By Yong Yi, Fluent Inc.

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The majority of gasoline-fueled automobiles today have an emission control system that uses a three-way catalytic converter. The purpose of the three-way catalyst is to convert carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons to carbon dioxide, water, and nitrogen so that emissions from gasoline engines can be rendered less harmful to the environment.

Geometry and mesh of the generic 3-way catalytic converter

Catalytic converters are built from structures called monoliths. The monolith forms the basic framework of the converter, and acts as an inert substrate for the catalytic coating. A layer of washcoat is first deposited on the substrate, and the catalysts (often precious metals such as platinum, palladium, and rhodium) are then deposited on the washcoat. In order to reach the required conversion efficiencies for a practical converter, the surface area for reactions must be very large, and this large area is provided by the monolith geometry and the highly porous washcoat. To optimize the design of a catalytic converter, it is important to investigate not only the flow field, but the chemical reactions and heat transfer in the system as well. The distributions of temperature and species throughout the device play an important role in its performance.

FLUENT 6.1 is a powerful tool for reaction simulations. In addition to offering a number of modeling options for treating reacting flow, a new reaction model is available for reactions and heat transfer inside porous regions, such as the monolith in the catalytic converter. With the parallel computing capability in FLUENT, this model can easily include the effects of multiple species and reactions. Interoperability with CHEMKIN is also available, allowing FLUENT to read complex gas or surface reaction mechanisms, if needed.

For the catalytic converter, the reaction mechanism is taken from Reference 1. Exhaust gas, consisting of O₂, N₂, C₃H₆, H₂, H₂O, CO, and NO enters the converter from one runner with a uniform flow rate of 0.01 kg/sec and temperature of 600K. The wall of the converter is assumed to be adiabatic. The surface-to-volume ratio of the porous media is assumed to be 3000 m^-1. The exhaust species diffuse to the surface of the washcoat, and are adsorbed by platinum and rhodium to become sites species. Surface reactions take place, and product species are released from the reacting surface by desorption. Sixty-one surface reactions were used to model the conversion of this mixture.

The temperature distribution on a plane cutting through the exhaust pipes shows a temperature rise due to the catalyst reaction taking place. This result is reflected in contour plots of other species concentrations as well.

Temperature distribution on a cutting plane through the exhaust pipes with only the left runner open

Mass fraction of CO on the cutting plane through the exhaust pipes with only the left runner open

References

1 Chatterjee D., Deutschmann O., and Warnatz J., "Detailed Surface Reaction Mechanism in a Three-Way Catalyst", Faraday Discuss., Vol. 119, pp. 371-384, 2001.

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