-CD's standard Lagrangian methodology.(fig. 7) The thermolysis is assumed to occur instantaneously relative to the evaporation process, and the hydrolysis surface chemistry can be modeled with an appropriate one-step mechanism from the literature.(4,5) In the SCR catalyst, NO X is reduced by ammonia over a vanadia/titania catalyst. Such systems have been of interest in stationary applications for some time, and these applications are where most of the kinetics development has occurred.(6) Fortunately, the chemistry is basically the same, and reasonably detailed mechanisms are available.(fig. 8) As with the three-way automotive catalysts, CHEMKIN Collection and single channel STAR-CD+CHEMKIN calculations are helpful for testing and calibration.(fig. 9) The most complex Diesel aftertreatment technology is the CSF. A filter is introduced into the Diesel exhaust stream to trap soot particles. However, without introducing a means of oxidizing the soot trapped in the filter, the filter would soon clog from the deposited soot. Because of the temperatures involved, the oxidation of the soot is often performed catalytically with NO 2. The NO 2 is converted from NO in the exhaust stream via another surface catalyzed process. Yet, the most challenging modeling aspect of the CSF is the filtration process itself. The filter material is treated as a porous media whose permeability is calculated from filtration theory (Fig. 10). The approach is to take an existing lumped parameter filtration model that treats the filter as a whole and apply it in a discretized fashion down the length of a wall-flow filter channel.(7) The regeneration process can be treated via a simple global reaction from the original work or via the STAR-CD+CHEMKIN chemistry calculation. Initial results show good agreement between the original model and the discretized application in STAR-CD (Fig. 11). References 1. Clarkson, R., "The Simulation of the Heat Transfer, Chemical Reactions, and Fluid Flow within a Catalytic Converter," Computational Dynamics Limited, 1996. 2. Clarkson, R., "The Implementation of a Reacting Catalyst Model within STAR-CD," Compuational Dynamics Limited, 1996. 3. Deur, J. M., et al., "The Combination of Detailed Kinetics and CFD in Automotive Applications," Eleventh International Engine Combustion Multi-Dimensional Modeling Conference, 2001. 4. Kleemann, M., et al., "Hydrolysis of Isocyanic Acid on SCR Catalysts," Industrial and Engineering Chemistry Research, 2000. 5. Brouwer, J., et al., "A Model for Prediction of Selective Non-Catalytic Reduction of Nitrogen Oxides by Ammonia, Urea, and Cyanuric Acid with Mixing Limitations in the Presence of CO," 26th International Symposium on Combustion, 1996. 6. Dumesic, J. A., et al., Kinetics of Selective Catalytic Reduction of Nitro Oxide by Ammonia over Vanadia/Titania, Journal of Catalysis, 1996. 7. Opris, C. N., A Computational Model Based on the Flow, Filtration, Heat Transfer, and Reaction Kinetics Theory in a Porous Ceramic Diesel Particulate Trap (Ph.D. Thesis)," Michigan Technological University, 1997. 8. Schindler, K. P., "Diesel Engines and the Environment," Conference onthermofluidynamic Processes in Diesel Engines (Thiesel), 2000. 9. Chatterjee, D., Detaillierte Modellierung von Abgskatalysatoren (Ph.D. Thesis), University of Heidelberg, 2001.
Figure 1. Diesel Aftertreatment Systems: (top, left, bottom) SCR, LNT, and CSF (8) Figure 2. Three-Way Catalytic Converter Reaction Mechanism (9)
Figure 3. Results of CHEMKIN Collection (CRESLAF) Calculations for Three-Way Catalytic Converter. Figure 4. Results of Single Channel STAR-CD+CHEMKIN Calculations for Three-Way Catalytic Converter. Figure 5. Example Showing Assignment of Representative Channels to Duct Boundary Face Region.
Figure 6. Results of STAR-CD Analysis of Representative Channel Coupling with Duct Boundary Face. Figure 7. Results of STAR-CD Urea Spray and Vaporization Calculations for Diesel SCR System. Figure 8. SCR System Reaction Mechanism.(6)
Figure 9. Results of Single Channel STAR-CD+CHEMKIN Calculations for Diesel SCR System. Figure 10. Example of STAR-CD Particulate Filter Trap Representation. Figure 11. Comparison STAR-CD and Lumped Parameter Filtration Results for Filter Pressure Drop.